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Title: Fragments of science, V. 1-2
Author: Tyndall, John, 1820-1893
Language: English
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Copyright Status: Not copyrighted in the United States. If you live elsewhere check the laws of your country before downloading this ebook. See comments about copyright issues at end of book.

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FRAGMENTS OF SCIENCE:

A Series of Detached

ESSAYS, ADDRESSES, AND REVIEWS.

BY

JOHN TYNDALL, F.R.S.

Printed By Spottiswoode AND CO.

NEW-STREET SQUARE

PARLIAMENT STREET

SIXTH EDITION,

VOL. 1.

LONDON: LONGMANS, GREEN, AND CO.

1879.

All rights reserved.

PREFACE TO THE SIXTH EDITION.

VOL. I.  INORGANIC NATURE.

I.      THE CONSTITUTION OF NATURE.

II.     RADIATION.

        1.  Visible and Invisible Radiation.

        2.  Origin and Character of Radiation.  The Aether.

        3.  The Atomic Theory in reference to the Aether.

        4.  Absorption of Radiant Heat by Gases.

        5.  Formation of Invisible Foci.

        6.  Visible and Invisible Rays of the Electric Light.

               Figure 1  Spectrum of Electric Light.

        7.  Combustion by Invisible Rays.

        8.  Transmutation of Rays: Calorescence.

        9.  Deadness of the Optic Nerve to the Calorific Rays.

        10. Persistence of Rays.

        11. Absorption of Radiant Heat by Vapours and Odours.

        12. Aqueous Vapour in relation to the Terrestrial Temperatures.

        13. Liquids and their Vapours in relation to Radiant Heat.

        14. Reciprocity of Radiation and Absorption.

        15. Influence of Vibrating Period and Molecular Form.

               Physical Analysis of the Human Breath.

        16. Summary and Conclusion.

III.    ON RADIANT HEAT IN RELATION TO THE COLOUR AND
CHEMICAL CONSTITUTION OF BODIES.


        1. DECOMPOSITION BY LIGHT.

           Physical Considerations.

           Production of Sky-blue by the Decomposition of Nitrite of Amyl.

        2. ON THE BLUE COLOUR OF THE SKY, & THE POLARISATION OF SKYLIGHT.

        3.  THE SKY OF THE ALPS.

V.      ON DUST AND DISEASE.

          Experiments on Dusty Air.

          The Germ Theory of Contagious Disease.

          Parasitic Diseases of Silkworms.  Pasteur's Researches.

          Origin and Propagation of Contagious Matter.

          The Germ Theory applied to Surgery.

          The Luminous beam as a means of Research.

          The Floating Matter of the Air.

          Dr. Bennett's Experiments.

          Application of Luminous beams to Water.

          Chalk-water.  Clark's Softening Process.

          Cotton-wool Respirator.

          Fireman's Respirator.

          Helmholtz on Hay Fever.

VI.     VOYAGE TO ALGERIA TO OBSERVE THE ECLIPSE.

VII.    NIAGARA.

VIII    THE PARALLEL ROADS OF GLEN ROY.

IX.     ALPINE SCULPTURE.

X.      RECENT EXPERIMENTS ON FOG-SIGNALS.

XI.     ON THE STUDY OF PHYSICS.

XII.    ON CRYSTALLINE AND SLATY CLEAVAGE.

XIII.   ON PARAMAGNETIC AND DIAMAGNETIC FORCES.

XIV.    PHYSICAL BASIS OF SOLAR CHEMISTRY.

XV.     ELEMENTARY MAGNETISM.

XVI.    ON FORCE.

XVII.   CONTRIBUTIONS TO MOLECULAR PHYSICS.

XVIII.  LIFE, AND LETTERS OF FARADAY.

XIX.    THE COPLEY MEDALIST OF 1870.

XX.     THE COPLEY MEDALIST OF 1871.

XXI.    DEATH BY LIGHTNING.

XXII.   SCIENCE AND THE 'SPIRITS'.

********************

VOL. II.

I.      REFLECTIONS ON PRAYER AND NATURAL LAW.

II      MIRACLES AND SPECIAL PROVIDENCES.

          ADDITIONAL REMARKS ON MIRACLES.

III     ON PRAYER AS A FORM OF PHYSICAL ENERGY.

IV.     VITALITY.

V.      MATTER AND FORCE.

VI.     SCIENTIFIC MATERIALISM.

VII.    AN ADDRESS TO STUDENTS.

VIII.   SCIENTIFIC USE OF THE IMAGINATION.

IX.     THE BELFAST ADDRESS.

X.      APOLOGY FOR THE BELFAST ADDRESS.

XI.     THE REV. JAMES MARTINEAU AND THE  BELFAST ADDRESS.

XII.    FERMENTATION, & ITS BEARINGS ON SURGERY & MEDICINE.

XIII.   SPONTANEOUS GENERATION.

XIV.    SCIENCE AND MAN.

XV.     PROFESSOR VIRCHOW AND EVOLUTION.

XVI. THE ELECTRIC LIGHT.

********************

PREFACE TO THE SIXTH EDITION.

TO AVOID unwieldiness of bulk this edition of the 'Fragments' is
published in two volumes, instead of, as heretofore, in one.

The first volume deals almost exclusively with the laws and phenomena
of matter.  The second trenches upon questions in which the phenomena
of matter interlace more or less with those of mind.

New Essays have been added, while old ones have been revised, and in
part recast. To be clear, without being superficial, has been my aim
throughout.

In neither volume have I aspired to sit in the seat of the scornful,
but rather to treat the questions touched upon with a tolerance, if
not a reverence, befitting their difficulty and weight.

Holding, as I do, the nebular hypothesis, I am logically bound to
deduce the life of the world from forces inherent in the nebula.  With
this view, which is set forth in the second volume, it seemed but fair
to associate the reasons which cause me to conclude that every attempt
made in our day to generate life independently of antecedent life has
utterly broken down.

A discourse on the Electric Light winds up the Second volume.  The
incongruity of its position is to be referred to the lateness of its
delivery.

********************

VOL. I. INORGANIC NATURE.

I. THE CONSTITUTION OF NATURE.

[Footnote: 'Fortnightly Review,' 1865, vol. iii. p. 129.]

WE cannot think of space as finite, for wherever in imagination we
erect a boundary, we are compelled to think of space as existing
beyond it.  Thus by the incessant dissolution of limits we arrive at a
more or less adequate idea of the infinity of space.  But, though
compelled to think of space as unbounded, there is no mental necessity
compelling us to think of it either as filled or empty; whether it is
so or not must be decided by experiment and observation.  That it is
not entirely void, the starry heavens declare; but the question still
remains, Are the stars themselves hung in vacuo?  Are the vast
regions which surround them, and across which their light is
propagated, absolutely empty?  A century ago the answer to this
question, founded on the Newtonian theory, would have been, 'No, for
particles of light are incessantly shot through space.' The reply of
modern science is also negative, but on different grounds.  It has the
best possible reasons for rejecting the idea of luminiferous
particles; but, in support of the conclusion that the celestial spaces
are occupied by matter, it is able to offer proofs almost as cogent as
those which can be adduced of the existence of an atmosphere round the
earth.  Men's minds, indeed, rose to a conception of the celestial and
universal atmosphere through the study of the terrestrial and local
one.  From the phenomena of sound, as displayed in the air, they
ascended to the phenomena of light, as displayed in the _aether_; which
is the name given to the interstellar medium.

The notion of this medium must not be considered as a vague or
fanciful conception on the part of scientific men.  Of its reality
most of them are as convinced as they are of the existence of the sun
and moon.  The luminiferous aether has definite mechanical properties.
It is almost infinitely more attenuated than any known gas, but its
properties are those of a solid rather than of a gas.  It resembles
jelly rather than air.  This was not the first conception of the
aether, but it is that forced upon us by a more complete knowledge of
its phenomena.  A body thus constituted may have its boundaries; but,
although the aether may not be co-extensive with space, it must at all
events extend as far as the most distant visible stars.  In fact it is
the vehicle of their light, and without it they could not be seen.
This all-pervading substance takes up their molecular tremors, and
conveys them with inconceivable rapidity to our organs of vision.  It
is the transported shiver of bodies countless millions of miles
distant, which translates itself in human consciousness into the
splendour of the firmament at night.

If the aether have a boundary, masses of ponderable matter might be
conceived to exist beyond it, but they could emit no light.  Beyond
the aether dark suns might burn; there, under proper conditions,
combustion might be carried on; fuel might consume unseen, and metals
be fused in invisible fires.  A body, moreover, once heated there,
would continue for ever heated; a sun or planet once molten, would
continue for ever molten.  For, the loss of heat being simply the
abstraction of molecular motion by the aether, where this medium is
absent no cooling could occur.  A sentient being on approaching a
heated body in this region, would be conscious of no augmentation of
temperature.  The gradations of warmth dependent on the laws of
radiation would not exist, and actual contact would first reveal the
heat of an extra ethereal sun.

Imagine a paddle-wheel placed in water and caused to rotate.  From it,
as a centre, waves would issue in all directions, and a wader as he
approached the place of disturbance would be met by stronger and
stronger waves.  This gradual augmentation of the impression made upon
the wader is exactly analogous to the augmentation of light when we
approach a luminous source.  In the one case, however, the coarse
common nerves of the body suffice; for the other we must have the
finer optic nerve.  But suppose the water withdrawn; the action at a
distance would then cease, and, as far as the sense of touch is
concerned, the wader would be first rendered conscious of the motion
of the wheel by the blow of the paddles.  The transference of motion
from the paddles to the water is mechanically similar to the
transference of molecular motion from the heated body to the aether;
and the propagation of waves through the liquid is mechanically
similar to the propagation of light and radiant heat.

As far as our knowledge of space extends, we are to conceive it as the
holder of the luminiferous aether, through which are interspersed, at
enormous distances apart, the ponderous nuclei of the stars.
Associated with the star that most concerns us we have a group of dark
planetary masses revolving at various distances round it, each again
rotating on its own axis; and, finally, associated with some of these
planets we have dark bodies of minor note--the moons.  Whether the
other fixed stars have similar planetary companions or not is to us a
matter of pure conjecture, which may or may not enter into our
conception of the universe.  But probably every thoughtful person
believes, with regard to those distant suns, that there is, in space,
something besides our system on which they shine.

From this general view of the present condition of space, and of the
bodies contained in it, we pass to the enquiry whether things were so
created at the beginning.  Was space furnished at once, by the fiat of
Omnipotence, with these burning orbs?  In presence of the revelations
of science this view is fading more and more.  Behind the orbs, we now
discern the nebulae from which they have been condensed.  And without
going so far back as the nebulae, the man of science can prove that
out of common non-luminous matter this whole pomp of stars might have
been evolved.

The law of gravitation enunciated by Newton is, that every particle of
matter in the universe attracts every other particle with a force
which diminishes as the square of the distance increases.  Thus the
sun and the earth mutually pull each other; thus the earth and the
moon are kept in company, the force which holds every respective pair
of masses together being the integrated force of their component
parts.  Under the operation of this force a stone falls to the ground
and is warmed by the shock; under its operation meteors plunge into
our atmosphere mid rise to incandescence.  Showers of such meteors
doubtless fall incessantly upon the sun.  Acted on by this force, the
earth, were it stopped in its orbit to-morrow, would rush towards, and
finally combine with, the sun.  Heat would also be developed by this
collision.  Mayer first, and Helmholtz and Thomson afterwards, have
calculated its amount.  It would equal that produced by the combustion
of more than 5,000 worlds of solid coal, all this heat being generated
at the instant of collision.  In the attraction of gravity, therefore,
acting upon non-luminous matter, we have a source of heat more
powerful than could be derived from any terrestrial combustion.  And
were the matter of the universe thrown in cold detached fragments into
space, and there abandoned to the mutual gravitation of its own parts,
the collision of the fragments would in the end produce the fires of
the stars.

The action of gravity upon matter originally cold may, in fact, be the
origin of all light and heat, and also the proximate source of such
other powers as are generated by light and heat.  But we have now to
enquire what is the light and what is the heat thus produced?  This
question has already been answered in a general way.  Both light and
heat are modes of motion.  Two planets clash and come to rest; their
motion, considered as that of masses, is destroyed, but it is in great
part continued as a motion of their ultimate particles.  It is this
latter motion, taken up by the rather, and propagated through it with
a velocity of 186,000 miles a second, that comes to its as the light
and heat of suns and stars.  The atoms of a hot body swing with
inconceivable rapidity--billions of times in a second--but this power
of vibration necessarily implies the operation of forces between the
atoms themselves.  It reveals to us that while they are held together
by one force, they are kept asunder by another, their position at any
moment depending on the equilibrium of attraction and repulsion.  The
atoms behave as if connected by elastic springs, which oppose at the
same time their approach and their retreat, but which tolerate the
vibration called heat.  The molecular vibration once set up is
instantly shared with the aether, and diffused by it throughout space.

We on the earth's surface live night and day in the midst of aethereal
commotion.  The medium is never still.  The cloud canopy above us may
be thick enough to shut out the light of the stars; but this canopy is
itself a warm body, which radiates its thermal motion through the
aether.  The earth also is warm, and sends its heat-pulses incessantly
forth.  It is the waste of its molecular motion in space that chills
the earth upon a clear night; it is the return of thermal motion from
the clouds which prevents the earth's temperature, on a cloudy night,
from falling so low.  To the conception of space being filled, we must
therefore add the conception of its being in a state of incessant
tremor.

The sources of this vibration are the ponderable masses of the
universe.  Let us take a sample of these and examine it in detail.
When we look to our planet, we find it to be an aggregate of solids,
liquids, and gases.  Subjected to a sufficiently low temperature, the
two last, would also assume the solid form.  When we look at any one
of these, we generally find it composed of still more elementary
parts.  We learn, for example, that the water of our rivers is formed
by the union, in definite proportions, of two gases, oxygen and
hydrogen.  We know how to bring these constituents together, so as to
form water: we also know how to analyse the water, and recover from it
its two constituents.  So, likewise, as regards the solid portions of
the earth.  Our chalk hills, for example, are formed by a combination
of carbon, oxygen, and calcium.  These are the so-called elements the
union of which, in definite proportions, has resulted in the formation
of chalk.  The flints within the chalk we know to be a compound of
oxygen and silicium, called silica; and our ordinary clay is, for the
most part, formed by the union of silicium, oxygen, and the well-known
light metal, aluminium.  By far the greater portion of the earth's
crust is compounded of the elementary substances mentioned in these
few lines.

The principle of gravitation has been already described as an
attraction which every particle of matter, however small, exerts on
every other particle.  With gravity there is no selection; no
particular atoms choose, by preference, other particular atoms as
objects of attraction; the attraction of gravitation is proportional
simply to the quantity of the attracting matter, regardless of its
quality.  But in the molecular world which we have now entered matters
are otherwise arranged.  Here we have atoms between which a strong
attraction is exercised, and also atoms between which a weak
attraction is exercised.  One atom can jostle another out of its place
in virtue of a superior force of attraction.  But, though the amount
of force exerted varies thus from atom to atom, it is still an
attraction of the same mechanical quality, if I may use the term, as
that of gravity itself.  Its intensity might be measured in the same
way, namely by the amount of motion which it can generate in a certain
time.  Thus the attraction of gravity at the earth's surface is
expressed by the number 32; because, when acting freely on a body for
a second of time, gravity imparts to the body a velocity of thirty-two
feet a second.  In like manner the mutual attraction of oxygen and
hydrogen might be measured by the velocity imparted to the atoms in
their rushing together.  Of course such a unit of time as a second is
not here to be thought of, the whole interval required by the atoms to
cross the minute spaces which separate them amounting only to an
inconceivably small fraction of a second.

It has been stated that when a body falls to the earth it is warmed by
the shock.  Here, to use the terminology of Mayer, we have a
_mechanical_ combination of the earth and the body.  Let us suffer the
falling body and the earth to dwindle in imagination to the size of
atoms, and for the attraction of gravity let us substitute that of
chemical affinity; we have then what is called a chemical combination.
The effect of the union in this case also is the development of heat,
and from the amount of heat generated we can infer the intensity of
the atomic pull.  Measured by ordinary mechanical standards, this is
enormous.  Mix eight pounds of oxygen with one of hydrogen, and pass a
spark through the mixture; the gases instantly combine, their atoms
rushing over the little distances which separate them.  Take a weight
of 47,000 pounds to an elevation of 1,000 feet above the earth's
surface, and let it fall; the energy with which it will strike the
earth will not exceed that of the eight pounds of oxygen atoms, as
they dash against one pound of hydrogen atoms to form water.

It is sometimes stated that gravity is distinguished from all other
forces by the fact of its resisting conversion into other forms of
force.  Chemical affinity, it is said, can be converted into heat and
light, and these again into magnetism and electricity: but gravity
refuses to be so converted; being a force maintaining itself under all
circumstances, and not capable of disappearing to give place to
another.  The statement arises from vagueness of thought.  If by it be
meant that a particle of matter can never be deprived of its weight,
the assertion is correct; but the law which affirms the convertibility
of natural forces was never intended, in the minds of those who
understood it, to affirm that such a conversion as that here implied
occurs in any case whatever.  As regards convertibility into heat,
gravity and chemical affinity stand on precisely the same footing.
The attraction in the one case is as indestructible as in the other.
Nobody affirms that when a stone rests upon the surface of the earth,
the mutual attraction of the earth and stone is abolished; nobody
means to affirm that the mutual attraction of oxygen for hydrogen
ceases, after the atoms have combined to form water.  What is meant,
in the case of chemical affinity, is, that the pull of that affinity,
acting through a certain space, imparts a motion of translation of the
one atom towards the other.  This motion is not heat, nor is the force
that produces it heat.  But when the atoms strike and recoil, the
motion of translation is converted into a motion of vibration, which
is heat.  The vibration, however, so far from causing the extinction
of the original attraction, is in part carried on by that attraction.
The atoms recoil, in virtue of the elastic force which opposes actual
contact, and in the recoil they are driven too far back.  The original
attraction then triumphs over the force of recoil, and urges the atoms
once more together.  Thus, like a pendulum, they oscillate, until
their motion is imparted to the surrounding aether; or, in other
words, until their heat becomes radiant heat.

In this sense, and in this sense only, is chemical affinity converted
into heat.  There is, first of all, the attraction between the atoms;
there is, secondly, space between them.  Across this space the
attraction urges them.  They collide, they recoil, they oscillate.
There is here a change in the form of the motion, but there is no real
loss.  It is so with the attraction of gravity.  To produce motion by
gravity space must also intervene between the attracting bodies.  When
they strike together motion is apparently destroyed, but in reality
there is no destruction.  Their atoms are suddenly urged together by
the shock; by their own perfect elasticity these atoms recoil; and
thus is set up the molecular oscillation which, when communicated to
the proper nerves, announces itself as heat.

It was formerly universally supposed that by the collision of
unelastic bodies force was destroyed.  Men saw, for example, that when
two spheres of clay, painter's putty, or lead for example, were urged
together, the motion possessed by the masses, prior to impact, was
more or less annihilated.  They believed in an absolute destruction of
the force of impact.  Until recent times, indeed, no difficulty was
experienced in believing this, whereas, at present, the ideas of force
and its destruction refuse to be united in most philosophic minds.  In
the collision of elastic bodies, on the contrary, it was observed that
the motion with which they clashed together was in great part restored
by the resiliency of the masses, the more perfect the elasticity the
more complete being the restitution.  This led to the idea of
perfectly elastic bodies--bodies competent to restore by their recoil
the whole of the motion which they possessed before impact--and this
again to the idea of the _conservation_ of force, as opposed to that
destruction of force which was supposed to occur when unelastic bodies
met in collision.

We now know that the principle of conservation holds equally good with
elastic and unelastic bodies.  Perfectly elastic bodies would develop
no heat on collision.  They would retain their motion afterwards,
though its direction might be changed; and it is only when sensible
motion is wholly or partly destroyed, that heat is generated.  This
always occurs in unelastic collision, the heat developed being the
exact equivalent of the sensible motion extinguished.  This heat
virtually declares that the property of elasticity, denied to the
masses, exists among their atoms; by the recoil and oscillation of
which the principle of conservation is vindicated.

But ambiguity in the use of the term 'force' makes itself more and
more felt as we proceed.  We have called the attraction of gravity a
force, without any reference to motion.  A body resting on a shelf is
as much pulled by gravity as when, after having been pushed off the
shelf, it falls towards the earth.  We applied the term force also to
that molecular attraction which we called chemical affinity.  When,
however, we spoke of the conservation of force, in the case of elastic
collision, we meant neither a pull nor a push, which, as just
indicated, might be exerted upon inert matter, but we meant force
invested in motion--the _vis viva_, as it is called, of the colliding
masses.

Force in this form has a definite mechanical measure, in the amount of
work that it can perform.  The simplest form of work is the raising of
a weight.  A man walking up-hill, or up-stairs, with a pound weight in
his hand, to an elevation say of sixteen feet, performs a certain
amount of work, over and above the lifting of his own body.  If he
carries the pound to a height of thirty-two feet, he does twice the
work; if to a height of forty-eight feet, he does three times the
work; if to sixty-four feet, he does four times the work, and so on.
If, moreover, he carries up two pounds instead of one, other things
being equal, he does twice the work; if three, four, or five pounds,
he does three, four, or five times the work.  In fact, it is plain
that the work performed depends on two factors, the weight raised and
the height to which it is raised.  It is expressed by the product of
these two factors.

But a body may be caused to reach a certain elevation in opposition to
the force of gravity, without being actually carried up.  If a hodman,
for example, wished to land a brick at an elevation of sixteen feet
above the place where he stood, he would probably pitch it up to the
bricklayer.  He would thus impart, by a sudden effort, a velocity to
the brick sufficient to raise it to the required height; the work
accomplished by that effort being precisely the same as if he had
slowly carried up the brick.  The initial velocity to be imparted, in
this case, is well known.  To reach a height of sixteen feet, the
brick must quit the man's hand with a velocity of thirty-two feet a
second.  It is needless to say, that a body starting with any
velocity, would, if wholly unopposed or unaided, continue to move for
ever with the same velocity.  But when, as in the case before us, the
body is thrown upwards, it moves in opposition to gravity, which
incessantly retards its motion, and finally brings it to rest at an
elevation of sixteen feet.  If not here caught by the bricklayer, it
would return to the hodman with an accelerated motion, and reach his
hand with the precise velocity it possessed on quitting it.

An important relation between velocity and work is here to be pointed
out.  Supposing the hodman competent to impart to the brick, at
starting, a velocity of sixty-four feet a second, or twice its former
velocity, would the amount of work performed be twice what it was in
the first instance?  No; it would be four times that quantity; for a
body starting with twice the velocity of another, will rise to four
times the height.  In like manner, a three-fold velocity will give a
nine-fold elevation, a four-fold velocity will give a sixteen-fold
elevation, and so on.  The height attained, then, is not proportional
to the initial velocity, but to the square of the velocity.  As
before, the work is also proportional to the weight elevated.  Hence
the work which any moving mass whatever is competent to perform, in
virtue of the motion which it at any moment possesses, is jointly
proportional to its weight and the square of its velocity.  Here,
then, we have a second measure of work-, in which we simply translate
the idea of height into its equivalent idea of motion.

In mechanics, the product of the mass of a moving body into the square
of its velocity, expresses what is called the _vis viva_, or living
force.  It is also sometimes called the 'mechanical effect.' If, for
example, a cannon pointed to the zenith urge a ball upwards with twice
the velocity imparted to a second ball, the former will rise to four
times the height attained by the latter.  If directed against a
target, it will also do four times the execution.  Hence the
importance of imparting a high velocity to projectiles in war.  Having
thus cleared our way to a perfectly definite conception of the _vis
viva_ of moving masses, we are prepared for the announcement that the
heat generated by the shock of a falling body against the earth is
proportional to the _vis viva_ annihilated.  The heat is proportional to
the square of the velocity.  In the case, therefore, of two
cannon-balls of equal weight, if one strike a target with twice the
velocity of the other, it will generate four times the heat, if with
three times the velocity, it will generate nine times the heat, and so
on.

Mr. Joule has shown that a pound weight falling from a height of 772
feet, or 772 pounds falling through one foot, will generate by its
collision with the earth an amount of heat sufficient to raise a pound
of water one degree Fahrenheit in temperature.  772 "foot-pounds"
constitute the mechanical equivalent of heat.  Now, a body falling
from a height of 772 feet, has, upon striking the earth, a velocity of
223 feet a second; and if this velocity were imparted to the body, by
any other means, the quantity of heat generated by the stoppage of its
motion would be that stated above.  Six times that velocity, or 1,338
feet, would not be an inordinate one for a cannon-ball as it quits the
gun.  Hence, a cannon-ball moving with a velocity of 1,338 feet a
second, would, by collision, generate an amount of heat competent to
raise its own weight of water 36 degrees Fahrenheit in temperature. If
composed of iron, and if all the heat generated were concentrated in
the ball itself, its temperature would be raised about 360 degrees
Fahrenheit; because one degree in the case of water is equivalent to
about ten degrees in the case of iron.  In artillery practice, the
heat generated is usually concentrated upon the front of the bolt, and
on the portion of the target first struck.  By this concentration the
heat developed becomes sufficiently intense to raise the dust of the
metal to incandescence, a flash of light often accompanying collision
with the target.

Let us now fix our attention for a moment on the gunpowder which urges
the cannon-ball.  This is composed of combustible matter, which if
burnt in the open air would yield a certain amount of heat.  It will
not yield this amount if it perform the work of urging a ball.  The
heat then generated by the gunpowder will fall short of that produced
in the open air, by an amount equivalent to the _vis viva_ of the ball;
and this exact amount is restored by the ball on its collision with
the target.  In this perfect way are heat and mechanical motion
connected.

Broadly enunciated, the principle of the conservation of force
asserts, that the quantity of force in the universe is as unalterable
as the quantity of matter; that it is alike impossible to create force
and to annihilate it.  But in what sense are we to understand this
assertion?  It would be manifestly inapplicable to the force of
gravity as defined by Newton; for this is a force varying inversely as
the square of the distance; and to affirm the constancy of a varying
force would be self-contradictory.  Yet, when the question is properly
understood, gravity forms no exception to the law of conservation.
Following the method pursued by Helmholtz, I will here attempt an
elementary exposition of this law.  Though destined in its
applications to produce momentous changes in human thought, it is not
difficult of comprehension.

For the sake of simplicity we will consider a particle of matter,
which we may call F, to be perfectly fixed, and a second movable
particle, D, placed at a distance from F.  We will assume that these
two particles attract each other according to the Newtonian law.  At a
certain distance, the attraction is of a certain definite amount,
which might be determined by means of a spring balance.  At half this
distance the attraction would be augmented four times; at a third of
the distance, nine times; at one-fourth of the distance, sixteen
times, and so on.  In every case, the attraction might be measured by
determining, with the spring balance, the amount of tension just
sufficient to prevent D from moving towards F.  Thus far we have
nothing whatever to do with motion; we deal with statics, not with
dynamics.  We simply take into account the _distance_ of D from F, and
the _pull_ exerted by gravity at that distance.

It is customary in mechanics to represent the magnitude of a force by
a line of a certain length, a force of double magnitude being
represented by a line of double length, and so on.  Placing then the
particle D at a distance from F, we can, in imagination, draw a
straight line from D to F, and at D erect a perpendicular to this
line, which shall represent the amount of the attraction exerted on D.
If D be at a very great distance from F, the attraction will be very
small, and the perpendicular consequently very short.  If the distance
be practically infinite, the attraction is practically _nil_.  Let us
now suppose at every point in the line joining F and D a perpendicular
to be erected, proportional in length to the attraction exerted at
that point; we thus obtain an infinite number of perpendiculars, of
gradually increasing length, as D approaches F.  Uniting the ends of
all these perpendiculars, we obtain a curve, and between this curve
and the straight line joining F and D we have an area containing all
the perpendiculars placed side by side.  Each one of this infinite
series of perpendiculars representing an attraction, or tension, as it
is sometimes called, the area just referred to represents the sum of
the tensions exerted upon the particle D, during its passage from its
first position to F.

Up to the present point we have been dealing with tensions, not with
motion.  Thus far _vis viva_ has been entirely foreign to our
contemplation of D and F.  Let us now suppose D placed at a
practically infinite distance from F; here, as stated, the pull of
gravity would be infinitely small, and the perpendicular representing
it would dwindle almost to a point.  In this position the sum of the
tensions capable of being exerted on D would be a maximum.  Let D now
begin to move in obedience to the infinitesimal attraction exerted
upon it.  Motion being once set up, the idea of _vis viva_ arises.  In
moving towards F the particle D consumes, as it were, the tensions.
Let us fix our attention on D, at any point of the path over which it
is moving.  Between that point and F there is a quantity of unused
tensions; beyond that point the tensions have been all consumed, but
we have in their place an equivalent quantity of _vis viva_.  After D
has passed any point, the tension previously in store at that point
disappears, but not without having added, during the infinitely small
duration of its action, a due amount of motion to that previously
possessed by D.  The nearer D approaches to F, the smaller is the sum
of the tensions remaining, but the greater is the _vis viva_; the
farther D is from F, the greater is the sum of the unconsumed
tensions, and the less is the living force.  Now the principle of
conservation affirms _not_ the constancy of the value of the tensions of
gravity, nor yet the constancy of the _vis viva_, taken separately, but
the absolute constancy of the value of both taken together.  At the
beginning the _vis viva_ was zero, and the tension area was a maximum;
close to F the _vis viva_ is a maximum, while the tension area is zero.
At every other point, the work-producing power of the particle D
consists in part of _vis viva_, and in part of tensions.

If gravity, instead of being attraction, were repulsion, then, with
the particles in contact, the sum of the tensions between D and F
would be a maximum, and the _vis viva_ zero.  If, in obedience to the
repulsion, D moved away from F, _vis viva_ would be generated; and the
farther D retreated from F the greater would be its _vis viva_, and the
less the amount of tension still available for producing motion.
Taking repulsion as well as attraction into account, the principle of
the conservation of force affirms that the mechanical value of the
_tensions_ and _vires vivae_ of the material universe, so far as we know
it, is a constant quantity.  The universe, in short, possesses two
kinds of property which are mutually convertible.  The diminution of
either carries with it the enhancement of the other, the total value
of the property remaining unchanged.

The considerations here applied to gravity apply equally to chemical
affinity.  Ina mixture of oxygen and hydrogen the atoms exist apart,
but by the application of proper means they may be caused to rush
together across that space that separates them.  While this space
exists, and as long as the atoms have not begun to move towards each
other, we have tensions and nothing else.  During their motion towards
each other the tensions, as in the case of gravity, are converted into
_vis viva_.  After they clash we have still _vis viva_, but in another
form.  It _was_ translation, it _is_ vibration.  It _was_ molecular
transfer, it _is_ heat.

It is possible to reverse these processes, to unlock the combined
atoms and replace them in their first positions.  But, to accomplish
this, as much heat would be required as was generated by their union.
Such reversals occur daily and hourly in nature.  By the solar waves,
the oxygen of water is divorced from its hydrogen in the leaves of
plants.  As molecular _vis viva_ the waves disappear, but in so doing
they re-endow the atoms of oxygen and hydrogen with tension.  The
atoms are thus enabled to recombine, and when they do so they restore
the precise amount of heat consumed in their separation.  The same
remarks apply to the compound of carbon and oxygen, called carbonic
acid, which is exhaled from our lungs, produced by our fires, and
found sparingly diffused everywhere throughout the air.  In the leaves
of plants the sunbeams also wrench the atoms of carbonic acid asunder,
and sacrifice themselves in the act; but when the plants are burnt,
the amount of heat consumed in their production is restored.

This, then, is the rhythmic play of Nature as regards her forces.
Throughout all her regions she oscillates from tension to _vis viva_,
from _vis viva_ to tension.  We have the same play in the planetary
system.  The earth's orbit is an ellipse, one of the foci of which is
occupied by the sun.  Imagine the earth at the most distant part of
the orbit.  Her motion, and consequently her _vis viva_, is then a
minimum.  The planet rounds the curve, and begins its approach to the
sun.  In front it has a store of tensions, which are gradually
consumed, an equivalent amount of _vis viva_ being generated.  When
nearest to the sun the motion, and consequently the _vis viva_, reach a
maximum.  But here the available tensions have been used up.  The
earth rounds this portion of the curve and retreats from the sun.
Tensions are now stored up, but _vis viva_ is lost, to be again restored
at the expense of the complementary force on the opposite side of the
curve.  Thus beats the heart of the universe, but without increase or
diminution of its total stock of force.

I have thus far tried to steer clear amid confusion, by fixing the
mind of the reader upon things rather than upon names.  But good names
are essential; and here, as yet, we are not provided with such.  We
have had the force of gravity and living force--two utterly distinct
things.  We have had pulls and tensions; and we might have had the
force of heat, the force of light, the force of magnetism, or the
force of electricity--all of which terms have been employed more or
less loosely by writers on physics.  This confusion is happily avoided
by the introduction of the term 'energy,' which embraces both _tension_
and _vis viva_.  Energy is possessed by bodies already in motion; it is
then actual, and we agree to call it actual or dynamic energy.  It is
our old _vis viva_.  On the other hand, energy is possible to bodies not
in motion, but which, in virtue of attraction or repulsion, possess a
power of motion which would realise itself if all hindrances were
removed.  Looking, for example, at gravity; a body on the earth's
surface in a position from which it cannot fall to a lower one
possesses no energy.  It has neither motion nor power of motion.  But
the same body suspended at a height above the earth has a power of
motion, though it may not have exercised it.  Energy is possible to
such a body, and we agree to call this potential energy.  It consists
of our old tensions.  We, moreover, speak of the conservation of
energy, instead of the conservation of force; and say that the sum of
the potential and dynamic energies of the material universe is a
constant quantity.

A body cast upwards consumes the actual energy of projection, and lays
up potential energy.  When it reaches its utmost height all its actual
energy is consumed, its potential energy being then a maximum.  When
it returns, there is a reconversion of the potential into the actual.
A pendulum at the limit of its swing possesses potential energy; at
the lowest point of its arc its energy is all actual.  A patch of snow
resting on a mountain slope has potential energy; loosened, and
shooting down as an avalanche, it possesses dynamic energy.  The
pine-trees growing on the Alps have potential energy; but rushing down
the _Holzrinne_ of the woodcutters they possess actual energy.  The same
is true of the mountains themselves.  As long as the rocks which
compose them can fall to a lower level, they possess potential energy,
which is converted into actual when the frost ruptures their cohesion
and hands them over to the action of gravity.  The stone avalanches of
the Matterhorn and Weisshorn are illustrations in point.  The hammer
of the great bell of Westminster, when raised before striking,
possesses potential energy; when it falls, the energy becomes dynamic;
and after the stroke, we have the rhythmic play of potential and
dynamic in the vibrations of the bell.  The same holds good for the
molecular oscillations of a heated body.  An atom is driven against
its neighbour, and recoils.  The ultimate amplitude of the recoil
being attained, the motion of the atom in that direction is checked,
and for an instant its energy is all potential.  It is then drawn
towards its neighbour with accelerated speed; thus, by attraction,
converting its potential into dynamic energy.  Its motion in this
direction is also finally checked, and again, for an instant, its
energy is all potential.  It once more retreats, converting, by
repulsion, its potential into dynamic energy, till the latter attains
a maximum, after which it is again changed into potential energy.
Thus, what is true of the earth, as she swings to and fro in her
yearly journey round the sun, is also true of her minutest atom.  We
have wheels within wheels, and rhythm within rhythm.

When a body is heated, a change of molecular arrangement always
occurs, and to produce this change heat is consumed.  Hence, a portion
only of the heat communicated to the body remains as dynamic energy.
Looking back on some of the statements made at the beginning of this
article, now that our knowledge is more extensive, we see the
necessity of qualifying them.  When, for example, two bodies clash,
heat is generated; but the heat, or molecular dynamic energy,
developed at the moment of collision, is not the exact equivalent of
the sensible dynamic energy destroyed.  The true equivalent is this
heat, plus the potential energy conferred upon the molecules by the
placing of greater distances between them.  This molecular potential
energy is afterwards, on the cooling of the body, converted into heat.

Wherever two atoms capable of uniting together by their mutual
attractions exist separately, they form a store of potential energy.
Thus our woods, forests, and coal-fields on the one hand, and our
atmospheric oxygen on the other, constitute a vast store of energy of
this kind--vast, but far from infinite.  We have, besides our
coal-fields, metallic bodies more or less sparsely distributed through
the earth's crust. These bodies can be oxydised; and hence they are,
so far as they go, stores of energy.  But the attractions of the great
mass of the earth's crust are already satisfied, and from them no
further energy can possibly be obtained.  Ages ago the elementary
constituents of our rocks clashed together and produced the motion of
heat, which was taken up by the aether and carried away through
stellar space.  It is lost for ever as far as we are concerned.  In
those ages the hot conflict of carbon, oxygen, and calcium produced
the chalk and limestone bills which are now cold; and from this
carbon, oxygen, and calcium no further energy can be derived.  So it
is with almost all the other constituents of the earth's crust. They
took their present form in obedience to molecular force; they turned
their potential energy into dynamic, and yielded it as radiant heat to
the universe, ages before man appeared upon this planet.  For him a
residue of potential energy remains, vast, truly, in relation to the
life and wants of an individual, but exceedingly minute in comparison
with the earth's primitive store.

To sum up.  The whole stock of energy or working-power in the world
consists of attractions, repulsions, and motions.  If the attractions
and repulsions be so circumstanced as to be able to produce motion,
they are sources of working-power, but not otherwise.  As stated a
moment ago, the attraction exerted between the earth and a body at a
distance from the earth's surface, is a source of working-power;
because the body can be moved by the attraction, and in falling can
perform work.  When it rests at its lowest level it is not a source of
power or energy, because it can fall no farther.  But though it has
ceased to be a source of _energy_, the attraction of gravity still acts
as a _force_, which holds the earth and weight together.

The same remarks apply to attracting atoms and molecules.  As long as
distance separates them, they can move across it in obedience to the
attraction; and the motion thus produced may, by proper appliances, be
caused to perform mechanical work.  When, for example, two atoms of
hydrogen unite with one of oxygen, to form water, the atoms are first
drawn towards each other--they move, they clash, and then by virtue of
their resiliency, they recoil and quiver.  To this quivering motion we
give the name of heat.  This atomic vibration is merely the
redistribution of the motion produced by the chemical affinity; and
this is the only sense in which chemical affinity can be said to be
converted into heat.  We must not imagine the chemical attraction
destroyed, or converted into anything else.  For the atoms, when
mutually clasped to form a molecule of water, are held together by the
very attraction which first drew them towards each other.  That which
has really been expended is the _pull_ exerted through the space by
which the distance between the atoms has been diminished.

If this be understood, it will be at once seen that gravity, as before
insisted on, may, in this sense, be said to be convertible into heat;
that it is in reality no more an outstanding and inconvertible agent,
as it is sometimes stated to be, than is chemical affinity.  By the
exertion of a certain pull through a certain space, a body is caused
to clash with a certain definite velocity against the earth.  Heat is
thereby developed, and this is the only sense in which gravity can be
said to be converted into heat.  In no case is the _force_, which
produces the motion annihilated or changed into anything else.  The
mutual attraction of the earth and weight exists when they are in
contact, as when they were separate but the ability of that attraction
to employ itself in the production of motion does not exist.

The transformation, in this case, is easily followed by the mind's
eye.  First, the weight as a whole is set in motion by the attraction
of gravity.  This motion of the mass is arrested by collision with the
earth, being broken up into molecular tremors, to which we give the
name of heat.

And when we reverse the process, and employ those tremors of heat to
raise a weight--which is done through the intermediation of an elastic
fluid in the steam-engine--a certain definite portion of the molecular
motion is consumed.  In this sense, and in this sense only, can the
heat be said to be converted into gravity; or, more correctly, into
potential energy of gravity.  Here the destruction of the heat has
created no new attraction; but the old attraction has conferred upon
it a power of exerting a certain definite pull, between the
starting-point of the falling weight and the earth.

When, therefore, writers on the conservation of energy speak of
tensions being 'consumed' and 'generated,' they do not mean thereby
that old attractions have been annihilated, and new ones brought into
existence, but that, in the one case, the power of the attraction to
produce motion has been diminished by the shortening of the distance
between the attracting bodies, while, in the other case, the power of
producing motion has been augmented by the increase of the distance.
These remarks apply to all bodies, whether they be sensible masses or
molecules.

Of the inner quality that enables matter to attract matter we know
nothing; and the law of conservation makes no statement regarding that
quality.  It takes the facts of attraction as they stand, and affirms
only the constancy of working-power.  That power may exist in the form
of MOTION; or it may exist in the form of FORCE, _with distance to act
through_.  The former is dynamic energy, the latter is potential
energy, the constancy of the sum of both being affirmed by the law of
conservation.  The convertibility of natural forces consists solely in
transformations of dynamic into potential, and of potential into
dynamic energy.  In no other sense has the convertibility of force any
scientific meaning.

Grave errors have been entertained as to what is really intended to be
conserved by the doctrine of conservation.  This exposition I hope
will tend to remove them.

********************

II. RADIATION.

[Footnote: The Rede Lecture delivered in the Senate House before the
University of Cambridge, May 16, 1865.]

*****

1.  Visible and Invisible Radiation.

BETWEEN the mind of man and the outer world are interposed the nerves
of the human body, which translate, or enable the mind to translate,
the impressions of that world into facts of consciousness and thought.

Different nerves are suited to the perception of different
impressions.  We do not see with the ear, nor hear with the eye, nor
are we rendered sensible of sound by the nerves of the tongue.  Out of
the general assemblage of physical actions, each nerve, or group of
nerves, selects and responds to those for the perception of which it
is specially organised.

The optic nerve passes from the brain to the back of the eyeball and
there spreads out, to form the retina, a web of nerve filaments, on
which the images of external objects are projected by the optical
portion of the eye.  This nerve is limited to the apprehension of the
phenomena of radiation, and, notwithstanding its marvellous
sensibility to certain impressions of this class, it is singularly
obtuse to other impressions.

Nor does the optic nerve embrace the entire range even of radiation.
Some rays, when they reach it, are incompetent to evoke its power,
while others never reach it at all, being absorbed by the humours of
the eye.  To all rays which, whether they reach the retina or not,
fail to excite vision, we give the name of invisible or obscure rays.
All non-luminous bodies emit such rays.  There is no body in nature
absolutely cold, and every body not absolutely cold emits rays of
heat.  But to render radiant heat fit to affect the optic nerve a
certain temperature is necessary.  A cool poker thrust into a fire
remains dark for a time, but when its temperature has become equal to
that of the surrounding coals, it glows like them.  In like manner, if
a current of electricity, of gradually increasing strength, be sent
through a wire of the refractory metal platinum, the wire first
becomes sensibly warm to the touch; for a time its heat augments,
still however remaining obscure; at length we can no longer touch the
metal with impunity; and at a certain definite temperature it emits a
feeble red light.  As the current augments in power the light augments
in brilliancy, until finally the wire appears of a dazzling white. The
light which it now emits is similar to that of the sun.

By means of a prism Sir Isaac Newton unravelled the texture of solar
light, and by the same simple instrument we can investigate the
luminous changes of our platinum wire.  In passing through the prism
all its rays (and they are infinite in variety) are bent or refracted
from their straight course; and, as different rays are differently
refracted by the prism, we are by it enabled to separate one class of
rays from another.  By such prismatic analysis Dr. Draper has shown,
that when the platinum wire first begins to glow, the light emitted is
sensibly red.  As the glow augments the red becomes more brilliant,
but at the same time orange rays are added to the emission. Augmenting
the temperature still further, yellow rays appear beside the orange;
after the yellow, green rays are emitted; and after the green come, in
succession, blue, indigo, and violet rays.  To display all these
colours at the same time the platinum wire must be _white-hot_: the
impression of whiteness being in fact produced by the simultaneous
action of all these colours on the optic nerve.

In the experiment just described we began with a platinum wire at an
ordinary temperature, and gradually raised it to a white heat.  At the
beginning, and even before the electric current had acted at all upon
the wire, it emitted invisible rays.  For some time after the action
of the current had commenced, and even for a time after the wire had
become intolerable to the touch, its radiation was still invisible.
The question now arises, What becomes of these invisible rays when the
visible ones make their appearance?  It will be proved in the sequel
that they maintain themselves in the radiation; that a ray once
emitted continues to be emitted when the temperature is increased, and
hence the emission from our platinum wire, even when it has attained
its maximum brilliancy, consists of a mixture of visible and invisible
rays.  If, instead of the platinum wire, the earth itself were raised
to incandescence, the obscure radiation which it now emits would
continue to be emitted.  To reach incandescence the planet would have
to pass through all the stages of non-luminous radiation, and the
final emission would embrace the rays of all these stages.  There can
hardly be a doubt that from the sun itself, rays proceed similar in
kind to those which the dark earth pours nightly into space.  In fact,
the various kind of obscure rays emitted by all the planets of our
system are included in the present radiation of the sun.

The great pioneer in this domain of science was Sir William Herschel.
Causing a beam of solar light to pass through a prism, he resolved it
into its coloured constituents; he formed what is technically called
the solar spectrum.  Exposing thermometers to the successive colours
he determined their heating power, and found it to augment from the
violet or most refracted end, to the red or least refracted end of the
spectrum.  But he did not stop here.  Pushing his thermometers into
the dark space beyond the red he found that, though the light had
disappeared, the radiant heat falling on the instruments was more
intense than that at any visible part of the spectrum.  In fact, Sir
William Herschel showed, and his results have been verified by various
philosophers since his time, that, besides its luminous rays, the sun
pours forth a multitude of other rays, more powerfully calorific than
the luminous ones, but entirely unsuited to the purposes of vision.

At the less refrangible end of the solar spectrum, then, the range of
the sun's radiation is not limited by that of the eye.  The same
statement applies to the more refrangible end.  Ritter discovered the
extension of the spectrum into the invisible region beyond the violet;
and, in recent times, this ultra-violet emission has had peculiar
interest conferred upon it by the admirable researches of Professor
Stokes.  The complete spectrum of the sun consists, therefore, of
three distinct parts: first, of ultra-red rays of high heating power,
but unsuited to the purposes of vision; secondly, of luminous rays
which display the succession of colours, red, orange, yellow, green,
blue, indigo, violet; thirdly, of ultra-violet rays which, like the
ultra-red ones, are incompetent to excite vision, but which, unlike
the ultra-red rays, possess a very feeble heating power.  In
consequence, however, of their chemical energy these ultra-violet rays
are of the utmost importance to the organic world.

********************

2.  Origin and Character of Radiation.  The Aether.

When we see a platinum wire raised gradually to a white heat, and
emitting in succession all the colours of the spectrum, we are simply
conscious of a series of changes in the condition of our own eyes.  We
do not see the actions in which these successive colours originate,
but the mind irresistibly infers that the appearance of the colours
corresponds to certain contemporaneous changes in the wire.  What is
the nature of these changes?  In virtue of what condition does the
wire radiate at all?  We must now look from the wire, as a whole, to
its constituent atoms.  Could we see those atoms, even before the
electric current has begun to act upon them, we should find them in a
state of vibration.  In this vibration, indeed, consists such warmth
as the wire then possesses.  Locke enunciated this idea with great
precision, and it has been placed beyond the pale of doubt by the
excellent quantitative researches of Mr. Joule.  'Heat,' says Locke,
'is a very brisk agitation of the insensible parts of the object,
which produce in us that sensation from which we denominate the object
hot: so what in our sensations is _heat_ in the object is nothing but
_motion_.' When the electric current, still feeble, begins to pass
through the wire, its first act is to intensify the vibrations already
existing, by causing the atoms to swing through wider ranges.
Technically speaking, the _amplitudes_ of the oscillations are
increased.  The current does this, however, without altering the
periods of the old vibrations, or the times in which they were
executed.  But besides intensifying the old vibrations the current
generates new and more rapid ones, and when a certain definite
rapidity has been attained, the wire begins to glow.  The colour first
exhibited is red, which corresponds to the lowest rate of vibration of
which the eye is able to take cognisance.  By augmenting the strength
of the electric current more rapid vibrations are introduced, and
orange rays appear.  A quicker rate of vibration produces yellow, a
still quicker green; and by further augmenting the rapidity, we pass
through blue, indigo, and violet, to the extreme ultra-violet rays.

Such are the changes recognised by the mind in the wire itself, as
concurrent with the visual changes taking place in the eye.  But what
connects the wire with this organ By what means does it send such
intelligence of its varying condition to the optic nerve?  Heat being
as defined by Locke, 'a very brisk agitation of the insensible parts
of an object,' it is readily conceivable that on touching a heated
body the agitation may communicate itself to the adjacent nerves, and
announce itself to them as light or heat.  But the optic nerve does
not touch the hot platinum, and hence the pertinence of the question,
By what agency are the vibrations of the wire transmitted to the eye?

The answer to this question involves one of the most important
physical conceptions that the mind of man has yet achieved: the
conception of a medium filling space and fitted mechanically for the
transmission of the vibrations of light and heat, as air is fitted for
the transmission of sound.  This medium is called the _luminiferous
aether_.  Every vibration of every atom of our platinum wire raises in
this aether a wave, which speeds through it at the rate of 186,000
miles a second.

The aether suffers no rupture of continuity at the surface of the eye,
the inter-molecular spaces of the various humours are filled with it;
hence the waves generated by the glowing platinum can cross these
humours and impinge on the optic nerve at the back of the eye.
[Footnote: The action here described is analogous to the passage of
sound-waves through thick felt whose interstices are occupied by air.]
Thus the sensation of light reduces itself to the acceptance of
motion.  Up to this point we deal with pure mechanics; but the
subsequent translation of the shock of the aethereal waves into
consciousness eludes mechanical science.  As an oar dipping into the
Cam generates systems of waves, which, speeding from the centre of
disturbance, finally stir the sedges on the river's bank, so do the
vibrating atoms generate in the surrounding aether undulations, which
finally stir the filaments of the retina.  The motion thus imparted is
transmitted with measurable, and not very great velocity to the brain,
where, by a process which the science of mechanics does not even tend
to unravel, the tremor of the nervous matter is converted into the
conscious impression of light.

Darkness might then be defined as aether at rest; light as aether in
motion.  But in reality the aether is never at rest, for in the
absence of light-waves we have heat-waves always speeding through it.
In the spaces of the universe both classes of undulations incessantly
commingle.  Here the waves issuing from uncounted centres cross,
coincide, oppose, and pass through each other, without confusion or
ultimate extinction.  Every star is seen across the entanglement of
wave-motions produced by all other stars.  It is the ceaseless thrill
caused by those distant orbs collectively in the aether, that
constitutes what we call the 'temperature of space.' As the air of a
room accommodates itself to the requirements of an orchestra,
transmitting each vibration of every pipe and string, so does the
inter-stellar aether accommodate itself to the requirements of light
and heat.  Its waves mingle in space without disorder, each being
endowed with an individuality as indestructible as if it alone had
disturbed the universal repose.

All vagueness with regard to the use of the terms 'radiation' and
'absorption' will now disappear.  Radiation is the communication of
vibratory motion to the aether; and when a body is said to be chilled
by radiation, as for example the grass of a meadow on a starlight
night, the meaning is, that the molecules of the grass have lost a
portion of their motion, by imparting it to the medium in which they
vibrate.  On the other hand, the waves of aether may so strike against
the molecules of a body exposed to their action as to yield up their
motion to the latter; and in this transfer of the motion from the
aether to the molecules consists the absorption of radiant heat.  All
the phenomena of heat are in this way reducible to interchanges of
motion; and it is purely as the recipients or the donors of this
motion, that we ourselves become conscious of the action of heat and
cold.

********************

3.  The Atomic Theory in reference to the Aether.

The word 'atoms' has been more than once employed in this discourse.
Chemists have taught us that all matter is reducible to certain
elementary forms to which they give this name.  These atoms are
endowed with powers of mutual attraction, and under suitable
circumstances they coalesce to form compounds.  Thus oxygen and
hydrogen are elements when separate, or merely _mixed_, but they may be
made to _combine_ so as to form molecules, each consisting of two atoms
of hydrogen and one of oxygen.  In this condition they constitute
water.  So also chlorine and sodium are elements, the former a pungent
gas, the latter a soft metal; and they unite together to form chloride
of sodium or common salt.  In the same way the element nitrogen
combines with hydrogen, in the proportion of one atom of the former to
three of the latter, to form ammonia.  Picturing in imagination the
atoms of elementary bodies as little spheres, the molecules of
compound bodies must be pictured as groups of such spheres.  This is
the atomic theory as Dalton conceived it.  Now if this theory have any
foundation in fact, and if the theory of an aether pervading space,
and constituting the vehicle of atomic motion, be founded in fact, it
is surely of interest to examine whether the vibrations of elementary
bodies are modified by the act of combination--whether as regards
radiation and absorption, or, in other words, whether as regards the
communication of motion to the aether, and the acceptance of motion
from it, the deportment of the uncombined atoms will be different from
that of the combined.

********************

4.  Absorption of Radiant Heat by Gases.

We have now to submit these considerations to the only test by which
they can be tried, namely, that of experiment.  An experiment is well
defined as a question put to Nature; but, to avoid the risk of asking
amiss, we ought to purify the question from all adjuncts which do not
necessarily belong to it.  Matter has been shown to be composed of
elementary constituents, by the compounding of which all its varieties
are produced.  But, besides the chemical unions which they form, both
elementary and compound bodies can unite in another and less intimate
way.  Gases and vapours aggregate to liquids and solids, without any
change of their chemical nature.  We do not yet know how the
transmission of radiant heat may be affected by the entanglement due
to cohesion; and, as our object now is to examine the influence of
chemical union alone, we shall render our experiments more pure by
liberating the atoms and molecules entirely from the bonds of
cohesion, and employing them in the gaseous or vaporous form.

Let us endeavour to obtain a perfectly clear mental image of the
problem now before us.  Limiting in the first place our enquiries to
the phenomena of absorption, we have to picture a succession of waves
issuing from a radiant source and passing through a gas; some of them
striking against the gaseous molecules and yielding up their motion to
the latter; others gliding round the molecules, or passing through the
intermolecular spaces without apparent hindrance.  The problem before
us is to determine whether such free molecules have any power whatever
to stop the waves of heat; and if so, whether different molecules
possess this power in different degrees.

In examining the problem let us fall back upon an actual piece of
work, choosing as the source of our heat-waves a plate of copper,
against the back of which a steady sheet of flame is permitted to
play.  On emerging from the copper, the waves, in the first instance,
pass through a space devoid of air, and then enter a hollow glass
cylinder, three feet long and three inches wide.  The two ends of this
cylinder are stopped by two plates of rock-salt, a solid substance
which offers a scarcely sensible obstacle to the passage of the
calorific waves.  After passing through the tube, the radiant heat
falls upon the anterior face of a thermo-electric pile, [Footnote: In
the Appendix to the first chapter of 'Heat as a Mode of 'Motion,' the
construction of the thermo-electric pile is fully explained.] which
instantly converts the heat into an electric current.  This current
conducted round a magnetic needle deflects it, and the magnitude of
the deflection is a measure of the heat falling upon the pile.  This
famous instrument, and not an ordinary thermometer, is what we shall
use in these enquiries, but we shall use it in a somewhat novel way.
As long as the two opposite faces of the thermo-electric pile are kept
at the same temperature, no matter how high that may be, there is no
current generated.  The current is a consequence of a difference of
temperature between the two opposite faces of the pile.  Hence, if
after the anterior face has received the heat from our radiating
source, a second source, which we may call the compensating source, be
permitted to radiate against the posterior face, this latter radiation
will tend to neutralise the former.  When the neutralisation is
perfect, the magnetic needle connected with the pile is no longer
deflected, but points to the zero of the graduated circle over which
it hangs.

And now let us suppose the glass tube, through which the waves from
the heated plate of copper are passing, to be exhausted by an
air-pump, the two sources of heat acting at the same time on the two
opposite faces of the pile.  When by means of an adjusting screen,
perfectly equal quantities of heat are imparted to the two faces, the
needle points to zero.  Let any gas be now permitted to enter the
exhausted tube; if its molecules possess any power of intercepting the
calorific waves, the equilibrium previously existing will be
destroyed, the compensating source will triumph, and a deflection of
the magnetic needle will be the immediate consequence.  From the
deflections thus produced by different gases, we can readily deduce
the relative amounts of wave-motion which their molecules intercept.

In this way the substances mentioned in the following table were
examined, a small portion only of each being admitted into the glass
tube.  The quantity admitted in each case was just sufficient to
depress a column of mercury associated with the tube one inch: in
other words, the gases were examined at a pressure of one-thirtieth of
an atmosphere.  The numbers in the table express the relative amounts
of wave-motion absorbed by the respective gases, the quantity
intercepted by air being taken as unity.

Radiation through Gases.

Name of gas      Relative absorption

Air                        1

Oxygen                     1

Nitrogen                   1

Hydrogen                   1

Carbonic oxide           750

Carbonic acid            972

Hydrochloric acid.     1,005

Nitric oxide           1,590

Nitrous oxide          1,860

Sulphide of hydrogen   2,100

Ammonia                5,460

Olefiant gas           6,030

Sulphurous acid        6,480

Every gas in this table is perfectly transparent to light, that is to
say, all waves within the limits of the visible spectrum pass through
it without obstruction; but for the waves of slower period, emanating
from our heated plate of copper, enormous differences of absorptive
power are manifested.  These differences illustrate in the most
unexpected manner the influence of chemical combination.  Thus the
elementary gases, oxygen, hydrogen, and nitrogen, and the mixture
atmospheric air, prove to be practical vacua to the rays of heat; for
every ray, or, more strictly speaking, for every unit of wave-motion,
which any one of them intercepts, perfectly transparent ammonia
intercepts 5,460 units, olefiant gas 6,030 units, while sulphurous
acid gas absorbs 6,480 units.  What, becomes of the wave-motion thus
intercepted?  It is applied to the heating of the absorbing gas.
Through air, oxygen, hydrogen, and nitrogen, the waves of aether pass
without absorption, and these gases are not sensibly changed in
temperature by the most powerful calorific rays.  The position of
nitrous oxide in the foregoing table is worthy of particular notice.
In this gas we have the same atoms in a state of chemical union, that
exist uncombined in the atmosphere; but the absorption of the compound
is 1,800 times that of air.

********************

5.  Formation of Invisible Foci.

This extraordinary deportment of the elementary gases naturally
directed attention to elementary bodies 'in other states of
aggregation.  Some of Melloni's results now attained a new
significance.  This celebrated experimenter had found crystals of
sulphur to be highly pervious to radiant heat; he had also proved that
lamp-black, and black glass, (which owes its blackness to the element
carbon) were to a considerable extent transparent to calorific rays of
low refrangibility.  These facts, harmonising so strikingly with the
deportment of the simple gases, suggested further enquiry.  Sulphur
dissolved in bisulphide of carbon was found almost perfectly
diathermic.  The dense and deeply-coloured element bromine was
examined, and found competent to cut off the light of our most
brilliant flames, while it transmitted the invisible calorific rays
with extreme freedom.  Iodine, the companion element of bromine, was
next thought of, but it was found impracticable to examine the
substance in its usual solid condition.  It however dissolves freely
in bisulphide of carbon.  There is no chemical union between the
liquid and the iodine; it is simply a case of solution, in which the
uncombined atoms of the element can act upon the radiant heat.  When
permitted to do so, it was found that a layer of dissolved iodine,
sufficiently opaque to cut off the light of the midday sun, was almost
absolutely transparent to the invisible calorific rays. [Footnote:
Professor Dewar has recently succeeded in producing a medium highly
opaque to light, and highly transparent to obscure heat, by fusing
together sulphur and iodine.]

By prismatic analysis Sir William Herschel separate the luminous from
the non-luminous rays of the sun, and he also sought to render the
obscure rays visible by concentration.  Intercepting the luminous
portion of his spectrum he brought, by a converging lens, the
ultra-red rays to a focus, but by this condensation he obtained no
light.  The solution of iodine offers a means of filtering the solar
beam, or failing it, the beam of the electric lamp, which renders
attainable far more powerful foci of invisible rays than could
possibly be obtained by the method of Sir William Herschel.  For to
form his spectrum he was obliged to operate upon solar light which had
passed through a narrow slit or through a small aperture, the amount
of the obscure heat being limited by this circumstance.  But with our
opaque solution we may employ the entire surface of the largest lens,
and having thus converged the rays, luminous and non-luminous, we can
intercept the former by the iodine, and do what we please with the
latter.  Experiments of this character, not only with the iodine
solution, but also with black glass and layers of lampblack, were
publicly performed at the Royal Institution in the early part of 1862,
and the effects at the foci of invisible rays, then obtained, were
such as had never been witnessed previously.

In the experiments here referred to, glass lenses were employed to
concentrate the rays.  But glass, though highly transparent to the
luminous, is in a high degree opaque to the invisible, heat-rays of
the electric lamp, and hence a large portion of those rays was
intercepted by the glass.  The obvious remedy here is to employ
rock-salt lenses instead of glass ones, or to abandon the use of
lenses wholly, and to concentrate the rays by a metallic mirror.  Both
of these improvements have been introduced, and, as anticipated, the
invisible foci have been thereby rendered more intense.  The mode of
operating remains however the same, in principle, as that made known
in 1862.  It was then found that an instant's exposure of the face of
the thermoelectric pile to the focus of invisible rays, dashed the
needles of a coarse galvanometer violently aside.  It is now found
that on substituting for the face of the thermo-electric pile a
combustible body, the invisible rays are competent to set that body on
fire.

********************

6.  Visible and Invisible Rays of the Electric Light.

We have next to examine what proportion the non-luminous rays of the
electric light bear to the luminous ones.  This the opaque solution of
iodine enables us to do with an extremely close approximation to the
truth.

The pure bisulphide of carbon, which is the solvent of the iodine, is
perfectly transparent to the luminous, and almost perfectly
transparent to the dark, rays of the electric lamp.  Supposing the
total radiation of the lamp to pass through the transparent
bisulphide, while through the solution of iodine only the dark rays
are transmitted.  If we determine, by means of a thermoelectric pile,
the total radiation, and deduct from it the purely obscure, we obtain
the value of the purely luminous emission.  Experiments, performed in
this way, prove that if all the visible rays of the electric light
were converged to a focus of dazzling brilliancy, its heat would only
be one-eighth of that produced at the unseen focus of the invisible
rays.

Exposing his thermometers to the successive colours of the solar
spectrum, Sir William Herschel determined the heating power of each,
and also that of the region beyond the extreme red.  Then drawing a
straight line to represent the length of the spectrum, he erected, at
various points, perpendiculars to represent the calorific intensity
existing at those points.  Uniting the ends of all his perpendiculars,
he obtained a curve which showed at a glance the manner in which the
heat was distributed in the solar spectrum.  Professor Müller of
Freiburg, with improved instruments, afterwards made similar
experiments, and constructed a more accurate diagram of the same kind.
We have now to examine the distribution of heat in the spectrum of the
electric light; and for this purpose we shall employ a particular form
of the thermo-electric pile, devised by Melloni.  Its face is a
rectangle, which by means of movable side-pieces can be rendered as
narrow as desired.  We can, for example, have the face of the pile the
tenth, the hundredth, or even the thousandth of an inch in breadth. By
means of an endless screw, this linear thermo-electric pile may be
moved through the entire spectrum, from the violet to the red, the
amount of heat falling upon the pile at every point of its march,
being declared by a magnetic needle associated with the pile.

When this instrument is brought up to the violet end of the spectrum
of the electric light, the heat is found to be insensible.  As the
pile is gradually moved from the violet end towards the red, heat soon
manifests itself, augmenting as we approach the red.  Of all the
colours of the visible spectrum the red possesses the highest heating
power.  On pushing the pile into the dark region beyond the red, the
heat, instead of vanishing, rises suddenly and enormously in
intensity, until at some distance beyond the red it attains a maximum.
Moving the pile still forward, the thermal power falls, somewhat more
rapidly than it rose.  It then gradually shades away, but, for a
distance beyond the red greater than the length of the whole visible
spectrum, signs of heat may be detected.

Drawing a datum line, and erecting along it perpendiculars,
proportional in length to the thermal intensity at the respective
points, we obtain the extraordinary curve, shown on the opposite page,
which exhibits the distribution of heat in the spectrum of the
electric light.  In the region of dark rays, beyond the red, the curve
shoots up to B, in a steep and massive peak--a kind of Matterhorn of
heat, which dwarfs the portion of the diagram C D E, representing the
luminous radiation.  Indeed the idea forced upon the mind by this
diagram is that the light rays are a mere insignificant appendage to
the heat-rays represented by the area A B C D, thrown in as it were by
nature for the purpose of vision.

Figure 1. Spectrum of Electric Light

The diagram drawn by Professor Müller to represent the distribution of
heat in the solar spectrum is not by any means so striking as that
just described, and the reason, doubtless, is that prior to reaching
the earth the solar rays have to traverse our atmosphere.  By the
aqueous vapour there diffused, the summit of the peak representing the
sun's invisible radiation is cut off.  A similar lowering of the
mountain of invisible heat is observed when the rays from the electric
light are permitted to pass through a film of water, which acts upon
them as the atmospheric vapour acts upon the rays of the sun.

********************

7.  Combustion by Invisible Rays.

The sun's invisible rays far transcend the visible ones in heating
power, so that if the alleged performances of Archimedes during the
siege of Syracuse had any foundation in fact, the dark solar rays
would have been the philosopher's chief agents of combustion.  On a
small scale we can readily produce, with the purely invisible rays of
the electric light, all that Archimedes is said to have performed with
the sun's total radiation.  Placing behind the electric light a small
concave mirror, the rays are converged, the cone of reflected rays and
their point of convergence being rendered clearly visible by the dust
always floating in the air.  Placing between the luminous focus and
the source of rays our solution of iodine, the light of the cone is
entirely cut away; but the intolerable heat experienced when the band
is placed, even for a moment, at the dark focus, shows that the
calorific rays pass unimpeded through the opaque solution.

Almost anything that ordinary fire can effect may be accomplished at
the focus of invisible rays; the air at the focus remaining at the
same time perfectly cold, on account of its transparency to the
heat-rays.  An air thermometer, with a hollow rack-salt bulb, would be
unaffected by the heat of the focus: there would be no expansion, and
in the open air there is no convection.  The aether at the focus, and
not the air, is the substance in which the heat is embodied.  A block
of wood, placed at the focus, absorbs the heat, and dense volumes of
smoke rise swiftly upwards, showing the manner in which the air itself
would rise, if the invisible rays were competent to heat it.  At the
perfectly dark focus dry paper is instantly inflamed: chips of wood
are speedily burnt up: lead, tin, and zinc are fused: and disks of
charred paper are raised to vivid incandescence.  It might be supposed
that the obscure rays would show no preference for black over white;
but they do show a preference, and to obtain rapid combustion, the
body, if not already black, ought to be blackened.  When metals are to
be burned, it is necessary to blacken or otherwise tarnish them, so as
to diminish their reflective power.  Blackened zinc foil, when brought
into the focus of invisible rays, is instantly caused to blaze, and
burns with its peculiar purple light.  Magnesium wire flattened, or
tarnished magnesium ribbon, also bursts into flame.  Pieces of
charcoal suspended in a receiver full of oxygen are also set on fire
when the invisible focus falls upon them; the dark rays after having
passed through the receiver, still possessing sufficient power to
ignite the charcoal, and thus initiate the attack of the oxygen.  If,
instead of being plunged in oxygen, the charcoal be suspended in
vacuo, it immediately glows at the place where the focus falls.

********************

8.  Transmutation of Rays: Calorescence.

[Footnote: I borrow this term from Professor Challis, 'Philosophical
Magazine,' vol. xii. P. 521]

Eminent experimenters were long occupied in demonstrating the
substantial identity of light and radiant heat, and we have now the
means of offering a new and striking proof of this identity.  A
concave mirror produces, beyond the object which it reflects, an
inverted and magnified image of the object.  Withdrawing, for example,
our iodine solution, an intensely luminous inverted image of the
carbon points of the electric light is formed at the focus of the
mirror employed in the foregoing experiments.  When the solution is
interposed, and the light is cut away, what becomes of this image?  It
disappears from sight; but an invisible thermograph remains, and it is
only the peculiar constitution of our eyes that disqualifies us from
seeing the picture formed by the calorific rays.  Falling on white
paper, the image chars itself out: falling on black paper, two holes
are pierced in it, corresponding to the images of the two coke points:
but falling on a thin plate of carbon in vacuo, or upon a thin sheet
of platinised platinum, either in vacuo or in air, radiant heat is
converted into light, and the image stamps itself in vivid
incandescence upon both the carbon and the metal.  Results similar to
those obtained with the electric light have also been obtained with
the invisible rays of the lime-light and of the sun.

Before a Cambridge audience it is hardly necessary to refer to the
excellent researches of Professor Stokes at the opposite end of the
spectrum.  The above results constitute a kind of complement to his
discoveries.  Professor Stokes named the phenomena which he has
discovered and investigated _Fluorescence_; for the new phenomena here
described I have proposed the term _Calorescence_.  He, by the
interposition of a proper medium, so lowered the refrangibility of the
ultraviolet rays of the spectrum as to render them visible.  Here, by
the interposition of the platinum foil, the refrangibility of the
ultra-red rays is so exalted as to render them visible.  Looking
through a prism at the incandescent image of the carbon points, the
light of the image is decomposed, and a complete spectrum is obtained.
The invisible rays of the electric light, remoulded by the atoms of
the platinum, shine thus visibly forth; ultra-red rays being converted
into red, orange, yellow, green, blue, indigo, violet, and ultraviolet
ones.  Could we, moreover, raise the original source of rays to a
sufficiently high temperature, we might not only obtain from the dark
rays of such a source a single incandescent image, but from the dark
rays of this image we might obtain a second one, from the dark rays of
the second a third, and so on--a series of complete images and spectra
being thus extracted from the invisible emission of the primitive
source. [Footnote: On investigating the calorescence produced by rays
transmitted through glasses of various colours, it was found that in
the case of certain specimens of blue glass, the platinum foil glowed
with a pink or purplish light.  The effect was not subjective, and
considerations of obvious interest are suggested by it.  Different
kinds of black glass differ notably as to their power of transmitting
radiant heat.  When thin, some descriptions tint the sun with a
greenish hue: others make it appear a glowing red without any trace of
green.  The latter are far more diathermic than the former.  In fact,
carbon when perfectly dissolved and incorporated with a good white
glass, is highly transparent to the calorific rays, and by employing
it as an absorbent the phenomena of 'calorescence' may be obtained,
though in a less striking form than with the iodine.  The black glass
chosen for thermometers, and intended to absorb completely the solar
heat, may entirely fail in this object, if the glass in which the
carbon is incorporated be colourless.  To render the bulb of a
thermometer a perfect absorbent, the glass ought in the first instance
to be green.  Soon after the discovery of fluorescence the late Dr.
William Allen Miller pointed to the lime-light as an illustration of
exalted refrangibility.  Direct experiments have since entirely
confirmed the view expressed at page 210 of his work on 'Chemistry,'
published in 1855.]

********************

9.  Deadness of the Optic Nerve to the Calorific Rays.

The layer of iodine used in the foregoing experiments intercepted the
rays of the noonday sun.  No trace of light from the electric lamp was
visible in the darkest room, even when a white screen was placed at
the focus of the mirror employed to concentrate the light.  It was
thought, however, that if the retina itself were brought into the
focus the sensation of light might be experienced.  The danger of this
experiment was twofold.  If the dark rays were absorbed in a high
degree by the humours of the eye the albumen of the humours might
coagulate along the line of the rays.  If, on the contrary, no such
high absorption took place, the rays might reach the retina with a
force sufficient to destroy it.  To test the likelihood of these
results, experiments were made on water and on a solution of alum, and
they showed it to be very improbable that in the brief time requisite
for an experiment any serious damage could be done.  The eye was
therefore caused to approach the dark focus, no defence, in the first
instance, being provided; but the heat, acting upon the parts
surrounding the pupil, could not be borne.  An aperture was therefore
pierced in a plate of metal, and the eye, placed behind the aperture,
was caused to approach the point of convergence of invisible rays. The
focus was attained, first by the pupil and afterwards by the retina.
Removing the eye, but permitting the plate of metal to remain, a sheet
of platinum foil was placed in the position occupied by the retina a
moment before.  The platinum became red-hot.  No sensible damage was
done to the eye by this experiment; no impression of light was
produced; the optic nerve was not even conscious of heat.

But the humours of the eye are known to be highly impervious to the
invisible calorific rays, and the question therefore arises, 'Did the
radiation in the foregoing experiment reach the retina at all?'  The
answer is, that the rays were in part transmitted to the retina, and
in part absorbed by the humours.  Experiments on the eye of an ox
showed that the proportion of obscure rays which reached the retina
amounted to 18 per cent.  of the total radiation; while the luminous
emission from the electric light amounts to no more than 10 per cent.
of the same total.  Were the purely luminous rays of the electric lamp
converged by our mirror to a focus, there can be no doubt as to the
fate of a retina placed there.  Its ruin would be inevitable; and yet
this would be accomplished by an amount of wave-motion but little more
than half of that which the retina, without exciting consciousness,
bears at the focus of invisible rays.

This subject will repay a moment's further attention.  At a common
distance of a foot the visible radiation of the electric light
employed in these experiments is 800 times the light of a candle.  At
the same distance, the portion of the radiation of the electric light
which reaches the retina, but fails to excite vision, is about 1,500
times the luminous radiation of the candle.' [Footnote: It will be
borne in mind that the heat which any ray, luminous or non-luminous,
is competent to generate is the true measure of the energy of the
ray.]  But a candle on a clear night can readily be seen at a distance
of a mile, its light at this distance being less than 1/20,000,000 of
its light at the distance of a foot.

Hence, to make the candle-light a mile off equal in power to the
non-luminous radiation received from the electric light at a foot
distance, its intensity would have to be multiplied by 1,500 x
20,000,000, or by thirty thousand millions.  Thus the thirty thousand
millionth part of the invisible radiation from the electric light,
received by the retina at the distance of a foot, would, if slightly
changed in character, be amply sufficient to provoke vision.  Nothing
could more forcibly illustrate that special relationship supposed by
Melloni and others to subsist between the optic nerve and the
oscillating periods of luminous bodies.  The optic nerve responds, as
it were, to the waves with which it is in consonance, while it refuses
to be excited by others of almost infinitely greater energy, whose
periods of recurrence are not in unison with its own.

********************

10.  Persistence of Rays.

At an early part of this lecture it was affirmed, that when a platinum
wire was, gradually raised to a state of high incandescence, new rays
were constantly added, while the intensity of the old ones was
increased.  Thus, in Dr. Draper's experiments, the rise of temperature
that generated the orange, yellow, green, and blue augmented the
intensity of the red.  What is true of the red is true of every other
ray of the spectrum, visible and invisible.  We cannot indeed see the
augmentation of intensity in the region beyond the red, but we can
measure it and express it numerically.  With this view the following
experiment was performed: A spiral of platinum wire was surrounded by
a small glass globe to protect it from currents of air; through an
orifice in the globe the rays could pass from the spiral and fall
afterwards upon a thermo-electric pile.  Placing in front of the
orifice an opaque solution of iodine, the platinum was gradually
raised from a low dark heat to the fullest incandescence, with the
following results:

Appearance of spiral    Energy of obscure radiation

Dark                                1

Dark, but hotter                    3

Dark, but still hotter              5

Dark, but still hotter             10

Feeble red                         19

Dull red                           25

Red                                37

Full red.                          62

Orange                             89

Bright orange                     144

Yellow                            202

White                             276

Intense white                     440

Thus the augmentation of the electric current, which raises the wire
from its primitive dark condition to an intense white heat, exalts at
the same time the energy of the obscure radiation, until at the end it
is fully 440 times what it was at the beginning.

What has been here proved true of the totality of the ultra-red rays
is true for each of them singly.  Placing our linear thermo-electric
pile in any part of the ultra-red spectrum, it may be proved that a
ray once emitted continues to be emitted with increased energy as the
temperature is augmented.  The platinum spiral, so often referred to,
being raised to whiteness by an electric current, a brilliant spectrum
was formed from its light.  A linear thermo-electric pile was placed
in the region of obscure rays beyond the red, and by diminishing the
current the spiral was reduced to a low temperature.  It was then
caused to pass through various degrees of darkness and incandescence,
with the following results:

Appearance of spiral      Energy of obscure rays

Dark                                 1

Dark                                 6

Faint red                           10

Dull red                            13

Red                                 18

Full red.                           27

Orange                              60

Yellow                              93

White                              122

Here, as in the former case, the dark and bright radiations reached
their maximum together; as the one augmented, the other augmented,
until at last the energy of the obscure rays of the particular
refrangibility here chosen, became 122 times what it was at first. To
reach a white heat the wire has to pass through all the stages of
invisible radiation, but in its most brilliant condition it embraces,
in an intensified form, the rays of all those stages.

And thus it is with all other kinds of matter, as far as they have
hitherto been examined.  Coke, whether brought to a white heat by the
electric current, or by the oxyhydrogen jet, pours out invisible rays
with augmented energy, as its light is increased.  The same is true of
lime, bricks, and 'other substances.  It is true of all metals which
are capable of being heated to incandescence.  It also holds good for
phosphorus burning in oxygen.  Every gush of dazzling light has
associated with it a gush of invisible radiant heat, which far
transcends the light in energy.  This condition of things applies to
all bodies capable of being raised to a white heat, either in the
solid or the molten condition.  It would doubtless also apply to the
luminous fogs formed by the condensation of incandescent vapours.  In
such cases when the curve representing the radiant energy of the body
is constructed, the obscure radiation towers upwards like a mountain,
the luminous radiation resembling a mere 'spur' at its base.  From the
very brightness of the light of some of the fixed stars we may infer
the intensity of that dark radiation, which is the precursor and
inseparable associate of their luminous rays.

We thus find the luminous radiation appearing when the radiant body
has attained a certain temperature; or, in other words, when the
vibrating atoms of the body have attained a certain width of swing. In
solid and molten bodies a certain amplitude cannot be surpassed
without the introduction of periods of vibration, which provoke the
sense of vision.  How are we to figure this?  If permitted to
speculate, we might ask, are not these more rapid vibrations the
progeny of the slower?  Is it not really the mutual action of the
atoms, when they swing through very wide spaces, and thus encroach
upon each other, that causes them to tremble in quicker periods?  If
so, whatever be the agency by which the large swinging space is
obtained, we shall have light-giving vibrations associated with it. It
matters not whether the large amplitudes be produced by the strokes of
a hammer, or by the blows of the molecules of a non-luminous gas, like
air at some height above a gas-flame; or by the shock of the aether
particles when transmitting radiant heat.  The result in all cases
will be incandescence.  Thus, the invisible waves of our filtered
electric beam may be regarded as generating synchronous vibrations
among the atoms of the platinum on which they impinge; but, once these
vibrations have attained a certain amplitude, the mutual jostling of
the atoms produces quicker tremors, and the light-giving waves follow
as the necessary product of the heat-giving ones.

********************

11. Absorption of Radiant Heat by Vapours and Odours.

We commenced the demonstrations brought forward in this lecture by
experiments on permanent gases, and we have now to turn our attention
to the vapours of volatile liquids.  Here, as in the case of the
gases, vast differences have been proved to exist between various
kinds of molecules, as regards their power of intercepting the
calorific waves.  While some vapours allow the waves a comparatively
free passage, the faintest mixture of other vapours causes a
deflection of the magnetic needle.  Assuming the absorption effected
by air, at a pressure of one atmosphere, to be unity, the following
are the absorptions effected by a series of vapours at a pressure of
1/60th of an atmosphere:

Name of vapour                        Absorption

Bisulphide of carbon                      47

Iodide of methyl                         115

Benzol                                   136

Amylene                                  321

Sulphuric ether                          440

Formic ether                             548

Acetic ether                             612

Bisulphide of carbon is the most transparent vapour in this list; and
acetic ether the most opaque; 1/60th of an atmosphere of the former,
however, produces 47 times the effect of a whole atmosphere of air,
while 1/60th of an atmosphere of the latter produces 612 times the
effect of a whole atmosphere of air.  Reducing dry air to the pressure
of the acetic ether here employed, and comparing them then together,
the quantity of wave-motion intercepted by the ether would be many
thousand times that intercepted by the air.

Any one of these vapours discharged into the free atmosphere, in front
of a body emitting obscure rays, intercepts more or less of the
radiation.  A similar effect is produced by perfumes diffused in the
air, though their attenuation is known to be almost infinite.
Carrying, for example, a current of dry air over bibulous paper,
moistened by patchouli, the scent taken up by the current absorbs 30
times the quantity of heat intercepted by the air which carries it;
and yet patchouli acts more feebly on radiant heat than any other
perfume yet examined.

Here follow the results obtained with various essential oils, the
odour, in each case, being carried by a current of dry air into the be
already employed for gases and vapours:

Name of perfume                       Absorption

Patchouli                                  30

Sandal wood                                32

Geranium                                   33

Oil of cloves                              34

Otto of roses                              37

Bergamot                                   44

Neroli                                     47

Lavender                                   60

Lemon                                      65

Portugal                                   67

Thyme                                      68

Rosemary                                   74

Oil of laurel                              80

Camomile flowers                           87

Cassia                                    109

Spikenard                                 355

Aniseed                                   372

Thus the absorption by a tube full of dry air being 1, that of the
odour of patchouli diffused in it is 30, at of lavender 60, that of
rosemary 74, whilst that of aniseed amounts to 372.  It would be idle
to speculate the quantities of matter concerned in these actions.

********************

12.  Aqueous Vapour in relation to the Terrestrial Temperatures.

We are now fully prepared for a result which, without such
preparation, might appear incredible.  Water is, to some extent, a
volatile body, and our atmosphere, resting as it does upon the surface
of the ocean, receives from it a continual supply of aqueous vapour.
It would be an error to confound clouds or fog or any visible mist
with the vapour of water, which is a perfectly impalpable gas,
diffused, even on the clearest days, throughout the atmosphere.
Compared with the great body of the air, the aqueous vapour it
contains is of almost infinitesimal amount, 99.5 out of every 100
parts of the atmosphere being composed of oxygen and nitrogen.  In the
absence of experiment, we should never think of ascribing to this
scant and varying constituent any important influence on terrestrial
radiation; and yet its influence is far more potent than that of the
great body of the air.  To say that on a day of average humidity in
England, the atmospheric vapour exerts 100 times the action of the air
itself, would certainly be an understatement of the fact.  Comparing a
single molecule of aqueous vapour with an atom of either of the main
constituents of our atmosphere, I am not prepared to say how many
thousand times the action of the former exceeds that of the latter.

But it must be borne in mind that these large numbers depend, in part,
on the extreme feebleness of the air; the power of aqueous vapour
seems vast, because that of the air with which it is compared is
infinitesimal.  Absolutely considered, however, this substance,
notwithstanding its small specific gravity, exercises a very potent
action.  Probably from 10 to 15 per cent.  of the heat radiated from
the earth is absorbed within 10 or 20 feet of the earth's surface.
This must evidently be of the utmost consequence to the life of the
world.  Imagine the superficial molecules of the earth agitated with
the motion of heat, and imparting it to the surrounding aether; this
motion would be carried rapidly away, and lost for ever to our planet,
if the waves of aether had nothing but the air to contend with in
their outward course.  But the aqueous vapour takes up the motion, and
becomes hereby heated, thus wrapping the earth like a warm garment,
and protecting its surface from the deadly chill which it would
otherwise sustain.  Various philosophers have speculated on the
influence of an atmospheric envelope.  De Saussure, Fourier,
M. Pouillet, and Mr. Hopkins have, one and all, enriched scientific
literature with contributions on this subject, but the considerations
which these eminent men have applied to atmospheric air, have, if my
experiments be correct, to be transferred to the aqueous vapour.

The observations of meteorologists furnish important, though hitherto
unconscious evidence of the influence of this agent.  Wherever the air
is dry we are liable to daily extremes of temperature.  By day, such
places, the sun's heat reaches the earth unimpeded, and renders the
maximum high; by night, on the other hand, the earth's heat escapes
unhindered to space, and renders the minimum low.  Hence the
difference between the maximum and minimum is greatest where the air
is driest. In the plains of India, the heights of the Himalaya, in
central Asia, in Australia--wherever drought reigns, we have the heat
of day forcibly contrasted with the chill of night.  In the Sahara
itself, when the sun's rays cease to impinge on the burning soil, the
temperature runs rapidly down to freezing, because there is no vapour
overhead to check the calorific drain.  And here another instance
might be added to the numbers already known, in which nature tends as
it were to check her own excess.  By nocturnal refrigeration, the
aqueous vapour of the air is condensed to water on the surface of the
earth; and, as only the superficial portions radiate, the act of
condensation makes water the radiating body.  Now experiment proves
that to the rays emitted by water, aqueous vapour is especially
opaque.  Hence the very act of condensation, consequent on terrestrial
cooling, becomes a safeguard to the earth, imparting to its radiation
that particular character which renders it most liable to be prevented
from escaping into space.

It might however be urged that, inasmuch as we derive all our heat
from the sun, the selfsame covering which protects the earth from
chill must also shut out the solar radiation.  This is partially true,
but only partially; the sun's rays are different in quality from the
earth's rays, and it does not at all follow that the substance which
absorbs the one must necessarily absorb the other.  Through a layer of
water, for example, one tenth of an inch in thickness, the sun's rays
are transmitted with comparative freedom; but through a layer half
this thickness, as Melloni has proved, no single ray from the warmed
earth could pass.  In like manner, the sun's rays pass with
comparative freedom through the aqueous vapour of the air: the
absorbing power of this substance being mainly exerted upon the
invisible heat that endeavours to escape from the earth.  In
consequence of this differential action upon solar and terrestrial
heat, the mean temperature of our planet is higher than is due to its
distance from the sun.

********************

13.  Liquids and their Vapours in relation to Radiant Heat.

The deportment here assigned to atmospheric vapour has been
established by direct experiments on it taken from the streets and
parks of London, from the downs of Epsom, from the hills and sea-beach
of the Isle of Wight, and also by experiments on air in the first
instance dried, and afterwards rendered artificially humid by pure
distilled water.  It has also en established in the following way: Ten
volatile quids were taken at random and the power of these quids, at a
common thickness, to intercept the waves f heat, was carefully
determined.  The vapours of the quids were next taken, in quantities
proportional to e quantities of liquid, and the power of the vapours
intercept the waves of heat was also determined.

Commencing with the substance which exerted the least absorptive
power, and proceeding onwards to the most energetic, the following
order of absorption was observed:

Liquids                             Vapours

Bisulphide of carbon.               Bisulphide of carbon.

Chloroform.                         Chloroform.

Iodide of methyl.                   Iodide of methyl.

Iodide of ethyl.                    Iodide of ethyl.

Benzol.                             Benzol.

Amylene.                            Amylene.

Sulphuric aether.                   Sulphuric aether.

Acetic aether.                      Acetic aether.

Formic aether.                      Formic aether.

Alcohol.                            Alcohol.

Water.

We here find the order of absorption in both cases be the same.  We
have liberated the molecules from the bonds which trammel them more or
less in a liquid condition; but this change in their state of
aggregation does not change their relative powers of absorption.
Nothing could more clearly prove that the act of absorption depends
upon the individual molecule, which equally asserts its power in the
liquid and the gaseous state.  We may safely conclude from the above
table that the position of a vapour is determined by that of its
liquid.  Now at the very foot of the list of liquids stands _water_,
signalising itself above all others by its enormous power of
absorption.  And from this fact, even if no direct experiment on the
vapour of water had ever been made, we should be entitled to rank that
vapour as our most powerful absorber of radiant heat.  Its
attenuation, however, diminishes its action.  I have proved that a
shell of air two inches in thickness surrounding our planet, and
saturated with the vapour of sulphuric aether, would intercept 35 per
cent.  of the earth's radiation.  And though the quantity of aqueous
vapour necessary to saturate air is much less than the amount of
sulphuric aether vapour which it can sustain, it is still extremely
probable that the estimate already made of the action of atmospheric
vapour within 10 feet of the earth's surface, is under the mark; and
that we are indebted to this wonderful substance, to an extent not
accurately determined, but certainly far beyond what has hitherto been
imagined, for the temperature now existing at the surface of the
globe.

********************

14.  Reciprocity of Radiation and Absorption.

Throughout the reflections which have hitherto occupied us, the image
before the mind has been that of a radiant source sending forth
calorific waves, which on passing among the molecules of a gas or
vapour were intercepted by those molecules in various degrees.  In all
cases it was the transference of motion from the aether to the
comparatively quiescent molecules of the gas or vapour that occupied
our thoughts.  We have now to change the form of our conception, and
to figure these molecules not as absorbers but as radiators, not as
the recipients but as the originators of wave-motion.  That is to say,
we must figure them vibrating, and generating in the surrounding
aether undulations which speed through it with the velocity of light.
Our object now is to enquire whether the act of chemical combination,
which proves so potent as regards the phenomena of absorption, does
not also manifest its power in the phenomena of radiation.  For the
examination of this question it is necessary, in the first place, to
heat our gases and vapours to the same temperature, and then examine
their power of discharging the motion thus imparted to them upon the
aether in which they swing.

A heated copper ball was placed above a ring gas-burner possessing a
great number of small apertures, the burner being connected by a tube
with vessels containing the various gases to be examined.  By gentle
pressure the gases were forced through the orifices of the burner
against the copper ball, where each of them, being heated, rose in an
ascending column.  A thermoelectric pile, entirely screened from the
hot ball, was exposed to the radiation of the warm gas, while the
deflection of a magnetic needle connected with the pile declared the
energy of the radiation.

By this mode of experiment it was proved that the selfsame molecular
arrangement which renders a gas a powerful absorber, renders it a
powerful radiator--that the atom or molecule which is competent to
intercept the calorific waves is, in the same degree, competent to
send them forth.  Thus, while the atoms of elementary gases proved
themselves unable to emit any sensible amount of radiant heat, the
molecules of compound gases were shown to be capable of powerfully
disturbing the surrounding aether.  By special modes of experiment the
same was proved to hold good for the vapours of volatile liquids, the
radiative power of every vapour being found proportional to its
absorptive power.

The method of experiment here pursued, though not of the simplest
character, is still easy to grasp.  When air is permitted to rush into
an exhausted tube, the temperature of the air is raised to a degree
equivalent to the _vis viva_ extinguished. [Footnote: See above for a
definition of _vis viva_.] Such air is said to be dynamically heated,
and, if pure, it shows itself incompetent to radiate, even when a
rock-salt window is provided for the passage of its rays.  But if
instead of being empty the tube contain a small quantity of vapour,
the warmed air communicates its heat by contact to the vapour, the
molecules of which convert into the radiant form the heat imparted to
them by the atoms of the air.  By this process also, which I have
called Dynamic Radiation, the reciprocity of radiation and absorption
has been conclusively proved. [Footnote: When heated air imparts its
motion to another gas or vapour, the transference of heat is
accompanied by a change of vibrating period.  The Dynamic Radiation of
vapours is rendered possible by this transmutation of vibrations.]

In the excellent researches of Leslie, De la Provostaye and Detains,
and Balfour Stewart, the same reciprocity, as regards solid bodies,
has been variously illustrated; while the labours, theoretical and
experimental, of Kirchhoff have given this subject a wonderful
expansion, and enriched it by applications of the highest kind.  To
their results are now to be added the foregoing, whereby gases and
vapours, which have been hitherto thought inaccessible to experiments
with the thermo-electric pile, are proved by it to exhibit the
indissoluble duality of radiation and absorption, the influence of
chemical combination on both being exhibited in the most decisive and
extraordinary way.

********************

15.  Influence of Vibrating Period and Molecular Form.  Physical
Analysis of the Human Breath.

In the foregoing experiments with gases and vapours have employed
throughout invisible rays, and found e of these bodies so impervious
to radiant heat, that lengths of a few feet they intercept every ray
as actually as a layer of pitch.  The substances, however, which show
themselves thus opaque to radiant heat perfectly transparent to light.
Now the rays of light differ from those of invisible heat merely in
point period, the former failing to affect the retina because their
periods of recurrence are too slow.  Hence, in one way or other, the
transparency of our gases and vapours depends upon the periods of the
waves which impinge upon them.  What is the nature of this dependence?
The admirable researches of Kirchhoff help us an answer.  The atoms
and molecules of every gas e certain definite rates of oscillation,
and those waves aether are most copiously absorbed whose periods
recurrence synchronise with those of the atomic ups amongst which they
pass.  Thus, when we find invisible rays absorbed and the visible ones
transmitted by a layer of gas, we conclude that the oscillating
periods of the atoms constituting the gaseous molecules coincide with
those of the invisible, and not with those of the visible spectrum.

It requires some discipline of the imagination to form a clear picture
of this process.  Such a picture is, however, possible, and ought to
be obtained.  When the waves of aether impinge upon molecules whose
periods of vibration coincide with the recurrence of the undulations,
the timed strokes of the waves augment the vibration of the molecules,
as a heavy pendulum is set in motion by well-timed puffs of breath.
Millions of millions of shocks are received every second from the
calorific waves; and it is not difficult to see that as every wave
arrives just in time to repeat the action of its predecessor, the
molecules must finally be caused to swing through wider spaces than if
the arrivals were not so timed.  In fact, it is not difficult to see
that an assemblage of molecules, operated upon by contending waves,
might remain practically quiescent.  This is actually the case when
the waves of the visible spectrum pass through a transparent gas or
vapour.  There is here no sensible transference of motion from the
aether to the molecules; in other words, there is no sensible
absorption of heat.

One striking example of the influence of period may be here recorded.
Carbonic acid gas is one of the feeblest absorbers of the radiant heat
emitted by solid bodies.  It is, for example, to a great extent
transparent to the rays emitted by the heated copper plate already
referred to.  There are, however, certain rays, comparatively few in
number, emitted by the copper, to which the carbonic acid is
impervious; and could we obtain a source of heat emitting such rays
only, we should find carbonic acid more opaque to the radiation from
that source, than any other gas.  Such a source is actually found in
the flame of carbonic oxide, where hot carbonic acid constitutes the
main radiating body.  Of the rays emitted by our heated plate of
copper, olefiant gas absorbs ten times the quantity absorbed by
carbonic acid.  Of the rays emitted by a carbonic oxide flame,
carbonic acid absorbs twice as much as olefiant gas.  This wonderful
change in the power of the former, as an absorber, is simply due to
the fact, that the periods of the hot and cold carbonic acid are
identical, and that the waves from the flame freely transfer their
motion to the molecules which synchronise with them.  Thus it is that
the tenth an atmosphere of carbonic acid, enclosed in a tube four feet
long, absorbs 60 per cent.  of the radiation from carbonic oxide
flame, while one-thirtieth of an atmosphere absorbs 48 per cent.  of
the heat from the same source.

In fact, the presence of the minutest quantity of carbonic acid may be
detected by its action on the rays from the carbonic oxide flame.
Carrying, for example, the dried human breath into a tube four feet
long, the absorption there effected by the carbonic acid of the breath
amounts to 50 per cent.  of the entire radiation.  Radiant heat may
indeed be employed as a means of determining practically the amount of
carbonic acid expired from the lungs.  My late assistant, Mr. Barrett,
while under my direction, made this determination.  The absorption
produced by the breath freed from its moisture, but retaining its
carbonic acid, was first determined.  Carbonic acid, artificially
prepared, was then mixed with dry air in such proportions that the
action of the mixture upon the rays of heat was the same as that of
the dried breath.  The percentage of the former being known,
immediately gave that of the latter.  The same breath, analysed
chemically by Dr. Frankland, and physically by Mr. Barrett, gave the
following results:

Percentage of Carbonic Acid in the Human Breath.

Chemical analysis              Physical analysis

4.66                                4.56

5.33                                5.22

It is thus proved that in the quantity of aethereal motion which it is
competent to take up, we have a practical measure of the carbonic acid
of the breath, and hence of the combustion going on in the human
lungs.

Still this question of period, though of the utmost importance, is not
competent to account for the whole of the observed facts.  The aether,
as far as we know, accepts vibrations of all periods with the same
readiness.  To it the oscillations of an atom of free oxygen are just
as acceptable as those of the atoms in a molecule of olefiant gas;
that the vibrating oxygen then stands so far below the olefiant gas in
radiant power must be referred not to period, but to some other
peculiarity.  The atomic group which constitutes the molecule of
olefiant gas, produces many thousand times the disturbance caused by
the oxygen, it may be because the group is able to lay a vastly more
powerful hold upon the aether than single atoms can.  Another, and
probably very potent cause of the difference may be, that the
vibrations, being those of the constituent atoms of the molecule,
[Footnote: See 'Physical Considerations,' Art. iv.] are generated
in highly condensed aether, which acts like: condensed air upon sound.
But whatever may be the fate of these attempts to visualise the physics
of the process, it will still remain true, that to account for the
phenomena of radiation and absorption we must take into consideration
the shape, size, and condition of the aether within the molecules, by
which the external aether is disturbed.

********************

16.  Summary and Conclusion.

Let us now cast a momentary glance over the ground that we have left
behind.  The general nature of light and heat was first briefly
described: the compounding of matter from elementary atoms, and the
influence of the act of combination on radiation and absorption, were
considered and experimentally illustrated.  Through the transparent
elementary gases radiant heat was found to pass as through a vacuum,
while many of the compound gases presented almost impassable obstacles
to the calorific-waves.  This deportment of the simple gases directed
our attention to other elementary bodies, the examination of which led
to the discovery that the element iodine, dissolved in bisulphide of
carbon, possesses the power detaching, with extraordinary sharpness,
the light of the spectrum from its heat, intercepting all luminous
rays up to the extreme red, and permitting the calorific rays beyond
the red to pass freely through it.  This substance was then employed
to filter the beams of the electric light, and to form foci of
invisible rays so intense as to produce almost all the effects
obtainable in ordinary fire.  Combustible bodies were burnt, and
refractory ones were raised to a white heat, by the concentrated
invisible rays.  Thus, by exalting their refrangibility, the invisible
rays of the electric light were rendered visible, and all the colours
of the solar spectrum were extracted from utter darkness.  The extreme
richness of the electric light in invisible rays of low refrangibility
was demonstrated, one-eighth only of its radiation consisting of
luminous rays.  The deadness of the optic nerve to those invisible
rays was proved, and experiments were then added to show that the
bright and the dark rays of a solid body, raised gradually to
incandescence, are strengthened together; intense dark heat being an
invariable accompaniment of intense white heat.  A sun could not be
formed, or a meteorite rendered luminous, on any other condition.  The
light-giving rays constituting only a small fraction of the total
radiation, their unspeakable importance to us is due to the fact, that
their periods are attuned to the special requirements of the eye.

Among the vapours of volatile liquids vast differences were also found
to exist, as regards their powers of absorption.  We followed various
molecules from a state of liquid to a state of gas, and found, in both
states of aggregation, the power of the individual molecules equally
asserted.  The position of a vapour as an absorber of radiant heat was
shown to be determined by that of the liquid from which it is derived.
Reversing our conceptions, and regarding the molecules of gases and
vapours not as the recipients but as the originators of wave-motion;
not as absorbers but as radiators; it was proved that the powers of
absorption and radiation went hand in hand, the self-same chemical act
which rendered a body competent to intercept the waves of aether,
rendering it competent, in the same degree, to generate them. Perfumes
were next subjected to examination, and, notwithstanding their
extraordinary tenuity, they were found vastly superior, in point of
absorptive power, to the body of the air in which they were diffused.
We were led thus slowly up to the examination of the most widely
diffused and most important of all vapours--the aqueous vapour of our
atmosphere, and we found in it a potent absorber of the purely
calorific rays.  The power of this substance to influence climate, and
its general influence on the temperature of the earth, were then
briefly dwelt upon.  A cobweb spread above a blossom is sufficient to
protect it from nightly chill; and thus the aqueous vapour of our
air, attenuated as it is, checks the drain of terrestrial heat, and
saves the surface of our planet from the refrigeration which would
assuredly accrue, were no such substance interposed between it and the
voids of space.  We considered the influence of vibrating period, and
molecular form, on absorption and radiation, and finally deduced, from
its action upon radiant heat, the exact amount of carbonic acid
expired by the human lungs.

Thus, in brief outline, were placed before you some ofthe results of
recent enquiries in the domain of Radiation, and my aim throughout has
been to raise in your minds distinct physical images of the various
processes involved in our researches.  It is thought by some that
natural science has a deadening influence on the imagination, and a
doubt might fairly be raised as to the value of any study which would
necessarily have this effect.  But the experience of the last hour
must, I think, have convinced you, that the study of natural science
goes hand in hand with the culture of the imagination.  Throughout the
greater part of this discourse we have been sustained by this faculty.
We have been picturing atoms, and molecules, and vibrations, and
waves, which eye has never seen nor ear heard, and which can only be
discerned by the exercise of imagination.  This, in fact, is the
faculty which enables us transcend the boundaries of sense, and
connect the phenomena of our visible world with those of an invisible
one.  Without imagination we never could have risen to the conceptions
which have occupied us here today; and in proportion to your power of
exercising this faculty aright, and of associating definite mental
images with the terms employed, will be the pleasure and the profit
which you will derive from this lecture.

The outward facts of nature are insufficient to satisfy the mind.  We
cannot be content with knowing that the light and heat of the sun
illuminate and warm the world.  We are led irresistibly to enquire,
'What is light, and what is heat?' and this question leads us at once
out of the region of sense into that of imagination. [Footnote: This
line of thought was pursued further five years subsequently.  See
'Scientific Use of the Imagination' in Vol. II.]

Thus pondering, and questioning, and striving to supplement that which
is felt and seen, but which is incomplete, by something unfelt and
unseen which is necessary to its completeness, men of genius have in
part discerned, not only the nature of light and heat, but also,
through them, the general relationship of natural phenomena.  The
working power of Nature consists of actual or potential motion, of
which all its phenomena are but special forms.  This motion manifests
itself in tangible and in intangible matter, being incessantly
transferred from the one to the other, and incessantly transformed by
the change.  It is as real in the waves of the aether as in the waves
of the sea; the latter--derived as they are from winds, which in their
turn are derived from the sun--are, indeed, nothing more than the
heaped-up motion of the aether waves.  It is the calorific waves
emitted by the sun which heat our air, produce our winds, and hence
agitate our ocean.  And whether they break in foam upon the shore, or
rub silently against the ocean's bed, or subside by the mutual
friction of their own parts, the sea waves, which cannot subside
without producing heat, finally resolve themselves into waves of
aether, thus regenerating the motion from which their temporary
existence was derived.  This connection is typical.  Nature is not an
aggregate of independent parts, but an organic whole.  If you open a
piano and sing into it, a certain string will respond.  Change the
pitch of our voice; the first string ceases to vibrate, but another
replies.  Change again the pitch; the first two strings are silent,
while another resounds.  Thus is sentient man acted on by Nature, the
optic, the auditory, and other nerves of the human body being so many
strings differently tuned, and responsive to different forms of the
universal power.

********************

III ON RADIANT HEAT IN RELATION TO THE COLOUR AND CHEMICAL
CONSTITUTION OF BODIES.

[Footnote: A discourse delivered in the Royal Institution of Great
Britain, Jan. 19, 1866.]

ONE of the most important functions of physical science, considered as
a discipline of the mind, is to enable us by means of the sensible
processes of Nature to apprehend the insensible.  The sensible
processes give direction to the line of thought; but this once given,
the length of the line is not limited by the boundaries of the senses.
Indeed, the domain of the senses, in Nature, is almost infinitely
small in comparison with the vast region accessible to thought which
lies beyond them.  From a few observations of a comet, when it comes
within the range of his telescope, an astronomer can calculate its
path in regions which no telescope can reach: and in like manner, by
means of data furnished in the narrow world of the senses, we make
ourselves at home in other and wider worlds, which are traversed by
the intellect alone.

From the earliest ages the questions, 'What is light?' and 'What is
heat?' have occurred to the minds of men; but these questions never
would have been answered had they not been preceded by the question,
'What is sound?' Amid the grosser phenomena of acoustics the mind was
first disciplined, conceptions being thus obtained from direct
observation, which were afterwards applied to phenomena of a character
far too subtle to be observed directly.  Sound we know to be due to
vibratory motion.  A vibrating tuning-fork, for example, moulds the
air around it into undulations or waves, which speed away on all sides
with a certain measured velocity, impinge upon the drum of the ear,
shake the auditory nerve, and awake in the brain the sensation of
sound.  When sufficiently near a sounding body we can feel the
vibrations of the air.  A deaf man, for example, plunging his hand
into a bell when it is sounded, feels through the common nerves of his
body those tremors which, when imparted to the nerves of healthy ears,
are translated into sound.  There are various ways of rendering those
sonorous vibrations not only tangible but visible; and it was not
until numberless experiments of this kind had been executed, that the
scientific investigator abandoned himself wholly, and without a shadow
of misgiving, to the conviction that what is sound within us is,
outside of us, a motion of the air.

But once having established this fact--once having proved beyond all
doubt that the sensation of sound is produced by an agitation of the
auditory nerve--the thought soon suggested itself that light might be
due to an agitation of the optic nerve.  This was a great step in
advance of that ancient notion which regarded light as something
emitted by the eye, and not as anything imparted to it.  But if light
be produced by an agitation of the retina, what is it that produces
the agitation?  Newton, you know, supposed minute particles to be shot
through the humours of the eye against the retina, which he supposed
to hang like a target at the back of the eye.  The impact of these
particles against the target, Newton believed to be the cause of
light. But Newton's notion has not held its ground, being entirely
driven from the field by the more wonderful and far more philosophical
notion that light, like sound, is a product of wave-motion.

The domain in which this motion of light is carried on lies entirely
beyond the reach of our senses.  The waves of light require a medium
for their formation and propagation; but we cannot see, or feel, or
taste, or smell this medium.  How, then, has its existence been
established?  By showing, that by the assumption of this wonderful
intangible aether, all the phenomena of optics are accounted for, with
a fulness, and clearness, and conclusiveness, which leave no desire of
the intellect unsatisfied.  When the law of gravitation first
suggested itself to the mind of Newton, what did he do?  He set
himself to examine whether it accounted for all the facts.  He
determined the courses of the planets; he calculated the rapidity of
the moon's fall towards the earth; he considered the precession of the
equinoxes, the ebb and flow of the tides, and found all explained by
the law of gravitation.  He therefore regarded this law as
established, and the verdict of science subsequently confirmed his
conclusion.  On similar, and, if possible, on stronger grounds, we
found our belief in the existence of the universal aether.  It
explains facts far more various and complicated than those on which
Newton based his law.  If a single phenomenon could be pointed out
which the aether is proved incompetent to explain, we should have to
give it up; but no such phenomenon has ever been pointed out.  It is,
therefore, at least as certain that space is filled with a medium, by
means of which suns and stars diffuse their radiant power, as that it
is traversed by that force which holds in its grasp, not only our
planetary system, but the immeasurable heavens themselves.

There is no more wonderful instance than this of the production of a
line of thought, from the world of the senses into the region of pure
imagination.  I mean by imagination here, not that play of fancy which
can give to airy nothings a local habitation and a name, but that
power which enables the mind to conceive realities which lie beyond
the range of the senses--to present to itself distinct images of
processes which, though mighty in the aggregate beyond all conception,
are so minute individually as to elude all observation.  It is the
waves of air excited by a tuning-fork which render its vibrations
audible.  It is the waves of aether sent forth from those lamps
overhead which render them luminous to us; but so minute are these
waves, that it would take from 30,000 to 60,000 of them placed end to
end to cover a single inch.  Their number, however, compensates for
their minuteness.  Trillions of them have entered your eyes, and hit
the retina at the backs of your eyes, in the time consumed in the
utterance of the shortest sentence of this discourse.  This is the
steadfast result of modern research; but we never could have reached
it without previous discipline.  We never could have measured the
waves of light, nor even imagined them to exist, had we not previously
exercised ourselves among the waves of sound.  Sound and light are now
mutually helpful, the conceptions of each being expanded,
strengthened, and defined by the conceptions of the other.

The aether which conveys the pulses of light and heat not only fills
celestial space, swathing suns, and planets, and moons, but it also
encircles the atoms of which these bodies are composed.  It is the
motion of these atoms, and not that of any sensible parts of bodies,
that the aether conveys.  This motion is the objective cause of what,
in our sensations, are light and heat.  An atom, then, sending its
pulses through the aether, resembles a tuning-fork sending its pulses
through the air.  Let us look for a moment at this thrilling medium,
and briefly consider its relation to the bodies whose vibrations it
conveys.  Different bodies, when heated to the same temperature,
possess very different powers of agitating the aether: some are good
radiators, others are bad radiators; which means that some are so
constituted as to communicate their atomic motion freely to the
aether, producing therein powerful undulations; while the atoms of
others are unable thus to communicate their motions, but glide through
the medium without materially disturbing its repose.  Recent
experiments have proved that elementary bodies, except under certain
anomalous conditions, belong to the class of bad radiators.  An atom,
vibrating in the aether, resembles a naked tuning-fork vibrating in
the air.  The amount of motion communicated to the air by the thin
prongs is too small to evoke at any distance the sensation of sound.
But if we permit the atoms to combine chemically and form molecules,
the result, in many cases, is an enormous change in the power of
radiation.  The amount of aethereal disturbance, produced by the
combined atoms of a body, may be many thousand times that produced by
the same atoms when uncombined.

The pitch of a musical note depends upon the rapidity of its
vibrations, or, in other words, on the length of its waves.  Now, the
pitch of a note answers to the colour of light.  Taking a slice of
white light from the sun, or from an electric lamp, and causing the
light to pass through an arrangement of prisms, it is decomposed.  We
have the effect obtained by Newton, who first unrolled the solar beam
into the splendours of the solar spectrum.  At one end of this
spectrum we have red light, at the other, violet; and between those
extremes lie the other prismatic colours.  As we advance along the
spectrum from the red to the violet, the pitch of the light--if I may
use the expression--heightens, the sensation of violet being produced
by a more rapid succession of impulses than that which produces the
impression of red.  The vibrations of the violet are about twice as
rapid as those of the red; in other words, the range of the visible
spectrum is about an octave.

There is no solution of continuity in this spectrum one colour changes
into another by insensible gradations.  It is as if an infinite number
of tuning-forks, of gradually augmenting pitch, were vibrating at the
same time.  But turning to another spectrum--that, namely, obtained
from the incandescent vapour of silver--you observe that it consists
of two narrow and intensely luminous green bands.  Here it is as if
two forks only, of slightly different pitch, were vibrating.  The
length of the waves which produce this first band is such that 47,460
of them, placed end to end, would fill an inch.  The waves which
produce the second band are a little shorter; it would take of these
47,920 to fill an inch.  In the case of the first band, the number of
impulses imparted, in one second, to every eye which sees it, is 677
millions of millions; while the number of impulses imparted, in the
same time, by the second band is 600 millions of millions.  We may
project upon a white screen the beautiful stream of green light from
which these bands were derived.  This luminous stream is the
incandescent vapour of silver.  The rates of vibration of the atoms of
that vapour are as rigidly fixed as those of two tuning-forks; and to
whatever height the temperature of the vapour may be raised, the
rapidity of its vibrations, and consequently its colour, which wholly
depends upon that rapidity, remain unchanged.

The vapour of water, as well as the vapour of silver, has its definite
periods of vibration, and these are such as to disqualify the vapour,
when acting freely as such, from being raised to a white heat.  The
oxyhydrogen flame, for example, consists of hot aqueous vapour.  It is
scarcely visible in the air of this room, and it would be still less
visible if we could burn the gas in a clean atmosphere.  But the
atmosphere, even at the summit of Mont Blanc, is dirty; in London it
is more than dirty; and the burning dirt gives to this flame the
greater portion of its present light.  But the heat of the flame is
enormous.  Cast iron fuses at a temperature of 2,000° Fahr; while the
temperature of the oxyhydrogen flame is 6,000° Fahr.  A piece of
platinum is heated to vivid redness, at a distance of two inches
beyond the visible termination of the flame.  The vapour which
produces incandescence is here absolutely dark.  In the flame itself
the platinum is raised to dazzling whiteness, and is even pierced by
the flame.  When this flame impinges on a piece of lime, we have the
dazzling Drummond light.  But the light is here due to the fact that
when it impinges upon the solid body, the vibrations excited in that
body by the flame are of periods different from its own.

Thus far we have fixed our attention on atoms and molecules in a state
of vibration, and surrounded by a medium which accepts their
vibrations, and transmits them through space.  But suppose the waves
generated by one system of molecules to impinge upon another system,
how will the waves be affected?  Will they be stopped, or will they be
permitted to pass?  Will they transfer their motion to the molecules
on which they impinge, or will they glide round the molecules, through
the intermolecular spaces, and thus escape?

The answer to this question depends upon a condition which may be
beautifully exemplified by an experiment on sound.  These two
tuning-forks are tuned absolutely alike.  They vibrate with the same
rapidity, and, mounted thus upon their resonant cases, you hear them
loudly sounding the same musical note.  Stopping one of the forks, I
throw the other into strong vibration, and bring that other near the
silent fork, but not into contact with it.  Allowing them to continue
in this position for four or five seconds, and then stopping the
vibrating fork, the sound does not cease.  The second fork has taken
up the vibrations of its neighbour, and is now sounding in its turn.
Dismounting one of the forks, and permitting the other to remain upon
its stand, I throw the dismounted fork into strong vibration.  You
cannot hear it sound.  Detached from its case, the amount of motion
which it can communicate to the air is too small to be sensible at any
distance.  When the dismounted fork is brought close to the mounted
one, but not into actual contact with it, out of the silence rises a
mellow sound.  Whence comes it?  From the vibrations which have been
transferred from the dismounted fork to the mounted one.

That the motion should thus transfer itself through the air it is
necessary that the two forks should be in perfect unison.  If a morsel
of wax not larger than a pea be placed on one of the forks, it is
rendered thereby powerless to affect, or to be affected by, the other.
It is easy to understand this experiment.  The pulses of the one fork
can affect the other, because they are _perfectly timed_.  A single
pulse causes the prong of the silent fork to vibrate through an
infinitesimal space.  But just as it has completed this small
vibration another pulse is ready to strike it.  Thus, the impulses add
themselves together.  In the five seconds during which the forks were
held near each other, the vibrating fork sent 1,280 waves against its
neighbour and those 1,280 shocks, all delivered at the proper moment,
all, as I have said, perfectly timed, have given such strength to the
vibrations of the mounted fork as to render them audible to all.

Another curious illustration of the influence of synchronism on
musical vibrations, is this: Three small gas-flames are inserted into
three glass tubes of different lengths.  Each of these flames can be
caused to emit a musical note, the pitch of which is determined by the
length of the tube surrounding the flame.  The shorter the tube the
higher is the pitch.  The flames are now silent within their
respective tubes, but each of them can be caused to respond to a
proper note sounded anywhere in this room.  With an instrument called
a syren, a powerful musical note, of gradually increasing pitch, can
be produced.  Beginning with a low note, and ascending gradually to a
higher one, we finally attain the pitch of the flame in the longest
tube.  The moment it is reached, the flame bursts into song.  The
other flames are still silent within their tubes.  But by urging the
instrument on to higher notes, the second flame is started, and the
third alone remains.  A still higher note starts it also.  Thus, as
the sound of the syren rises gradually in pitch, it awakens every
flame in passing, by striking it with a series of waves whose periods
of recurrence are similar to its own.

Now the wave-motion from the syren is in part taken up by the flame
which synchronises with the waves; and were these waves to impinge
upon a multitude of flames, instead of upon one flame only, the
transference might be so great as to absorb the whole of the original
wave motion.  Let us apply these facts to radiant heat.  This blue
flame is the flame of carbonic oxide; this transparent gas is carbonic
acid gas.  In the blue flame we have carbonic acid intensely heated,
or, in other words, in a state of intense vibration.  It thus
resembles the sounding fork, while this cold carbonic acid resembles
the silent one.  What is the consequence?  Through the synchronism of
the hot and cold gas, the waves emitted by the former are intercepted
by the latter, the transmission of the radiant heat being thus
prevented.  The cold gas is intensely opaque to the radiation from
this particular flame, though highly transparent to heat of every
other kind.  We are here manifestly dealing with that great principle
which lies at the basis of spectrum analysis, and which has enabled
scientific men to determine the substances of which the sun, the
stars, and even the nebulae are composed; the principle, namely, that
a body which is competent to emit any ray, whether of heat or light,
is competent in the same degree to absorb that ray.  The absorption
depends on the synchronism existing between the vibrations of the toms
from which the rays, or more correctly the waves, sue, and those of
the atoms on which they impinge.

To its almost total incompetence to emit white light, aqueous vapour
adds a similar incompetence to absorb bite light.  It cannot, for
example, absorb the luminous rays of the sun, though it can absorb the
non-luminous rays of the earth.  This incompetence of the vapour to
absorb luminous rays is shared by water and ice--in fact, by all
really transparent substances.  Their transparency is due to their
inability to absorb luminous rays.  The molecules of such substances
are in dissonance with luminous waves; and hence such waves pass
through transparent bodies without disturbing the molecular rest. A
purely luminous beam, however intense may be its heat, is sensibly
incompetent to melt ice.  We can, for example, converge a powerful
luminous beam upon a surface covered with hoar frost, without melting
a single spicula of the crystals.  How then, it may be asked, are the
snows of the Alps swept away by the sunshine of summer?  I answer,
they are not swept away by sunshine at all, but by rays which have no
sunshine whatever in them.  The luminous rays of the sun fall upon the
snow-fields and are flashed in echoes from crystal to crystal, but
they find next to no lodgment within the crystals.  They are hardly at
all absorbed, and hence they cannot produce fusion.  But a body of
powerful dark rays is emitted by the sun; and it is these that cause
the glaciers to shrink and the snows to disappear; it is they that
fill the banks of the Arve and Arveyron, and liberate from their
frozen captivity the Rhone and the Rhine.

Placing a concave silvered mirror behind the electric light its rays
are converged to a focus of dazzling brilliancy.  Placing in the path
of the rays, between the light and the focus, a vessel of water, and
introducing at the focus a piece of ice, the ice is not melted by the
concentrated beam.  Matches, at the same place, are ignited, and wood
is set on fire.  The powerful heat, then, of this luminous beam is
incompetent to melt the ice.  On withdrawing the cell of water, the
ice immediately liquefies, and the water trickles from it in drops.
Reintroducing the cell of water, the fusion is arrested, and the drops
cease to fall.  The transparent water of the cell exerts no sensible
absorption on the luminous rays, still it withdraws something from the
beam, which, when permitted to act, is competent to melt the ice. This
something is the dark radiation of the electric light.  Again, I place
a slab of pure ice in front of the electric lamp; send a luminous beam
first through our cell of water and then through the ice.  By means of
a lens an image of the slab is cast upon a white screen.  The beam,
sifted by the water, has little power upon the ice. But observe what
occurs when the water is removed; we have here a star and there a
star, each star resembling a flower of six petals, and growing visibly
larger before our eyes.  As the leaves enlarge, their edges become
serrated, but there is no deviation from the six-rayed type.  We have
here, in fact, the crystallisation of the ice reversed by the
invisible rays of the electric beam.  They take the molecules down in
this wonderful way, and reveal to us the exquisite atomic structure of
the substance with which Nature every winter roofs our ponds and
lakes.

Numberless effects, apparently anomalous, might be adduced in
illustration of the action of these lightless rays.  These two
powders, for example, are both white, and undistinguishable from each
other by the eye.  The luminous rays of the sun are unabsorbed by
both--from such rays these powders acquire no heat; still one of them,
sugar, is heated so highly by the concentrated beam of the electric
lamp, that it first smokes and then violently inflames, while the
other substance, salt, is barely warmed at the focus.  Placing two
perfectly transparent liquids in test-tubes at the focus, one of them
boils in a couple of seconds, while the other, in a similar position,
is hardly warmed.  The boiling-point of the first liquid is 78°C,
which is speedily reached; that of the second liquid is only 48°C,
which is never reached at all.  These anomalies are entirely due to
the unseen element which mingles with the luminous rays of the
electric beam, and indeed constitutes 90 per cent.  of its calorific
power.

A substance, as many of you know, has been discovered, by which these
dark rays may be detached from the total emission of the electric
lamp.  This ray-filter is a liquid, black as pitch to the luminous,
but bright as a diamond to the non-luminous, radiation.  It
mercilessly cuts off the former, but allows the latter free
transmission.  When these invisible rays are brought to a focus, at a
distance of several feet from the electric lamp, the dark rays form an
invisible image of their source.  By proper means, this image may be
transformed into a visible one of dazzling brightness.  It might,
moreover, be shown, if time permitted, how, out of those perfectly
dark rays, could be extracted, by a process of transmutation, all the
colours of the solar spectrum.  It might also be proved that those
rays, powerful as they are, and sufficient to fuse many metals, can be
permitted to enter the eye, and to break upon the retina, without
producing the least luminous impression.

The dark rays being thus collected, you see nothing at their place of
convergence.  With a proper thermometer it could be proved that even
the air at the focus is just as cold as the surrounding air.  And mark
the conclusion to which this leads.  It proves the aether at the focus
to be practically detached from the air,--that the most violent
aethereal motion may there exist, without the least aerial motion.
But, though you see it not, there is sufficient heat at that focus to
set London on fire.  The heat there is competent to raise iron to a
temperature at which it throws off brilliant scintillations.  It can
heat platinum to whiteness, and almost fuse that refractory metal.  It
actually can fuse gold, silver, copper, and aluminium.  The moment,
moreover, that wood is placed at the focus it bursts into a blaze.

It has been already affirmed that, whether as regards radiation or
absorption, the elementary atoms possess but little power.  This might
be illustrated by a long array of facts; and one of the most singular
of these is furnished by the deportment of that extremely combustible
substance, phosphorus, when placed at the dark focus.  It is
impossible to ignite there a fragment of amorphous phosphorus.  But
ordinary phosphorus is a far quicker combustible, and its deportment
towards radiant heat is still more impressive.  It may be exposed to
the intense radiation of an ordinary fire without bursting into flame.
It may also be exposed for twenty or thirty seconds at an obscure
focus, of sufficient power to raise platinum to a red heat, without
ignition.  Notwithstanding the energy of the aethereal waves here
concentrated, notwithstanding the extremely inflammable character of
the elementary body exposed to their action, the atoms of that body
refuse to partake of the motion of the powerful waves of low
refrangibility, and consequently cannot be affected by their heat.

The knowledge we now possess will enable us to analyse with profit a
practical question.  White dresses are worn in summer, because they
are found to be cooler than dark ones.  The celebrated Benjamin
Franklin placed bits of cloth of various colours upon snow, exposed
them to direct sunshine, and found that they sank to different depths
in the snow.  The black cloth sank deepest, the white did not sink at
all.  Franklin inferred from this experiment that black-bodies are the
best absorbers, and white ones the worst absorbers, of radiant heat.
Let us test the generality of this conclusion.  One of these two cards
is coated with a very dark powder, and the other with a perfectly
white one.  I place the powdered surfaces before a fire, and leave
them there until they have acquired as high a temperature as they can
attain in this position.  Which of the cards is then most highly
heated?  It requires no thermometer to answer this question. Simply
pressing the back of the card, on which the white powder is strewn,
against the cheek or forehead, it is found intolerably hot. Placing
the dark card in the same position, it is found cool.  The white
powder has absorbed far more heat than the dark one.  This simple
result abolishes a hundred conclusions which have been hastily drawn
from the experiments of Franklin.  Again, here are suspended two
delicate mercurial thermometers at the same distance from a gas-flame.
The bulb of one of them is covered by a dark substance, the bulb of
the other by a white one.  Both bulbs have received the radiation from
the flame, but the white bulb has absorbed most, and its mercury
stands much higher than that of the other thermometer.  This
experiment might be varied in a hundred ways: it proves that from the
darkness of a body you can draw no certain conclusion regarding its
power of absorption.

The reason of this simply is, that colour gives us intelligence of
only one portion, and that the smallest one, of the rays impinging on
the coloured body.  Were the rays all luminous, we might with
certainty infer from the colour of a body its power of absorption; but
the great mass of the radiation from our fire, our gas-flame, and even
from the sun itself, consists of invisible calorific rays, regarding
which colour teaches us nothing.  A body may be highly transparent to
the one class of rays, and highly opaque to the other.  Thus the white
powder, which has shown itself so powerful an absorber, has been
specially selected on account of its extreme perviousness to the
visible rays, and its extreme imperviousness to the invisible ones;
while the dark powder was chosen on account of its extreme
transparency to the invisible, and its extreme opacity to the visible,
rays.  In the case of the radiation from our fire, about 98 per cent
of the whole emission consists of invisible rays; the body, therefore,
which was most opaque to these triumphed as an absorber, though that
body was a white one.

And here it is worth while to consider the manner in which we obtain
from natural facts what may be called their intellectual value.
Throughout the processes of Nature we have interdependence and
harmony; and the main value of physics, considered as a mental
discipline, consists in the tracing out of this interdependence, and
the demonstration of this harmony.  The outward and visible phenomena
are the counters of the intellect; and our science would not be worthy
of its name and fame if it halted at facts, however practically
useful, and neglected the laws which accompany and rule the phenomena.
Let us endeavour, then, to extract from the experiment of Franklin all
that it can yield, calling to our aid the knowledge which our
predecessors have already stored.  Let us imagine two pieces of cloth
of the same texture, the one black and the other white, placed upon
sunned snow.  Fixing our attention on the white piece, let us enquire
whether there is any reason to expect that it will sink in the snow at
all.  There is knowledge at hand which enables us to reply at once in
the negative.  There is, on the contrary, reason to expect that, after
a sufficient exposure, the bit of cloth will be found on an eminence
instead of in a hollow; that instead of a depression, we shall have a
relative elevation of the bit of cloth.  For, as regards the luminous
rays of the sun, the cloth and the snow are alike powerless; the one
cannot be warmed, nor the other melted, by such rays.  The cloth is
white and the snow is white, because their confusedly mingled fibres
and particles are incompetent to absorb the luminous rays.  Whether,
then, the cloth will sink or not depends entirely upon the dark rays
of the sun.  Now the substance which absorbs these dark rays with the
greatest avidity is ice,--or snow, which is merely ice in powder.
Hence, a less amounts of heat will be lodged in the cloth than in the
surrounding snow.  The cloth must therefore act as a shield to the
snow on which it rests; and, in consequence of the more rapid fusion
of the exposed snow, its shield must, in due time, be left behind,
perched upon an eminence like a glacier-table.

But though the snow transcends the cloth, both as a radiator and
absorber, it does not much transcend it.  Cloth is very powerful in
both these respects.  Let us now turn our attention to the piece of
black cloth, the texture and fabric of which I assume to be the same
as that of the white.  For our object being to compare the effects of
colour, we must, in order to study this effect in its purity, preserve
all the other conditions constant.  Let us then suppose the black
cloth to be obtained from the dyeing of the white.  The cloth itself,
without reference to the dye, is nearly as good an absorber of heat as
the snow around it.  But to the absorption of the dark solar rays by
the undyed cloth, is now added the absorption of the whole of the
luminous rays, and this great additional influx of heat is far more
than sufficient to turn the balance in favour of the black cloth.  The
sum of its actions on the dark and luminous rays, exceeds the action
of the snow on the dark rays alone.  Hence the cloth will sink in the
snow, and this is the complete analysis of Franklin's experiments.

Throughout this discourse the main stress has been laid on chemical
constitution, as influencing most powerfully the phenomena of
radiation and absorption.

With regard to gases and vapours, and to the liquids from which these
vapours are derived, it has been proved by the most varied and
conclusive experiments that the acts of radiation and absorption are
molecular--that they depend upon chemical, and not upon mechanical,
condition.  In attempting to extend this principle to solids I was met
by a multitude of facts, obtained by celebrated experimenters, which
seemed flatly to forbid such an extension.  Mellon, for example, had
found the same radiant and absorbent power for chalk and lamp-black.
MM. Masson and Courtépée had performed a most elaborate series of
experiments on chemical precipitates of various kinds, and found that
they one and all manifested the same power of radiation.  They
concluded from their researches, that when bodies are reduced to an
extremely fine state of division, the influence of this state is so
powerful as entirely to mask and override whatever influence may be
due to chemical constitution.

But it appears to me that through the whole of these researches an
oversight has run, the mere mention of which will show what caution is
essential in the operations of experimental philosophy; while an
experiments or two will make clear wherein the oversight consists.
Filling a brightly polished metal cube with boiling water, I determine
the quantity of heat emitted by two of the bright surfaces.  As a
radiator of heat one of them far transcends the other.  Both surfaces
appear to be metallic; what, then, is the cause of the observed
difference in their radiative power?  Simply this: one of the surfaces
is coated with transparent gum, through which, of course, is seen the
metallic lustre behind; and this varnish, though so perfectly
transparent to luminous rays, is as opaque as pitch, or lamp-black, to
non-luminous ones.  It is a powerful emitter of dark rays; it is also
a powerful absorber.  While, therefore, at the present moment, it is
copiously pouring forth radiant heat itself, it does not allow a
single ray from the metal behind to pass through it.  The varnish
then, and not the metal, is the real radiator.

Now Melloni, and Masson, and Courtépée experimented thus: they mixed
their powders and precipitates with gum-water, and laid them, by means
of a brush, upon the surfaces of a cube like this.  True, they saw
their red powders red, their white ones white, and their black ones
black, but they saw these colours _through the coat of varnish which
surrounded every particle_.  When, therefore, it was concluded that
colour had no influence on radiation, no chance had been given to it
of asserting its influence; when it was found that all chemical
precipitates radiated alike, it was the radiation from a varnish,
common to them all, which showed the observed constancy.  Hundreds,
perhaps thousands, of experiments on' radiant heat have been performed
in this way, by various enquirers, but the work will, I fear, have to
be done over again.  I am not, indeed, acquainted with an instance in
which an oversight of so trivial a character has been committed by so
many able men in succession, vitiating so large an amounts of
otherwise excellent work.  Basing our reasonings thus on demonstrated
facts, we arrive at the extremely probable conclusion that the
envelope of the particles, and not the particles themselves, was the
real radiator in the experiments just referred to.  To reason thus,
and deduce their more or less probable consequences from experimental
facts, is an incessant exercise of the student of physical science.
But having thus followed, for a time, the light of reason alone
through a series of phenomena, and emerged from them with a purely
intellectual conclusion, our duty is to bring that conclusion to an
experimental test. In this way we fortify our science.

For the purpose of testing our conclusion regarding the influence of
the gum, I take two powders presenting the same physical appearance;
one of them is a compound of mercury, and the other a compound of
lead.  On two surfaces of a cube are spread these bright red powders,
without varnish of any kind.  Filling the cube with boiling water, and
determining the radiation from the' two surfaces, one of them is found
to emit thirty-nine units of heat, while the other emits seventy-four.
This, surely, is a great difference.  Here, however, is a second cube,
having two of its surfaces coated with the same powders, the only
difference being that the powders are laid on by means of a
transparent gum.  Both surfaces are now absolutely alike in radiative
power.  Both of them emit somewhat more than was emitted by either of
the unvarnished powders, simply because the gum employed is a better
radiator than either of them.  Excluding all varnish, and comparing
white with white, vast differences are found; comparing black with
black, they are also different; and when black and white are compared,
in some cases the black radiates far more than the white, while in
other cases the white radiates far more than the black.  Determining,
moreover, the absorptive power of those powders, it is found to go
hand-in-hand with their radiative power.  The good radiator is a good
absorber, and the bad radiator is a bad absorber.  From all this it is
evident that as regards the radiation and absorption of non-luminous
heat, colour teaches us nothing; and that even as regards the
radiation of the sun, consisting as it does mainly of non-luminous
rays, conclusions as to the influence of colour may be altogether
delusive.  This is the strict scientific upshot of our researches. But
it is not the less true that in the case of wearing apparel--and this
for reasons which I have given in analysing the experiments of
Franklin--black dresses are more potent than white ones as absorbers
of solar heat.

Thus, in brief outline, have been brought before you a few of the
results of recent enquiry.  If you ask me what is the use of them, I
can hardly answer you, unless you define the term use.  If you meant
to ask whether those dark rays which clear away the Alpine snows, will
ever be applied to the roasting of turkeys, or the driving of
steam-engines--while affirming their power to do both, I would frankly
confess that they are not at present capable of competing profitably
with coal in these particulars.  Still they may have great uses
unknown to me; and when our coal-fields are exhausted, it is possible
that a more aethereal race than we are may cook their victuals, and
perform their work, in this transcendental way.  But is it necessary
that the student of science should have his labours tested by their
possible practical applications?  What is the practical value of
Homer's Iliad?  You smile, and possibly think that Homer's Iliad is
good as a means of culture.  There's the rub.  The people who demand
of science practical uses, forget, or do not know, that it also is
great as a means of culture--that the knowledge of this wonderful
universe is a thing profitable in itself, and requiring no practical
application to justify its pursuit.

But while the student of Nature distinctly refuses to have his labours
judged by their practical issues, unless the term practical be made to
include mental as well as material good, he knows full well that the
greatest practical triumphs have been episodes in the search after
pure natural truth.  The electric telegraph is the standing wonder of
this age, and the men whose scientific knowledge, and mechanical
skill, have made the telegraph what it is, are deserving of all
honour.  In fact, they have had their reward, both in reputation and
in those more substantial benefits which the direct service of the
public always carries in its train.  But who, I would ask, put the
soul into this telegraphic body?  Who snatched from heaven the fire
that flashes along the line?  This, I am bound to say, was done by two
men, the one a dweller in Italy, [Footnote: Volta] the other a
dweller in England, [Footnote: Faraday] who never in their enquiries
consciously set a practical object before them--whose only stimulus
was the fascination which draws the climber to a never-trodden peak,
and would have made Caesar quit his victories for the sources of the
Nile.  That the knowledge brought to us by those prophets, priests,
and kings of science is what the world calls 'useful knowledge,' the
triumphant application of their discoveries proves.  But science has
another function to fulfil, in the storing and the training of the
human mind; and I would base my appeal to you on the specimen which
has this evening been brought before you, whether any system of
education at the present day can be deemed even approximately
complete, in which the knowledge of Nature is neglected or ignored.

********************

IV.  NEW CHEMICAL REACTIONS PRODUCED BY LIGHT.

1868-69.

1 DECOMPOSITION BY LIGHT.

MEASURED by their power, not to excite vision, but to produce heat--in
other words, measured by their absolute energy--the ultra-red waves of
the sun and of the electric light, as shown in the preceding articles,
far transcend the visible.  In the domain of chemistry, however, there
are numerous cases in which the more powerful waves are ineffectual,
while the more minute waves, through what may be called their
timeliness of application, are able to produce great effects.  A
series of these, of a novel and beautiful character, discovered in
1868, and further illustrated in subsequent years, may be exhibited by
subjecting the vapours of volatile liquids to the action of
concentrated sunlight, or to the concentrated beam of the electric
light.  Their investigation led up to the discourse on 'Dust and
Disease' which follows in this volume; and for this reason some
account of them is introduced here.

*****

A glass tube 3 feet long and 3 inches wide, which had been frequently
employed in my researches on radiant heat, was supported horizontally
on two stands.  At one end of the tube was placed an electric lamp,
the height and position of both being so arranged, that the axis of
the tube, and that of the beam issuing from the lamp, were coincident.
In the first experiments the two ends of the tube were closed by
plates of rock-salt, and subsequently by plates of glass.  For the
sake of distinction, I call this tube the experimental tube.  It was
connected with an air-pump, and also with a series of drying and other
tubes used for the purification of the air.

A number of test-tubes, like F, fig. 2 (I have used at least fifty of
them), were converted into Woulf's flasks.  Each of them was stopped
by a cork, through which passed two glass tubes: one of these tubes
(a) ended immediately below the cork, while the other (b) descended to
the bottom of the flask, being drawn out at its lower end to an
orifice about 0.03 of an inch in diameter.  It was found necessary to
coat the cork carefully with cement.  In the later experiments corks
of vulcanised India-rubber were invariably employed.

The little flask, thus formed, being partially filled with the liquid
whose vapour was to be examined, was introduced into the path of the
purified current Of air.  The experimental tube being exhausted, and
the cock hick cut off the supply of purified air being cautiously
turned on, the air entered the flask through the tube b, and escaped
by the small orifice at the lower end of into the liquid.  Through
this it bubbled, loading itself with vapour, after which the mixed air
and vapour, passing from the flask by the tube a, entered the
experimental tube, where they were subjected to the action of light.

The whole arrangement is shown in fig. 3, where L represents the
electric lamp, ss' the experimental tube, pp' the pipe leading to the
air-pump, and F the test-tube containing the volatile liquid.  The
tube tt' is plugged with cotton-wool intended to intercept the
floating matter of the air; the bent tube T' contains caustic potash,
the tube T sulphuric acid, the one intended to remove the carbonic
acid and the other the aqueous vapour of the air.

The power of the electric beam to reveal the existence of anything
within the experimental tube, or the impurities of the tube itself, is
extraordinary.  When the experiments is made in a darkened room, a
tube which in ordinary daylight appears absolutely clean, is often
shown by the present mode of examination to be exceedingly filthy.

The following are some of the results obtained with this arrangement:

Nitrite of amyl.  The vapour of this liquid was in the first instance
permitted to enter the experimental tube, while the beam from the
electric lamp was passing through it.  Curious clouds, the cause of
which was then unknown, were observed to form near the place of entry,
being afterwards whirled through the tube.

The tube being again exhausted, the mixed air and vapour were allowed
to enter it in the dark.  The slightly convergent beam of the electric
light was then sent through the mixture.  For a moment the tube was
_optically empty_, nothing whatever being seen within it; but before a
second had elapsed a shower of particles was precipitated on the beam.
The cloud thus generated became denser as the light continued to act,
slowing at some places vivid iridescence.

The lens of the electric lamp was now placed so as to form within the
tube a strongly convergent cone of rays.  The tube was cleansed and
again filled in darkness.  When the light was sent through it, the
precipitation upon the beam was so rapid and intense that the cone,
which a moment before was invisible, flashed suddenly forth like a
solid luminous spear.  The effect was the same when the air and vapour
were allowed to enter the tube in diffuse daylight.  The cloud,
however, which shone with such extraordinary radiance under the
electric beam, was invisible in the ordinary light of the laboratory.

The quantity of mixed air and vapour within the experimental tube
could of course be regulated at pleasure.  The rapidity of the action
diminished with the attenuation of the vapour.  When, for example, the
mercurial column associated with the experimental tube was depressed
only five inches, the action was not nearly so rapid as when the tube
was full.  In such cases, however, it was exceedingly interesting to
observe, after some seconds of waiting, a thin streamer of delicate
bluish-white cloud slowly forming along the axis of the tube, and
finally swelling so as to fill it.

Fig.  2.

Fig.  3.

When dry oxygen was employed to carry in the vapour the effect was the
same as that obtained with air.

When dry hydrogen was used as a vehicle, the effect was also the same.

The effect, therefore, is not due to any interaction between the
vapour of the nitrite and its vehicle.

This was further demonstrated by the deportment of the vapour itself.
When it was permitted to enter the experimental tube unmixed with air
or any other gas, the effect was substantially the same.  Hence the
seat of the observed action is the vapour.

This action is not to be ascribed to heat.  As regards the glass of
the experimental tube, and the air within the tube, the beam employed
in these experiments was perfectly cold.  It had been sifted by
passing it through a solution of alum, and through the thick
double-convex lens of the lamp.  When the unsifted beam of the lamp
was employed, the effect was still the same; the obscure calorific
rays did not appear to interfere with the result.

My object here being simply to point out to chemists a method of
experiments which reveals a new and beautiful series of reactions, I
left to them the examination of the products of decomposition.  The
group of atoms forming the molecule of nitrite of amyl is obviously
shaken asunder by certain specific waves of the electric beam, nitric
oxide and other products, of which the _nitrate_ of amyl is probably
one, being the result of the decomposition.  The brown fumes of
nitrous acid were seen mingling with the cloud within the experimental
tube.  The nitrate of amyl, being less volatile than the nitrite, and
not being able to maintain itself in the condition of vapour, would be
precipitated as a visible cloud along the track of the beam.

In the anterior portions of the tube a powerful sifting of the beam by
the vapour occurs, which diminishes the chemical action in the
posterior portions.  In some experiments the precipitated cloud only
extended halfway down the tube.  When, under these circumstances, the
lamp was shifted so as to send the beam through the other end of the
tube, copious precipitation occurred there also.

Solar light also effects the decomposition of the nitrite-of-amyl
vapour.  On October 10, 1868, I partially darkened a small room in the
Royal Institution, into which the sun shone, permitting the light to
enter through an open portion of the window-shutter.  In the track of
the beam was placed a large plano-convex lens, which formed a fine
convergent cone in the dust of the room behind it.  The experimental
tube was filled in the laboratory, covered with a black cloth, and
carried into the partially darkened room.  On thrusting one end of the
tube into the cone of rays behind the lens, precipitation within the
cone was copious and immediate.  The vapour at the distant end of the
tube was in part shielded by that in front, and was also more feebly
acted on through the divergence of the rays.  On reversing the tube, a
second and similar cone was precipitated.

Physical Considerations.

I sought to determine the particular portion of the light which
produced the foregoing effects.  When, previous to entering the
experimental tube, the beam was caused to pass through a red glass,
the effect was greatly weakened, but not extinguished.  This was also
the case with various samples of yellow glass.  A blue glass being
introduced before the removal of the yellow or the red, on taking the
latter away prompt precipitation occurred along the track of the blue
beam.  Hence, in this case, the more refrangible rays are the most
chemically active.  The colour of the liquid nitrite of amyl indicates
that this must be the case; it is a feeble but distinct yellow: in
other words, the yellow portion of the beam is most freely
transmitted.  It is not, however, the transmitted portion of any beam
which produces chemical action, but the absorbed portion.  Blue, as
the complementary colour to yellow, is here absorbed, and hence the
more energetic action of the blue rays.

This reasoning, however, assumes that the same rays are absorbed by
the liquid and its vapour.  The assumption is worth testing.  A
solution of the yellow chromate of potash, the colour of which may be
made almost, if not altogether, identical with that of the liquid
nitrite of amyl, was found far more effective in stopping the chemical
rays than either the red or the yellow glass.  But of all substances
the liquid nitrite itself is most potent in arresting the rays which
act upon its vapour.  A layer one-eighth of an inch in thickness,
which scarcely perceptibly affected the luminous intensity, absorbed
the entire chemical energy of the concentrated beam of the electric
light.

The close relation subsisting between a liquid and its vapour, as
regards their action upon radiant heat, has been already amply
demonstrated. [Footnote: 'Phil. Trans.' 1864; 'Heat, a Mode of
Motion,' chap, xii; and P. 61 of this volume.]  As regards the nitrite
of amyl, this relation is more specific than in the cases hitherto
adduced; for here the special constituent of the beam, which provokes
the decomposition of the vapour, is shown to be arrested by the
liquid.

A question of extreme importance in molecular physics here arises:
What is the real mechanism of this absorption, and where is its seat?
[Footnote: My attention was very forcibly directed to this subject
some years ago by a conversation with my excellent friend Professor
Clausius.]

I figure, as others do, a molecule as a group of atoms, held together
by their mutual forces, but still capable of motion among themselves.
The vapour of the nitrite of amyl is to be regarded as an assemblage
of such molecules.  The question now before us is this: In the act of
absorption, is it the molecules that are effective, or is it their
constituent atoms?  Is the _vis viva_ of the intercepted light-waves
transferred to the molecule as a whole, or to its constituent parts?

The molecule, as a whole, can only vibrate in virtue of the forces
exerted between it and its neighbour molecules.  The intensity of
these forces, and consequently the rate of vibration, would, in this
case, be a Junction of the distance between the molecules.  Now the
identical absorption of the liquid and of the vaporous nitrite of amyl
indicates an identical vibrating period on the part of liquid and
vapour, and this, to my mind, amounts to an experimental proof that
the absorption occurs in the main _within_ the molecule.  For it can
hardly be supposed, if the absorption were the act of the molecule as
a whole, that it could continue to affect waves of the same period
after the substance had passed from the vaporous to the liquid state.

In point of fact, the decomposition of the nitrite of amyl is itself
to some extent an illustration of this internal molecular absorption;
for were the absorption the act of the molecule as a whole, the
relative motions of its constituent atoms would remain unchanged, and
there would be no mechanical cause for their separation.  It is
probably the synchronism of the vibrations of one portion of the
molecule with the incident waves, that enables the amplitude of those
vibrations to augment, until the chain which binds the parts of the
molecule together is snapped asunder.

I anticipate wide, if not entire, generality for the fact that a
liquid and its vapour absorb the same rays.  A cell of liquid chlorine
would, I imagine, deprive light more effectually of its power of
causing chlorine and hydrogen to combine than any other filter of the
luminous rays.  The rays which give chlorine its colour have nothing
to do with this combination, those that are absorbed by the chlorine
being really effective rays.  A highly sensitive bulb, containing
chlorine and hydrogen, in the exact proportions necessary for the
formation of hydrochloric acid, was placed at one end of an
experimental tube, the beam of the electric lamp being sent through it
from the other.  The bulb did not explode when the tube was filled
with chlorine, while the explosion was violent and immediate when the
tube was filled with air.  I anticipate for the liquid chlorine an
action similar to, but still more energetic than, that exhibited by
the gas.  If this should prove to be the case, it will favour the view
that chlorine itself is _molecular_ and not _monatomic_.

Production of Sky-blue by the Decomposition of Nitrite of Amyl.

When the quantity of nitrite vapour is considerable, and the light
intense, the chemical action is exceedingly rapid, the particles
precipitated being so large as to whiten the luminous beam.  Not so,
however, when a well-mixed and highly attenuated vapour fills the
experimental tube.  The effect now to be described was first obtained
when the vapour of the nitrite was derived from a portion of its
liquid which had been accidentally introduced into the passage through
which the dry air flowed into the experimental tube.

In this case, the electric beam traversed the tube for several seconds
before any action was visible.  Decomposition then visibly commenced,
and advanced slowly.  When the light was very strong, the cloud
appeared of a milky blue.  When, on the contrary, the intensity was
moderate, the blue was pure and deep.  In Brücke's important
experiments on the blue of the sky and the morning and evening red,
pure mastic is dissolved in alcohol, and then dropped into water well
stirred.  When the proportion of mastic to alcohol is correct, the
resin is precipitated so finely as to elude the highest microscopic
power.  By reflected light, such a medium appears bluish, by
transmitted light yellowish, which latter colour, by augmenting the
quantity of the precipitate, can be caused to pass into orange or red.

But the development of colour in the attenuated nitrite-of-amyl vapour
is doubtless more similar to what takes place in our atmosphere.  The
blue, moreover, is far purer and more sky-like than that obtained from
Bruecke's turbid medium.  Never, even in the skies of the Alps, have I
seen a richer or a purer blue than that attainable by a suitable
disposition of the light falling upon the precipitated vapour.

Iodide of Allyl.--Among the liquids hitherto subjected to the
concentrated electric light, iodide of allyl, in point of rapidity and
intensity of action, comes next to the nitrite of amyl.  With the
iodide I have employed both oxygen and hydrogen, as well as air, as a
vehicle, and found the effect in all cases substantially the same. The
cloud-column here was exquisitely beautiful.  It revolved round the
axis of the decomposing beam; it was nipped at certain places like an
hour-glass, and round the two bells of the glass delicate
cloud-filaments twisted themselves in spirals.  It also folded itself
into convolutions resembling those of shells.  In certain conditions
of the atmosphere in the Alps I have often observed clouds of a
special pearly lustre; when hydrogen was made the vehicle of the
iodide-of allyl vapour a similar lustre was most exquisitely shown.
With a suitable disposition of the light, the purple hue of
iodine-vapour came out very strongly in the tube.

The remark already made, as to the bearing of the decomposition of
nitrite of amyl by light on the question of molecular absorption,
applies here also; for were the absorption the work of the molecule as
a whole, the iodine would not be dislodged from the allyl with which
it is combined.  The non-synchronism of iodine with the waves of
obscure heat is illustrated by its marvellous transparency to such
heat.  May not its synchronism with the waves of light in the present
instance be the cause of its divorce from the allyl?

Iodide of Isopropyl.--The action of light upon the vapour of this
liquid is, at first, more languid than upon iodide of allyl; indeed
many beautiful reactions may be overlooked, in consequence of this
languor at the commencement.  After some minutes' exposure, however,
clouds begin to form, which grow in density and in beauty as the light
continues to act.  In every experiments hitherto made with this
substance the column of cloud filling the experimental tube, was
divided into two distinct parts near the middle of the tube.  In one
experiments a globe of cloud formed at the centre, from which, right
and left, issued an axis uniting the globe with two adjacent
cylinders.  Both globe and cylinders were animated by a common motion
of rotation.  As the action continued, paroxysms of motion were
manifested; the various parts of the cloud would rush through each
other with sudden violence.  During these motions beautiful and
grotesque cloud-forms were developed.  At some places the nebulous
mass would become ribbed so as to resemble the graining of wood; a
longitudinal motion would at times generate in it a series of curved,
transverse bands, the retarding influence of the sides the tube
causing an appearance resembling, on a small scale, the dirt-bands of
the Mer de Glace.  In the anterior portion of the tube those sudden
commotion were most intense; here buds of cloud would sprout forth,
and grow in a few seconds into perfect flower-like forms.  The cloud
of iodide of isopropyl had a character Of its own, and differed
materially from all others that I had seen.  A gorgeous mauve colour
was observed in the last twelve inches of the tube; the vapour of
iodine was present, and it may have been the sky-blue scattered by the
precipitated particles which, mingling with the purple of the iodine,
produced the mauve.  As in all other cases here adduced, the effects
were proved to be due to the light; they never occurred in darkness.

The forms assumed by some of those actinic clouds, as I propose to
call them, in consequence of rotations and other motions, due to
differences of temperature, are perfectly astounding.  I content
myself here with a meagre description of one more of them.

The tube being filled with the sensitive mixture, the beam was sent
through it, the lens at the same time being so placed as to produce a
cone of very intense light.  Two minutes elapsed before anything was
visible; but at the end of this time a faint bluish cloud appeared to
hang itself on the most concentrated portion of the beam.

Soon afterwards a second cloud was formed five inches farther down the
experimental tube.  Both clouds were united by a slender cord of the
same bluish tint as themselves.

As the action of the light continued, the first cloud gradually
resolved itself into a series of parallel disks of exquisite delicacy,
which rotated round an axis perpendicular to their surfaces, and
finally blended to a screw surface with an inclined generatrix.  This
gradually changed into a filmy funnel, from the narrow end of which
the 'cord' extended to the cloud in advance.

The latter also underwent slow but incessant modification.  It first
resolved itself into a series of strata resembling those of the
electric discharge.  After a little time, and through changes which it
was difficult to follow, both clouds presented the appearance of a
series of concentric funnels set one within the other, the interior
ones being seen through the outer ones.  Those of the distant cloud
resembled claret-glasses in shape.  As many as six funnels were thus
concentrically set together, the two series being united by the
delicate cord of cloud already referred to.  Other cords and Blender
tubes were afterwards formed, which coiled themselves in delicate
spirals around the funnels.

Rendering the light along the connecting-cord more intense, it
diminished in thickness and became whiter; this was a consequence of
the enlargement of its particles.  The cord finally disappeared, while
the funnels melted into two ghost-like films, shaped like parasols.
They were barely visible, being of an exceedingly delicate blue tint.
They seemed woven of blue air.  To compare them with cobweb or with
gauze would be to liken them to something infinitely grosser than
themselves.

In all cases a distant candle-flame, when looked at through the cloud,
was sensibly undimmed.

2.  ON THE BLUE COLOUR OF THE SKY, AND THE POLARISATION OF SKYLIGHT.

[Footnote: In my 'Lectures on Light' (Longman), the polarisation of
light will be found briefly, but, I trust, clearly explained.]

1869.

After the communication to the Royal Society of the foregoing brief
account of a new Series of Chemical Reactions produced by Light, the
experiments upon this subject were continued, the number of substances
thus acted on being considerably increased.

I now, however, beg to direct attention to two questions glanced at
incidentally in the preceding pages--the blue colour of the sky, and
the polarisation of skylight.  Reserving the historic treatment of the
subject for a more fitting occasion, I would merely mention now that
these questions constitute, in the opinion of our most eminent
authorities, the two great standing enigmas of meteorology.  Indeed it
was the interest manifested in them by Sir John Herschel, in a letter
of singular speculative power, addressed to myself, that caused me to
enter upon the consideration of these questions so soon.

The apparatus with which I work consists, as already stated, of a
glass tube about a yard in length, and from 2.5 to 3 inches internal
diameter.  The vapour to be examined is introduced into this tube in
the manner already described, and upon it the condensed beam of the
electric lamp is permitted to act, until the neutrality or the
activity of the substance has been declared.

It has hitherto been my aim to render the chemical action of light
upon vapours visible.  For this purpose substances have been chosen,
one at least of whose products of decomposition under light shall have
a boiling-point so high, that as soon as the substance is formed it
shall be precipitated.  By graduating the quantity of the vapour, this
precipitation may be rendered of any degree of fineness, forming
particles distinguishable by the naked eye, or far beyond the reach of
our highest microscopic powers.  I have no reason to doubt that
particles may be thus obtained, whose diameters constitute but a small
fraction of the length of a wave of violet light.

In all cases when the vapours of the liquids employed are sufficiently
attenuated, no matter what the liquid may be, the visible action
commences with the formation of a _blue cloud_.  But here I must guard
myself against all misconception as to the use of this term.  The
'cloud' here referred to is totally invisible in ordinary daylight. To
be seen, it requires to be surrounded by darkness, _it only_ being
illuminated by a powerful beam of light.  This blue cloud differs in
many important particulars from the finest ordinary clouds, and might
justly have assigned to it an intermediate position between such
clouds and true vapour.  With this explanation, the term 'cloud,' or
'incipient cloud,' or 'actinic cloud,' as I propose to employ it,
cannot, I think, be misunderstood.

I had been endeavouring to decompose carbonic acid gas by light.  A
faint bluish cloud, due it may be, or it may not be, to the residue of
some vapour previously employed, was formed in the experimental tube.
On looking across this cloud through a Nicol's prism, the line of
vision being horizontal, it was found that when the short diagonal of
the prism was vertical, the quantity of light reaching the eye was
greater than when the long diagonal was vertical.  When a plate of
tourmaline was held between the eye and the bluish cloud, the quantity
of light reaching the eye when the axis of the prism was perpendicular
to the axis of the illuminating beam, was greater than when the axes
of the crystal and of the beam were parallel to each other.

This was the result all round the experimental tube.  Causing the
crystal of tourmaline to revolve round the tube, with its axis
perpendicular to the illuminating beam, the quantity of light that
reached the eye was in all its positions a maximum.  When the
crystallographic axis was parallel to the axis of the beam, the
quantity of light transmitted by the crystal was a minimum.

From the illuminated bluish cloud, therefore, polarised light was
discharged, the direction of maximum polarisation being at right
angles to the illuminating beam; the plane of vibration of the
polarised light was perpendicular to the beam. [Footnote: This is
still an undecided point; but the probabilities are so much in its
favour, and it is in my opinion so much preferable to have a physical
image on which the mind can rest, that I do not hesitate to employ the
phraseology in the text.]

Thin plates of selenite or of quartz, placed between the Nicol and the
actinic cloud, displayed the colours of polarised light, these colours
being most vivid when the line of vision was at right angles to the
experimental tube.  The plate of selenite usually employed was a
circle, thinnest at the centre, and augmenting uniformly in thickness
from the centre outwards.  When placed in its proper position between
the Nicol and the cloud, it exhibited a system of splendidly-coloured
rings.

The cloud here referred to was the first operated upon in the manner
described.  It may, however, be greatly improved upon by the choice of
proper substances, and by the application, in proper quantities, of
the substances chosen.  Benzol, bisulphide of carbon, nitrite of amyl,
nitrite of butyl, iodide of allyl, iodide of isopropyl, and many other
substances may be employed.  I will take the nitrite of butyl as
illustrative of the means adopted to secure the best result, with
reference to the present question.

And here it may be mentioned that a vapour, which when alone, or mixed
with air in the experimental tube, resists the action of light, or
shows but a feeble result of this action, may, when placed in
proximity with another gas or vapour, exhibit vigorous, if not violent
action.  The case is similar to that of carbonic acid gas, which,
diffused in the atmosphere, resists the decomposing action of solar
light, but when placed in contiguity with chlorophyl in the leaves of
plants, has its molecules shaken asunder.

Dry air was permitted to bubble through the liquid nitrite of butyl,
until the experimental tube, which had been previously exhausted, was
filled with the mixed air and vapour.  The visible action of light
upon the mixture after fifteen minutes' exposure was slight.  The tube
was afterwards filled with half an atmosphere of the mixed air and
vapour, and a second half-atmosphere of air which had been permitted
to bubble through fresh commercial hydrochloric acid.  On sending the
beam through this mixture, the tube, for a moment, was optically
empty.  But the pause amounted only to a small fraction of a second, a
dense cloud being immediately precipitated upon the beam.

This cloud began blue, but the advance to whiteness was so rapid as
almost to justify the application of the term instantaneous.  The
dense cloud, looked at perpendicularly to its axis, showed scarcely
any signs of polarisation.  Looked at obliquely the polarisation was
strong.

The experimental tube being again cleansed and exhausted, the mixed
air and nitrite-of-butyl vapour was permitted to enter it until the
associated mercury column was depressed 1/10 of an inch.  In other
words, the air and vapour, united, exercised a pressure not exceeding
1/300th of an atmosphere.  Air, passed through a solution of
hydrochloric acid, was then added, till the mercury column was
depressed three inches.  The condensed beam of the electric light was
passed for some time through this mixture without revealing anything
within the tube competent to scatter the light.  Soon, however, a
superbly blue cloud was formed along, the track of the beam, and it
continued blue sufficiently long to permit of its thorough
examination.  The light discharged from the cloud, at right angles to
its own length, was at first perfectly polarised.  It could be totally
quenched by the Nicol.  By degrees the cloud became of whitish blue,
and for a time the selenite colours, obtained by looking at it
normally, were exceedingly brilliant.  The direction of maximum
polarisation was distinctly at right angles to the illuminating beam.
This continued to be the case as long as the cloud maintained a
decided blue colour, and even for some time after the blue had changed
to whitish blue.  But, as the light continued to act, the cloud became
coarser and whiter, particularly at its centre, where it at length
ceased to discharge polarised light in the direction of the
perpendicular, while it continued to do so at both ends.

But the cloud which had thus ceased to polarise the light emitted
normally, showed vivid selenite colours when looked at obliquely,
proving that the direction of maximum polarisation changed with the
texture of the cloud.  This point shall receive further illustration
subsequently.

A blue, equally rich and more durable, was obtained by employing the
nitrite-of-butyl vapour in a still more attenuated condition.  The
instance here cited is representative.  In all cases, and with all
substances, the cloud formed at the commencement, when the
precipitated particles are sufficiently fine, is _blue_, and it can be
made to display a colour rivalling that of the purest Italian sky.  In
all cases, moreover, this fine blue cloud polarises _perfectly_ the beam
which illuminates it, the direction of polarisation enclosing an angle
of 90° with the axis of the illuminating beam.

It is exceedingly interesting to observe both the perfection and the
decay of this polarisation.  For ten or fifteen minutes after its
first appearance the light from a vividly illuminated actinic cloud,
looked at perpendicularly, is absolutely quenched by a Nicol's prism
with its longer diagonal vertical.  But as the sky-blue is gradually
rendered impure by the growth of the particles--in other words, as
real clouds begin to be formed--the polarisation begins to decay, a
portion of the light passing through the prism in all its positions.
It is worthy of note, that for some time after the cessation of
perfect polarisation, the residual light which passes, when the Nicol
is in its position of minimum transmission, is of a gorgeous blue, the
whiter light of the cloud being extinguished. [Footnote: This shows
that particles too large to polarise the blue, polarise perfectly
light of lower refrangibility.]  When the cloud texture has become
sufficiently coarse to approximate to that of ordinary clouds, the
rotation of the Nicol ceases to have any sensible effect on the
quantity of light discharged normally.

The perfection of the polarisation, in a direction perpendicular to
the illuminating beam, is also illustrated by the following
experiments: A Nicol's prism, large enough to embrace the entire beam
of the electric lamp, was placed between the lamp and the experimental
tube.  A few bubbles of air, carried through the liquid nitrite of
butyl, were introduced into the tube, and they were followed by about
three inches (measured by the mercurial gauge) of air which had passed
through aqueous hydrochloric acid.  Sending the polarised beam through
the tube, I placed myself in front of it, my eye being on a level with
its axis, my assistant occupying a similar position behind the tube.
The short diagonal of the large Nicol was in the first instance
vertical, the plane of vibration of the emergent beam being therefore
also vertical.  As the light continued to act, a superb blue cloud,
visible to both my assistant and myself, was slowly formed.  But this
cloud, so deep and rich when looked at from the positions mentioned,
_utterly disappeared when looked at vertically downwards, or vertically
upwards_.  Reflection from the cloud was not possible in these
directions.  When the large Nicol was slowly turned round its axis,
the eye of the observer being on the level of the beam, and the line
of vision perpendicular to it, entire extinction of the light emitted
horizontally occurred when the longer diagonal of the large Nicol was
vertical.  But now a vivid blue cloud was seen when looked at
downwards or upwards.  This truly fine experiments, which I
contemplated making on my own account, was first definitely suggested
by a remark in a letter addressed to me by Professor Stokes.

As regards the polarisation of skylight, the greatest stumbling-block
has hitherto been, that, in accordance with the law of Brewster, which
makes the index of refraction the tangent of the polarising angle, the
reflection which produces perfect polarisation would require to be
made in air upon air; and indeed this led many of our most eminent
men, Brewster himself among the number, to entertain the idea of
aerial molecular reflection. [Footnote: 'The cause of the
polarisation is evidently a reflection of the sun's light upon
something.  The question is on what?  Were the angle of maximum
polarisation 76°, we should look to water or ice as the reflecting
body, however inconceivable the existence in a cloudless atmosphere
and a hot summer's day of unevaporated molecules (particles?) of
water.  But though we were once of this opinion, careful observation
has satisfied us that 90°, or thereabouts, is the correct angle, and
that therefore whatever be the body on which the light has been
reflected, if polarised by a single reflection, the polarising angle
must be 45°, and the index of refraction, which is the tangent of that
angle, unity; in other words, the reflection would require to be made
in air upon air!' (Sir John Herschel, 'Meteorology,' par.  233.)

Any particles, if small enough, will produce both the colour and the
polarisation of the sky.  But is the existence of small
water-particles on a hot summer's day in the higher regions of our
atmosphere inconceivable?  It is to be remembered that the oxygen and
nitrogen of the air behave as a vacuum to radiant heat, the
exceedingly attenuated vapour of the higher atmosphere being therefore
in practical contact with the cold of space.]

I have, however, operated upon substances of widely different
refractive indices, and therefore of very different polarising angles
as ordinarily defined, but the polarisation of the beam, by the
incipient cloud, has thus far proved itself to be absolutely
independent of the polarising angle.  The law of Brewster does not
apply to matter in this condition, and it rests with the undulatory
theory to explain why.  Whenever the precipitated particles are
sufficiently fine, no matter what the substance forming the particles
may be, the direction of maximum polarisation is at right angles to
the illuminating beam, the polarising angle for matter in this
condition being invariably 45°.

Suppose our atmosphere surrounded by an envelope impervious to light,
but with an aperture on the sunward side through which a parallel beam
of solar light could enter and traverse the atmosphere.  Surrounded by
air not directly illuminated, the track of such a beam would resemble
that of the parallel beam of the electric lamp through an incipient
cloud.  The sunbeam would be blue, and it would discharge laterally
light in precisely the same condition as that discharged by the
incipient cloud.  In fact, the azure revealed by such a beam would be
to all intents and purposes that which I have called a 'blue cloud.'
Conversely our 'blue cloud' is, to all intents and purposes, an
_artificial sky_.' [Footnote: The opinion of Sir John Herschel,
connecting the polarisation and the blue colour of the sky, is
verified by the foregoing results.  'The more the subject [the
polarisation of skylight] is considered,' writes this eminent
philosopher, 'the more it will be found beset with difficulties, and
its explanation when arrived at will probably be found to carry with
it that of the blue colour of the sky itself, and of the great
quantity of light it actually does send down to us.'  'We may observe,
too,' he adds, 'that it is only where the purity of the sky is most
absolute that the polarisation is developed in its highest degree, and
that where there is the slightest perceptible tendency to cirrus it is
materially impaired.' This applies word for word to our 'incipient
clouds.']

But, as regards the polarisation of the sky, we know that not only is
the direction of maximum polarisation at right angles to the track of
the solar beams, but that at certain angular distances, probably
variable ones, from the sun, 'neutral points,' or points of no
polarisation, exist, on both sides of which the planes of atmospheric
polarisation are at right angles to each other.  I have made various
observations upon this subject which are reserved for the present;
but, pending the more complete examination of the question, the
following facts bearing upon it may be submitted.

The parallel beam employed in these experiments tracked its way
through the laboratory air, exactly as sunbeams are seen to do in the
dusty air of London.  I have reason to believe that a great portion of
the matter thus floating in the laboratory air consists of organic
particles, which are capable of imparting a perceptibly bluish tint to
the air.  These also showed, though far less vividly, all the effects
of polarisation obtained with the incipient clouds.  The light
discharged laterally from the track of the illuminating beam was
polarised, though not perfectly, the direction of maximum polarisation
being at right angles to the beam.  At all points of the beam,
moreover, throughout its entire length, the light emitted normally was
in the same state of polarisation.  Keeping the positions of the Nicol
and the selenite constant, the same colours were observed throughout
the entire beam, when the line of vision was perpendicular to its
length.

The horizontal column of air, thus illuminated, was 18 feet long, and
could therefore be looked at very obliquely.  I placed myself near the
end of the beam, as it issued from the electric lamp, and, looking
through the Nicol and selenite more and more obliquely at the beam,
observed the colours fading until they disappeared.  Augmenting the
obliquity the colours appeared once more, but they were now
complementary to the former ones.

Hence this beam, like the sky, exhibited a neutral point, on opposite
sides of which the light was polarised in planes at right angles to
each other.

Thinking that the action observed in the laboratory might be caused,
in some way, by the vaporous fumes diffused in its air, I had the
light removed to a room at the top of the Royal Institution.  The
track of the beam was seen very finely in the air of this room, a
length of 14 or 15 feet being attainable.  This beam exhibited all the
effects observed with the beam in the laboratory.  Even the
uncondensed electric light falling on the floating matter showed,
though faintly, the effects of polarisation.

When the air was so sifted as to entirely remove the visible floating
matter, it no longer exerted any sensible action upon the light, but
behaved like a vacuum.  The light is scattered and polarised by
_particles_, not by molecules or atoms.

By operating upon the fumes of chloride of ammonium, the smoke of
brown paper, and tobacco-smoke, I had varied and confirmed in many
ways those experiments on neutral points, when my attention was drawn
by Sir Charles Wheatstone to an important observation communicated to
the Paris Academy in 1860 by Professor Govi, of Turin. [Footnote:
Comptes Rendus,' tome li, pp. 360 and 669.] M. Govi had been led to
examine a beam of light sent through a room in which were successively
diffused the smoke of incense, and tobacco-smoke.  His first brief
communication stated the fact of polarisation by such smoke; but in
his second communication he announced the discovery of a neutral point
in the beam, at the opposite sides of which the light was polarised in
planes at right angles to each other.

But unlike my observations on the laboratory air, and unlike the
action of the sky, the direction of maximum polarisation in M. Govi's
experiments enclosed a very small angle with the axis of the
illuminating beam.  The question was left in this condition, and I am
not aware that M. Govi or any other investigator has pursued it
further.

I had noticed, as before stated, that as the clouds formed in the
experimental tube became denser, the polarisation of the light
discharged at right angles to the beam became weaker, the direction of
maximum polarisation becoming oblique to the beam.  Experiments on the
fumes of chloride of ammonium gave me also reason to suspect that the
position of the neutral point was not constant, but that it varied
with the density of the illuminated fumes.

The examination of these questions led to the following new and
remarkable results: The laboratory being well filled with the fumes of
incense, and sufficient time being allowed for their uniform
diffusion, the electric beam was sent through the smoke.  From the
track of the beam polarised light was discharged; but the direction of
maximum polarisation, instead of being perpendicular, now enclosed an
angle of only 12° or 13° with the axis of the beam.

A neutral point, with complementary effects at opposite sides of it,
was also exhibited by the beam.  The angle enclosed by the axis of the
beam, and a line drawn from the neutral point to the observer's eye,
measured in the first instance 66°.

The windows of the laboratory were now opened for some minutes, a
portion of the incense-smoke being permitted to escape.  On again
darkening the room and turning on the light, the line of vision to the
neutral point was found to enclose, with the axis of the beam, an
angle of 63°.

The windows were again opened for a few minutes, more of the smoke
being permitted to escape.  Measured as before, the angle referred to
was found to be 54°.

This process was repeated three additional times the neutral point was
found to recede lower and lower down the beam, the angle between a
line drawn from the eye to the neutral point and the axis of the beam
falling successively from 54° to 49°, 43° and 33°.

The distances, roughly measured, of the neutral point from the lamp,
corresponding to the foregoing series of observations, were these:

1st observation   2 feet 2 inches.

2nd observation   2 feet 6 inches.

3rd observation   2 feet 10 inches.

4th observation   3 feet 2 inches.

5th observation   3 feet 7 inches.

6th observation   4 feet 6 inches.

At the end of this series of experiments the direction of maximum
polarisation had again become normal to the beam.

The laboratory was next filled with the fumes of gunpowder.  In five
successive experiments, corresponding to five different densities of
the gunpowder-smoke, the angles enclosed between the line of vision to
the neutral point and the axis of the beam, were 63 degrees, 50°, 47°,
42°, and 38° respectively.

After the clouds of gunpowder had cleared away, the laboratory was
filled with the fumes of common resin, rendered so dense as to be very
irritating to the lungs.  The direction of maximum polarisation
enclosed, in this case, an angle of 12°, or thereabouts, with the axis
of the beam.  Looked at, as in the former instances, from a position
near the electric lamp, no neutral point was observed throughout the
entire extent of the beam.

When this beam was looked at normally through the selenite and Nicol,
the ring-system, though not brilliant, was distinct.  Keeping the eye
upon the plate of selenite, and the line of vision perpendicular, the
windows were opened, the blinds remaining undrawn.  The resinous fumes
slowly diminished, and as they did so the ring-system became paler. It
finally disappeared.  Continuing to look in the same direction, the
rings revived, but now the colours were complementary to the former
ones.  _The neutral point had passed me in its motion down the beam,
consequent upon the attenuation of the fumes of resin_.

With the fumes of chloride of ammonium substantially the same results
were obtained.  Sufficient, however, has been here stated to
illustrate the variability of the position of the neutral
point. [Footnote: Brewster has proved the variability of the position
of the neutral point for skylight with the sun's altitude, a result
obviously connected with the foregoing experiments.]

By a puff of tobacco-smoke, or of condensed steam, blown into the
illuminated beam, the brilliancy of the selenite colours may be
greatly enhanced.  But with different clouds two different effects are
produced.  Let the ring-system observed in the common air be brought
to its maximum strength, and then let an attenuated cloud of chloride
of ammonium be thrown into the beam at the point looked at; the ring
system flashes out with augmented brilliancy, but the character of the
polarisation remains unchanged.  This is also the case when
phosphorus, or sulphur, is burned underneath the beam, so as to cause
the fine particles of phosphorus or of sulphur to rise into the light.
With the sulphur-fumes the brilliancy of the colours is exceedingly
intensified; but in none of these cases is there any change in the
character of the polarisation.

But when a puff of the fumes of hydrochloric acid, hydriodic acid, or
nitric acid is thrown into the beam, there is a complete reversal of
the selenite tints.  Each of these clouds twists the plane of
polarisation 90°, causing the centre of the ring-system to change from
black to white, and the rings themselves to emit their complementary
colours. [Footnote: Sir John Herschel suggested to me that this
change of the polarisation from positive to negative may indicate a
change from polarisation by reflection to polarisation by refraction.
This thought repeatedly occurred to me while looking at the effects;
but it will require much following up before it emerges into
clearness.]

Almost all liquids have motes in them sufficiently numerous to
polarise sensibly the light, and very beautiful effects may be
obtained by simple artificial devices.  When, for example, a cell of
distilled water is placed in front of the electric lamp, and a thin
slice of the beam is permitted to pass through it, scarcely any
polarised light is discharged, and scarcely any colour produced with a
plate of selenite. But if a bit of soap be agitated in the water above
the beam, the moment the infinitesimal particles reach the light the
liquid sends forth laterally almost perfectly polarised light; and if
the selenite be employed, vivid colours flash into existence.  A still
more brilliant result is obtained with mastic dissolved in a great
excess of alcohol.

The selenite rings, in fact, constitute an extremely delicate test as
to the collective quantity of individually invisible particles in a
liquid.  Commencing with distilled water, for example, a thick slice
of light is necessary to make the polarisation of its suspended
particles sensible.  A much thinner slice suffices for common water;
while, with Bruecke's precipitated mastic, a slice too thin to produce
any sensible effect with most other liquids, suffices to bring out
vividly the selenite colours.

3.  THE SKY OF THE ALPS.

The vision of an object always implies a differential action on the
retina of the observer.  The object is distinguished from surrounding
space by its excess or defect of light in relation to that space.  By
altering the illumination, either of the object itself or of its
environment, we alter the appearance of the object.  Take the case of
clouds floating in the atmosphere with patches of blue between them.
Anything that changes the illumination of either alters the appearance
of both, that appearance depending, as stated, upon differential
action.

Now the light of the sky, being polarised, may, as the reader of the
foregoing pages knows, be in great part quenched by a Nicol's prism,
while the light of a common cloud, being unpolarised, cannot be thus
extinguished.  Hence the possibility of very remarkable variations,
not only in the aspect of the firmament, which is really changed, but
also in the aspect of the clouds, which have that firmament as a
background.  It is possible, for example, to choose clouds of such a
depth of shade that when the Nicol quenches the light behind them,
they shall vanish, being undistinguishable from the residual dull tint
which outlives the extinction of the brilliancy of the sky.  A cloud
less deeply shaded, but still deep enough, when viewed with the naked
eye, to appear dark on a bright ground, is suddenly changed to a white
cloud on a dark ground by the quenching of the light behind it.  When
a reddish cloud at sunset chances to float in the region of maximum
polarisation, the quenching of the surrounding light causes it to
flash with a brighter crimson. Last Easter eve the Dartmoor sky, which
had just been cleansed by a snow-storm, wore a very wild appearance.
Round the horizon it was of steely brilliancy, while reddish cumuli
and cirri floated southwards. When the sky was quenched behind them
these floating masses seemed like dull embers suddenly blown upon;
they brightened like a fire.

In the Alps we have the most magnificent examples of crimson clouds
and snows, so that the effects just referred to may be here studied
under the best possible conditions.  On August 23, 1869, the evening
Alpenglow was very fine, though it did not reach its maximum depth and
splendour.  The side of the Weisshorn seen from the Bel Alp, being
turned from the sun, was tinted mauve; but I wished to observe one of
the rose-coloured buttresses of the mountain.  Such a one was visible
from a point a few hundred feet above the hotel.  The Matterhorn also,
though for the most part in shade, had a crimson projection, while a
deep ruddy red lingered along its western shoulder.  Four distinct
peaks and buttresses of the Dom, in addition to its dominant head--all
covered with pure snow--were reddened by the light of sunset.  The
shoulder of the Alphubel was similarly coloured, while the great mass
of the Fletschorn was all a-glow, and so was the snowy spine of the
Monte Leone.

Looking at the Weisshorn through the Nicol, the glow of its
protuberance was strong or weak according to the position of the
prism.  The summit also underwent striking changes.  In one position
of the prism it exhibited a pale white against a dark background; in
the rectangular position it was a dark mauve against a light
background.  The red of the Matterhorn changed in a similar manner;
but the whole mountain also passed through wonderful changes of
definition.  The air at the time was filled with a silvery haze, in
which the Matterhorn almost disappeared.  This could be wholly
quenched by the Nicol, and then the mountain sprang forth with
astonishing solidity and detachment from the surrounding air.  The
changes of the Dom were still more wonderful.  A vast amounts of light
could be removed from the sky behind it, for it occupied the position
of maximum polarisation.  By a little practice with the Nicol it was
easy to render the extinction of the light, or its restoration, almost
instantaneous.  When the sky was quenched, the four minor peaks and
buttresses, and the summit of the Dom, together with the shoulder of
the Alphubel, glowed as if set suddenly on fire.  This was immediately
dimmed by turning the Nicol through an angle of 90°.  It was not the
stoppage of the light of the sky behind the mountains alone which
produced this startling effect; the air between them and me was highly
opalescent, and the quenching of this intermediate glare augmented
remarkably the distinctness of the mountains.

On the morning of August 24 similar effects were finely shown.  At 10
A.M. all three mountains, the Dom, the Matterhorn, and the Weisshorn,
were powerfully affected by the Nicol.  But in this instance also, the
line drawn to the Dom being very nearly perpendicular to the solar
beams, the effects on this mountain were most striking.  The grey
summit of the Matterhorn, at the same time, could scarcely be
distinguished from the opalescent haze around it; but when the Nicol
quenched the haze, the summit became instantly isolated, and stood out
in bold definition.  It is to be remembered that in the production of
these effects the only things changed are the sky behind, and the
luminous haze in front of the mountains; that these are changed
because the light emitted from the sky and from the haze is plane
polarised light, and that the light from the snows and from the
mountains, being sensibly unpolarised, is not directly affected by the
Nicol.  It will also be understood that it is not the interposition of
the haze _as an opaque body_ that renders the mountains indistinct, but
that it is the _light_ of the haze which dims and bewilders the eye, and
thus weakens the definition of objects seen through it.

These results have a direct bearing upon what artists call 'aerial
perspective.' As we look from the summit of Mont Blanc, or from a
lower elevation, at the serried crowd of peaks, especially if the
mountains be darkly coloured--covered with pines, for example--every
peak and ridge is separated from the mountains behind it by a thin
blue haze which renders the relations of the mountains as to distance
unmistakable.  When this haze is regarded through the Nicol
perpendicular to the sun's rays, it is in many cases wholly quenched,
because the light which it emits in this direction is wholly
polarised.  When this happens, aerial perspective is abolished, and
mountains very differently distant appear to rise in the same vertical
plane.  Close to the Bel Alp for instance, is the gorge of the Massa,
and beyond the gorge is a high ridge darkened by pines.  This ridge
may be projected upon the dark slopes at the opposite side of the
Rhone valley, and between both we have the blue haze referred to,
throwing the distant mountains far away.  But at certain hours of the
day the haze may be quenched, and then the Massa ridge and the
mountains beyond the Rhone seem almost equally distant from the eye.
The one appears, as it were, a vertical continuation of the other. The
haze varies with the temperature and humidity of the atmosphere. At
certain times and places it is almost as blue as the sky itself; but
to see its colour, the attention must be withdrawn from the mountains
and from the trees which cover them.  In point of fact, the haze is a
piece of more or less perfect sky; it is produced in the same manner,
and is subject to the same laws, as the firmament itself. We live _in_
the sky, not _under_ it.

These points were further elucidated by the deportment of the
selenite, plate, with which the readers of the foregoing pages
are so well acquainted.  On some of the sunny days of August the
haze in the valley of the Rhone, as looked at from the Bel Alp,
was very remarkable.  Towards evening the sky above the mountains
opposite to my place of observation yielded a series of the most
splendidly-coloured iris-rings; but on lowering the selenite until it
had the darkness of the pines at the opposite side of the Rhone
'valley, instead of the darkness of space, as a background, the
colours were not much diminished in brilliancy.  I should estimate the
distance across the valley, as the crow flies, to the opposite
mountain, at nine miles; so that a body of air of this thickness can,
under favourable circumstances, produce chromatic effects of
polarisation almost as vivid as those produced by the sky itself.

Again: the light of a landscape, as of most other things, consists of
two parts; the one, coming purely from superficial reflection, is
always of the same colour as the light which falls upon the landscape;
the other part reaches us from a certain depth within the objects
which compose the landscape, and it is this portion of the total light
which gives these objects their distinctive colours.  The white light
of the sun enters all substances to a certain depth, and is partly
ejected by internal reflection; each distinct substance absorbing and
reflecting the light, in accordance with the laws of its own molecular
constitution. Thus the solar light is _sifted_ by the landscape, which
appears in such colours and variations of colour as, after the sifting
process, reach the observer's eye.  Thus the bright green of grass, or
the darker colour of the pine, never comes to us alone, but is always
mingled with an amounts of light derived from superficial reflection.
A certain hard brilliancy is conferred upon the woods and meadows by
this superficially-reflected light.  Under certain circumstances, it
may be quenched by a Nicol's prism, and we then obtain the true colour
of the grass and foliage.  Trees and meadows, thus regarded, exhibit a
richness and softness of tint which they never show as long as the
superficial light is permitted to mingle with the true interior
emission.  The needles of the pines show this effect very well,
large-leaved trees still better; while a glimmering field of maize
exhibits the most extraordinary variations when looked at through the
rotating Nicol.

Thoughts and questions like those here referred to took me, in August
1869, to the top of the Aletschhorn.  The effects described in the
foregoing paragraphs were for the most part reproduced on the summit
of the mountain.  I scanned the whole of the sky with my Nicol.  Both
alone, and in conjunction with the selenite, it pronounced the
perpendicular to the solar beams to be the direction of maximum
polarisation.

But at no portion of the firmament was the polarisation complete.  The
artificial sky produced in the experiments recorded in the preceding
pages could, in this respect, be rendered far more perfect than the
natural one; while the gorgeous 'residual blue' which makes its
appearance when the polarisation of the artificial sky ceases to be
perfect, was strongly contrasted with the lack-lustre hue which, in
the case of the firmament, outlived the extinction of the brilliancy.
With certain substances, however, artificially treated, this dull
residue may also be obtained.

All along the arc from the Matterhorn to Mont Blanc the light of the
sky immediately above the mountains was powerfully acted upon by the
Nicol.  In some cases the variations of intensity were astonishing.  I
have already said that a little practice enables the observer to shift
the Nicol from one position to another so rapidly as to render the
alternative extinction and restoration of the light immediate.  When
this was done along the arc to which I have referred, the alternations
of light and darkness resembled the play of sheet lightning behind the
mountains.  There was an element of awe connected with the suddenness
with which the mighty masses, ranged along the line referred to,
changed their aspect and definition under the operation of the prism.

*****

The physical reason of the blueness of both natural and artificial
skies is, I trust, correctly given in the essay on the Scientific use
of the Imagination published in the second volume of these Fragments.

********************

V. ON DUST AND DISEASE.

[Footnote: A discourse delivered before the Royal Institution of Great
Britain, January 21, 1870.]

Experiments on Dusty Air.

SOLAR light, in passing through a dark room, reveals its track by
illuminating the dust floating in the air.  'The sun,' says Daniel
Culverwell, 'discovers atomes, though they be invisible by
candle-light, and makes them dance naked in his beams.'

In my researches on the decomposition of vapours by light, I was
compelled to remove these 'atoms' and this dust. It was essential that
the space containing the vapours should embrace no visible thing--that
no substance capable of scattering light in the slightest sensible
degree should, at the outset of an experiments, be found in the wide
'experimental tube' in which the vapour was enclosed.

For a long time I was troubled by the appearance there of floating
matter, which, though invisible in diffuse daylight, was at once
revealed by a powerfully condensed beam.  Two U-tubes were placed in
succession in the path of the air, before it entered the liquid whose
vapour was to be carried into the experimental tube.  One of the
U-tubes contained fragments of marble wetted with a strong solution of
caustic potash; the other, fragments of glass wetted with concentrated
sulphuric acid which, while yielding no vapour of its own, powerfully
absorbs the aqueous vapour of the air. [Footnote: The apparatus is
figured in Fig. 3.]  To my astonishment, the air of the Royal
Institution, sent through these tubes at a rate sufficiently slow to
dry it, and to remove its carbonic acid, carried into the experimental
tube a considerable amounts of mechanically suspended matter, which
was illuminated when the beam passed through the tube.  The effect was
substantially the same when the air was permitted to bubble through
the liquid acid, and through the solution of potash.

I tried to intercept this floating matter in various ways; and on
October 5, 1868, prior to sending the air through the drying
apparatus, it was carefully permitted to pass over the tip of a
spirit-lamp flame.  The floating matter no longer appeared, having
been burnt up by the flame.  It was therefore _organic matter_.  I was
by no means prepared for this result; having previously thought that
the dust of our air was, in great part, inorganic and non-combustible.
[Footnote: According to an analysis kindly furnished to me by Dr.
Percy, the dust collected _from the walls_ of the British Museum
contains fully 50 per cent.  of inorganic matter.  I have every
confidence in the results of this distinguished chemist; they show
that the _floating_ dust of our rooms is, as it were, winnowed from the
heavier matter.  As bearing directly upon this point I may quote the
following passage from Pasteur: 'Mais ici se présente une remarque: la
poussière que Pon trouve à la surface de tous les corps est soumise
constamment à des courants d'air, qui doivent soulever des particules
les plus légères, au nombre desquelles se trouvent, sans doute, de
préférence les corpuscules organisés, oeufs ou spores, moins lourds
généralement que les particules minérales.']

I had constructed a small gas-furnace, now much employed by chemists,
containing a platinum tube, which could be heated to vivid redness.
[Footnote: Pasteur was, I believe, the first to employ such a tube.]
The tube contained a roll of platinum gauze, which, while it permitted
the air to pass through it, ensured the practical contact of the dust
with the incandescent metal.  The air of the laboratory was permitted
to enter the experimental tube, sometimes through the cold, and
sometimes through the heated, tube of platinum.  In the first column
of the following fragment of a long table the quantity of air operated
on is expressed by the depression of the mercury gauge of the
air-pump.  In the second column the condition of the platinum tube is
mentioned, and in the third the state of the air in the experimental
tube.

Quantity of air   State of platinum tube  State of experimental tube

15 inches                 Cold                  Full of particles.

30 inches                 Red-hot               Optically empty.

The phrase 'optically empty' shows that when the conditions of perfect
combustion were present, the floating matter totally disappeared.

*****

In a cylindrical beam, which strongly illuminated the dust of the
laboratory, I placed an ignited spirit-lamp.  Mingling with the flame,
and round its rim, were seen curious wreaths of darkness resembling an
intensely black smoke.  On placing the flame at some distance below
the beam, the same dark masses stormed upwards.  They were blacker
than the blackest smoke ever seen issuing from the funnel of a
steamer; and their resemblance to smoke was so perfect as to lead the
most practised observer to conclude that the apparently pure flame of
the alcohol lamp required but a beam of sufficient intensity to reveal
its clouds of liberated carbon.  But is the blackness smoke?  This
question presented itself in a moment and was thus answered: A red-hot
poker was placed underneath the beam: from it the black wreaths also
ascended.  A large hydrogen flame was next employed, and it produced
those whirling masses of darkness, far more copiously than either the
spirit-flame or poker. Smoke was therefore out of the question.
[Footnote: In none of the public rooms of the United States where I
had the honour to lecture was this experiment made.  The organic dust
was too scanty.  Certain rooms in England--the Brighton Pavilion, for
example--also lack the necessary conditions.]

What, then, was the blackness?  It was simply that of stellar space;
that is to say, blackness resulting from the absence from the track of
the beam of all matter competent to scatter its light.  When the flame
was placed below the beam the floating matter was destroyed _in situ_;
and the air, freed from this matter, rose into the beam, jostled aside
the illuminated particles, and substituted for their light the
darkness due to its own perfect transparency.  Nothing could more
forcibly illustrate the invisibility of the agent which renders all
things visible.  The beam crossed, unseen, the black chasm formed by
the transparent air, while, at both sides of the gap, the thick-strewn
particles shone out like a luminous solid under the powerful
illumination.

It is not, however, necessary to burn the particles to produce a
stream of darkness.  Without actual combustion, currents may be
generated which shall displace the floating matter, and appear dark
amid the surrounding brightness.  I noticed this effect first on
placing a red-hot copper ball below the beam, and permitting it to
remain there until its temperature had fallen below that of boiling
water.  The dark currents, though much enfeebled, were still produced.
They may also be produced by a flask filled with hot water.

To study this effect a platinum wire was stretched across the beam,
the two ends of the wire being connected with the two poles of a
voltaic battery.  To regulate the strength of the current a rheostat
was placed in the circuit.  Beginning with a feeble current the
temperature of the wire was gradually augmented; but long before it
reached the heat of ignition, a flat stream of air rose from it, which
when looked at edgeways appeared darker and sharper than one of the
blackest lines of Fraunhofer in the purified spectrum.  Right and left
of this dark vertical band the floating matter rose upwards, bounding
definitely the non-luminous stream of air.  What is the explanation?
Simply this: The hot wire rarefied the air in contact with it, but it
did not equally lighten the floating matter.  The convection current
of pure air therefore passed upwards among the inert particles,
dragging them after it right and left, but forming between them an
impassable black partition.  This elementary experiments enables us to
render an account of the dark currents produced by bodies at a
temperature below that of combustion.

But when the platinum wire is intensely heated, the floating matter is
not only displaced, but destroyed.  I stretched a wire about 4 inches
long through the air of an ordinary glass shade resting on
cotton-wool, which also surrounded the rim.  The wire being raised to
a white heat by an electric current, the air expanded, and some of it
was forced through the cotton-wool.  When the current was interrupted,
and the air within the shade cooled, the returning air did not carry
motes along with it, being filtered by the wool.  At the beginning of
this experiments the shade was charged with floating matter; at the
end of half an hour it was optically empty.

On the wooden base of a cubical glass shade, a cubic foot in volume,
upright supports were fixed, and from one support to the other 38
inches of platinum wire were stretched in four parallel lines.  The
ends of the platinum wire were soldered to two stout copper wires
which passed through the base of the shade and could be connected with
a battery.  As in the last experiments the shade rested upon
cotton-wool.  A beam sent through the shade revealed the suspended
matter.  The platinum wire was then raised to whiteness.  In five
minutes there was a sensible diminution of the matter, and in ten
minutes it was totally consumed.

Oxygen, hydrogen, nitrogen, carbonic acid, so prepared as to exclude
all floating particles, produce, when poured or blown into the beam,
the darkness of stellar space.  Coal-gas does the same.  An ordinary
glass shade, placed in the air with its mouth downwards, permits the
track of the beam to be seen crossing it.  When coal-gas or hydrogen
is allowed to enter the shade by a tube reaching to its top, the gas
gradually fills the shade from above downwards.  As soon as it
occupies the space crossed by the beam, the luminous track is
abolished.  Lifting the shade so as to bring the common boundary of
gas and air above the beam, the track flashes forth.  After the shade
is full, if it be inverted, the pure gas passes upwards like a black
smoke among the illuminated particles.

The Germ Theory of Contagious Disease.

There is no respite to our contact with the floating matter of the
air; and the wonder is, not that we should suffer occasionally from
its presence, but that so small a portion of it, and even that but
rarely diffused over large areas, should appear to be deadly to man.
And what is this portion?  It was some time ago the current belief
that epidemic diseases generally were propagated by a kind of malaria,
which consisted of organic matter in a state of motor-decay; that when
such matter was taken into the body through the lungs, skin, or
stomach, it had the power of spreading there the destroying process by
which itself had been assailed.  Such a power was visibly exerted in
the case of yeast. A little leaven was seen to leaven the whole
lump--a mere speck of matter, in this supposed state of decomposition,
being apparently competent to propagate indefinitely its own decay.
Why should not a bit of rotten malaria act in a similar manner within
the human frame?  In 1836 a very wonderful reply was given to this
question.  In that year Cagniard de la Tour discovered the
yeast-plant--a living organism, which when placed in a proper medium
feeds, grows, and reproduces itself, and in this way carries on the
process which we name fermentation.  By this striking discovery
fermentation was connected with organic growth.

Schwann, of Berlin, discovered the yeast-plant independently about the
same time; and in February, 1837, he also announced the important
result, that when a decoction of meat is effectually screened from
ordinary air, and supplied solely with calcined air, putrefaction
never sets in.  Putrefaction, therefore, he affirmed to be caused, not
by the air, but by something which could be destroyed by a
sufficiently high temperature.  The results of Schwann were confirmed
by the independent experiments of Helmholtz, Ure, and Pasteur, while
other methods, pursued by Schultze, and by Schroeder and Dusch, led to
the same result.

But as regards fermentation, the minds of chemists, influenced
probably by the great authority of Gay-Lussac, fell back upon the old
notion of matter in a state of decay.  It was not the living
yeast-plant, but the dead or dying parts of it, which, assailed by
oxygen, produced the fermentation.  Pasteur, however, proved the real
'ferments,' mediate or immediate, to be organised beings which find in
the reputed ferments their necessary food.

Side by side with these researches and discoveries, and fortified by
them and others, has run the germ theory of epidemic disease.  The
notion was expressed by Kircher, and favoured by Linnaeus, that
epidemic diseases may be due to germs which float in the atmosphere,
enter the body, and produce disturbance by the development within the
body of parasitic life.  The strength of this theory consists in the
perfect parallelism of the phenomena of contagious disease with those
of life.  As a planted acorn gives birth to an oak, competent to
produce a whole crop of acorns, each gifted with the power of
reproducing its parent tree; and as thus from a single seedling a
whole forest may spring; so, it is contended, these epidemic diseases
literally plant their seeds, grow, and shake abroad new germs, which,
meeting in the human body their proper food and temperature, finally
take possession of whole populations.  There is nothing to my
knowledge in pure chemistry which resembles the power of propagation
and self-multiplication possessed by the matter which produces
epidemic disease.  If you sow wheat you do not get barley; if you sow
small-pox you do not get scarlet-fever, but small-pox indefinitely
multiplied, and nothing else.  The matter of each contagious disease
reproduces itself as rigidly as if it were (as Miss Nightingale puts
it) dog or cat.

Parasitic Diseases of Silkworms.  Pasteur's Researches.

It is admitted on all hands that some diseases are the product of
parasitic growth.  Both in man and in lower creatures, the existence
of such diseases has been demonstrated.  I am enabled to lay before
you an account of an epidemic of this kind, thoroughly investigated
and successfully combated by M. Pasteur.  For fifteen years a plague
had raged among the silkworms of France.  They had sickened and died
in multitudes, while those that succeeded in spinning their cocoons
furnished only a fraction of the normal quantity of silk.  In 1853 the
silk culture of France produced a revenue of one hundred and thirty
millions of francs.  During the twenty previous years the revenue had
doubled itself, and no doubt was entertained as to its further
augmentation.  The weight of the cocoons produced in 1853 was
26,000,000 kilogrammes; in 1865 it had fallen to 4,000,000, the fall
entailing, in a single year, a loss of 100,000,000 francs.

The country chiefly smitten by this calamity happened to be that of
the celebrated chemist Dumas, now perpetual secretary of the French
Academy of Sciences.  He turned to his friend, colleague, and pupil,
Pasteur, and besought him, with an earnestness which the circumstances
rendered almost personal, to undertake the investigation of the
malady.  Pasteur at this time had never seen a silkworm, and he urged
his inexperience in reply to his friend.  But Dumas knew too well the
qualities needed for such an enquiry to accept Pasteur's reason for
declining it.  'Je mets,' said he, 'un prix extréme à voir votre
attention fixée sur la question qui intéresse mon pauvre pays; la
misére surpasse tout ce que vous pouvez imaginer.' Pamphlets about the
plague had been showered upon the public, the monotony of waste paper
being broken, at rare intervals, by a more or less useful publication.
'The Pharmacopoeia of the Silkworm,' wrote M. Cornalia in 1860, 'is
now as complicated as that of man.  Gases, liquids, and solids have
been laid under contribution.  From chlorine to sulphurous acid, from
nitric acid to rum, from sugar to sulphate of quinine,--all has been
invoked in behalf of this unhappy insect.' The helpless cultivators,
moreover, welcomed with ready trustfulness every new remedy, if only
pressed upon them with sufficient hardihood. It seemed impossible to
diminish their blind confidence in their blind guides.  In 1863 the
French Minister of Agriculture signed an agreement to pay 500,000
francs for the use of a remedy, which its promoter declared to be
infallible.  It was tried in twelve different departments of France,
and found perfectly useless.  In no single instance was it successful.
It was under these circumstances that M. Pasteur, yielding to the
entreaties of his friend, betook himself to Alais in the beginning of
June, 1865.  As regards silk husbandry, this was the most important
department in France, and it was the most sorely smitten by the
plague.

The silkworm had been previously attacked by muscardine, a disease
proved by Bassi to be caused by a vegetable parasite.  This malady was
propagated annually by the parasitic spores.  Wafted by winds they
often sowed the disease in places far removed from the centre of
infection.  Muscardine is now said to be very rare, a deadlier malady
having taken its place.  This new disease is characterised by the
black spots which cover the silkworms; hence the name _pébrine_, first
applied to the plague by M. de Quatrefages, and adopted by Pasteur.
_pébrine_ declares itself in the stunted and unequal growth of the
worms, in the languor of their movements, in their fastidiousness as
regards food, and in their premature death.  The course of discovery
as regards the epidemic is this: In 1849 Guérin Méneville noticed in
the blood of silkworms vibratory corpuscles, which he supposed from
their motions to be endowed with independent life.  Filippi, however,
showed that the motion of the corpuscles was the well-known Brownian
motion; but he committed the error of supposing the corpuscles to be
normal to the life of the insect.  Possessing the power of indefinite
self-multiplication, they are really the cause of its mortality--the
form and substance of its disease.  This was well described by
Cornalia; while Lebert and Frey subsequently found the corpuscles not
only in the blood, but in all the tissues of the insect.  Osimo, in
1857, discovered them in the eggs; and on this observation Vittadiani
founded, in 1859, a practical method of distinguishing healthy from
diseased eggs.  The test often proved fallacious, and it was never
extensively applied.

These living corpuscles take possession of the intestinal canal, and
spread thence throughout the body of the worm.  They fill the silk
cavities, the stricken insect often going automatically through the
motions of spinning, without any material to work upon.  Its organs,
instead of being filled with the clear viscous liquid of the silk, are
packed to distension by the corpuscles.  On this feature of the plague
Pasteur fixed his entire attention.  The cycle of the silkworm's life
is briefly this: From the fertile egg comes the little worm, which
grows, and casts its skin.  This process of moulting is repeated two
or three times at intervals during the life of the insect.  After the
last moulting the worm climbs the brambles placed to receive it, and
spins among them its cocoon.  It passes thus into a chrysalis; the
chrysalis becomes a moth, and the moth, when liberated, lays the eggs
which form the starting-point of a new cycle.  Now Pasteur proved that
the plague-corpuscles might be incipient in the egg, and escape
detection; they might also be germinal in the worm, and still baffle
the microscope.  But as the worm grows, the corpuscles grow also,
becoming larger and more defined.  In the aged chrysalis they are more
pronounced than in the worm; while in the moth, if either the egg or
the worm from which it comes should have been at all stricken, the
corpuscles infallibly appear, offering no difficulty of detection.
This was the first great point made out in 1865 by Pasteur.  The
Italian naturalists, as aforesaid, recommended the examination of the
eggs before risking their incubation.  Pasteur showed that both eggs
and worms might be smitten, and still pass muster, the culture of such
eggs or such worms being sure to entail disaster.  He made the moth
his starting-point in seeking to regenerate the race.

Pasteur made his first communication on this subject to the Academy of
Sciences in September, 1865.  It raised a cloud of criticism.  Here,
forsooth, was a chemist rashly quitting his proper _métier_ and
presuming to lay down the law for the physician and biologist on a
subject which was eminently theirs.  'On trouva étrange que je fusse
si peu au courant de la question; on m'opposa des travaux qui avaient
paru depuis longtemps en Italie, dont les résultats montraient
l'inutilité de mes efforts, et l'impossibilité d'arriver à un résultat
pratique dans la direction que je m'étais engagé.  Que mon ignorance
fut grande au sujet des recherches sans nombre qui avaient paru depuis
quinze années.' Pasteur heard the buzz, but he continued his work.  In
choosing the eggs intended for incubation, the cultivators selected
those produced in the successful 'educations' of the year.  But they
could not understand the frequent and often disastrous failures of
their selected eggs; for they did not know, and nobody prior to
Pasteur was competent to tell them, that the finest cocoons may
envelope doomed corpusculous moths.  It was not, however, easy to make
the cultivators accept new guidance.  To strike their imagination, and
if possible determine their practice, Pasteur hit upon the expedient
of prophecy.  In 1866 he inspected, at St. Hippolyte-du-Fort, fourteen
different parcels of eggs intended for incubation.  Having examined a
sufficient number of the moths which produced these eggs, he wrote out
the prediction of what would occur in 1867, and placed the prophecy as
a sealed letter in the hands of the Mayor of St. Hippolyte.

In 1867 the cultivators communicated to the mayor their results.  The
letter of Pasteur was then opened and read, and it was found that in
twelve out of fourteen cases there was absolute conformity between his
prediction and the observed facts.  Many of the groups had perished
totally; the others had perished almost totally; and this was the
prediction of Pasteur.  In two out of the fourteen cases, instead of
the prophesied destruction, half an average crop was obtained.  Now,
the parcels of eggs here referred to were considered healthy by their
owners.  They had been hatched and tended in the firm hope that the
labour expended on them would prove remunerative.  The application of
the moth-test for a few minutes in 1866, would have saved the labour
and averted the disappointment.  Two additional parcels of eggs were
at the same time submitted to Pasteur.  He pronounced them healthy;
and his words were verified by the production of an excellent crop.
Other cases of prophecy still more remarkable, because more
circumstantial, are recorded in Pasteur's work.

Pasteur subjected the development of the corpuscles to a searching
investigation, and followed out with admirable skill and completeness
the various modes by which the plague was propagated. From moths
perfectly free from corpuscles he obtained healthy worms, and
selecting 10, 20, 30, 50, as the case might be, he introduced into the
worms the corpusculous matter. It was first permitted to accompany the
food. Let its take a single example out of many. Rubbing up a small
corpusculous worm in water, he smeared the mixture over the
mulberry-leaves. Assuring himself that the leaves had been eaten, he
watched the consequences from day to day. Side by side with the
infected worms he reared their fellows, keeping them as much as
possible out of the way of infection. These constituted his 'lot
témoin,'--his standard of comparison. On April 16, 1868, he thus
infected thirty worms. Up to the 23rd they remained quite well. On the
25th they seemed well, but on that day corpuscles were found in the
intestines of two of them. On the 27th, or eleven days after the
infected repast, two fresh worms were examined, and not only was the
intestinal canal found in each case invaded, but the silk organ itself
was charged with corpuscles. On the 28th the twenty-six remaining
worms were covered by the black spots of _pébrine_. On the 30th the
difference of size between the infected and non-infected worms was
very striking, the sick worms being not more than two-thirds of the
bulk of the healthy ones. On May 2 a worm which had just finished its
fourth moulting was examined. Its whole body was so filled with the
parasite as to excite astonishment that it could live.

The disease advanced, the worms died and were examined, and on May 11
only six out of the thirty remained. They were the strongest of the
lot, but on being searched they also were found charged with
corpuscles. Not one of the thirty worms had escaped; a single meal had
poisoned them all. The standard lot, on the contrary, spun their fine
cocoons, two only of their moths being proved to contain any trace of
the parasite, which had doubtless been introduced during the rearing
of the worms.

As his acquaintance with the subject increased, Pasteur's desire for
precision augmented, and he finally counted the growing number of
corpuscles seen in the field of his microscope from day to day. After
a contagious repast the number of worms containing the parasite
gradually augmented until finally it became cent. per cent. The number
of corpuscles would at the same time rise from 0 to 1, to 10, to 100,
and sometimes even to 1,000 or 1,500 in the field of his microscope.
He then varied the mode of infection. He inoculated healthy worms with
the corpusculous matter, and watched the consequent growth of the
disease. He proved that the worms inoculate each other by the
infliction of visible wounds with their claws. In various cases he
washed the claws, and found corpuscles in the water. He demonstrated
the spread of infection by the simple association of healthy and
diseased worms. By their claws and their dejections, the diseased
worms spread infection.  It was no hypothetical infected medium--no
problematical pythogenic gas--that killed the worms, but a definite
organism. The question of infection at a distance was also examined,
and its existence demonstrated. As might be expected from Pasteur's
antecedents, the investigation was exhaustive, the skill and beauty of
his manipulation finding fitting correlatives in the strength and
clearness of his thought.

The following quotation from Pasteur's work clearly shows the relation
in which his researches stand to the important question on which he
was engaged:

*****

Place (he says) the most skilful educator, even the most expert
microscopist, in presence of large educations which present the
symptoms described in our experiments; his judgment will necessarily
be erroneous if he confines himself to the knowledge which preceded my
researches.  The worms will not present to him the slightest spot of
_pébrine_; the microscope will not reveal the existence of corpuscles;
the mortality of the worms will be null or insignificant; and the
cocoons leave nothing to be desired.  Our observer would, therefore,
conclude without hesitation that the eggs produced will be good for
incubation.  The truth is, on the contrary, that all the worms of
these fine crops have been poisoned; that from the beginning they
carried in them the germ of the malady; ready to multiply itself
beyond measure in the chrysalides and the moths, thence to pass into
the eggs and smite with sterility the next generation.  And what is
the first cause of the evil concealed under so deceitful an exterior?
In our experiments we can, so to speak, touch it with our fingers.  It
is entirely the effect of a single corpusculous repast; an effect more
or less prompt according to the epoch of life of the worm that has
eaten the poisoned food.

*****

Pasteur describes in detail his method of securing healthy eggs.  It
is nothing less than a mode of restoring to France her ancient silk
husbandry.  The justification of his work is to be found in the
reports which reached him of the application and the unparalleled
success of his method, while editing his researches for final
publication.  In both France and Italy his method has been pursued
with the most surprising results.  But it was an up-hill fight which
led to this triumph.

'Ever,' he says, 'since the commencement of these researches, I have
been exposed to the most obstinate and unjust contradictions; but I
have made it a duty to leave no trace of these conflicts in this
book.' And in reference to parasitic diseases, generally, he uses the
following weighty words: 'Il est au pouvoir de l'homme de faire
disparaitre de la surface du globe les maladies parasitaires, si,
comme c'est ma conviction, la doctrine des générations spontanées est
une chimère.'

Pasteur dwells upon the ease with which an island like Corsica might
be absolutely isolated from the silkworm epidemic.  And with regard to
other epidemics, Mr. Simon describes an extraordinary case of insular
exemption, for the ten years extending from 1851 to 1860.  Of the 627
registration districts of England, one only had an entire escape from
diseases which, in whole or in part, were prevalent in all the others:
'In all the ten years it had not a single death by measles, nor a
single death by small-pox, nor a single death by scarlet-fever.  And
why?  Not because of its general sanitary merits, for it had an
average amounts of other evidence of unhealthiness.  Doubtless, the
reason of its escape was that it was insular.  It was the district of
the Scilly Isles; to which it was most improbable that any febrile
contagion should come from without.  And its escape is an
approximative proof that, at least for those ten years, no _contagium_
of measles, nor any _contagium_ of scarlet-fever, nor any _contagium_ of
smallpox had arisen spontaneously within its limits.' It may be added
that there were only seven districts in England in which no death from
diphtheria occurred, and that, of those seven districts, the district
of the Scilly Isles was one.

A second parasitic disease of silkworms, called in France _la
Flacherie_, co-existent with _pébrine_, but quite distinct from it, has
also been investigated by Pasteur.  Enough, however, has been said to
send the reader interested in these questions to the original volumes
for further information.  To one important practical point M. Pasteur,
in a letter to myself, directs attention:

*****

Permettez-moi de terminer ces quelques lignes que je dois dicter,
vaincu que je suis par la maladie, en vous faisant observer que vous
rendriez service aux Colonies de la Grande-Bretagne en répandant la
connaissance de ce livre, et des principes que j'établis touchant la
maladie des vers à soie.  Beaucoup de ces colonies pourraient cultiver
le mûrier avec succés, et, en jetant les yeux sur mon ouvrage, vous
vous convaincrez aisement qu'il est facile aujourd'hui, nonseulement
d'éloigner la maladie régnante, mais en outre de donner aux récoltes
de la soie une prospérité qu'elles n'ont jamais eue.

Origin and Propagation of Contagious Matter.

Prior to Pasteur, the most diverse and contradictory opinions were
entertained as to the contagious character of _pébrine_; some stoutly
affirmed it, others as stoutly denied it.  But on one point all were
agreed.  I They believed in the existence of a deleterious medium,
rendered epidemic by some occult and mysterious influence, to which
was attributed the cause of the disease.' Those acquainted with our
medical literature will not fail to observe an instructive analogy
here.  We have on the one side accomplished writers ascribing epidemic
diseases to 'deleterious media' which arise spontaneously in crowded
hospitals and ill-smelling drains.  According to them, the _contagia_ of
epidemic disease are formed _de novo_ in a putrescent atmosphere.  On
the other side we have writers, clear, vigorous, with well-defined
ideas and methods of research, contending that the matter which
produces epidemic disease comes always from a parent stock.  It
behaves as germinal matter, and they do not hesitate to regard it as
such.  They no more believe in the spontaneous generation of such
diseases, than they do in the spontaneous generation of mice. Pasteur,
for example, found that _pébrine_ had been known for an indefinite time
as a disease among silkworms.  The development of it which he combated
was merely the expansion of an already existing power--the bursting
into open conflagration of a previously smouldering fire.  There is
nothing surprising in this.  For though epidemic disease requires a
special _contagium_ to produce it, surrounding conditions must have a
potent influence on its development.  Common seeds may be duly sown,
but the conditions of temperature and moisture may be such as to
restrict, or altogether prevent, the subsequent growth.  Looked at,
therefore, from the point of view of the germ theory, the exceptional
energy which epidemic disease from time to time exhibits, is in
harmony with the method of Nature.  We sometimes hear diphtheria
spoken of as if it were a new disease of the last twenty years; but
Mr. Simon tells me that about three centuries ago tremendous epidemics
of it began to rage in Spain (where it was named _Garrotillo_), and soon
afterwards in Italy; and that since that time the disease has been
well known to all successive generations of doctors.  In or about
1758, for instance, Dr. Starr, of Liskeard, in a communication to the
Royal Society, particularly described the disease, with all the
characters which have recently again become familiar, but under the
name of _morbus strangulatorius_, as then severely epidemic in Cornwall.
This fact is the more interesting, as diphtheria, in its more modern
reappearance, again showed predilection for that remote county.  Many
also believe that the Black Death, of five centuries ago, has
disappeared as mysteriously as it came; but Mr. Simon finds that it is
believed to be prevalent at this hour in some of the north-western
parts of India.

Let me here state an item of my own experience.  When I was at the Bel
Alp in 1869, the English chaplain received letters informing him of
the breaking out of scarlet-fever among his children.  He lived, if I
remember rightly, on the healthful eminence of Dartmoor, and it was
difficult to imagine how scarlet-fever could have been wafted to the
place.  A drain ran close to his house, and on it his suspicions were
manifestly fixed.  Some of our medical writers would fortify him in
this notion, and thus deflect him from the truth, while those of
another, and, in my opinion, a wiser school, would deny to a drain,
however foul, the power of generating _de novo_ a specific disease.
After close enquiry he recollected that a hobby-horse had been used
both by his boy and another, who, a short time previously, had passed
through scarlet-fever.

Drains and cesspools, indeed, are by no means in such evil odour as
they used to be.  A fetid Thames and a low death-rate occur from time
to time together in London.  For, if the special matter or germs of
epidemic disorder be not present, a corrupt atmosphere, however
obnoxious otherwise, will not produce the disorder.  But, if the germs
be present, defective drains and cesspools become the potent
distributors of disease and death.  Corrupted air may promote an
epidemic, but cannot produce it.  On the other hand, through the
transport of the special germ or virus, disease may develop itself in
regions where the drainage is good and the atmosphere pure.

If you see a new thistle growing in your field, you feel sure that its
seed has been wafted thither.  Just as sure does it seem that the
contagious matter of epidemic disease has been transplanted to the
place where it newly appears.  With a clearness and conclusiveness s
not to be surpassed, Dr. William Budd has traced such diseases from
place to place; showing how they plant themselves, at distinct foci,
among populations subjected to the same atmospheric influences, just
as grains of corn might be carried in the pocket and sown. Hildebrand,
to whose remarkable work, 'Du Typhus contagieux,' Dr. de Mussy has
directed my attention, gives the following striking case, both of the
durability and the transport of the virus of scarlatina: 'Un habit
noir que j'avais en visitant une malade attaquée de scarlatina, et que
je portai de Vienne en Podolie, sans l'avoir mis depuis plus d'un an
et demi, me communiqua, dès que je fus arrivé, cette maladie
contagieuse, que je répandis ensuite dans cette province, où elle
était jusqu'alors presque inconnue.' Some years ago Dr. de Mussy
himself was summoned to a country house in Surrey, to see a young lady
who was suffering from a dropsy, evidently the consequence of
scarlatina.  The original disease, being of a very mild character, had
been quite overlooked; but circumstances were recorded which could
leave no doubt upon the mind as to the nature and cause of the
complaint.  But then the question arose, How did the young lady catch
the scarlatina?  She had come there on a visit two months previously,
and it was only after she had been a month in the house that she was
taken ill.  The housekeeper at length cleared up the mystery.  The
young lady, on her arrival, had expressed a wish to occupy a room in
an isolated tower.  Her desire was granted; and in that room, six
months previously, a visitor had been confined with an attack of
scarlatina.  The room had been swept and whitewashed, but the carpets
had been permitted to remain.

Thousands of cases could probably be cited in which the disease has
shown itself in this mysterious way, but where a strict examination
has revealed its true parentage and extraction.  Is it, then,
philosophical to take refuge in the fortuitous concourse of atoms as a
cause of specific disease, merely because in special cases the
parentage may be indistinct?  Those best acquainted with atomic
nature, and who are most ready to admit, as regards even higher things
than this, the potentialities of matter, will be the last to accept
these rash hypotheses.

The Germ Theory applied to Surgery.

Not only medical but still more especially surgical science is now
seeking light and guidance from this germ theory.  Upon it the
antiseptic system of Professor Lister of Edinburgh is founded.  As
already stated, the germ theory of putrefaction was started by
Schwann; but the illustrations of this theory adduced by Professor
Lister are of such public moment as not only to justify, but to render
imperative, their introduction here.

Schwann's observations (says Professor Lister) did not receive the
attention which they appeared to me to have deserved.  The
fermentation of sugar was generally allowed to be occasioned by the
_Torula cerevisiae_; but it was not admitted that putrefaction was due
to an analogous agency.  And yet the two cases present a very striking
parallel.  In each a stable chemical compound, sugar in the one case,
albumen in the other, undergoes extraordinary chemical changes under
the influence of an excessively minute quantity of a substance which,
regarded chemically, we should suppose inert.  As an example of this
in the case of putrefaction, let us take a circumstance often
witnessed in the treatment of large chronic abscesses.  In order to
guard against the access of atmospheric air, we used to draw off the
matter by means of a canula and trocar, such as you see here,
consisting of a silver tube with a sharp-pointed steel rod fitted into
it, and projecting beyond it.  The instrument, dipped in oil, was
thrust into the cavity of the abscess, the trocar was withdrawn, and
the pus flowed out through the canula, care being taken by gentle
pressure over the part to prevent the possibility of regurgitation.
The canula was then drawn out with due precaution against the reflux
of air.  This method was frequently successful as to its immediate
object, the patient being relieved from the mass of the accumulated
fluid, and experiencing no inconvenience from the operation.  But the
pus was pretty certain to reaccumulate in course of time, and it
became necessary again and again to repeat the process.  And unhappily
there was no absolute security of immunity from bad consequences.
However carefully the procedure was conducted, it sometimes happened,
even though the puncture seemed healing by first intention, that
feverish symptoms declared themselves in the course of the first or
second day, and, on inspecting the seat of the abscess, the skin was
perhaps seen to be red, implying the presence of some cause of
irritation, while a rapid reaccumulation of the fluid was found to
have occurred.  Under these circumstances, it became necessary to open
the abscess by free incision, when a quantity, large in proportion to
the size of the abscess, say, for example, a quart, of pus escaped,
fetid from putrefaction.  Now, how had this change been brought about?
Without the germ theory, I venture to say, no rational explanation of
it could have been given.  It must have been caused by the
introduction of something from without.  Inflammation of the punctured
wound, even supposing it to have occurred, would not explain the
phenomenon.  For mere inflammation, whether acute or chronic, though
it occasions the formation of pus, does not induce Putrefaction.  The
pus originally evacuated was perfectly sweet, and we know of nothing
to account for the alteration in its quality but the influence of
something derived from the external world.  And what could that
something be?  The dipping of the instrument in oil, and the
subsequent precautions, prevented the entrance of oxygen.  Or even if
you allowed that a few atoms of the gas did enter, it would be an
extraordinary assumption to make that these could in so short a time
effect such changes in so large a mass of albuminous material.
Besides, the pyogenic membrane is abundantly supplied with capillary
vessels, through which arterial blood, rich in oxygen, is perpetually
flowing; and there can be little doubt that the pus, before it was
evacuated at all, was liable to any action which the element might be
disposed to exert upon it.

On the oxygen theory, then, the occurrence of putrefaction under these
circumstances is quite inexplicable.  But if you admit the germ
theory, the difficulty vanishes at once.  The canula and trocar having
been lying exposed to the air, dust will have been deposited upon
them, and will be present in the angle between the trocar and the
silver tube, and in that protected situation will fail to be wiped off
when the instrument is thrust through the tissues.  Then when the
trocar is withdrawn, some portions of this dust will naturally remain
upon the margin of the canula, which is left projecting into the
abscess, and nothing is more likely than that some particles may fail
to be washed off by the stream of out-flowing pus, but may be
dislodged when the tube is taken out, and left behind in the cavity.
The germ theory tells us that these particles of dust will be pretty
sure to contain the germs of putrefactive organisms, and if one such
is left in the albuminous liquid, it will rapidly develop at the high
temperature of the body, and account for all the phenomena.

But striking as is the parallel between putrefaction in this instance
and the vinous fermentation, as regards the greatness of the effect
produced, compared with the minuteness and the inertness, chemically
speaking, of the cause, you will naturally desire further evidence of
the similarity of the two processes.  You can see with the microscope
the Torula of fermenting must or beer.  Is there, you may ask, any
organism to be detected in the putrefying pus?  Yes, gentlemen, there
is.  If any drop of the putrid matter is examined with a good glass,
it is found to be teeming with myriads of minute jointed bodies,
called vibrios, which indubitably proclaim their vitality by the
energy of their movements.  It is not an affair of probability, but a
fact, that the entire mass of that quart of pus has become peopled
with living organisms as the result of the introduction of the canula
and trocar; for the matter first let out was as free from vibrios as
it was from putrefaction.  If this be so, the greatness of the
chemical changes that have taken place in the pus ceases to be
surprising.  We know that it is one of the chief peculiarities of
living structures that they possess extraordinary powers of effecting
chemical changes in materials in their vicinity, out of all proportion
to their energy as mere chemical compounds.  And we can hardly doubt
that the animalcules which have been developed in the albuminous
liquid, and have grown at its expense, must have altered its
constitution, just as we ourselves alter that of the materials on
which we feed. [Footnote: 'Introductory Lecture before the University
of Edinburgh.']

In the operations of Professor Lister care is taken that every portion
of tissue laid bare by the knife shall be defended from germs; that if
they fall upon the wound they should be killed as they fall.  With
this in view he showers upon his exposed surfaces the spray of dilute
carbolic acid, which is particularly deadly to the germs, and he
surrounds the wound in the most careful manner with antiseptic
bandages.  To those accustomed to strict experiment it is manifest
that we have a strict experimenter here--a man with a perfectly
distinct object in view, which he pursues with never-tiring patience
and unwavering faith.  And the result, in his hospital practice, as
described by himself, has been, that even in the midst of abominations
too shocking to be mentioned here, and in the neighbourhood of wards
where death was rampant from pyaemia, erysipelas, and hospital
gangrene, he was able to keep his patients absolutely free from these
terrible scourges.  Let me here recommend to your attention Professor
Lister's 'Introductory Lecture before the University of Edinburgh,'
which I have already quoted; his paper on The Effect of the Antiseptic
System of Treatment on the Salubrity of a Surgical Hospital;' and the
article in the 'British Medical Journal' of January 14, 1871.

If, instead of using carbolic acid spray, he could surround his wounds
with properly filtered air, the result would, he contends, be the
same.  In a room where the germs not only float but cling to clothes
and walls, this would be difficult, if not impossible.  But surgery is
acquainted with a class of wounds in which the blood is freely mixed
with air that has passed through the lungs, and it is a most
remarkable fact that such air does not produce putrefaction. Professor
Lister, as far as I know, was the first to give a philosophical
interpretation of this fact, which he describes and comments upon
thus:

I have explained to my own mind the remarkable fact that in simple
fracture of the ribs, if the lung be punctured by a fragment, the
blood effused into the pleural cavity, though freely mixed with air,
undergoes no decomposition.  The air is sometimes pumped into the
pleural cavity in such abundance that, making its way through the
wound in the pleura costalis, it inflates the cellular tissue of the
whole body.  Yet this occasions no alarm to the surgeon (although if
the blood in the pleura were to putrefy, it would infallibly occasion
dangerous suppurative pleurisy).  Why air introduced into the pleural
cavity through a wounded lung, should have such wholly different
effects from that entering directly through a wound in the chest, was
to me a complete mystery until I heard of the germ theory of
putrefaction, when it at once occurred to me that it was only natural
that air should be filtered of germs by the air-passages, one of whose
offices is to arrest inhaled particles of dust, and prevent them from
entering the air-cells.

*****

I shall have occasion to refer to this remarkable hypothesis farther
on.

The advocates of the germ theory, both of putrefaction and epidemic
disease, hold that both arise, not from the air, but from something
contained in the air.  They hold, moreover, that this 'something' is
not a vapour nor a gas, nor indeed a molecule of any kind, but a
_particle_. [Footnote: As regards size, there is probably no sharp line
of division between molecules and particles; the one gradually shades
into the other.  But the distinction that I would draw is this: the
atom or the molecule, if free, is always part of a gas, the particle
is never so.  A particle is a bit of liquid or solid matter, formed by
the Aggregation of atoms or molecules.]  The term 'particulate 'has
been used in the Reports of the Medical Department of the Privy
Council to describe this supposed constitution of contagious matter;
and Dr. Sanderson's experiments render it in the highest degree
probable, if they do not actually demonstrate, that the virus of
small-pox is 'particulate.' Definite knowledge upon this point is of
exceeding importance, because in the treatment of _particles_ methods
are available which it would be futile to apply to _molecules_.

The Luminous beam as a means of Research.

My own interference with this great question, while sanctioned by
eminent names, has been also an object of varied and ingenious attack.
On this point I will only say that when angry feeling escapes from
behind the intellect, where it may be useful as an urging force, and
places itself athwart the intellect, it is liable to produce all
manner of delusions.  Thus my censors, for the most part, have
levelled their remarks against positions which were never assumed, and
against claims which were never made.  The simple history of the
matter is this: During the autumn of 1868 I was much occupied with the
observations referred to at the beginning of this discourse, and in
part described in the preceding article.  For fifteen years it had
been my habit to make use of floating dust to reveal the paths of
luminous beams through the air; but until 1868 I did not intentionally
reverse the process, and employ a luminous beam to reveal and examine
the dust. In a paper presented to the Royal Society in December, 1869,
the observations which induced me to give more special attention to
the question of spontaneous generation, and the germ theory of
epidemic disease, are thus described:

The Floating Matter of the Air.

Prior to the discovery of the foregoing action (the chemical action of
light upon vapours, Fragment IV.), and also during the experiments
just referred to, the nature of my work compelled me to aim at
obtaining experimental tubes absolutely clean upon the surface, and
absolutely free within from suspended matter.  Neither condition is,
however, easily attained.

For however well the tubes might be washed and polished, and however
bright and pure they might appear in ordinary daylight, the electric
beam infallibly revealed signs and tokens of dirt.  The air was always
present, and it was sure to deposit some impurity.  All chemical
processes, not conducted in a vacuum, are open to this disturbance.
When the experimental tube was exhausted, it exhibited no trace of
floating matter, but on admitting the air through the U-tubes
(containing caustic potash and sulphuric acid), a _dust-cone_ more or
less distinct was always revealed by the powerfully condensed electric
beam.

The floating motes resembled minute particles of liquid which had been
carried mechanically from the U-tubes into the experimental tube.
Precautions were therefore taken to prevent any such transfer.  They
produced little or no mitigation.  I did not imagine, at the time,
that the dust of the external air could find such free passage through
the caustic potash and sulphuric acid.  This, however, was the case;
the motes really came from without.  They also passed with freedom
through a variety of aethers and alcohols.  In fact, it requires
long-continued action on the part of an acid first to wet the motes
and afterwards to destroy them.  By carefully passing the air through
the flame of a spirit lamp, or through a platinum tube heated to
bright redness, the floating matter was sensibly destroyed.  It was
therefore combustible, in other words, organic, matter.  I tried to
intercept it by a large respirator of cotton-wool.  Close pressure was
necessary to render the wool effective.  A plug of the wool, rammed
pretty tightly into the tube through which the air passed, was finally
found competent to hold back the motes.  They appeared from time to
time afterwards, and gave me much trouble; but they were invariably
traced in the end to some defect in the purifying apparatus--to some
crack or flaw in the sealing-wax employed to render the tubes
air-tight.  Thus through proper care, but not without a great deal of
searching out of disturbances, the experimental tube, even when filled
with air or vapour, contains nothing competent to scatter the light.
The space within it has the aspect of an absolute vacuum.

An experimental tube in this condition I call _optically empty_.

The simple apparatus employed in these experiments will be at once
understood by reference to a figure printed in the last article (Fig.
3.)  s s' is the glass experimental tube, which has varied in length
from 1 to 5 feet, and which may be from 2 to 3 inches in diameter.
From the end s, the pipe pp' passes to an air-pump.  Connected with
the other end s' we have the flask F, containing the liquid whose
vapour is to be examined; then follows a U-tube, T, filled with
fragments of clean glass, wetted with sulphuric acid; then a second
U-tube, T, containing fragments of marble, wetted with caustic potash;
and finally a narrow straight tube t t', containing a tolerably
tightly fitting plug of cotton-wool.  To save the air-pump gauge from
the attack of such vapours as act on mercury, as also to facilitate
observation, a separate barometer tube was employed.

Through the cork which stops the flask F two glass tubes, a and b,
pass air-tight.  The tube a ends immediately under the cork; the tube
b, on the contrary, descends to the bottom of the flask and dips into
the liquid.  The end of the tube b is drawn out so as to render very
small the orifice through which the air escapes into the liquid.

The experimental tube s s' being exhausted, a cock at the end s' is
turned carefully on.  The air passes slowly through the cotton-wool,
the caustic potash, and the sulphuric acid in succession.  Thus
purified, it enters the flask F and bubbles through the liquid.
Charged with vapour, it finally passes into the experimental tube,
where it is submitted to examination.  The electric lamp L placed at
the end of the experimental tube furnishes the necessary beam.

*****

The facts here forced upon my attention had a bearing too evident to
be overlooked.  The inability of air which had been filtered through
cotton-wool to generate animalcular life, had been demonstrated by
Schroeder and Pasteur: here the cause of its impotence was rendered
evident to the eye.  The experiment proved that no sensible amount of
light was scattered by the molecules of the air; that the scattered
light always arose from suspended particles; and the fact that the
removal of these abolished simultaneously the power of scattering
light and of originating life, obviously detached the life-originating
power from the air, and fixed it on something suspended in the air.
Gases of all kinds passed with freedom through the plug of
cotton-wool; hence the thing whose removal by the cotton-wool rendered
the gas impotent, could not itself have been matter in the gaseous
condition.  It at once occurred to me that the retina, protected as it
was, in these experiments, from all extraneous light, might be
converted into a new and powerful instrument of demonstration in
relation to the germ theory.

But the observations also revealed the danger incurred in experiments
of this nature; showing that without an amount of care far beyond that
hitherto bestowed upon them, such experiments left the door open to
errors of the gravest description.  It was especially manifest that
the chemical method employed by Schultze in his experiments, and so
often resorted to since, might lead to the most erroneous
consequences; that neither acids nor alkalies had the power of rapid
destruction hitherto ascribed to them.  In short, the employment of
the luminous beam rendered evident the cause of success in experiments
rigidly conducted like those of Pasteur; while it made equally evident
the certainty of failure in experiments less severely carried out.

Dr. Bennett's Experiments.

But I do not wish to leave an assertion of this kind without
illustration.  Take, then', the well-conceived experiments of Dr.
Hughes Bennett, described before the Royal Society of Surgeons in
Edinburgh on January 17, 1868. [Footnote: 'British Medical Journal,'
13, pt. ii. 1868.]  Into flasks containing decoctions of
liquorice-root, hay, or tea, Dr. Bennett, by an ingenious method,
forced air.  The air was driven through two U-tubes, the one
containing a solution of caustic potash, the other sulphuric acid.
'All the bent tubes were filled with fragments of pumice-stone to
break up the air, so as to prevent the possibility of any germs
passing through in the centre of bubbles.'  The air also passed
through a Liebig's bulb containing sulphuric acid, and also through a
bulb containing gun-cotton.

It was only natural for Dr. Bennett to believe that his 'bent tubes'
entirely cut off the germs.  Previous to the observations just
referred to, I also believed in their efficacy.  But these
observations destroy any such notion.  The gun-cotton, moreover, will
fail to arrest the whole of the floating matter, unless it is tightly
packed, and there is no indication in Dr. Bennett's memoir that it was
so packed.  On the whole, I should infer, from the mere inspection of
Dr. Bennett's apparatus, the very results which he has described--a
retardation of the development of life, a total absence of it in some
cases, and its presence in others.

In his first series of experiments, eight flasks were fed with sifted
air, and five with common air.  In ten or twelve days all the five had
fungi in them; whilst it required from four to nine months to develop
fungi in the others.  In one of the eight, moreover, even after this
interval no fungi appeared.  In a second series of experiments there
was a similar exception.  In a third series the cork stoppers used in
the first and second series were abandoned, and glass stoppers
employed.  Flasks containing decoctions of tea, beef, and hay were
filled with common air, and other flasks with sifted air.  In every
one of the former fungi appeared and in not one of the latter.  These
experiments simply ruin the doctrine that Dr. Bennett finally
espouses.

In all these negative cases, the prepared air was forced into the
infusion when it was boiling hot.  Dr. Bennett made a fourth series of
experiments, in which, previous to forcing in the air, he permitted
the flasks to cool.  Into four bottles thus treated he forced prepared
air, and after a time found fungi in all of them.  What is his
conclusion?  Not that the boiling hot liquid, employed in his first
experiments, had destroyed such germs as had run the gauntlet of his
apparatus; but that air which, previous to being sealed up, had been
exposed to a temperature of 212°, _is too rare to support life_.  This
conclusion is so remarkable that it ought to be stated in Dr.
Bennett's own words.  'It may be easily conceived that air subjected
to a boiling temperature is so expanded as scarcely to merit the name
of air, and that it is more or less unfit for the purpose of
sustaining animal or vegetable life.'

Now numerical data are attainable here, and as a matter of fact I live
and flourish for a considerable portion of each year in a medium of
less density than that which Dr. Bennett describes as scarcely
meriting the name of air.  The inhabitants of the higher Alpine
chalets, with their flocks and herds, and the grasses which support
these, do the same; while the chamois rears its kids in air rarer
still.  Insect life, moreover, is sometimes exhibited with monstrous
prodigality at Alpine heights.

In a fifth series of experiments sixteen bottles were filled with
infusions.  Into four of them, while cold, ordinary unheated and
unsifted air was pumped.  In these four bottles fungi were developed.
Into four other bottles, containing a boiling infusion, ordinary air
was also pumped--no fungi were here developed.  Into four other
bottles containing an infusion which had been boiled and permitted to
cool, sifted air was pumped--no fungi were developed.  Finally, into
four bottles containing a boiling infusion sifted air was pumped no
fungi were developed.  Only, therefore, in the four cases where the
infusions were cold infusions, and the air ordinary air, did fungi
appear.

Dr. Bennett does not draw from his experiments the conclusion to which
they so obviously point.  On them, on the contrary, he founds a
defence of the doctrine of spontaneous generation, and a general
theory of spontaneous development.  So strongly was he impressed with
the idea that the germs could not possibly pass through his potash and
sulphuric acid tubes, that the appearance of fungi, even in a small
minority of cases, where the air had been sent through these tubes,
was to him conclusive evidence of the spontaneous origin of such
fungi.  And he accounts for the absence of life in many of his
experiments by an hypothesis which will not bear a moment's
examination.  But, knowing that organic particles may pass unscathed
through alkalies and acids, the results of Dr. Bennett are precisely
what ought wider the circumstances to be expected.  Indeed, their
harmony with the conditions now revealed is a proof of the honesty and
accuracy with which they were executed.

The caution exercised by Pasteur both in the execution of his
experiments, and in the reasoning based upon them, is perfectly
evident to those who, through the practice of severe experimental
enquiry, have rendered themselves competent to judge of good
experimental work.  He found germs in the mercury used to isolate his
air.  He was never sure that they did not cling to the instruments he
employed, or to his own person.  Thus when he opened his hermetically
sealed flasks upon the Mer de Glace, he had his eye upon the file used
to detach the drawn-out necks of his bottles; and he was careful to
stand to leeward when each flask was opened.  Using these precautions,
he found the glacier air incompetent, in nineteen cases out of twenty,
to generate life; while similar flasks, opened amid the vegetation of
the lowlands, were soon crowded with living things.  M. Pouchet
repeated Pasteur's experiments in the Pyrenees, adopting the
precaution of holding his flasks above his head, and obtaining a
different result.  Now great care would be needed to render this
procedure a real precaution.  The luminous beam at once shows us its
possible effect.  Let smoking brown paper be placed at the open mouth
of a glass shade, so that the smoke shall ascend and fill the shade. A
beam sent through the shade forms a bright track through the smoke.
When the closed fist is placed underneath the shade, a vertical wind
of surprising violence, considering the small elevation of
temperature, rises from the band, displacing by comparatively dark air
the illuminated smoke.  Unless special care were taken such a wind
would rise from M. Pouchet's body as he held his flasks above his
head, and thus the precaution of Pasteur, of not coming between the
wind and the flask, would be annulled.

Let me now direct attention to another result of Pasteur, the cause
and significance of which are at once revealed by the luminous beam.
He prepared twenty one flasks, each containing a decoction of yeast,
filtered and clear.  He boiled the decoction so as to destroy whatever
germs it might contain, and, while the space above the liquid was
filled with pure steam, he sealed his flasks with a blow-pipe.  He
opened ten of them in the deep, damp caves of the Paris Observatory,
and eleven of them in the courtyard of the establishment.  Of the
former, one only showed signs of life subsequently.  In nine out of
the ten flasks no organisms of any kind were developed.  In all the
others organisms speedily appeared.

Now here is an experiment conducted in Paris, on which we can throw
obvious light in London.  Causing our luminous beam to pass through a
large flask filled with the air of this room, and charged with its
germs and its dust, the beam is seen crossing the flask from side to
side.  But here is another similar flask, which cuts a clear gap out
of the beam.  It is filled with _unfiltered_ air, and still no trace of
the beam is visible.  Why?  By pure accident I stumbled on this flask
in our apparatus room, where it had remained quiet for some time.
Acting upon this obvious suggestion I set aside three other flasks,
filled, in the first instance, with mote-laden air.  They are now
optically empty.  Our former experiments proved that the
life-producing particles attach themselves to the fibres of
cotton-wool.  In the present experiment the motes have been brought by
gentle air-currents, established by slight differences of temperature
within our closed vessels, into contact with the interior surface, to
which they adhere.  The air of these flasks has deposited its dust,
germs and all, and is practically free from suspended matter.

I had a chamber erected, the lower half of which is of wood, its upper
half being enclosed by four glazed window-frames.  It tapers to a
truncated cone at the top.  It measures in plan 3 ft.  by 2 ft.  6 in,
and its height is 5 ft.  10 in.  On February 6 it was closed, every
crevice that could admit dust, or cause displacement of the air, being
carefully pasted over with paper.  The electric beam at first revealed
the dust within the chamber as it did in the air of the laboratory.
The chamber was examined almost daily; a perceptible diminution of the
floating matter being noticed as time advanced.  At the end of a week
the chamber was optically empty, exhibiting no trace of matter
competent to scatter the light.  Such must have been the case in the
stagnant caves of the Paris Observatory.  Were our electric beam sent
through the air of these caves its track would be invisible; thus
showing the indissoluble association of the scattering of light by air
and its power to generate life.

I will now turn to what seems to me a more interesting application of
the luminous beam than any hitherto described.  My reference to
Professor Lister's interpretation of the fact, that air which has
passed through the lungs cannot produce putrefaction, is fresh in your
memories.  'Why air,' said he, 'introduced into the pleural cavity,
through a wounded lung, should have such wholly different effects from
that entering through a permanently open wound, penetrating from
without, was to me a complete mystery, till I heard of the germ
theory of putrefaction, when it at once occurred to me that it was
only natural that the air should be filtered of germs by the air
passages, one of whose offices is to arrest inhaled particles of
dust, and prevent them from entering the air-cells.'

Here is a surmise which bears the stamp of genius, but which needs
verification.  If, for the words 'it is only natural' we were
authorised to write 'it is perfectly certain,' the demonstration would
be complete.  Such demonstration is furnished by experiments with a
beam of light.  One evening, towards the close of 1869, while pouring
various pure gases across the dusty track of a luminous beam, the
thought occurred to me of using my breath instead of the gases.  I
then noticed, for the first time, the extraordinary darkness produced
by the expired air, _towards the end of the expiration_.  Permit me to
repeat the experiment in your presence.  I fill my lungs with ordinary
air and breathe through a glass tube across the beam.  The
condensation of the aqueous vapour of the breath is shown by the
formation of a luminous white cloud of delicate texture.  We abolish
this cloud by drying the breath previous to its entering the beam; or,
still more simply, by warming the glass tube.  The luminous track of
the beam is for a time uninterrupted by the breath, because the dust
returning from the lungs makes good, in great part, the particles
displaced.  After a time, however, an obscure disk appears in the
beam, the darkness of which increases, until finally, towards the end
of the expiration, the beam is, as it were, pierced by an intensely
black hole, in which no particles whatever can be discerned.  The
deeper air of the lungs is thus proved to be absolutely free from
suspended matter.  It is therefore in the precise condition required
by Professor Lister's explanation.  This experiment may be repeated
any number of times with the same result.  I think it must be regarded
as a crowning piece of evidence both of the correctness of Professor
Lister's views and of the impotence, as regards vital development, of
optically pure air. [Footnote: Dr. Burden Sanderson draws attention to
the important observation of Brauell, which shows that the _contagium_
of a pregnant animal, suffering from splenic fever, is not found in
the blood of the foetus; the placental apparatus acting as a filter,
and holding back the infective particles.]

Application of Luminous beams to Water.

The method of examination here pursued is also applicable to water. It
is in some sense complementary to that of the microscope, and may, I
think, materially aid enquiries conducted with that instrument.  In
microscopic examination attention is directed to a small portion of
the liquid, and the aim is to detect the individual particles.  By the
present method a large portion of the liquid is illuminated, the
collective action of the particles being revealed, by the scattered
light.  Care is taken to defend the eye from the access of all other
light, and, thus defended, it becomes an organ of inconceivable
delicacy.  Indeed, an amount of impurity so infinitesimal as to be
scarcely expressible in numbers, and the individual particles of which
are so small as wholly to elude the microscope, may, when examined by
the method alluded to, produce not only sensible, but striking,
effects upon the eye.

We will apply the method, in the first place, to an experiment of M.
Pouchet intended to prove conclusively that animalcular life is
developed in cases where no antecedent germs could possibly exist. He
produced water from the combustion of hydrogen in air, justly arguing
that no germ could survive the heat of a hydrogen flame.  But he
overlooked the fact that his aqueous vapour was condensed in the air,
and was allowed as water to trickle through the air.  Indeed the
experiment is one of a number by which workers like M. Pouchet are
differentiated from workers like Pasteur.  I will show you some water,
produced by allowing a hydrogen flame to play upon a polished silver
condenser, formed by the bottom of a silver basin, containing ice. The
collected liquid is pellucid in the common light; but in the condensed
electric beam it is seen to be laden with particles, so thick-strewn
and minute as to produce a continuous luminous cone.  In passing
through the air the water loaded itself with this matter; and the
deportment of such water could obviously have no influence in deciding
this great question.

We are invaded with dirt not only in the air we breathe, but in the
water we drink.  To prove this I take the bottle of water intended to
quench your lecturer's thirst; which, in the track of the beam, simply
reveals itself as dirty water.  And this water is no worse than the
other London waters.  Thanks to the kindness of Professor Frankland, I
have been furnished with specimens of the water of eight London
companies.  They are all laden with impurities mechanically suspended.
But you will ask whether filtering will not remove the suspended
matter?  The grosser matter, undoubtedly, but not the more finely
divided matter.  Water may be passed any number of times through
bibulous paper, it will continue laden with fine matter.  Water passed
through Lipscomb's charcoal filter, or through the filters of the
Silicated Carbon Company, has its grosser matter removed, but it is
thick with fine matter.  Nine-tenths of the light scattered by these
suspended particles is perfectly polarised in a direction at right
angles to the beam, and this release of the particles from the
ordinary law of polarisation is a demonstration of their smallness.  I
should say by far the greater number of the particles concerned in
this scattering are wholly beyond the range of the microscope, and no
ordinary filter can intercept such particles.  It is next to
impossible, by artificial means, to produce a pure water.  Mr.
Hartley, for example, some time ago distilled water while surrounded
by hydrogen, but the water was not free from floating matter.  It is
so hard to be clean in the midst of dirt.  In water from the Lake of
Geneva, which has remained long without being stirred, we have an
approach to the pure liquid.  I have a bottle of it here, which was
carefully filled for me by my distinguished friend Soret.  The track
of the beam through it is of a delicate sky-blue; there is scarcely a
trace of grosser matter.

The purest water that I have seen--probably the purest which has been
seen hitherto--has been obtained from the fusion of selected specimens
of ice.  But extraordinary precautions are required to obtain this
degree of purity.  The following apparatus has been constructed for
this purpose: Through the plate of an air-pump passes the shank of a
large funnel, attached to which below the plate is a clean glass bulb.
In the funnel is placed a block of the most transparent ice, and over
the funnel a glass receiver.  This is first exhausted and refilled
several times with air, filtered by its passage through cotton-wool,
the ice being thus surrounded by pure moteless air.  But the ice has
previously been in contact with mote-filled air; it is therefore
necessary to let it wash its own surface, and also to wash the bulb
which is to receive the water of liquefaction.  The ice is permitted
to melt, the bulb is filled and emptied several times, until finally
the large block dwindles to a small one.  We may be sure that all
impurity has been thus removed from the surface of the ice. The water
obtained in this way is the purest hitherto obtained.  Still I should
hesitate to call it absolutely pure.  When condensed light is sent
through it, the track of the beam is not invisible, but of the most
exquisitely delicate blue.  This blue is purer than that of the sky,
so that the matter which produces it must be finer than that of the
sky.  It may be and indeed has been, contended that this blue is
scattered by the very molecules of the water, and not by matter
suspended in the water.  But when we remember that this perfection of
blue is approached gradually through stages of less perfect blue; and
when we consider that a blue in all respects similar is demonstrably
obtainable from particles mechanically suspended, we should hesitate,
I think, to conclude that we have arrived here at the last stage of
purification.  The evidence, I think, points distinctly to the
conclusion that, could we push the process of purification still
farther, even this last delicate trace of blue would disappear.

Chalk-water.  Clark's Softening Process.

But is it not possible to match the water of the Lake of Geneva here
in England?  Undoubtedly it is.  We have in England a kind of rock
which constitutes at once an exceedingly clean recipient and a natural
filter, and from which we can obtain water extremely free from
mechanical impurities.  I refer to the chalk formation, in which large
quantities of water are held in store.  Our chalk hills are in most
cases covered with thin layers of soil, and with very scanty
vegetation.  Neither opposes much obstacle to the entry of the rain
into the chalk, where any organic impurity which the water may carry
in is soon oxidised and rendered harmless.  Those who have scampered
like myself over the downs of Hants and Wilts will remember the
scarcity of water in these regions.  In fact, the rainfall, instead of
washing the surface and collecting in streams, sinks into the fissured
chalk and percolates through it.  When this formation is suitably
tapped, we obtain water of exceeding briskness and purity.  A large
glass globe, filled with the water of a well near Tring, shows itself
to be wonderfully free from mechanical impurity.  Indeed, it stands to
reason that water wholly withdrawn from surface contamination, and
percolating through so clean a substance, should be pure.  It has been
a subject much debated, whether the supply of excellent water which
the chalk holds in store could not be rendered available for London.
Many of the most eminent engineers and chemists have ardently
recommended this source, and have sought to show, not only that its
purity is unrivalled, but that its quantity is practically
inexhaustible.  Data sufficient to test this are now, I believe, in
existence; the number of wells sunk in the chalk being so
considerable, and the quantity of water which they yield so well
known.

But this water, so admirable as regards freedom from mechanical
impurity, labours under the disadvantage of being rendered very hard
by the carbonate of lime which it holds in solution.  The chalk-water
in the neighbourhood of Watford contains about seventeen grains of
carbonate of lime per gallon.  This, in the old terminology, used to
be called seventeen degrees of hardness.  This hard water is bad for
tea, bad for washing, and it furs our boilers, because the lime held
in solution is precipitated by boiling.  If the water be used cold,
its hardness must be neutralised at the expense of soap, before it
will give a lather.  These are serious objections to the use of
chalk-water in London.  But they are successfully met by the fact that
such water can be softened inexpensively, and on a grand scale.  I had
long known the method of softening water called Clark's process, but
not until recently, under the guidance of Mr. Homersham, did I see
proof of its larger applications.  The chalk-water is softened for the
supply of the city of Canterbury; and at the Chiltern Hills it is
softened for the supply of Tring and Aylesbury.  Caterham also enjoys
the luxury.

I have visited all these places, and made myself acquainted with the
works.  At Canterbury there are three reservoirs covered in and
protected, by a concrete roof and layers of pebbles, both from the
summer's heat and the winter's cold.  Each reservoir holds 120,000
gallons of water.  Adjacent to these reservoirs are others containing
pure slaked lime--the so-called 'cream of lime.' These being filled
with water, the lime and water are thoroughly mixed by air forced by
an engine through apertures in the bottom of the reservoir.  The water
soon dissolves all the lime it is capable of dissolving.  The
mechanically suspended lime is then allowed to subside to the bottom,
leaving a perfectly transparent lime-water behind.

The softening process is this: Into one of the empty reservoirs is
introduced a certain quantity of the clear lime-water, and after this
about nine times the quantity of the chalk-water.  The transparency
immediately disappears--the mixture of the two clear liquids becoming
thickly turbid, through the precipitation of carbonate of lime.  The
precipitate is crystalline and heavy, and in about twelve hours a
layer of pure white carbonate of lime is formed at the bottom of the
reservoir, with a water of extraordinary beauty and purity overhead. A
few days ago I pitched some halfpence into a reservoir sixteen feet
deep at the Chiltern Hills.  This depth hardly dimmed the coin.  Had I
cast in a pin, it could have been seen at the bottom.  By this process
of softening, the water is reduced from about seventeen degrees of
hardness, to three degrees of hardness.  It yields a lather
immediately.  Its temperature is constant throughout the year.  In the
hottest summer it is cool, its temperature being twenty degrees above
the freezing point; and it does not freeze in winter if conveyed in
proper pipes.  The reservoirs are covered; a leaf cannot blow into
them, and no surface contamination can reach the water.  It passes
direct from the main into the house tap; no cisterns are employed, and
the supply is always fresh and pure.  This is the kind of water which
is supplied to the fortunate people of Tring, Caterham, and
Canterbury.

*****

The foregoing article, as far as it relates to the theory which
ascribes epidemic disease to the development of low parasitic life
within the human life, was embodied in a discourse delivered before
the Royal Institution in January 1870.  In June 1871, after a brief
reference to the polarisation of light by cloudy matter, I ventured to
recur to the subject in these terms: What is the practical use of
these curiosities?  If we exclude the interest attached to the
observation of new facts, and the enhancement of that interest through
the knowledge that facts often become the exponents of laws, these
curiosities are in themselves worth little.  They will not enable us
to add to our stock of food, or drink, or clothes, or jewellery.  But
though thus shorn of all usefulness in themselves, they may, by
carrying thought into places which it would not otherwise have
entered, become the antecedents of practical consequences.  In
looking, for example, at our illuminated dust, we may ask ourselves
what it is.  How does it act, not upon a beam of light, but upon our
own bodies?  The question then assumes a practical character.  We find
on examination that this dust is mainly organic matter--in part
living, in part dead.  There are among it particles of ground straw,
torn rags, smoke, the pollen of flowers, the spores of fungi, and the
germs of other things.  But what have they to do with the animal
economy?  Let me give you an illustration to which my attention has
been lately drawn by Mr. George Henry Lewes, who writes to me thus:

'I wish to direct your attention to the experiments of von
Recklingshausen should you happen not to know them.  They are striking
confirmations of what you say of dust and disease.  Last spring, when
I was at his laboratory in Wuerzburg, I examined with him blood that
had been three weeks, a month, and five weeks, out of the body,
preserved in little porcelain cups under glass shades.  This blood was
living and growing.  Not only were the Amoeba-like movements of the
white corpuscles present, but there were abundant evidences of the
growth and development of the corpuscles.  (I also saw a frog's heart
still pulsating which had been removed from the body I forget how many
days, but certainly more than a week).  There were other examples of
the same persistent vitality, or absence of putrefaction.  Von
Recklingshausen did not attribute this to the absence of germs--germs
were not mentioned by him; but when I asked him how he represented the
thing to himself, he said the whole mystery of his operation consisted
in keeping the blood _free from dirt_.  The instruments employed were
raised to a red heat just before use; the thread was silver thread and
was similarly treated; and the porcelain cups, though not kept free
from air, were kept free from currents.  He said he often had
failures, and these he attributed to particles of dust having escaped
his precautions.'

Professor Lister, who has founded upon the removal or destruction of
this 'dirt' momentous improvements in surgery, tells us the effect of
its introduction into the blood of wounds.  The blood would putrefy
and become fetid; and when you examine more closely what putrefaction
means, you find the putrefying substance swarming with infusorial
life, the germs of which have been derived from the atmospheric dust.

We are now assuredly in the midst of practical matters; and with your
permission I will refer once more to a question which has recently
occupied a good deal of public attention.  As regards the lowest forms
of life, the world is divided, and has for a long time been divided,
into two parties, the one affirming that we have only to submit
absolutely dead matter to certain physical conditions, to evolve from
it living things; the other (without wishing to set bounds to the
power of matter) affirming that, in our day, life has never been found
to arise independently of pre-existing life.  I belong to the party
which claims life as a derivative of life.  The question has two
factors--the evidence, and the mind that judges of the evidence; and
it may be purely a mental set or bias on my part that causes me
throughout this long discussion, to see, on the one side, dubious
facts and defective logic, and on the other side firm reasoning and a
knowledge of what rigid experimental enquiry demands.  But, judged of
practically, what, again, has the question of Spontaneous Generation
to do with us?  Let us see.  There are numerous diseases of men and
animals that are demonstrably the products of parasitic life, and such
diseases may take the most terrible epidemic forms, as in the case of
the silkworms of France, referred to at an earlier part of this
article.  Now it is in the highest degree important to know whether
the parasites in question are spontaneously developed, or whether they
have been wafted from without to those afflicted with the disease. The
means of prevention, if not of cure, would be widely different in the
two cases.

But this is not all.  Besides these universally admitted cases, there
is the broad theory, now broached and daily growing in strength and
clearness--daily, indeed, gaining more and more of assent from the
most successful workers and profound thinkers of the medical
profession itself--the theory, namely, that contagious disease,
generally, is of this parasitic character.  Had I any cause to regret
having introduced this theory to your notice more than a year ago,
that regret should now be expressed.  I would certainly renounce in
your presence whatever leaning towards the germ theory my words might
then have betrayed.  But since the time referred to nothing has
occurred to shake my conviction of the truth of the theory.  Let me
briefly state the grounds on which its supporters rely.  From their
respective viruses you may plant typhoid fever, scarlatina, or
small-pox.  What is the crop that arises from this husbandry?  As
surely as a thistle rises from a thistle seed, as surely as the fig
comes from the fig, the grape from the grape, the thorn from the
thorn, so surely does the typhoid virus increase and multiply into
typhoid fever, the scarlatina virus into scarlatina, the small-pox
virus into small-pox.  What is the conclusion that suggests itself
here?  It is this: That the thing which we vaguely call a virus is to
all intents and purposes a seed.  Excluding the notion of vitality, in
the whole range of chemical science you cannot point to an action
which illustrates this perfect parallelism with the phenomena of
life--this demonstrated power of self-multiplication and reproduction.
The germ theory alone accounts for the phenomena.

In cases of epidemic disease, it is not on bad air or foul drains that
the attention of the physician of the future will primarily be fixed,
but upon disease germs, which no bad air or foul drains can create,
but which may be pushed by foul air into virulent energy of
reproduction.  You may think I am treading on dangerous ground, that I
am putting forth views that may interfere with salutary practice.  No
such thing.  If you wish to learn the impotence of medical practice in
dealing with contagious diseases, you have only to refer to the
Harveian oration for 1871, by Sir William Gull.  Such diseases defy
the physician.  They must run their course, and the utmost that can be
done for them is careful nursing.  And this, though I do not specially
insist upon it, would favour the idea of their vital origin.  For if
the seeds of contagious disease be themselves living things, it may be
difficult to destroy either them or their progeny, without involving
their living habitat in the same destruction.

It has been said, and it is sure to be repeated, that I am quitting my
own métier, in speaking of these things.  Not so.  I am dealing with
a question on which minds accustomed to weigh the value of
experimental evidence are alone competent to decide, and regarding
which, in its present condition, minds so trained are as capable of
forming an opinion as regarding the phenomena of magnetism or radiant
heat.  'The germ theory of disease,' it has been said, 'appertains to
the biologist and the physician.' Where, I would ask in reply, is the
biologist or physician, whose researches, in connection with this
subject, could for one instant be compared to those of the chemist
Pasteur?  It is not the philosophic members of the medical profession
who are dull to the reception of truth not originated within the pale
of the profession itself.  I cannot better conclude this portion of my
story than by reading to you an extract from a letter addressed to me
some time ago by Dr. William Budd, of Clifton, to whose insight and
energy the town of Bristol owes so much in the way of sanitary
improvement.

'As to the germ theory itself,' writes Dr. Budd, that is a matter on
which I have long since made up my mind.  From the day when I first
began to think of these subjects I have never had a doubt that the
specific cause of contagious fevers must be living organisms.

'It is impossible, in fact, to make any statement bearing upon the
essence or distinctive characters of these fevers, without using terms
which are of all others _the most distinctive of life_.  Take up the
writings of the most violent opponent of the germ theory, and, ten to
one, you will find them full of such terms as "propagation,"
"self-propagation," "reproduction," "self-multiplication," and so on.
Try as he may--if he has anything to say of those diseases which is
characteristic of them--he cannot evade the use of these terms, or the
exact equivalents to them.  While perfectly applicable to living
things, these terms express qualities which are not only inapplicable
to common chemical agents, but, as far as I can see, actually
inconceivable of them.'

Cotton-wool Respirator.

Once, then, established within the body, this evil form of life, if
you will allow me to call it so, must run its course.  Medicine as yet
is powerless to arrest its progress, and the great point to be aimed
at is to prevent its access to the body.  It was with this thought in
my mind that I ventured to recommend, more than a year ago, the use of
cotton-wool respirators in infectious places.  I would here repeat my
belief in their efficacy if properly constructed.  But I do not wish
to prejudice the use of these respirators, by connecting them
indissolubly with the germ theory.  There are too many trades in
England where life is shortened and rendered miserable by the
introduction of matters into the lungs which might be kept out of
them.  Dr. Greenhow has shown the stony grit deposited in the lungs of
stonecutters.  The black lungs of colliers is another case in point.
In fact, a hundred obvious cases might be cited, and others that are
not obvious might be added to them.  We should not, for example, think
that printing implied labour where the use of cotton-wool respirators
might come into play; but the fact is that the dust arising from the
sorting of the type is very destructive of health.  I went some time
ago into a manufactory in one of our large towns, where iron vessels
are enamelled by coating them with a mineral powder, and subjecting
them to a heat sufficient to fuse the powder.  The organisation of the
establishment was excellent, and one thing only was needed to make it
faultless.  In a large room a number of women were engaged covering
the vessels.  The air was laden with the fine dust, and their faces
appeared as white and bloodless as the powder with which they worked.
By the use of cotton-wool respirators these women might be caused to
breathe air as free from suspended matter as that of the open street.
Over a year ago a Lancashire seedsman wrote to me, stating that during
the seed season his men suffered horribly from irritation and fever,
so that many of them left his service.  He asked for help, and I gave
him my advice.  At the conclusion of the season, this year, he wrote
to inform me that he had folded a little cotton-wool in muslin, and
tied it in front of the mouth; and that with this simple defence he
had passed through the season in comfort, and without a single
complaint from his men.

Against the use of such a respirator the obvious objection arises,
that it becomes wet and heated by the breath.  While casting about for
a remedy for this, a friend forwarded to me from Newcastle a form of
respirator invented by Mr. Carrick, a hotel-keeper at Glasgow, which,
by a slight modification, may be caused to meet the case perfectly.
The respirator, with its back in part removed, is shown in fig. 4.
Under the partition of wire-gauze q r, is a space intended by Mr.
Carrick for 'medicated substances,' and which may be filled with
cotton-wool.  The mouth is placed against the aperture o, which fits
closely round the lips, and the filtered air enters the mouth through
a light valve v, which is lifted by the act of inhalation.

During exhalation this valve closes; the breath escapes by a second
valve, v', into the open air.  The wool is thus kept dry and cool; the
air in passing through it being filtered of everything it holds in
suspension.  The respirator has since taken other forms.

FIG.  4.

*****

Fireman's Respirator.

We have thus been led by our first unpractical experiments into a
thicket of practical considerations.  But another step is possible.
Admiring, as I do, the bravery of our firemen, and hearing that smoke
was a more serious enemy than flame itself, I thought of devising a
fireman's respirator.

Our fire-escapes are each in charge of a single man, and it would be
of obvious importance to place it in the power of each of those men to
penetrate through the densest smoke, into the recesses of a house, and
there to rescue those who would otherwise be suffocated or burnt.
Cotton-wool, which so effectually arrested dust, was first tried; but,
though found soothing in certain gentle kinds of smoke, it was no
match for the pungent fumes of a resinous fire.  For the purpose of
catching the atmospheric germs, M. Pouchet spread a film of glycerine
on a plate of glass, urged air against the film, and examined the dust
which stuck to it.  The moistening of the cotton-wool with glycerine
was a decided improvement; still the respirator only enabled us to
remain in dense smoke for three or four minutes, after which the
irritation became unendurable.  Reflection suggested that, besides the
smoke, there must be numerous hydrocarbons produced, which, being in a
state of vapour, would be very imperfectly arrested by the
cotton-wool.  These, in all probability, were the cause of the
residual irritation; and if these could be removed, a practically
perfect respirator might possibly be obtained.

I state the reasoning exactly as it occurred to my mind.  Its result
will be anticipated by many present.  All bodies possess the power of
condensing, in a greater or less degree, gases and vapours upon their
surfaces, and when the condensing body is very porous, or in a fine
state of division, the force of condensation may produce very
remarkable effects.  Thus, a clean piece of platinum-foil placed in a
mixture of oxygen and hydrogen so squeezes the gases together as to
cause them to combine; and if the experiment be made with care, the
heat of combination may raise the platinum to bright redness.  The
promptness of this action is greatly augmented by reducing the
platinum to a state of fine division.  A pellet of 'spongy platinum,'
for instance, plunged into a mixture of oxygen and hydrogen, causes
the gases to explode instantly.  In virtue of its extreme porosity, a
similar power is possessed by charcoal.  It is not strong enough to
cause the oxygen and hydrogen to combine like the spongy platinum, but
it so squeezes the more condensable vapours, and acts with such
condensing power upon the oxygen of the air, as to bring both within
the combining distance, thus enabling the oxygen to attack and destroy
the vapours in the pores of the charcoal.  In this way, effluvia of
all kinds may be virtually burnt up; and this is the principle of the
excellent charcoal respirators invented by Dr. Stenhouse.  Armed with
one of these, you may go into the foulest-smelling places without
having your nose offended.

But, while powerful to arrest vapours, the charcoal respirator is
ineffectual as regards smoke.  The smoke-particles get freely through
the respirator.  With a number of such respirators, tested in a proper
room, from half a minute to a minute was the limit of endurance.  This
might be exceeded by Faraday's simple method of emptying the lungs
completely, and then filling them before going into a smoky
atmosphere.  In fact, each solid smoke particle is itself a bit of
charcoal, and carries on it, and in it, its little load of irritating
vapour.  It is this, far more than the particles of carbon themselves,
that produces the irritation.  Hence two causes of offence are to be
removed: the carbon particles which convey the irritant by adhesion
and condensation, and the free vapour which accompanies the particles.
The cotton-wool moistened with glycerine I knew would arrest the
first; fragments of charcoal I hoped would stop the second.  In the
first fireman's respirator, Mr. Carrick's arrangement of two valves,
the one for inhalation, the other for exhalation, was preserved.  But
the portion of the respirator which holds the filtering and absorbent
substances, was prolonged to a depth of four or five inches (see fig.
5).  Under the partition of wire-gauze q r at the bottom of the space
which fronts the mouth was placed a layer of cotton-wool, c, moistened
with glycerine; then a thin layer of dry wool, c'; then a layer of
charcoal fragments; and finally a second thin layer of dry
cotton-wool.  The succession of the layers may be changed without
prejudice to the action.  A wire-gauze cover, shown in plan under fig.
5, keeps the substances from falling out of the respirator.  A layer
of caustic lime may be added for the absorption of carbonic acid; but
in the densest smoke that we have hitherto employed, it has not been
found necessary, nor is it shown in the figure.  In a flaming
building, indeed, the mixture of air with the smoke never permits the
carbonic acid to become so dense as to be irrespirable; but in a place
where the gas is present in undue quantity, the fragments of lime
would materially mitigate its action.

In a small cellar-like chamber with a stone flooring and stone walls,
the first experiments were made.  We Placed there furnaces containing
resinous pine-wood, lighted the wood, and, placing over it a lid which
prevented too brisk a circulation of the air, generated dense volumes
of smoke.  With our eyes protected by suitable glasses, my assistant
and I have remained for half an hour and more in smoke so dense and
pungent that a single inhalation, through the undefended mouth, would
be perfectly unendurable.  We might have prolonged our stay for hours.

FIG.  5.

Having thus far perfected the instrument, I wrote to the chief officer
of the Metropolitan Fire Brigade, asking him whether such a respirator
would be of use to him.  His reply was prompt; it would be most
valuable.  He had, however, made himself acquainted with every
contrivance of the kind in this and other countries, and had found
none of them of any practical use.  He offered to come and test it
here, or to place a room at my disposal in the City.  At my request he
came here, accompanied by three of his men.  Our small room was filled
with smoke to their entire satisfaction.  The three men went
successively into it, and remained there as long as Captain Shaw
wished them.  On coming out they said that they had not suffered the
slightest inconvenience; that they could have remained all day in the
smoke.  Captain Shaw then tested the respirator with the same result,
and he afterwards took great interest in the perfecting of the
instrument.

*****

Various ameliorations and improvements have recently been introduced
into the smoke respirator.  The hood of Captain Shaw has been improved
upon by the simple and less expensive mouthpiece of Mr. Sinclair; and
this, in its turn, has been simplified and improved by my assistant
Mr. John Cottrell.  The respirator is now in considerable demand, and
it has already done good practical service.  Care is, however,
necessary, in moistening the wool with glycerine.  It must be
carefully teazed, so that the individual fibres may be moistened, and
_clots_ must be avoided.  I cannot recommend the layers of moistened
flannel which, in some cases, have been used instead of cotton-wool:
nothing equals the wool, when carefully treated.

An experiment made last year brought out very conspicuously the
necessity of careful packing, and the enormous comparative power of
resisting smoke irritation possessed by our firemen, and the able
officer who commands them.  Having heard from Captain Shaw that, in
some recent very trying experiments, he had obtained the best effects
from dry cotton-wool, and thinking that I could not have been mistaken
in my first results, which proved the dry so much inferior to the
moistened wool and its associated charcoal, I proposed to Captain Shaw
to bring the matter to a test at his workshops in the City.  He was
good enough to accept my proposal, and thither I went on May 7, 1874.
The smoke was generated in a confined space from wet straw, and it was
certainly very diabolical.

At this season of the year I am usually somewhat shorn of vigour, and
therefore not in the best condition for severe experiments; still I
wished to test the matter in my own person.  With a respirator which
had been in use some days previously, and which was not carefully
packed, I followed a fireman into the smoke, he being provided with a
dry-wool respirator.  I was compelled to quit the place in about three
minutes, while the fireman remained there for six or seven minutes.

I then tried his respirator upon myself, and found that with it I
could not remain more than a minute in the smoke; in fact the first
inhalation provoked coughing.

Thinking that Captain Shaw himself might have lungs more like mine
than those of his fireman, I proposed that we should try the
respirators together; but he informed me that his lungs were very
strong.  He was, however, good enough to accede to my request. Before
entering the den a second time I repacked my respirator, with due
care, and entered the smoke in company with Captain Shaw.  I could
hear him breathe long slow inhalations; his labour was certainly
greater than mine, and after the lapse of seven minutes I heard him
cough.  In seven and a half minutes he had to quit the place, thus
proving that his lungs were able to endure the irritation seven times
as long as mine could bear it.  I continued in the smoke, with hardly
any discomfort, for sixteen minutes, and certainly could have remained
in it much longer.  The advantage arising from the glycerine was thus
placed beyond question.

During this time I was in a condition to render very material
assistance to a person in danger of suffocation.

Helmholtz on Hay Fever.

In my lecture on Dust and Disease in 1870, I referred to an experiment
made by Helmholtz upon himself which strikingly connected hay fever
with animalcular life.  About a year ago I received from Professor
Binz of Bonn a short, but important paper, embracing Helmholtz's
account of his observation, to which Professor Binz has added some
remarks of his own.  The paper, being mainly intended for English
medical men, was published in English, and though here and there its
style might be amended, I think it better to publish it unaltered.

From what I have observed (says Professor Binz) of recent English
publications on the subject of hay fever, I am led to suppose that
English authorities are inaccurately acquainted with the discovery of
Professor Helmholtz, as far back as 1868, of the existence of uncommon
low organisms in the nasal secretions in this complaint, and of the
possibility of arresting their action by the local employment of
quinine.  I therefore purpose to republish the letter in which he
originally announced these facts to myself, and to add some further
observations on this topic.  The letter is as follows: [Footnote:
Cf.  Virchow's 'Archiv.' vol. xlvi.]

'I have suffered, as well as I can remember, since the year 1847, from
the peculiar catarrh called by the English "hay fever," the speciality
of which consists in its attacking its victims regularly in the hay
season (myself-between May 20 and the end of June), that it ceases in
the cooler weather, but on the other hand quickly reaches a great
intensity if the patients expose themselves to heat and sunshine.  An
extraordinary violent sneezing then sets in, and a strongly corrosive
thin discharge, with which much epithelium is thrown off.  This
increases, after a few hours, to a painful inflammation of the mucous
membrane and of the outside of the nose, and excites fever with severe
headache and great depression, if the patient cannot withdraw himself
from the heat and the sunshine.  In a cool room, however, these
symptoms vanish as quickly as they come on, and there then only
remains for a few days a lessened discharge and soreness, as if caused
by the loss of epithelium.  I remark, by the way, that in all my other
years I had very little tendency to catarrh or catching cold, while
the hay fever has never failed during the twenty-one years of which I
have spoken, and has never attacked me earlier or later in the year
than the times named.  The condition is extremely troublesome, and
increases, if one is obliged to be much exposed to the sun, to an
excessively severe malady.

'The curious dependence of the disease on the season of the year
suggested to me the thought that organisms might be the origin of the
mischief.  In examining the secretion I regularly found, in the last
five years, certain vibrio-like bodies in it, which _at other times I
could not observe_ in my nasal secretion...  They are very
small, and can only be recognised with the immersion-lens of a very
good Hartnack's microscope.  It is characteristic of the common
isolated single joints that they contain four nuclei in a row, of
which two pairs are more closely united.  The length of the joints is
0.004 millimetre.  Upon the warm objective-stage they move with
moderate activity, partly in, mere vibration, partly shooting
backwards and forwards in the direction of their long axis; in lower
temperatures they are very inactive.  Occasionally one finds them
arranged in rows upon each other, or in branching series.  Observed
some days in the moist chamber, they vegetated again, and appeared
somewhat larger and more conspicuous than immediately after their
excretion.  It is to be noticed that only that kind of secretion
contains them which is expelled by violent sneezings; that which drops
slowly does not contain any.  They stick tenaciously enough in the
lower cavities and recesses of the nose.

'When I saw your first notice respecting the poisonous action of
quinine upon infusoria, I determined at once to make an experiment
with that substance, thinking that these vibrionic bodies, even if
they did not cause the whole illness, still could render it much more
unpleasant through their movements and the decompositions caused by
them.  For that reason I made a neutral solution of sulphate of
quinine, which did not contain much of the salt (1·800), but still was
effective enough, and caused moderate irritation on the mucus
membrane of the nose.  I then lay flat on my back, keeping my head
very low, and poured with a pipette about four cubic centimetres into
both nostrils.  Then I turned my head about in order to let the liquid
flow in all directions.

'The desired effect was obtained immediately, and remained for some
hours; I could expose myself to the sun without fits of sneezing and
the other disagreeable symptoms coming on.  It was sufficient to
repeat the treatment three times a day, even under the most
unfavourable circumstances, in order to keep myself quite free.
[Footnote: There is no foundation for the objection that syringing the
nose could not cure the asthma which accompanies hay fever; for this
asthma is only the reflex effect arising from the irritation of the
nose.--B.]  There were then no such vibrios in the secretion.  If I
only go out in the evening, it suffices to inject the quinine once a
day, just before going.  After continuing this treatment for some days
the symptoms disappear completely, but if I leave off they return till
towards the end of June.

'My first experiments with quinine date from the summer of 1867; this
year (1868) I began at once as soon as the first traces of the illness
appeared, and I have thus been able to stop its development
completely.

'I have hesitated as yet in publishing the matter, because I have
found no other patient [Footnote: Helmholtz, now Professor of
Physics at the University of Berlin, is, although M.D., no medical
practitioner.--B.] on whom I could try the experiment.  There is, it
seems to me, no doubt, considering the extraordinary regularity in the
recurrence and course of the illness, that quinine had here a most
quick and decided effect.  And this again makes my hypothesis very
probable, that the vibrios, although of no specific form but a very
frequent one, are at least the cause of the rapid increase of the
symptoms in warm air, as heat excites them to lively action.

I should be very glad if the above lines would induce medical men in
England--the haunt of hay fever--to test the observation of Helmholtz.
To most patients the application with the pipette may be too difficult
or impossible; I have therefore already suggested the use of Weber's
very simple but effective nose-douche.  Also it will be advisable to
apply the solution of quinine _tepid_.  It can, further, not be repeated
often enough that quinine is frequently adulterated, especially with
cinchona, the action of which is much less to be depended upon.

Dr. Frickhoefer, of Schwalbach, has communicated to me a second case
in which hay fever was cured by local application of quinine.
[Footnote: Cf.  Virchow's 'Archiv.' (1870), vol. li. p. 176.]
Professor Busch, of Bonn, authorises me to say that he succeeded in
two cases of 'catarrhus aestivus' by the same method: a third patient
was obliged to abstain from the use of quinine, as it produced an
unbearable irritation of the sensible nerves of the nose.  In the
autumn of 1872 Helmholtz told me that his fever was quite cured, and
that in the meantime two other patients had, by his advice, tried this
method, and with the same success. [Footnote: Prof. Helmholtz, whom I
had the pleasure of meeting in Switzerland last year, then told me
that he was quite convinced that hay fever was produced by the pollen
afloat in early summer in the atmosphere.]

********************

VI. VOYAGE TO ALGERIA TO OBSERVE THE ECLIPSE.

1870.

THE opening of the Eclipse Expedition was not propitious.  Portsmouth,
on Monday, December 5, 1870, was swathed by fog, which was intensified
by smoke, and traversed by a drizzle of fine rain.  At six P.M. I was
on board the "Urgent." On Tuesday morning the weather was too thick to
permit of the ship's being swung and her compasses calibrated.  The
Admiral of the port, a man of very noble presence, came on board.
Under his stimulus the energy which the weather had damped appeared to
become more active, and soon after his departure we steamed down to
Spithead.  Here the fog had so far lightened as to enable the officers
to swing the ship.

At three P.M. on Tuesday, December 6, we got away, gliding
successively past Whitecliff Bay, Bembridge, Sandown, Shanklin,
Ventnor, and St. Catherine's Lighthouse.  On Wednesday morning we
sighted the Isle of Ushant, on the French side of the Channel.  The
northern end of the island has been fretted by the waves into detached
tower-like masses of rock of very remarkable appearance.  In the
Channel the sea was green, and opposite Ushant it was a brighter
green.  On Wednesday evening we committed ourselves to the Bay of
Biscay.  The roll of the Atlantic was full, but not violent.  There
had been scarcely a gleam of sunshine throughout the day, but the
cloud-forms were fine, and their apparent solidity impressive.  On
Thursday morning the green of the sea was displaced by a deep indigo
blue.  The whole of Thursday we steamed across the bay.  We had little
blue sky, but the clouds were again grand and varied--cirrus, stratus,
cumulus, and nimbus, we had them all.  Dusky hair-like trails were
sometimes dropped from the distant clouds to the sea.

These were falling showers, and they sometimes occupied the whole
horizon, while we steamed across the rainless circle which was thus
surrounded.  Sometimes we plunged into the rain, and once or twice, by
slightly changing our course, avoided a heavy shower.  From time to
time perfect rainbows spanned the heavens from side to side.  At times
a bow would appear in fragments, showing the keystone of the arch
midway in air, and its two buttresses on the horizon.  In all cases
the light of the bow could be quenched by a Nicol's prism, with its
long diagonal tangent to the arc.  Sometimes gleaming patches of the
firmament were seen amid the clouds.  When viewed in the proper
direction, the gleam could be quenched by a Nicol's prism, a dark
aperture being thus opened into stellar space.

At sunset on Thursday the denser clouds were fiercely fringed, while
through the lighter ones seemed to issue the glow of a conflagration.
On Friday morning we sighted Cape Finisterre--the extreme end of the
arc which sweeps from Ushant round the Bay of Biscay.  Calm spaces of
blue, in which floated quietly scraps of cumuli, were behind us, but
in front of us was a horizon of portentous darkness.  It continued
thus threatening throughout the day.  Towards evening the wind
strengthened to a gale, and at dinner it was difficult to preserve the
plates and dishes from destruction.  Our thinned company hinted that
the rolling had other consequences.  It was very wild when we went to
bed.  I slumbered and slept, but after some time was rendered
anxiously conscious that my body had become a kind of projectile, with
the ship's side for a target.  I gripped the edge of my berth to save
myself from being thrown out.  Outside, I could hear somebody say that
he had been thrown from his berth, and sent spinning to the other side
of the saloon.  The screw laboured violently amid the lurching; it
incessantly quitted the water, and, twirling in the air, rattled
against its bearings, causing the ship to shudder from stem to stern.
At times the waves struck us, not with the soft impact which might be
expected from a liquid, but with the sudden solid shock of
battering-rams.  'No man knows the force of water,' said one of the
officers,' until he has experienced a storm at sea.' These blows
followed each other at quicker intervals, the screw rattling after
each of them, until, finally, the delivery of a heavier stroke than
ordinary seemed to reduce the saloon to chaos.  Furniture crashed,
glasses rang, and alarmed enquiries immediately followed.  Amid the
noises I heard one note of forced laughter; it sounded very ghastly.
Men tramped through the saloon, and busy voices were heard aft, as if
something there had gone wrong.

I rose, and not without difficulty got into my clothes.  In the
after-cabin, under the superintendence of the able and energetic
navigating lieutenant, Mr. Brown, a group of blue-jackets were working
at the tiller-ropes.  These had become loose, and the helm refused to
answer the wheel.  High moral lessons might be gained on shipboard, by
observing what steadfast adherence to an object can accomplish, and
what large effects are heaped up by the addition of infinitesimals.
The tiller-rope, as the blue-jackets strained in concert, seemed
hardly to move; still it did move a little, until finally, by timing
the pull to the lurching of the ship, the mastery of the rudder was
obtained.  I had previously gone on deck.  Round the saloon-door were
a few members of the eclipse party, who seemed in no mood for
scientific observation.  Nor did I; but I wished to see the storm.  I
climbed the steps to the poop, exchanged a word with Captain Toynbee,
the only member of the party to be seen on the poop, and by his
direction made towards a cleat not far from the wheel. [Footnote: The
cleat is a T-shaped mass of metal employed for the fastening of
ropes.] Round it I coiled my arms.  With the exception of the men at
the wheel, who stood as silent as corpses, I was alone.

I had seen grandeur elsewhere, but this was a new form of grandeur to
me.  The "Urgent" is long and narrow, and during our expedition she
lacked the steadying influence of sufficient ballast. She was for a
time practically rudderless, and lay in the trough of the sea.  I
could see the long ridges, with some hundreds of feet between their
crests, rolling upon the ship perfectly parallel to her sides.  As
they approached, they so grew upon the eye as to render the expression
'mountains high' intelligible.  At all events, there was no mistaking
their mechanical might, as they took the ship upon their shoulders,
and swung her like a pendulum.  The deck sloped sometimes at an angle
which I estimated at over forty-five degrees; wanting my previous
Alpine practice, I should have felt less confidence in my grip of the
cleat.  Here and there the long rollers were tossed by interference
into heaps of greater height.  The wind caught their crests, and
scattered them over the sea, the whole surface of which was seething
white.  The aspect of the clouds was a fit accompaniment to the fury
of the ocean.  The moon was almost full--at times concealed, at times
revealed, as the scud flew wildly over it.  These things appealed to
the eye, while the ear was filled by the groaning of the screw and the
whistle and boom of the storm.

Nor was the outward agitation the only object of interest to me.  I
was at once subject and object to myself, and watched with intense
interest the workings of my own mind.  The "Urgent" is an elderly
ship.  She had been built, I was told, by a contracting firm for some
foreign Government, and had been diverted from her first purpose when
converted into a troop-ship.  She had been for some time out of work,
and I had heard that one of her boilers, at least, needed repair.  Our
scanty but excellent crew, moreover, did not belong to the "Urgent,"
but had been gathered from other ships.  Our three lieutenants were
also volunteers.  All this passed swiftly through my mind as the
steamer shook under the blows of the waves, and I thought that
probably no one on board could say how much of this thumping and
straining the "Urgent" would be able to bear.  This uncertainty caused
me to look steadily at the worst, and I tried to strengthen myself in
the face of it.

But at length the helm laid hold of the water, and the ship was got
gradually round to face the waves.  The rolling diminished, a certain
amount of pitching taking its place.  Our speed had fallen from eleven
knots to two.  I went again to bed.  After a space of calm, when we
seemed crossing the vortex of a storm, heavy tossing recommenced.  I
was afraid to allow myself to fall asleep, as my berth was high, and
to be pitched out of it might be attended with bruises, if not with
fractures.  From Friday at noon to Saturday at noon we accomplished
sixty-six miles, or an average of less than three miles an hour.  I
overheard the sailors talking about this storm.  The "Urgent,"
according to those that knew her, had never previously experienced
anything like it. [Footnote: 'There is, it will be seen, a fair
agreement between these impressions and those so vigorously described
by a scientific correspondent of the 'Times.']

All through Saturday the wind, though somewhat sobered, blew dead
against us.  The atmospheric effects were exceedingly fine.  The
cumuli resembled mountains in shape, and their peaked summits
shone as white as Alpine snows.  At one place this resemblance was
greatly strengthened by a vast area of cloud, uniformly illuminated,
and lying like a _névé_ below the peaks.  From it fell a kind of
cloud-river strikingly like a glacier.  The horizon at sunset was
remarkable--spaces of brilliant green between clouds of fiery red.
Rainbows had been frequent throughout the day, and at night a
perfectly continuous lunar bow spanned the heavens from side to side.
Its colours were feeble; but, contrasted with the black ground against
which it rested, its luminousness was extraordinary.

Sunday morning found us opposite to Lisbon, and at midnight we rounded
Cape St. Vincent, where the lurching seemed disposed to recommence.
Through the kindness of Lieutenant Walton, a cot had been slung for
me.  It hung between a tiller-wheel and a flue, and at one A.M. I was
roused by the banging of the cot against its boundaries.  But the wind
was now behind us, and we went along at a speed of eleven knots.  We
felt certain of reaching Cadiz by three.  But a new lighthouse came in
sight, which some affirmed to be Cadiz Lighthouse, while the
surrounding houses were declared to be those of Cadiz itself. Out of
deference to these statements, the navigating lieutenant changed his
course, and steered for the place.  A pilot came on board, and he
informed us that we were before the mouth of the Guadalquivir, and
that the lighthouse was that of Cipiòna.  Cadiz was still some
eighteen miles distant.

We steered towards the city, hoping to get into the harbour before
dark.  But the pilot who would have guided us had been snapped up by
another vessel, and we did not get in.  We beat about during the
night, and in the morning found ourselves about fifteen miles from
Cadiz.  The sun rose behind the city, and we steered straight into the
light.  The three-towered cathedral stood in the midst, round which
swarmed apparently a multitude of chimney-stacks.  A nearer approach
showed the chimneys to be small turrets.  A pilot was taken on board;
for there is a dangerous shoal in the harbour.  The appearance of the
town as the sun shone upon its white and lofty walls was singularly
beautiful.  We cast anchor; some officials arrived and demanded a
clean bill of health.  We had none.  They would have nothing to do
with us; so the yellow quarantine flag was hoisted, and we waited for
permission to land the Cadiz party.  After some hours' delay the
English consul and vice-consul came on board, and with them a Spanish
officer ablaze with gold lace and decorations.  Under slight pressure
the requisite permission had been granted.  We landed our party, and
in the afternoon weighed anchor.  Thanks to the kindness of our
excellent paymaster, I was here transferred to a more roomy berth.

Cadiz soon sank beneath the sea, and we sighted in succession Cape
Trafalgar, Tarifa, and the revolving light of Ceuta.  The water was
very calm, and the moon rose in a quiet heaven.  She swung with her
convex surface downwards, the common boundary between light and shadow
being almost horizontal.  A pillar of reflected light shimmered up to
us from the slightly rippled sea.  I had previously noticed the
phosphorescence of the water, but tonight it was stronger than usual,
especially among the foam at the bows.  A bucket let down into the sea
brought up a number of the little sparkling organisms which caused the
phosphorescence.  I caught some of them in my hand.  And here an
appearance was observed which was new to most of us, and strikingly
beautiful to all.  Standing at the bow and looking forwards, at a
distance of forty or fifty yards from the ship, a number of luminous
streamers were seen rushing towards us.  On nearing the vessel they
rapidly turned, like a comet round its perihelion, placed themselves
side by side, and, in parallel trails of light, kept up with the ship.
One of them placed itself right in front of the bow as a pioneer.
These comets of the sea were joined at intervals by others.  Sometimes
as many as six at a time would rush at us, bend with extraordinary
rapidity round a sharp curve, and afterwards keep us company.  I
leaned over the bow, and scanned the streamers closely.  The frontal
portion of each of them revealed the outline of a porpoise.  The rush
of the creatures through the water had started the phosphorescence,
every spark of which was converted by the motion of the retina into a
line of light.  Each porpoise was thus wrapped in a luminous sheath.
The phosphorescence did not cease at the creature's tail, but was
carried many porpoise-lengths behind it.

To our right we had the African hills, illuminated by the moon.
Gibraltar Rock at length became visible, but the town remained long
hidden by a belt of haze, through which at length the brighter lamps
struggled.  It was like the gradual resolution of a nebula into stars.
As the intervening depth became gradually less, the mist vanished more
and more, and finally all the lamps shone through it They formed a
bright foil to the sombre mass of rock above them.  The sea was so
calm and the scene so lovely that Mr. Huggins and myself stayed on
deck till near midnight, when the ship was moored.  During our walking
to and fro a striking enlargement of the disk of Jupiter was
observed, whenever the heated air of the funnel came between us and
the planet.  On passing away from the heated air, the flat dim disk
would immediately shrink to a luminous point.  The effect was one of
visual persistence.  The retinal image of the planet was set quivering
in all azimuths by the streams of heated air, describing in quick
succession minute lines of light, which summed themselves to a disk of
sensible area.

At six o'clock next morning, the gun at the Signal Station on the
summit of the rock, boomed.  At eight the band on board the
'Trafalgar' training-ship, which was in the harbour, struck up the
national anthem; and immediately afterwards a crowd of mite-like
cadets swarmed up the rigging.  After the removal of the apparatus
belonging to the Gibraltar party we went on shore.  Winter was in
England when we left, but here we had the warmth of summer.  The
vegetation was luxuriant--palm-trees, cactuses, and aloes, all ablaze
with scarlet flowers.  A visit to the Governor was proposed, as an act
of necessary courtesy, and I accompanied Admiral Ommaney and Mr.
Huggins to 'the Convent,' or Government House.  We sent in our cards,
waited for a time, and were then conducted by an orderly to his
Excellency.  He is a fine old man, over six feet high, and of frank
military bearing.  He received us and conversed with us in a very
genial manner.  He took us to see his garden, his palms, his shaded
promenades, and his orange-trees loaded with fruit, in all of which he
took manifest delight.  Evidently 'the hero of Kars' had fallen upon
quarters after his own heart.  He appeared full of good nature, and
engaged us on the spot to dine with him that day.

We sought the town-major for a pass to visit the lines.  While
awaiting his arrival I purchased a stock of white glass bottles, with
a view to experiments on the colour of the sea.  Mr. Huggins and
myself, who wished to see the rock, were taken by Captain Salmond to
the library, where a model of Gibraltar is kept, and where we had a
useful preliminary lesson.  At the library we met Colonel Maberly, a
courteous and kindly man, who gave us good advice regarding our
excursion.  He sent an orderly with us to the entrance of the lines.
The orderly handed us over to an intelligent Irishman, who was
directed to show us everything that we desired to see, and to hide
nothing from us.  We took the 'upper line,' traversed the galleries
hewn through the limestone; looked through the embrasures, which
opened like doors in the precipice, towards the hills of Spain;
reached St. George's hall, and went still higher, emerging on the
summit of one of the noblest cliffs I have ever seen.

Beyond were the Spanish lines, marked by a line of white sentry-boxes;
nearer were the English lines, less conspicuously indicated; and
between both was the neutral ground.  Behind the Spanish lines rose
the conical hill called the Queen of Spain's Chair.  The general
aspect of the mainland from the rock is bold and rugged.  Doubling
back from the galleries, we struck upwards towards the crest, reached
the Signal Station, where we indulged in 'shandy-gaff' and bread and
cheese.  Thence to O'Hara's Tower, the highest point of the rock.  It
was built by a former Governor, who, forgetful of the laws of
terrestrial curvature, thought he might look from the tower into-the
port of Cadiz.  The tower is riven, and it may be climbed along the
edges of the crack.  We got to the top of it; thence descended the
curious Mediterranean Stair--a zigzag, mostly of steps down a steeply
falling slope, amid palmetto brush, aloes, and prickly pear.

Passing over the Windmill Hill, we were joined at the 'Governor's
Cottage' by a car, and drove afterwards to the lighthouse at Europa
Point.  The tower was built, I believe, by Queen Adelaide, and it
contains a fine dioptric apparatus of the first order, constructed by
Messrs.  Chance, of Birmingham.  At the appointed hour we were at the
Convent.  During dinner the same genial traits which appeared in the
morning were still more conspicuous.  The freshness of the Governor's
nature showed itself best when he spoke of his old antagonist in arms,
Mouravieff.  Chivalry in war is consistent with its stern prosecution.
These two men were chivalrous, and after striking the last blow became
friends for ever.  Our kind and courteous reception at Gibraltar is a
thing to be remembered with pleasure.

On December 15 we committed ourselves to the Mediterranean.  The views
of Gibraltar with which we are most acquainted represent it as a huge
ridge; but its aspect, end on, both from the Spanish lines and from
the other side, is truly noble.  There is a sloping bank of sand at
the back of the rock, which I was disposed to regard simply as the
_débris_ of the limestone.  I wished to let myself down upon it, but had
not the time.  My friend Mr. Busk, however, assures me that it is
silica, and that the same sand constitutes the adjacent neutral
ground.  There are theories afloat as to its having been blown from
Sahara.  The Mediterranean throughout this first day, and indeed
throughout the entire voyage to Oran, was of a less deep blue than the
Atlantic.  Possibly the quantity of organisms may have modified the
colour.  At night the phosphorescence was startling, breaking suddenly
out along the crests of the waves formed by the port and starboard
bows.  Its strength was not uniform.  Having flashed brilliantly for a
time, it would in part subside, and afterwards regain its vigour.
Several large phosphorescent masses of weird appearance also floated
past.

On the morning of the 16th we sighted the fort and lighthouse of Marsa
el Kibir, and beyond them the white walls of Oran lying in the bight
of a bay, sheltered by dominant hills.  The sun was shining brightly;
during our whole voyage we had not had so fine a day.  The wisdom
which had led us to choose Oran as our place of observation seemed
demonstrated.  A rather excitable pilot came on board, and he guided
us in behind the Mole, which had suffered much damage the previous
year from an unexplained outburst of waves from the Mediterranean.
Both port and bow anchors were cast in deep water.  With three huge
hawsers the ship's stem was made fast to three gun-pillars fixed in
the Mole; and here for a time the "Urgent" rested from her labours.

M. Janssen, who had rendered his name celebrated by his observations
of the eclipse in India in 1868, when he showed the solar flames to be
eruptions of incandescent hydrogen, was already encamped in the open
country about eight miles from Oran.  On December 2 he had quitted
Paris in a balloon, with a strong young sailor as his assistant, had
descended near the mouth of the Loire, seen M. Gambetta, and received
from him encouragement and aid.  On the day of our arrival his
encampment was visited by Mr. Huggins, and the kind and courteous
Engineer of the Port drove me subsequently, in his own phaeton, to the
place.  It bore the best repute as regards freedom from haze and fog,
and commanded an open outlook; but it was inconvenient for us on
account of its distance from the ship.  The place next in repute was
the railway station, between two and three miles distant from the
Mole.  It was inspected, but, being enclosed, was abandoned for an
eminence in an adjacent garden, the property of Mr. Hinshelwood, a
Scotchman who had settled some years previously as an Esparto merchant
in Oran. [Footnote: Esparto is a kind of grass now much used in the
manufacture of paper.] He, in the most liberal manner, placed his
ground at the disposition of the party.  Here the tents were pitched,
on the Saturday, by Captain Salmond and his intelligent corps of
sappers, the instruments being erected on the Monday under cover of
the tents.

Close to the railway station runs a new loopholed wall of defence,
through which the highway passes into the open country.  Standing on
the highway, and looking southwards, about twenty yards to the right
is a small bastionet, intended to carry a gun or two.  Its roof I
thought would form an admirable basis for my telescope, while the view
of the surrounding country was unimpeded in all directions.  The
authorities kindly allowed me the use of this bastionet.  Two men, one
a blue-jacket named Elliot, and the other a marine named Hill, were
placed at my disposal by Lieutenant Walton; and, thus aided, on Monday
morning I mounted my telescope.  The instrument was new to me, and
some hours of discipline were spent in mastering all the details of
its manipulation.

Mr. Huggins joined me, and we visited together the Arab quarter of
Oran.  The flat-roofed houses appeared very clean and white.  The
street was filled with loiterers, and the thresholds were occupied by
picturesque groups.  Some of the men were very fine.  We saw many
straight, manly fellows who must have been six feet four in height.
They passed us with perfect indifference, evincing no anger,
suspicion, or curiosity, hardly caring in fact to glance at us as we
passed.  In one instance only during my stay at Oran was I spoken to
by an Arab.  He was a tall, good-humoured fellow, who came smiling up
to me, and muttered something about 'les Anglais.' The mixed
population of Oran is picturesque in the highest degree: the Jews,
rich and poor, varying in their costumes as their wealth varies; the
Arabs more picturesque still, and of all shades of complexion--the
negroes, the Spaniards, the French, all grouped together, each race
preserving its own individuality, formed a picture intensely
interesting to me.

On Tuesday, the 20th, I was early at the bastionet.  The night had
been very squally.  The sergeant of the sappers had taken charge of
our key, and on Tuesday morning Elliot went for it.  He brought back
the intelligence that the tents had been blown down, and the
instruments overturned.  Among these was a large and valuable
equatorial from the Royal Observatory, Greenwich.  It seemed hardly
possible that this instrument, with its wheels and verniers and
delicate adjustments, could have escaped uninjured from such a fall.
This, however, was the case; and during the day all the overturned
instruments were restored to their places, and found to be in
practical working order.  This and the following day were devoted to
incessant schooling.  I had come out as a general stargazer, and not
with the intention of devoting myself to the observation of any
particular phenomenon.  I wished to see the whole--the first contact,
the advance of the moon, the successive swallowing up of the solar
spots, the breaking of the last line of crescent by the lunar
mountains into Bailey's beads, the advance of the shadow through the
air, the appearance of the corona and prominences at the moment of
totality, the radiant streamer; of the corona, the internal structure
of the flames, a glance through a polariscope, a sweep round the
landscape with the naked eye, the reappearance of the soar limb
through Bailey's beads, and, finally, the retreat of the lunar shadow
through the air.

I was provided with a telescope of admirable definition, mounted,
adjusted, packed, and most liberally placed at my disposal by Mr.
Warren De La Rue.  The telescope grasped the whole of the sun, and a
considerable portion of the space surrounding it.  But it would not
take in the extreme limits of the corona.  For this I had lashed on to
the large telescope a light but powerful instrument, constructed by
Ross, and lent to me by Mr. Huggins.  I was also furnished with an
excellent binocular by Mr. Dallmeyer.  In fact, no man could have been
more efficiently supported.

It required a strict parcelling out of the interval of totality to
embrace in it the entire series of observations.  These, while the sun
remained visible, were to be made with an unsilvered diagonal
eye-piece, which reflected but a small fraction of the sun's light,
this fraction, being still further toned down by a dark glass.  At the
moment of totality the dark glass was to be removed, and a silver
reflector pushed in, so as to get the maximum of light from the corona
and prominences The time of totality was distributed as follows:

1.  Observe approach of shadow through the air: totality.

2.  Telescope              30 seconds.

3.  Finder                 30 seconds.

4.  Double image prism     15 seconds.

5.  Naked eye              10 seconds.

6.  Finder or binocular    20 seconds.

7.  Telescope              20 seconds.

8.  Observe retreat of shadow.

In our rehearsals Elliot stood beside me, watch in hand, and
furnished with a lantern.  He called out at the end of each interval,
while I moved from telescope to finder, from finder to polariscope,
from polariscope to naked eye, from naked eye back to finder, from
finder to telescope, abandoning the instrument finally to observe the
retreating shadow.  All this we went over twenty times, while looking
at the actual sun, and keeping him in the middle of the field.  It was
my object to render the repetition of the lesson so mechanical as to
leave no room for flurry, forgetfulness, or excitement.  Volition was
not to be called upon, nor judgment exercised, but a well-beaten path
of routine was to be followed.  Had the opportunity occurred, I think
the programme would have been strictly carried out.

But the opportunity did not occur.  For several days the weather had
been ill-natured.  We had wind so strong As to render the hawsers at
the stern of the "Urgent" as rigid as iron, and to destroy the
navigating lieutenant's sleep.  We had clouds, a thunder-storm, and
some rain.  Still the hope was held out that the atmosphere would
cleanse itself, and if it did we were promised air of extraordinary
limpidity.  Early on the 22nd we were all at our posts.  Spaces of
blue in the early morning gave us some encouragement, but all depended
on the relation of these spaces to the surrounding clouds. Which of
them were to grow as the day advanced?  The wind was high, and to
secure the steadiness of my instrument I was forced to retreat behind
a projection of the bastionet, place stones upon its stand, and,
further, to avail myself of the shelter of a sail.  My practised men
fastened the sail at the top, and loaded it with boulders at the
bottom.  It was tried severely, but it stood firm.

The clouds and blue spaces fought for a time with varying success. The
sun was bidden and revealed at intervals, hope oscillating in
synchronism with the changes of the sky.  At the moment of first
contact a dense cloud intervened; but a minute or two afterwards the
cloud had passed, and the encroachment of the black body of the moon
was evident upon the solar disk.  The moon marched onward, and I saw
it at frequent intervals; a large group of spots were approached and
swallowed up.  Subsequently I caught sight of the lunar limb as it cut
through the middle of a large spot.  The spot was not to be
distinguished from the moon, but rose like a mountain above it.  The
clouds, when thin, could be seen as grey scud drifting across the
black surface of the moon; but they thickened more and more, and made
the intervals of clearness scantier.  During these moments I watched
with an interest bordering upon fascination the march of the silver
sickle of the sun across the field of the telescope.  It was so sharp
and so beautiful.  No trace of the lunar limb could be observed beyond
the sun's boundary.  Here, indeed, it could only be relieved by the
corona, which was utterly cut off by the dark glass.  The blackness of
the moon beyond the sun was, in fact, confounded with the blackness of
space.

Beside me was Elliot with the watch and lantern, while Lieutenant
Archer, of the Royal Engineers, had the kindness to take charge of my
note-book.  I mentioned, and he wrote rapidly down, such things as
seemed worthy of remembrance.  Thus my hands and mind were entirely
free; but it was all to no purpose.  A patch of sunlight fell and
rested upon the landscape some miles away.  It was the only
illuminated spot within view.  But to the north-west there was still a
space of blue which might reach us in time.  Within seven minutes of
totality another space towards the zenith became very dark.  The
atmosphere was, as it were, on the brink of a precipice, being charged
with humidity, which required but a slight chill to bring it down in
clouds.  This was furnished by the withdrawal of the solar beams: the
clouds did come down, covering up the space of blue on which our hopes
had so long rested.  I abandoned the telescope and walked to and fro
in despair.  As the moment of totality approached, the descent towards
darkness was as obvious as a falling stone.  I looked towards a
distant ridge, where the darkness would first appear.  At the moment a
fan of beams, issuing from the hidden sun, was spread out over the
southern heavens.  These beams are bars of alternate light and shade,
produced in illuminated haze by the shadows of floating cloudlets of
varying density.  The beams are practically parallel, but by an effect
of perspective they appear divergent, having the sun, in fact, for
their point of convergence.  The darkness took possession of the ridge
referred to, lowered upon M. Janssen's observatory, passed over the
southern heavens, blotting out the beams as if a sponge had been drawn
across them.  It then took successive possession of three spaces of
blue sky in the south-eastern atmosphere.  I again looked towards the
ridge.  A glimmer as of day-dawn was behind it, and immediately
afterwards the fan of beams, which had been for more than two minutes
absent, revived.  The eclipse of 1870 had ended, and, as far as the
corona and flames were concerned, we had been defeated.

Even in the heart of the eclipse the darkness was by no means perfect.
Small print could be read.  In fact, the clouds which rendered the day
a dark one, by scattering light into the shadow, rendered the darkness
less intense than it would have been had the atmosphere been without
cloud.  In the more open spaces I sought for stars, but could find
none.  There was a lull in the wind before and after totality, but
during the totality the wind was strong.  I waited for some time on
the bastionet, hoping to get a glimpse of the moon on the opposite
border of the sun, but in vain.  The clouds continued, and some rain
fell.  The day brightened somewhat afterwards, and, having packed all
up, in the sober twilight Mr. Crookes and myself climbed the heights
above the fort of Vera Cruz.  From this eminence we had a very noble
view over the Mediterranean and the flanking African hills.  The
sunset was remarkable, and the whole outlook exceedingly fine.

The able and well-instructed medical officer of the "Urgent," Mr.
Goodman, observed the following temperatures during the progress of
the eclipse:

Hour         Deg.

11.45        56

11.55        55

12.10        54

12.37        53

12.39        52

12.43        51

 1.5         52

  1.27       53

  1.44       56

  2.10       57

The minimum temperature occurred some minutes after totality, when a
slight rain fell.

The wind was so strong on the 23rd that Captain Henderson would not
venture out.  Guided by Mr. Goodman, I visited a cave in a remarkable
stratum of shell-breccia, and, thanks to my guide, secured specimens.
Mr. Busk informs me that a precisely similar breccia, is found at
Gibraltar, at approximately the same level.  During the afternoon,
Admiral Ommaney and myself drove to the fort of Marsa el Kibir.  The
fortification is of ancient origin, the Moorish arches being still
there in decay, but the fort is now very strong.  About four or five
hundred fine-looking dragoons were looking after their horses, waiting
for a lull to enable them to embark for France.  One of their officers
was wandering in a very solitary fashion over the fort.  We had some
conversation with him.  He had been at Sedan, had been taken prisoner,
but had effected his escape.  He shook his head when we spoke of the
termination of the war, and predicted its long continuance.  There was
bitterness in his tone as he spoke of the charges of treason so
lightly levelled against French commanders.

The green waves raved round the promontory on which the fort stands,
smiting the rocks, breaking into foam, and jumping, after impact, to a
height of a hundred feet and more into the air.  As we returned our
vehicle broke down through the loss of a wheel.  The Admiral went on
board, while I remained long watching the agitated sea.  The little
horses of Oran well merit a passing word.  Their speed and endurance,
both of which are heavily drawn upon by their drivers, are
extraordinary.

The wind sinking, we lifted anchor on the 24th.  For some hours we
went pleasantly along; but during the afternoon the storm revived, and
it blew heavily against us all the night.  When we came opposite the
Bay of Almeria, on the 25th, the captain turned the ship, and steered
into the bay, where, under the shadow of the Sierra Nevada, we passed
Christmas night in peace.  Next morning 'a rose of dawn' rested on the
snows of the adjacent mountains, while a purple haze was spread over
the lower hills.  I had no notion that Spain possessed so fine a range
of mountains as the Sierra Nevada.  The height is considerable, but
the form also is such as to get the maximum of grandeur out of the
height.  We weighed anchor at eight A.M., passing for a time through
shoal water, the bottom having been evidently stirred up.  The
adjacent land seemed eroded in a remarkable manner.  It has its
floods, which excavate these valleys and ravines, and leave those
singular ridges behind.  Towards evening I climbed the mainmast, and,
standing on the cross-trees, saw the sun set amid a blaze of fiery
clouds.  The wind was strong and bitterly cold, and I was glad to
slide back to the deck along a rope, which stretched from the
mast-head to the ship's side.  That night we cast anchor beside the
Mole of Gibraltar.

On the morning of the 27th, in company with two friends, I drove to
the Spanish lines, with the view of seeing the rock from that side. It
is an exceedingly noble mass.  The Peninsular and Oriental mail-boat
had been signalled and had come.  Heavy duties called me homeward, and
by transferring myself from the "Urgent" to the mail-steamer I should
gain three days.  I hired a boat, rowed to the steamer, learned that
she was to start at one, and returned with all speed to the "Urgent."
Making known to Captain Henderson my wish to get away, he expressed
doubts as to the possibility of reaching the mail-steamer in time.
With his accustomed kindness, he however placed a boat at my disposal.
Four hardy fellows and one of the ship's officers jumped into it; my
luggage, hastily thrown together, was tumbled in, and we were
immediately on our way.  We had nearly four miles to row in about
twenty minutes; but we hoped the mail-boat might not be punctual.  For
a time we watched her anxiously; there was no motion; we came nearer,
but the flags were not yet hauled in.  The men put forth all their
strength, animated by the exhortations of the officer at the helm.
The roughness of the sea rendered their efforts to some extent
nugatory: still we were rapidly approaching the steamer.  At length
she moved, punctual almost to the minute, at first slowly, but soon
with quickened pace.

We turned to the left, so as to cut across her bows.  Five minutes'
pull would have brought us up to her.  The officer waved his cap and I
my hat.  'If they could only see us, they might back to us in a
moment.' But they did not see us, or if they did, they paid us no
attention.  I returned to the "Urgent," discomfited, but grateful to
the fine fellows who had wrought so hard to carry out my wishes.

Glad of the quiet, in the sober afternoon I took a walk towards Europa
Point.  The sky darkened and heavy squalls passed at intervals.
Private theatricals were at the Convent, and the kind and courteous
Governor had sent cards to the eclipse party.  I failed in my duty in
not going.  St. Michael's Cave is said to rival, if it does not
outrival, the Mammoth Cave of Kentucky.  On the 28th Mr. Crookes, Mr.
Carpenter, and myself, guided by a military policeman who understood
his work, explored the cavern.  The mouth is about 1,100 feet above
the sea.  We zigzagged up to it, and first were led into an aperture
in the rock, at some height above the true entrance of the cave.  In
this upper cavern we saw some tall and beautiful stalactite pillars.

The water drips from the roof charged with bicarbonate of lime.
Exposed to the air, the carbonic acid partially escapes, and the
simple carbonate of lime, which is hardly at all soluble in water,
deposits itself as a solid, forming stalactites and stalagmites.  Even
the exposure of chalk or limestone water to the open air partially
softens it.  A specimen of the Redbourne water exposed by Professors
Graham, Miller, and Hofmann, in a shallow basin, fell from eighteen
degrees to nine degrees of hardness.  The softening process of Clark
is virtually a hastening of the natural process.  Here, however,
instead of being permitted to evaporate, half the carbonic acid is
appropriated by lime, the half thus taken up, as well as the remaining
half, being precipitated.  The solid precipitate is permitted to sink,
and the clear supernatant liquid is limpid soft water.

We returned to the real mouth of St. Michael's Cave, which is entered
by a wicket.  The floor was somewhat muddy, and the roof and walls
were wet.  We soon found ourselves in the midst of a natural temple,
where tall columns sprang complete from floor to roof, while incipient
columns were growing to meet each other, upwards and downwards.  The
water which trickles from the stalactite, after having in part yielded
up its carbonate of lime, falls upon the floor vertically underneath,
and there builds the stalagmite.  Consequently, the pillars grow from
above and below simultaneously, along the same vertical.  It is easy
to distinguish the stalagmitic from the stalactitic portion of the
pillars.  The former is always divided into short segments by
protuberant rings, as if deposited periodically, while the latter
presents a uniform surface.  In some cases the points of inverted
cones of stalactite rested on the centres of pillars of stalagmite.
The process of solidification and the consequent architecture were
alike beautiful.

We followed our guide through various branches and arms of the cave,
climbed and descended steps, halted at the edges of dark shafts and
apertures, and squeezed ourselves through narrow passages.  From time
to time we halted, while Mr. Crookes illuminated with ignited
magnesium wire, the roof, columns, dependent spears, and graceful
drapery of the stalactites.  Once, coming to a magnificent cluster of
icicle-like spears, we helped ourselves to specimens.  There was some
difficulty in detaching the more delicate ones, their fragility was so
great.  A consciousness of vandalism, which smote me at the time,
haunts me still; for, though our requisitions were moderate, this
beauty ought not to be at all invaded.  Pendent from the roof, in
their natural habitat, nothing can exceed their delicate beauty; they
_live_, as it were, surrounded by organic connections.  In London they
are curious, but not beautiful.  Of gathered shells Emerson writes:

   I wiped away the weeds and foam,
   And brought my sea-born treasures home
   But the poor, unsightly, noisome things
   Had left their beauty on the shore,
   With the sun, and the sand, and the wild uproar.

The promontory of Gibraltar is so burrowed with caverns that it has
been called the Hill of Caves.  They are apparently related to the
geologic disturbances which the rock has undergone.  The earliest of
these is the tilting of the once horizontal strata.  Suppose a force
of torsion to act upon the promontory at its southern extremity near
Europa Point, and suppose the rock to be of a partially yielding
character; such a force would twist the strata into screw-surfaces,
the greatest amount of twisting being endured near the point of
application of the force.  Such a twisting the rock appears to have
suffered; but instead of the twist fading gradually and uniformly off,
in passing from south to north, the want of uniformity in the material
has produced lines of dislocation where there are abrupt changes in
the amount of twist. Thus, at the northern end of the rock the dip to
the west is nineteen degrees; in the Middle Hill, it is thirty-eight
degrees; in the centre of the South hill, or Sugar Loaf, it is
fifty-seven degrees.  At the southern extremity of the Sugar Loaf
strata are vertical, while farther to the south they actually turn
over and dip to the east.

The rock is thus divided into three sections, separated from each
other by places of dislocation, where the strata are much wrenched and
broken.  These are called the Northern and Southern Quebrada, from the
Spanish 'Tierra Quebrada,' or broken ground.  It is at these places
that the inland caves of Gibraltar are almost exclusively found. Based
on the observations of Dr. Falconer and himself, an excellent and most
interesting account of these 'caves, and of the human remains and
works of art which they contain, was communicated by Mr. Busk to the
meeting of the Congress of Prehistoric Archaeology at Norwich, and
afterwards printed in the 'Transactions' of the Congress. [Footnote:
In this essay Mr. Busk refers to the previous labours of Mr. Smith, of
Jordan Hill, to whom we owe most of our knowledge of the geology of
the rock.]  Long subsequent to the operation of the twisting force
just referred to, the promontory underwent various changes of level.
There are sea-terraces and layers of shell-breccia along its flanks,
and numerous caves which, unlike the inland ones, are the product of
marine erosion.  The Ape's Hill, on the African side of the strait,
Mr. Busk informs me has undergone similar disturbances. [Footnote: No
one can rise from the perusal of Mr. Busk's paper without a feeling of
admiration for the principal discoverer and indefatigable explorer of
the Gibraltar caves, the late Captain Frederick Brome.]

*****

In the harbour of Gibraltar, on the morning of our departure, I
resumed a series of observations on the colour of the sea.  On the way
out a number of specimens had been collected, with a view to
subsequent examination.  But the bottles were claret bottles, of
doubtful purity.  At Gibraltar, therefore, I purchased fifteen white
glass bottles, with ground glass stoppers, and at Cadiz, thanks to the
friendly guidance of Mr. Cameron, I secured a dozen more.  These
seven-and-twenty bottles were filled with water, taken at different
places between Oran and Spithead.

And here let me express my warmest acknowledgments to Captain
Henderson, the commander of H.M.S. "Urgent," who aided me in my
observations in every possible way.  Indeed, my thanks are due to all
the officers for their unfailing courtesy and help.  The captain
placed at my disposal his own coxswain, an intelligent fellow named
Thorogood, who skilfully attached a cord to each bottle, weighted it
with lead, cast it into the sea, and, after three successive rinsings,
filled it under my own eyes.  The contact of jugs, buckets, or other
vessels was thus avoided; and even the necessity of pouring out the
water, afterwards, through the dirty London air.

The mode of examination applied to these bottles has been already
described. [Footnote: On Dust and Disease, p. 168.]  The liquid is
illuminated by a powerfully condensed beam, its condition being
revealed through the light scattered by its suspended particles. 'Care
is taken to defend the eye from the access of all other light, and,
thus defended, it becomes an organ of inconceivable delicacy.' Were
water of uniform density perfectly free from suspended matter, it
would, in my opinion, scatter no light at all.  The track of a
luminous beam could not be seen in such water.  But 'an amount of
impurity so infinitesimal as to be scarcely expressible in numbers,
and the individual particles of which are so small as wholly to elude
the microscope, may, when examined by the method alluded to, produce
not only sensible, but striking, effects upon the eye.'

The results of the examination of nineteen bottles filled at various
places between Gibraltar and Spithead, are here tabulated:

No.  Locality            Colour of Sea     Appearance in Luminous beam

1    Gibraltar Harbour      Green           Thick with fine particles

2    Two miles              Clearer green   Thick with very
     from Gibraltar                         fine particles

3    Off Cabreta Point      Bright green     Still thick, but less so

4    Off Cabreta Point      Black-indigo     Much less thick, very pure

5    Off Tarifa             Undecided        Thicker than No. 4

6    Beyond Tarifa          Cobalt-blue      Much purer than No. 5

7    Twelve miles           Yellow-green           Very thick
     from Cadiz

8    Cadiz Harbour          Yellow-green     Exceedingly thick

9    Fourteen miles         Yellow-green     Thick, but less so
      from Cadiz

10   Fourteen miles         Bright green     Much less thick
      from Cadiz

11   Between Capes          Deep Indigo      Very little matter,
     St. Mary and Vincent                    very pure

12   Off the Burlings.      Strong green     Thick, with fine matter

13   Beyond the Burlings    Indigo           Very little matter, pure

14   Off Cape Finisterre    Undecided        Less pure

15   Bay of Biscay          Black-indigo     Very little matter,
                                             very pure

16   Bay of Biscay          Indigo           Very fine matter.
                                             Iridescent

17   Off Ushant             Dark green       A good deal of matter

18   Off St. Catherine's    Yellow-green     Exceedingly thick

19   Spithead               Green            Exceedingly thick

Here we have three specimens of water, described as green, a clearer
green, and bright green, taken in Gibraltar Harbour, at a point two
miles from the harbour, and off Cabreta Point.  The home examination
showed the first to be thick with suspended matter, the second less
thick, and the third still less thick.  Thus the green brightened as
the suspended matter diminished in amount.

Previous to the fourth observation our excellent navigating
lieutenant, Mr. Brown, steered along the coast, thus avoiding the
adverse current which sets in, through the Strait, from the Atlantic
to the Mediterranean.  He was at length forced to cross the boundary
of the Atlantic current, which was defined with extraordinary
sharpness.  On the one side of it the water was a vivid green, on the
other a deep blue.  Standing at the bow of the ship, a bottle could be
filled with blue water, while at the same moment a bottle cast from
the stern could be filled with green water.  Two bottles were secured,
one on each side of this remarkable boundary.  In the distance the
Atlantic had the hue called ultra-marine; but looked fairly down upon,
it was of almost inky blackness--black qualified by a trace of
indigo.

What change does the home examination here reveal?  In passing to
indigo, the water becomes suddenly augmented in purity, the suspended
matter becoming suddenly less.  Off Tarifa, the deep indigo
disappears, and the sea is undecided in colour.  Accompanying this
change, we have a rise in the quantity of suspended matter.  Beyond
Tarifa, we change to cobalt-blue, the suspended matter falling at the
same time in quantity.  This water is distinctly purer than the green.
We approach Cadiz, and at twelve miles from the city get into
yellow-green water; this the London examination shows to be thick with
suspended matter.  The same is true of Cadiz harbour, and also of a
point fourteen miles from Cadiz in the homeward direction.  Here there
is a sudden change from yellow-green to a bright emerald-green, and
accompanying the change a sudden fall in the quantity of suspended
matter.  Between Cape St. Mary and Cape St: Vincent the water changes
to the deepest indigo, a further diminution of the suspended matter
being the concomitant phenomenon.

We now reach the remarkable group of rocks called the Burlings, and
find the water between the shore and the rocks a strong green; the
home examination shows it to be thick with fine matter.  Fifteen or
twenty miles beyond the Burlings we come again into indigo water, from
which the suspended matter has in great part disappeared.  Off Cape
Finisterre, about the place where the 'Captain' went down, the water
becomes green, and the home examination pronounces it to be thicker.
Then we enter the Bay of Biscay, where the indigo resumes its power,
and where the home examination shows the greatly augmented purity of
the water.  A second specimen of water, taken from the Bay of Biscay,
held in suspension fine particles of a peculiar kind; the size of them
was such as to render the water richly iridescent.  It showed itself
green, blue, or salmon-coloured, according to the direction of the
line of vision.  Finally, we come to our last two bottles, the one
taken opposite St. Catherine's lighthouse, in the Isle of Wight, the
other at Spithead.  The sea at both these places was green, and both
specimens, as might be expected, were pronounced by the home
examination to be thick with suspended matter.

Two distinct series of observations are here referred to--the one
consisting of direct observations of the colour of the sea, conducted
during the voyage from Gibraltar to Portsmouth: the other carried out
in the laboratory of the Royal Institution.  And here it is to be
noted that in the home examination I never knew what water was placed
in my hands.  The labels, with the names of the localities written
upon them, had been tied up, all information regarding the source of
the water being thus held back.  The bottles were simply numbered, and
not till all of them had been examined, and described, were the labels
opened, and the locality and sea-colour corresponding to the various
specimens ascertained.  The home observations, therefore, must have
been perfectly unbiassed, and they clearly establish the association
of the green colour with fine suspended matter, and of the ultramarine
colour, and more especially of the black-indigo hue of the Atlantic,
with the comparative absence of such matter.

So much for mere observation; but what is the cause of the dark hue of
the deep ocean? [Footnote: A note, written to me on October 22, by my
friend Canon Kingsley, contains the following reference to this point:
'I have never seen the Lake of Geneva, but I thought of the brilliant
dazzling dark blue of the mid-Atlantic under the sunlight, and its
black-blue under cloud, both so solid that one might leap off the
sponson on to it without fear; this was to me the most wonderful thing
which I saw on my voyages to and from the West Indies.']

A preliminary remark or two will clear our way towards an explanation.
Colour resides in white light, appearing when any constituent of the
white light is withdrawn.  The hue of a purple liquid, for example, is
immediately accounted for by its action on a spectrum.  It cuts out
the yellow and green, and allows the red and blue to pass through. The
blending of these two colours produces the purple.  But while such a
liquid attacks with special energy the yellow and green, it enfeebles
the whole spectrum.  By increasing the thickness of the stratum we may
absorb the whole of the light.  The colour of a blue liquid is
similarly accounted for.  It first extinguishes the red; then, as the
thickness augments, it attacks the orange, yellow, and green in
succession; the blue alone finally remaining.  But even it might be
extinguished by a sufficient depth of 'the liquid.

And now we are prepared for a brief, but tolerably complete, statement
of that action of sea-water upon light, to which it owes its darkness.
The spectrum embraces three classes of rays--the thermal, the visual,
and the chemical.  These divisions overlap each other; the thermal
rays are in part visual, the visual rays in part chemical, and vice
versa.  The vast body of thermal rays lie beyond the red, being
invisible.  These rays are attacked with exceeding energy by water.
They are absorbed close to the surface of the sea, and are the great
agents in evaporation.  At the same time the whole spectrum suffers
enfeeblement; water attacks all its rays, but with different degrees
of energy.  Of the visual rays, the red are first extinguished.  As
the solar beam plunges deeper into the sea, orange follows red, yellow
follows orange, green follows yellow, and the various shades of blue,
where the water is deep enough, follow green.  Absolute extinction of
the solar beam would be the consequence if the water were deep and
uniform.  If it contained no suspended matter, such water would be as
black as ink.  A reflected glimmer of ordinary light would reach us
from its surface, as it would from the surface of actual ink; but no
light, hence no colour, would reach us from the body of the water.

In very clear and deep sea-water this condition is approximately
fulfilled, and hence the extraordinary darkness of such water.  The
indigo, already referred to, is, I believe, to be ascribed in part to
the suspended matter, which is never absent, even in the purest
natural water; and in part to the slight reflection of the light from
the limiting surfaces of strata of different densities.  A modicum of
light is thus thrown back to the eye, before the depth necessary to
absolute extinction has been attained.  An effect precisely similar
occurs under the moraines of glaciers.  The ice here is exceptionally
compact, and, owing to the absence of the internal scattering common
in bubbled ice, the light plunges into the mass, where it is
extinguished, the perfectly clear ice presenting an appearance of
pitchy blackness. [Footnote: I learn from a correspondent that certain
Welsh tarns, which are reputed bottomless, have this inky hue.]

The green colour of the sea has now to be accounted for; and here,
again, let us fall back upon the sure basis of experiment.  A strong
white dinner-plate had a lead weight securely fastened to it.  Fifty
or sixty yards of strong hempen line were attached to the plate.

My assistant, Thorogood, occupied a boat, fastened as usual to the
davits of the "Urgent," while I occupied a second boat nearer the
stern of the ship.  He cast the plate as a mariner heaves the lead,
and by the time it reached me it had sunk a considerable depth in the
water.  In all cases the hue of this plate was green.  Even when the
sea was of the darkest indigo, the green, was vivid and pronounced.  I
could notice the gradual deepening of the colour as the plate sank,
but at its greatest depth, even in indigo water, the colour was still
a blue-green. [Footnote: In no case, of course, is the green pure, but
a mixture of green and blue.]

Other observations confirmed this one.  The "Urgent" is a screw
steamer, and right over the blades of the screw was an orifice called
the screw-well, through which one could look from the poop down upon
the screw.  The surface-glimmer, which so pesters the eye, was here in
a great measure removed.  Midway down, a plank crossed the screw-well
from side to side; on this I placed myself and observed the action of
the screw underneath.  The eye was rendered sensitive by the
moderation of the light; and, to remove still further all disturbing
causes, Lieutenant Walton had a sail and tarpaulin thrown over the
mouth of the well.  Underneath this I perched myself on the plank and
watched the screw.  In an indigo sea the play of colour was
indescribably beautiful, and the contrast between the water, which had
the screw-blades, and that which had the bottom of the ocean, as a
background, was extraordinary.  The one was of the most brilliant
green, the other of the deepest ultramarine.  The surface of the water
above the screw-blade was always ruffled.  Liquid lenses were thus
formed, by which the coloured light was withdrawn from some places and
concentrated upon others, the water flashing with metallic lustre. The
screw-blades in this case played the part of the dinner-plate in the
former case, and there were other instances of a similar kind. The
white bellies of porpoises showed the green hue, varying in intensity
as the creatures swung to and fro between the surface and the deeper
water.  Foam, at a certain depth below the surface, was also green.
In a rough sea the light which penetrated the summit of a wave
sometimes reached the eye, a beautiful green cap being thus placed
upon the wave, even in indigo water.

But how is this colour to be connected with the suspended particles?
Thus.  Take the dinner-plate which showed so brilliant a green when
thrown into indigo water.  Suppose it to diminish in size, until it
reaches an almost microscopic magnitude.  It would still behave
substantially as the larger plate, sending to the eye its modicum of
green light.  If the plate, instead of being a large coherent mass,
were ground to a powder sufficiently fine, and in this condition
diffused through the clear sea-water, it would also send green light
to the eye.  In fact, the suspended particles which the home
examination reveals, act in all essential particulars like the plate,
or like the screw-blades, or like the foam, or like the bellies of the
porpoises.  Thus I think the greenness of the sea is physically
connected with the matter which it holds in suspension.

We reached Portsmouth on January 5, 1871.  Then ended a voyage which,
though its main object was not realised, has left behind it pleasant
memories, both of the aspects of nature and the kindliness of men.

********************

VII. NIAGARA.

[Footnote: A Discourse delivered at the Royal Institution of Great
Britain, April 4, 1873.]

It is one of the disadvantages of reading books about natural scenery
that they fill the mind with pictures, often exaggerated, often
distorted, often blurred, and, even when well drawn, injurious to the
freshness of first impressions.  Such has been the fate of most of us
with regard to the Falls of Niagara.  There was little accuracy in the
estimates of the first observers of the cataract.  Startled by an
exhibition of power so novel and so grand, emotion leaped beyond the
control of the judgment, and gave currency to notions which have often
led to disappointment.

A record of a voyage in 1535 by a French mariner named Jacques
Cartier, contains, it is said, the first printed allusion to Niagara.
In 1603 the first map of the district was constructed by a Frenchman
named Champlain.  In 1648 the Jesuit Rageneau, in a letter to his
superior at Paris, mentions Niagara as 'a cataract of frightful
height.' [Footnote: From an interesting little book presented to me at
Brooklyn by its author, Mr. Holly, some of these data are derived:
Hennepin, Kalm, Bakewell, Lyell, Hall, and others I have myself
consulted.]  In the winter of 1678 and 1679 the cataract was visited
by Father Hennepin, and described in a book dedicated 'to the King of
Great Britain.' He gives a drawing of the waterfall, which shows that
serious changes have taken place since his time.  He describes it as
'a great and prodigious cadence of water, to which the universe does
not offer a parallel.' The height of the fall, according to Hennepin,
was more than 600 feet.  'The waters,' he says, 'which fall from this
great precipice do foam and boil in the most astonishing manner,
making a noise more terrible than that of thunder.  When the wind
blows to the south its frightful roaring may be heard for more than
fifteen leagues.' The Baron la Hontan, who visited Niagara in 1687,
makes the height 800 feet.  In 1721 Charlevois, in a letter to Madame
de Maintenon, after referring to the exaggerations of his
predecessors, thus states the result of his own observations: 'For my
part, after examining it on all sides, I am inclined to think that we
cannot allow it less than 140 or 150 feet,'--a remarkably close
estimate.  At that time, viz.  a hundred and fifty years ago, it had
the shape of a horseshoe, and reasons will subsequently be given for
holding that this has been always the form of the cataract, from its
origin to its present site.

As regards the noise of the fall, Charlevois declares the accounts of
his predecessors, which, I may say, are repeated to the present hour,
to be altogether extravagant.  He is perfectly right.  The thunders of
Niagara are formidable enough to those who really seek them at the
base of the Horseshoe Fall; but on the banks of the river, and
particularly above the fall, its silence, rather than its noise, is
surprising.  This arises, in part, from the lack of resonance; the
surrounding country being flat, and therefore furnishing no echoing
surfaces to reinforce the shock of the water.  The resonance from the
surrounding rocks causes the Swiss Reuss at the Devil's Bridge, when
full, to thunder more loudly than the Niagara.

On Friday, November 1, 1872, just before reaching the village of
Niagara Falls, I caught, from the railway train, my first glimpse of
the smoke of the cataract.  Immediately after my arrival I went with a
friend to the northern end of the American Fall.  It may be that my
mood at the time toned down the impression produced by the first
aspect of this grand cascade; but I felt nothing like disappointment,
knowing, from old experience, that time and close acquaintanceship,
the gradual interweaving of mind and nature, must powerfully influence
my final estimate of the scene.  After dinner we crossed to Goat
Island, and, turning to the right, reached the southern end of the
American Fall.  The river is here studded with small islands. Crossing
a wooden bridge to Luna Island, and clasping a tree which grows near
its edge, I looked long at the cataract, which here shoots down the
precipice like an avalanche of foam.  It grew in power and beauty.
The channel spanned by the wooden bridge was deep, and the river there
doubled over the edge of the precipice, like the swell of a muscle,
unbroken.  The ledge here overhangs, the water being poured out far
beyond the base of the precipice.  A space, called the Cave of the
Winds, is thus enclosed between the wall of rock and the falling
water.

Goat Island ends in a sheer dry precipice, which connects the American
and Horseshoe Falls.  Midway between both is a wooden hut, the
residence of the guide to the Cave of the Winds, and from the hut a
winding staircase, called Biddle's Stair, descends to the base of the
precipice.  On the evening of my arrival I went down this stair, and
wandered along the bottom of the cliff.  One well-known factor in the
formation and retreat of the cataract was immediately observed.  A
thick layer of limestone formed the upper portion of the cliff.  This
rested upon a bed of soft shale, which extended round the base of the
cataract.  The violent recoil of the water against this yielding
substance crumbles it away, undermining the ledge above, which,
unsupported, eventually breaks off, and produces the observed
recession.

At the southern extremity of the Horseshoe is a promontory, formed by
the doubling back of the gorge excavated by the cataract, and into
which it plunges.  On the promontory stands a stone building, called
the Terrapin Tower, the door of which had been nailed up because of
the decay of the staircase within it.  Through the kindness of Mr.
Townsend, the superintendent of Goat Island, the door was opened for
me.  From this tower, at all hours of the day, and at some hours of
the night, I watched and listened to the Horseshoe Fall.  The river
here is evidently much deeper than the American branch; and instead of
bursting into foam where it quits the ledge, it bends solidly over,
and falls in a continuous layer of the most vivid green.  The tint is
not uniform; long stripes of deeper hue alternating with bands of
brighter colour.  Close to the ledge over which the water rolls, foam
is generated, the light falling upon which, and flashing back from it,
is sifted in its passage to and fro, and changed from white to
emerald-green.  Heaps of superficial foam are also formed at intervals
along the ledge, and are immediately drawn into long white striae.
[Footnote: The direction of the wind with reference to the course of a
ship may be inferred with accuracy from the foam-streaks on the
surface of the sea.]  Lower down, the surface, shaken by the reaction
from below, incessantly rustles into whiteness.  The descent finally
resolves itself into a rhythm, the water reaching the bottom of the
fall in periodic gushes.  Nor is the spray uniformly diffused through
the air, but is wafted through it in successive veils of gauze-like
texture.  From all this it is evident that beauty is not absent from
the Horseshoe Fall, but majesty is its chief attribute.  The plunge of
the water is not wild, but deliberate, vast, and fascinating.  From
the Terrapin Tower, the adjacent arm of the Horseshoe is seen
projected against the opposite one, midway down; to the imagination,
therefore, is left the picturing of the gulf into which the cataract
plunges.

The delight which natural scenery produces in some minds is difficult
to explain, and the conduct which it prompts can hardly be fairly
criticised by those who have never experienced it.  It seems to me a
deduction from the completeness of the celebrated Thomas Young, that
he was unable to appreciate natural scenery.  'He had really,' says
Dean Peacock, 'no taste for life in the country; he was one of those
who thought that no one who was able to live in London would be
content to 'live elsewhere.' Well, Dr. Young, like Dr. Johnson, had a
right to his delights; but I can understand a hesitation to accept
them, high as they were, to the exclusion of

    That o'erflowing joy which Nature yields
    To her true lovers.

To all who are of this mind, the strengthening of desire on my part to
see and know Niagara Falls, as far as it is possible for them to be
seen and known, will be intelligible.

On the first evening of my visit, I met, at the head of Biddle's
Stair, the guide to the Cave of the Winds.  He was in the prime of
manhood--large, well built, firm and pleasant in mouth and eye.  My
interest in the scene stirred up his, and made him communicative.

Turning to a photograph, he described, by reference to it, a feat
which he had accomplished some time previously, and which had brought
him almost under the green water of the Horseshoe Fall.  'Can you lead
me there to-morrow?' I asked.  He eyed me enquiringly, weighing,
perhaps, the chances of a man of light build, and with grey in his
whiskers, in such an undertaking.  'I wish,' I added, 'to see as much
of the fall as can be seen, and where you lead I will endeavour to
follow.' His scrutiny relaxed into a smile, and he said, 'Very well;
I shall be ready for you to-morrow.'

On the morrow, accordingly, I came.  In the hut at the head of
Biddle's Stair I stripped wholly, and re-dressed according to
instructions,--drawing on two pairs of woollen pantaloons, three
woollen jackets, two pairs of socks, and a pair of felt shoes.  Even
if wet, my guide assured me that the clothes would keep me from being
chilled; and he was right.  A suit and hood of yellow oilcloth covered
all.  Most laudable precautions were taken by the young assistant who
helped to dress me to keep the water out; but his devices broke down
immediately when severely tested.

We descended the stair; the handle of a pitchfork doing, in my case,
the duty of an alpenstock.  At the bottom, the guide enquired whether
we should go first to the Cave of the Winds, or to the Horseshoe,
remarking that the latter would try us most. I decided on getting the
roughest done first, and he turned to the left over the stones.  They
were sharp and trying.  The base of the first portion of the cataract
is covered with huge boulders, obviously the ruins of the limestone
ledge above.  The water does not distribute itself uniformly among
these, but seeks out channels through which it pours torrentially.  We
passed some of these with wetted feet, but without difficulty.  At
length we came to the side of a more formidable current.  My guide
walked along its edge until he reached its least turbulent portion.
Halting, he said, 'This is our greatest difficulty; if we can cross
here, we shall get far towards the Horseshoe.'

He waded in.  It evidently required all his strength to steady him.
The water rose above his loins, and it foamed still higher.  He had to
search for footing, amid unseen boulders, against which the torrent
rose violently.  He struggled and swayed, but he struggled
successfully, and finally reached the shallower water at the other
side.  Stretching out his arm, he said to me, 'Now come on.' I looked
down the torrent, as it' rushed to the river below, which was seething
with the tumult of the cataract.  De Saussure recommended the
inspection of Alpine dangers, with the view of making them familiar to
the eye before they are encountered; and it is a wholesome custom in
places of difficulty to put the possibility of an accident clearly
before the mind, and to decide beforehand what ought to be done should
the accident occur.  Thus wound up in the present instance, I entered
the water.  Even where it was not more than knee-deep, its power was
manifest. As it rose around me, I sought to split the torrent by
presenting a side to it; but the insecurity of the footing enabled it
to grasp my loins, twist me fairly round, and bring its impetus to
bear upon my back.  Further struggle was impossible; and feeling my
balance hopelessly gone, I turned, flung myself towards the bank just
quitted, and was instantly, as expected, swept into shallower water.

The oilcloth covering was a great incumbrance; it had been made for a
much stouter man, and, standing upright after my submersion, my legs
occupied the centre of two bags of water.  My guide exhorted me to try
again.  Prudence was at my elbow, whispering dissuasion; but, taking
everything into account, it appeared more immoral to retreat than to
proceed.  Instructed by the first misadventure, I once more entered
the stream.  Had the alpenstock been of iron it might have helped me;
but, as it was, the tendency of the water to sweep it out of my hands
rendered it worse than useless.  I, however, clung to it by habit.
Again the torrent rose, and again I wavered; but, by keeping the left
hip well against it, I remained upright, and at length grasped the
hand of my leader at the other side.  He laughed pleasantly.  The
first victory was gained, and he enjoyed it.  'No traveller,' he said,
'was ever here before.'  Soon afterwards, by trusting to a piece of
drift-wood which seemed firm, I was again taken off my feet, but was
immediately caught by a protruding rock.

We clambered over the boulders towards the thickest spray, which soon
became so weighty as to cause us to stagger under its shock.  For the
most part nothing could be seen; we were in the midst of bewildering
tumult, lashed by the water, which sounded at times like the cracking
of innumerable whips.  Underneath this was the deep resonant roar of
the cataract.  I tried to shield my eyes with my hands, and look
upwards; but the defence was useless.  The guide continued to move on,
but at a certain place he halted, desiring me to take shelter in his
lee, and observe the cataract.  The spray did not come so much from
the upper ledge, as from the rebound of the shattered water when it
struck the bottom.  Hence the eyes could be protected from the
blinding shock of the spray, while the line of vision to the upper
ledges remained to some extent clear.  On looking upwards over the
guide's shoulder I could see the water bending over the ledge, while
the Terrapin Tower loomed fitfully through the intermittent
spray-gusts.  We were right under the tower.  A little farther on the
cataract, after its first plunge, hit a protuberance some way down,
and flew from it in a prodigious burst of spray; through this we
staggered.  We rounded the promontory on which the Terrapin Tower
stands, and moved, amid the wildest commotion, along the arm of the
Horse-hoe, until the boulders failed us, and the cataract fell into
the profound gorge of the Niagara River.

Here the guide sheltered me again, and desired me to look up; I did
so, and could see, as before, the green gleam of the mighty curve
sweeping over the uipper ledge, and the fitful plunge of the water, as
the spray between us and it alternately gathered and disappeared. An
eminent friend of mine often speaks of the mistake of those physicians
who regard man's ailments as purely chemical, to be met by chemical
remedies only.  He contends for the psychological element of cure.  By
agreeable emotions, he says, nervous currents are liberated which
stimulate blood, brain, and viscera.  The influence rained from
ladies' eyes enables my friend to thrive on dishes which would kill
him if eaten alone.  A sanative effect of the same order I experienced
amid the spray and thunder of Niagara.  Quickened by the emotions
there aroused, the blood sped exultingly through the arteries,
abolishing introspection, clearing the heart of all bitterness, and
enabling one to think with tolerance, if not with tenderness, on the
most relentless and unreasonable foe.  Apart from its scientific
value, and purely as a moral agent, the play was worth the candle.  My
companion knew no more of me than that I enjoyed the wildness of the
scene; but as I bent in the shelter of his large frame he said, 'I
should like to see you attempting to describe all this.' He rightly
thought it indescribable.  The name of this gallant fellow was Thomas
Conroy.

We returned, clambering at intervals up and down, so as to catch
glimpses of the most impressive portions of the cataract.  We passed
under ledges formed by tabular masses of limestone, and through some
curious openings formed by the falling together of the summits of the
rocks.  At length we found ourselves beside our enemy of the morning.
Conroy halted for a minute or two, scanning the torrent thoughtfully.
I said that, as a guide, he ought to have a rope in such a place; but
he retorted that, as no traveller had ever thought of coming there, he
did not see the necessity of keeping a rope.  He waded in.  The
struggle to keep himself erect was evident enough; he swayed, but
recovered himself again and again.  At length he slipped, gave way,
did as I had done, threw himself towards the bank, and was swept into
the shallows.  Standing in the stream near its edge, he stretched his
arm towards me.  I retained the pitchfork handle, for it had been
useful among the boulders.  By wading some way in, the staff could be
made to reach him, and I proposed his seizing it.  'If you are sure,'
he replied, 'that, in case of giving way, you can maintain your grasp,
then I will certainly hold you.'  Remarking that he might count on
this, I waded in, and stretched the staff to my companion.  It was
firmly grasped by both of us.  Thus helped, though its onset was
strong, I moved safely across the torrent.  All danger ended here.  We
afterwards roamed sociably among the torrents and boulders below the
Cave of the Winds.  The rocks were covered with organic slime, which
could not have been walked over with bare feet, but the felt shoes
effectually prevented slipping.  We reached the cave and entered it,
first by a wooden way carried over the boulders, and then along a
narrow ledge, to the point eaten deepest into the shale.  When the
wind is from the south, the falling water, I am told, can be seen
tranquilly from this spot; but when we were there, a blinding
hurricane of spray was whirled against us.  On the evening of the same
day, I went behind the water on the Canada side, which, after the
experiences of the morning, struck me as an imposture.

Still even this latter is exciting to some nerves.  Its effect upon
himself is thus vividly described by Bakewell, jun: 'On turning a
sharp angle of the rock, a sudden gust of wind met us, coming from
the hollow between the fall and the rock, which drove the spray
directly in our faces, with such force that in an instant we were wet
through.  When in the midst of this shower-bath the shock took away my
breath: I turned back and scrambled over the loose stones to escape
the conflict.  The guide soon followed, and told me that I had passed
the worst part.  With that assurance I made a second attempt; but so
wild and disordered was my imagination that when I had reached half
way I could bear it no longer.' [Footnote: 'Mag. of Nat. Hist,' 1830,
pp. 121, 122.]

To complete my knowledge I desired to see the fall from the river
below it, and long negotiations were necessary to secure the means of
doing so.  The only boat fit for the undertaking had been laid up for
the winter; but this difficulty, through the kind intervention of Mr.
Townsend, was overcome.  The main one was to secure oarsmen
sufficiently strong and skilful to urge the boat where I wished it to
be taken.  The son of the owner of the boat, a finely-built young
fellow, but only twenty, and therefore not sufficiently hardened, was
willing to go; and up the river, it was stated, there lived another
man who could do anything with the boat which strength and daring
could accomplish.  He came.  His figure and expression of face
certainly indicated extraordinary firmness and power.  On Tuesday,
November 5, we started, each of us being clad in oilcloth.  The elder
oarsman at once assumed a tone of authority over his companion, and
struck immediately in amid the breakers below the American Fall.  He
hugged the cross freshets instead of striking out into the smoother
water.  I asked him why he did so, and he replied that they were
directed outwards, not downwards.  The struggle, however, to prevent
the bow of the boat from being turned by them, was often very severe.

The spray was in general blinding, but at times it disappeared and
yielded noble views of the fall.  The edge of the cataract is crimped
by indentations which exalt its beauty.  Here and there, a little
below the highest ledge, a secondary one juts out; the water strikes
it and bursts from it in huge protuberant masses of foam and spray. We
passed Goat Island, came to the Horseshoe, and worked for a time along
its base, the boulders over which Conroy and myself had scrambled a
few days previously lying between us and the cataract.  A rock was
before us, concealed and revealed at intervals, as the waves passed
over it.  Our leader tried to get above this rock, first on the
outside of it.  The water, however, was here in violent motion.  The
men struggled fiercely, the older one ringing out an incessant peal of
command and exhortation to the younger.  As we were just clearing the
rock, the bow came obliquely to the surge; the boat was turned
suddenly round and shot with astonishing rapidity down the river.  The
men returned to the charge, now trying to get up between the
half-concealed rock and the boulders to the left.  But the torrent set
in strongly through this channel.  The tugging was quick and violent,
but we made little way.  At length, seizing a rope, the principal
oarsman made a desperate attempt to get upon one of the boulders,
hoping to be able to drag the boat through the channel; but it bumped
so violently against the rock, that the man flung himself back and
relinquished the attempt.

We returned along the base of the American Fall, running in and out
among the currents which rushed, from it laterally into the river.
Seen from below the American Fall is certainly exquisitely beautiful,
but it is a mere frill of adornment to its nobler neighbour the
Horseshoe.  At times we took to the river, from the centre of which
the Horseshoe Fall appeared especially magnificent.  A streak of cloud
across the neck of Mont Blanc can double its apparent height, so here
the green summit of the cataract shining above the smoke of spray
appeared lifted to an extraordinary elevation.  Had Hennepin and La
Hontan seen the fall from this position, their estimates of the height
would have been perfectly excusable.

*****

From a point a little way below the American Fall, a ferry crosses
the river, in summer, to the Canadian side.  Below the ferry is a
suspension bridge for carriages and foot-passengers, and a mile or two
lower down is the railway suspension bridge.  Between ferry and bridge
the river Niagara flows unruffled; but at the suspension bridge the
bed steepens and the river quickens its motion.  Lower down the gorge
narrows, and the rapidity and turbulence increase.  At the place
called the' Whirlpool Rapids' I estimated the width of the river at
300 feet, an estimate confirmed by the dwellers on the spot.  When it
is remembered that the drainage of nearly half a continent is
compressed into this space, the impetuosity of the river's rush may be
imagined.  Had it not been for Mr. Bierstädt, the distinguished
photographer of Niagara, I should have quitted the place without
seeing these rapids; for this, and for his agreeable company to the
spot, I have to thank him.  From the edge of the cliff above the
rapids, we descended, a little, I confess, to a climber's disgust, in
an 'elevator,' because the effects are best seen from the water level.

Two kinds of motion are here obviously active, a motion of translation
and a motion of undulation--the race of the river through its gorge,
and the great waves generated by its collision with, and rebound from,
the obstacles in its way.  In the middle of the river the rush and
tossing are most violent; at all events, the impetuous force of the
individual waves is here most strikingly displayed.  Vast pyramidal
heaps leap incessantly from the river, some of them with such energy
as to jerk their summits into the air, where they hang momentarily
suspended in crowds of liquid spherules.  The sun shone for a few
minutes.  At times the wind, coming up the river, searched and sifted
the spray, carrying away the lighter drops, and leaving the heavier
ones behind.  Wafted in the proper direction, rainbows appeared and
disappeared fitfully in the lighter mist. In other directions the
common gleam of the sunshine from the waves and their shattered crests
was exquisitely beautiful.  The complexity of the action was still
further illustrated by the fact, that in some cases, as if by the
exercise of a local explosive force, the drops were shot radially from
a particular centre, forming around it a kind of halo.

The first impression, and, indeed, the current explanation of these
rapids is, that the central bed of the river is cumbered with large
boulders, and that the jostling, tossing, and wild leaping of the
water there, are due to its impact against these obstacles.  I doubt
this explanation.  At all events, there is another sufficient reason
to be taken into account.  Boulders derived from the adjacent cliffs
visibly cumber the sides of the river.  Against these the water rises
and sinks rhythmically but violently, large waves being thus produced.
On the generation of each wave, there is an immediate compounding of
the wave-motion with he river-motion.  The ridges, which in still
water would proceed in circular curves round the centre of
disturbance, cross the river obliquely, and the result is that at the
centre waves commingle, which have really been generated at the sides.
In the first instance, we had a composition of wave-motion with
river-motion; here we have the coalescence of waves with waves.  Where
crest and furrow cross each other, the motion is annulled; where
furrow and furrow cross, the river is ploughed to a greater depth; and
where crest and crest aid each other, we have that astonishing leap of
the water which breaks the cohesion of the crests, and tosses them
shattered into the air.  From the water level the cause of the action
is not so easily seen; but from the summit of the cliff the lateral
generation of the waves, and their propagation to the perfectly
obvious.  If this explanation be correct, the phenomena observed at
the Whirlpool Rapids form one of the grandest illustrations of the
principle of _interference_.  The Nile 'cataract,' Mr.  Huxley informs
me, offers more moderate examples of the same action.

At some distance below the Whirlpool Rapids we have the celebrated
whirlpool itself.  Here the river makes a sudden bend to the
north-east, forming nearly a right angle with its previous direction.
The water strikes the concave bank with great force, and scoops it
incessantly away.  A vast basin has been thus formed, in which the
sweep of the river prolongs itself in gyratory currents.  Bodies and
trees which have come over the falls, are stated to circulate here for
days without finding the outlet.  From various points of the cliffs
above, this is curiously hidden.  The rush of the river into the
whirlpool is obvious enough; and though you imagine the outlet must be
visible, if one existed, you cannot find it.  Turning, however, round
the bend of the precipice to the north-east, the outlet comes into
view.

The Niagara season was over; the chatter of sightseers had ceased, and
the scene presented itself as one of holy seclusion and beauty.  I
went down to the river's edge, where the weird loneliness seemed to
increase.  The basin is enclosed by high and almost precipitous
banks--covered, at the time, with russet woods.  A kind of mystery
attaches itself to gyrating water, due perhaps to the fact that we are
to some extent ignorant of the direction of its force.  It is said
that at certain points of the whirlpool, pine-trees are sucked down,
to be ejected mysteriously elsewhere.  The 'water is of the brightest
emerald-green.  The gorge through which it escapes is narrow, and the
motion of the river swift though silent.  The surface is steeply
inclined, but it is perfectly unbroken.  There are no lateral waves,
no ripples with their breaking bubbles to raise a murmur; while the
depth is here too great to allow the inequality of the bed to ruffle
the surface.  Nothing can be more beautiful than this sloping liquid
mirror formed by the Niagara, in sliding from the whirlpool.

The green colour is, I think, correctly accounted for in the last
Fragment.  While crossing the Atlantic in 1872-73 I had frequent
opportunities of testing the explanation there given.  Looked properly
down upon, there are portions of the ocean to which we should hardly
ascribe a trace of blue; at the most, a mere hint of indigo reaches
the eye.  The water, indeed, is practically black, and this is an
indication both of its depth and of its freedom from mechanically
suspended matter.  In small thicknesses water is sensibly transparent
to all kinds of light; but, as the thickness increases, the rays of
low refrangibility are first absorbed, and after them the other rays.
Where, therefore, the water is very deep and very pure, all the
colours are absorbed, and such water ought to appear black, as no
light is sent from its interior to the eye.  The approximation of the
Atlantic Ocean to this condition is an indication of its extreme
purity.

Throw a white pebble into such water; as it sinks it becomes greener
and greener, and, before it disappears, it reaches a vivid blue-green.
Break such a pebble into fragments, each of these will behave like the
unbroken mass; grind the pebble to powder, every particle will yield
its modicum of green; and if the particles be so fine as to remain
suspended in the water, the scattered light will be a uniform green.
Hence the greenness of shoal water.  You go to bed with the black
Atlantic around you.  You rise in the morning, find it a vivid green,
and correctly infer that you are crossing the bank of Newfoundland.
Such water is found charged with fine matter in a state of mechanical
suspension.  The light from the bottom may sometimes come into play,
but it is not necessary.  A storm can render the water muddy, by
rendering the particles too numerous and gross.  Such a case occurred
towards the close of my visit to Niagara.  There had been rain and
storm in the upper lake-regions, and the quantity of suspended matter
brought down quite extinguished the fascinating green of the
Horseshoe.

Nothing can be more superb than the green of the Atlantic waves, when
the circumstances are favourable to the exhibition of the colour.  As
long as a wave remains unbroken no colour appears; but when the foam
just doubles over the crest, like an Alpine snow-cornice, under the
cornice we often see a display of the most exquisite green.  It is
metallic in its brilliancy.  But the foam is necessary to its
production.  The foam is first illuminated, and it scatters the light
in all directions; the light which passes through the higher portion
of the wave alone reaches the eye, and gives to that portion its
matchless colour.  The folding of the wave, producing as it does, a
series of longitudinal protuberances and furrows which act like
cylindrical lenses, introduces variations in the intensity of the
light, and materially enhances its beauty.

*****

We have now to consider the genesis and proximate destiny of the Falls
of Niagara.  We may open our way to this subject by a few preliminary
remarks upon erosion.  Time and intensity are the main factors of
geologic change, and they are in a certain sense convertible.  A
feeble force acting through long periods, and an intense force acting
through short ones, may produce approximately the same results.  To
Dr. Hooker I have been indebted for some specimens of stones, the
first examples of which were picked up by Mr. Hackworth on the shores
of Lyell's Bay, near Wellington, in New Zealand.  They were described
by Mr. Travers in the 'Transactions of the New Zealand Institute.'
Unacquainted with their origin, you would certainly ascribe their
forms to human workmanship.  They resemble knives and spear-heads,
being apparently chiselled off into facets, with as much attention to
symmetry as if a tool, guided by human intelligence, had passed over
them.  But no human instrument has been brought to bear upon these
stones.  They have been wrought into their present shape by the
wind-blown sand of Lyell's Bay.  Two winds are, dominant here, and
they in succession urged the sand against opposite sides of the stone;
every little particle of sand chipped away its infinitesimal bit of
stone, and in the end sculptured these singular forms. [Footnote:
'These stones, which have a strong resemblance to works of human art,
occur in great abundance, and of various sizes, from half-an-inch to
several inches in length.  A large number were exhibited showing the
various forms, which are those of wedges, knives, arrow-heads, &c, and
all with sharp cutting edges.

'Mr. Travers explained that, notwithstanding their artificial
appearance, these stones were formed by the cutting action of the
wind-driven sand, as it passed to and fro over an exposed
boulder-bank.  He gave a minute account of the manner in which the
varieties of form are produced, and referred to the effect which the
erosive action thus indicated would have on railway and other works
executed on sandy tracts.

'Dr. Hector stated that although, as a group, the specimens on the
table could not well be mistaken for artificial productions, still the
forms are so peculiar, and the edges, in a few of them, so perfect,
that if they were discovered associated with human works, there is no
doubt that they would have been referred to the so-called "stone
period."'--Extracted from the Minutes of the Wellington Philosophical
Society, February 9, 1869.]

The Sphynx of Egypt is nearly covered up by the sand of the desert.
The neck of the Sphynx is partly cut across, not, as I am assured by
Mr. Huxley, by ordinary weathering, but by the eroding action of the
fine sand blown against it.  In these cases Nature furnishes us with
hints which may be taken advantage of in art; and this action of sand
has been recently turned to extraordinary account in the United
States.  When in Boston, I was taken by my courteous and helpful
friend, Mr. Josiah Quincey, to see the action of the sand-blast. A
kind of hopper containing fine silicious sand was connected with a
reservoir of compressed air, the pressure being variable at pleasure.
The hopper ended in a long slit, from which the sand was blown.  A
plate of glass was placed beneath this slit, and caused to pass slowly
under it; it came out perfectly depolished, with a bright opalescent
glimmer, such as could only be produced by the most careful grinding.
Every little particle of sand urged against the glass, having all its
energy concentrated on the point of impact, formed there a little pit,
the depolished surface consisting of innumerable hollows of this
description.

But this was not all.  By protecting certain portions of the surface,
and exposing others, figures and tracery of any required form could be
etched upon the glass.  The figures of open iron-work could be thus
copied; while wire-gauze placed over the glass produced a reticulated
pattern.  But it required no such resisting substance as iron to
shelter the glass.  The patterns of the finest lace could be thus
reproduced; the delicate filaments of the lace itself offering a
sufficient protection.  All these effects have been obtained with a
simple model of the sand-blast devised by my assistant.  A fraction of
a minute suffices to etch upon glass a rich and beautiful lace
pattern.  Any yielding substance may be employed to protect the glass.
By diffusing the shock of the particle, such substances practically
destroy the local erosive power.  The hand can bear, without
inconvenience, a sand-shower which would pulverise glass.  Etchings
executed on glass with suitable kinds of ink are accurately worked out
by the sandblast. In fact, within certain limits, the harder the
surface, the greater is the concentration of the shock, and the more
effectual is the erosion.  It is not necessary that the sand should be
the harder substance of the two; corundum, for example, is much harder
than quartz; still, quartz-sand can not only depolish, but actually
blow a hole through a plate of corundum.  Nay, glass may be depolished
by the impact of fine shot; the grains in this case bruising the
glass, before they have time to flatten and turn their energy into
heat.

And here, in passing, we may tie together one or two apparently
unrelated facts.  Supposing you turn on, at the lower part of a house,
a cock which is fed by a pipe from a cistern at the top of the house,
the column of water, from the cistern downwards, is set in motion.  By
turning off the cock, this motion is stopped; and, when the turning
off is very sudden, the pipe, if not strong, may be burst by the
internal impact of the water.  By distributing the turning of the cock
over half a second of time, the shock and danger of rupture may be
entirely avoided.  We have here an example of the concentration of
energy in time.  The sand-blast illustrates the concentration of
energy in space.  The action of flint and steel is an illustration of
the same principle.  The heat required to generate the spark is
intense; and the mechanical action, being moderate, must, to produce
fire, be in the highest degree concentrated.  This concentration is
secured by the collision of hard substances.  Calc-spar will not
supply the place of flint, nor lead the place of steel, in the
production of fire by collision.  With the softer substances, the
total heat produced may be greater than with the hard ones, but, to
produce the spark, the heat must be intensely localised.

We can, however, go far beyond the mere depolishing of glass; indeed I
have already said that quartz-sand can wear a hole through corundum.
This leads me to express my acknowledgments to General Tilghman, who
is the inventor of the sand-Blast. [Footnote: The absorbent power,
if I may use the phrase, exerted by the industrial arts in the United
States, is forcibly illustrated by the rapid transfer of men like
Mr. Tilghman from the life of the soldier to that of the civilian.
General McClellan, now a civil engineer, whom I had the honour of
frequently meeting in New York, is a most eminent example of the same
kind.  At the end of the war, indeed, a million and a half of men were
thus drawn, in an astonishingly short time, from military to civil
life.]  To his spontaneous kindness I am indebted for some beautiful
illustrations of his process.  In one thick plate of glass a figure
has been worked out to a depth of three eighths of an inch.  A second
plate, seven eighths of an inch thick, is entirely perforated.  In a
circular plate of marble, nearly half an inch thick, open work of most
intricate and elaborate description has been executed.  It would
probably take many days to perform this work by any ordinary process;
with the sand-blast it was accomplished in an hour.  So much for the
strength of the blast; its delicacy is illustrated by this beautiful
example of line engraving, etched on glass by means of the Blast.

This power of erosion, so strikingly displayed when sand is urged by
air, renders us better able to conceive its action when urged by
water.  The erosive power of a river is vastly augmented by the solid
matter carried along with it.  Sand or pebbles, caught in a river
vortex, can wear away the hardest rock potholes' and deep cylindrical
shafts being thus produced.  An extraordinary instance of this kind of
erosion is to be seen in the Val Tournanche, above the village of this
name.  The gorge at Handeck has been thus cut out.  Such waterfalls
were once frequent in the valleys of Switzerland; for hardly any
valley is without one or more transverse barriers of resisting
material, over which the river flowing through the valley once fell as
a cataract.  Near Pontresina, in the Engadin, there is such a case; a
hard gneiss being there worn away to form a gorge, through which the
river from the Morteratsch glacier rushes.  The barrier of the Kirchet
above Meyringen is also a case in point.  Behind it was a lake,
derived from the glacier of the Aar, and over the barrier the lake
poured its excess of water.  Here the rock, being limestone, was in
part dissolved; but added to this we had the action of the sand and
gravel carried along by the water, which, on striking the rock,
chipped it away like the particles of the sand-Blast. Thus, by
solution and mechanical erosion, the great chasm of the
Finsteraarschlucht was formed.  It is demonstrable that the water
which flows at the bottoms of such deep fissures once flowed at the
level of their present edges, and tumbled down the lower faces of the
barriers.  Almost every valley in Switzerland furnishes examples of
this kind; the untenable hypothesis of earthquakes, once so readily
resorted to in accounting for these gorges, being now for the most
part abandoned.  To produce the Canons of Western America, no other
cause is needed than the integration of effects individually
infinitesimal.

And now we come to Niagara.  Soon after Europeans had taken possession
of the country, the conviction appears to have arisen that the deep
channel of the river Niagara below the falls had been excavated by the
cataract.  In Mr. Bakewell's 'Introduction to Geology,' the prevalence
of this belief has been referred to.  It is expressed thus by
Professor Joseph Henry in the 'Transactions of the Albany Institute:'
[Footnote: Quoted by Bakewell.] 'In viewing the position of the
falls, and the features of the country round, it is impossible not to
be impressed with the idea that this great natural raceway has been
formed by the continued action of the irresistible Niagara, and that
the falls, beginning at Lewiston, have, in the course of ages, worn
back the rocky strata to their present site.' The same view is
advocated by Sir Charles Lyell, by Mr. Hall, by M. Agassiz, by
Professor Ramsay, indeed by most of those who have inspected the
place.

A connected image of the origin and progress of the cataract is easily
obtained.  Walking northward from the village of Niagara Falls by the
side of the river, we have to our left the deep and comparatively
narrow gorge, through which the Niagara flows.  The bounding cliffs of
this gorge are from 300 to 350 feet high.  We reach the whirlpool,
trend to the north-east, and after a little time gradually resume our
northward course.  Finally, at about seven miles from the present
falls, we come to the edge of a declivity, which informs us that we
have been hitherto walking on table-land.  At some hundreds of feet
below us is a comparatively level plain, which stretches to Lake
Ontario.  The declivity marks the end of the precipitous gorge of the
Niagara.  Here the river escapes from its steep mural boundaries, and
in a widened bed pursues its way to the lake which finally receives
its waters.

The fact that in historic times, even within the memory of man, the
fall has sensibly receded, prompts the question, How far has this
recession gone?  At what point did the ledge which thus continually
creeps backwards begin its retrograde course?  To minds disciplined in
such researches the answer has been, and will be--At the precipitous
declivity which crossed the Niagara from Lewiston on the American to
Queenston on the Canadian side.  Over this transverse barrier the
united affluents of all the upper lakes once poured their waters, and
here the work of erosion began.  The dam, moreover, was demonstrably
of sufficient height to cause the river above it to submerge Goat
Island; and this would perfectly account for the finding by Sir
Charles Lyell, Mr. Hall, and others, in the sand and gravel of the
island, the same fluviatile shells as are now found in the Niagara
River higher up.  It would also account for those deposits along the
sides of the river, the discovery of which enabled Lyell, Hall, and
Ramsay to reduce to demonstration the popular belief that the Niagara
once flowed through a shallow valley.

The physics of the problem of excavation, which I made clear to my
mind before quitting Niagara, are revealed by a close inspection of
the present Horseshoe Fall.  We see evidently that the greatest weight
of water bends over the very apex of the Horseshoe.  In a passage in
his excellent chapter on Niagara Falls, Mr. Hall alludes to this fact.
Here we have the most copious and the most violent whirling of the
shattered liquid; here the most powerful eddies recoil against the
shale.  From this portion of the fall, indeed, the spray sometimes
rises without solution of continuity to the region of clouds, becoming
gradually more attenuated, and passing finally through the condition
of true cloud into invisible vapour, which is sometimes reprecipitated
higher up.  All the phenomena point distinctly to the centre of the
river as the place of greatest mechanical energy, and from the centre
the vigour of the fall gradually dies away towards the sides.  The
Horseshoe form, with the concavity facing downwards, is an obvious and
necessary consequence of this action.  Right along the middle of the
river the apex of the curve pushes its way backwards, cutting along
the centre a deep and comparatively narrow groove, and draining the
sides as it passes them. [Footnote: In the discourse the excavation of
the centre and drainage of the sides action was illustrated by a model
devised by my assistant, Mr. John Cottrell.]  Hence the remarkable
discrepancy between the widths of the Niagara above and below the
Horseshoe.  All along its course, from Lewiston Heights to its present
position, the form of the fall was probably that of a horseshoe; for
this is merely the expression of the greater depth, and consequently
greater excavating power, of the centre of the river.  The gorge,
moreover, varies in width, as the depth of the centre of the ancient
river varied, being narrowest where that depth was greatest.

The vast comparative erosive energy of the Horseshoe Fall comes
strikingly into view when it and the American Fall are compared
together.  The American branch of the river is cut at a right angle by
the gorge of the Niagara.  Here the Horseshoe Fall was the real
excavator.  It cut the rock, and formed the precipice, over which the
American Fall tumbles.  But since its formation, the erosive action of
the American Fall has been almost nil, while the Horseshoe has cut its
way for 600 yards across the end of Goat Island, and is now doubling
back to excavate its channel parallel to the length of the island.
This point, which impressed me forcibly, has not, I have just learned,
escaped the acute observation of Professor Ramsay. [Footnote: His
words are: 'Where the body of water is small in the American Fall, the
edge has only receded a few yards (where most eroded) during the time
that the Canadian Fall has receded from the north corner of Goat
Island to the innermost curve of the Horseshoe Fall.'--Quarterly
Journal of Geological Society, May 1859.]  The river bends; the
Horseshoe immediately accommodates itself to the bending, and will
follow implicitly the direction of the deepest water in the upper
stream.  The flexures of the gorge are determined by those of the
river channel above it.  Were the Niagara centre above the fall
sinuous, the gorge would obediently follow its sinuosities.  Once
suggested, no doubt geographers will be able to point out many
examples of this action.  The Zambesi is thought to present a great
difficulty to the erosion theory, because of the sinuosity of the
chasm below the Victoria Falls.  But, assuming the basalt to be of
tolerably uniform texture, had the river been examined before the
formation of this sinuous channel, the present zigzag course of the
gorge below the fall could, I am persuaded, have been predicted, while
the sounding of the present river would enable us to predict the
course to be pursued by the erosion in the future.

But not only has the Niagara River cut the gorge; it has carried away
the chips of its own workshop.  The shale, being probably crumbled, is
easily carried away.  But at the base of the fall we find the huge
boulders already described, and by some means or other these are
removed down the river.  The ice which fills the gorge in winter, and
which grapples with the boulders, has been regarded as the
transporting agent.  Probably it is so to some extent.  But erosion
acts without ceasing on the abutting points of the boulders, thus
withdrawing their support and urging them gradually down the river.
Solution also does its portion of the work.  That solid matter is
carried down is proved by the difference of depth between the Niagara
River and Lake Ontario, where the river enters it.  The depth falls
from 72 feet to 20 feet, in consequence of the deposition of solid
matter caused by the diminished motion of the river. [Footnote: Near
the mouth of the gorge at Queenston, the depth, according to the
Admiralty Chart, is 180 feet; well within the gorge it is 132 feet.]

The annexed highly instructive map has been reduced from one published
in Mr. Hall's 'Geology of New York.' It is based on surveys executed
in 1842, by Messrs.  Gibson and Evershed.  The ragged edge of the
American Fall north of Goat Island marks the amount of erosion which
it has been able to accomplish, while the Horseshoe Fall was cutting
its way southward across the end of Goat Island to its present
position.  The American Fall is 168 feet high, a precipice cut down,
not by itself, but by the Horseshoe Fall.  The latter in 1842 was 159
feet high, and, as shown by the map, is already turning eastward, to
excavate its gorge along the centre of the upper river.  P is the apex
of the Horseshoe, and T marks the site of the Terrapin Tower, with the
promontory adjacent, round which I was conducted by Conroy.  Probably
since 1842 the Horseshoe has worked back beyond the position here
assigned to it.

In conclusion, we may say a word regarding the proximate future of
Niagara.  At the rate of excavation assigned to it by Sir Charles
Lyell, namely, a foot a year, five thousand years or so will carry the
Horseshoe Fall far higher than Goat Island.  As the gorge recedes it
will drain, as it has hitherto done, the banks right and left of it,
thus leaving a nearly level terrace between Goat Island and the edge
of the gorge.  Higher up it will totally drain the American branch of
the river; the channel of which in due time will become cultivable
land.  The American Fall will then be transformed into a dry
precipice, forming a simple continuation of the cliffy boundary of the
Niagara gorge.  At the place occupied by the fall at this moment we
shall have the gorge enclosing a right angle, a second whirlpool being
the consequence.  To those who visit Niagara a few millenniums hence I
leave the verification of this prediction.  All that can be said is,
that if the causes now in action continue to act, it will prove itself
literally true.

*****

Fig.  6.

POSTSCRIPT.

A year or so after I had quitted the United States, a man sixty years
of age, while engaged in painting one of the bridges which connect
Goat Island with the Three Sisters, slipped through the rails of the
bridge into the rapids, and was carried impetuously towards the
Horseshoe Fall.  He was urged against a rock which rose above the
water, and with the grasp of desperation he clung to it.  The
population of the village of Niagara Falls was soon upon the island,
and ropes were brought, but there was none to use them.  In the midst
of the excitement, a tall powerful young fellow was observed making
his way silently through the crowd.  He reached a rope; selected from
the bystanders a number of men, and placed one end of the rope in
their hands.  The other end he fastened round himself, and choosing a
point considerably above that to which the man clung, he plunged into
the rapids.  He was carried violently downwards, but he caught the
rock, secured the old painter and saved him.  Newspapers from all
parts of the Union poured in upon me, describing this gallant act of
my guide Conroy.

********************

VIII.  THE PARALLEL ROADS OF GLEN ROY.

[Footnote: A discourse delivered at the Royal Institution of Great
Britain on June 9, 1876.]

THE first published allusion to the Parallel Roads of Glen Roy occurs
in the appendix to the third volume of Pennant's 'Tour in Scotland,' a
work published in 1776.  'In the face of these hills,' says this
writer, 'both sides of the glen, there are three roads at small
distances from each other and directly opposite on each side.  These
roads have been measured in the complete parts of them, and found to
be 26 paces of a man 5 feet 10 inches high.  The two highest are
pretty near each other, about 50 yards, and the lowest double that
distance from the nearest to it.  They are carried along the sides of
the glen with the utmost regularity, nearly as exact as drawn with a
line of rule and compass.'

The correct heights of the three roads of Glen Roy are respectively
1150, 1070, and 860 feet above the sea.  Hence a vertical distance of
80 feet separates the two highest, while the lowest road is 210 feet
below the middle one.

These 'roads' are usually shelves or terraces formed in the yielding
drift which here covers the slopes of the mountains.  They are all
sensibly horizontal and therefore parallel.  Pennant accepted as
reasonable the explanation of them given by the country people in his
time.  They thought that the roads 'were designed for the chase, and
that the terraces were made after the spots were cleared in lines from
wood, in order to tempt the animals into the open paths after they
were rouzed, in order that they might come within reach of the bowmen
who might conceal themselves in the woods above and below.'

In these attempts of 'the country people' we have an illustration of
that impulse to which all scientific knowledge is due--the desire to
know the causes of things; and it is a matter of surprise that in the
case of the parallel roads, with their weird appearance challenging
enquiry, this impulse did not make itself more rapidly and
energetically felt.  Their remoteness may perhaps account for the fact
that until the year 1817 no systematic description of them, and no
scientific attempt at an explanation of them, appeared.  In that year
Dr. MacCulloch, who was then President of the Geological Society,
presented to that Society a memoir, in which the roads were discussed,
and pronounced to be the margins of lakes once embosomed in Glen Roy.
Why there should be three roads, or why the lakes should stand at
these particular levels, was left unexplained.

To Dr. MacCulloch succeeded a man, possibly not so learned as a
geologist, but obviously fitted by nature to grapple with her facts
and to put them in their proper setting.  I refer to Sir Thomas
Dick-Lauder, who presented to the Royal Society of Edinburgh, on the
2nd of March, 1818, his paper on the Parallel Roads of Glen.  Roy.  In
looking over the literature of this subject, which is now copious, it
is interesting to observe the differentiation of minds, and to single
out those who went by a kind of instinct to the core of the question,
from those who erred in it, or who learnedly occupied themselves with
its analogies, adjuncts, and details.  There is no man, in my opinion,
connected with the history of the subject, who has shown, in relation
to it, this spirit of penetration, this force of scientific insight,
more conspicuously than Sir Thomas Dick-Lauder.  Two distinct mental
processes are involved in the treatment of such a question.  Firstly,
the faithful and sufficient observation of the data; and secondly,
that higher mental process in which the constructive imagination comes
into play, connecting the separate facts of observation with their
common cause, and weaving them into an organic whole.  In neither of
these requirements did Sir Thomas Dick-Lauder fail.

Adjacent to Glen Roy is a valley called Glen Gluoy, along the sides of
which ran a single shelf, or terrace, formed obviously in the same
manner as the parallel roads of Glen Roy.  The two shelves on the
opposing sides of the glen were at precisely the same level, and
Dick-Lauder wished to see whether, and how, they became united at the
head of the glen.  He followed the shelves into the recesses of the
mountains.  The bottom of the valley, as it rose, came ever nearer to
them, until finally, at the head of Glen Gluoy, he reached a col, or
watershed, of precisely the same elevation as the road which swept
round the glen.

The correct height of this col is 1170 feet above the sea; that is to
say, 20 feet above the highest road in Glen Roy.

From this col a lateral branch-valley--Glen Turrit--led down to Glen
Roy.  Our explorer descended from the col to the highest road of the
latter glen, and pursued it exactly as he had pursued the road in Glen
Gluoy.  For a time it belted the mountain sides at a considerable
height above the bottom of the valley; but this rose as he proceeded,
coming ever nearer to the highest shelf, until finally he reached a
col, or watershed, looking into Glen Spey, and of precisely the same
elevation as the highest road of Glen Roy.

He then dropped down to the lowest of these roads, and followed it
towards the mouth of the glen.  Its elevation above the bottom of the
valley gradually increased; not because the shelf rose, but because it
remained level while the valley sloped downwards.  He found this
lowest road doubling round the hills at the mouth of Glen Roy, and
running along the sides of the mountains which flank Glen Spean.  He
followed it eastwards.

PARALLEL ROADS OF GLEN ROY.

After a Sketch by Sir Thomas Dick-Lauder.

The bottom of the Spean Valley, like the others, gradually rose, and
therefore gradually approached the road on the adjacent mountain-side.
He came to Loch Laggan, the surface of which rose almost to the level
of the road, and beyond the head of this lake he found, as in the
other two cases, a col, or watershed, at Makul, of exactly the same
level as the single road in Glen Spean, which, it will be remembered,
is a continuation of the lowest road in Glen Roy.

Here we have a series of facts of obvious significance as regards the
solution of this problem.  The effort of the mind to form a coherent
image from such facts may be compared with the effort of the eyes to
cause the pictures of a stereoscope to coalesce.  For a time we
exercise a certain strain, the object remaining vague and indistinct.
Suddenly its various parts seem to run together, the object starting
forth in clear and definite relief.  Such, I take it, was the effect
of his ponderings upon the mind of Sir Thomas Dick-Lauder.  His
solution was this: Taking all their features into account, he was
convinced that water only could have produced the terraces.  But how
had the water been collected?  He saw clearly that, supposing the
mouth of Glen Gluoy to be stopped by a barrier sufficiently high, if
the waters from the mountains flanking the glen were allowed to
collect, they would form behind the barrier a lake, the surface of
which would gradually rise until it reached the level of the col at
the head of the glen.  The rising would then cease; the superfluous
water of Glen Gluoy discharging itself over the col into Glen Roy.  As
long as the barrier stopping the mouth of Glen Gluoy continued high
enough, we should have in that glen a lake at the precise level of its
shelf, which lake, acting upon the loose drift of the flanking
mountains, would form the shelf revealed by observation.

So much for Glen Gluoy.  But suppose the mouth of Glen Roy also
stopped by a similar barrier.  Behind it also the water from the
adjacent mountains would collect.  The surface of the lake thus formed
would gradually rise, until it had reached the level of the col which
divides Glen Roy from Glen Spey.  Here the rising of the lake would
cease; its superabundant water being poured over the col into the
valley of the Spey.  This state of things would continue as long as a
sufficiently high barrier remained at the mouth of Glen Roy.  The lake
thus dammed in, with its surface at the level of the highest parallel
road, would act, as in Glen Gluoy, upon the friable drift
overspreading the mountains, and would form the highest road or
terrace of Glen Roy.

And now let us suppose the barrier to be so far removed from the mouth
of Glen Roy as to establish a connection between it and the upper part
of Glen Spean, while the lower part of the latter glen still continued
to be blocked up.  Upper Glen Spean and Glen Roy would then be
occupied by a continuous lake, the level of which would obviously be
determined by the col at the head of Loch Laggan.  The water in Glen
Roy would sink from the level it had previously maintained, to the
level of its new place of escape.  This new lake-surface would
correspond exactly with the lowest parallel road, and it would form
that road by its action upon the drift of the adjacent mountains.

In presence of the observed facts, this solution commends itself
strongly to the scientific mind.  The question next occurs, What was
the character of the assumed barrier which stopped the glens?  There
are at the present moment vast masses of detritus in certain portions
of Glen Spean, and of such detritus Sir Thomas Dick-Lauder imagined
his barriers to have been formed.  By some unknown convulsion, this
detritus had been heaped up.  But, once given, and once granted that
it was subsequently removed in the manner indicated, the single road
of Glen Gluoy and the highest and lowest roads of Glen Roy would be
explained in a satisfactory manner.

To account for the second or middle road of Glen Roy, Sir Thomas
Dick-Lauder invoked a new agency.  He supposed that at a certain point
in the breaking down or waste of his dam, a halt occurred, the barrier
holding its ground at a particular level sufficiently long to dam a
lake rising to the height of, and forming the second road.  This point
of weakness was at once detected by Mr. Darwin, and adduced by him as
proving that the levels of the cols did not constitute an essential
feature in the phenomena of the parallel roads.  Though not destroyed,
Sir Thomas Dick-Lauder's theory was seriously shaken by this argument,
and it became a point of capital importance, if the facts permitted,
to remove such source of weakness.  This was done in 1847 by Mr. David
Milne, now Mr. Milne-Home.  On walking up Glen Roy from Roy Bridge, we
pass the mouth of a lateral glen, called Glen Glaster, running
eastward from Glen Roy.  There is nothing in this lateral glen to
attract attention, or to suggest that it could have any conspicuous
influence in the production of the parallel roads.  Hence, probably,
the failure of Sir Thomas Dick-Lauder to notice it.  But Mr.
Milne-Home entered this glen, on the northern side of which the middle
and lowest roads are fairly shown.  The principal stream running
through the glen turns at a certain point northwards and loses itself
among hills too high to offer any outlet.  But another branch of the
glen turns to the south-east; and, following up this branch, Mr.
Milne-Home reached a col, or watershed, of the precise level of the
second Glen Roy road.  When the barrier blocking the glens had been so
far removed as to open this col, the water in Glen Roy would sink to
the level of the second road.  A new lake of diminished depth would be
thus formed, the surplus water of which would escape over the Glen
Glaster col into Glen Spean.  The margin of this new lake, acting upon
the detrital matter, would form the second road.  The theory of Sir
Thomas Dick-Lauder, as regards the part played by the cols, was
re-riveted by this new and unexpected discovery.

I have referred to Mr. Darwin, whose powerful mind swayed for a time
the convictions of the scientific world in relation to this question.
His notion was--and it is a notion which very naturally presents
itself--that the parallel roads were formed by the sea; that this
whole region was once submerged and subsequently upheaved; that there
were pauses in the process of upheaval, during which these glens
constituted so many fiords, on the sides of which the parallel
terraces were formed.  This theory will not bear close criticism; nor
is it now maintained by Mr. Darwin himself.  It would not account for
the sea being 20 feet higher in Glen Gluoy than in Glen Roy.  It would
not account for the absence of the second and third Glen Roy roads
from Glen Gluoy, where the mountain flanks are quite as impressionable
as in Glen Roy.  It would not account for the absence of the shelves
from the other mountains in the neighbourhood, all of which 'would
have been clasped by the sea had the sea been there.  Here then, and
no doubt elsewhere, Mr. Darwin has shown himself to be fallible; but
here, as elsewhere, he has shown himself equal to that discipline of
surrender to evidence which girds his intellect with such unassailable
moral strength.

But, granting the significance of Sir Thomas Dick-Lauder's facts, and
the reasonableness, on the whole, of the views which he has founded on
them, they will not bear examination in detail.  No such barriers of
detritus as he assumed could have existed without leaving traces
behind them; but there is no trace left.  There is detritus enough in
Glen Spean, but not where it is wanted.  The two highest parallel
roads stop abruptly at different points near the mouth of Glen Roy,
but no remnant of the barrier against which they abutted is to be
seen.  It might be urged that the subsequent invasion of the valley by
glaciers has swept the detritus away; but there have been no glaciers
in these valleys since the disappearance of the lakes.  Professor
Geikie has favoured me with a drawing of the Glen Spean 'road' near
the entrance to Glen Trieg.  The road forms a shelf round a great
mound of detritus which, had a glacier followed the formation of the
shelf, must have been cleared away.  Taking all the circumstances into
account, you may, I think, with safety dismiss the detrital barrier as
incompetent to account for the present condition of Glen Gluoy and
Glen Roy.

Hypotheses in science, though apparently transcending experience, are
in reality experience modified by scientific thought and pushed into
an ultra experiential region.  At the time that he wrote, Sir Thomas
Dick-Lauder could not possibly have discerned the cause subsequently
assigned for the blockage of these glens.  A knowledge of the action
of ancient glaciers was the necessary antecedent to the new
explanation, and experience of this nature was not possessed by the
distinguished writer just mentioned.  The extension of Swiss glaciers
far beyond their present limits, was first made known by a Swiss
engineer named Venetz, who established, by the marks they had left
behind them, their former existence in places which they had long
forsaken.  The subject of glacier extension was subsequently followed
up with distinguished success by Charpentier, Studer, and others. With
characteristic vigour Agassiz grappled with it, extending his
observations far beyond the domain of Switzerland.  He came to this
country in 1840, and found in various places indubitable marks of
ancient glacier action.  England, Scotland, Wales, and Ireland he
proved to have once given birth to glaciers.  He visited Glen Roy,
surveyed the surrounding neighbourhood, and pronounced, as a
consequence of his investigation, the barriers which stopped the glens
and produced the parallel roads to have been barriers of ice.  To Mr.
Jamieson, above all others, we are indebted for the thorough testing
and confirmation of this theory.

And let me here say that Agassiz is only too likely to be misrated and
misjudged by those who, though accurate within a limited sphere, fail
to grasp in their totality the motive powers invoked in scientific
investigation.  True he lacked mechanical precision, but he abounded
in that force and freshness of the scientific imagination which in
some sciences, and probably in some stages of all sciences, are
essential to the creator of knowledge.  To Agassiz was given, not the
art of the refiner, but the instinct of the discoverer, and the
strength of the delver who brings ore from the recesses of the mine.
That ore may contain its share of dross, but it also contains the
precious metal which gives employment to the refiner, and without
which his occupation would depart.

Let us dwell for a moment upon this subject of ancient glaciers. Under
a flask containing water, in which a thermometer is immersed, is
placed a Bunsen's lamp.  The water is heated, reaches a temperature of
212°, and then begins to boil.  The rise of the thermometer then
ceases, although heat continues to be poured by the lamp into the
water.  What becomes of that heat?  We know that it is consumed in the
molecular work of vaporization.  In the experiment here arranged, the
steam passes from the flask through a tube into a second vessel kept
at a low temperature.  Here it is condensed, and indeed congealed to
ice, the second vessel being plunged in a mixture cold enough to
freeze the water.  As a result of the process we obtain a mass of ice.
That ice has an origin very antithetical to its own character.  Though
cold, it is the child of heat.  If we removed the lamp, there would be
no steam, and if there were no steam there would be no ice.  The mere
cold of the mixture surrounding the second vessel would not produce
ice.  The cold must have the proper material to work upon; and this
material--aqueous vapour--is, as we here see, the direct product of
heat.

It is now, I suppose, fifteen or sixteen years since I found myself
conversing with an illustrious philosopher regarding that glacial
epoch which the researches of Agassiz and others had revealed.  This
profoundly thoughtful man maintained the fixed opinion that, at a
certain stage in the history of the solar system, the sun's radiation
had suffered diminution, the glacial epoch being a consequence of this
solar chill.  The celebrated French mathematician Poisson had another
theory.  Astronomers have shown that the solar system moves through
space, and 'the temperature of space' is a familiar expression with
scientific men.  It was considered probable by Poisson that our
system, during its motion, had traversed portions of space of
different temperatures; and that, during its passage through one of
the colder regions of the universe, the glacial epoch occurred.
Notions such as these were more or less current everywhere not many
years ago, and I therefore thought it worth while to show how
incomplete they were.  Suppose the temperature of our planet to be
reduced, by the subsidence of solar heat, the cold of space, or any
other cause, say one hundred degrees.  Four-and-twenty hours of such a
chill would bring down as, snow nearly all the moisture of our
atmosphere.  But this would not produce a glacial epoch.  Such an
epoch would require the long-continued generation of the material from
which the ice of glaciers is derived.  Mountain snow, the nutriment of
glaciers, is derived from aqueous vapour raised mainly from the
tropical ocean by the sun.  The solar fire is as necessary a factor in
the process as our lamp in the experiment referred to a moment ago.
Nothing is easier than to calculate the exact amount of heat expended
by the sun in the production of a glacier.  It would, as I have
elsewhere shown, [Footnote: 'Heat a Mode of Motion,' fifth edition,
chap. vi: Forms of Water, sections 55 and 56.]  raise a quantity of
cast iron five times the weight of the glacier not only to a white
heat, but to its point of fusion.  If, as I have already urged,
instead of being filled with ice, the valleys of the Alps were filled
with white-hot metal, of quintuple the mass of the present glaciers,
it is the heat, and not the cold, that would arrest our attention and
solicit our explanation.  The process of glacier making is obviously
one of distillation, in which the fire of the sun, which generates the
vapour, plays as essential a part as the cold of the mountains which
condenses it. [Footnote: In Lyell's excellent 'Principles of Geology,'
the remark occurs that 'several writers have fallen into the strange
error of supposing that the glacial period must have been one of
higher mean temperature than usual.'  The really strange error was the
forgetfulness of the fact that without the heat the substance
necessary to the production of glaciers would be wanting.]

It was their ascription to glacier action that first gave the parallel
roads of Glen Roy an interest in my eyes; and in 1867, with a view to
self-instruction, I made a solitary pilgrimage to the place, and
explored pretty thoroughly the roads of the principal glen.  I traced
the highest road to the col dividing Glen Roy from Glen Spey, and,
thanks to the civility of an Ordnance surveyor, I was enabled to
inspect some of the roads with a theodolite, and to satisfy myself
regarding the common level of the shelves at opposite sides of the
valley.  As stated by Pennant, the width of the roads amounts
sometimes to more than twenty yards; but near the head of Glen Roy the
highest road ceases to have any width, for it runs along the face of a
rock, the effect of the lapping of the water on the more friable
portions of the rock being perfectly distinct to this hour.  My
knowledge of the region was, however, far from complete, and nine
years had dimmed the memory even of the portion which had been
thoroughly examined.  Hence my desire to see the roads once more
before venturing to talk to you about them.  The Easter holidays of
1876 were to be devoted to this purpose; but at the last moment a
telegram from Roy Bridge informed me that the roads were snowed up.
Finding books and memories poor substitutes for the flavour of facts,
I resolved subsequently to make another effort to see the roads.
Accordingly last Thursday fortnight, after lecturing here, I packed
up, and started (not this time alone) for the North.  Next day at noon
my wife and I found ourselves at Dalwhinnie, whence a drive of some
five-and-thirty miles brought us to the excellent hostelry of Mr.
Macintosh, at the mouth of Glen Roy.

We might have found the hills covered with mist, which would have
wholly defeated us; but Nature was good-natured, and we had two
successful working days among the hills.  Guided by the excellent
ordnance map of the region, on the Saturday morning we went up the
glen, and on reaching the stream called Allt Bhreac Achaidh faced the
hills to the west. At the watershed between Glen Roy and Glen Fintaig
we bore northwards, struck the ridge above Glen Gluoy, came in view of
its road, which we persistently followed as long as it continued
visible.  It is a feature of all the roads that they vanish before
reaching the cola over which fell the waters of the lakes which formed
them.  One reason doubtless is that at their upper ends the lakes were
shallow, and incompetent on this account to raise wavelets of any
strength to act upon the mountain drift.  A second reason is that they
were land-locked in the higher portions and protected from the
south-westerly winds, the stillness of their waters causing them to
produce but a feeble impression upon the mountain sides.  From Glen
Gluoy we passed down Glen Turrit to Glen Roy, and through it
homewards, thus accomplishing two or three and twenty miles of rough
and honest work.

Next day we thoroughly explored Glen Glaster, following its two roads
as far as they were visible.  We reached the col discovered by Mr.
Milne-Home, which stands at the level of the middle road of Glen Roy.
Thence we crossed southwards over the mountain _Creag Dhubh_, and
examined the erratic blocks upon its sides, and the ridges and mounds
of moraine matter which cumber the lower flanks of the mountain.  The
observations of Mr. Jamieson upon this region, including the mouth of
Glen Trieg, are in the highest degree interesting.  We entered Glen
Spean, and continued a search begun on the evening of our arrival at
Roy Bridge--the search, namely, for glacier polishings and markings.
We did not find them copious, but they are indubitable.

One of the proofs most convenient for reference, is a great rounded
rock by the roadside, 1,000 yards east of the milestone marked
three-quarters of a mile from Roy Bridge.  Farther east other cases
occur, and they leave no doubt upon the mind that Glen Spean was at
one time filled by a great glacier.  To the disciplined eye the aspect
of the mountains is perfectly conclusive on this point; and in no
position can the observer more readily and thoroughly convince himself
of this than at the head of Glen Glaster.  The dominant hills here are
all intensely glaciated.

But the great collecting ground of the glaciers which dammed the glens
and produced the parallel roads, were the mountains south and west of
Glen Spean.  The monarch of these is Ben Nevis, 4,370 feet high.  The
position of Ben Nevis and his colleagues, in reference to the
vapour-laden winds of the Atlantic, is a point of the first
importance.  It is exactly similar to that of Carrantual and the
Macgillicuddy Reeks in the south-west of Ireland.  These mountains
are, and were, the first to encounter the south-western Atlantic
winds, and the precipitation, even at present, in the neighbourhood of
Killarney, is enormous.  The winds, robbed of their vapour, and
charged with the heat set free by its precipitation, pursue their
direction obliquely across Ireland; and the effect of the drying
process may be understood by comparing the rainfall at Cahirciveen
with that at Portarlington.  As found by Dr. Lloyd, the ratio is as 59
to 21--fifty-nine inches annually at Cahirciveen to twenty-one at
Portarlington.  During the glacial epoch this vapour fell as snow, and
the consequence was a system of glaciers which have left traces and
evidences of the most impressive character in the region of the
Killarney Lakes.  I have referred in other places to the great glacier
which, descending from the Reeks, moved through the Black Valley, took
possession of the lake-basins, and left its traces on every rock and
island emergent from the waters of the upper lake.  They are all
conspicuously glaciated.  Not in Switzerland itself do we find clearer
traces of ancient glacier action.

What the Macgillicuddy Reeks did in Ireland, Ben Nevis and the
adjacent mountains did, and continue to do, in Scotland.  We had an
example of this on the morning we quitted Roy Bridge.  From the bridge
westward rain fell copiously, and the roads were wet; but the
precipitation ceased near Loch Laggan, whence eastward the roads were
dry.  Measured by the gauge, the rainfall Fort William is 86 inches,
while at Laggan it is only 46 inches annually.  The difference between
west and east is forcibly brought out by observations at the two ends
of the Caledonian Canal.  Fort William at the south-western end has,
as just stated, 86 inches, while Culloden, at its north-eastern end,
has only 24.  To the researches of that able and accomplished
meteorologist, Mr. Buchan, we are indebted for these and other data of
the most interesting and valuable kind.

Adhering to the facts now presented to us, it is not difficult to
restore in idea the process by which the glaciers of Lochaber were
produced and the glens dammed by ice.  When the cold of the glacial
epoch began to invade the Scottish hills, the sun at the same time
acting with sufficient power upon the tropical ocean, the vapours
raised and drifted on to these 'northern mountains were more and more
converted into snow.  This slid down the slopes, and from every
valley, strath, and corry, south of Glen Spean, glaciers were poured
into that glen.  The two great factors here brought into play are the
nutrition of the glaciers by the frozen material above, and their
consumption in the milder air below.  For a period supply exceeded
consumption, and the ice extended, filling Glen Spean to an
ever-increasing height, and abutting against the mountains to the
north of that glen.  But why, it may be asked, should the valleys
south of Glen Spean be receptacles of ice at a time when those north
of it were receptacles of water?  The answer is to be found in the
position and the greater elevation of the mountains south of Glen
Spean.  They first received the loads of moisture carried by the
Atlantic winds, and not until they had been in part dried, and also
warmed by the liberation of their latent heat, did these winds touch
the hills north of the Glen.

An instructive observation bearing upon this point is here to be
noted.  Had our visit been in the winter we should have found all the
mountains covered; had it been in the summer we should have found the
snow all gone.  But happily it was at a season when the aspect of the
mountains north and south of Glen Spean exhibited their relative
powers as snow collectors.  Scanning the former hills from many points
of view, we were hardly able to detect a fleck of snow, while heavy
swaths and patches loaded the latter.  Were the glacial epoch to
return, the relation indicated by this observation would cause Glen
Spean to be filled with glaciers from the south, while the hills and
valleys on the north, visited by warmer and drier winds, would remain
comparatively free from ice.  This flow from the south would be
reinforced from the west, and as long as the supply was in excess of
the consumption the glaciers would extend, the dams which closed the
glens increasing in height.  By-and-by supply and consumption becoming
approximately equal, the height of the glacier barriers would remain
constant.  Then, as milder weather set in, consumption would be in
excess, a lowering of the barriers and a retreat of the ice being the
consequence.  But for a long time the conflict between supply and
consumption would continue, retarding indefinitely the disappearance
of the barriers, and keeping the imprisoned lakes in the northern
glens.  But however slow its retreat, the ice in the long run would be
forced to yield.  The dam at the mouth of Glen Roy, which probably
entered the glen sufficiently far to block up Glen Glaster, would
gradually retreat.  Glen Glaster and its col being opened, the
subsidence of the lake eighty feet, from the level of the highest to
that of the second parallel road, would follow as a consequence.  I
think this the most probable course of things, but it is also possible
that Glen Glaster may have been blocked by a glacier from Glen Trieg.
The ice dam continuing to retreat, at length permitted Glen Roy to
connect itself with upper Glen Spean.  A continuous lake then filled
both glens, the level of which, as already explained, was determined
by the col at Makul, above the head of Loch Laggan.  The last to yield
was the portion of the glacier which derived nutrition from Ben Nevis,
and probably also from the mountains north and south of Loch Arkaig.
But it at length yielded, and the waters in the glens resumed the
courses which they pursue to-day.

For the removal of the ice barriers no cataclysm is to be invoked; the
gradual melting of the dam would produce the entire series of
phenomena.  In sinking from col to col the water would flow over a
gradually melting barrier, the surface of the imprisoned lake not
remaining sufficiently long at any particular level to produce a shelf
comparable to the parallel roads.  By temporary halts in the process
of melting due to atmospheric conditions or to the character of the
dam itself, or through local softness in the drift, small
pseudo-terraces would be formed, which, to the perplexity of some
observers, are seen upon the flanks of the glens to-day.

In presence then of the fact that the barriers which stopped these
glens to a height, it may be, of 1,500 feet above the bottom of Glen
Spean, have dissolved and left not a wreck behind; in presence of the
fact, insisted on by Professor Geikie, that barriers of detritus would
undoubtedly have been able to maintain themselves had they ever been
there; in presence of the fact that great glaciers once most certainly
filled these valleys--that the whole region, as proved by Mr.
Jamieson, is filled with the traces of their action; the theory which
ascribes the parallel roads to lakes dammed by barriers of ice has, in
my opinion, a degree of probability on its side which amounts to a
practical demonstration of its truth.

Into the details of the terrace formation I do not enter.  Mr. Darwin
and Mr. Jamieson on the one side, and Sir John Lubbock on the other,
deal with true causes.  The terraces, no doubt, are due in part to the
descending drift arrested by the water, and in part to the fretting of
the wavelets, and the rearrangement of the stirred detritus, along the
belts of contact of lake and bill.  The descent of matter must have
been frequent when the drift was unbound by the rootlets which hold it
together now.  In some cases, it may be remarked, the visibility of
the roads is materially augmented by differences of vegetation.  The
grass upon the terraces is not always of the same character as that
above and below them, while on heather-covered hills the absence of
the dark shrub from the roads greatly enhances their conspicuousness.

The annexed sketch of a model will enable the reader to grasp the
essential features of the problem and its solution.  Glen Gluoy and
Glen Roy are lateral valleys which open into Glen Spean.  Let us
suppose Glen Spean filled from v to w with ice of a uniform elevation
of 1,500 feet above the sea, the ice not filling the upper part of
that glen.  The ice would thrust itself for some distance up the
lateral valleys, closing all their mouths.  The streams from the
mountains right and left of Glen Gluoy would pour their waters into
that glen, forming a lake, the level of which would be determined by
the height of the col at A, 1170 feet above the sea.  Over this col
the water would flow into Glen Roy.  But in Glen Roy it could not rise
higher than 1150 feet, the height of the col at B, over which it would
flow into Glen Spey.

The water halting at these levels for a sufficient time, would form
the single road in Glen Gluoy and the highest road in Glen Roy.  This
state of things would continue as long as the ice dam was sufficiently
high to dominate the cols at A and B; but when through change of
climate the gradually sinking dam reached, in succession, the levels
of these cols, the water would then begin to flow over the dam instead
of over the cols.  Let us suppose the wasting of the ice to continue
until a connection was established between Glen Roy and Glen Glaster,
a common lake would then fill both these glens, the level of which
would be determined by that of the col c, over which the water would
pour for an indefinite period into Glen Spean.  During this period the
second Glen Roy road and the highest road of Glen Glaster would be
formed.  The ice subsiding still further, a connection would
eventually be established between Glen Roy, Glen Glaster, and the
upper part of Glen Spean.  A common lake would fill all three glens,
the level of which would be that of the col D, over which for an
indefinite period the lake would pour its water.  During this period
the lowest Glen Roy road, which is common also to Glen Glaster and
Glen Spean, would be formed.  Finally, on the disappearance of the ice
from the lower part of Glen Spean the waters would flow down their
respective valleys as they do to-day.

Fig.  7

Reviewing our work, we find three considerable steps to have marked
the solution of the problem of the Parallel Roads of Glen Roy.  The
first of these was taken by Sir Thomas Dick-Lauder, the second was the
pregnant conception of Agassiz regarding glacier action, and the third
was the testing and verification of this conception by the very
thorough researches of Mr. Jamieson.  No circumstance or incident
connected with this discourse gives me greater pleasure than the
recognition of the value of these researches.  They are marked
throughout by unflagging industry, by novelty and acuteness of
observation, and by reasoning power of a high and varied kind.  These
pages had been returned 'for press' when I learned that the relation
of Ben Nevis and his colleagues to the vapour-laden winds of the
Atlantic had not escaped Mr. Jamieson.  To him obviously the
exploration of Lochaber, and the development of the theory of the
Parallel Roads, has been a labour of love.

Thus ends our rapid survey of this brief episode in the physical
history of the Scottish hills,--brief, that is to say, in comparison
with the immeasurable lapses of time through which, to produce its
varied structure and appearances, our planet must have passed.  In the
survey of such a field two things are specially worthy to be taken
into account--the widening of the intellectual horizon and the
reaction of expanding knowledge upon the intellectual organ itself.

At first, as in the case of ancient glaciers, through sheer want of
capacity, the mind refuses to take in revealed facts.  But by degrees
the steady contemplation of these facts so strengthens and expands the
intellectual powers, that where truth once could not find an entrance
it eventually finds a home. [Footnote: The formation, connection,
successive subsidence, and final disappearance of the glacial lakes of
Lochaber were illustrated in the discourse here reported by the model
just described, constructed under the supervision of my assistant, Mr.
John Cottrell.  Glen Gluoy with its lake and road and the cataract
over its col; Glen Roy and its three roads with their respective
cataracts at the head of Glen Spey, Glen Glaster, and Glen Spean, were
all represented.  The successive shiftings of the barriers, which were
formed of plate glass, brought each successive lake and its
corresponding road into view, while the entire removal of the barriers
caused the streams to flow down the glens of the model as they flow
down the real glens of to-day.]

A map of the district, with the parallel roads shown in red, is
annexed.

LITERATURE OF THE SUBJECT.

THOMAS PENNANT.--A Tour in Scotland.  Vol. iii. 1776, p. 394. JOHN
MACCULLOCH.--On the Parallel Roads of Glen Roy.  Geol. Soc. Trans.
vol. iv. 1817, p. 314.

THOMAS LAUDER DICK (afterwards SIR THOMAS DICK-LAUDER, Bart.)--On the
Parallel Roads of Lochaber.  Edin. Roy. Soc. Trans. 1818, vol. ix.
p. 1.

CHARLES DARWIN.--Observations on the Parallel Roads of Glen Roy, and
of the other parts of Lochaber in Scotland, with an attempt to prove
that they are of marine origin.  Phil. Trans. 1839, vol. cxxix. p. 39.

SIR CHARLES LYELL.--Elements of Geology. Second edition, 1841.

Louis AGASSIZ.--The Glacial Theory and its Recent Progress--Parallel
Terraces. Edin. New Phil. Journal, 1842, vol. xxxiii. p. 236.

DAVID MILNE (afterwards DAVID MILNE-HOME).--On the Parallel Roads of
Lochaber; with Remarks on the Change of Relative Levels of Sea and
Land in Scotland, and on the Detrital Deposits in that Country. Edin.
Roy. Soc. Trans. 1847, vol. xvi. p. 395.

ROBERT CHAMBERS.--Ancient Sea Margins.  Edinburgh, 1848.

H. D. ROGERS.--On the Parallel Roads of Glen Roy. Royal Inst.
Proceedings, 1861, vol. iii. p. 341.

THOMAS F. JAMIESON.--On the Parallel Roads of Glen Roy, and their
Place in the History of the Glacial Period.  Quart. Journal Geol.
Soc. 1863, vol. xix. p. 235.

SIR CHARLES LYELL.--Antiquity of Man.  1863, p. 253.

REV. R. B. WATSON.--On the Marine Origin of the Parallel Roads of Glen
Roy. Quart. Journ. Geol. Soc. 1865, vol. xxii. p. 9.

SIR JOHN LUBBOCK.--On the Parallel Roads of Glen Roy. Quart. Journ.
Geol. Soc. 1867, vol. xxiv. p. 83.

CHARLES BABBAGE.--Observations on the Parallel Roads of Glen Roy.
Quart. Journ. Geol. Soc. 1868, vol. xxiv. p. 273.

JAMES NICOL.--On the Origin of the Parallel Roads of Glen Roy. 1869.
Geol. Soc. Journal, vol. xxv. p. 282.

JAMES NICOL.--How the Parallel Roads of Glen Roy were formed.  1872.
Geol. Soc. Journal, vol. xxviii. p. 237.

MAJOR-GENERAL SIR HENRY JAMES, R.E.--Notes on the Parallel Roads of
Lochaber. 4to. 1874.

********************

IX.  ALPINE SCULPTURE.

1864.

TO account for the conformation of the Alps, two hypotheses have been
advanced, which may be respectively named the hypothesis of fracture
and the hypothesis of erosion.  The former assumes that the forces by
which the mountains were elevated produced fissures in the earth's
crust, and that the valleys of the Alps are the tracks of these
fissures; while the latter maintains that the valleys have been cut
out by the action of ice and water, the mountains themselves being the
residual forms of this grand sculpture.  I had heard the Via Mala
cited as a conspicuous illustration of the fissure theory--the
profound chasm thus named, and through which the Hinter-Rhein now
flows, could, it was alleged, be nothing else than a crack in the
earth's crust. To the Via Mala I therefore went in 1864 to instruct
myself upon the point in question.

The gorge commences about a quarter of an hour above Tusis; and, on
entering it, the first impression certainly is that it must be a
fissure.  This conclusion in my case was modified as I advanced.  Some
distance up the gorge I found upon the slopes to my right quantities
of rolled stones, evidently rounded by water-action.  Still further
up, and just before reaching the first bridge which spans the chasm, I
found more rolled stones, associated with sand and gravel.  Through
this mass of detritus, fortunately, a vertical cutting had been made,
which exhibited a section showing perfect stratification.  There was
no agency in the place to roll these stones, and to deposit these
alternating layers of sand and pebbles, but the river which now rushes
some hundreds of feet below them.  At one period of the Via Mala's
history the river must have run at this high level.  Other evidences
of water-action soon revealed themselves.  From the parapet of the
first bridge I could see the solid rock 200 feet above the bed of the
river scooped and eroded.

It is stated in the guide-books that the river, which usually runs
along the bottom of the gorge, has been known almost to fill it during
violent thunder-storms; and it may be urged that the marks of erosion
which the sides of the chasm exhibit are due to those occasional
floods.  In reply to this, it may be stated that even the existence of
such floods is not well authenticated, and that if the supposition
were true, it would be an additional argument in favour of the cutting
power of the river.  For if floods operating at rare intervals could
thus erode the rock, the same agency, acting without ceasing upon the
river's bed, must certainly be competent to excavate it.

I proceeded upwards, and from a point near another bridge (which of
them I did not note) had a fine view of a portion of the gorge.  The
river here runs at the bottom of a cleft of profound depth, but so
narrow that it might be leaped across.  That this cleft must be a
crack is the impression first produced; but a brief inspection
suffices to prove that it has been cut by the river.  From top to
bottom we have the unmistakable marks of erosion.  This cleft was best
seen on looking downwards from a point near the bridge; but looking
upwards from the bridge itself, the evidence of aqueous erosion was
equally convincing.

The character of the erosion depends upon the rock as well as upon the
river.  The action of water upon some rocks is almost purely
mechanical; they are simply ground away or detached in sensible
masses.  Water, however, in passing over limestone, charges itself
with carbonate of lime without damage to its transparency; the rock is
dissolved in the water; and the gorges cut by water in such rocks
often resemble those cut in the ice of glaciers by glacier streams. To
the solubility of limestone is probably to be ascribed the fantastic
forms which peaks of this rock usually assume, and also the grottos
and caverns which interpenetrate limestone formations.  A rock capable
of being thus dissolved will expose a smooth surface after the water
has quitted it; and in the case of the Via Mala it is the polish of
the surfaces and the curved hollows scooped in the sides of the gorge,
which assure us that the chasm has been the work of the river.

About four miles from Tusis, and not far from the little village of
Zillis, the Via Mala opens into a plain bounded by high terraces.  It
occurred to me the moment I saw it that the plain had been the bed of
an ancient lake; and a farmer, who was my temporary companion,
immediately informed me that such was the tradition of the
neighbourhood.  This man conversed with intelligence, and as I drew
his attention to the rolled stones, which rest not only above the
river, but above the road, and inferred that the river must once have
been there to have rolled those stones, he saw the force of the
evidence perfectly.  In fact, in former times, and subsequent to the
retreat of the great glaciers, a rocky barrier crossed the valley at
this place, damming the river which came from the mountains higher up.
A lake was thus formed which poured its waters over the barrier.  Two
actions were here at work, both tending to obliterate the lake--the
raising of its bed by the deposition of detritus, and the cutting of
its dam by the river.  In process of time the cut deepened into the
Via Mala; the lake was drained, and the river now flows in a definite
channel through the plain which its waters once totally covered.

From Tusis I crossed to Tiefenkasten by the Schien Pass, and thence
over the Julier Pass to Pontresina.  There are three or four ancient
lake-beds between Tiefenkasten and the summit of the Julier.  They are
all of the same type--a more or less broad and level valley-bottom,
with a barrier in front through which the river has cut a passage, the
drainage of the lake being the consequence.  These lakes were
sometimes dammed by barriers of rock, sometimes by the moraines of
ancient glaciers.

An example of this latter kind occurs in the Rosegg valley, about
twenty minutes below the end of the Rosegg glacier, and about an hour
from Pontresina.  The valley here is crossed by a pine-covered moraine
of the noblest dimensions; in the neighbourhood of London it might be
called a mountain.  That it is a moraine, the inspection of it from a
point on the Surlei slopes above it will convince any person
possessing an educated eye.  Where, moreover, the interior of the
mound is exposed, it exhibits moraine-matter--detritus pulverised by
the ice, with boulders entangled in it.  It stretched quite across the
valley, and at one time dammed the river up.  But now the barrier is
cut through, the stream having about one-fourth of the moraine to its
right, and the remaining three-fourths to its left.  Other moraines of
a more resisting character hold their ground as barriers to the
present day.

In the Val di Campo, for example, about three-quarters of an hour from
Pisciadello, there is a moraine composed of large boulders, which
interrupt the course of a river and compel the water to fall over them
in cascades.  They have in great part resisted its action since the
retreat of the ancient glacier which formed the moraine.  Behind the
moraine is a lake-bed, now converted into a level meadow, which rests
on a deep layer of mould.

At Pontresina a very fine and instructive gorge is to be seen.  The
river from the Morteratsch glacier rushes through a deep and narrow
chasm which is spanned at one place by a stone bridge.  The rock is
not of a character to preserve smooth polishing; but the larger
features of water-action are perfectly evident from top to bottom.
Those features are in part visible from the bridge, but still better
from a point a little distance from the bridge in the direction of the
upper village of Pontresina.  The hollowing out of the rock by the
eddies of the water is here quite manifest. A few minutes' walk
upwards brings us to the end of the gorge; and behind it we have the
usual indications of an ancient lake, and terraces of distinct water
origin.  From this position indeed the genesis of the gorge is clearly
revealed.  After the retreat of the ancient glacier, a transverse
ridge of comparatively resisting material crossed the valley at this
place.  Over the lowest part of this ridge the river flowed, rushing
steeply down to join at the bottom of the slope the stream which
issued from the Rosegg glacier.  On this incline the water became a
powerful eroding agent, and finally cut the channel to its present
depth.

Geological writers of reputation assume at this place the existence of
a fissure, the 'washing out' of which resulted in the formation of the
gorge.  Now no examination of the bed of the river ever proved the
existence of this fissure; and it is certain that water, particularly
when charged with solid matter in suspension, can cut a channel
through unfissured rock.  Cases of deep cutting can be pointed out
where the clean bed of the stream is exposed, the rock which forms the
floor of the river not exhibiting a trace of fissure.  An example of
this kind on a small scale occurs near the Bernina Gasthaus, about two
hours from Pontresina.  A little way below the junction of the two
streams from the Bernina Pass and the Heuthal the river flows through
a channel cut by itself, and 20 or 30 feet in depth.  At some places
the river-bed is covered with rolled stones; at other places it is
bare, but shows no trace of fissure.  The abstract power of water, if
I may use the term, to cut through rock is demonstrated by such
instances.  But if water be competent to form a gorge without the aid
of a fissure, why assume the existence of such fissures in cases like
that at Pontresina?  It seems far more philosophical to accept the
simple and impressive history written on the walls of those gorges by
the agent which produced them.

Numerous cases might be pointed out, varying in magnitude, but all
identical in kind, of barriers which crossed valleys and formed lakes
having been cut through by rivers, narrow gorges being the
consequence.  One of the most famous examples of this kind is the
Finsteraarschlucht in the valley of Hash.  Here the ridge called the
Kirchet seems split across, and the river Aar rushes through the
fissure.  Behind the barrier we have the meadows and pastures of Imhof
resting on the sediment of an ancient lake.  Were this an isolated
case, one might with an apparent show of reason conclude that the
Finsteraarschlucht was produced by an earthquake, as some suppose it
to have been; but when we find it to be a single sample of actions
which are frequent in the Alps--when probably a hundred cases of the
same kind, though different in magnitude, can be pointed out--it seems
quite unphilosophical to assume that in each particular case an
earthquake was at hand to form a channel for the river.  As in the case
of the barrier at Pontresina, the Kirchet, after the retreat of the
Aar glacier, dammed the waters flowing from it, thus forming a lake,
on the bed of which now stands the village of Imhof.  Over this
barrier the Aar tumbled towards Meyringen, cutting, as the centuries
passed, its bed ever deeper, until finally it became deep enough to
drain the lake, leaving in its place the alluvial plain, through which
the river now flows in a definite channel.

In 1866 I subjected the Finsteraarschlucht to a close examination. The
earthquake theory already adverted to was then prevalent regarding it,
and I wished to see whether any evidences existed of aqueous erosion.
Near the summit of the Kirchet is a signboard inviting the traveller
to visit the Aarenschlucht, a narrow lateral gorge which runs down to
the very bottom of the principal one.  The aspect of this smaller
chasm from bottom to top proves to demonstration that water had in
former ages been there at work.  It is scooped, rounded, and polished,
so as to render palpable to the most careless eye that it is a gorge
of erosion.  But it was regarding the sides of the great chasm that
instruction was needed, and from its edge nothing to satisfy me could
be seen.  I therefore stripped and waded into the river until a point
was reached which commanded an excellent view of both sides of the
gorge.  The water was cutting cold, but I was repaid.  Below me on the
left-hand side was a jutting cliff which bore the thrust of the river
and caused the Aar to swerve from its direct course.  From top to
bottom this cliff was polished, rounded, and scooped.  There was no
room for doubt.  The river which now runs so deeply down had once been
above.  It has been the delver of its own channel through the barrier
of the Kirchet.

But the broad view taken by the advocates of the fracture theory is,
that the valleys themselves follow the tracks of primeval fissures
produced by the upheaval of the land, the cracks across the barriers
referred to being in reality portions of the great cracks which formed
the valleys.  Such an argument, however, would virtually concede the
theory of erosion as applied to the valleys of the Alps.  The narrow
gorges, often not more than twenty or thirty feet across, sometimes
even narrower, frequently occur at the bottom of broad valleys.  Such
fissures might enter into the list of accidents which gave direction
to the real erosive agents which scooped the valley out; but the
formation of the valley, as it now exists, could no more be ascribed
to such cracks than the motion of a railway train could be ascribed to
the finger of the engineer which turns on the steam.

These deep gorges occur, I believe, for the most part in limestone
strata; and the effects which the merest driblet of water can produce
on limestone are quite astonishing.  It is not uncommon to meet chasms
of considerable depth produced by small streams the beds of which are
dry for a large portion of the year.  Right and left of the larger
gorges such secondary chasms are often found.  The idea of time must,
I think, be more and more included in our reasonings on these
phenomena.  Happily, the marks which the rivers have, in most cases,
left behind them, and which refer, geologically considered, to actions
of yesterday, give us ground and courage to conceive what may be
effected in geologic periods.  Thus the modern portion of the Via Mala
throws light upon the whole.  Near Bergün, in the valley of the
Albula, there is also a little Via Mala, which is not less significant
than the great one.  The river flows here through a profound limestone
gorge, and to the very edges of the gorge we have the evidences of
erosion.  But the most striking illustration of water-action upon
limestone rock that I have ever seen is the gorge at Pfaeffers.  Here
the traveller passes along the side of the chasm midway between top
and bottom.  Whichever way he looks, backwards or forwards, upwards or
downwards, towards the sky or towards the river, he meets everywhere
the irresistible and impressive evidence that this wonderful fissure
has been sawn through the mountain by the waters of the Tamina.

I have thus far confined myself to the consideration of the gorges
formed by the cutting through of the rock-barriers which frequently
cross the valleys of the Alps; as far as they have been examined by me
they are the work of erosion.  But the larger question still remains,
To what action are we to ascribe the formation of the valleys
themselves?  This question includes that of the formation of the
mountain-ridges, for were the valleys wholly filled, the ridges would
disappear.  Possibly no answer can be given to this question which is
not beset with more or less of difficulty.  Special localities might
be found which would seem to contradict every solution which, refers
the conformation of the Alps to the operation of a single cause.

Still the Alps present features of a character sufficiently definite
to bring the question of their origin within the sphere of close
reasoning.  That they were in whole or in part once beneath the sea
will not be disputed; for they are in great part composed of
sedimentary rocks which required a sea to form them.  Their present
elevation above the sea is due to one of those local changes in the
shape of the earth which have been of frequent occurrence throughout
geologic time, in some cases depressing the land, and in others
causing the sea-bottom to protrude beyond its surface.  Considering
the inelastic character of its materials, the protuberance of the Alps
could hardly have been pushed out without dislocation and fracture;
and this conclusion gains in probability when we consider the
foldings, contortions, and even reversals in position of the strata in
many parts of the Alps.  Such changes in the position of beds which
were once horizontal could not have been effected without dislocation.
Fissures would be produced by these changes; and such fissures, the
advocates of the fracture theory contend, mark the positions of the
valleys of the Alps.

Imagination is necessary to the man of science, and we could not
reason on our present subject without the power of presenting mentally
a picture of the earth's crust cracked and fissured by the forces
which produced its upheaval.  Imagination, however, must be strictly
checked by reason and by observation.  That fractures occurred cannot,
I think, be doubted, but that the valleys of the Alps are thus formed
is a conclusion not at all involved in the admission of dislocations.
I never met with a precise statement of the manner in which the
advocates of the fissure theory suppose the forces to have
acted--whether they assume a general elevation of the region, or a
local elevation of distinct ridges; or whether they assume local
subsidences after a general elevation, or whether they would superpose
upon the general upheaval minor and local upheavals.

In the absence of any distinct statement, I will assume the elevation
to be general--that a swelling out of the earth's crust occurred here,
sufficient to place the most prominent portions of the protuberance
three miles above the sea-level.  To fix the ideas, let us consider a
circular portion of the crust, say one hundred miles in diameter, and
let us suppose, in the first instance, the circumference of this
circle to remain fixed, and that the elevation was confined to the
space within it.  The upheaval would throw the crust into a state of
strain; and, if it were inflexible, the strain must be relieved by
fracture.  Crevasses would thus intersect the crust. Let us now
enquire what proportion the area of these open fissures is likely to
bear to the area of the unfissured crust. An approximate answer is all
that is here required; for the problem is of such a character as to
render minute precision unnecessary.

No one, I think, would affirm that the area of the fissures would be
one-hundredth the area of the land.  For let us consider the strain
upon a single line drawn over the summit of the protuberance from a
point on its rim to a point opposite.  Regarding the protuberance as a
spherical swelling, the length of the arc corresponding to a chord of
100 miles and a versed sine of 3 miles is 100.24 miles; consequently
the surface to reach its new position must stretch 0.24 of a mile, or
be broken.  A fissure or a number of cracks with this total width
would relieve the strain; that is to say, the sum of the widths of all
the cracks over the length of 100 miles would be 420 yards.  If,
instead of comparing the width of the fissures with the length of the
lines of tension, we compared their areas with the area of the
unfissured land, we should of course find the proportion much less.
These considerations will help the imagination to realise what a small
ratio the area of the open fissures must bear to the unfissured crust.
They enable us to say, for example, that to assume the area of the
fissures to be one-tenth of the area of the land would be quite
absurd, while that the area of the fissures could be one-half or more
than one-half that of the land would be in a proportionate degree
unthinkable.  If we suppose the elevation to be due to the shrinking
or subsidence of the land all round our assumed circle, we arrive
equally at the conclusion that the area of the open fissures would be
altogether insignificant as compared with that of the unfissured
crust.

To those who have seen them from a commanding elevation, it is
needless to say that the Alps themselves bear no sort of resemblance
to the picture which this theory presents to us.  Instead of deep
cracks with approximately vertical walls, we have ridges running into
peaks, and gradually sloping to form valleys.  Instead of a fissured
crust, we have a state of things closely resembling the surface of the
ocean when agitated by a storm.  The valleys, instead of being much
narrower than the ridges, occupy the greater space.  A plaster cast of
the Alps turned upside down, so as to invert the elevations and
depressions, would exhibit blunter and broader mountains, with
narrower valleys between them, than the present ones.  The valleys
that exist cannot, I think, with any correctness of language be called
fissures.  It may be urged that they originated in fissures: but even
this is unproved, and, were it proved, the fissures would still play
the subordinate part of giving direction to the agents which are to be
regarded as the real sculptors of the Alps.

The fracture theory, then, if it regards the elevation of the Alps as
due to the operation of a force acting throughout the entire region,
is, in my opinion, utterly incompetent to account for the conformation
of the country.  If, on the other hand, we are compelled to resort to
local disturbances, the manipulation of the earth's crust necessary to
obtain the valleys and the mountains will, I imagine, bring the
difficulties of the theory into very strong relief.  Indeed an
examination of the region from many of the more accessible
eminences--from the Galenstock, the Grauhaupt, the Pitz Languard, the
Monte Confinale--or, better still, from Mont Blanc, Monte Rosa, the
Jungfrau, the Finsteraarhorn, the Weisshorn, or the Matterhorn, where
local peculiarities are toned down, and the operations of the powers
which really made this region what it is are alone brought into
prominence--must, I imagine, convince every physical geologist of the
inability of any fracture theory to account for the present
conformation of the Alps.

A correct model of the mountains, with an unexaggerated vertical
scale, produces the same effect upon the mind as the prospect from one
of the highest peaks.  We are apt to be influenced by local phenomena
which, though insignificant in view of the general question of Alpine
conformation, are, with reference to our customary standards, vast and
impressive.  In a true model those local peculiarities disappear; for
on the scale of a model they are too small to be visible; while the
essential facts and forms are presented to the undistracted attention.

A minute analysis of the phenomena strengthens the conviction which
the general aspect of the Alps fixes in the mind.  We find, for
example, numerous valleys which the most ardent plutonist would not
think of ascribing to any other agency than erosion.  That such is
their genesis and history is as certain as that erosion produced the
Chines in the Isle of Wight.  From these indubitable cases of
erosion--commencing, if necessary, with the small ravines which run
down the flanks of the ridges, with their little working navigators at
their bottoms--we can proceed, by almost insensible gradations, to the
largest valleys of the Alps; and it would perplex the plutonist to fix
upon the point at which fracture begins to play a material part.

In ascending one of the larger valleys, we enter it where it is wide
and where the eminences are gentle on either side.  The flanking
mountains become higher and more abrupt as we ascend, and at length we
reach a place where the depth of the valley is a maximum.  Continuing
our walk upwards, we find ourselves flanked by gentler slopes, and
finally emerge from the valley and reach the summit of an open col, or
depression in the chain of mountains.  This is the common character of
the large valleys.  Crossing the col, we descend along the opposite
slope of the chain, and through the same series of appearances in the
reverse order.  If the valleys on both sides of the col were produced
by fissures, what prevents the fissure from prolonging itself across
the col?  The case here cited is representative; and I am not
acquainted with a single instance in the Alps where the chain has been
cracked in the manner indicated.  The cols are simply depressions; in
many of which the unfissured rock can be traced from side to side.

The typical instance just sketched follows as a natural consequence
from the theory of erosion.  Before either ice or water can exert
great power as an erosive agent, it must collect in sufficient mass.
On the higher slopes and plateaus--in the region of cols--the power is
not fully developed; but lower down tributaries unite, erosion is
carried on with increased vigour, and the excavation gradually reaches
a maximum.  Lower still the elevations diminish and the slopes become
more gentle; the cutting power gradually relaxes, until finally the
eroding agent quits the mountains altogether, and the grand effects
which it produced in the earlier portions of its course entirely
disappear.

I have hitherto confined myself to the consideration of the broad
question of the erosion theory as compared with the fracture theory;
and all that I have been able to observe and think with reference to
the subject leads me to adopt the former.  Under the term erosion I
include the action of water, of ice, and of the atmosphere, including
frost and rain.  Water and ice, however, are the principal agents, and
which of these two has produced the greatest effect it is perhaps
impossible to say.  Two years ago I wrote a brief note 'On the
Conformation of the Alps,' [Footnote: Phil. Mag. vol. xxiv. p. 169]
in which I ascribed the paramount influence to glaciers.  The facts on
which that opinion was founded are, I think, unassailable; but whether
the conclusion then announced fairly follows from the facts is, I
confess, an open question.

The arguments which have been thus far urged against the conclusion
are not convincing.  Indeed, the idea of glacier erosion appears so
daring to some minds that its boldness alone is deemed its sufficient
refutation.  It is, however, to be remembered that a precisely similar
position was taken up by many excellent workers when the question of
ancient glacier extension was first mooted.  The idea was considered
too hardy to be entertained; and the evidences of glacial action were
sought to be explained by reference to almost any process rather than
the true one.  Let those who so wisely took the side of 'boldness' in
that discussion beware lest they place themselves, with reference to
the question of glacier erosion, in the position formerly occupied by
their opponents.

Looking at the little glaciers of the present day--mere pigmies as
compared to the giants of the glacial epoch--we find that from every
one of them issues a river more or less voluminous, charged with the
matter which the ice has rubbed from the rocks.  Where the rocks are
soft, the amount of this finely pulverised matter suspended in the
water is very great.  The water, for example, of the river which flows
from Santa Catarina to Bormio is thick with it.  The Rhine is charged
with this matter, and by it has so silted up the Lake of Constance as
to abolish it for a large fraction of its length.  The Rhone is
charged with it, and tens of thousands of acres of cultivable land are
formed by the silt above the Lake of Geneva.

In the case of every glacier we have two agents at work--the ice
exerting a crushing force on every point of its bed which bears its
weight, and either rasping this point into powder or tearing it bodily
from the rock to which it belongs; while the water which everywhere
circulates upon the bed of the glacier continually washes the detritus
away and leaves the rock clean for further abrasion.  Confining the
action of glaciers to the simple rubbing away of the rocks, and
allowing them sufficient time to act, it is not a matter of opinion,
but a physical certainty, that they will scoop out valleys.  But the
glacier does more than abrade.  Rocks are not homogeneous; they are
intersected by joints and places of weakness, which divide them into
virtually detached masses.  A glacier is undoubtedly competent to root
such masses bodily away.  Indeed the mere _à priori_ consideration of
the subject proves the competence of a glacier to deepen its bed.
Taking the case of a glacier 1,000 feet deep (and some of the older
ones were probably three times this depth), and allowing 40 feet of
ice to an atmosphere, we find that on every square inch of its bed
such a glacier presses with a weight of 375 lbs, and on every square
yard of its bed with a weight of 486,000 lbs. With a vertical pressure
of this amount the glacier is urged down its valley by the pressure
from behind.  We can hardly, I think, deny to such a tool a power of
excavation.

The retardation of a glacier by its bed has been referred to as
proving its impotence as an erosive agent; but this very retardation
is in some measure an expression of the magnitude of the erosive
energy.  Either the bed must give way, or the ice must slide over
itself.  We get indeed some idea of the crushing pressure which the
moving glacier exercises against its bed-from the fact that the
resistance, and the effort to overcome it, are such as to make the
upper layers of a glacier move bodily over the lower ones--a portion
only of the total motion being due to the progress of the entire mass
of the glacier down its valley.

The sudden bend in the valley of the Rhone at Martigny has also been
regarded as conclusive evidence against the theory of erosion.  'Why,'
it has been asked, I did not the glacier of the Rhone go straight
forward instead of making this awkward bend?' But if the valley be a
crack, why did the crack make this bend?  The crack, I submit, had at
least as much reason to prolong itself in a straight line as the
glacier had.  A statement of Sir John Herschel with reference to
another matter is perfectly applicable here: 'A crack once produced
has a tendency to run--for this plain reason, that at its momentary
limit, at the point at which it has just arrived, the divellent force
on the molecules there situated is counteracted only by half of the
cohesive force which acted when there was no crack, viz.  the cohesion
of the uncracked portion alone' ('Proc.  Roy.  Soc.' vol. xii.
p. 678).  To account, then, for the bend, the adherent of the fracture
theory must assume the existence of some accident which turned the
crack at right angles to itself; and he surely will permit the
adherent of the erosion theory to make a similar assumption.

The influence of small accidents on the direction of rivers is
beautifully illustrated in glacier streams, which are made to cut
either straight or sinuous channels by causes apparently of the most
trivial character.  In his interesting paper 'On the Lakes of
Switzerland,' M. Studer also refers to the bend of the Rhine at
Sargans in proof that the river must there follow a pre-existing
fissure.  I made a special expedition to the place in 1864; and though
it was plain that M. Studer had good grounds for the selection of this
spot, I was unable to arrive at his conclusion as to the necessity of
a fissure.

Again, in the interesting volume recently published by the Swiss
Alpine Club, M. Desor informs us that the Swiss naturalists who met
last year at Samaden visited the end of the Morteratsch glacier, and
there convinced themselves that a glacier had no tendency whatever to
imbed itself in the soil.  I scarcely think that the question of
glacier erosion, as applied either to lakes or valleys, is to be
disposed of so easily.  Let me record here my experience of the
Morteratsch glacier.

I took with me in 1864 a theodolite to Pontresina, and while there had
to congratulate myself on the aid of my friend Mr. Hirst, who in 1857
did such good service upon the Mer de Glace and its tributaries.  We
set out three lines across the Morteratsch glacier, one of which
crossed the ice-stream near the well-known hut of the painter Georgei,
while the two others were staked out, the one above the hut and the
other below it.  Calling the highest line A, the line which crossed
the glacier at the hut B, and the lowest line C, the following are the
mean hourly motions of the three lines, deduced from observations
which extended over several days.  On each line eleven stakes were
fixed, which are designated by the figures 1, 2, 3, &c.  in the
Tables.

Morteratsch Glacier, Line A.

No. of Stake.    Hourly Motion.

1                 0.35 inch.

2                 0.49 inch.

3                 0.53 inch.

4                 0.54 inch.

5                 0.56 inch.

6                 0.54 inch.

7                 0.52 inch.

8                 0.49 inch.

9                 0.40 inch.

10                0.29 inch.

11                0.20 inch.

As in all other measurements of this kind, the retarding influence of
the sides of the glacier is manifest: the centre moves with the
greatest velocity.

Morteratsch Glacier, Line B.

No.  of Stake.    Hourly Motion.

1                 0.05 inch.

2                 0.14 inch.

3                 0.24 inch.

4                 0.32 inch.

5                 0-41 inch.

6                 0.44 inch.

7                 0.44 inch.

8                 0.45 inch.

9                 0.43 inch.

10                0.44 inch.

11                0.44 inch.

The first stake of this line was quite close to the edge of the
glacier, and the ice was thin at the place, hence its slow motion.
Crevasses prevented us from carrying the line sufficiently far across
to render the retardation of the further side of the glacier fully
evident.

Morteratsch Glacier, Line C.

No.  of Stake     Hourly Motion.

1                 0.05 inch.

2                 0.09 inch.

3                 0.18 inch.

4                 0.20 inch.

5                 0.25 inch.

6                 0.27 inch.

7                 0.27 inch.

8                 0.30 inch.

9                 0.21 inch.

10                0.20 inch.

11                0.16 inch.

Comparing the three lines together, it will be observed that the
velocity diminishes as we descend the glacier.  In 100 hours the
maximum motion of three lines respectively is as follows:

Maximum Motion in 100 hours.

Line A            56 inches

Line B            45 inches.

Line C            30 inches.

This deportment explains an appearance which must strike every
observer who looks upon the Morteratsch from the Piz Languard, or from
the new Bernina Road.  A medial moraine runs along the glacier,
commencing as a narrow streak, but towards the end the moraine
extending in width, until finally it quite covers the terminal portion
of the glacier.  The cause of this is revealed by the foregoing
measurements, which prove that a stone on the moraine where it is
crossed by the line A approaches a second stone on the moraine where
it is crossed by the line C with a velocity of twenty-six inches per
one hundred hours.  The moraine is in a state of longitudinal
compression.  Its materials are more and more squeezed together, and
they must consequently move laterally and render the moraine at the
terminal portion of the glacier wider than above.

The motion of the Morteratsch glacier, then, diminishes as we descend.
The maximum motion of the third line is thirty inches in one hundred
hours, or seven inches a day--a very slow motion; and had we run a
line nearer to the end of the glacier, the motion would have been
slower still.  At the end itself it is nearly insensible. [Footnote:
The snout of the Aletsch Glacier has a diurnal motion of less than two
inches, while a mile or so above the snout the velocity is eighteen
inches.  The spreading out of the moraine is here very striking.]  Now
I submit that this is not the Place to seek for the scooping power of
a glacier.  The opinion appears to be prevalent that it is the snout
of a glacier that must act the part of ploughshare; and it is
certainly an erroneous opinion.  The scooping power will exert itself
most where the weight and the motion are greatest. A glacier's snout
often rests upon matter which has been scooped from the glacier's bed
higher up.  I therefore do not think that the inspection of what the
end of a glacier does or does not accomplish can decide this question.

The snout of a glacier is potent to remove anything against which it
can fairly abut; and this power, notwithstanding the slowness of the
motion, manifests itself at the end of the Morteratsch glacier.  A
hillock, bearing pine-trees, was in front of the glacier when Mr.
Hirst and myself inspected its end; and this hillock is being bodily
removed by the thrust of the ice.  Several of the trees are
overturned; and in a few years, if the glacier continues its reputed
advance, the mound will certainly be ploughed away.

The question of Alpine conformation stands, I think, thus: We have,
in the first place, great valleys, such as those of the Rhine and the
Rhone, which we might conveniently call valleys of the first order.
The mountains which flank these main valleys are also cut by lateral
valleys running into the main ones, and which may be called valleys of
the second order.  When these latter are examined, smaller valleys are
found running into them, which may be called valleys of the third
order.  Smaller ravines and depressions, again, join the latter, which
may be called valleys of the fourth order, and so on until we reach
streaks and cuttings so minute as not to merit the name of valleys at
all.  At the bottom of every valley we have a stream, diminishing in
magnitude as the order of the valley ascends, carving the earth and
carrying its materials to lower levels.  We find that the larger
valleys have been filled for untold ages by glaciers of enormous
dimensions, always moving, grinding down and tearing away the rocks
over which they passed.  We have, moreover, on the plains at the feet
of the mountains, and in enormous quantities, the very matter derived
from the sculpture of the mountains themselves.

The plains of Italy and Switzerland are cumbered by the _débris_ of the
Alps.  The lower, wider, and more level valleys are also filled to
unknown depths with the materials derived from the higher ones.  In
the vast quantities of moraine-matter which cumber many even of the
higher valleys we have also suggestions as to the magnitude of the
erosion which has taken place.  This moraine-matter, moreover, can
only in small part have been derived from the falling of rocks upon
the ancient glacier; it is in great part derived from the grinding and
the ploughing-out of the glacier itself.  This accounts for the
magnitude of many of the ancient moraines, which date from a period
when almost all the mountains were covered with ice and snow, and
when, consequently, the quantity of moraine-matter derived from the
naked crests cannot have been considerable.

The erosion theory ascribes the formation of Alpine valleys to the
agencies here briefly referred to.  It invokes nothing but true
causes.  Its artificers are still there, though, it may be, in
diminished strength; and if they are granted sufficient time, it is
demonstrable that they are competent to produce the effects ascribed
to them.  And what does the fracture theory offer in comparison?  From
no possible application of this theory, pure and simple, can we obtain
the slopes and forms of the mountains.  Erosion must in the long run
be invoked, and its power therefore conceded.  The fracture theory
infers from the disturbances of the Alps the existence of fissures;
and this is a probable inference.  But that they were of a magnitude
sufficient to produce the conformation of the Alps, and that they
followed, as the Alpine valleys do, the lines of natural drainage of
the country, are assumptions which do not appear to me to be justified
either by reason or by observation.

There is a grandeur in the secular integration of small effects
implied by the theory of erosion almost superior to that involved in
the idea of a cataclysm.  Think of the ages which must have been
consumed in the execution of this colossal sculpture.  The question
may, of course, be pushed further.  Think of the ages which the molten
earth required for its consolidation.  But these vaster epochs lack
sublimity through our inability to grasp them.  They bewilder us, but
they fail to make a solemn impression.  The genesis of the mountains
comes more within the scope of the intellect, and the majesty of the
operation is enhanced by our partial ability to conceive it.  In the
falling of a rock from a mountain-head, in the shoot of an avalanche,
in the plunge of a cataract, we often see more impressive
illustrations of the power of gravity than in the motions of the
stars.  When the intellect has to intervene, and calculation is
necessary to the building up of the conception, the expansion of the
feelings ceases to be proportional to the magnitude of the phenomena.

*****

I will here record a few other measurements executed on the Rosegg
glacier: the line was staked out across the trunk formed by the
junction of the Rosegg proper with the Tschierva glacier, a short
distance below the rocky promontory called Agaliogs.

Rosegg Glacier.

No.  of Stake.    Hourly Motion.

1                 0.01 inch.

2                 0.05

3                 0.07

4                 0.10

5                 0.11

6                 0.13

7                 0.14

8                 0.18

9                 0.24

10                0.23

11                0.24

This is an extremely slowly moving glacier; the maximum motion hardly
amounts to seven inches a day.  Crevasses prevented us from continuing
the line quite across the glacier.

********************

X. RECENT EXPERIMENTS ON FOG-SIGNALS.

[Footnote: A discourse delivered in the Royal Institution, March 22,
1878.]

The care of its sailors is one of the first duties of a maritime
people, and one of the sailor's greatest dangers is his proximity to
the coast at night.  Hence, the idea of warning him of such proximity
by beacon-fires placed sometimes on natural eminences and sometimes on
towers built expressly for the purpose.  Close to Dover Castle, for
example, stands an ancient Pharos of this description.

As our marine increased greater skill was invoked, and lamps
reinforced by parabolic reflectors poured their light upon the sea.
Several of these lamps were sometimes grouped together so as to
intensify the light, which at a little distance appeared as if it
emanated from a single source.  This 'catoptric' form of apparatus is
still to some extent employed in our lighthouse-service, but for a
long time past it has been more and more displaced by the great lenses
devised by the illustrious Frenchman, Fresnel.

In a first-class 'dioptric' apparatus the light emanates from a lamp
with several concentric wicks, the flame of which, being kindled by a
very active draught, attains to great intensity.  In fixed lights the
lenses refract the rays issuing from the lamp so as to cause them to
form a luminous sheet which grazes the sea-horizon.  In revolving
lights the lenses gather up the rays into distinct beams, resembling
the spokes of a wheel, which sweep over the sea and strike the eye of
the mariner in succession.

It is not for clear weather that the greatest strengthening of the
light is intended, for here it is not needed.  Nor is it for densely
foggy weather, for here it is ineffectual.  But it is for the
intermediate stages of hazy, snowy, or rainy weather, in which a
powerful light can assert itself, while a feeble one is extinguished.
The usual first-order lamp is one of four wicks, but Mr. Douglass, the
able and indefatigable engineer of the Trinity House, has recently
raised the number of the wicks to six, which produce a very noble
flame.  To Mr. Wigham, of Dublin, we are indebted for the successful
application of gas to lighthouse illumination.  In some lighthouses
his power varies from 28 jets to 108 jets, while in the lighthouse of
Galley Head three burners of the largest size can be employed, the
maximum number of jets being 324.  These larger powers are invoked
only in case of fog, the 28-jet burner being amply sufficient for
clear weather.  The passage from the small burner to the large, and
from the large burner to the small, is made with ease, rapidity, and
certainty.  This employment of gas is indigenous to Ireland, and the
Board of Trade has exercised a wise liberality in allowing every
facility to Mr. Wigham for the development of his invention.

The last great agent employed in lighthouse illumination is
electricity.  It was in this Institution, beginning in 1831, that
Faraday proved the existence and illustrated the laws of those induced
currents which in our day have received such astounding development.
In relation to this subject Faraday's words have a prophetic ring.  'I
have rather,' he writes in 1831, 'been desirous of discovering new
facts and new relations dependent on magneto-electric induction than
of exalting the force of those already obtained, being assured that
the latter would find their full development hereafter.'  The labours
of Holmes, of the Paris Alliance Company, of Wilde, and of Gramme,
constitute a brilliant fulfilment of this prediction.

But, as regards the augmentation of power, the greatest step hitherto
made was independently taken a few years ago by Dr. Werner Siemens and
Sir Charles Wheatstone.  Through the application of their discovery a
machine endowed with an infinitesimal charge of magnetism may, by a
process of accumulation at compound interest, be caused so to enrich
itself magnetically as to cast by its performance all the older
machines into the shade.  The light now before you is that of a small
machine placed downstairs, and worked there by a minute steam-engine.
It is a light of about 1000 candles; and for it, and for the
steam-engine that 'works it, our members are indebted to the
liberality of Dr. William Siemens, who in the most generous manner has
presented the machine to this Institution.  After an exhaustive trial
at the South Foreland, machines on the principle of Siemens, but of
far greater power than this one, have been recently chosen by the
Elder Brethren of the Trinity House for the two light-houses at the
Lizard Point.

Our most intense lights, including the six-wick lamp, the Wigham
gas-light, and the electric light, being intended to aid the mariner
in heavy weather, may be regarded, in a certain sense, as fog-signals.
But fog, when thick, is intractable to light.  The sun cannot
penetrate it, much less any terrestrial source of illumination.  Hence
the necessity of employing sound-signals in dense fogs.  Bells, gongs,
horns, whistles, guns, and syrens have been used for this purpose; but
it is mainly, if not wholly, with explosive signals that we have now
to deal.  The gun has been employed with useful effect at the North
Stack, near Holyhead, on the Kish Bank near Dublin, at Lundy Island,
and at other points on our coasts.  During the long, laborious, and I
venture to think memorable series of observations conducted under the
auspices of the Elder Brethren of the Trinity House at the South
Foreland in 1872 and 1873, it was proved that a short 5.5-inch
howitzer, firing 3 lbs. of powder, yielded a louder report than a long
18-pounder firing the same charge.  Here was a hint to be acted on by
the Elder Brethren.  The effectiveness of the sound depended on the
shape of the gun, and as it could not be assumed that in the howitzer
we had hit accidentally upon the best possible shape, arrangements
were made with the War Office for the construction of a gun specially
calculated to produce the loudest sound attainable from the combustion
of 3 lbs. of powder.  To prevent the unnecessary landward waste of the
sound, the gun was furnished with a parabolic muzzle, intended to
project the sound over the sea, where it was most needed.  The
construction of this gun was based on a searching series of
experiments executed at Woolwich with small models, provided with
muzzles of various kinds.  A drawing of the gun is annexed (p. 309).
It was constructed on the principle of the revolver, its various
chambers being loaded and brought in rapid succession into the firing
position.  The performance of the gun proved the correctness of the
principles on which its construction was based.

An incidental point of some interest was decided by the earliest
Woolwich experiments.  It had been a widely spread opinion among
artillerists, that a bronze gun produces a specially loud report.  I
doubted from the outset whether this would help us; and in a letter
dated 22nd April, 1874, I ventured to express myself thus: 'The
report of a gun, as affecting an observer close at hand, is made up of
two factors--the sound due to the shock of the air by the violently
expanding gas, and the sound derived from the vibrations of the gun,
which, to some extent, rings like a bell.  This latter, I apprehend,
will disappear at considerable distances.'

FIG.  8. Breech-loading Fog-signal Gun, with Bell Mouth, proposed by
Major Maitland, R.A. Assistant Superintendent. [Footnote: The carriage
of this gun has been modified in construction since this drawing was
made.]

The result of subsequent trial, as reported by General Campbell, is,
'that the sonorous qualities of bronze are greatly superior to those
of cast iron at short distances, but that the advantage lies with the
baser metal at long ranges.' [Footnote: General Campbell assigns a
true cause for this difference.  The ring of the bronze gun represents
so much energy withdrawn from the explosive force of the gunpowder.
Further experiments would, however, be needed to place the superiority
of the cast-iron gun at a distance beyond question.]

Coincident with these trials of guns at Woolwich, gun-cotton was
thought of as a probably effective sound-producer.  From the first,
indeed, theoretic considerations caused me to fix my attention
persistently on this substance; for the remarkable experiments of Mr.
Abel, whereby its rapidity of combustion and violently explosive
energy are demonstrated, seemed to single it out as a substance
eminently calculated to fulfil the conditions necessary to the
production of an intense wave of sound.  What those conditions are we
shall now more particularly enquire, calling to our aid a brief but
very remarkable paper, published by Professor Stokes in the
'Philosophical Magazine' for 1868.

The explosive force of gunpowder is known to depend on the sudden
conversion of a solid body into an intensely heated gas.  Now the work
which the artillerist requires the expanding gas to perform is the
displacement of the projectile, besides which it has to displace the
air in front of the projectile, which is backed by the whole pressure
of the atmosphere.  Such, however, is not the work that we want our
gunpowder to perform.  We wish to transmute its energy not into the
mere mechanical translation of either shot or air, but into vibratory
motion.  We want _pulses_ to be formed which shall propagate themselves
to vast distances through the atmosphere, and this requires a certain
choice and management of the explosive material.

A sound-wave consists essentially of two parts--a condensation and a
rarefaction.  Now air is a very mobile fluid, and if the shock
imparted to it lack due promptness, the wave is not produced. Consider
the case of a common clock pendulum, which oscillates to and fro, and
which might be expected to generate corresponding pulses in the air.
When, for example, the bob moves to the right, the air to the right of
it might be supposed to be condensed, while a partial vacuum might be
supposed to follow the bob.  As a matter of fact, we have nothing of
the kind.  The air particles in front of the bob retreat so rapidly,
and those behind it close so rapidly in, that no sound-pulse is
formed.  The mobility of hydrogen, moreover, being far greater than
that of air, a prompter action is essential to the formation of
sonorous waves in hydrogen than in air.  It is to this rapid power of
readjustment, this refusal, so to speak, to allow its atoms to be
crowded together or to be drawn apart, that Professor Stokes, with
admirable penetration, refers the damping power, first described by
Sir John Leslie, of hydrogen upon sound.

A tuning-fork which executes 256 complete vibrations in a second, if
struck gently on a pad and held in free air, emits a scarcely audible
note.  It behaves to some extent like the pendulum bob just referred
to.  This feebleness is due to the prompt 'reciprocating flow' of the
air between the incipient condensations and rarefactions, whereby the
formation of sound-pulses is forestalled.  Stokes, however, has taught
us that this flow may be intercepted by placing the edge of a card in
close proximity to one of the corners of the fork.  An immediate
augmentation of the sound of the fork is the consequence.

The more rapid the shock imparted to the air, the greater is the
fractional part of the energy of the shock converted into wave motion.
And as different kinds of gunpowder vary considerably in their
rapidity of combustion, it may be expected that they will also vary as
producers of sound.  This theoretic inference is completely verified
by experiment.  In a series of preliminary trials conducted at
Woolwich on the 4th of June, 1875, the sound-producing powers of four
different kinds of powder were determined.  In the order of the size
of their grains they bear the names respectively of Fine-grain
(F.G.), Large-grain (L.G.), Rifle Large-grain (R.L.G.), and
Pebble-powder (P.) (See annexed figures.) The charge in each case
amounted to 4.5 lbs. four 24-lb. howitzers being employed to fire the
respective charges.

FIG.  9.

There were eleven observers, all of whom, without a single
dissentient, pronounced the sound of the fine-grain powder loudest of
all.  In the opinion of seven of the eleven the large-grain powder
came next; seven also of the eleven placed the rifle large-grain third
on the list; while they were again unanimous in pronouncing the
pebble-powder the worst sound-producer.  These differences are
entirely due to differences in the rapidity of combustion.  All who
have witnessed the performance of the 80-ton gun must have been
surprised at the mildness of its thunder.  To avoid the strain
resulting from quick combustion, the powder employed is composed of
lumps far larger than those of the pebble-powder above referred to. In
the long tube of the gun these lumps of solid matter gradually resolve
themselves into gas, which on issuing from muzzle imparts a kind of
push to the air, instead of the sharp shock necessary to form the
condensation of an intensely sonorous wave.

These are some of the physical reasons why guncotton might be regarded
as a promising fog-signal.  Firing it as we have been taught to do by
Mr. Abel, its explosion is more rapid than that of gunpowder.  In its
case the air particles, alert as they are, will not, it might be
presumed, be able to slip from condensation to rarefaction with a
rapidity sufficient to forestall the formation of the wave.  On _à
priori_ grounds then, we are entitled to infer the effectiveness of
gun-cotton, while in a great number of comparative experiments,
stretching from 1874 to the present time, this inference has been
verified in the most conclusive manner.

As regards explosive material, and zealous and accomplished help in
the use of it, the resources of Woolwich Arsenal have been freely
placed at the disposal of the Elder Brethren.  General Campbell,
General Younghusband, Colonel Fraser, Colonel Maitland, and other
officers, have taken an active personal part in the investigation, and
in most cases have incurred the labour of reducing and reporting on
the observations.  Guns of various forms and sizes have been invoked
for gunpowder, while gun-cotton has been fired in free air and in the
foci of parabolic reflectors.

On the 22nd of February, 1875, a number of small guns, cast specially
for the purpose--some with plain, some with conical, and some with
parabolic muzzles--firing 4 oz. of fine-grain powder, were pitted
against 4 oz. of gun-cotton detonated both in the open, and in the
focus of a parabolic reflector. [Footnote: For charges of this
weight the reflector is of moderate size, and may be employed without
fear of fracture.]

The sound produced by the gun-cotton, reinforced by the reflector, was
unanimously pronounced loudest of all.  With equal unanimity, the
gun-cotton detonated in free air was placed second in intensity.
Though the same charge was used throughout, the guns differed notably
among themselves, but none of them came up to the gun-cotton, either
with or without the reflector.  A second series, observed from a
different distance on the same day, confirmed to the letter the
foregoing result.

As a practical point, however, the comparative cost of gun-cotton and
gunpowder has to be taken into account, though considerations of cost
ought not to be stretched too far in cases involving the safety of
human life.  In the earlier experiments, where quantities of equal
price were pitted against each other, the results were somewhat
fluctuating.  Indeed, the perfect manipulation of the gun-cotton
required some preliminary discipline--promptness, certainty, and
effectiveness of firing, augmenting as experience increased.  As 1 lb.
of gun-cotton costs as much as 3 lbs. of gunpowder, these quantities
were compared together on the 22nd of February.  The guns employed to
discharge the gunpowder were a 12-lb.  brass howitzer, a 24-lb.
cast-iron howitzer, and the long 18-pounder employed at the South
Foreland.  The result was, that the 24-lb. howitzer, firing 3 lbs. of
gunpowder, had a slight advantage over 1 lb.  of gun-cotton detonated
in the open; while the 12-lb.  howitzer and the 18-pounder were both
beaten by the gun-cotton.  On the end of May, on the other hand, the
gun-cotton is reported as having been beaten by all the guns.

Meanwhile, the parabolic-muzzle gun, expressly intended for
fog-signalling, was pushed rapidly forward, and on March 22 and 23,
1876, its power was tested at Shoeburyness.  Pitted against it were a
16-pounder, a 5.5-inch howitzer, 1.5 lb.  of gun-cotton detonated in
the focus of a reflector (see annexed figure), and 1.5 lb. of
gun-cotton detonated in free air.  On this occasion nineteen different
series of experiments were made, when the new experimental gun, firing
a 3-lb.  charge, demonstrated its superiority over all guns previously
employed to fire the same charge.  As regards the comparative merits
of the gun-cotton fired in the open, and the gunpowder fired from the
new gun, the mean values of their sounds were the same.  Fired in the
focus of the reflector, the gun-cotton clearly dominated over all the
other sound-producers. [Footnote: The reflector was fractured by the
explosion, but it did good service afterwards.]

FIG.  10.

Gun-cotton Slab (1.5 lb.) Detonated in the Focus of a Cast-iron
Reflector.

The whole of the observations here referred to were embraced by an
angle of about 70°, of which 50' lay on the one side and 20° on the
other side of the line of fire.  The shots were heard by eleven
observers on board the 'Galatea,' which took up positions varying from
2 miles to 13.5 miles from the firing-point.  In all these
observations, the reinforcing action of the reflector, and of the
parabolic muzzle of the gun, came into play.  But the reinforcement of
the sound in one direction implies its withdrawal from some other
direction, and accordingly it was found that at a distance of 5.25
miles from the firing-point, and on a line including nearly an angle
of 90° with the line of fire, the gun-cotton in the open beat the new
gun; while behind the station, at distances of 8.5 miles and 13.5
miles respectively, the gun-cotton in the open beat both the gun and
the gun-cotton in the reflector.  This result is rendered more
important by the fact that the sound reached the Mucking Light, a
distance of 13.5 miles, against a light wind which was blowing at the
time.

Most, if not all, of our ordinary sound-producers send forth waves
which are not of uniform intensity throughout.  A trumpet is loudest
in the direction of its axis.  The same is true of a gun.  A bell,
with its mouth pointed upwards or downwards, sends forth waves which
are far denser in the horizontal plane passing through the bell than
at an angular distance of 90° from that plane.  The oldest bellbangers
must have been aware of the fact that the sides of the bell, and not
its mouth, emitted the strongest sound, their practice being probably
determined by this knowledge.  Our slabs of gun-cotton also emit waves
of different densities in different parts.  It has occurred in the
experiments at Shoeburyness that when the broad side of a slab was
turned towards the suspending wire of a second slab six feet distant,
the wire was cut by the explosion, while when the edge of the slab was
turned to the wire this never occurred.

To the circumstance that the broadsides of the slabs faced the sea is
probably to be ascribed the remarkable fact observed on March 23, that
in two directions, not far removed from the line of fire, the
gun-cotton detonated in the open had a slight advantage over the new
gun.

Theoretic considerations rendered it probable that the shape and size
of the exploding mass would affect the constitution of the wave of
sound.  I did not think large rectangular slabs the most favourable
shape, and accordingly proposed cutting a large slab into fragments of
different sizes, and pitting them against each other The differences
between the sounds were by no means so great as the differences in the
quantities of explosive material might lead one to expect.  The mean
values of eighteen series of observations made on board the 'Galatea,'
at distances varying from 1.75 mile to 4.8 miles, were as follows:

Weights          4 oz.  6 oz.  9 oz.  12 oz.

Value of sound   3.12   3.34   4.0    4.03

These charges were cut from a slab of dry gun-cotton about 1.75 inch
thick: they were squares and rectangles of the following dimensions:

4 oz,           2 inches by 2 inches;

6 oz,           2 inches by 3 inches;

9 oz,           3 inches by 3 inches;

12 oz,          2 inches by 6 inches.

The numbers under the respective weights express the recorded value of
the sounds.  They must be simply taken as a ready means of expressing
the approximate relative intensity of the sounds as estimated by the
ear.  When we find a 9-oz. charge marked 4, and a 12-oz. charge marked
4.03, the two sounds may be regarded as practically equal in
intensity, thus proving that an addition of 30 per cent.  in the
larger charges produces no sensible difference in the sound.  Were the
sounds estimated by some physical means, instead of by the ear, the
values of the sounds at the distances recorded would not, in my
opinion, show a greater advance with the increase of material than
that indicated by the foregoing numbers.  Subsequent experiments
rendered still more certain the effectiveness, as well as the economy,
of the smaller charges of gun-cotton.

It is an obvious corollary from the foregoing experiments that on our
'nesses' and promontories, where the land is clasped on both sides for
a considerable distance by the sea--where, therefore, the sound has to
propagate itself rearward as well as forward--the use of the parabolic
gun, or of the parabolic reflector, might be a disadvantage rather
than an advantage.  Here guncotton, exploded in the open, forms the
most appropriate source of sound.  This remark is especially
applicable to such lightships as are intended to spread the sound all
round them as from central foci.

As a signal in rock lighthouses, where neither syren, steam-whistle,
nor gun could be mounted; and as a handy fleet-signal, dispensing with
the lumber of special signal-guns, the gun-cotton will prove
invaluable.  But in most of these cases we have the drawback that
local damage may be done by the explosion.  The lantern of the rock
lighthouse might suffer from concussion near at hand, and though
mechanical arrangements might be devised, both in the case of the
lighthouse and of the ship's deck, to place the firing-point of the
gun-cotton at a safe distance, no such arrangement could compete, as
regards simplicity and effectiveness, with the expedient of a
gun-cotton rocket.  Had such a means of signalling existed at the
Bishop's Rock lighthouse, the ill-fated 'Schiller' might have been
warned of her approach to danger ten, or it may be twenty, miles
before she reached the rock which wrecked her.  Had the fleet
possessed such a signal, instead of the ubiquitous but ineffectual
whistle, the 'Iron Duke' and 'Vanguard' need never have come into
collision.

It was the necessity of providing a suitable signal for rock
lighthouses, and of clearing obstacles which cast an acoustic shadow,
that suggested the idea of the gun-cotton rocket to Sir Richard
Collinson, Deputy Master of the Trinity House.  His idea was to place
a disk or short cylinder of gun-cotton in the head of a rocket, the
ascensional force of which should be employed to carry the disk to an
elevation of 1000 feet or thereabouts, where by the ignition of a fuse
associated with a detonator, the gun-cotton should be fired, sending
its sound in all directions vertically and obliquely down upon earth
and sea.  The first attempt to realise this idea was made on July 18,
1876, at the firework manufactory of the Messrs.  Brock, at Nunhead.
Eight rockets were then fired, four being charged with 5 oz. and four
with 7.5 oz. of gun-cotton.  They ascended to a great height, and
exploded with a very loud report in the air.  On July 27, the rockets
were tried at Shoeburyness.

The most noteworthy result on this occasion was the hearing of the
sounds at the Mouse Lighthouse, 8.5 miles E.  by S, and at the Chapman
Lighthouse, 8.5 miles W.  by N; that is to say, at opposite sides of
the firing-point.  It is worthy of remark that, in the case of the
Chapman Lighthouse, land and trees intervened between the firing-point
and the place of observation.  This,' as General Younghusband justly
remarked at the time, 'may prove to be a valuable consideration if it
should be found necessary to place a signal station in a position
whence the sea could not be freely observed.' Indeed, the clearing of
such obstacles was one of the objects which the inventor of the rocket
had in view.

With reference to the action of the wind, it was thought desirable to
compare the range of explosions produced near the surface of the earth
with others produced at the elevation attainable by the gun-cotton
rockets.  Wind and weather, however, are not at our command; and hence
one of the objects of a series of experiments conducted on December
13, 1876, was not fulfilled.  It is worthy, however, of note that on
this day, with smooth water and a calm atmosphere, the rockets were
distinctly heard at a distance of 11.2 miles from the firing-point.
The quantity of gun-cotton employed was 7.5 oz. On Thursday, March 8,
1877, these comparative experiments of firing at high and low
elevations were pushed still further.  The gun-cotton near the ground
consisted of 0.5-lb. disks, suspended from a horizontal iron bar about
4.5 feet above the ground.

The rockets carried the same quantity of gun-cotton in their heads,
and the height to which they attained, as determined by a theodolite,
was from 800 to 900 feet.  The day was cold, with occasional squalls
of snow and hail, the direction of the sound being at right angles to
that of the wind.  Five series of observations were made on board the
'Vestal,' at distances varying from 3 to 6 miles.  The mean value of
the explosions in the air exceeded that of the explosions near the
ground by a small but sensible quantity.  At Windmill Hill, Gravesend,
however, which was nearly to leeward, and 5.5 miles from the
firing-point, in nineteen cases out of twenty-four the disk fired near
the ground was loudest; while in the remaining five the rocket had the
advantage.

Towards the close of the day the atmosphere became very serene.  A few
distant cumuli sailed near the horizon, but the zenith and a vast
angular space all round it were absolutely free from cloud.  From the
deck of the 'Galatea' a rocket was discharged, which reached a great
elevation, and exploded with a loud report.  Following this solid
nucleus of sound was a continuous train of echoes, which retreated to
a continually greater distance, dying gradually off into silence after
seven seconds' duration.  These echoes were of the same character as
those so frequently noticed at the South Foreland in 1872-73, and
called by me 'aerial echoes.'

On the 23rd of March the experiments were resumed, the most noteworthy
results of that day's observations being that the sounds were heard at
Tillingham, 10 miles to the N.E.; at West Mersea, 15.75 miles to the
N.E. by E; at Brightlingsea, 17.5 miles to the N.E.; and at Clacton
Wash, 20.5 miles to the N.E. by 1/2 E. The wind was blowing at the
time from the S.E.  Some of these sounds were produced by rockets,
some by a 24-lb.  howitzer, and some by an 8-inch Maroon.

In December, 1876, Mr. Gardiner, the managing director of the
Cotton-powder Company, had proposed a trial of this material against
the gun-cotton.  The density of the cotton he urged was only 1.03,
while that of the powder was 1.70.  A greater quantity of explosive
material being thus compressed into the same volume, Mr. Gardiner
thought that a greater sonorous effect must be produced by the powder.
At the instance of Mr. Mackie, who had previously gone very
thoroughly into the subject, a Committee of the Elder Brethren visited
the cotton-powder manufactory, on the banks of the Swale, near
Faversham, on the 16th of June, 1877.  The weights of cotton-powder
employed were 2 oz, 8 oz, 1 lb, and 2 lbs, in the form of rockets and
of signals fired a few feet above the ground.  The experiments
throughout were arranged and conducted by Mr. Mackie.  Our desire on
this occasion was to get 'as near to windward as possible, but the
Swale and other obstacles limited our distance to 1.5 mile.  We stood
here E.S.E. from the firing-point while the wind blew fresh from the
N.E.

The cotton-powder yielded a very effective report.  The rockets in
general had a slight advantage over the same quantities of material
fired near the ground.  The loudness of the sound was by no means
proportional to the quantity of the material exploded, 8 oz. yielding
very nearly as loud a report as 1 lb.  The 'aerial echoes,' which
invariably followed the explosion of the rockets, were loud and
long-continued.

On the 17th of October, 1877, another series of experiments with
howitzers and rockets was carried out at Shoeburyness.  The charge of
the howitzer was 3 lbs. of L. G. powder.  The charges of the rockets
were 12 oz, 8 oz, 4 oz, and 2 oz. of gun-cotton respectively.  The gun
and the four rockets constituted a series, and eight series were fired
during the afternoon of the 17th.  The observations were made from the
'Vestal' and the 'Galatea,' positions being successively assumed which
permitted the sound to reach the observers with the Wind, against the
wind, and across the wind.  The distance of the 'Galatea' varied from
3 to 7 miles, that of the 'Vestal,' which was more restricted in her
movements, being 2 to 3 miles.  Briefly summed up, the result is that
the howitzer, firing a 3-lb.  charge, which it will be remembered was
our best gun at 'the South Foreland, was beaten by the 12-oz. rocket,
by the 8-oz. rocket, and by the 4-oz. rocket. The 2-oz. rocket alone
fell behind the howitzer.

It is worth while recording the distances at which some of the sounds
were heard on the day now referred to:

1.  Leigh         6.5 miles W.N.W.      24 out of 40 sounds heard.

2.  Girdler       12 miles S.E. by E.    5 out of 40 sounds heard.
    Light-vessel

3.  Reculvers    171 miles S.E. by S.   18 out of 40 sounds heard.

4.  St. Nicholas 20 miles S.E.           3 out of 40 sounds heard.

5.  Epple Bay    22 miles S.E. by E.    19 out of 40 sounds heard.

6.  Westgate     23 miles S.E. by E.     9 out of 40 sounds heard.

7.  Kingsgate    25 miles S.E.  by E.    8 out of 40 sounds heard.

The day was cloudy, with occasional showers of drizzling rain; the
wind about N.W. by N. all day; at times squally, rising to a force of
6 or 7 and sometimes dropping to a force of 2 or 3.  The station at
Leigh excepted, all these places were to leeward of Shoeburyness.  At
four other stations to leeward, varying in distance from 15.5 to 24.5
miles, nothing was heard, while at eleven stations to windward,
varying from 8 to 26 miles, the sounds were also inaudible.  It was
found, indeed, that the sounds proceeding directly against the wind
did not penetrate much beyond 3 miles.

On the following day, viz. the 18th October, we proceeded to Dungeness
with the view of making a series of strict comparative experiments
with gun-cotton and cotton-powder.  Rockets containing 8 oz, 4 oz, and
2 oz. of gun-cotton had been prepared at the Royal Arsenal; while
others, containing similar quantities of cotton-powder, had been
supplied by the Cotton-powder Company at Faversham.  With these were
compared the ordinary 18-pounder gun, which happened to be mounted at
Dungeness, firing the usual charge of 3 lbs. of powder, and a syren.

From these experiments it appeared that the guncotton and
cotton-powder were practically equal as producers of sound.

The effectiveness of small charges was illustrated in a very striking
manner, only a single unit separating the numerical value of the 8-oz.
rocket from that of the 2-oz. rocket.  The former was recorded as 6.9
and the latter as 5.9, the value of the 4-oz. rocket being
intermediate between them.  These results were recorded by a number of
very practised observers on board the 'Galatea.' They were completely
borne out by the observations of the Coastguard, who marked the value
of the 8-oz rocket 6-1, and that of the 2-oz. rocket 5.2.  The
18-pounder gun fell far behind all the rockets, a result, possibly, to
be in part ascribed to the imperfection of the powder.  The
performance of the syren was, on the whole, less satisfactory than
that of the rocket.  The instrument was worked, not by steam of 70
lbs. pressure, as at the South Foreland, but by compressed air,
beginning with 40 lbs. and ending with 30 lbs. pressure.  The trumpet
was pointed to windward, and in the axis of the instrument the sound
was about as effective as that of the 8-oz. rocket.  But in a
direction at right angles to the axis, and still more in the rear of
this direction, the syren fell very sensibly behind even the 2-oz.
rocket.

These are the principal comparative trials made between the gun-cotton
rocket and other fog-signals; but they are not the only ones.  On the
2nd of August, 1877, for example, experiments were made at Lundy
Island with the following results.  At 2 miles distant from the
firing-point, with land intervening, the 18-pounder, firing a 3-lb.
charge, was quite unheard.  Both the 4-oz. rocket and the 8-oz.
rocket, however, reached an elevation which commanded the acoustic
shadow, and yielded loud reports.  When both were in view the rockets
were still superior to the gun.  On the 6th of August, at St. Ann's,
the 4-oz. and 8-oz. rockets proved superior to the syren.  On the
Shambles Light-vessel, when a pressure of 13 lbs. was employed to
sound the syren, the rockets proved greatly superior to that
instrument.  Proceeding along the sea margin at Flamboro' Head, Mr.
Edwards states that at a distance of 1.25 mile, with the 18-pounder
previously used as a fog-signal hidden behind the cliffs, its report
was quite unheard, while the 4-oz. rocket, rising to an elevation
which brought it clearly into view, yielded a powerful sound in the
face of an opposing wind.

On the evening of February 9th, 1877, a remarkable series of
experiments were made by Mr. Prentice at Stowmarket with the
gun-cotton rocket.  From the report with which he has kindly furnished
me I extract the following particulars.  The first column in the
annexed statement contains the name of the place of observation, the
second its distance from the firing-point, and the third the result
observed:

Stoke Hill, Ipswich  10 miles Rockets clearly seen and sounds
distinctly heard 53 seconds after the flash.

Melton               15 miles Signals distinctly heard.  Thought at
                              first that sounds were reverberated
                              from the sea.

Framlingham          18 miles Signals very distinctly heard, both in
                              the open air and in a closed room.
                              Wind in favour of sound.

Stratford.           19 miles St. Andrews Reports loud; startled
                              pheasants  in a cover close by.

Tuddenham.           10 miles St. Martin Reports very loud; rolled
                              away like thunder.

Christ Church Park.  11 miles Report arrived a little more than a
                              minute after flash.

Nettlestead Hall      6 miles Distinct in every part of observer's
                              house.  Very loud in the open air.

Bildestone            6 miles Explosion very loud, wind against sound.

Nacton               14 miles Reports quite distinct--mistaken by
                              inhabitants for claps of thunder.

Aldboro'             25 miles Rockets seen through a very hazy
                              atmosphere; a rumbling detonation heard.

Capel Mills          11 miles Reports heard within and without the
                              observer's house.  Wind opposed to sound.

Lawford              15.5 miles Reports distinct: attributed to
                                distant thunder.

In the great majority of these cases, the direction of the sound
enclosed a large angle with the direction of the wind.  In some cases,
indeed, the two directions were at right angles to each other.  It is
needless to dwell for a moment on the advantage of possessing a signal
commanding ranges such as these.

The explosion of substances in the air, after having been carried to a
considerable elevation by rockets, is a familiar performance.  In
1873, moreover, the Board of Trade proposed a light-and-sound rocket
as a signal of distress, which proposal was subsequently realized, but
in a form too elaborate and expensive for practical use.  The idea of
a gun-cotton rocket fit for signalling in fogs is, I believe, wholly
due to Sir Richard Collinson, the Deputy Master of the Trinity House.
Thanks to the skilful aid given by the authorities of Woolwich, by Mr.
Prentice, and Mr. Brock, that idea is now an accomplished fact; a
signal of great power, handiness, and economy, being thus placed at
the service of our mariners.  Not only may the rocket be applied in
association with lighthouses and lightships, but in the Navy also it
may be turned to important account.  Soon after the loss of the
'Vanguard' I ventured to urge upon an eminent naval officer the
desirability of having an organized code of fog-signals for the fleet.
He shook his head doubtingly, and referred to the difficulty of
finding room for signal guns.  The gun-cotton rocket completely
surmounts this difficulty, It is manipulated with ease and rapidity,
while its discharges may be so grouped and combined as to give a most
important extension to the voice of the admiral in command.  It is
needless to add that at any point upon our coasts, or upon any other
coast, where its establishment might be desirable, a fog-signal
station might be extemporised without difficulty.

*****

I have referred more than once to the train of echoes which
accompanied the explosion of gun-cotton in free air, speaking of them
as similar in all respects to those which were described for the first
time in my Report on Fog-signals, addressed to the Corporation of
Trinity House in 1874. [Footnote: See also 'Philosophical
Transactions' for 1874, p. 183.]  To these echoes I attached a
fundamental significance.  There was no visible reflecting surface
from which they could come.  On some days, with hardly a cloud in the
air and hardly a ripple on the sea, they reached a magical intensity.
As far as the sense of hearing could judge, they came from the body of
the air in front of the great trumpet which produced them.  The
trumpet blasts were five seconds in duration, but long before the
blast had ceased the echoes struck in, adding their strength to the
primitive note of the trumpet.  After the blast had ended the echoes
continued, retreating further and further from the point of
observation, and finally dying away at great distances.  The echoes
were perfectly continuous as long as the sea was clear of ships,
'tapering' by imperceptible gradations into absolute silence.  But
when a ship happened to throw itself athwart the course of the sound,
the echo from the broadside of the vessel was returned as a shock
which rudely interrupted the continuity of the dying atmospheric
music.

These echoes have been ascribed to reflection from the crests of the
sea-waves.  But this hypothesis is negatived by the fact, that the
echoes were produced in great intensity and duration when no waves
existed--when the sea, in fact, was of glassy smoothness.  It has been
also shown that the direction of the echoes depended not on that of
waves, real or assumed, but on the direction of the axis of the
trumpet.  Causing that axis to traverse an arc of 210°, and the
trumpet to sound at various points of the arc, the echoes were always,
at all events in calm weather, returned from that portion of the
atmosphere towards which the trumpet was directed.  They could not,
under the circumstances, come from the glassy sea; while both their
variation of direction and their perfectly continuous fall into
silence, are irreconcilable with the notion that they came from fixed
objects on the land.  They came from that portion of the atmosphere
into which the trumpet poured its maximum sound, and fell in intensity
as the direct sound penetrated to greater atmospheric distances.

The day on which our latest observations were made was particularly
fine.  Before reaching Dungeness, the smoothness of the sea and the
serenity of the air caused me to test the echoing power of the
atmosphere.  A single ship lay about half a mile distant between us
and the land.  The result of the proposed experiment was clearly
foreseen.  It was this.  The rocket being sent up, it exploded at a
great height; the echoes retreated in their usual fashion, becoming
less and less intense as the distances of the invisible surfaces of
reflection from the observers increased.  About five seconds after the
explosion, a single loud shock was sent back to us from the side of
the vessel lying between us and the land.  Obliterated for a moment by
this more intense echo the aerial reverberation continued its retreat,
dying away into silence in two or three seconds afterwards. [Footnote:
The echoes of the gun fired on shore this day were very brief; those
of the 12-oz. gun-cotton rocket were 12" and those of the 8-oz.
cotton-powder rocket 11" in duration.]

I have referred to the firing of an 8-oz. rocket from the deck of the
'Galatea' on March 8, 1877, stating the duration of its echoes to be
seven seconds.  Mr. Prentice, who was present at the time, assured me
that in his experiments similar echoes had been frequently heard of
more than twice this duration.  The ranges of his sounds alone would
render this result in the highest degree probable.

To attempt to interpret an experiment which I have not had an
opportunity of repeating, is an operation of some risk; and it is not
without a consciousness of this that I refer here to a result
announced by Professor Joseph Henry, which he considers adverse to the
notion of aerial echoes.  He took the trouble to point the trumpet of
a syren towards the zenith, and found that when the syren was sounded
no echo was returned.  Now the reflecting surfaces which give rise to
these echoes are for the most part due to differences of temperature
between sea and air.  If, through any cause, the air above be chilled,
we have descending streams--if the air below be warmed, we have
ascending streams as the initial cause of atmospheric flocculence.  A
sound proceeding vertically does not cross the streams, nor impinge
upon the reflecting surfaces, as does a sound proceeding horizontally
across them.  Aerial echoes, therefore, will not accompany the
vertical sound as they accompany the horizontal one.  The experiment,
as I interpret it, is not opposed to the theory of these echoes which
I have ventured to enunciate.  But, as I have indicated, not only to
see but to vary such an experiment is a necessary prelude to grasping
its full significance.

In a paper published in the 'Philosophical Transactions' for 1876,
Professor Osborne Reynolds refers to these echoes in the following
terms Without attempting to explain the reverberations and echoes
which have been observed, I will merely call attention to the fact
that in no case have I heard any attending the reports of the rockets,
[Footnote: These carried 12 oz. of gunpowder, which has been found by
Col. Fraser to require an iron case to produce an effective
explosion.]  although they seem to have been invariable with the guns
and pistols.  These facts suggest that the echoes are in some way
connected with the direction given to the sound.  They are caused by
the voice, trumpets, and the syren, all of which give direction to the
sound; but I am not aware that they have ever been observed in the
case of a sound which has no direction of greatest intensity.' The
reference to the voice, and other references in his paper, cause me to
think that, in speaking of echoes, Professor Osborne Reynolds and
myself are dealing with different phenomena.  Be that as it may, the
foregoing observations render it perfectly certain that the condition
as to direction here laid down is not necessary to the production of
the echoes.

There is not a feature connected with the aerial echoes which cannot
be brought out by experiments in the air of the laboratory.  I have
recently made the following experiment: A rectangle, x Y (p. 331),
22 inches by 12, was crossed by twenty-three brass tubes (half the
number would suffice and only eleven are shown in the figure), each
having a slit along it from which gas can issue.  In this way
twenty-three low flat flames were obtained.  A sounding reed a fixed
in a short tube was placed at one end of the rectangle, and a
'sensitive flame,' [Footnote: Fully described in my 'Lectures on
Sound,' 3rd edition, p. 227.]  f, at some distance beyond the other
end.  When the reed sounded, the flame in front of it was violently
agitated, and roared boisterously.  Turning on the gas, and lighting
it as it issued from the slits, the air above the flames became so
heterogeneous that the sensitive flame was instantly stilled, rising
from a height of 6 inches to a height of 18 inches.  Here we had the
acoustic opacity of the air in front of the South Foreland strikingly
imitated. [Footnote: Lectures on Sound, 3rd ed, p. 268.]  Turning off
the gas, and removing the sensitive flame to f, some distance behind
the reed, it burned there tranquilly, though the reed was sounding.
Again lighting the gas as it issued from the brass tubes, the sound
reflected from the heterogeneous air threw the sensitive flame into
violent agitation.  Here we had imitated the aerial echoes heard when
standing behind the syren-trumpet at the South Foreland. The
experiment is extremely simple, and in the highest degree impressive.

Fig.  11.

*****

The explosive rapidity of dynamite marks it as a substance specially
suitable for the production of sound.  At the suggestion of Professor
Dewar, Mr. McRoberts has carried out a series of experiments on
dynamite, with extremely promising results.  Immediately after the
delivery of the foregoing lecture I was informed that Mr. Brock
proposed the employment of dynamite in the Collinson rocket.



********************

XI.  ON THE STUDY OF PHYSICS.

[Footnote: From a lecture delivered in the Royal Institution of Great
Britain in the Spring of 1854.]

I HOLD in my hand an uncorrected proof of the syllabus of this course
of lectures, and the title of the present lecture A there stated to be
'On the Importance of the Study of Physics as a Means of Education.'
The corrected proof, however, contains the title: 'On the Importance
of the Study of Physics as a Branch of Education.' Small as this
editorial alteration may seem, the two words suggest two radically
distinct modes of viewing the subject before us.  The term Education
is sometimes applied to a single faculty or organ, and if we know
wherein the education of a single faculty consists, this will help us
to clearer notions regarding the education of the sum of all the
faculties, or of the mind.  When, for example, we speak of the
education of the voice, what do we mean?  There are certain membranes
at the top of the windpipe which throw into vibration the air forced
between them from the lungs, thus producing musical sounds.  These
membranes are, to some extent, under the control of the will, and it
is found that they can be so modified by exercise as to produce notes
of a clearer and more melodious character.  This exercise we call the
education of the voice.  We may choose for our exercise songs new or
old, festive or solemn; the education of the voice being the object
aimed at, the songs may be regarded as the means by which this
education is accomplished.  I think this expresses the state of the
case more clearly than if we were to call the songs a branch of
education.  Regarding also the education of the human mind as the
improvement and development of the mental faculties, I shall consider
the study of Physics as a means towards the attainment of this end.
From this point of view, I degrade Physics into an implement of
culture, and this is my deliberate design.

The term Physics, as made use of in the present Lecture, refers to
that portion of natural science which lies midway between astronomy
and chemistry.  The former, indeed, is Physics applied to 'masses of
enormous weight,' while the latter is Physics applied to atoms and
molecules.  The subjects of Physics proper are therefore those which
lie nearest to human perception: light and heat, colour, sound,
motion, the loadstone, electrical attractions and repulsions, thunder
and lightning, rain, snow, dew, and so forth.  Our senses stand
between these phenomena and the reasoning mind.  We observe the fact,
but are not satisfied with the mere act of observation: the fact must
be accounted for--fitted into its position in the line of cause and
effect.  Taking our facts from Nature we transfer them to the domain
of thought: look at them, compare them, observe their mutual relations
and connexions, and bringing them ever clearer before the mental eye,
finally alight upon the cause which unites them.  This is the last act
of the mind, in this centripetal direction--in its progress from the
multiplicity of facts to the central cause on which they depend.  But,
having guessed the cause, we are not yet contented.  We set out from
the centre and travel in the other direction.  If the guess be true,
certain consequences must follow from it, and we appeal to the law and
testimony of experiment whether the thing is so.  Thus is the circuit
of thought completed,--from without inward, from multiplicity to
unity, and from within outward, from unity to multiplicity.  In thus
traversing both ways the line between cause and effect, all our
reasoning powers are called into play.  The mental effort involved in
these processes may be compared to those exercises of the body which
invoke the co-operation of every muscle, and thus confer upon the
whole frame the benefits of healthy action.

The first experiment a child makes is a physical experiment: the
suction-pump is but an imitation of the first act of every new-born
infant.  Nor do I think it calculated to lessen that infant's
reverence, or to make him a worse citizen, when his riper experience
shows him that the atmosphere was his helper in extracting the first
draught from his mother's breast. The child grows, but is still an
experimenter: he grasps at the moon, and his failure teaches him to
respect distance.  At length his little fingers acquire sufficient
mechanical tact to lay hold of a spoon.  He thrusts the instrument
into his mouth, hurts his gums, and thus learns the impenetrability of
matter.  He lets the spoon fall, and jumps with delight to hear it
rattle against the table.  The experiment made by accident is repeated
with intention, and thus the young student receives his first lessons
upon sound and gravitation.  There are pains and penalties, however,
in the path of the enquirer: he is sure to go wrong, and Nature is
just as sure to inform him of the fact.  He falls downstairs, burns
his fingers, cuts his hand, scalds his tongue, and in this way learns
the conditions of his physical well being.  This is Nature's way of
proceeding, and it is wonderful what progress her pupil makes.  His
enjoyments for a time are physical, and the confectioner's shop
occupies the foreground of human happiness; but the blossoms of a
finer life are already beginning to unfold themselves, and the
relation of cause and effect dawns upon the boy.  He begins to see
that the present condition of things is not final, but depends upon
one that has gone before, and will be succeeded by another.  He
becomes a puzzle to himself; and to satisfy his newly-awakened
curiosity, asks all manner of inconvenient questions.  The needs and
tendencies of human nature express themselves through these early
yearnings of the child.  As thought ripens, he desires to know the
character and causes of the phenomena presented to his observation;
and unless this desire has been granted for the express purpose of
having it repressed, unless the attractions of natural phenomena be
like the blush of the forbidden fruit, conferred merely for the
purpose of exercising our self-denial in letting them alone; we may
fairly claim for the study of Physics the recognition that it answers
to an impulse implanted by nature in the constitution of man.

A few days ago, a Master of Arts, who is still a young man, and
therefore the recipient of a modern education, stated to me that until
he had reached the age of twenty years he had never been taught
anything whatever regarding natural phenomena, or natural law.  Twelve
years of his life previously had been spent exclusively among the
ancients.  The case, I regret to say, is typical.  Now, we cannot,
without prejudice to humanity, separate the present from the past. The
nineteenth century strikes its roots into the centuries gone by, and
draws nutriment from them.  The world cannot afford to lose the record
of any great deed or utterance; for such are prolific throughout all
time.  We cannot yield the companionship of our loftier brothers of
antiquity,--of our Socrates and Cato,--whose lives provoke us to
sympathetic greatness across the interval of two thousand years.  As
long as the ancient languages are the means of access to the ancient
mind, they must ever be of priceless value to humanity; but surely
these avenues might be kept open without making such sacrifices as
that above referred to, universal.  We have conquered and possessed
ourselves of continents of land, concerning which antiquity knew
nothing; and if new continents of thought reveal themselves to the
exploring human spirit, shall we not possess them also?  In these
latter days, the study of Physics has given us glimpses of the methods
of Nature which were quite hidden from the ancients, and we should be
false to the trust committed to us, if we were to sacrifice the hopes
and aspirations of the Present out of deference to the Past.

The bias of my own education probably manifests itself in a desire I
always feel to seize upon every possible opportunity of checking my
assumptions and conclusions by experience.  In the present case, it is
true, your own consciousness might be appealed to in proof of the
tendency of the human mind to inquire into the phenomena presented to
it by the senses; but I trust you will excuse me if, instead of doing
this, I take advantage of the facts which have fallen in my way
through life, referring to your judgment to decide whether such facts
are truly representative and general, and not merely individual and
local.

At an agricultural college in Hampshire, with which I was connected
for some time, and which is now converted into a school for the
general education of youth, a Society was formed among the boys, who
met weekly for the purpose of reading reports and papers upon various
subjects.  The Society had its president and treasurer; and abstracts
of its proceedings were published in a little monthly periodical
issuing from the school press.  One of the most remarkable features of
these weekly meetings was, that after the general business had been
concluded, each member enjoyed the right of asking questions on any
subject on which he desired information.  The questions were either
written out previously in a book, or, if a question happened to
suggest itself during the meeting, it was written upon a slip of paper
and handed in to the Secretary, who afterwards read all the questions
aloud.  A number of teachers were usually present, and they and the
boys made a common stock of their wisdom in furnishing replies.  As
might be expected from an assemblage of eighty or ninety boys, varying
from eighteen to eight years old, many odd questions were proposed. To
the mind which loves to detect in the tendencies of the young the
instincts of humanity generally, such questions are not without a
certain philosophic interest, and I have therefore thought it not
derogatory to the present course of Lectures to copy a few of them,
and to introduce them here.  They run as follows:

What are the duties of the Astronomer Royal?

What is frost?

Why are thunder and lightning more frequent in summer than in winter?

What occasions falling stars?

What is the cause of the sensation called 'pins and needles '?

What is the cause of waterspouts?

What is the cause of hiccup?

If a towel be wetted with water, why does the wet portion become
darker than before?

What is meant by Lancashire witches?

Does the dew rise or fall?

What is the principle of the hydraulic press?

Is there more oxygen in the air in summer than in winter?

What are those rings which we see round the gas and sun?

What is thunder?

How is it that a black hat can be moved by forming round it a magnetic
circle, while a white hat remains stationary?

What is the cause of perspiration?

Is it true that men were once monkeys?

What is the difference between the soul and the mind?

Is it contrary to the rules of Vegetarianism to eat eggs?

In looking over these questions, which were wholly unprompted, and
have been copied almost at random from the book alluded to, we see
that many of them are suggested directly by natural objects, and are
not such as had an interest conferred on them' by previous culture.
Now the fact is beyond the boy's control, and so certainly is the
desire to know its cause.  The sole question then is, whether this
desire is to be gratified or not.  Who created the fact?  Who
implanted the desire?  Certainly not man.  Who then will undertake to
place himself between the desire and its fulfilment, and proclaim a
divorce between them?  Take, for example, the case of the wetted
towel, which at first sight appears to be one of the most unpromising
questions in the list. Shall we tell the proposer to repress his
curiosity, as the subject is improper for him to know, and thus
interpose our wisdom to rescue the boy from the consequences of a wish
which acts to his prejudice?  Or, recognising the propriety of the
question, how shall we answer it?  It is impossible to answer it
without reference to the laws of optics--without making the boy to
some extent a natural philosopher.  You may say that the effect is due
to the reflection of light at the common surface of two media of
different refractive indices.  But this answer presupposes on the part
of the boy a knowledge of what reflection and refraction are, or
reduces you to the necessity of explaining them.

On looking more closely into the matter, we find that our wet towel
belongs to a class of phenomena which have long excited the interest
of philosophers.  The towel is white for the same reason that snow is
white, that foam is white, that pounded granite or glass is white, and
that the salt we use at table is white.  On quitting one medium and
entering another, a portion of light is always reflected, but on this
condition--the media must possess different refractive indices. Thus,
when we immerse a bit of glass in water, light is reflected from the
common surface of both, and it is this light which enables us to see
the glass.  But when a transparent solid is immersed in a liquid of
the same refractive index as itself, it immediately disappears.  I
remember once dropping the eyeball of an ox into water; it vanished as
if by magic, with the exception of the crystalline lens, and the
surprise was so great as to cause a bystander to suppose that the
vitreous humour had been instantly dissolved.  This, however, was not
the case, and a comparison of the refractive index of the humour with
that of water cleared up the whole matter.  The indices were
identical, and hence the light pursued its way through both as if they
formed one continuous mass.

In the case of snow, powdered quartz, or salt, we have a transparent
solid mixed with air.  At every transition from solid to air, or
from air to solid, a portion of light is reflected, and this takes
place so often that the light is wholly intercepted.  Thus from the
mixture of two transparent bodies we obtain an opaque one.  Now the
case of the towel is precisely similar.  The tissue is composed of
semi-transparent vegetable fibres, with the interstices between them
filled with air; repeated reflection takes place at the limiting
surfaces of air and fibre, and hence the towel becomes opaque like
snow or salt.  But if we fill the interstices with water, we diminish
the reflection; a portion of the light is transmitted, and the
darkness of the towel is due to its increased transparency.  Thus the
deportment of various minerals, such as hydrophane and tabasheer, the
transparency of tracing paper used by engineers, and many other
considerations of the highest scientific interest, are involved in the
simple enquiry of this unsuspecting little boy.

Again, take the question regarding the rising or falling of the dew--a
question long agitated, and finally set at rest by the beautiful
researches of Wells.  I do not think that any boy of average
intelligence will be satisfied with the simple answer that the dew
falls.  He will wish to learn how you know that it falls, and, if
acquainted with the notions of the middle ages, he may refer to the
opinion of Father Laurus, that a goose egg filled in the morning with
dew and exposed to the sun, will rise like a balloon--a swan's egg
being better for the experiment than a goose egg.  It is impossible to
give the boy a clear notion of the beautiful phenomenon to which his
question refers, without first making him acquainted with the
radiation and conduction of heat.  Take, for example, a blade of
grass, from which one of these orient pearls is depending.

During the day the grass, and the earth beneath it, possess a certain
amount of warmth imparted by the sun; during a serene night, heat is
radiated from the surface of the grass into space, and to supply the
loss, there is a flow of heat from the earth to the blade.  Thus the
blade loses heat by radiation, and gains heat by conduction.  Now, in
the case before us, the power of radiation is great, whereas the power
of conduction is small; the consequence is that the blade loses more
than it gains, and hence becomes more and more refrigerated.  The
light vapour floating around the surface so cooled is condensed upon
it, and there accumulates to form the little pearly globe which we
call a dew-drop.

Thus the boy finds the simple and homely fact which addressed his
senses to be the outcome and flower of the deepest laws.  The fact
becomes, in a measure, sanctified as an object of thought, and
invested for him with a beauty for evermore.  He thus learns that
things which, at first sight, seem to stand isolated and without
apparent brotherhood in Nature are organically united, and finds the
detection of such analogies a source of perpetual delight.  To enlist
pleasure on the side of intellectual performance is a point of the
utmost importance; for the exercise of the mind, like that of the
body, depends for its value upon the spirit in which it is
accomplished.  Every physician knows that something more than mere
mechanical motion is comprehended under the idea of healthful
exercise--that, indeed, being most healthful which makes us forget all
ulterior ends in the mere enjoyment of it.  What, for example, could
be substituted for the action of the playground, where the boy plays
for the mere love of playing, and without reference to physiological
laws; while kindly Nature accomplishes her ends unconsciously, and
makes his very indifference beneficial to him.  You may have more
systematic motions, you may devise means for the more perfect traction
of each particular muscle, but you cannot create the joy and gladness
of the game, and where these are absent, the charm and the health of
the exercise are gone.  The case is similar with the education of the
mind.

The study of Physics, as already intimated, consists of two processes,
which are complementary to each other--the tracing of facts to their
causes, and the logical advance from the cause to the fact.  In the
former process, called _induction_, certain moral qualities come into
play.  The first condition of success is patient industry, an honest
receptivity, and a willingness to abandon all preconceived notions,
however cherished, if they be found to contradict the truth.  Believe
me, a self-renunciation which has something lofty in it, and of which
the world never hears, is often enacted in the private experience of
the true votary of science.  And if a man be not capable of this
self-renunciation--this loyal surrender of himself to Nature and to
fact, he lacks, in my opinion, the first mark of a true philosopher.

Thus the earnest prosecutor of science, who does not work with the
idea of producing a sensation in the world, who loves the truth better
than the transitory blaze of to-day's fame, who comes to his task with
a single eye, finds in that task an indirect means of the highest
moral culture.  And although the virtue of the act depends upon its
privacy, this sacrifice of self, this upright determination to accept
the truth, no matter how it may present itself--even at the hands of a
scientific foe, if necessary--carries with it its own reward.  When
prejudice is put under foot and the stains of personal bias have been
washed away--when a man consents to lay aside his vanity and to become
Nature's organ--his elevation is the instant consequence of his
humility.

I should not wonder if my remarks provoked a smile, for they seem to
indicate that I regard the man of science as a heroic, if not indeed
an angelic, character; and cases may occur to you which indicate the
reverse.  You may point to the quarrels of scientific men, to their
struggles for priority, to that unpleasant egotism which screams
around its little property of discovery like a scared plover about its
young.  I will not deny all this; but let it be set down to its proper
account, to the weakness--or, if you will--to the selfishness of Man,
but not to the charge of Physical Science.

The second process in physical investigation is _deduction_, or the
advance of the mind from fixed principles to the conclusions which
flow from them.  The rules of logic are the formal statement of this
process, which, however, was practised by every healthy mind before
ever such rules were written.  In the study of Physics, induction and
deduction are perpetually wedded to each other.  The man observes,
strips facts of their peculiarities of form, and tries to unite them
by their essences; having effected this, he at once deduces, and thus
checks his induction.

Here the grand difference between the methods at present followed, and
those of the ancients, becomes manifest. They were one-sided in these
matters: they omitted the process of induction, and substituted
conjecture for observation.  They could never, therefore, fulfil the
mission of Man to 'replenish the earth, and subdue it.' The
subjugation of Nature is only to be accomplished by the penetration of
her secrets and the patient mastery of her laws.  This not only
enables us to protect ourselves from the hostile action of natural
forces, but makes them our slaves.  By the study of Physics we have
indeed opened to us treasuries of power of which antiquity never
dreamed.  But while we lord it over Matter, we have thereby become
better acquainted with the laws of Mind; for to the mental philosopher
the study of Physics furnishes a screen against which the human spirit
projects its own image, and thus becomes capable of self-inspection.

Thus, then, as a means of intellectual culture, the study of Physics
exercises and sharpens observation: it brings the most exhaustive
logic into play: it compares, abstracts, and generalizes, and provides
a mental scenery appropriate to these processes.  The strictest
precision of thought is everywhere enforced, and prudence, foresight,
and sagacity are demanded.  By its appeals to experiment, it
continually checks itself, and thus walks on a foundation of facts.
Hence the exercise it invokes does not end in a mere game of
intellectual gymnastics, such as the ancients delighted in, but tends
to the mastery of Nature.  This gradual conquest of the external
world, and the consciousness of augmented strength which accompanies
it, render the study of Physics as delightful as it is important.

With regard to the effect on the imagination, certain it is that the
cool results of physical induction furnish conceptions which transcend
the most daring flights of that faculty.  Take for example the idea of
an all-pervading aether which transmits a tingle, so to speak, to the
finger ends of the universe every time a street lamp is lighted.  The
invisible billows of this aether can be measured with the same ease
and certainty as that with which an engineer measures a base and two
angles, and from these finds the distance across the Thames.  Now it
is to be confessed that there may be just as little poetry in the
measurement of an aethereal undulation as in that of the river; for
the intellect, during the acts of measurement and calculation,
destroys those notions of size which appeal to the poetic sense.  It
is a mistake to suppose, with Dr. Young, that

    An undevout astronomer is mad;

there being no necessary connexion between a devout state of mind and
the observations and calculations of a practical astronomer.  It is
not until the man withdraws from his calculation, as a painter from
his work, and thus realizes the great idea on which he has been
engaged, that imagination and wonder are excited.  There is, I admit,
a possible danger here.  If the arithmetical processes of science be
too exclusively pursued, they may impair the imagination, and thus the
study of Physics is open to the same objection as philological,
theological, or political studies, when carried to excess.  But even
in this case, the injury done is to the investigator himself: it does
not reach the mass of mankind.  Indeed, the conceptions furnished by
his cold unimaginative reckonings may furnish themes for the poet, and
excite in the highest degree that sentiment of wonder which,
notwithstanding all its foolish vagaries, table-turning included, I,
for my part, should be sorry to see banished from the world.

I have thus far dwelt upon the study of Physics as an agent of
intellectual culture; but like other things in Nature, this study
subserves more than a single end.  The colours of the clouds delight
the eye, and, no doubt, accomplish moral purposes also, but the
selfsame clouds hold within their fleeces the moisture by which our
fields are rendered fruitful.  The sunbeams excite our interest and
invite our investigation; but they also extend their beneficent
influences to our fruits and corn, and thus accomplish, not only
intellectual ends, but minister, at the same time, to our material
necessities.  And so it is with scientific research.

While the love of science is a sufficient incentive to the pursuit of
science, and the investigator, in the prosecution of his enquiries, is
raised above all material considerations, the results of his labours
may exercise a potent influence upon the physical condition of the
community.  This is the arrangement of Nature, and not that of the
scientific investigator himself; for he usually pursues his object
without regard to its practical applications.

And let him who is dazzled by such applications--who sees in the
steam-engine and the electric telegraph the highest embodiment of
human genius and the only legitimate object of scientific research,
beware of prescribing conditions to the investigator.  Let him beware
of attempting to substitute for that simple love with which the votary
of science pursues his task, the calculations of what he is pleased to
call utility.  The professed utilitarian is unfortunately, in most
cases, the very last man to see the occult sources from which useful
results are derived.  He admires the flower, but is ignorant of the
conditions of its growth.  The scientific man must approach Nature in
his own way; for if you invade his freedom by your so-called practical
considerations, it may be at the expense of those qualities on which
his success as a discoverer depends.  Let the self-styled practical
man look to those from the fecundity of whose thought be, and
thousands like him, have sprung into existence.  Were they inspired in
their first enquiries by the calculations of utility?  Not one of
them.  They were often forced to live low and lie hard, and to seek
compensation for their penury in the delight which their favourite
pursuits afforded them.

In the words of one well qualified to speak upon this subject, 'I say
not merely look at the pittance of men like John Dalton, or the
voluntary starvation of the late Graff; but compare what is considered
as competency or affluence by your Faradays, Liebigs, and Herschels,
with the expected results of a life of successful commercial
enterprise: then compare the amount of mind put forth, the work done
for society in either case, and you will be constrained to allow that
the former belong to a class of workers who, properly speaking, are
not paid, and cannot be paid for their work, as indeed it is of a sort
to which no payment could stimulate.'

But while the scientific investigator, standing upon the frontiers of
human knowledge, and aiming at the conquest of fresh soil from the
surrounding region of the unknown, makes the discovery of truth his
exclusive object for the time, he cannot but feel the deepest interest
in the practical application of the truth discovered.  There is
something ennobling in the triumph of Mind over Matter.  Apart even
from its uses to society, there is something elevating in the idea of
Man having tamed that wild force which flashes through the telegraphic
wire, and made it the minister of his will.  Our attainments in these
directions appear to be commensurate with our needs.  We had already
subdued horse and mule, and obtained from them all the service which
it was in their power to render: we must either stand still, or find
more potent agents to execute our purposes.  At this point the
steam-engine appears.  These are still new things; it is not long
since we struck into the scientific methods which have produced these
results.  We cannot for an instant regard them as the final
achievements of Science, but rather as an earnest of what she is yet
to do.  They mark our first great advances upon the dominion of
Nature.  Animal strength fails, but here are the forces which hold the
world together, and the instincts and successes of Man assure him that
these forces are his when he is wise enough to command them.

As an instrument of intellectual culture, the study of Physics is
profitable to all: as bearing upon special functions, its value,
though not so great, is still more tangible.  Why, for example, should
Members of Parliament be ignorant of the subjects concerning which
they are called upon to legislate?  In this land of practical physics,
why should they be unable to form an independent opinion upon a
physical question?  Why should the member of a parliamentary committee
be left at the mercy of interested disputants when a scientific
question is discussed, until he deems the nap a blessing which rescues
him from the bewilderments of the committee-room?  The education which
does not supply the want here referred to, fails in its duty to
England.  With regard to our working people, in the ordinary sense of
the term working, the study of Physics would, I imagine, be
profitable, not only as a means of intellectual culture, but also as a
moral influence to woo them from pursuits which now degrade them.  A
man's reformation oftener depends upon the indirect, than upon the
direct action of the will.  The will must be exerted in the choice of
employment which shall break the force of temptation by erecting a
barrier against it.  The drunkard, for example, is in a perilous
condition if he content himself merely with saying, or swearing, that
he will avoid strong drink.  His thoughts, if not attracted by another
force, will revert to the public-house, and to rescue him permanently
from this, you must give him an equivalent.

By investing the objects of hourly intercourse with an interest which
prompts reflection, new enjoyments would be opened to the working man,
and every one of these would be a point of force to protect him
against temptation.  Besides this, our factories and our foundries
present an extensive field of observation, and were those who work in
them rendered capable, by previous culture, of _observing_ what they
see, the results might be incalculable.  Who can say what intellectual
Samsons are at the present moment toiling with closed eyes in the
mills and forges of Manchester and Birmingham?  Grant these Samsons
sight, and you multiply the chances of discovery, and with them the
prospects of national advancement.  In our multitudinous technical
operations we are constantly playing with forces our ignorance of
which is often the cause of our destruction.  There are agencies at
work in a locomotive of which the maker of it probably never dreamed,
but which nevertheless may be sufficient to convert it into an engine
of death.  When we reflect on the intellectual condition of the people
who work in our coal mines, those terrific explosions which occur from
time to time need not astonish us.  If these men possessed sufficient
physical knowledge, from the operatives themselves would probably
emanate a system by which these shocking accidents might be avoided.
Possessed of the knowledge, their personal interests would furnish the
necessary stimulus to its practical application, and thus two ends
would be served at the same time the elevation of the men and the
diminution of the calamity.

Before the present Course of Lectures was publicly announced, I had
many misgivings as to the propriety of my taking a part in them,
thinking that my place might be better filled by an older and more
experienced man.  To my experience, however, such as it was, I
resolved to adhere, and I have therefore described things as they
revealed themselves to my own eyes, and have been enacted in my own
limited practice.  There is one mind common to us all; and the true
expression of this mind, even in small particulars, will attest itself
by the response which it calls forth in the convictions of my hearers.
I ask your permission to proceed a little further in this fashion, and
to refer to a fact or two in addition to those already cited, which
presented themselves to my notice during my brief career as a teacher
in the college already alluded to.  The facts, though extremely
humble, and deviating in some slight degree from the strict subject of
the present discourse, may yet serve to illustrate an educational
principle.

One of the duties which fell to my share was the instruction of a
class in mathematics, and I usually found that Euclid and the ancient
geometry generally, when properly and sympathetically addressed to the
understanding, formed a most attractive study for youth.  But it was
my habitual practice to withdraw the boys from the routine of the
book, and to appeal to their self-power in the treatment of questions
not comprehended in that routine.  At first, the change from the
beaten track usually excited aversion: the youth felt like a child
amid strangers; but in no single instance did this feeling continue.
When utterly disheartened, I have encouraged the boy by the anecdote
of Newton, where he attributes the difference between him and other
men, mainly to his own patience; or of Mirabeau, when he ordered his
servant, who had stated something to be impossible, never again to use
that blockhead of a word.  Thus cheered, the boy has returned to his
task with a smile, which perhaps had something of doubt in it, but
which, nevertheless, evinced a resolution to try again.  I have seen
his eye brighten, and, at length, with a pleasure of which the ecstasy
of Archimedes was but a simple expansion, heard him exclaim, 'I have
it, sir.' The consciousness of self-power, thus awakened, was of
immense value; and, animated by it, the progress of the class was
astonishing.  It was often my custom to give the boys the choice of
pursuing their propositions in the book, or of trying their strength
at others not to be found there.  Never in a single instance was the
book chosen.  I was ever ready to assist when help was needful, but my
offers of assistance were habitually declined.  The boys had tasted
the sweets of intellectual conquest and demanded victories of their
own.  Their diagrams were scratched on the walls, cut into the beams
upon the playground, and numberless other illustrations were afforded
of the living interest they took in the subject.  For my own part, as
far as experience in teaching goes, I was a mere fledgling--knowing
nothing of the rules of pedagogics, as the Germans name it; but
adhering to the spirit indicated at the commencement of this
discourse, and endeavouring to make geometry a means rather than a
branch of education.  The experiment was successful, and some of the
most delightful hours of my existence have been spent in marking the
vigorous and cheerful expansion of mental power, when appealed to in
the manner here described.

Our pleasure was enhanced when we applied our mathematical knowledge
to the solution of physical problems.  Many objects of hourly contact
had thus a new interest and significance imparted to them.  The swing,
the see-saw, the tension of the giant-stride ropes, the fall and
rebound of the football, the advantage of a small boy over a large one
when turning short, particularly in slippy weather; all became
subjects of investigation.  A lady stands before a looking-glass, of
her own height; it was required to know how much of the glass was
really useful to her?  We learned with pleasure the economic fact that
she might dispense with the lower half and see her whole figure
notwithstanding.  It was also pleasant to prove by mathematics, and
verify by experiment, that the angular velocity of a reflected beam is
twice that of the mirror which reflects it.  From the hum of a bee we
were able to determine the number of times the insect flaps its wings
in a second.  Following up our researches upon the pendulum, we
learned how Colonel Sabine had made it the means of determining the
figure of the earth; and we were also startled by the inference which
the pendulum enabled us to draw, that if the diurnal velocity of the
earth were seventeen times its present amount, the centrifugal force
at the equator would be precisely equal to the force of gravitation,
so that an inhabitant of those regions would then have the same
tendency to fall upwards as downwards.  All these things were sources
of wonder and delight to us: and when we remembered that we were
gifted with the powers which had reached such results, and that the
same great field was ours to work in, our hopes arose that at some
future day we might possibly push the subject a little further, and
add our own victories to the conquests already won.

I ought to apologise to you for dwelling so long upon this subject;
but the days spent among these young philosophers made a deep
impression on me.  I learned among them something of myself and of
human nature, and obtained some notion of a teacher's vocation.  If
there be one profession in England of paramount importance, I believe
it to be that of the schoolmaster; and if there be a position where
selfishness and incompetence do most serious mischief, by lowering the
moral tone and exciting irreverence and cunning where reverence and
noble truthfulness ought to be the feelings evoked, it is that of the
principal of a school.  When a man of enlarged heart and mind comes
among boys, when he allows his spirit to stream through them, and
observes the operation of his own character evidenced in the elevation
of theirs,--it would be idle to talk of the position of such a man
being honourable.  It is a blessed position.  The man is a blessing to
himself and to all around him.  Such men, I believe, are to be found
in England, and it behoves those who busy themselves with the
mechanics of education at the present day, to seek them out.  For no
matter what means of culture may be chosen, whether physical or
philological, success must ever mainly depend upon the amount of life,
love, and earnestness, which the teacher himself brings with him to
his vocation.

Let me again, and finally, remind you that the claims of that science
which finds in me to-day its unripened advocate, are those of the
logic of Nature upon the reason of her child--that its disciplines, as
an agent of culture, are based upon the natural relations subsisting
between Man and the universe of which he forms a part.  On the one
side, we have the apparently lawless shifting of phenomena; on the
other side, mind, which requires law for its equilibrium, and through
its own indestructible instincts, as well as through the teachings of
experience, knows that these phenomena are reducible to law.  To
chasten this apparent chaos is a problem which man has set before him.
The world was built in order: and to us are trusted the will and power
to discern its harmonies, and to make them the lessons of our lives.
From the cradle to the grave we are surrounded with objects which
provoke inquiry.  Descending for a moment from this high plea to
considerations which lie closer to us as a nation--as a land of gas
and furnaces, of steam and electricity: as a land which science,
practically applied, has made great in peace and mighty in war: I ask
you whether this 'land of old and just renown' has not a right to
expect from her institutions a culture more in accordance with her
present needs than that supplied by declension and conjugation?  And
if the tendency should be to lower the estimate of science, by
regarding it exclusively as the instrument of material prosperity, let
it be the high mission of our universities to furnish the proper
counterpoise by pointing out its nobler uses--lifting the national
mind to the contemplation of it as the last development of that
'increasing purpose' which runs through the ages and widens the
thoughts of men.

********************

XII.  ON CRYSTALLINE AND SLATY CLEAVAGE.

[Footnote: From a discourse delivered in the Royal Institution of
Great Britain, June 6, 1856.]

WHEN the student of physical science has to investigate the character
of any natural force, his first care must be to purify it from the
mixture of other forces, and thus study its simple action.  If, for
example, he wishes to know how a mass of liquid would shape itself if
at liberty to follow the bent of its own molecular forces, he must see
that these forces have free and undisturbed exercise.  We might
perhaps refer him to the dewdrop for a solution of the question; but
here we have to do, not only with the action of the molecules of the
liquid upon each other, but also with the action of gravity upon the
mass, which pulls the drop downwards and elongates it.  If he would
examine the problem in its purity, he must do as Plateau has done,
detach the liquid mass from the action of gravity; he would then find
the shape to be a perfect sphere.  Natural processes come to us in a
mixed manner, and to the uninstructed mind are a mass of
unintelligible confusion.  Suppose half-a-dozen of the best musical
performers to be placed in the same room, each playing his own
instrument to perfection, but no two playing the same tune; though
each individual instrument might be a source of perfect music, still
the mixture of all would produce mere noise.

Thus it is with the processes of nature, where mechanical and
molecular laws intermingle and create apparent confusion.  Their
mixture constitutes what may be called the _noise_ of natural laws, and
it is the vocation of the man of science to resolve this noise into
its components, and thus to detect the underlying music.

The necessity of this detachment of one force from all other forces is
nowhere more strikingly exhibited than in the phenomena of
crystallisation.  Here, for example, is a solution of common sulphate
of soda or Glauber salt.  Looking into it mentally, we see the
molecules of that liquid, like disciplined squadrons under a governing
eye, arranging themselves into battalions, gathering round distinct
centres, and forming themselves into solid masses, which after a time
assume the visible shape of the crystal now held in my hand.  I may,
like an ignorant meddler wishing to hasten matters, introduce
confusion into this order.  This may be done by plunging a glass rod
into the vessel; the consequent action is not the pure expression of
the crystalline forces; the molecules rush together with the confusion
of an unorganised mob, and not with the steady accuracy of a
disciplined host. In this mass of bismuth also we have an example of
confused crystallisation; but in the crucible behind me a slower
process is going on: here there is an architect at work 'who makes no
chips, no din,' and who is now building the particles into crystals,
similar in shape and structure to those beautiful masses which we see
upon the table.  By permitting alum to crystallise in this slow way,
we obtain these perfect octahedrons; by allowing carbonate of lime to
crystallise, nature produces these beautiful rhomboids; when silica
crystallises, we have formed these hexagonal prisms capped at the ends
by pyramids; by allowing saltpetre to crystallise we have these
prismatic masses, and when carbon crystallises, we have the diamond.
If we wish to obtain a perfect crystal we must allow the molecular
forces free play; if the crystallising mass be permitted to rest upon
a surface it will be flattened, and to prevent this a small crystal
must be so suspended as to be surrounded on all sides by the liquid,
or, if it rest upon the surface, it must be turned daily so as to
present all its faces in succession to the working builder.

In building up crystals these little atomic bricks often arrange
themselves into layers which are perfectly parallel to each other, and
which can be separated by mechanical means; this is called the
cleavage of the crystal.  The crystal of sugar I hold in my hand has
thus far escaped the solvent and abrading forces which sooner or later
determine the fate of sugar-candy.  I readily discover that it cleaves
with peculiar facility in one direction.  Again I lay my knife upon
this piece of rocksalt, and with a blow cleave it in one direction.
Laying the knife at right angles to its former position, the crystal
cleaves again; and finally placing the knife at right angles to the
two former positions, we find a third cleavage.  Rocksalt cleaves in
three directions, and the resulting solid is this perfect cube, which
may be broken up into any number of smaller cubes.  Iceland spar also
cleaves in three directions, not at right angles, but oblique to each
other, the resulting solid being a rhomboid.  In each of these cases
the mass cleaves with equal facility in all three directions.  For the
sake of completeness I may say that many crystals cleave with unequal
facility in different directions: heavy spar presents an example of
this kind of cleavage.

Turn we now to the consideration of some other phenomena to which the
term cleavage may be applied.  Beech, deal, and other woods cleave
with facility along the fibre, and this cleavage is most perfect when
the edge of the axe is laid across the rings which mark the growth of
the tree.  If you look at this bundle of hay severed from a rick, you
will see a sort of cleavage in it also; the stalks lie in horizontal
planes, and only a small force is required to separate them laterally.
But we cannot regard the cleavage of the tree as the same in character
as that of the hayrick.  In the one case it is the molecules arranging
themselves according to organic laws which produce a cleavable
structure, in the other case the easy separation in one direction is
due to the mechanical arrangement of the coarse sensible stalks of
hay.

This sandstone rock was once a powder held in mechanical suspension by
water.  The powder was composed of two distinct parts, fine grains of
sand and small plates of mica.  Imagine a wide strand covered by a
tide, or an estuary with water which holds such powder in suspension:
how will it sink?  The rounded grains of sand will reach the bottom
first, because they encounter least resistance, the mica afterwards,
and when the tide recedes we have the little plates shining like
spangles upon the surface of the sand.  Each successive tide brings
its charge of mixed powder, deposits its duplex layer day after day,
and finally masses of immense thickness are piled up, which by
preserving the alternations of sand and mica tell the tale of their
formation.  Take the sand and mica, mix them together in water, and
allow them to subside; they will arrange themselves in the manner
indicated, and by repeating the process you can actually build up a
mass which shall be the exact counterpart of that presented by nature.
Now this structure cleaves with readiness along the planes in which
the particles of mica are strewn.  Specimens of such a rock sent to me
from Halifax, and other masses from the quarries of Over Darwen in
Lancashire, are here before you.  With a hammer and chisel I can
cleave them into flags; indeed these flags are employed for roofing
purposes in the districts from which the specimens have come, and
receive the name of 'slatestone.' But you will discern without a word
from me, that this cleavage is not a crystalline cleavage any more
than that of a hayrick is.  It is molar, not molecular.

This, so far as I am aware of, has never been imagined, and it has
been agreed among geologists not to call such splitting as this
cleavage at all, but to restrict the term to a phenomenon of a totally
different character.

Those who have visited the slate quarries of Cumberland and North
Wales will have witnessed the phenomenon to which I refer.  We have
long drawn our supply of roofing-slates from such quarries;
school-boys ciphered on these slates, they were used for tombstones in
churchyards, and for billiard-tables in the metropolis; but not until
a comparatively late period did men begin to enquire how their
wonderful structure was produced.  What is the agency which enables us
to split Honister Crag, or the cliffs of Snowdon, into laminae from
crown to base?  This question is at the present moment one of the
great difficulties of geologists, and occupies their attention perhaps
more than any other.  You may wonder at this.  Looking into the quarry
of Penrhyn, you may be disposed to offer the explanation I heard given
two years ago.  'These planes of cleavage,' said a friend who stood
beside me on the quarry's edge, 'are the planes of stratification
which have been lifted by some convulsion into an almost vertical
position.' But this was a mistake, and indeed here lies the grand
difficulty of the problem.  The planes of cleavage stand in most cases
at a high angle to the bedding.  Thanks to Sir Roderick Murchison, I
am able to place the proof of this before you.  Here is a specimen of
slate in which both the planes of cleavage and of bedding are
distinctly marked, one of them making a large angle with the other.
This is common.  The cleavage of slates then is not a question of
stratification; what then is its cause?

In an able and elaborate essay published in 1835, Prof. Sedgwick
proposed the theory that cleavage is due to the action of crystalline
or polar forces subsequent to the consolidation of the rock.  'We may
affirm,' he says, 'that no retreat of the parts, no contraction of
dimensions in passing to a solid state, can explain such phenomena.
They appear to me only resolvable on the supposition that crystalline
or polar forces acted upon the whole mass simultaneously in one
direction and with adequate force.'  And again, in another place:
'Crystalline forces have re-arranged whole mountain masses, producing
a beautiful crystalline cleavage, passing alike through all the
strata.' [Footnote: Transactions of the Geological Society, ser. ii,
vol. iii. p. 477.]

The utterance of such a man struck deep, as it ought to do, into the
minds of geologists, and at the present day there are few who do not
entertain this view either in whole or in part. [Footnote: In a letter
to Sir Charles Lyell, dated from the Cape of Good Hope February 20,
1836, Sir John Herschel writes as follows: 'If rocks have been so
heated as to allow of a commencement of crystallisation, that is to
say, if they have been heated to a point at which the particles can
begin to move amongst themselves, or at least on their own axes, some
general law must then determine the position in which these particles
will rest on cooling.  Probably that position will have some relation
to the direction in which the heat escapes.  Now when all or a
majority of particles of the same nature have a general tendency to
one position, that must of course determine a cleavage plane.']  The
boldness of the theory, indeed, has, in some cases, caused speculation
to run riot, and we have books published on the action of polar forces
and geologic magnetism, which rather astonish those who know something
about the subject.  According to this theory whole districts of North
Wales and Cumberland, mountains included, are neither more nor less
than the parts of a gigantic crystal.  These masses of slate were
originally fine mud, composed of the broken and abraded particles of
older rocks.  They contain silica, alumina, potash, soda, and mica
mixed mechanically together.  In the course of ages the mixture became
consolidated, and the theory before us assumes that a process of
crystallisation afterwards rearranged the particles and developed in
it a single plane of cleavage.  Though a bold, and I think
inadmissible, stretch of analogies, this hypothesis has done good
service.  Right or wrong, a thoughtfully uttered theory has a dynamic
power which operates against intellectual stagnation; and even by
provoking opposition is eventually of service to the cause of truth.
It would, however, have been remarkable if, among the ranks of
geologists themselves, men were not found to seek an explanation of
slate-cleavage involving a less hardy assumption.

The first step in an enquiry of this kind is to seek facts.  This has
been done, and the labours of Daniel Sharpe (the late President of the
Geological Society, who, to the loss of science and the sorrow of all
who knew him, has so suddenly been taken away from us), Mr. Henry
Clifton Sorby, and others, have furnished us with a body of facts
associated with slaty cleavage, and having a most important bearing
upon the question.

Fossil shells are found in these slate-rocks.  I have here several
specimens of such shells in the actual rock, and occupying various
positions in regard to the cleavage planes.  They are squeezed,
distorted, and crushed; in all cases the distortion leads to the
inference that the rock which contains these shells has been subjected
to enormous pressure in a direction at right angles to the planes of
cleavage.  The shells are all flattened and spread out in these
planes.  Compare this fossil trilobite of normal proportions with
these others which have suffered distortion.  Some have lain across,
some along, and some oblique to the cleavage of the slate in which
they are found; but in all cases the distortion is such, as required
for its production a compressing force acting, at right angles to the
planes of cleavage.  As the trilobites lay in the mud, the jaws of a
gigantic vice appear to have closed upon them and squeezed them into
the shapes you see.

We sometimes find a thin layer of coarse gritty material, between two
layers of finer rock, through which and across the gritty layer pass
the planes of lamination.  The coarse layer is found bent by the
pressure into sinuosities like a contorted ribbon.  Mr. Sorby has
described a striking case of this kind.  This crumpling can be
experimentally imitated; the amount of compression might, moreover, be
roughly estimated by supposing the contorted bed to be stretched out,
its length measured and compared with the shorter distance into which
it has been squeezed.  We find in this way that the yielding of the
mass has been considerable.

Let me now direct your attention to another proof of pressure; you see
the varying colours which indicate the bedding on this mass of slate.
The dark portion is gritty, being composed of comparatively coarse
particles, which, owing to their size, shape and gravity, sink first
and constitute the bottom of each layer.  Gradually, from bottom to
top the coarseness diminishes, and near the upper surface we have a
layer of exceedingly fine grain.  It is the fine mud thus consolidated
from which are derived the German razor-stones, so much prized for the
sharpening of surgical instruments.

When a bed is thin, the fine-grain slate is permitted to rest upon a
slab of the coarse slate in contact with it; when the fine bed is
thick, it is cut into slices which are cemented to pieces of ordinary
slate, and thus rendered stronger.  The mud thus deposited is, as
might be expected, often rolled up into nodular masses, carried
forward, and deposited among coarser material by the rivers from which
the slate-mud has subsided.  Here are such nodules enclosed in
sandstone.  Everybody, moreover, who has ciphered upon a school-slate
must remember the whitish-green spots which sometimes dotted the
surface of the slate, and over which the pencil usually slid as if the
spots were greasy.  Now these spots are composed of the finer mud, and
they could not, on account of their fineness, bite the pencil like the
surrounding gritty portions of the slate.  Here is a beautiful example
of these spots: you observe them, on the cleavage surface, in broad
round patches.  But turn the slate edgeways and the section of each
nodule is seen to be a sharp oval with its longer axis parallel to the
cleavage.  This instructive fact has been adduced by Mr. Sorby.  I
have made excursions to the quarries of Wales and Cumberland, and to
many of the slate yards of London, and found the fact general.  Thus
we elevate a common experience of our boyhood into evidence of the
highest significance as regards a most important geological problem.
From the magnetic deportment of these slates, I was led to infer that
these spots contain a less amount of iron than the surrounding dark
slate.  An analysis was made for me by Mr. Hambly in the laboratory of
Dr. Percy at the School of Mines with the following result:

ANALYSIS OF SLATE.

Dark Slate, two analyses.

1.  Percentage of iron     5.85

2. Percentage of iron      6.13

                    Mean   5.99

Whitish Green Slate.

1.  Percentage of iron     3.24

2.  Percentage of iron     3.12

                    Mean   3.18

According to these analyses the quantity of iron in the dark slate
immediately adjacent to the greenish spot is nearly double the
quantity contained in the spot itself.  This is about the proportion
which the magnetic experiments suggested.

Let me now remind you that the facts brought before you are
typical--each is the representative of a class.  We have seen shells
crushed; the trilobites squeezed, beds contorted, nodules of greenish
marl flattened; and all these sources of independent testimony point
to one and the same conclusion, namely, that slate-rocks have been
subjected to enormous pressure in a direction at right angles to the
Planes of cleavage.

In reference to Mr. Sorby's contorted bed, I have said that by
supposing it to be stretched out and its length measured, it would
give us an idea of the amount of yielding of the mass above and below
the bed.  Such a measurement, however, would not give the exact amount
of yielding.  I hold in my hand a specimen of slate with its bedding
marked upon it; the lower portions of each layer being composed of a
comparatively coarse gritty material something like what you may
suppose the contorted bed to be composed of.  Now in crossing these
gritty portions, the cleavage turns, as if tending to cross the
bedding at another angle.  When the pressure began to act, the
intermediate bed, which is not entirely unyielding, suffered
longitudinal pressure; as it bent, the pressure became gradually more
transverse, and the direction of its cleavage is exactly such as you
would infer from an action of this kind--it is neither quite across
the bed, nor yet in the same direction as the cleavage of the slate
above and below it, but intermediate between both.  Supposing the
cleavage to be at right angles to the pressure, this is the direction
which it ought to take across these more unyielding strata.

Thus we have established the concurrence of the phenomena of cleavage
and pressure--that they accompany each other; but the question still
remains, Is the pressure sufficient to account for the cleavage?  A
single geologist, as far as I am aware, answers boldly in the
affirmative.  This geologist is Sorby, who has attacked the question
in the true spirit of a physical investigator.  Call to mind the
cleavage of the flags of Halifax and Over Darwen, which is caused by
the interposition of layers of mica between the gritty strata.  Mr.
Sorby finds plates of mica to be also a constituent of slate-rock.  He
asks himself, what will be the effect of pressure upon a mass
containing such plates confusedly mixed up in it?  It will be, he
argues, and he argues rightly, to place the plates with their flat
surfaces more or less perpendicular to the direction in which the
pressure is exerted.  He takes scales of the oxide of iron, mixes them
with a fine powder, and on squeezing the mass finds that the tendency
of the scales is to set themselves at right angles to the line of
pressure.  Along the planes of weakness produced by the scales the
mass cleaves.

By tests of a different character from those applied by Mr. Sorby, it
might be shown how true his conclusion is--that the effect of pressure
on elongated particles, or plates, will be such as he describes it.
But while the scales must be regarded as a true cause, I should not
ascribe to them a large share in the production of the cleavage.  I
believe that even if the plates of mica were wholly absent, the
cleavage of slate-rocks would be much the same as it is at present.

Here is a mass of pure white wax: it contains no mica particles, no
scales of iron, or anything analogous to them.  Here is the selfsame
substance submitted to pressure.  I would invite the attention of the
eminent geologists now before me to the structure of this wax.  No
slate ever exhibited so clean a cleavage; it splits into laminae of
surpassing tenuity, and proves at a single stroke that pressure is
sufficient to produce cleavage, and that this cleavage is independent
of intermixed plates or scales.  I have purposely mixed this wax with
elongated particles, and am unable to say at the present moment that
the cleavage is sensibly affected by their presence--if anything, I
should say they rather impair its fineness and clearness than promote
it.

The finer the slate is the more perfect will be the resemblance of its
cleavage to that of the wax.  Compare the surface of the wax with the
surface of this slate from Borrodale in Cumberland.  You have
precisely the same features in both: you see flakes clinging to the
surfaces of each, which have been partially torn away in cleaving. Let
any close observer compare these two effects, he will, I am persuaded,
be led to the conclusion that they are the product of a common cause.
[Footnote: I have usually softened the wax by warming it, kneaded it
with the fingers, and pressed it between thick plates of glass
previously wetted.  At the ordinary summer temperature the pressed wax
is soft, and tears rather than cleaves; on this account I cool my
compressed specimens in a mixture of pounded ice and salt, and when
thus cooled they split cleanly.]

But you will ask me how, according to my view, does pressure produce
this remarkable result?  This may be stated in a very few words.

There is no such thing in nature as a body of perfectly homogeneous
structure.  I break this clay which seems so uniform, and find that
the fracture presents to my eyes innumerable surfaces along which it
has given way, and it has yielded along those surfaces because in them
the cohesion of the mass is less than elsewhere.  I break this marble,
and even this wax, and observe the same result; look at the mud at the
bottom of a dried pond; look at some of the ungravelled walks in
Kensington Gardens on drying after rain,--they are cracked and split,
and other circumstances being equal, they crack and split where the
cohesion is a minimum.  Take then a mass of partially consolidated
mud.  Such a mass is divided and subdivided by interior surfaces along
which the cohesion is comparatively small.  Penetrate the mass in
idea, and you will see it composed of numberless irregular polyhedra
bounded by surfaces of weak cohesion.  Imagine such a mass subjected
to pressure,--it yields and spreads out in the direction of least
resistance; the little polyhedra become converted into laminae,
separated from each other by surfaces of weak cohesion, and the
infallible result will be a tendency to cleave at right angles to the
line of pressure. [Footnote: It is scarcely necessary to say that if the
mass were squeezed equally in all directions no laminated structure
could be produced; it must have room to yield in a lateral direction.
Mr. Warren De la Rue informs me that he once wished to obtain
white-lead in a fine granular state, and to accomplish this he first
compressed it.  The mould was conical, and permitted the lead to
spread out a little laterally.  The lamination was as perfect as that
of slate, and it quite defeated him in his effort to obtain a granular
powder.]

Further, a mass of dried mud is full of cavities and fissures.  If you
break dried pipe-clay you see them in great numbers, and there are
multitudes of them so small that you cannot see them.  A flattening of
these cavities must take place in squeezed mud, and this must to some
extent facilitate the cleavage of the mass in the direction indicated.

Although the time at my disposal has not permitted me duly to develope
these thoughts, yet for the last twelve months the subject has
presented itself to me almost daily under one aspect or another.  I
have never eaten a biscuit during this period without remarking the
cleavage developed by the rolling-pin.  You have only to break a
biscuit across, and to look at the fracture, to see the laminated
structure.  We have here the means of pushing the analogy further.  I
invite you to compare the structure of this slate, which was subjected
to a high temperature during the conflagration of Mr. Scott Russell's
premises, with that of a biscuit.  Air or vapour within the slate has
caused it to swell, and the mechanical structure it reveals is
precisely that of a biscuit.  During these enquiries I have received
much instruction in the manufacture of puff-paste.  Here is some such
paste baked under my own superintendence.  The cleavage of our hills
is accidental cleavage, but this is cleavage with intention.  The
volition of the pastrycook has entered into its formation.  It has
been his aim to preserve a series of surfaces of structural weakness,
along which the dough divides into layers.  Puff-paste in preparation
must not be handled too much; it ought, moreover, to be rolled on a
cold slab, to prevent the butter from melting, and diffusing itself,
thus rendering the paste more homogeneous and less liable to split.
Puff-paste is, then, simply an exaggerated case of slaty cleavage.

The principle here enunciated is so simple as to be almost trivial;
nevertheless, it embraces not only the cases mentioned, but, if time
permitted, it might be shown you that the principle has a much wider
range of application.  When iron is taken from the puddling furnace it
is more or less spongy, an aggregate in fact of small nodules: it is
at a welding heat, and at this temperature is submitted to the process
of rolling.  Bright smooth bars are the result.  But notwithstanding
the high heat the nodules do not perfectly blend together.  The
process of rolling draws them into fibres.  Here is a mass acted upon
by dilute sulphuric acid, which exhibits in a striking manner this
fibrous structure.  The experiment was made by my friend Dr. Percy,
without any reference to the question of cleavage.

Break a piece of ordinary iron and you have a granular fracture; heat
the iron, you elongate these granules, and finally render the mass
fibrous.  Here are pieces of rails along which the wheels of
locomotives have slid-den; the granules have yielded and become
plates.  They exfoliate or come off in leaves; all these effects
belong, I believe, to the great class of phenomena of which slaty
cleavage forms the most prominent example. [Footnote: For some further
observations on this subject by Mr. Sorby and myself, see
Philosophical Magazine for August, 1856.]

We have now reached the termination of our task.  You have witnessed
the phenomena of crystallisation, and have had placed before you the
facts which are found associated with the cleavage of slate rocks.
Such facts, as expressed by Helmholtz, are so many telescopes to our
spiritual vision, by which we can see backward through the night of
antiquity, and discern the forces which have been in operation upon
the earth's surface

        Ere the lion roared,
        Or the eagle soared.

From evidence of the most independent and trustworthy character, we
come to the conclusion that these slaty masses have been subjected to
enormous pressure, and by the sure method of experiment we have
shown--and this is the only really new point which has been brought
before you--how the pressure is sufficient to produce the cleavage.
Expanding our field of view, we find the self-same law, whose
footsteps we trace amid the crags of Wales and Cumberland, extending
into the domain of the pastrycook and ironfounder; nay, a wheel cannot
roll over the half-dried mud of our streets without revealing to us
more or less of the features of this law.  Let me say, in conclusion,
that the spirit in which this problem has been attacked by geologists,
indicates the dawning of a new day for their science.  The great
intellects who have laboured at geology, and who have raised it to its
present pitch of grandeur, were compelled to deal with the subject in
mass; they had no time to look after details.  But the desire for more
exact knowledge is increasing; facts are flowing in which, while they
leave untouched the intrinsic wonders of geology, are gradually
supplanting by solid truths the uncertain speculations which beset the
subject in its infancy.  Geologists now aim to imitate, as far as
possible, the conditions of nature, and to produce her results; they
are approaching more and more to the domain of physics, and I trust
the day will soon come when we shall interlace our friendly arms
across the common boundary of our sciences, and pursue our respective
tasks in a spirit of mutual helpfulness, encouragement and goodwill.

[I would now lay more stress on the lateral yielding, referred to in
the footnote concerning Mr. Warren De la Rue's attempt to produce
finely granular white-lead, accompanied as it is by tangential
sliding, than I was prepared to do when this lecture was given.  This
sliding is, I think, the principal cause of the planes of weakness,
both in pressed wax and slate rock.  J. T. 1871.]

********************

XIII.  ON PARAMAGNETIC AND DIAMAGNETIC FORCES

[Footnote: Abstract of a discourse delivered in the Royal Institution,
February 1, 1856.]

THE notion of an attractive force, which draws bodies towards the
centre of the earth, was entertained by Anaxagoras and his pupils, by
Democritus, Pythagoras, and Epicurus; and the conjectures of these
ancients were renewed by Galileo, Huyghens, and others, who stated
that bodies attract each other as a magnet attracts iron.  Kepler
applied the notion to bodies beyond the surface of the earth, and
affirmed the extension of this force to the most distant stars.  Thus
it would appear, that in the attraction of iron by a magnet originated
the conception of the force of gravitation.  Nevertheless, if we look
closely at the matter, it will be seen that the magnetic force
possesses characters strikingly distinct from those of the force which
holds the universe together.  The theory of gravitation is, that every
particle of matter attracts every other particle; in magnetism also we
have attraction, but we have always, at the same time, repulsion, the
final effect being due to the difference of these two forces.  A body
may be intensely acted on by a magnet, and still no motion of
translation will follow, if the repulsion be equal to the attraction.
Previous to magnetization, a dipping needle, when its centre of
gravity is supported, stands accurately level; but, after
magnetization, one end of it, in our latitude, is pulled towards the
north pole of the earth.  The needle, however, being suspended from
the arm of a fine balance, its weight is found unaltered by its
magnetization.  In like manner, when the needle is permitted to float
upon a liquid, and thus to follow the attraction of the north magnetic
pole of the earth, there is no motion of the mass towards that pole.
The reason is known to be, that although the marked end of the needle
is attracted by the north pole, the unmarked end is repelled by an
equal force, the two equal and opposite forces neutralizing each
other.

When the pole of an ordinary magnet is brought to act upon the
swimming needle, the latter is attracted,--the reason being that the
attracted end of the needle being nearer to the pole of the magnet
than the repelled end, the force of attraction is the more powerful of
the two.  In the case of the earth, its pole is so distant that the
length of the needle is practically zero.  In like manner, when a
piece of iron is presented to a magnet, the nearer parts are
attracted, while the more distant parts are repelled; and because the
attracted portions are nearer to the magnet than the repelled ones, we
have a balance in favour of attraction.  Here then is the special
characteristic of the magnetic force, which distinguishes it from that
of gravitation.  The latter is a simple unpolar force, while the
former is duplex or polar.  Were gravitation like magnetism, a stone
would no more fall to the ground than a piece of iron towards the
north magnetic pole: and thus, however rich in consequences the
supposition of Kepler and others may have been, it is clear that a
force like that of magnetism would not be able to transact the
business of the universe.

The object of this discourse is to enquire whether the force of
diamagnetism, which manifests itself as a repulsion of certain bodies
by the poles of a magnet, is to be ranged as a polar force, beside
that of magnetism; or as an unpolar force, beside that of gravitation.
When a cylinder of soft iron is placed within a wire helix, and
surrounded by an electric current, the antithesis of its two ends, or,
in other words, its polar excitation, is at once manifested by its
action upon a magnetic needle; and it may be asked why a cylinder of
bismuth may not be substituted for the cylinder of iron, and its state
similarly examined.  The reason is, that the excitement of the bismuth
is so feeble, that it would be quite masked by that of the helix in
which it is enclosed; and the problem that now meets us is, so to
excite a diamagnetic body that the pure action of the body upon a
magnetic needle may be observed, unmixed with the action of the body
used to excite the diamagnetic.

How this has been effected may be illustrated in the following
manner:

When through an upright helix of covered copper wire, a voltaic
current is sent, the top of the helix attracts, while its bottom
repels, the same pole of a magnetic needle; its central point, on the
contrary, is neutral, and exhibits neither attraction nor repulsion.
Such a helix is caused to stand between the two poles N'S' of an
astatic system. [Footnote: The reversal of the poles of the two
magnets, which were of the same strength, completely annulled the
action of the earth as a magnet.]  The two magnets S N' and S'N are
united by a rigid cross piece at their centres, and are suspended from
the point a, so that both magnets swing in the same horizontal plane.
It is so arranged that the poles N' s' are opposite to the central or
neutral point of the helix, so that when a current is sent through the
latter, the magnets, as before explained, are unaffected.  Here then
we have an excited helix which itself has no action upon the magnets,
and we are thus enabled to examine the action of a body placed within
the helix and excited by it, undisturbed by the influence of the
latter.  The helix being 12 inches high, a cylinder of soft iron 6
inches long, suspended from a string and passing over a pulley, can be
raised or lowered within the helix.  When it is so far sunk that its
lower end rests upon the table, the upper end finds itself between the
poles N´S´ of the astatic system.  The iron cylinder is thus converted
into a strong magnet, attracting one of the poles, and repelling the
other, and consequently deflecting the entire astatic system.  When
the cylinder is raised so that the upper end is at the level of the
top of the helix, its lower end comes between the poles N´S´; and a
deflection opposed in direction to the former one is the immediate
consequence.  To render these deflections more easily visible, a
mirror m is attached to the system of magnets; a beam of light thrown
upon the mirror being reflected and projected as a bright disk against
the wall.  The distance of this image from the mirror being
considerable, and its angular motion double that of the latter, a very
slight motion of the magnet is sufficient to produce a displacement of
the image through several yards.

This then is the principle of the beautiful apparatus [Footnote:
Devised by Prof. W.  Weber, and constructed by M. Leyser, of Leipzig.]
by which the investigation was conducted.  It is manifest that if a
second helix be placed between the poles SN with a cylinder within
it, the action upon the astatic magnet may be exalted.  This was the
arrangement made use of in the actual enquiry.  Thus to intensify the
feeble action, which it is here our object to seek, we have in the
first place neutralized the action of the earth upon the magnets, by
placing them astatically.  Secondly, by making use of two cylinders,
and permitting them to act simultaneously on the four poles of the
magnets, we have rendered the deflecting force four times what it
would be, if only a single pole were used.  Finally, the whole
apparatus was enclosed in a suitable case which protected the magnets
from air-currents, and the deflections were read off through a glass
plate in the case, by means of a telescope and scale placed at a
considerable distance from the instrument.

A pair of bismuth cylinders was first examined.  Sending a current
through the helices, and observing that the magnets swung perfectly
free, it was first arranged that the bismuth cylinders within the
helices had their central or neutral points opposite to the poles of
the magnets.  All being at rest the number on the scale marked by the
cross wire of the telescope was 572.  The cylinders were then moved,
one up the other down, so that two of their ends were brought to bear
simultaneously upon the magnetic poles: the magnet moved promptly, and
after some oscillations [Footnote: To lessen these a copper damper was
made use of.]  came to rest at the number 612; thus moving from a
smaller to a larger number.  The other two ends of the bars were next
brought to bear upon the magnet: a prompt deflection was the
consequence, and the final position of equilibrium was 526; the
movement being from a larger to a smaller number.  We thus observe a
manifest polar action of the bismuth cylinders upon the magnet; one
pair of ends deflecting it in one direction, and the other pair
deflecting it in the opposite direction.

Substituting for the cylinders of bismuth thin cylinders of iron, of
magnetic slate, of sulphate of iron, carbonate of iron, protochloride
of iron, red ferrocyanide of potassium, and other magnetic bodies, it
was found that when the position of the magnetic cylinders was the
same as that of the cylinders of bismuth, the deflection produced by
the former was always opposed in direction to that produced by the
latter; and hence the disposition of the force in the diamagnetic body
must have been precisely antithetical to its disposition in the
magnetic ones.

But it will be urged, and indeed has been urged against this
inference, that the deflection produced by the bismuth cylinders may
be due to induced currents excited in the metal by its motion within
the helices.  In reply to this objection, it may be stated, in the
first place, that the deflection is permanent, and cannot therefore be
due to induced currents, which are only of momentary duration.  It has
also been urged that such experiments ought to be made with other
metals, and with better conductors than bismuth; for if due to
currents of induction, the better the conductor the more exalted will
be the effect.  This requirement was complied with.

Cylinders of antimony were substituted for those of bismuth.  This
metal is a better conductor of electricity, but less strongly
diamagnetic than bismuth.  If therefore the action referred to be due
to induced currents we ought to have it greater in the case of
antimony than with bismuth; but if it springs from a true diamagnetic
polarity, the action of the bismuth ought to exceed that of the
antimony.  Experiment proves this to be the case.  Hence the
deflection produced by these metals is due to their diamagnetic, and
not to their conductive capacity.  Copper cylinders were next
examined: here we have a metal which conducts electricity fifty times
better than bismuth, but its diamagnetic power is nearly null; if the
effects be due to induced currents we ought to have them here in an
enormously exaggerated degree, but no sensible deflection was produced
by the two cylinders of copper.

It has also been proposed by the opponents of diamagnetic polarity to
coat fragments of bismuth with some insulating substance, so as to
render the formation of induced currents impossible, and to test the
question with cylinders of these fragments.  This requirement was also
fulfilled.  It is only necessary to reduce the bismuth to powder and
expose it for a short time to the air to cause the particles to become
so far oxidised as to render them perfectly insulating.  The
insulating power of the powder was exhibited experimentally;
nevertheless, this powder, enclosed in glass tubes, exhibited an
action scarcely less powerful than that of the massive bismuth
cylinders.

But the most rigid proof, a proof admitted to be conclusive by those
who have denied the antithesis of magnetism and diamagnetism, remains
to be stated.  Prisms of the same heavy glass as that with which the
diamagnetic force was discovered, were substituted for the metallic
cylinders, and their action upon the magnet was proved to be precisely
the same in kind as that of the cylinders of bismuth.  The enquiry was
also extended to other insulators: to phosphorus, sulphur, nitre,
calcareous spar, statuary marble, with the same invariable result:
each of these substances was proved to be polar, the disposition of
the force being the same as that of bismuth and the reverse of that of
iron.  When a bar of iron is set erect, its lower end is known to be a
north pole, and its upper end a south pole, in virtue of the earth's
induction.  A marble statue, on the contrary, has its feet a south
pole, and its head a north pole, and there is no doubt that the same
remark applies to its living archetype; each man walking over the
earth's surface is a true diamagnet, with its poles the reverse of
those of a mass of magnetic matter of the same shape and position.

An experiment of practical value, as affording a ready estimate of the
different conductive powers of two metals for electricity, was
exhibited in the lecture, for the purpose of proving experimentally
some of the statements made in reference to this subject.  A cube of
bismuth was suspended by a twisted string between the two poles of an
electro-magnet.  The cube was attached by a short copper wire to a
little square pyramid, the base of which was horizontal, and its sides
formed of four small triangular pieces of looking-glass.  A beam of
light was suffered to fall upon this reflector, and as the reflector
followed the motion of the cube the images cast from its sides
followed each other in succession, each describing a circle about
thirty feet in diameter.  As the velocity of rotation augmented, these
images blended into a continuous ring of light.  At a particular
instant the electro-magnet was excited, currents were evolved in the
rotating cube, and the strength of these currents, which increases
with the conductivity of the cube for electricity, was practically
estimated by the time required to bring the cube and its associated
mirrors to a state of rest. With bismuth this time amounted to a score
of seconds or more: a cube of copper, on the contrary, was struck
almost instantly motionless when the circuit was established.

********************

XIV. PHYSICAL BASIS OF SOLAR CHEMISTRY.

[Footnote: From a discourse delivered at the Royal Institution of
Great Britain, June 7, 1861.]

OMITTING all preface, attention was first drawn to an experimental
arrangement intended to prove that gaseous bodies radiate heat in
different degrees.  Near a double screen of polished tin was placed an
ordinary ring gas-burner, and on this was placed a hot copper ball,
from which a column of heated air ascended.  Behind the screen, but so
situated that no ray from the ball could reach the instrument, was an
excellent Thermo-electric pile, connected by wires with a very
delicate galvanometer.  The pile was known to be an instrument whereby
heat is applied to the generation of electric currents; the strength
of the current being an accurate measure of the quantity of the heat.
As long as both faces of the pile are at the same temperature, no
current is produced; but the slightest difference in the temperature
of the two faces at once declares itself by the production of a
current, which, when carried through the galvanometer, indicates by
the deflection of the needle both its strength and its direction.

The two faces of the pile were in the first instance brought to the
same temperature; the equilibrium being shown by the needle of the
galvanometer standing at zero.  The rays emitted by the current of hot
air already referred to were permitted to fall upon one of the faces
of the pile; and an extremely slight movement of the needle showed
that the radiation from the hot air, though sensible, was extremely
feeble.  Connected with the ring-burner was a holder containing oxygen
gas; and by turning a cock, a stream of this gas was permitted to
issue from the burner, strike the copper ball, and ascend in a heated
column in front of the pile.  The result was, that oxygen showed
itself, as a radiator of heat, to be quite as feeble as atmospheric
air.

A second holder containing olefiant gas was then connected with the
ring-burner.  Oxygen and air had already flowed over the ball and
cooled it in some degree.  Hence the olefiant gas laboured under a
disadvantage.  But on permitting the gas to rise from the ball, it
casts an amount of heat against the adjacent face of the pile
sufficient to impel the needle of the galvanometer almost to 90°. This
experiment proved the vast difference between two equally invisible
gases with regard to their power of emitting radiant heat.

The converse experiment was now performed.  The thermo-electric pile
was removed and placed between two cubes filled with water kept in a
state of constant ebullition; and it was so arranged that the
quantities of heat falling from the cubes on the opposite faces of the
pile were exactly equal, thus neutralising each other.  The needle of
the galvanometer being at zero, a sheet of oxygen gas was caused to
issue from a slit between one of the cubes and the adjacent face of
the pile.  If this sheet of gas possessed any sensible power of
intercepting the thermal rays from the cube, one face of the pile
being deprived of the heat thus intercepted, a difference of
temperature between its two faces would instantly set in, and the
result would be declared by the galvanometer.  The quantity absorbed
by the oxygen under those circumstances was too feeble to affect the
galvanometer; the gas, in fact, proved perfectly transparent to the
rays of heat.  It had but a feeble power of radiation: it had an
equally feeble power of absorption.

The pile remaining in its position, a sheet of olefiant gas was caused
to issue from the same slit as that through which the oxygen had
passed.  No one present could see the gas; it was quite invisible, the
light went through it as freely as through oxygen or air; but its
effect upon the thermal rays emanating from the cube was what might be
expected from a sheet of metal.  A quantity so large was cut off, that
the needle of the galvanometer, promptly quitting the zero line, moved
with energy to its stops.  Thus the olefiant gas, so light and clear
and pervious to luminous rays, was proved to be a most potent
destroyer of the rays emanating from an obscure source.  The
reciprocity of action established in the case of oxygen comes out
here; the good radiator is found by this experiment to be the good
absorber.

This result, now exhibited before a public audience for the first
time, was typical of what had been obtained with gases generally.
Going through the entire list of gases and vapours in this way, we
find radiation and absorption to be as rigidly associated as positive
and negative in electricity, or as north and south polarity in
magnetism.  So that if we make the number which expresses the
absorptive power the numerator of a fraction, and that which expresses
its radiative power the denominator, the result would be, that on
account of the numerator and denominator varying in the same,
proportion, the value of that fraction would always remain the same,
whatever might be the gas or vapour experimented with.

But why should this reciprocity exist?  What is the meaning of
absorption?  what is the meaning of radiation?  When you cast a stone
into still water, rings of waves surround the place where it falls;
motion is radiated on all sides from the centre of disturbance.  When
a hammer strikes a bell, the latter vibrates; and sound, which is
nothing more than an undulatory motion of the air, is radiated in all
directions.  Modern philosophy reduces light and heat to the same
mechanical category.  A luminous body is one with its atoms in a state
of vibration; a hot body is one with its atoms also vibrating, but at
a rate which is incompetent to excite the sense of vision; and, as a
sounding body has the air around it, through which it propagates its
vibrations, so also the luminous or heated body has a medium, called
aether, which accepts its motions and carries them forward with
inconceivable velocity.  Radiation, then, as regards both light and
heat, is the transference of motion from the vibrating body to the
aether in which it swings: and, as in the case of sound, the motion
imparted to the air is soon transferred to surrounding objects,
against which the aerial undulations strike, the sound being, in
technical language, absorbed; so also with regard to light and heat,
absorption consists in the transference of motion from the agitated
aether to the molecules of the absorbing body.

The simple atoms are found to be bad radiators; the compound atoms
good ones: and the higher the degree of complexity in the atomic
grouping, the more potent, as a general rule, is the radiation and
absorption.  Let us get definite ideas here, however gross, and purify
them afterwards by the process of abstraction.  Imagine our simple
atoms swinging like single spheres in the aether; they cannot create
the swell which a group of them united to form a system can produce.
An oar runs freely edgeways through the water, and imparts far less of
its motion to the water than when its broad flat side is brought to
bear upon it.  In our present language the oar, broad side vertical,
is a good radiator; broad side horizontal, it is a bad radiator.
Conversely the waves of water, impinging upon the flat face of the
oar-blade, will impart a greater amount of motion to it than when
impinging upon the edge.  In the position in which the oar radiates
well, it also absorbs well.  Simple atoms glide through the aether
without much resistance; compound ones encounter resistance, and hence
yield up more speedily their motion to the aether.  Mix oxygen and
nitrogen mechanically, they absorb and radiate a certain amount of
heat.  Cause these gases to combine chemically and form nitrous oxide,
both the absorption and radiation are thereby augmented hundreds of
times!

In this way we look with the telescope of the intellect into atomic
systems, and obtain a conception of processes which the eye of sense
can never reach.  But gases and vapours possess a power of choice as
to the rays which they absorb.  They single out certain groups of rays
for destruction, and allow other groups to pass unharmed.  This is
best illustrated by a famous experiment of Sir David Brewster's,
modified to suit present requirements.  Into a glass cylinder, with
its ends stopped by discs of plate-glass, a small quantity of nitrous
acid gas is introduced; the presence of the gas being indicated by its
rich brown colour.  The beam from an electric lamp being sent through
two prisms of bisulphide of carbon, a spectrum seven feet long and
eighteen inches wide is cast upon the screen.  Introducing the
cylinder containing the nitrous acid into the path of the beam as it
issues from the lamp, the splendid and continuous spectrum becomes
instantly furrowed by numerous dark bands, the rays answering to which
are intercepted by the nitric gas, while the light which falls upon
the intervening spaces is permitted to pass with comparative impunity.

Here also the principle of reciprocity, as regards radiation and
absorption, holds good; and could we, without otherwise altering its
physical character, render that nitrous gas luminous, we should find
that the very rays which it absorbs are precisely those which it would
emit.  When atmospheric air and other gases are brought to a state of
intense incandescence by the passage of an electric spark, the spectra
which we obtain from them consist of a series of bright bands.  But
such spectra are produced with the greatest brilliancy when, instead
of ordinary gases, we make use of metals heated so highly as to
volatilise them.  This is easily done by the voltaic current.  A
capsule of carbon filled with mercury, which formed the positive
electrode of the electric lamp, has a carbon point brought down upon
it.  On separating the one from the other, a brilliant arc containing
the mercury in a volatilised condition passes between them.  The
spectrum of this arc is not continuous like that of the solid carbon
points, but consists of a series of vivid bands, each corresponding in
colour to that particular portion of the spectrum to which its rays
belong.  Copper gives its system of bands; zinc gives its system; and
brass, which is an alloy of copper and zinc, gives a spectrum made up
of the bands belonging to both metals.

Not only, however, when metals are united like zinc and copper to form
an alloy, is it possible to obtain the bands which belong to them.  No
matter how we may disguise the metal--allowing it to unite with oxygen
to form an oxide, and this again with an acid to form a salt; if the
heat applied be sufficiently intense, the bands belonging to the metal
reveal themselves with perfect definition.  Into holes drilled in a
cylinder of retort carbon, pure culinary salt is introduced.  When the
carbon is made the positive electrode of the lamp, the resultant
spectrum shows the brilliant yellow lines of the metal sodium.
Similar experiments made with the chlorides of strontium, calcium,
lithium, [Footnote: The vividness of the colours of the lithium
spectrum is extraordinary; the spectrum, moreover, contained a blue
band of indescribable splendour.  It was thought by many, during the
discourse, that I had mistaken strontium for lithium, as this blue
band had never before been seen.  I have obtained it many times since;
and my friend Dr. Miller, having kindly analysed the substance made
use of, pronounces it pure chloride of lithium.--J. T.]  and other
metals, give the bands due to the respective metals. When different
salts are mixed together, and rammed into holes in the carbon; a
spectrum is obtained which contains the bands of them all.

The position of these bright bands never varies, and each metal has
its own system.  Hence the competent observer can infer from the bands
of the spectrum the metals which produce it.  It is a language
addressed to the eye instead of the ear; and the certainty would not
be augmented if each metal possessed the power of audibly calling out,
'I am here!' Nor is this language affected by distance.  If we find
that the sun or the stars give us the bands of our terrestrial metals,
it is a declaration on the part of these orbs that such metals enter
into their composition.  Does the sun give us any such intimation?
Does the solar spectrum exhibit bright lines which we might compare
with those produced by our terrestrial metals, and prove either their
identity or difference?  No.  The solar spectrum, when closely
examined, gives us a multitude of fine dark lines instead of bright
ones.  They were first noticed by Dr. Wollaston, but were multiplied
and investigated with profound skill by Fraunhofer, and named after
him Fraunhofer's lines.  They had been long a standing puzzle to
philosophers.  The bright lines yielded by metallic vapours had been
also known to us for years; but the connection between both classes of
phenomena was wholly unknown, until Kirchhoff, with admirable
acuteness, revealed the secret, and placed it at the same time in our
power to chemically analyse the sun.

We have now some difficult work before us.  Hitherto we have been
delighted by objects which addressed themselves as much to our
aesthetic taste as to our scientific faculty; we have ridden
pleasantly to the base of the final cone of Etna, and must now
dismount and march through ashes and lava, if we would enjoy the
prospect from the summit.  Our problem is to connect the dark lines of
Fraunhofer with the bright ones of the metals.  The white beam of the
lamp is refracted in passing through our two prisms, but its different
components are refracted in different degrees, and thus its colours
are drawn apart.

Now the colour depends solely upon the rate of oscillation of the
atoms of the luminous body; red light being produced by one rate, blue
light by a much quicker rate, and the colours between red and blue by
the intermediate rates.  The solid incandescent coal-points give us a
continuous spectrum; or in other words they emit rays of all possible
periods between the two extremes of the spectrum.  Colour, as many of
you know, is to light what _pitch_ is to sound.  When a violin-player
presses his finger on a string he makes it shorter and tighter, and
thus, causing it to vibrate more speedily, heightens the pitch.
Imagine such a player to move his fingers slowly along the string,
shortening it gradually as he draws his bow, the note would rise in
pitch by a regular gradation; there would be no gap intervening
between note and note.  Here we have the analogue to the continuous
spectrum, whose colours insensibly blend together without gap or
interruption, from the red of the lowest pitch to the violet of the
highest. But suppose the player, instead of gradually shortening his
string, to press his finger on a certain point, and to sound the
corresponding note; then to pass on to another point more or less
distant, and sound its note; then to another, and so on, thus sounding
particular notes separated from each other by gaps which correspond to
the intervals of the string passed over; we should then have the exact
analogue of a spectrum composed of separate bright bands with
intervals of darkness between them.  But this, though a perfectly true
and intelligible analogy, is not sufficient for our purpose; we must
look with the mind's eye at the oscillating atoms of the volatilised
metal.

Figure these atoms as connected together by springs of a certain
tension, which, if the atoms are squeezed together, push them again
asunder, and if the atoms are drawn apart, pull them again together,
causing them, before coming to rest, to quiver for a certain time at a
certain definite rate determined by the strength of the spring.  Now
the volatilised metal which gives us one bright band is to be figured
as having its atoms united by springs all of the same tension, its
vibrations are all of one kind.  The metal which gives us two bands
may be figured as having some of its atoms united by springs of one
tension, and others by springs of a different tension.  Its vibrations
are of two distinct kinds; so also when we have three or more bands we
are to figure as many distinct sets of springs, each capable of
vibrating in its own particular time and at a different rate from the
others.  If we seize this idea definitely, we shall have no difficulty
in dropping the metaphor of springs, and substituting for it mentally
the forces by which the atoms act upon each other.  Having thus far
cleared our way, let us make another effort to advance.

A heavy ivory ball is here suspended from a string.  I blow against
this ball; a single puff of my breath moves it a little way from its
position of rest; it swings back towards me, and when it reaches the
limit of its swing I puff again.  It now swings further; and thus by
timing the puffs I can so accumulate their action as to produce
oscillations of large amplitude.  The ivory ball here has absorbed the
motion which my breath communicated to the air.  I now bring the ball
to rest. Suppose, instead of the breath, a wave of air to strike
against it, and that this wave is followed by a series of others which
succeed each other exactly in the same intervals as my puffs; it is
obvious that these waves would communicate their motion to the ball
and cause it to swing as the puffs did.  And it is equally manifest
that this would not be the case if the impulses of the waves were not
properly timed; for then the motion imparted to the pendulum by one
wave would be neutralised by another, and there could not be the
accumulation of effect obtained when the periods of the waves
correspond with the periods of the pendulum.  So much for the
particular impulses absorbed by the pendulum.  But if such a pendulum
set oscillating in air could produce waves in the air, it is evident
that the waves it would produce would be of the same period as those
whose motions it would take up or absorb most completely, if they
struck against it.  Perhaps the most curious effect of these timed
impulses ever described was that observed by a watchmaker, named
Ellicott, in the year 1741. He left two clocks leaning against the
same rail; one of them, which we may call A, was set going; the other,
B, not.  Some time afterwards he found, to his surprise, that B was
ticking also.  The pendulums being of the same length, the shocks
imparted by the ticking of A to the rail against which both clocks
rested were propagated to B, and were so timed as to set B going.
Other curious effects were at the same time observed.  When, the
pendulums differed from each other a certain amount, set B going, but
the reaction of B stopped A.  Then B set A going, and the re-action of
A stopped B.  When the periods of oscillation were close to each
other, but still not quite alike, the clocks mutually controlled each
other, and by a kind of compromise they ticked in perfect unison.

But what has all this to do with our present subject?  The varied
actions of the universe are all modes of motion; and the vibration of
a ray claims strict brotherhood with the vibrations of our pendulum.
Suppose aethereal waves striking upon atoms which oscillate in the
same periods as the waves, the motion of the waves will be absorbed by
the atoms; suppose we send our beam of white light through a sodium
flame, the atoms of that flame will be chiefly affected by those
undulations which are synchronous with their own periods of vibration.
There will be on the part of those particular rays a transference of
motion from the agitated aether to the atoms of the volatilised metal,
which, as already defined, is absorption.

The experiment justifying this conclusion is now for the first time to
be made before a public audience.  I pass a beam through our two
prisms, and the spectrum spreads its colours upon the screen.  Between
the lamp and the prism I interpose a snapdragon light.  Alcohol and
water are here mixed with common salt, and the metal dish that holds
them is heated by a spirit-lamp.  The vapour from the mixture ignites
and we have a monochromatic flame.  Through this flame the beam from
the lamp is now passing; and observe the result upon the spectrum. You
see a shady band cut out of the yellow,--not very dark, but
sufficiently so to be seen by everybody present.

But let me exalt this effect.  Placing in front of the electric lamp
the intense flame of a large Bunsen's burner, a platinum capsule
containing a bit of sodium less than a pea in magnitude is plunged
into the flame.  The sodium soon volatilises and burns with brilliant
incandescence.  The beam crosses the flame, and at the same time the
yellow band of the spectrum is clearly and sharply cut out, a band of
intense darkness occupying its place.  On withdrawing the sodium, the
brilliant yellow of the spectrum takes its proper place, while the
reintroduction of the flame causes the band to reappear.

Let me be more precise: The yellow colour of the spectrum extends
over a sensible space, blending on one side with the orange and on the
other with the green.  The term 'yellow band' is therefore somewhat
indefinite.  This vagueness may be entirely removed.  By dipping the
carbon-point used for the positive electrode into a solution of common
salt, and replacing it in the lamp, the bright yellow band produced by
the sodium vapour stands out from the spectrum.  When the sodium flame
is caused to act upon the beam it is that particular yellow band that
is obliterated, an intensely black streak occupying its place.

An additional step of reasoning leads to the conclusion that if,
instead of the flame of sodium alone, we were to introduce into the
path of the beam a flame in which lithium, strontium, magnesium,
calcium, &c, are in a state of volatilisation, each metallic vapour
would cut out a system of bands, corresponding exactly in position
with the bright bands of the same metallic vapour.  The light of our
electric lamp shining through such a composite flame would give us a
spectrum cut up by dark lines, exactly as the solar spectrum is cut up
by the lines of Fraunhofer.

Thus by the combination of the strictest reasoning with the most
conclusive experiment, we reach the solution of one of the grandest of
scientific problems--the constitution of the sun.  The sun consists of
a nucleus surrounded by a flaming atmosphere.  The light of the
nucleus would give us a continuous spectrum, like that of our common
carbon-points; but having to pass through the photosphere, as our beam
had to pass through the flame, those rays of the nucleus which the
photosphere can itself emit are absorbed, and shaded spaces,
corresponding to the particular rays absorbed, occur in the spectrum.
Abolish the solar nucleus, and we should have a spectrum showing a
bright line in the place of every dark line of Fraunhofer.  These
lines are therefore not absolutely dark, but dark by an amount
corresponding to the difference between the light of the nucleus
intercepted by the photosphere, and the light which issues from the
latter.

The man to whom we owe this noble generalisation is Kirchhoff,
Professor of Natural Philosophy in the University of Heidelberg;
[Footnote: Now Professor in the University of Berlin.] but, like
every other great discovery, it is compounded of various elements. Mr.
Talbot observed the bright lines in the spectra of coloured flames.
Sixteen years ago Dr. Miller gave drawings and descriptions of the
spectra of various coloured flames.  Wheatstone, with his accustomed
ingenuity, analysed the light of the electric spark, and showed that
the metals between which the spark passed determined the bright bands
in the spectrum of the spark.  Masson published a prize essay on these
bands; Van der Willigen, and more recently Plucker, have given us
beautiful drawings of the spectra, obtained from the discharge of
Ruhmkorff's coil.  But none of these distinguished men betrayed the
least knowledge of the connection between the bright bands of the
metals and the dark lines of the solar spectrum.  The man who came
nearest to the philosophy of the subject was Angstrom.  In a paper
translated from Poggendorff's 'Annalen' by myself, and published in
the 'Philosophical Magazine' for 1855, he indicates that the rays
which a body absorbs are precisely those which it can emit when
rendered luminous.  In another place, he speaks of one of his spectra
giving the general impression of a reversal of the solar spectrum.
Foucault, Stokes, and Thomson, have all been very close to the
discovery; and, for my own part, the examination of the radiation and
absorption of heat by gases and vapours, some of the results of which
I placed before you at the commencement of this discourse, would have
led me in 1859 to the law on which all Kirchhoff's speculations are
founded, had not an accident withdrawn me from the investigation.  But
Kirchhoff's claims are unaffected by these circumstances.  True, much
that I have referred to formed the necessary basis of his discovery;
so did the laws of Kepler furnish to Newton the basis of the theory of
gravitation.  But what Kirchhoff has done carries us far beyond all
that had before been accomplished.  He has introduced the order of law
amid a vast assemblage of empirical observations, and has ennobled our
previous knowledge by showing its relationship to some of the most
sublime of natural phenomena.

********************

XV.  ELEMENTARY MAGNETISM.

A LECTURE TO SCHOOLMASTERS.

WE have no reason to believe that the sheep or the dog, or indeed any
of the lower animals, feel an interest in the laws by which natural
phenomena are regulated.  A herd may be terrified by a thunderstorm;
birds may go to roost, and cattle return to their stalls, during a
solar eclipse; but neither birds nor cattle, as far as we know, ever
think of enquiring into the causes of these things.  It is otherwise
with Man.  The presence of natural objects, the occurrence of natural
events, the varied appearances of the universe in which he dwells
penetrate beyond his organs of sense, and appeal to an inner power of
which the senses are the mere instruments and excitants.  No fact is
to him either original or final.  He cannot limit himself to the
contemplation of it alone, but endeavours to ascertain its position in
a series to which uniform experience assures him it must belong.  He
regards all that he witnesses in the present as the efflux and
sequence of something that has gone before, and as the source of a
system of events which is to follow.  The notion of spontaneity, by
which in his ruder state he accounted for natural events, is
abandoned; the idea that nature is an aggregate of independent parts
also disappears, as the connection and mutual dependence of physical
powers become more and more manifest: until he is finally led to
regard Nature as an organic whole--as a body each of whose members
sympathises with the rest, changing, it is true, from age to age, but
changing without break of continuity in the relation of cause and
effect.

The system of things which we call Nature is, however, too vast and
various to be studied first-hand by any single mind.  As knowledge
extends there is always a tendency to subdivide the field of
investigation.  Its various parts are taken up by different minds, and
thus receive a greater amount of attention than could possibly be
bestowed on them if each investigator aimed at the mastery of the
whole.  The centrifugal form in which knowledge, as a whole, advances,
spreading ever wider on all sides, is due in reality to the exertions
of individuals, each of whom directs his efforts, more or less, along
a single line.  Accepting, in many respects, his culture from his
fellow-men--taking it from spoken words or from written books--in some
one direction, the student of Nature ought actually to touch his work.
He may otherwise be a distributor of knowledge, but not a creator, and
he fails to attain that vitality of thought, and correctness of
judgment, which direct and habitual contact with natural truth can
alone impart.

One large department of the system of Nature which forms the chief
subject of my own studies, and to which it is my duty to call your
attention this evening, is that of physics, or natural philosophy.
This term is large enough to cover the study of Nature generally, but
it is usually restricted to a department which, perhaps, lies closer
to our perceptions than any other.  It deals with the phenomena and
laws of light and heat--with the phenomena and laws of magnetism and
electricity--with those of sound--with the pressures and motions of
liquids and gases, whether at rest or in a state of translation or of
undulation.  The science of mechanics is a portion of natural
philosophy, though at present so large as to need the exclusive
attention of him who would cultivate it profoundly.  Astronomy is the
application of physics to the motions of the heavenly bodies, the
vastness of the field causing it, however, to bed regarded as a
department in itself.  In chemistry physical agents play important
parts.  By heat and light we cause atoms and molecules to unite or to
fall asunder.  Electricity exerts a similar power.  Through their
ability to separate nutritive compounds into their constituents, the
solar beams build up the whole vegetable world, and by it the animal
world.  The touch of the self-same beams causes hydrogen and chlorine
to; unite with sudden explosion, and to form by their combination a
powerful acid.  Thus physics and chemistry intermingle.  Physical
agents are, however, employed by the chemist as a means to an end;
while in physics proper the laws and phenomena of the agents
themselves, both qualitative and quantitative, are the primary objects
of attention.

My duty here to-night is to spend an hour in telling how this subject
is to be studied, and how a knowledge of it is to be imparted to
others.  From the domain of physics, which would be unmanageable as a
whole, I select as a sample the subject of magnetism.  I might readily
entertain you on the present occasion with an account of what natural
philosophy has accomplished.  I might point to those applications of
science of which we hear so much in the newspapers, and which are so
often mistaken for science itself.  I might, of course, ring changes
on the steam-engine and the telegraph, the electrotype and the
photograph, the medical applications of physics, and the various other
inlets by which scientific thought filters into practical life.  That
would be easy compared with the task of informing you how you are to
make the study of physics the instrument of your pupil's culture; how
you are to possess its facts and make them living seeds which shall
take root and grow in the mind, and not lie like dead lumber in the
storehouse of memory.  This is a task much heavier than the mere
recounting of scientific achievements; and it is one which, feeling my
own want of time to execute it aright, I might well hesitate to
accept.

But let me sink excuses, and attack the work before me.  First and
foremost, then, I would advise you to get a knowledge of facts from
actual observation.  Facts looked at directly are vital; when they
pass into words half the sap is taken out of them.  You wish, for
example, to get a knowledge of magnetism; well, provide yourself with
a good book on the subject, if you can, but do not be content with
what the book tells you; do not be satisfied with its descriptive
woodcuts; see the operations of the force yourself.  Half of our book
writers describe experiments which they never made, and their
descriptions often lack both force and truth; but, no matter how
clever or conscientious they may be, their written words cannot supply
the place of actual observation.  Every fact has numerous radiations,
which are shorn off by the man who describes it.

Go, then, to a philosophical instrument maker, and give a shilling or
half a crown for a straight bar-magnet, or, if you can afford it,
purchase a pair of them; or get a smith to cut a length of ten inches
from a bar of steel an inch wide and half an inch thick; file its ends
smoothly, harden it, and get somebody like myself to magnetise it.
Procure some darning needles, and also a little unspun silk, which
will give you a suspending fibre void of torsion.  Make little loop
of paper, or of wire, and attach your fibre to it.  Do it neatly.  In
the loop place a darning-needle, and bring the two ends or poles, as
they are called, of your bar-magnet successively up to the ends of the
needle.  Both the poles, you find, attract both ends of the needle.
Replace the needle by a bit of annealed iron wire; the same effects
ensue.  Suspend successively little rods of lead, copper, silver,
brass, wood, glass, ivory, or whalebone; the magnet produces no
sensible effect upon any of the substances.  You thence infer a
special property in the case of steel and iron.  Multiply your
experiments, However, and you will find that some other substances,
besides iron and steel, are acted upon by your magnet.  A rod of the
metal nickel, or of the metal cobalt, from which the blue colour used
by painters is derived, exhibits powers similar to those observed with
the iron and steel.

In studying the character of the force you may, however, confine
yourself to iron and steel, which are always at hand. Make your
experiments with the darning-needle over and over again; operate on
both ends of the needle; try both ends of the magnet.  Do not think
the work dull; you are conversing with Nature, and must acquire over
her language a certain grace and mastery, which practice can alone
impart.  Let every movement be made with care, and avoid slovenliness,
from the outset.  Experiment, as I have said, is the language by which
we address Nature, and through which she sends her replies; in the use
of this language a lack of straightforwardness is as possible, and as
prejudicial, as in the spoken language of the tongue.  If, therefore,
you wish to become acquainted with the truth of Nature, you must from
the first resolve to deal with her sincerely.

Now remove your needle from its loop, and draw it from eye to point
along one of the ends of the magnet; resuspend it, and repeat your
former experiment.  You now find that each extremity of the magnet
attracts one end of the needle, and repels the other.  The simple
attraction observed in the first instance, is now replaced by a _dual_
force.  Repeat the experiment till you have thoroughly observed the
ends which attract and those which repel each other.

Withdraw the magnet entirely from the vicinity of your needle, and
leave the latter freely suspended by its fibre.  Shelter it as well as
you can from currents of air, and if you have iron buttons on your
coat, or a steel penknife in your pocket, beware of their action.  If
you work at night, beware of iron candlesticks, or of brass ones with
iron rods inside.  Freed from such disturbances, the needle takes up a
certain determinate position.  It sets its length nearly north and
south.  Draw it aside and let it go.  After several oscillations it
will again come to the same position.  If you have obtained your
magnet from a philosophical instrument maker, you will see a mark on
one of its ends.  Supposing, then, that you drew your needle along the
end thus marked, and that the point of your needle was the last to
quit the magnet, you will find that the point turns to the south, the
eye of the needle turning towards the north.  Make sure of this, and
do not take the statement on my authority.

Now take a second darning-needle like the first, and magnetise it in
precisely the same manner: freely suspended it also will turn its eye
to the north and its point to the south.  Your next step is to examine
the action of the two needles which you have thus magnetised upon each
other.

Take one of them in your hand, and leave the other suspended; bring
the eye-end of the former near the eye-end of the latter; the
suspended needle retreats: it is repelled.  Make the same experiment
with the two points; you obtain the same result, the suspended needle
is repelled.  Now cause the dissimilar ends to act on each other--you
have attraction--point attracts eye, and eye attracts point.  Prove
the reciprocity of this action by removing the suspended needle, and
putting the other in its place.  You obtain the same result.  The
attraction, then, is mutual, and the repulsion U mutual.  You have
thus demonstrated in the clearest manner the fundamental law of
magnetism, that like poles repel, and that unlike poles attract, each
other.  You may say that this is all easily understood without doing;
but _do it_, and your knowledge will not be confined to what I have
uttered here.

I have said that one end of your bar magnet has a mark upon it; lay
several silk fibres together, so as to get sufficient strength, or
employ a thin silk ribbon, and form a loop large enough to hold your
magnet.  Suspend it; it turns its marked end towards the north.  This
marked end is that which in England is called the north pole.  If a
common smith has made your magnet, it will be convenient to determine
its north pole yourself, and to mark it with a file.  Vary your
experiments by causing your magnetised darning-needle to attract and
repel your large magnet; it is quite competent to do so.  In
magnetising the needle, I have supposed the point to be the last to
quit the marked end of the magnet; the point of the needle is a south
pole.  The end which last quits the magnet is always opposed in
polarity to the end of the magnet with which it, has been last in
contact.

You may perhaps learn all this in a single hour; but spend several at
it, if necessary; and remember, understanding it is not sufficient:
you must obtain a manual aptitude in addressing Nature.  If you speak
to your fellow-man you are not entitled to use jargon.  Bad
experiments are jargon addressed to Nature, and just as much to be
deprecated.  Manual dexterity in illustrating the interaction of
magnetic poles is of the utmost importance at this stage of your
progress; and you must not neglect attaining this power over your
implements.  As you proceed, moreover, you will be tempted to do more
than I can possibly suggest. Thoughts will occur to you which you will
endeavour to follow out: questions will arise which you will try to
answer.  The same experiment may be twenty different things to twenty
people.  Having witnessed the action of pole on pole, through the air,
you will perhaps try whether the magnetic power is not to be screened
off.  You use plates of glass, wood, slate, pasteboard, or
gutta-percha, but find them all pervious to this wondrous force.  One
magnetic pole acts upon another through these bodies as if they were
not present.  Should you ever become a patentee for the regulation of
ships' compasses, you will not fall, as some projectors have done,
into the error of screening off the magnetism of the ship by the
interposition of such substances.

If you wish to teach a class you must contrive that the effects which
you have thus far witnessed for yourself shall be witnessed by twenty
or thirty pupils.  And here your private ingenuity must come into
play.  You will attach bits of paper to your needles, so as to render
their movements visible at a distance, denoting the north and south
poles by different colours, say green and red.  You may also improve
upon your darning-needle.  Take a strip of sheet steel, heat it to
vivid redness and plunge it into cold water.  It is thereby hardened;
rendered, in fact, almost as brittle as glass.  Six inches of this,
magnetised in the manner of the darning-needle, will be better able to
carry your paper indexes.  Having secured such a strip, you proceed
thus:

Magnetise a small sewing-needle and determine its poles; or, break
half an inch, or an inch, off your magnetised darning-needle and
suspend it by a fine silk fibre.  The sewing-needle, or the fragment
of the darning needle, is now to be used as a test-needle, to examine
the distribution of the magnetism in your strip of steel.  Hold the
strip upright in your left hand, and cause the test-needle to approach
the lower end of your strip; one end of the test-needle is attracted,
the other is repelled.  Raise your needle along the strip; its
oscillations, which at first were quick, become slower; opposite the
middle of the strip they cease entirely; neither end of the needle is
attracted; above the middle the test-needle turns suddenly round, its
other end being now attracted.  Go through the experiment thoroughly:
you thus learn that the entire lower half of the strip attracts one
end of the needle, while the entire upper half attracts the opposite
end.  Supposing the north end of your little needle to be that
attracted below, you infer that the entire lower half of your
magnetised strip exhibits south magnetism, while the entire upper half
exhibits north magnetism.  So far, then, you have determined the
distribution of magnetism in your strip of steel.

You look at this fact, you think of it; in its suggestiveness the
value of an experiment chiefly consists.  The thought naturally
arises: 'What will occur if I break my strip of steel across in the
middle?  Shall I obtain two magnets each possessing a single pole?'
Try the experiment; break your strip of steel, and test each half as
you tested the whole.  The mere presentation of its two ends in
succession to your test-needle, suffices to show that you have _not_ a
magnet with a single pole--that each half possesses two poles with a
neutral point between them.  And if you again break the half into two
other halves, you will find that each quarter of the original strip
exhibits precisely the same magnetic distribution as the whole strip.
You may continue the breaking process: no matter how small your
fragment may be, it still possesses two opposite poles and a neutral
point between them.  Well, your hand ceases to break where breaking
becomes a mechanical impossibility; but does the mind stop there?  No:
you follow the breaking process in idea when you can no longer realise
it in fact; your thoughts wander amid the very atoms of your steel,
and you conclude that each atom is a magnet, and that the force
exerted by the strip of steel is the mere summation, or resultant, of
the forces of its ultimate particles.

Here, then, is an exhibition of power which we can call forth at
pleasure or cause to disappear.  We magnetise our strip, of steel by
drawing it along the pole of a magnet; we can demagnetise it, or
reverse its magnetism, by properly drawing it along the same pole in
the opposite direction.  What, then, is the real nature of this
wondrous change?  What is it that takes place among the atoms of the
steel when the substance is magnetised?  The question leads us beyond
the region of sense, and into that of imagination.  This faculty,
indeed, is the divining-rod of the man of science.  Not, however, an
imagination which catches its creations from the air, but one informed
and inspired by facts; capable of seizing firmly on a physical image
as a principle, of discerning its consequences, and of devising means
whereby these forecasts of thought may be brought to an experimental
test. If such a principle be adequate to account for all the
phenomena--if from an assumed cause the observed acts necessarily
follow, we call the assumption a theory, and, once possessing it, we
can not only revive at pleasure facts already known, but we can
predict others which we have never seen.  Thus, then, in the
prosecution of physical science, our powers of observation, memory,
imagination, and inference, are all drawn upon.  We observe facts and
store them up; the constructive imagination broods upon these
memories, tries to discern their interdependence and weave them to an
organic whole.  The theoretic principle flashes or slowly dawns upon
the mind; and then the deductive faculty interposes to carry out the
principle to its logical consequences.  A perfect theory gives
dominion over natural facts; and even an assumption which can only
partially stand the test of a comparison with facts, may be of eminent
use in enabling us to connect and classify groups of phenomena.  The
theory of magnetic fluids is of this latter character, and with it we
must now make ourselves familiar.

With the view of stamping the thing more firmly on your minds, I will
make use of a strong and vivid image.  In optics, red and green are
called complementary colours; their mixture produces white.  Now I ask
you to imagine each of these colours to possess a self-repulsive
power; that red repels red, that green repels green; but that red
attracts green and green attracts red, the attraction of the
dissimilar colours being equal to the repulsion of the similar ones.
Imagine the two colours mixed so as to produce white, and suppose two
strips of wood painted with this white-; what will be their action
upon each other?  Suspend one of them freely as we suspended our
darning-needle, and bring the other near it; what will occur?  The red
component of the strip you hold in your hand will repel the red
component of your suspended strip; but then it will attract the green,
and, the forces being equal, they neutralise each other.  In fact, the
least reflection shows you that the strips will be as indifferent to
each other as two unmagnetised darning-needles would be under the same
circumstances.

But suppose, instead of mixing the colours, we painted one half of
each strip from centre to end red, and the other half green, it is
perfectly manifest that the two strips would now behave towards each
other exactly as our two magnetised darning-needles--the red end would
repel the red and attract the green, the green would repel the green
and attract the red; so that, assuming two colours thus related to
each other, we could by their mixture produce the neutrality of an
unmagnetised body, while by their separation we could produce the
duality of action of magnetised bodies.

But you have already anticipated a defect in my conception; for if we
break one of our strips of wood in the middle we have one half
entirely red, and the other entirely green, and with these it would be
impossible to imitate the action of our broken magnet.  How, then,
must we modify our conception?  We must evidently suppose _each
molecule of the wood_ painted green on one face and red on the opposite
one.  The resultant action of all the atoms would then exactly
resemble the action of a magnet.  Here also, if the two opposite
colours of each atom could be caused to mix so as to produce white, we
should have, as before, perfect neutrality.

For these two self-repellent and mutually attractive colours,
substitute in your minds two invisible self-repellent and mutually
attractive fluids, which in ordinary steel are mixed to form a neutral
compound, but which the act of magnetisation separates from each
other, placing the opposite fluids on the opposite face of each
molecule.  You have then a perfectly distinct conception of the
celebrated theory of magnetic fluids.  The strength of the magnetism
excited is supposed to be proportional to the quantity of neutral
fluid decomposed.  According to this theory nothing is actually
transferred from the exciting magnet to the excited steel.  The act of
magnetisation consists in the forcible separation of two fluids which
existed in the steel before it was magnetised, but which then
neutralised each other by their coalescence.  And if you test your
magnet, after it has excited a hundred pieces of steel, you will find
that it has lost no force--no more, indeed, than I should lose, had my
words such a magnetic influence on your minds as to excite in them a
strong resolve to study natural philosophy.  I should rather be the
gainer by my own utterance, and by the reaction of your fervour.  The
magnet also is the gainer by the reaction of the body which it
magnetises.

Look now to your excited piece of steel; figure each molecule with its
opposed fluids spread over its opposite faces.  How can this state of
things be permanent?  The fluids, by hypothesis, attract each other;
what, then, keeps them apart?  Why do they not instantly rush together
across the equator of the atom, and thus neutralise each other?  To
meet this question philosophers have been obliged to infer the
existence of a special force, which holds the fluids asunder.  They
call it _coercive force_; and it is found that those kinds of steel
which offer most resistance to being magnetised--which require the
greatest amount of 'coercion' to tear their fluids asunder--are the
very ones which offer the greatest resistance to the reunion of the
fluids, after they have been once separated.  Such kinds of steel are
most suited to the formation of _permanent_ magnets.  It is manifest,
indeed, that without coercive force a permanent magnet would not be at
all possible.

Probably long before this you will have dipped the end of your magnet
among iron filings, and observed how they cling to it; or into a
nail-box, and found how it drags the nails after it.  I know very well
that if you are not the slaves of routine, you will have by this time
done many things that I have not told you to do, and thus multiplied
your experience beyond what I have indicated.  You are almost sure to
have caused a bit of iron to hang from the end of your magnet, and you
have probably succeeded in causing a second bit to attach itself to
the first, a third to the second; until finally the force has become
too feeble to bear the weight of more.  If you have operated with
nails, you may have observed that the points and edges hold together
with the greatest tenacity; and that a bit of iron clings more firmly
to the corner of your magnet than to one of its flat surfaces.  In
short, you will in all likelihood have enriched your experience in
many ways without any special direction from me.

Well, the magnet attracts the nail, and the nail attracts a second
one.  This proves that the nail in contact with the magnet has had the
magnetic quality developed in it by that contact.  If it be withdrawn
from the magnet its power to attract its fellow nail ceases.  Contact,
however, is not necessary.  A sheet of glass or paper, or a space of
air, may exist between the magnet and the nail; the latter is still
magnetised, though not so forcibly as when in actual contact.  The
nail thus presented to the magnet is itself a temporary magnet.  That
end which is turned towards the magnetic pole has the opposite
magnetism of the pole which excites it; the end most remote from the
pole has the same magnetism as the pole itself, and between the two
poles the nail, like the magnet, possesses a magnetic equator.

Conversant as you now are with the theory of magnetic fluids, you have
already, I doubt not, anticipated me in imagining the exact condition
of an iron nail under the influence of the magnet.  You picture the
iron as possessing the neutral fluid in abundance; you picture the
magnetic pole, when brought near, decomposing the fluid; repelling the
fluid of a like kind with itself, and attracting the unlike fluid;
thus exciting in the parts of the iron nearest to itself the opposite
polarity.  But the iron is incapable of becoming a permanent magnet.
It only shows its virtue as long as the magnet acts upon it.  What,
then, does the iron lack which the steel possesses?  It lacks coercive
force.  Its fluids are separated with ease; but, once the separating
cause is removed, they flow together again, and neutrality is
restored.  Imagination must be quite nimble in picturing these
changes--able to see the fluids dividing and reuniting, according as
the magnet is brought near or withdrawn.  Fixing a definite pole in
your mind, you must picture the precise arrangement of the two fluids
with reference to this pole, and be able to arouse similar pictures in
the minds of your pupils.  You will cause them to place magnets and
iron in various positions, and describe the exact magnetic state of
the iron in each particular case.  The mere facts of magnetism will
have their interest immensely augmented by an acquaintance with the
principles whereon the facts depend.  Still, while you use this theory
of magnetic fluids to track out the phenomena and link them together,
you will not forget to tell your pupils that it is to be regarded as a
symbol merely,--a symbol, moreover, which is incompetent to cover all
the facts, but which does good practical service whilst we are waiting
for the actual truth. [Footnote: This theory breaks down when applied
to diamagnetic bodies which are repelled by magnets.  Like soft iron,
such bodies are thrown into a state of temporary excitement, in virtue
of which they are repelled; but any attempt to explain such a
repulsion by the decomposition of a fluid will demonstrate its own
futility.]

The state of excitement into which iron is thrown by the influence, of
a magnet, is sometimes called 'magnetisation by influence.' More
commonly, however, the magnetism is said to be 'induced' in the iron,
and hence this mode of magnetising is called 'magnetic induction.'
Now, there is nothing theoretically perfect in Nature: there is no
iron so soft as not to possess a certain amount of coercive force, and
no steel so hard as not to be capable, in some degree, of magnetic
induction.  The quality of steel is in some measure possessed by iron,
and the quality of iron is shared in some degree by steel.  It is in
virtue of this latter fact that the unmagnetised darning-needle was
attracted in your first experiment; and from this you may at once
deduce the consequence that, after the steel has been magnetised, the
repulsive action of a magnet must be always less than its attractive
action.  For the repulsion is opposed by the inductive action of the
magnet on the steel, while the attraction is assisted by the same
inductive action.  Make this clear to your minds, and verify it by
your experiments.  In some cases you can actually make the attraction
due to the temporary magnetism overbalance the repulsion due to the
permanent magnetism, and thus cause two poles of the same kind
apparently to attract each other.  When, however, good hard magnets
act on each other from a sufficient distance, the inductive action
practically vanishes, and the repulsion of like poles is sensibly
equal to the attraction of unlike ones.

I dwell thus long on elementary principles, because they are of the
first importance, and it is the temptation of this age of unhealthy
cramming to neglect them.  Now follow me a little farther.  In
examining the distribution of magnetism in your strip of steel you
raised the needle slowly from bottom to top, and found what we called
a neutral point at the centre.

Now does the magnet really exert no influence on the pole presented to
its centre?  Let us see.

Let SN, fig. 13, be our magnet, and let n represent a particle of
north magnetism placed exactly opposite the middle of the magnet.  Of
course this is an imaginary case, as you can never in reality thus
detach your north magnetism from its neighbour.  But supposing us to
have done so, what would be the action of the two poles of the magnet
on n?  Your reply will of course be that the pole S attracts n while
the pole N repels it.  Let the magnitude and direction of the
attraction be expressed by the line n m, and the magnitude and
direction of the repulsion by the line n o.  Now, the particle n being
equally distant from s and N, the line n o, expressing the repulsion,
will be equal to m n, which expresses the attraction.  Acted upon by
two such forces, the particle n must evidently move in the direction n
p, exactly midway between m n and n o.  Hence you see that, although
there is no tendency of the particle n to move towards the magnetic
equator, there is a tendency on its part to move parallel to the
magnet.  If, instead of a particle of north magnetism, we placed a
particle of south magnetism opposite to the magnetic equator, it would
evidently be urged along the line n q; and if, instead of two separate
particles of magnetism, we place a little magnetic needle, containing
both north and south magnetism, opposite the magnetic equator, its
south pole being urged along n q, and its north along n p, the little
needle will be compelled to set itself parallel to the magnet s N.
Make the experiment, and satisfy yourselves that this is a true
deduction.

Substitute for your magnetic needle a bit of iron wire, devoid of
permanent magnetism, and it will set itself exactly as the needle
does.  Acted upon by the magnet, the wire, as you know, becomes a
magnet and behaves as such; it will turn its north pole towards p, and
south pole towards q, just like the needle.

But supposing you shift the position of your particle of north
magnetism, and bring it nearer to one end of your magnet than to the
other; the forces acting on the particle are no longer equal; the
nearest pole of the magnet will act more powerfully on the particle
than the more distant one.  Let SN, fig. 14, be the magnet, and n the
particle of north magnetism, in its new position.  It is repelled by
N, and attracted by S.  Let the repulsion be represented in magnitude
and direction by the line n o, and the attraction by the shorter line
n M. The resultant of these two forces will be found by completing the
parallelogram m n o p, and drawing its diagonal n p.  Along n p, then,
a particle of north magnetism would be urged by the simultaneous
action of S and N.  Substituting a particle of south magnetism for n,
the same reasoning would lead to the conclusion that the particle
would be urged along it q.  If we place at n a short magnetic needle,
its north pole will be urged along n p, its south pole along n q, the
only position possible to the needle, thus acted on, being along the
line p q, which is no longer parallel to the magnet.  Verify this
deduction by actual experiment.

In this way we might go round the entire magnet; and, considering its
two poles as two centres from which the force emanates, we could, in
accordance with ordinary mechanical principles, assign a definite
direction to the magnetic needle at every particular place.  And
substituting, as before, a bit of iron wire for the magnetic needle,
the positions of both will be the same.

Now, I think, without further preface, you will be able' to comprehend
for yourselves, and explain to others, one of the most interesting
effects in the whole domain of magnetism.  Iron filings you know are
particles of iron, irregular in shape, being longer in some directions
than in others.  For the present experiment, moreover, instead of the
iron filings, very small scraps of thin iron wire might be employed. I
place a sheet of paper over the magnet; it is all the better if the
paper be stretched on a wooden frame as this enables us to keep it
quite level.  I scatter the filings, or the scraps of wire, from a
sieve upon the paper, and tap the latter gently, so as to liberate the
particles for a moment from its friction.  The magnet acts on the
filings through the paper, and see how it arranges them!  They
embrace the magnet in a series of beautiful curves, which are
technically called 'magnetic curves,' or 'lines of magnetic force.'
Does the meaning of these lines yet flash upon you?  Set your magnetic
needle, or your suspended bit of wire, at any point of one of the
curves, and you will find the direction of the needle, or of the wire,
to be exactly that of the particle of iron, or of the magnetic curve,
at that point.  Go round and round the magnet; the direction of your
needle always coincides with the direction of the curve on which it is
placed.  These, then, are the lines along which a particle of south
magnetism, if you could detach it, would move to the north pole, and a
bit of north magnetism to the south pole.  They are the lines along
which the decomposition of the neutral fluid takes place.  In the case
of the magnetic needle, one of its poles being urged in one direction,
and the other pole in the opposite direction, the needle must
necessarily set itself as a _tangent_ to the curve.  I will not seek to
simplify this subject further.  If there be anything obscure or
confused or incomplete in my statement, you ought now, by patient
thought, to be able to clear away the obscurity, to reduce the
confusion to order, and to supply what is needed to render the
explanation complete.  Do not quit the subject until you thoroughly
understand it; and if you are then able to look with your mind's eye
at the play of forces around a magnet, and see distinctly the
operation of those forces in the production of the magnetic curves,
the time which we have spent together will not have been spent in
vain.

FIG.  15.

In this thorough manner we must master our materials, reason upon
them, and, by determined study, attain to clearness of conception.
Facts thus dealt with exercise an expansive force upon the intellect;
they widen the mind to generalisation.  We soon recognise a
brotherhood between the larger phenomena of Nature and the minute
effects which we have observed in our private chambers.  Why, we
enquire, does the magnetic needle set north and south?  Evidently it
is compelled to do so by the earth; the great globe which we inherit
is itself a magnet.  Let us learn a little more about it.  By means of
a bit of wax, or otherwise, attach the end of your silk fibre to the
middle point of your magnetic needle; the needle will thus be
uninterfered with by the paper loop, and will enjoy to some extent a
power of dipping' its point, or its eye, below the horizon.  Lay your
bar magnet on a table, and hold the needle over the equator of the
magnet.  The needle sets horizontal.  Move it towards the north end of
the magnet; the south end of the needle dips, the dip augmenting as
you approach the north pole, over which the needle, if free to move,
will set itself exactly vertical.  Move it back to the centre, it
resumes its horizontality; pass it on towards the south pole, its
north end now dips, and directly over the south pole the needle
becomes vertical, its north end being now turned downwards.  Thus we
learn that on the one side of the magnetic equator the north end of
the needle dips; on the other side the south end dips, the dip varying
from nothing to 90°.  If we go to the equatorial regions of the earth
with a suitably suspended needle we shall find there the position of
the needle horizontal.  If we sail north one end of the needle dips;
if we sail south the opposite end dips; and over the north or south
terrestrial magnetic pole the needle sets vertical.  The south
magnetic pole has not yet been found, but Sir James Ross discovered
the north magnetic pole on June 1, 1831.  In this manner we establish
a complete parallelism between the action of the earth and that of an
ordinary magnet.

The terrestrial magnetic poles do not coincide with the geographical
ones; nor does the earth's magnetic equator quite coincide with the
geographical equator.  The direction of the magnetic needle in London,
which is called the magnetic meridian, encloses an angle of 24° with
the astronomical meridian, this angle being called the Declination of
the needle for London.  The north, pole of the needle now lies to the
west of the true meridian; the declination is westerly.  In the year
1660, however, the declination was nothing, while before that time it
was easterly.  All this proves that the earth's magnetic constituents
are gradually changing their distribution.  This change is very slow:
it is therefore called the secular change, and the observation of it
has not yet extended over a sufficient period to enable us to guess,
even approximately, at its laws.

Having thus discovered, to some extent, the secret of the earth's
magnetic power, we can turn it to account.  In the line of 'dip' I
hold a poker formed of good soft iron.  The earth, acting as a magnet,
is at this moment constraining the two fluids of the poker to
separate, making the lower end of the poker a north pole, and the
upper end a south pole.  Mark the experiment: When the knob is
uppermost, it attracts the north end of a magnetic needle; when
undermost it attracts the south end of a magnetic needle.  With such a
poker repeat this experiment and satisfy yourselves that the fluids
shift their position according to the manner in which the poker is
presented to the earth.  It has already been stated that the softest
iron possesses a certain amount of coercive force.  The earth, at this
moment, finds in this force an antagonist which opposes the
decomposition of the neutral fluid, The component fluids may be
figured as meeting an amount of friction, or possessing an amount of
adhesion, which prevents them from gliding over the molecules of the
poker.  Can we assist the earth in this case?  If we wish to remove
the residue of a powder from the interior surface of a glass to which
the powder clings, we invert the glass, tap it, loosen the hold of the
powder, and thus enable the force of gravity to pull it down.  So also
by tapping the end of the poker we 'loosen the adhesion of the
magnetic fluids to the molecules and enable the earth to pull them
apart.  But, what is the consequence?  The portion of fluid which has
been thus forcibly dragged over the molecules refuses to return when
the poker has been removed from the line of dip; the iron, as you see,
has become a permanent magnet.  By reversing its position and tapping
it again we reverse its magnetism.  A thoughtful and competent teacher
will know how to place these remarkable facts before his pupils in a
manner which will excite their interest. By the use of sensible
images, more or less gross, he will first give those whom he teaches
definite conceptions, purifying these conceptions afterwards, as the
minds of his pupils become more capable of abstraction.  By thus
giving them a distinct substratum for their reasonings, he will confer
upon his pupils a profit and a joy which the mere exhibition of facts
without principles, or the appeal to the bodily senses and the power
of memory alone, could never inspire.

*****

As an expansion of the note on magnetic fluids, the following extract
may find a place here: 'It is well known that a voltaic current
exerts an attractive force upon a second current, flowing in the same
direction; and that when the directions are opposed to each other the
force exerted is a repulsive one.  By coiling wires into spirals,
Ampère was enabled to make them produce all the phenomena of
attraction and repulsion exhibited by magnets, and from this it was
but a step to his celebrated theory of molecular currents.  He
supposed the molecules of a magnetic body to be surrounded by such
currents, which, however, in the natural state of the body mutually
neutralised each other, on account of their confused grouping.  The
act of magnetisation he supposed to consist in setting these molecular
currents parallel to each other; and, starting from this principle, he
reduced all the phenomena of magnetism to the mutual action of
electric currents.

'If we reflect upon the experiments recorded in the foregoing pages
from first to last, we can hardly fail to be convinced that
diamagnetic bodies operated on by magnetic forces possess a polarity
"the same in kind as, but the reverse in direction of, that acquired
by magnetic bodies." But if this be the case, how are we to conceive
the _physical mechanism_ of this polarity?  According to Coulomb's and
Poisson's theory, the act of magnetisation consists in the
decomposition of a neutral magnetic fluid; the north pole of a magnet,
for example, possesses an attraction for the south fluid of a piece of
soft iron submitted to its influence, draws the said fluid towards it,
and with it the material particles with which the fluid is associated.
To account for diamagnetic phenomena this theory seems to fail
altogether; according to it, indeed, the oft-used phrase, "a north
pole exciting a north pole, and a south pole a south pole," involves a
contradiction.  For if the north fluid be supposed to be _attracted_
towards the influencing north pole, it is absurd to suppose that its
presence there could produce _repulsion_.  The theory of Ampère is
equally at a loss to explain diamagnetic action; for if we suppose the
particles of bismuth surrounded by molecular currents, then, according
to all that is known of electrodynamic laws, these currents would set
themselves parallel to, and in the same direction as, those of the
magnet, and hence attraction, and not repulsion, would be the result.
The fact, however, of this not being the case, proves that these
molecular currents are not the mechanism by which diamagnetic
induction is effected.  The consciousness of this, I doubt not, drove
M. Weber to the assumption that the phenomena of diamagnetism are
produced by molecular currents, not _directed_, but actually _excited_ in
the bismuth by the magnet.  Such induced currents would, according to
known laws, have a direction opposed to those of the inducing magnet,
and hence would produce the phenomena of repulsion.  To carry out the
assumption here made, M. Weber is obliged to suppose that the
molecules of diamagnetic bodies are surrounded by channels, in which
the induced molecular currents, once excited, continue to flow without
resistance.' [Footnote: In assuming these non-resisting channels M.
Weber, it must be admitted, did not go beyond the assumptions of
Ampère.]--Diamagnetism and Magne-crystallic Action, p. 136-7.

********************

XVI.  ON FORCE.

[Footnote: A discourse delivered in the Royal Institution, June 6,
1862.]

A SPHERE of lead was suspended at a height of 16 feet above the
theatre floor of the Royal Institution.  It was liberated, and fell by
gravity.  That weight required a second to fall to the floor from that
elevation; and the instant before it touched the floor, it had a
velocity of 32 feet a second.  That is to say, if at that instant the
earth were annihilated, and its attraction annulled, the weight would
proceed through space at the uniform velocity of 32 feet a second.

If instead of being pulled downward by gravity, the weight be cast
upward in opposition to gravity, then, to reach a height of 16 feet it
must start with a velocity of 32 feet a second.  This velocity
imparted to the weight by the human hand, or by any other mechanical
means, would carry it to the precise height from which we saw it fall.

Now the lifting of the weight may be regarded as so much mechanical
work performed.  By means of a ladder placed against the wall, the
weight might be carried up to a height of 16 feet; or it might be
drawn up to this height by means of a string and pulley, or it might
be suddenly jerked up to a height of 16 feet.  The amount of work done
in all these cases, as far as the raising of the weight is concerned,
would be absolutely the same.  The work done at one and the same
place, and neglecting the small change of gravity with the height,
depends solely upon two things; on the quantity of matter lifted, and
on the height to which it is lifted.  If we call the quantity or mass
of matter m, and the height through which it is lifted h, then the
product of m into h, or mh, expresses, or is proportional to, the
amount of work done.

Supposing, instead of imparting a velocity of 32 feet a second we
impart at starting twice this velocity.  To what height will the
weight rise?  You might be disposed to answer, 'To twice the height;'
but this would be quite incorrect.  Instead of twice 16, or 32 feet,
it would reach a height of four times 16, or 64 feet.  So also, if we
treble the starting velocity, the weight would reach nine times the
height; if we quadruple the speed at starting, we attain sixteen times
the height.  Thus, with a four-fold velocity of 128 feet a second at
starting, the weight would attain an elevation of 256 feet.  With a
seven-fold velocity at starting, the weight would rise to 49 times the
height, or to an elevation of 784 feet.

Now the work done--or, as it is sometimes called, the _mechanical
effect_--other things being constant, is, as before explained,
proportional to the height, and as a double velocity gives four times
the height, a treble velocity nine times the height, and so on, it is
perfectly plain that the mechanical effect increases as the square of
the velocity.  If the mass of the body be represented by the letter m,
and its velocity by v, the mechanical effect would be proportional to
or represented by m v2.  In the case considered, I have supposed the
weight to be cast upward, being opposed in its flight by the
resistance of gravity; but the same holds true if the projectile be
sent into water, mud, earth, timber, or other resisting material.  If,
for example, we double the velocity of a cannon-ball, we quadruple its
mechanical effect.  Hence the importance of augmenting the velocity of
a projectile, and hence the philosophy of Sir William Armstrong in
using a large charge of powder in his recent striking experiments.

The measure then of mechanical effect is the mass of the body
multiplied by the square of its velocity.

Now in firing a ball against a target the projectile, after collision,
is often found hot.  Mr. Fairbairn informs me that in the experiments
at Shoeburyness it is a common thing to see a flash, even in broad
daylight, when the ball strikes the target.  And if our lead weight be
examined after it has fallen from a height it is also found heated.
Now here experiment and reasoning lead us to the remarkable law that,
like the mechanical effect, the amount of heat generated is
proportional to the product of the mass into the square of the
velocity.  Double your mass, other things being equal, and you double
your amount of heat; double your velocity, other things remaining
equal, and you quadruple your amount of heat.  Here then we have
common mechanical motion destroyed and heat produced.  When a violin
bow is drawn across a string, the sound produced is due to motion
imparted to the air, and to produce that motion muscular force has
been expended.  We may here correctly say, that the mechanical force
of the arm is converted into music.  In a similar way we say that the
arrested motion of our descending weight, or of the cannon-ball, is
converted into heat.  The mode of motion changes, but motion still
continues; the motion of the mass is converted into a motion of the
atoms of the mass; and these small motions, communicated to the
nerves, produce the sensation we call heat.

We know the amount of heat which a given amount of mechanical force
can develope.  Our lead ball, for example, in falling to the earth
generated a quantity of heat sufficient to raise its own temperature
three-fifths of a Fahrenheit degree.  It reached the earth with a
velocity of 32 feet a second, and forty times this velocity would be
small for a rifle bullet; multiplying 0.6 by the square of 40, we find
that the amount of heat developed by collision with the target would,
if wholly concentrated in the lead, raise its temperature 960 degrees.
This would be more than sufficient to fuse the lead.  In reality,
however, the heat developed is divided between the lead and the body
against which it strikes; nevertheless, it would be worth while to pay
attention to this point, and to ascertain whether rifle bullets do
not, under some circumstances, show signs of fusion. [Footnote: Eight
years subsequently this surmise was proved correct.  In the
Franco-German War signs of fusion were observed in the case of bullets
impinging on bones.]

From the motion of sensible masses, by gravity and other means, we now
pass to the motion of atoms towards each other by chemical affinity. A
collodion balloon filled with a mixture of chlorine and hydrogen being
hung in the focus of a parabolic mirror, in the focus of a second
mirror 20 feet distant a strong electric light was suddenly generated;
the instant the concentrated light fell upon the balloon, the gases
within it exploded, hydrochloric acid being the result. Here the atoms
virtually fell together, the amount of heat produced showing the
enormous force of the collision.  The burning of charcoal in oxygen is
an old experiment, but it has now a significance beyond what it used
to have; we now regard the act of combination on the part of the atoms
of oxygen and coal as we regard the clashing of a falling weight
against the earth.  The heat produced in both cases is referable to a
common cause.  A diamond, which burns in oxygen as a star of white
light, glows and burns in consequence of the falling of the atoms of
oxygen against it.  And could we measure the velocity of the atoms
when they clash, and could we find their number and weights,
multiplying the weight of each atom by the square of its velocity, and
adding all together, we should get a number representing the exact
amount of heat developed by the union of the oxygen and carbon.

Thus far we have regarded the heat developed by the clashing of
sensible masses and of atoms.  Work is expended in giving motion to
these atoms or masses, and heat is developed.  But we reverse this
process daily, and by the expenditure of heat execute work.  We can
raise a weight by heat; and in this agent we possess an enormous store
of mechanical power.  A pound of coal produces by its combination with
oxygen an amount of heat which, if mechanically applied, would suffice
to raise a weight of 100 lbs. to a height of 20 miles above the
earth's surface.  Conversely, 100 lbs. falling from a height of 20
miles, and striking against 'the earth, would generate an amount of
heat equal to that developed by the combustion of a pound of coal.
Wherever work is done by heat, heat disappears.  A gun which fires a
ball is less heated than one which fires blank cartridge.  The
quantity of heat communicated to the boiler of a working steam-engine
is greater than that which could be obtained from the re-condensation
of the steam, after it had done its work; and the amount of work
performed is the exact equivalent of the amount of heat lost. Mr.
Smyth informed us in his interesting discourse, that we dig annually
84 millions of tons of coal from our pits.  The amount of mechanical
force represented by this quantity of coal seems perfectly fabulous.
The combustion of a single pound of coal, supposing it to take place
in a minute, would be equivalent to the work of 300 horses; and if we
suppose 108 millions of horses working day and night with unimpaired
strength, for a year, their united energies would enable them to
perform an amount of work just equivalent to that which the annual
produce of our coal-fields would be able to accomplish.

Comparing with ordinary gravity the force with which oxygen and carbon
unite together, chemical affinity seems almost infinite.  But let us
give gravity fair play by permitting it to act throughout its entire
range.  Place a body at such a distance from the earth that the
attraction of our planet is barely sensible, and let it fall to the
earth from this distance.  It would reach the earth with a final
velocity of 36,747 feet a second; and on collision with the earth the
body would generate about twice the amount of heat generated by the
combustion of an equal weight of coal.  We have stated that by falling
through a space of 16 feet our lead bullet would be heated
three-fifths of a degree; but a body falling from an infinite distance
has already used up 1,299,999 parts out of 1,300,000 of the earth's
pulling power, when it has arrived within 16 feet of the surface; on
this space only 1/1,300,000 of the whole force is exerted.

Let us now turn our thoughts for a moment from the earth to the sun.
The researches of Sir John Herschel and M. Pouillet have informed us
of the annual expenditure of the sun as regards heat; and by an easy
calculation we ascertain the precise amount of the expenditure which
falls to the share of our planet.  Out of 2300 million parts of light
and heat the earth receives one.  The whole heat emitted by the sun in
a minute would be competent to boil 12,000 millions of cubic miles of
ice-cold water.  How is this enormous loss made good--whence is the
sun's heat derived, and by what means is it maintained?  No
combustion--no chemical affinity with which we are acquainted, would
be competent to produce the temperature of the sun's surface.
Besides, were the sun a burning body merely, its light and heat would
speedily come to an end.  Supposing it to be a solid globe of coal,
its combustion would only cover 4600 years of expenditure.  In this
short time it would burn itself out.  What agency then can produce the
temperature and maintain the outlay?  We have already regarded the
case of a body falling from a great distance towards the earth, and
found that the heat generated by its collision would be twice that
produced by the combustion of an equal weight of coal.  How much
greater must be the heat developed by a body falling against the sun!
The maximum velocity with which a body can strike the earth is about 7
miles in a second; the maximum velocity with which it can strike the
sun is 390 miles in a second.  And as the heat developed by the
collision is proportional to the square of the velocity destroyed, an
asteroid falling into the sun with the above velocity would generate
about 10,000 times the quantity of heat produced by the combustion of
an asteroid of coal of the same weight.

Have we any reason to believe that such bodies exist in space, and
that they may be raining down upon the sun?  The meteorites flashing
through the air are small planetary bodies, drawn by the earth's
attraction.  They enter our atmosphere with planetary velocity, and by
friction against the air they are raised to incandescence and caused
to emit light and heat.  At certain seasons of the year they shower
down upon us in great numbers.  In Boston 240,000 of them were
observed in nine hours.  There is no reason to suppose that the
planetary system is limited to 'vast masses of enormous weight;' there
is, on the contrary, reason to believe that space is stocked with
smaller masses, which obey the same laws as the larger ones.  That
lenticular envelope which surrounds the sun, and which is known to
astronomers as the Zodiacal light, is probably a crowd of meteors; and
moving as they do in a resisting medium, they must continually
approach the sun.  Falling into it, they would produce enormous heat,
and this would constitute a source from which the annual loss of heat
might be made good.  The sun, according to this hypothesis, would
continually grow larger; but how much larger?  Were our moon to fall
into the sun, it would develope an amount of heat sufficient to cover
one or two years' loss; and were our earth to fall into the sun a
century's loss would be made good.  Still, our moon and our earth, if
distributed over the surface of the sun, would utterly vanish from
perception.  Indeed, the quantity of matter competent to produce the
required effect would, during the range of history, cause no
appreciable augmentation in the sun's magnitude.  The augmentation of
the sun's attractive force would be more sensible.  However this
hypothesis may fare as a representant of what is going on in nature,
it certainly shows how a sun _might_ be formed and maintained on known
thermo-dynamic principles.

Our earth moves in its orbit with a velocity of 68,040 miles an hour.
Were this motion stopped, an amount of heat would be developed
sufficient to raise the temperature of a globe of lead of the same
size as the earth 384,000 degrees of the centigrade thermometer.  It
has been prophesied that 'the elements shall melt with fervent heat.'
The earth's own motion embraces the conditions of fulfilment; stop
that motion, and the greater part, if not the whole, of our planet
would be reduced to vapour.  If the earth fell into the sun, the
amount of heat developed by the shock would be equal to that developed
by the combustion of a mass of solid coal 6435 times the earth in
size.

There is one other consideration connected with the permanence of our
present terrestrial conditions, which is well worthy of our attention.
Standing upon one of, the London bridges, we observe the current of
the Thames reversed, and the water poured upward twice a-day.  The
water thus moved rubs against the river's bed, and heat is the
consequence of this friction.  The heat thus generated is in part
radiated into space and lost, as far as the earth is concerned.  What
supplies this incessant loss?  The earth's rotation.  Let us look a
little more closely at the matter.  Imagine the moon fixed, and the
earth turning like a wheel from west to east in its diurnal rotation.
Suppose a high mountain on the earth's surface approaching the earth's
meridian; that mountain is, as it were, laid hold of by the moon; it
forms a kind of handle by which the earth is pulled more quickly
round.  But when the meridian is passed the pull of the moon on the
mountain would be in the opposite direction, it would tend to diminish
the velocity of rotation as much as it previously augmented it; thus
the action of all fixed bodies on the earth's surface is neutralised.
But suppose the mountain to lie always to the east of the moon's
meridian, the pull then would be always exerted against the earth's
rotation, the velocity of which would be diminished in a degree
corresponding to the strength of the pull.  _The tidal wave occupies
this position_--it lies always to the east of the moon's meridian. The
waters of the ocean are in part dragged as a brake along the surface
of the earth; and as a brake they must diminish the velocity of the
earth's rotation. [Footnote: Kant surmised an action of this kind.]
Supposing then that we turn a mill by the action of the tide, and
produce heat by the friction of the millstones; that heat has an
origin totally different from the heat produced by another mill which
is turned by a mountain stream.  The former is produced at the expense
of the earth's rotation, the latter at the expense of the sun's
radiation.

The sun, by the act of vaporisation, lifts mechanically all the
moisture of our air, which when it condenses falls in the form of
rain, and when it freezes falls as snow.  In this solid form it is
piled upon the Alpine heights, and furnishes materials for glaciers.
But the sun again interposes, liberates the solidified liquid, and
permits it to roll by gravity to the sea.  The mechanical force of
every river in the world as it rolls towards the ocean, is drawn from
the heat of the sun.  No streamlet glides to a lower level without
having been first lifted to the elevation from which it springs by the
power of the sun.  The energy of winds is also due entirely to the
same power.

But there is still another work which the sun performs, and its
connection with which is not so obvious.  Trees and vegetables grow
upon the earth, and when burned they give rise to heat, and hence to
mechanical energy.  Whence is this power derived?  You see this oxide
of iron, produced by the falling together of the atoms of iron and
oxygen; you cannot see this transparent carbonic acid gas, formed by
the falling together of carbon and oxygen.  The atoms thus in close
union resemble our lead weight while resting on the earth; but we can
wind up the weight and prepare it for another fall, and so these atoms
can be wound up and thus enabled to repeat the process of combination.
In the building of plants carbonic acid is the material from which the
carbon of the plant is derived; and the solar beam is the agent which
tears the atoms asunder, setting the oxygen free, and allowing the
carbon to aggregate in woody fibre.  Let the solar rays fall upon a
surface of sand; the sand is heated, and finally radiates away as much
heat as it receives; let the same beams fall upon a forest, the
quantity of heat given back is less than the forest receives; for the
energy of a portion of the sunbeams is invested in building the trees.
Without the sun the reduction of the carbonic acid cannot be effected,
and an amount of sunlight is consumed exactly equivalent to the
molecular work done.  Thus trees are formed; thus the cotton on which
Mr. Bazley discoursed last Friday is produced.  I ignite this cotton,
and it flames; the oxygen again unites with the carbon; but an amount
of heat equal to that produced by its combustion was sacrificed by the
sun to form that bit of cotton.

We cannot, however, stop at vegetable life, for it is the source,
mediate or immediate, of all animal life.  The sun severs the carbon
from its oxygen and builds the vegetable; the animal consumes the
vegetable thus formed, a reunion of the severed elements takes place,
producing animal heat.  The process of building a vegetable is one of
winding up; the process of building an animal is one of running down.
The warmth of our bodies, and every mechanical energy which we exert,
trace their lineage directly to the sun.

The fight of a pair of pugilists, the motion of an army, or the
lifting of his own body by an Alpine climber up a mountain slope, are
all cases of mechanical energy drawn from the sun.  A man weighing 150
pounds has 64 pounds of muscle; but these, when dried, reduce
themselves to 15 pounds.  Doing an ordinary day's work, for eighty
days, this mass of muscle would be wholly oxidised.  Special organs
which do more work would be more quickly consumed: the heart, for
example, if entirely unsustained, would be oxidised in about a week.
Take the amount of heat due to the direct oxidation of a given weight
of food; less heat is developed by the oxidation of the same amount of
food in the working animal frame, and the missing quantity is the
equivalent of the mechanical work accomplished by the muscles.

I might extend these considerations; the work, indeed, is done to my
hand--but I am warned that you have been already kept too long.  To
whom then are we indebted for the most striking generalisations of
this evening's discourse?  They are the work of a man of whom you have
scarcely ever heard--the published labours of a German doctor, named
Mayer.  Without external stimulus, and pursuing his profession as town
physician in Heilbronn, this man was the first to raise the conception
of the interaction of heat and other natural forces to clearness in
his own mind.  And yet he is scarcely ever heard of, and even to
scientific men his merits are but partially known.  Led by his own
beautiful researches, and quite independent of Mayer, Mr. Joule
published in 1843 his first paper on the 'Mechanical Value of Heat;'
but in 1842 Mayer had actually calculated the mechanical equivalent of
heat from data which only a man of the rarest penetration could turn
to account.

In 1845 he published his memoir on 'Organic Motion,' and applied the
mechanical theory of heat in the most fearless and precise manner to
vital processes.  He also embraced the other natural agents in his
chain of conservation.  In 1853 Mr. Waterston proposed, independently,
the meteoric theory of the sun's heat, and in 1854 Professor William
Thomson applied his admirable mathematical powers to the development
of the theory; but six years previously the subject had been handled
in a masterly manner by Mayer, and all that I have said about it has
been derived from him.  When we consider the circumstances of Mayer's
life, and the period at which he wrote, we cannot fail to be struck
with astonishment at what he has accomplished.  Here was a man of
genius working in silence, animated solely by a love of his subject,
and arriving at the most important results in advance of those whose
lives were entirely devoted to Natural Philosophy.  It was the
accident of bleeding a feverish patient at Java in 1840 that led Mayer
to speculate on these subjects.  He noticed that the venous blood in
the tropics was of a brighter red than in colder latitudes, and his
reasoning on this fact led him into the laboratory of natural forces,
where he has worked with such signal ability and success.  Well, you
will desire to know what has become of this man.  His mind, it is
alleged, gave way; it is said he became insane, and he was certainly
sent to a lunatic asylum.  In a biographical dictionary of his country
it is stated that he died there, but this is incorrect.  He recovered;
and, I believe, is at this moment a cultivator of vineyards in
Heilbronn.

====================

June 20, 1862.

While preparing for publication my last course of lectures on Heat, I
wished to make myself acquainted with all that Dr. Mayer had done in
connection with this subject.  I accordingly wrote to two gentlemen
who above all others seemed likely to give me the information which I
needed. [Footnote: Helmholtz and Clausius.]  Both of them are Germans,
and both particularly distinguished in connection with the Dynamical
Theory of Heat.  Each of them kindly furnished me with the list of
Mayer's publications, and one of them [Clausius] was so friendly as to
order them from a bookseller, and to send them to me.  This friend, in
his reply to my first letter regarding Mayer, stated his belief that I
should not find anything very important in Mayer's writings; but
before forwarding the memoirs to me he read them himself.  His letter
accompanying them contains the following words: 'I must here retract
the statement in my last letter, that you would not find much matter
of importance in Mayer's writings: I am astonished at the multitude of
beautiful and correct thoughts which they contain;' and he goes on to
point out various important subjects, in the treatment of which Mayer
had anticipated other eminent writers.  My other friend, in whose own
publications the name of Mayer repeatedly occurs, and whose papers
containing these references were translated some years ago by myself,
was, on the 10th of last month, unacquainted with the thoughtful and
beautiful essay of Mayer's, entitled 'Beitraege zur Dynamik des
Himmels,' and in 1854, when Professor William Thomson developed in so
striking a manner the meteoric theory of the sun's heat, he was
certainly not aware of the existence of that essay, though from a
recent article in 'Macmillan's Magazine' I infer that he is now aware
of it.  Mayer's physiological writings have been referred to by
physiologists--by Dr. Carpenter, for example--in terms of honouring
recognition.  We have hitherto, indeed, obtained fragmentary glimpses
of the man, partly from physicists and partly from physiologists; but
his total merit has never yet been recognised as it assuredly would
have been had he chosen a happier mode of publication.  I do not think
a greater disservice could be done to a man of science, than to
overstate his claims: such overstatement is sure to recoil to the
disadvantage of him in whose interest it is made.  But when Mayer's
opportunities, achievements, and fate are taken into account, I do not
think that I shall be deeply blamed for attempting to place him in
that honourable position, which I believe to be his due.

Here, however, are the titles of Mayer's papers, the perusal of which
will correct any error of judgment into which I may have fallen
regarding their author.  'Bemerkungen ueber die Kraefte der unbelebten
Natur,' Liebig's 'Annalen,' 1842, Vol. 42, p. 231; 'Die Organische
Bewegung in ihrem Zusammenhange mit dem Stoffwechsel,' Heilbronn,
1845; 'Beitraege zur Dynamik des Himmels,' Heilbronn, 1848;
'Bemerkungen ueber das Mechanische Equivalent der Waerme,' Heilbronn,
1851.

====================

IN MEMORIAM.--Dr. Julius Robert Mayer died at Heilbronn on March 20,
1878, aged 63 years.  It gives me pleasure to reflect that the great
positionwhich he will for ever occupy in the annals of science was
first virtually assigned to him in the foregoing discourse.  He was
subsequently hosen by acclamation a member of the French Academy of
Sciences; and he received from the Royal Society the Copley medal-its
Highest reward. [Footnote: See 'The Copley Medalist for 1871,' p.479.]

====================

November 1878.

At the meeting of the British Association at Glasgow in 1876--that is
to say, more than fourteen years after its delivery and
publication--the foregoing lecture was made the cloak for an unseemly
personal attack by Professor Tait.  The anger which found this
uncourteous vent dates from 1863, when it fell to my lot to maintain,
in opposition to him and a more eminent colleague, the position which
in 1862 I had assigned to Dr. Mayer. [Footnote: See 'Philosophical
Magazine' for this and the succeeding years.]  In those days Professor
Tait denied to Mayer all originality, and he has since, I regret to say,
never missed an opportunity, however small, of carping at Mayer's
claims.  The action of the Academy of Sciences and of the Royal Society
summarily disposes of this detraction, to which its object, during his
lifetime, never vouchsafed either remonstrance or reply.

Some time ago Professor Tait published a volume of lectures entitled
'Recent Advances in Physical Science,' which I have reason to know has
evoked an amount of censure far beyond that hitherto publicly
expressed.  Many of the best heads on the continent of Europe agree in
their rejection and condemnation of the historic portions of this
book.  In March last it was subjected to a brief but pungent critique
by Du Bois-Reymond, the celebrated Perpetual Secretary of the Academy
of Sciences in Berlin.  Du Bois-Reymond's address was on 'National
Feeling,' and his critique is thus wound up: 'The author of the
"Lectures" is not, perhaps, sufficiently well acquainted with the
history on which he professes to throw light, and on the later phases
of which he passes so unreserved (schroff) a judgment.  He thus
exposes himself to the suspicion--which, unhappily, is not weakened by
his other writings--that the fiery Celtic blood of his country
occasionally runs away with him, converting him for the time into a
scientific Chauvin.  Scientific Chauvinism,' adds the learned
secretary, 'from which German investigators have hitherto kept free,
is more reprehensible (gehaessig) than political Chauvinism, inasmuch
as self-control (_sittliche Haltung_) is more to be expected from men of
science, than from the politically excited mass.' [Footnote: Festrede,
delivered before the Academy of Sciences of Berlin, in celebration of
the birthday of the Emperor and King, March 28, 1878.]

In the case before this 'expectation' would, I fear, be doomed to
disappointment.  But Du Bois-Reymond and his countrymen must not
accept the writings of Professor Tait as representative of the thought
of England.  Surely no nation in the world has more effectually shaken
itself free from scientific Chauvinism.  From the day that Davy, on
presenting the Copley medal to Arago, scornfully brushed aside that
spurious patriotism which would run national boundaries through the
free domain of science, chivalry towards foreigners has been a guiding
principle with the Royal Society.

On the more private amenities indulged in by Professor Tait, I do not
consider it necessary to say a word.

********************

XVII.  CONTRIBUTIONS TO MOLECULAR PHYSICS.

[Footnote: A discourse delivered at the Royal Institution, March 18,
1864--supplementing, though of prior date, the Rede Lecture on
Radiation.]

HAVING on previous occasions dwelt upon the enormous differences which
exist among gaseous bodies both as regards their power of absorbing
and emitting radiant heat, I have now to consider the effect of a
change of aggregation.  When a gas is condensed to a liquid, or a
liquid congealed to a solid, the molecules coalesce, and grapple with
each other by forces which are insensible as long as the gaseous state
is maintained.  But, even in the solid and liquid conditions, the
luminiferous aether still surrounds the molecules: hence, if the acts
of radiation and absorption depend on them individually, regardless of
their state of aggregation, the change from the gaseous to the liquid
state ought not materially to affect the radiant and absorbent power.
If, on the contrary, the mutual entanglement of the molecular by the
force of cohesion be of paramount influence, then we may expect that
liquids will exhibit a deportment towards radiant heat altogether
different from that of the vapours from which they are derived.

The first part of an enquiry conducted in 1863-64 was devoted to an
exhaustive examination of this question.  Twelve different liquids
were employed, and five different layers of each, varying in thickness
from 0.02 of an inch to 0.27 of an inch.  The liquids were enclosed,
not in glass vessels, which would have materially modified the
incident heat, but between plates of transparent rock-salt, which only
slightly affected the radiation.  The source of heat throughout these
comparative experiments consisted of a platinum wire, raised to
incandescence by an electric current of unvarying strength.  The
quantities of radiant heat absorbed and transmitted by each of the
liquids at the respective thicknesses were first determined.  The
vapours of these liquids were subsequently examined, the quantities of
vapour employed being rendered proportional to the quantities of
liquid previously traversed by the radiant heat.  The result was that,
for heat from the same source, the order of absorption of liquids and
of their vapours proved absolutely the same.  There is no known
exception to this law; so that, to determine the position of a vapour
as an absorber or a radiator, it is only necessary to determine the
position of its liquid.

This result proves that the state of aggregation, as far at all events
as the liquid stage is concerned, is of altogether subordinate
moment--a conclusion which will probably prove to be of cardinal
importance in molecular physics.  On one important and contested point
it has a special bearing.  If the position of a liquid as an absorber
and radiator determine that of its vapour, the position of water fixes
that of aqueous vapour.  Water has been compared with other liquids in
a multitude of experiments, and it has been found, both as a radiant
and as an absorbent, to transcend them all.  Thus, for example, a
layer of bisulphide of carbon 0.02 of an inch in thickness absorbs 6
per cent, and allows 94 per cent of the radiation from the red-hot
platinum spiral to pass through it; benzol absorbs 43 and transmits 57
per cent.  of the same radiation; alcohol absorbs 67 and transmits 33
per cent, and alcohol, as an absorber of radiant heat, stands at the
head of all liquids except one.  The exception is water.  A layer of
this substance, of the thickness above given, absorbs 81 per cent, and
permits only 19 per cent.  of the radiation to pass through it. Had no
single experiment ever been made upon the vapour of water, its
vigorous action upon radiant heat might be inferred from the
deportment of the liquid.

The relation of absorption and radiation to the chemical constitution
of the radiating and absorbing substances was next briefly considered.
For the first six substances in the list of liquids examined, the
radiant and absorbent powers augment as the number of atoms in the
compound molecule augments.  Thus, bisulphide of carbon has 3 atoms,
chloroform 5, iodide of ethyl 8, benzol 12, and amylene 15 atoms in
their respective molecules.  The order of their power as radiants and
absorbents is that here indicated, bisulphide of carbon being the
feeblest, and amylene the strongest of the six.  Alcohol, however,
excels benzol as an absorber, though it has but 9 atoms in its
molecule; but, on the other hand, its molecule is rendered more
complex by the introduction of a new element.  Benzol contains carbon
and hydrogen, while alcohol contains carbon, hydrogen and oxygen.
Thus, not only does atomic _multitude_ come into play in absorption and
radiation--atomic _complexity_ must also be taken into account.  I would
recommend to the particular attention of chemists the molecule of
water; the deportment of this substance towards radiant heat being
perfectly anomalous, if the chemical formula at present ascribed to it
be correct.

Sir William Herschel made the important discovery that, beyond the
limits of the red end of the solar spectrum, rays of high heating
power exist which are incompetent to excite vision.  The discovery is
capable of extension.  Dissolving iodine in the bisulphide of carbon,
a solution is obtained which entirely intercepts the light of the most
brilliant flames, while to the ultra-red rays of such flames the same
iodine is found to be perfectly diathermic.  The transparent
bisulphide, which is highly pervious to invisible heat, exercises on
it the same absorption as the perfectly opaque solution.  A hollow
prism filled with the opaque liquid being placed in the path of the
beam from an electric lamp, the light-spectrum is completely
intercepted, but the heat spectrum may be received upon a screen and
there examined.  Falling upon a thermo-electric pile, its invisible
presence is shown by the prompt deflection of even a coarse
galvanometer.

What, then, is the physical meaning of opacity and transparency as
regards light and radiant heat?  The visible rays of the spectrum
differ from the invisible ones simply in period.  The sensation of
light is excited by waves of aether shorter and more quickly recurrent
than the non-visual waves which fall beyond 'the extreme red.  But why
should iodine stop the former and allow the latter to pass?  The
answer to this question no doubt is, that the intercepted waves are
those whose periods of recurrence coincide with the periods of
oscillation possible to the atoms of the dissolved iodine.  The
elastic forces which keep these atoms apart compel them to vibrate in
definite periods, and, when these periods synchronise with those of
the aethereal waves, the latter are absorbed.  Briefly defined, then,
transparency in liquids, as well as in gases, is synonymous with
discord, while opacity is synonymous with accord, between the periods
of the waves of aether and those of the molecules on which they
impinge.

According to this view transparent and colourless substances owe their
transparency to the dissonance existing between the oscillating
periods of their atoms and those of the waves of the whole visible
spectrum.  From the prevalence of transparency in compound bodies, the
general discord of the vibrating periods of their atoms with the
light-giving waves of the spectrum, may be inferred; while their
synchronism with the ultra-red periods is to be inferred from their
opacity to the ultra-red rays.  Water illustrates this in a most
striking manner.  It is highly transparent to the luminous rays, which
proves that its atoms do not readily oscillate in the periods which
excite vision.  It is highly opaque to the ultra-red undulations,
which proves the synchronism of its vibrating periods with those of
the longer waves.

If, then, to the radiation from any source water shows itself
eminently or perfectly opaque, we may infer that the atoms whence the
radiation emanates oscillate in ultra-red periods.  Let us apply this
test to the radiation from a flame of hydrogen.  This flame consists
mainly of incandescent aqueous vapour, the temperature of which, as
calculated by Bunsen, is 3259°C, so that, if the penetrative power of
radiant heat, as generally supposed, augment with the temperature of
its source, we may expect the radiation from this flame to be
copiously transmitted by water.  While, however, a layer of the
bisulphide of carbon 0.07 of an inch in thickness transmits 72 per
cent.  of the incident radiation, and while every other liquid
examined transmits more or less of the heat, a layer of water of the
above thickness is entirely opaque to the radiation from the hydrogen
flame.  Thus we establish accord between the periods of the atoms of
cold water and those of aqueous vapour at a temperature of 3259°C. But
the periods of water have already been proved to be ultra red--hence
those of the hydrogen flame must be sensibly ultra-red also. The
absorption by dry air of the heat emitted by a platinum spiral raised
to incandescence by electricity is insensible, while that by the
ordinary undried air is 6 per cent.  Substituting for the platinum
spiral a hydrogen flame, the absorption by dry air still remains
insensible, while that of the undried air rises to 20 per cent.  of
the entire radiation.  The temperature of the hydrogen flame is, as
stated, 3259°C; that of the aqueous vapour of the air 20°C.  Suppose,
then, the temperature of aqueous vapour to rise from 20°C.  to 3259°C,
we must conclude that the augmentation of temperature is applied to an
increase of amplitude or width of swing, and not to the introduction
of quicker periods into the radiation.

The part played by aqueous vapour in the economy of nature is far more
wonderful than has been hitherto supposed.  To nourish the vegetation
of the earth the actinic and luminous rays of the sun must penetrate
our atmosphere; and to such rays aqueous vapour is eminently
transparent.  The violet and the ultra-violet rays pass through it
with freedom.  To protect vegetation from destructive chills the
terrestrial rays must be checked in their transit towards stellar
space; and this is accomplished by the aqueous vapour diffused through
the air.  This substance is the great moderator of the earth's
temperature, bringing its extremes into proximity, and obviating
contrasts between day and night which would render life insupportable.
But we can advance beyond this general statement, now that we know the
radiation from aqueous vapour is intercepted, in a special degree, by
water, and, reciprocally, the radiation from water by aqueous vapour;
for it follows from this that the very act of nocturnal refrigeration
which produces the condensation of aqueous vapour at the surface of
the earth--giving, as it were, a varnish of water to that
surface--imparts to terrestrial radiation that particular character
which disqualifies it from passing through the earth's atmosphere and
losing itself in space.

And here we come to a question in molecular physics which at the
present moment occupies attention.  By allowing the violet and
ultra-violet rays of the spectrum to fall upon sulphate of quinine and
other substances Professor Stokes has changed the periods of those
rays.  Attempts have been made to produce a similar result at the
other end of the spectrum--to convert the ultra-red periods into
periods competent to excite vision--but hitherto without success. Such
a change of period, I agree with Dr. Miller in believing, occurs when
the limelight is produced by an oxy-hydrogen flame.  In this common
experiment there is an actual breaking up of long periods into short
ones--a true rendering of unvisual periods visual.  The change of
refrangibility here effected differs from that of Professor Stokes;
firstly, by its being in the opposite direction--that is, from a lower
refrangibility to a higher; and, secondly, in the circumstance that
the lime is heated by the collision of the molecules of aqueous
vapour, before their heat has assumed the radiant form.  But it cannot
be doubted that the same effect would be produced by radiant heat of
the same periods, provided the motion of the aether could be rendered
sufficiently intense. [Footnote: This was soon afterwards
accomplished.  See the section on 'Transmutation of Rays'.]  The
effect in principle is the same, whether we consider the lime to be
struck by a particle of aqueous vapour oscillating at a certain rate,
or by a particle of aether oscillating at the same rate.

By plunging a platinum wire into a hydrogen flame we cause it to glow,
and thus introduce shorter periods into the radiation.  These, as
already stated, are in discord with the atomic vibrations of water;
hence we may infer that the transmission through water will be
rendered more copious by the introduction of the wire into the flame.
Experiment proves this conclusion to be true.  Water, from being
opaque, opens a passage to 6 per cent.  of the radiation from the
spiral.  A thin plate of colourless glass, moreover, transmits 68 per
cent.  of the radiation from the hydrogen flame; but when the flame
and spiral are employed, 78 per cent.  of the heat is transmitted.

For an alcohol flame Knoblauch and Melloni found glass to be less
transparent than for the same flame with a platinum spiral immersed in
it; but Melloni afterwards showed that the result was not
general--that black glass and black mica were decidedly more
diathermic to the radiation from the pure alcohol flame. Melloni did
not explain this, but the reason is now obvious. The mica and glass
owe their blackness to the carbon diffused through them.  This carbon,
as first proved by Melloni, is in some measure transparent to the
ultra-red rays, and I have myself succeeded in transmitting between 40
and 50 per cent. of the radiation from a hydrogen flame through a
layer of carbon which intercepted the light of an intensely brilliant
flame.  The products of combustion of alcohol are carbonic acid and
aqueous vapour, the heat of which is almost wholly ultra-red.  For
this radiation, then, the carbon is in a considerable degree
transparent, while for the radiation from the platinum spiral, it is
in a great measure opaque. The platinum wire, therefore, which
augmented the radiation through the pure glass, augmented the
absorption of the black glass and mica.

No more striking or instructive illustration of the influence of
coincidence could be adduced than that furnished by the radiation from
a carbonic oxide flame. Here the product of combustion is carbonic
acid; and on the radiation from this flame even the ordinary carbonic
acid of the atmosphere exerts a powerful effect. A quantity of the
gas, only one-thirtieth of an atmosphere in density, contained in a
polished brass tube four feet long, intercepts 50 per cent. of the
radiation from the carbonic oxide flame. For the heat emitted by
lampblack, olefiant gas is a far more powerful absorber than carbonic
acid; in fact, for such heat, with one exception, carbonic acid is the
most feeble absorber to be found among the compound gases. Moreover,
for the radiation from a hydrogen flame olefiant gas possesses twice
the absorbent power of carbonic acid, while for the radiation from the
carbonic oxide flame, at a common pressure of one inch of mercury, the
absorption by carbonic acid is more than twice that of olefiant gas.
Thus we establish the coincidence of period between carbonic acid at a
temperature of 20°C. and carbonic acid at a temperature of over
3000°C, the periods of oscillation of both the incandescent and the
cold gas belonging to the ultra-red portion of the spectrum.

It will be seen from the foregoing remarks and experiments how
impossible it is to determine the effect of temperature pure and
simple on the transmission of radiant heat if different sources of
heat be employed. Throughout such an examination the same oscillating
atoms ought to be retained. This is done by beating a platinum spiral
by an electric current, the temperature meanwhile varying between the
widest possible limits. Their comparative opacity to the ultra-red
rays shows the general accord of the oscillating periods of the
vapours referred to at the commencement of this lecture with those of
the ultra-red undulations. Hence, by gradually heating a platinum wire
from darkness up to whiteness, we ought gradually to augment the
discord between it and these vapours, and thus augment the
transmission. Experiment entirely confirms this conclusion. Formic
nether, for example, absorbs 45 per cent. of the radiation from a
platinum spiral heated to barely visible redness; 32 per cent. of the
radiation from the same spiral at a red heat; 26 per cent. of the
radiation from a white-hot spiral, and only 21 per cent. when the
spiral is brought near its point of fusion. Remarkable cases of
inversion as to transparency also occur. For barely visible redness
formic aether is more opaque than sulphuric; for a bright red heat
both are equally transparent; while, for a white heat, and still more
for a higher temperature, sulphuric aether is more opaque than formic.
This result gives us a clear view of the relationship of the two
substances to the luminiferous aether. As we introduce waves of
shorter period the sulphuric aether augments most rapidly in opacity;
that is to say, its accord with the shorter waves is greater than that
of the formic. Hence we may infer that the atoms of formic aether
oscillate, on the whole, more slowly than those of sulphuric aether.

When the source of heat is a Leslie's cube coated with lampblack and
filled with boiling water, the opacity of formic aether in comparison
with sulphuric is very decided. With this source also the positions of
chloroform and iodide of methyl are inverted. For a white-hot spiral,
the absorption of chloroform vapour being 10 per cent, that of iodide
of methyl is 16; with the blackened cube as source, the absorption by
chloroform is 22 per cent, while that by the iodide of methyl is only
19.  This inversion is not the result of temperature merely; for when
a platinum wire, heated to the temperature of boiling water, is
employed as a source, the iodide continues to be the most powerful
absorber.  All the experiments hitherto made go to prove that from
heated lampblack an emission takes place which synchronises in an
especial manner with chloroform.  For the cube at 100' C, coated with
lampblack, the absorption by chloroform is more than three times that
by bisulphide of carbon; for the radiation from the most luminous
portion of a gas-flame the absorption by chloroform is also
considerably in excess of that by bisulphide of carbon; while, for the
flame of a Bunsen's burner, from which the incandescent carbon
particles are removed by the free admixture of air, the absorption by
bisulphide of carbon is nearly twice that by chloroform.  _The removal
of the carbon particles more than doubles the relative transparency of
the chloroform_.  Testing, moreover, the radiation from various parts
of the same flame, it was found that for the blue base of the flame
the bisulphide of carbon was most opaque, while for all other parts of
the flame the chloroform was most opaque.  For the radiation from a
very small gas flame, consisting of a blue base and a small white tip,
the bisulphide was also most opaque, and its opacity very decidedly
exceeded that of the chloroform when the source of heat was the flame
of bisulphide of carbon.  Comparing the radiation from a Leslie's cube
coated with isinglass with that from a similar cube coated with
lampblack, at the common temperature of 100°C, it was found that, out
of eleven vapours, all but one absorbed the radiation from the
isinglass most powerfully; the single exception was chloroform.

It is worthy of remark that whenever, through a change of source, the
position of a vapour as an absorber of radiant heat was altered, the
position of the liquid from which the vapour was derived underwent a
similar change.

It is still a point of difference between eminent investigators
whether radiant heat, up to a temperature of 100°C, is monochromatic
or not.  Some affirm this; some deny it.  A long series of experiments
enables me to state that probably no two substances at a temperature
of 100°C.  emit heat of the same quality.  The heat emitted by
isinglass, for example, is different from that emitted by lampblack,
and the heat emitted by cloth, or paper, differs from both.  It is
also a subject of discussion whether rock-salt is equally diathermic
to all kinds of calorific rays; the differences affirmed to exist by
some investigators being ascribed by others to differences of
incidence from the various sources employed.  MM. de la Provostaye
and Desains maintain the former view, Melloni and M. Knoblauch
maintain the latter.  I tested this point without changing anything
but the temperature of the source; its size, distance, and
surroundings remaining the same.  The experiments proved rock-salt to
be coloured thermally.  It is more opaque, for example, to the
radiation from a barely visible spiral than to that from a white-hot
one.

In regard to the relation of radiation to conduction, if we define
radiation, internal as well as external, as the communication of
motion from the vibrating atoms to the aether, we may, I think, by
fair theoretic reasoning, reach the conclusion that the best radiators
ought to prove the worst conductors.  A broad consideration of the
subject shows at once the general harmony of this conclusion with
observed facts.  Organic substances are all excellent radiators; they
are also extremely bad conductors.  The moment we pass from the metals
to their compounds we pass from good conductors to bad ones, and from
bad radiators to good ones.  Water, among liquids, is probably the
worst conductor; it is the best radiator.  Silver, among solids, is
the best conductor; it is the worst radiator.  The excellent
researches of MM. de la Provostaye and Desains furnish a striking
illustration of what I am inclined to regard as a natural law--that
those atoms which transfer the greatest amount of motion to the
aether, or, in other words, radiate most powerfully, are the least
competent to communicate motion to each other, or, in other words, to
propagate by conduction readily.

********************

XVIII.  LIFE, AND LETTERS OF FARADAY.

1870.

UNDERTAKEN and executed in a reverent and loving spirit, the work of
Dr. Bence Jones makes Faraday the virtual writer of his own life.
Everybody now knows the story of the philosopher's birth; that his
father was a smith; that he was born at Newington Butts in 1791; that
he ran along the London pavements, a bright-eyed errand boy, with a
load of brown curls upon his head and a packet of newspapers under his
arm; that the lad's master was a bookseller and bookbinder--a kindly
man, who became attached to the little fellow, and in due time made
him his apprentice without fee; that during his apprenticeship he
found his appetite for knowledge provoked and strengthened by the
books he stitched and covered.  Thus he grew in wisdom and stature to
his year of legal manhood, when he appears in the volumes before us as
a writer of letters, which reveal his occupation, acquirements, and
tone of mind.  His correspondent was Mr. Abbott, a member of the
Society of Friends, who, with a forecast of his correspondent's
greatness, preserved his letters and produced them at the proper time.

In later years Faraday always carried in his pocket a blank card, on
which he jotted down in pencil his thoughts and memoranda.  He made
his notes in the laboratory, in the theatre, and in the streets.  This
distrust of his memory reveals itself in his first letter to Abbot. To
a proposition that no new enquiry should be started between them
before the old one had been exhaustively discussed, Faraday objects.
'Your notion,' he says, 'I can hardly allow, for the following
reason: ideas and thoughts spring up in my mind which are irrevocably
lost for want of noting at the time.' Gentle as he seemed, he wished
to have his own way, and he had it throughout his life.  Differences
of opinion sometimes arose between the two friends, and then they
resolutely faced each other.  'I accept your offer to fight it out
with joy, and shall in the battle of experience cause not pain, but, I
hope, pleasure.' Faraday notes his own impetuosity, and incessantly
checks it.  There is at times something almost mechanical in his
self-restraint.  In another nature it would have hardened into mere
'correctness' of conduct; but his overflowing affections prevented
this in his case.  The habit of self control became a second nature to
him at last, and lent serenity to his later years.

In October 1812 he was engaged by a Mr. De la Roche as a journeyman
bookbinder; but the situation did not suit him.  His master appears to
have been an austere and passionate man, and Faraday was to the last
degree sensitive.  All his life he continued so.  He suffered at times
from dejection; and a certain grimness, too, pervaded his moods.  'At
present,' he writes to Abbott, 'I am as serious as you can be, and
would not scruple to speak a truth to any human being, whatever
repugnance it might give rise to.  Being in this state of mind, I
should have refrained from writing to you, did I not conceive from the
general tenor of your letters that your mind is, at proper times,
occupied upon serious subjects to the exclusion of those that are
frivolous.'  Plainly he had fallen into that stern Puritan mood, which
not only crucifies the affections and lusts of him who harbours it,
but is often a cause of disturbed digestion to his friends.

About three months after his engagement with De la Roche, Faraday
quitted him and bookbinding together.  He had heard Davy, copied his
lectures, and written to him, entreating to be released from Trade,
which he hated, and enabled to pursue Science.  Davy recognised the
merit of his correspondent, kept his eye upon him, and, when occasion
offered, drove to his door and sent in a letter, offering him the post
of assistant in the laboratory of the Royal Institution.  He was
engaged March 1, 1813, and on the 8th we find him extracting the sugar
from beet-root.  He joined the City Philosophical Society which had
been founded by Mr. Tatum in 1808.  'The discipline was very sturdy,
the remarks very plain, and the results most valuable.' Faraday
derived great profit from this little association.  In the laboratory
he had a discipline sturdier still.  Both Davy and himself were at
this time frequently cut and bruised by explosions of chloride of
nitrogen.  One explosion was so rapid 'as to blow my hand open, tear
away a part of one nail, and make my fingers so sore that I cannot use
them easily.' In another experiment 'the tube and receiver were blown
to pieces, I got a cut on the head, and Sir Humphry a bruise on his
hand.' And again speaking of the same substance, he says, 'when put in
the pump and exhausted, it stood for a moment, and then exploded with
a fearful noise.  Both Sir H.  and I had masks on, but I escaped this
time the best. Sir H. had his face cut in two places about the chin,
and a violent blow on the forehead struck through a considerable
thickness of silk and leather.' It was this same substance that blew
out the eye of Dulong.

Over and over again, even at this early date, we can discern the
quality which, compounded with his rare intellectual power, made
Faraday a great experimental philosopher.  This was his desire to see
facts, and not to rest contented with the descriptions of them.  He
frequently pits the eye against the ear, and affirms the enormous
superiority of the organ of vision.  Late in life I have heard him say
that he could never fully understand an experiment until he had seen
it.  But he did not confine himself to experiment.  He aspired to be a
teacher, and reflected and wrote upon the method of scientific
exposition.  'A lecturer,' he observes, 'should appear easy and
collected, undaunted and unconcerned:' still 'his whole behaviour
should evince respect for his audience.'  These recommendations were
afterwards in great part embodied by himself.  I doubt his
'unconcern,' but his fearlessness was often manifested.  It used to
rise within him as a wave, which carried both him and his audience
along with it.  On rare occasions also, when he felt himself and his
subject hopelessly unintelligible, he suddenly evoked a certain
recklessness of thought, and, without halting to extricate his
bewildered followers, he would dash alone through the jungle into
which he had unwittingly led them; thus saving them from ennui by the
exhibition of a vigour which, for the time being, they could neither
share nor comprehend.

In October 1813 he quitted England with Sir Humphry and Lady Davy.
During his absence he kept a journal, from which copious and
interesting extracts have been made by Dr. Bence Jones.  Davy was
considerate, preferring at times to be his own servant rather than
impose on Faraday duties which he disliked.  But Lady Davy was the
reverse.  She treated him as an underling; he chafed under the
treatment, and was often on the point of returning home.  They halted
at Geneva.  De la Rive, the elder, had known Davy in 1799, and, by his
writings in the 'Bibliothéque Britannique,' had been the first to make
the English chemist's labours known abroad.  He welcomed Davy to his
country residence in 1814.  Both were sportsmen, and they often went
out shooting together.  On these occasions Faraday charged Davy's gun
while De la Rive charged his own.  Once the Genevese philosopher found
himself by the side of Faraday, and in his frank and genial way
entered into conversation with the young man.  It was evident that a
person possessing such a charm of manner and such high intelligence
could be no mere servant.  On enquiry De la Rive was somewhat shocked
to find that the _soi-disant domestique_ was really _préparateur_ in the
laboratory of the Royal Institution; and he immediately proposed that
Faraday thenceforth should join the masters instead of the servants at
their meals.  To this Davy, probably out of weak deference to his
wife, objected; but an arrangement was come to that Faraday
thenceforward should have his food in his own room.  Rumour states
that a dinner in honour of Faraday was given by De la Rive.  This is a
delusion; there was no such banquet; but Faraday never forgot the
kindness of the friend who saw his merit when he was a mere _garcon de
laboratoire_. [Footnote: While confined last autumn at Geneva by the
effects of a fall in the Alps, my friends, with a kindness I can never
forget, did all that friendship could suggest to render my captivity
pleasant to me.  M. de la Rive then wrote out for me the full account,
of which the foregoing is a condensed abstract.  It was at the desire
of Dr. Bence Jones that I asked him to do so.  The rumour of a banquet
at Geneva illustrates the tendency to substitute for the youth of 1814
the Faraday of later years.]

He returned in 1815 to the Royal Institution.  Here he helped Davy for
years; he worked also for himself, and lectured frequently at the City
Philosophical Society.  He took lessons in elocution, happily without
damage to his natural force, earnestness, and grace of delivery.  He
was never pledged to theory, and he changed in opinion as knowledge
advanced.  With him life was growth.  In those early lectures we hear
him say, 'In knowledge, that man only is to be contemned and despised
who is not in a state of transition.'  And again: 'Nothing is more
difficult and requires more caution than philosophical deduction, nor
is there anything more adverse to its accuracy than fixity of
opinion.' Not that he was wafted about by every wind of doctrine; but
that he united flexibility with his strength.  In striking contrast
with this intellectual expansiveness was his fixity in religion, but
this is a subject which cannot be discussed here.

Of all the letters published in these volumes none possess a greater
charm than those of Faraday to his wife.  Here, as Dr. Bence Jones
truly remarks, 'he laid open all his mind and the whole of his
character, and what can be made known can scarcely fail to charm every
one by its loveliness, its truthfulness, and its earnestness.' Abbott
and he sometimes swerved into wordplay about love; but up to 1820, or
thereabouts, the passion was potential merely.  Faraday's journal
indeed contains entries which show that he took pleasure in the
assertion of his contempt for love; but these very entries became
links in his destiny.  It was through them that he became acquainted
with one who inspired him with a feeling which only ended with his
life.  His biographer has given us the means of tracing the varying
moods which preceded his acceptance.  They reveal more than the common
alternations of light and gloom; at one moment he wishes that his
flesh might melt and that he might become nothing; at another he is
intoxicated with hope.  The impetuosity of his character was then
unchastened by the discipline to which it was subjected in after
years.  The very strength of his passion proved for a time a bar to
its advance, suggesting, as it did, to the conscientious mind of Miss
Barnard, doubts of her capability to return it with adequate force.
But they met again and again, and at each successive meeting he found
his heaven clearer, until at length he was able to say, 'Not a
moment's alloy of this evening's happiness occurred.  Everything was
delightful to the last moment of my stay with my companion, because
she was so.' The turbulence of doubt subsided, and a calm and
elevating confidence took its place.  'What can I call myself,' he
writes to her in a subsequent letter, 'to convey most perfectly my
affection and love for you?  Can I or can truth say more than that for
this world I am yours?  Assuredly he made his profession good, and no
fairer light falls upon his character than that which reveals his
relations to his wife.  Never, I believe, existed a manlier, purer,
steadier love.  Like a burning diamond, it continued to shed, for
six-and-forty years, its white and smokeless glow.

Faraday was married on June 12, 1821; and up to this date Davy appears
throughout as his friend.  Soon afterwards, however, disunion occurred
between them, which, while it lasted, must have given Faraday intense
pain.  It is impossible to doubt the honesty of conviction with which
this subject has been treated by Dr. Bence Jones, and there may be
facts known to him, but not appearing in these volumes, which justify
his opinion that Davy in those days had become jealous of Faraday.
This, which is the prevalent belief, is also reproduced in an
excellent article in the March number of 'Framer's Magazine.' But the
best analysis I can make of the data fails to present Davy in this
light to me.  The facts, as I regard them, are briefly these.

In 1820, Oersted of Copenhagen made the celebrated discovery which
connects electricity with magnetism, and immediately afterwards the
acute mind of Wollaston perceived that a wire carrying a current ought
to rotate round its own axis under the influence of a magnetic pole.
In 1821 'he tried, but failed, to realise this result in the
laboratory of the Royal Institution.  Faraday was not present at the
moment, but he came in immediately afterwards and heard the
conversation of Wollaston and Davy about the experiment.  He had also
heard a rumour of a wager that Dr. Wollaston would eventually succeed.

This was in April.  In the autumn of the same year Faraday wrote a
history of electro-magnetism, and repeated for himself the experiments
which he described.  It was while thus instructing himself that he
succeeded in causing a wire, carrying an electric current, to rotate
round a magnetic pole.  This was not the result sought by Wollaston,
but it was closely related to that result.

The strong tendency of Faraday's mind to look upon the reciprocal
actions of natural forces gave birth to his greatest discoveries; and
we, who know this, should be justified in concluding that, even had
Wollaston not preceded him, the result would have been the same.  But
in judging Davy we ought to transport ourselves to his time, and
carefully exclude from our thoughts and feelings that noble subsequent
life, which would render simply impossible the ascription to Faraday
of anything unfair.  It would be unjust to Davy to put our knowledge
in the place of his, or to credit him with data which he could not
have possessed.  Rumour and fact had connected the name of Wollaston
with these supposed interactions between magnets and currents.  When,
therefore, Faraday in October published his successful experiment,
without any allusion to Wollaston, general, though really ungrounded,
criticism followed.  I say ungrounded because, firstly, Faraday's
experiment was not that of Wollaston, and secondly, Faraday, before he
published it, had actually called upon Wollaston, and not finding him
at home, did not feel himself authorised to mention his name.

In December, Faraday published a second paper on the same subject,
from which, through a misapprehension, the name of Wollaston was also
omitted.  Warburton and others thereupon affirmed that Wollaston's
ideas had been appropriated without acknowledgment, and it is plain
that Wollaston himself, though cautious in his utterance, was also
hurt.  Censure grew till it became intolerable.  'I hear,' writes
Faraday to his friend Stodart, 'every day more and more of these
sounds, which, though only whispers to me, are, I suspect, spoken
aloud among scientific men.' He might have written explanations and
defences, but he went straighter to the point.  He wished to see the
principals face to face--to plead his cause before them personally.
There was a certain vehemence in his desire to do this.  He saw
Wollaston, he saw Davy, he saw Warburton; and I am inclined to think
that it was the irresistible candour and truth of character which
these viva voce defences revealed, as much as the defences themselves,
that disarmed resentment at the time.

As regards Davy, another cause of dissension arose in 1823.  In the
spring of that year Faraday analysed the hydrate of chlorine, a
substance once believed to be the element chlorine, but proved by Davy
to be a compound of that element and water.  The analysis was looked
over by Davy, who then and there suggested to Faraday to heat the
hydrate in a closed glass tube.  This was done, the substance was
decomposed, and one of the products of decomposition was proved by
Faraday to be chlorine liquefied by its own pressure.  On the day of
its discovery he communicated this result to Dr. Paris.  Davy, on
being informed of it, instantly liquefied another gas in the same way.
Having struck thus into Faraday's enquiry, ought he not to have left
the matter in Faraday's hands?  I think he ought.  But, considering
his relation to both Faraday and the hydrate of chlorine, Davy, I
submit, may be excused for thinking differently.  A father is not
always wise enough to see that his son has ceased to be a boy, and
estrangement on this account is not rare; nor was Davy wise enough to
discern that Faraday had passed the mere assistant stage, and become a
discoverer.  It is now hard to avoid magnifying this error.  But had
Faraday died or ceased to work at this time, or had his subsequent
life been devoted to money-getting, instead of to research, would
anybody now dream of ascribing jealousy to Davy?  Assuredly not.  Why
should he be jealous?  His reputation at this time was almost without
a parallel: his glory was without a cloud.  He had added to his other
discoveries that of Faraday, and after having been his teacher for
seven years, his language to him was this: 'It gives me great pleasure
to hear that you are comfortable at the Royal Institution, and I trust
that you will not only do something good and honourable for yourself,
but also for science.' This is not the language of jealousy, potential
or actual.  But the chlorine business introduced irritation and anger,
to which, and not to any ignobler motive, Davy's opposition to the
election of Faraday to the Royal Society is, I am persuaded, to be
ascribed.

These matters are touched upon with perfect candour, and becoming
consideration, in the volumes of Dr. Bence Jones; but in 'society'
they are not always so handled.  Here a name of noble intellectual
associations is surrounded by injurious rumours which I would
willingly scatter for ever.  The pupil's magnitude, and the splendour
of his position, are too great and absolute to need as a foil the
humiliation of his master.  Brothers in intellect, Davy and Faraday,
however, could never have become brothers in feeling; their characters
were too unlike.  Davy loved the pomp and circumstance of fame;
Faraday the inner consciousness that he had fairly won renown.  They
were both proud men.  But with Davy pride projected itself into the
outer world; while with Faraday it became a steadying and dignifying
inward force.  In one great particular they agreed.  Each of them
could have turned his science to immense commercial profit, but
neither of them did so.  The noble excitement of research, and the
delight of discovery, constituted their reward.  I commend them to the
reverence which great gifts greatly exercised ought to inspire.  They
were both ours; and through the coming centuries England will be able
to point with just pride to the possession of such men.

====================

The first volume of the 'Life and Letters' reveals to us the youth who
was to be father to the man.  Skilful, aspiring, resolute, he grew
steadily in knowledge and in power.  Consciously or unconsciously, the
relation of Action to Reaction was ever present to Faraday's mind.  It
had been fostered by his discovery of Magnetic Rotations, and it
planted in him more daring ideas of a similar kind.  Magnetism he knew
could be evoked by electricity, and he thought that electricity, in
its turn, ought to be capable of evolution by magnetism.  On August
29, 1831, his experiments on this subject began.  He had been
fortified by previous trials, which, though failures, had begotten
instincts directing him towards the truth.  He, like every strong
worker, might at times miss the outward object, but he always gained
the inner light, education, and expansion.  Of this Faraday's life was
a constant illustration.  By November he had discovered and colligated
a multitude of the most wonderful and unexpected phenomena.  He had
generated currents by currents; currents by magnets, permanent and
transitory; and he afterwards generated currents by the earth itself.
Arago's 'Magnetism of Rotation,' which had for years offered itself
as a challenge to the best scientific intellects of Europe, now fell
into his hands.  It proved to be a beautiful, but still special,
illustration of the great principle of Magneto-electric Induction.
Nothing equal to this latter, in the way of pure experimental enquiry,
had previously been achieved.

Electricities from various sources were next examined, and their
differences and resemblances revealed.  He thus assured himself of
their substantial identity.  He then took up Conduction, and gave many
striking illustrations of the influence of Fusion on Conducting Power.
Renouncing professional work, from which at this time he might have
derived an income of many thousands a year, he poured his whole
momentum into his researches.  He was long entangled in
Electrochemistry.  The light of law was for a time obscured by the
thick umbrage of novel facts; but he finally emerged from his
researches with the great principle of Definite Electro-chemical
Decomposition in his hands.  If his discovery of Magneto-electricity
may be ranked with that of the pile by Volta, this new discovery, may
almost stand beside that of Definite Combining Proportions in
Chemistry.  He passed on to Static Electricity--its Conduction,
Induction, and Mode of Propagation.  He discovered and illustrated the
principle of Inductive Capacity; and, turning to theory, he asked
himself how electrical attractions and repulsions are transmitted. Are
they, like gravity, actions at a distance, or do they require a
medium?  If the former, then, like gravity, they will act in straight
lines; if the latter, then, like sound or light, they may turn a
corner.  Faraday held--and his views are gaining ground--that his
experiments proved the fact of curvilinear propagation, and hence the
operation of a medium.  Others denied this; but none can deny the
profound and philosophic character of his leading thought. [Footnote:
In a very remarkable paper published in Poggendorff's 'Annalen' for
1857, Werner Siemens accepts and develops Faraday's theory of
Molecular Induction.]  The first volume of the Researches contains all
the papers here referred to.

Faraday had heard it stated that henceforth physical discoveries would
be made solely by the aid of mathematics; that we had our data, and
needed only to work deductively.  Statements of a similar character
crop out from time to time in our day.  They arise from an imperfect
acquaintance with the nature, present condition, and prospective
vastness of the field of physical enquiry.  The tendency of natural
science doubtless is to bring all physical phenomena under the
dominion of mechanical laws; to give them, in other words,
mathematical expression.  But our approach to this result is
asymptotic; and for ages to come--possibly for all the ages of the
human race--Nature will find room for both the philosophical
experimenter and the mathematician.  Faraday entered his protest
against the foregoing statement by labelling his investigations
'Experimental Researches in Electricity.' They were completed in 1854,
and three volumes of them have been published.  For the sake of
reference, he numbered every paragraph, the last number being 3362. In
1859 he collected and published a fourth volume of papers, under the
title, 'Experimental Researches in Chemistry and Physics.' Thus did
this apostle of experiment illustrate its power, and magnify his
office.

The second volume of the Researches embraces memoirs on the
Electricity of the Gymnotus; on the Source of Power in the Voltaic
Pile; on the Electricity evolved by the Friction of Water and Steam,
in which the phenomena and principles of Sir William Armstrong's
Hydro-electric machine are described and developed; a paper on
Magnetic Rotations, and Faraday's letters in relation to the
controversy it aroused.  The contribution of most permanent value
here, is that on the Source of Power in the Voltaic Pile.  By it the
Contact Theory, pure and simple, was totally overthrown, and the
necessity of chemical action to the maintenance of the current
demonstrated.

The third volume of the Researches opens with a memoir entitled 'The
Magnetisation of Light,' and the Illumination of Magnetic Lines of
Force.' It is difficult even now to affix a definite meaning to this
title; but the discovery of the rotation of the plane of polarisation,
which it announced, seems pregnant with great results.  The writings
of William Thomson on the theoretic aspects of the discovery; the
excellent electrodynamic measurements of Wilhelm Weber, which are
models of experimental completeness and skill; Weber's labours in
conjunction with his lamented friend Kohlrausch--above all, the
researches of Clerk Maxwell on the Electro-magnetic Theory of
Light--point to that wonderful and mysterious medium, which is the
vehicle of light and radiant heat, as the probable basis also of
magnetic and electric phenomena.  The hope of such a connection was
first raised by the discovery here referred to. [Footnote: A letter
addressed to me by Professor Weber on March 18 last contains the
following reference to the connection here mentioned: 'Die Hoffnung
einer solchen Combination ist durch Faraday's Entdeckung der
Drehung er Polarisationsebene durch magnetische Directionskraft
zuerst, und sodann durch die Uebereinstimmung derjenigen
Geschwindigkeit, welche das Verhaeltniss der electro-dynamischen
Einheit zur lectro-statischen ausdrueckt, mit der Geschwindigkeit
des Lichts angeregt worden; und mir scheint von allen Versuchen,
welche zur erwirklichung dieser Hoffnung gemacht worden sind, das von
Herrn Maxwell gemachte am erfolgreichsten.'] Faraday himself seemed to
cling with particular affection to this discovery.  He felt that there
was more in it than he was able to unfold.  He predicted that it would
grow in meaning with the growth of science.  This it has done; this it
is doing now.  Its right interpretation will probably mark an epoch in
scientific history.

Rapidly following it is the discovery of Diamagnetism, or the
repulsion of matter by a magnet.  Brugmans had shown that bismuth
repelled a magnetic needle.  Here he stopped.  Le Bailliff proved that
antimony did the same.  Here he stopped.  Seebeck, Becquerel, and
others, also touched the discovery.  These fragmentary gleams excited
a momentary curiosity and were almost forgotten, when Faraday
independently alighted on the same facts; and, instead of stopping,
made them the inlets to a new and vast region of research.  The value
of a discovery is to be measured by the intellectual action it calls
forth; and it was Faraday's good fortune to strike such lodes of
scientific truth as give occupation to some of the best intellects of
our age.

The salient quality of Faraday's scientific character reveals itself
from beginning to end of these volumes; a union of ardour and
patience--the one prompting the attack, the other holding him on to
it, till defeat was final or victory assured.  Certainty in one sense
or the other was necessary to his peace of mind.  The right method of
investigation is perhaps incommunicable; it depends on the individual
rather than on the system, and the mark is missed when Faraday's
researches are pointed to as merely illustrative of the power of the
inductive philosophy.  The brain may be filled with that philosophy;
but without the energy and insight which this man possessed, and which
with him were personal and distinctive, we should never rise to the
level of his achievements.  His power is that of individual genius,
rather than of philosophic method; the energy of a strong soul
expressing itself after its own fashion, and acknowledging no mediator
between it and Nature.

The second volume of the 'Life and Letters,' like the first, is a
historic treasury as regards Faraday's work and character, and his
scientific and social relations.  It contains letters from Humboldt,
Herschel, Hachette, De la Rive, Dumas, Liebig, Melloni, Becquerel,
Oersted, Plucker, Du Bois Reymond, Lord Melbourne, Prince Louis
Napoleon, and many other distinguished men.  I notice with particular
pleasure a letter from Sir John Herschel, in reply to a sealed packet
addressed to him by Faraday, but which he had permission to open if he
pleased.  The packet referred to one of the many unfulfilled hopes
which spring up in the minds of fertile investigators:

'Go on and prosper, "from strength to strength," like a victor
marching with assured step to further conquests; and be certain that
no voice will join more heartily in the peans that already begin to
rise, and will speedily swell into a shout of triumph, astounding even
to yourself, than that of J.  F.  W.  Herschel.'

Faraday's behaviour to Melloni in 1835 merits a word of notice.  The
young man was a political exile in Paris.  He had newly fashioned and
applied the thermo-electric pile, and had obtained with it results of
the greatest importance.  But they were not appreciated.  With the
sickness of disappointed hope Melloni waited for the report of the
Commissioners, appointed by the Academy of Sciences to examine the
Primier.  At length he published his researches in the 'Annales de
Chimie.' They thus fell into the hands of Faraday, who, discerning at
once their extraordinary merit, obtained for their author the Rumford
Medal of the Royal Society.  A sum of money always accompanies this
medal; and the pecuniary help was, at this time, even more essential
than the mark of honour to the young refugee.  Melloni's gratitude was
boundless:

'Et vous, monsieur,' he writes to Faraday, 'qui appartenez à une
société à laquelle je n'avais rien offert, vous qui me connaissiez à
peine de nom; vous n'avez pas demandé si j'avais des ennemis faibles
ou puissants, ni calculé quel en était le nombre; mais vous avez parlé
pour l'opprimé étranger, pour celui qui n'avait pas le moindre droit à
tant de bienveillance, et vos paroles ont été accueillies
favorablement par des collègues consciencieux!  Je reconnais bien là
des hommes dignes de leur noble mission, les véritable représentants
de la science d'un pays libre et généreux.'

Within the prescribed limits of this article it would be impossible to
give even the slenderest summary of Faraday's correspondence, or to
carve from it more than the merest fragments of his character.  His
letters, written to Lord Melbourne and others in 1836, regarding his
pension, illustrate his uncompromising independence.  The Prime
Minister had offended him, but assuredly the apology demanded and
given was complete.  I think 'it certain that, notwithstanding the
very full account of this transaction given by Dr. Bence Jones,
motives and influences were at work which even now are not entirely
revealed.  The minister was bitterly attacked, but he bore the censure
of the press with great dignity.  Faraday, while he disavowed having
either directly or indirectly furnished the matter of those attacks,
did not publicly exonerate the Premier.  The Hon.  Caroline Fox had
proved herself Faraday's ardent friend, and it was she who had healed
the breach between the philosopher and the minister.  She manifestly
thought that Faraday ought to have come forward in Lord Melbourne's
defence, and there is a flavour of resentment in one of her letters to
him on the subject.  No doubt Faraday had good grounds for his
reticence, but they are to me unknown.

In 1841 his health broke down utterly, and he went to Switzerland with
his wife and brother-in-law.  His bodily vigour soon revived, and he
accomplished feats of walking respectable even for a trained
mountaineer.  The published extracts from his Swiss journal contain
many beautiful and touching allusions.  Amid references to the tints
of the Jungfrau, the blue rifts of the glaciers, and the noble Niesen
towering over the Lake of Thun, we come upon the charming little scrap
which I have elsewhere quoted: 'Clout-nail making goes on here rather
considerably, and is a very neat and pretty operation to observe.  I
love a smith's shop and anything relating to smithery.  My father was
a smith.' This is from his journal; but he is unconsciously speaking
to somebody--perhaps to the world.

His description of the Staubbach, Giessbach, and of the scenic effects
of sky and mountain, are all fine and sympathetic.  But amid it all,
and in reference to it all, he tells his sister that 'true enjoyment
is from within, not from without.' In those days Agassiz was living
under a slab of gneiss on the glacier of the Aar.  Faraday met Forbes
at the Grimsel, and arranged with him an excursion to the 'Hôtel des
Neufchâtelois'; but indisposition put the project out.

From the Fort of Ham, in 1843, Faraday received a letter addressed to
him by Prince Louis Napoleon Bonaparte.  He read this letter to me
many years ago, and the desire, shown in various ways by the French
Emperor, to turn modern science to account, has often reminded me of
it since.  At the age of thirty-five the prisoner of Ham speaks of
'rendering his captivity less sad by studying the great discoveries'
which science owes to Faraday; and he asks a question which reveals
his cast of thought at the time: 'What is the most simple combination
to give to a voltaic battery, in order to produce a spark capable of
setting fire to powder under water or under ground?' Should the
necessity arise, the French Emperor will not lack at the outset the
best appliances of modern science; while we, I fear, shall have to
learn the magnitude of the resources we are now neglecting amid the
pangs of actual war.' [Footnote: The 'science' has since been applied,
with astonishing effect, by those who had studied it far more
thoroughly than the Emperor of the French.  We also, I am happy to
think, have improved the time since the above words were written
[1878].]

One turns with renewed pleasure to Faraday's letters to his wife,
published in the second volume.  Here surely the loving essence of the
man appears more distinctly than anywhere else. From the house of Dr.
Percy, in Birmingham, he writes thus:

'Here--even here the moment I leave the table, I wish I were with you
IN QUIET.  Oh, what happiness is ours!  My runs into the world in this
way only serve to make me esteem that happiness the more.'

And again:

'We have been to a grand conversazione in the town-hall, and I have
now returned to my room to talk with you, as the pleasantest and
happiest thing that I can do.  Nothing rests me so much as communion
with you.  I feel it even now as I write, and catch myself saying the
words aloud as I write them.'

Take this, moreover, as indicative of his love for Nature:

'After writing, I walk out in the evening hand in hand with my dear
wife to enjoy the sunset; for to me who love scenery, of all that I
have seen or can see, there is none surpasses that of heaven.  A
glorious sunset brings with it a thousand thoughts that delight me.'

Of the numberless lights thrown upon him by the Life and Letters,'
some fall upon his religion.  In a letter to Lady Lovelace, he
describes himself as belonging to 'a very small and despised sect of
Christians, known, if known at all, as _Sandemanians_, and our hope is
founded on the faith that is in Christ.' He adds: 'I do not think it
at all necessary to tie the study of the natural sciences and religion
together, and in my intercourse with my fellow-creatures, that which
is religious, and that which is philosophical, have ever been two
distinct things.' He saw clearly the danger of quitting his moorings,
and his science acted indirectly as the safeguard of his faith.  For
his investigations so filled his mind as to leave no room for
sceptical questionings, thus shielding from the assaults of
philosophy, the creed of his youth.  His religion was constitutional
and hereditary.  It was implied in the eddies of his blood and in the
tremors of his brain; and, however its outward and visible form might
have changed, Faraday would still have possessed its elemental
constituents--awe, reverence, truth, and love.

It is worth enquiring how so profoundly religious a mind, and so great
a teacher, would be likely to regard our present discussions on the
subject of education.  Faraday would be a 'secularist' were he now
alive.  He had no sympathy with those who contemn knowledge unless it
be accompanied by dogma.  A lecture delivered before the City
Philosophical Society in 1818, when he was twenty-six years of age,
expresses the views regarding education which he entertained to the
end of his life. 'First, then,' he says, 'all theological
considerations are banished from the society, and of course from my
remarks; and whatever I may say has no reference to a future state, or
to the means which are to be adopted in this world in anticipation of
it.  Next, I have no intention of substituting anything for religion,
but I wish to take that part of human nature which is independent of
it.  Morality, philosophy, commerce, the various institutions and
habits of society, are independent of religion, and may exist either
with or without it.  They are always the same, and can dwell alike in
the breasts of those who, from opinion, are entirely opposed in the
set of principles they include in the term religion, or in those who
have none.

'To discriminate more closely, if possible, I will observe that we
have no right to judge religious opinions; but the human nature of
this evening is that part of man which we have a right to judge.  And
I think it will be found on examination, that this humanity--as it may
perhaps be called--will accord with what I have before described as
being in our own hands so improvable and perfectible.'

In an old journal I find the following remarks on one of my earliest
dinners with Faraday: 'At two o'clock he came down for me.  He, his
niece, and myself, formed the party, "I never give dinners," he said.
"I don't know how to give dinners, and I never dine out.  But I should
not like my friends to attribute this to a wrong cause.  I act thus
for the sake of securing time for work, and not through religious
motives, as some imagine."  He said grace.  I am almost ashamed to
call his prayer a "saying Of grace."  In the language of Scripture, it
might be described as the petition of a son, into whose heart God had
sent the Spirit of His Son, and who with absolute trust asked a
blessing from his father.  We dined on roast beef, Yorkshire pudding,
and potatoes; drank sherry, talked of research and its requirements,
and of his habit of keeping himself free from the distractions of
society.  He was bright and joyful--boy-like, in fact, though he is
now sixty-two.  His work excites admiration, but contact with him
warms and elevates the heart.  Here, surely, is a strong man.  I love
strength; but let me not forget the example of its union with modesty,
tenderness, and sweetness, in the character of Faraday.'

Faraday's progress in discovery, and the salient points of his
character, are well brought out by the wise choice of letters and
extracts published in the volumes before us.  I will not call the
labours of the biographer final.  So great a character will challenge
reconstruction.  In the coming time some sympathetic spirit, with the
requisite strength, knowledge, and solvent power, will, I doubt not,
render these materials plastic, give them more perfect organic form,
and send through them, with less of interruption, the currents of
Faraday's life.  'He was too good a man,' writes his present
biographer, 'for me to estimate rightly, and too great a philosopher
for me to understand thoroughly.' That may be: but the reverent
affection to which we owe the discovery, selection, and arrangement of
the materials here placed before us, is probably a surer guide than
mere literary skill.  The task of the artist who may wish in future
times to reproduce the real though unobtrusive grandeur, the purity,
beauty, and childlike simplicity of him whom we have lost, will find
his chief treasury already provided for him by Dr. Bence Jones's
labour of love.

********************

XIX.  THE COPLEY MEDALIST OF 1870.

THIRTY years ago Electro-magnetism was looked to as a motive power,
which might possibly compete with steam.  In centres of industry, such
as Manchester, attempts to investigate and apply this power were
numerous.  This is shown by the scientific literature of the time.
Among others Mr. James Prescot Joule, a resident of Manchester, took
up the subject, and, in a series of papers published in Sturgeon's
'Annals of Electricity' between 1839 and 1841, described various
attempts at the construction and perfection of electro-magnetic
engines.  The spirit in which Mr. Joule pursued these enquiries is
revealed in the following extract: 'I am particularly anxious,' he
says, 'to communicate any new arrangement in order, if possible, to
forestall the monopolising designs of those who seem to regard this
most interesting subject merely in the light of pecuniary
speculation.' He was naturally led to investigate the laws of
electro-magnetic attractions, and in 1840 he announced the important
principle that the attractive force exerted by two electromagnets, or
by an electro-magnet and a mass of annealed iron, is directly
proportional to the square of the strength of the magnetising current;
while the attraction exerted between, an electro-magnet and the pole
of a permanent steel magnet, varies simply as the strength of the
current.  These investigations were conducted independently of, though
a little subsequently to, the celebrated enquiries of Henry, Jacobi,
and Lenz and Jacobi, on the same subject.

On December 17, 1840, Mr. Joule communicated to the Royal Society a
paper on the production of heat by Voltaic electricity.  In it he
announced the law that the calorific effects of equal quantities of
transmitted electricity are proportional to the resistance overcome by
the current, whatever may be the length, thickness, shape, or
character of the metal which closes the circuit; and also proportional
to the square of the quantity of transmitted electricity.  This is a
law of primary importance.  In another paper, presented to, but
declined by, the Royal Society, he confirmed this law by new
experiments, and materially extended it.  He also executed experiments
on the heat consequent on the passage of Voltaic electricity through
electrolytes, and found, in all cases, that the heat evolved by the
proper action of any Voltaic current is proportional to the square of
the intensity of that current, multiplied by the resistance to
conduction which it experiences.  From this law he deduced a number of
conclusions of the highest importance to electrochemistry.

It was during these enquiries, which are marked throughout by rare
sagacity and originality, that the great idea of establishing
quantitative relations between Mechanical Energy and Heat arose and
assumed definite form in his mind.  In 1843 Mr. Joule read before the
meeting of the British Association at Cork a.  paper' On the Calorific
Effects of Magneto-Electricity, and on the Mechanical Value of Heat.'
Even at the present day this memoir is tough reading, and at the time
it was written it must have appeared hopelessly entangled.  This, I
should think, was the reason why Faraday advised Mr. Joule not to
submit the paper to the Royal Society.  But its drift and results are
summed up in these memorable words by its author, written some time
subsequently: 'In that paper it was demonstrated experimentally, that
the mechanical power exerted in turning a magneto-electric machine is
converted into the heat evolved by the passage of the currents of
induction through its coils; and, on the other hand, that the motive
power of the electromagnetic engine is obtained at the expense of the
heat due to the chemical reaction of the battery by which it is
worked.' [Footnote: Phil. Mag. May, 1845.]  It is needless to dwell
upon the weight and importance of this statement.

Considering the imperfections incidental to a first determination, it
is not surprising that the 'mechanical values of heat,' deduced from
the different series of experiments published in 1843, varied widely
from each other.  The lowest limit was 587, and the highest 1,026
foot-pounds, for 1 degree Fahr.  of temperature.

One noteworthy result of his enquiries, which was pointed out at the
time by Mr. Joule, had reference to the exceedingly small fraction of
the heat actually converted into useful effect in the steam-engine.
The thoughts of the celebrated Julius Robert Mayer, who was then
engaged in Germany upon the same question, had moved independently in
the same groove; but to his labours due reference will be made on a
future occasion. [Footnote: See the next Fragment.]  In the memoir now
referred to, Mr. Joule also announced that he had proved heat to be
evolved during the passage of water through narrow tubes; and he
deduced from these experiments an equivalent of 770 foot-pounds, a
figure remarkably near the one now accepted.  A detached statement
regarding the origin and convertibility of animal heat strikingly
illustrates the penetration of Mr. Joule, and his mastery of
principles, at the period now referred to.  A friend had mentioned to
him Haller's hypothesis, that animal heat might arise from the
friction of the blood in the veins and arteries.  'It is
unquestionable,' writes Mr. Joule,'  that heat is produced by such
friction; but it must be understood that the mechanical force expended
in the friction is a part of the force of affinity which causes the
venous blood to unite with oxygen, so that the whole heat of the
system must still be referred to the chemical changes.  But if the
animal were engaged in turning a piece of machinery, or in ascending a
mountain, I apprehend that in proportion to the muscular effort put
forth for the purpose, a _diminution_ of the heat evolved in the system
by a given chemical action would be experienced.' The italics in this
memorable passage, written, it is to be remembered, in 1843, are Mr.
Joule's own.

The concluding paragraph of this British Association paper equally
illustrates his insight and precision, regarding the nature of
chemical and latent heat.  'I had,' he writes, 'endeavoured to prove
that when two atoms combine together, the heat evolved is exactly that
which would have been evolved by the electrical current due to the
chemical action taking place, and is therefore proportional to the
intensity of the chemical force causing the atoms to combine.  I now
venture to state more explicitly, that it is not precisely the
attraction of affinity, but rather the mechanical force expended by
the atoms in falling towards one another, which determines the
intensity of the current, and, consequently, the quantity of heat
evolved; so that we have a simple hypothesis by which we may explain
why heat is evolved so freely in the combination of gases, and by
which indeed we may account "latent heat" as a mechanical power,
prepared for action, as a watch-spring is when wound up.  Suppose, for
the sake of illustration, that 8 lbs. of oxygen and 1 lb.  of hydrogen
were presented to one another in the gaseous state, and then exploded;
the heat evolved would be about 1 degree Fahr.  in 60,000 lbs. of
water, indicating a mechanical force, expended in the combination,
equal to a weight of about 50,000,000 lbs. raised to the height of one
foot.  Now if the oxygen and hydrogen could be presented to each other
in a liquid state, the heat of combination would be less than before,
because the atoms in combining would fall through less space.'  No
words of mine are needed to point out the commanding grasp of
molecular physics, in their relation to the mechanical theory of heat,
implied by this statement.

Perfectly assured of the importance of the principle which his
experiments aimed at establishing, Mr. Joule did not rest content with
results presenting such discrepancies as those above referred to.  He
resorted in 1844 to entirely new methods, and made elaborate
experiments on the thermal changes produced in air during its
expansion: firstly, against a pressure, and therefore performing work;
secondly, against no pressure, and therefore performing no work. He
thus established anew the relation between the heat consumed and the
work done.  From five different series of experiments he deduced five
different mechanical equivalents, the agreement between them being far
greater than that attained in his first experiments.  The mean of them
was 802 foot-pounds.  From experiments with water agitated by a
paddle-wheel, he deduced, in 1845, an equivalent of 890 foot-pounds.
In 1847 he again operated upon water and sperm-oil, agitated them by a
paddle-wheel, determined their elevation of temperature, and the
mechanical power which produced it.  From the one he derived an
equivalent of 781.6 foot-pounds; from the other an equivalent of 782.1
foot-pounds.  The mean of these two very close determinations is 781.8
foot-pounds.

By this time the labours of the previous ten years had made Mr. Joule
completely master of the conditions essential to accuracy and success.
Bringing his ripened experience to bear upon the subject, he executed
in 1849 a series of 40 experiments on the friction of water, 50
experiments on the friction of mercury, and 20 experiments on the
friction of plates of cast-iron.  He deduced from these experiments
our present mechanical equivalent of heat, justly recognised all over
the world as 'Joule's equivalent.'

There are labours so great and so pregnant in consequences, that they
are most highly praised when they are most simply stated.  Such are
the labours of Mr. Joule.  They constitute the experimental foundation
of a principle of incalculable moment, not only to the practice, but
still more to the philosophy of Science.  Since the days of Newton,
nothing more important than the theory, of which Mr. Joule is the
experimental demonstrator, has been enunciated.

I have omitted all reference to the numerous minor papers with which
Mr. Joule has enriched scientific literature.  Nor have I alluded to
the important investigations which he has conducted jointly with Sir
William Thomson.  But sufficient, I think, has been here said to show
that, in conferring upon Mr. Joule the highest honour of the Royal
Society, the Council paid to genius not only a well-won tribute, but
one which had been fairly earned twenty years previously. [Footnote:
Lord Beaconsfield has recently honoured himself and England by
bestowing an annual pension of 200 pounds on Dr. Joule.]

********************

XX.  THE COPLEY MEDALIST OF 1871.

DR. JULIUS ROBERT MAYER was educated for D the medical profession. In
the summer of 1840, as he himself informs us, he was at Java, and
there observed that the venous blood of some of his patients had a
singularly bright red colour.  The observation riveted his attention;
he reasoned upon it, and came to the conclusion that the brightness of
the colour was due to the fact that a less amount of oxidation
sufficed to keep up the temperature of the body in a hot climate than
in a cold one.  The darkness of the venous blood he regarded as the
visible sign of the energy of the oxidation.

It would be trivial to remark that accidents such as this, appealing
to minds prepared for them, have often led to great discoveries.
Mayer's attention was thereby drawn to the whole question of animal
heat.  Lavoisier had ascribed this heat to the oxidation of the food.
'One great principle,' says Mayer, 'of the physiological theory of
combustion, is that under all circumstances the same amount of fuel
yields, by its perfect combustion, the same amount of heat; that this
law holds good even for vital processes; and that hence the living
body, notwithstanding all its enigmas and wonders, is incompetent to
generate heat out of nothing.'

But beyond the power of generating internal heat, the animal organism
can also generate heat outside of itself.  A blacksmith, for example,
by hammering can heat a nail, and a savage by friction can warm wood
to its point of ignition.  Now, unless we give up the physiological
axiom that the living body cannot create heat out of nothing, 'we are
driven,' says Mayer, 'to the conclusion that it is the total heat
generated within and without that is to be regarded as the true
calorific effect of the matter oxidised in the body.'

From this, again, he inferred that the heat generated externally must
stand in a fixed relation to the work expended in its production. For,
supposing the organic processes to remain the same; if it were
possible, by the mere alteration of the apparatus, to generate
different amounts of heat by the same amount of work, it would follow
that the oxidation of the same amount of material would sometimes
yield a less, sometimes a greater, quantity of heat.  'Hence,' says
Mayer, 'that a fixed relation subsists between heat and work, is a
postulate of the physiological theory of combustion.'

This is the simple and natural account, given subsequently by Mayer
himself, of the course of thought started by his observation in Java.
But the conviction once formed, that an unalterable relation subsists
between work and heat, it was: inevitable that Mayer should seek to
express it numerically.  It was also inevitable that a mind like his,
having raised itself to clearness on this important point, should push
forward to consider the relationship of natural forces generally.  At
the beginning of 1842 his work had made considerable progress; but he
had become physician to the town of Heilbronn, and the duties of his
profession limited the time which he could devote to purely scientific
enquiry.  He thought it wise, therefore, to secure himself against
accident, and in the spring of 1842 wrote to Liebig, asking him to
publish in his 'Annalen' a brief preliminary notice of the work then
accomplished.  Liebig did so, and Dr. Mayer's first paper is contained
in the May number of the 'Annalen' for 1842.

Mayer had reached his conclusions by reflecting on the complex
processes of the living body; but his first step in public was to
state definitely the physical principles on which his physiological
deductions were to rest. He begins, therefore, with the forces of
inorganic nature.  He finds in the universe two systems of causes
which are not mutually convertible;--the different kinds of matter and
the different forms of force.  The first quality of both he affirms to
be indestructibility.  A force cannot become nothing, nor can it arise
from nothing.  Forces are convertible but not destructible.  In the
terminology of his time, he then gives clear expression to the ideas
of potential and dynamic energy, illustrating his point by a weight
resting upon the earth, suspended at a height above the earth, and
actually falling to the earth.  He next fixes his attention on cases
where motion is apparently destroyed, without producing other motion;
on the shock of inelastic bodies, for example. Under what form does
the vanished motion maintain itself?  Experiment alone, says Mayer,
can help us here.  He warms water by stirring it; he refers to the
force expended in overcoming friction.  Motion in both cases
disappears; but heat is generated, and the quantity generated is the
equivalent of the motion destroyed.  'Our locomotives,' he observes
with extraordinary sagacity, 'may be compared to distilling apparatus:
the heat beneath the boiler passes into the motion of the train, and
is again deposited as heat in the axles and wheels.

A numerical solution of the relation between heat and work was what
Mayer aimed at, and towards the end of his first paper he makes the
attempt.  It was known that a definite amount of air, in rising one
degree in temperature, can take up two different amounts of heat.  If
its volume be kept constant, it takes up one amount: if its pressure
be kept constant it takes up a different amount.  These two amounts
are called the specific heat under constant volume and under constant
pressure.  The ratio of the first to the second is as 1: 1.421.  No
man, to my knowledge, prior to Dr. Mayer, penetrated the significance
of these two numbers.  He first saw that the excess 0.421 was not, as
then universally supposed, heat actually lodged in the gas, but heat
which had been actually consumed by the gas in expanding against
pressure.  The amount of work here performed was accurately known, the
amount of heat consumed was also accurately known, and from these data
Mayer determined the mechanical equivalent of heat.  Even in this
first paper he is able to direct attention to the enormous discrepancy
between the theoretic power of the fuel consumed in steam-engines, and
their useful effect.

Though this paper contains but the germ of his further labours, I
think it may be safely assumed that, as regards the mechanical theory
of heat, this obscure Heilbronn physician, in the year 1842, was in
advance of all the scientific men of the time.

Having, by the publication of this paper, secured himself against what
he calls 'Eventualitaeten,' he devoted every hour of his spare time
to his studies, and in 1845 published a memoir which far transcends
his first one in weight and fulness, and, indeed, marks an epoch in
the history of science.  The title of Mayer's first paper was,
'Remarks on the Forces of Inorganic Nature.' The title of his second
great essay was, 'Organic Motion in its Connection with Nutrition.' In
it he expands and illustrates the physical principles laid down in his
first brief paper.

He goes fully through the calculation of the mechanical equivalent of
heat.  He calculates the performances of steam-engines, and finds that
100 lbs. of coal, in a good working engine, produce only the same
amount of heat as 95 lbs. in an unworking one; the 5 missing lbs.
having been converted into work.  He determines the useful effect of
gunpowder, and finds nine per cent. of the force of the consumed
charcoal invested on the moving ball.  He records observations on the
heat generated in water agitated by the pulping engine of a paper
manufactory, and calculates the equivalent of that heat in
horse-power.  He compares chemical combination with mechanical
combination--the union of atoms with the union of falling bodies with
the earth.  He calculates the velocity with which a body starting at
an infinite distance would strike the earth's surface, and finds that
the heat generated by its collision would raise an equal weight of
water 17,356' C.  in temperature.  He then determines the thermal
effect which would be produced by the earth itself falling into the
sun.  So that here, in 1845, we have the germ of that meteoric theory
of the sun's heat which Mayer developed with such extraordinary
ability three years afterwards.  He also points to the almost
exclusive efficacy of the sun's heat in producing mechanical motions
upon the earth, winding up with the profound remark, that the heat
developed by friction in the wheels of our wind and water mills comes
from the sun in the form of vibratory motion; while the heat produced
by mills driven by tidal action is generated at the expense of the
earth's axial rotation.

Having thus, with firm step, passed through the powers of inorganic
nature, his next object is to bring his principles to bear upon the
phenomena of vegetable and animal life.  Wood and coal can burn;
whence come their heat, and the work producible by that heat?  From
the immeasurable reservoir of the sun.  Nature has proposed to herself
the task of storing up the light which streams earthward from the sun,
and of casting into a permanent form the most fugitive of all powers.
To this end she has overspread the earth with organisms which, while
living, take in the solar light, and by its consumption generate
forces of another kind.  These organisms are plants.  The vegetable
world, indeed, constitutes the instrument whereby the wave-motion of
the sun is changed into the rigid form of chemical tension, and thus
prepared for future use.  With this prevision, as shall subsequently
be shown, the existence of the human race itself is inseparably
connected.  It is to be observed that Mayer's utterances are far from
being anticipated by vague statements regarding the 'stimulus' of
light, or regarding coal as 'bottled sunlight.' He first saw the full
meaning of De Saussure's observation as to the reducing power of the
solar rays, and gave that observation its proper place in the doctrine
of conservation.  In the leaves of a tree, the carbon and oxygen of
carbonic acid, and the hydrogen and oxygen of water, are forced
asunder at the expense of the sun, and the amount of power thus
sacrificed is accurately restored by the combustion of the tree.  The
heat and work potential in our coal strata are so much strength
withdrawn from the sun of former ages.  Mayer lays the axe to the root
of the notions regarding 'vital force' which were prevalent when he
wrote.  With the plain fact before us that in the absence of the solar
rays plants cannot perform the work of reduction, or generate chemical
tensions, it is, he contends, incredible that these tensions should be
caused by the mystic play of the vital force.  Such an hypothesis
would cut off all investigation; it would land us in a chaos of
unbridled phantasy.

'I count,' he says, 'therefore, upon your agreement with me when I
state, as an axiomatic truth, that during vital processes the
conversion only, and never the creation of matter or force occurs.'

Having cleared his way through the vegetable world, as he had
previously done through inorganic nature, Mayer passes on to the other
organic kingdom.  The physical forces collected by plants become the
property of animals.  Animals consume vegetables, and cause them to
reunite with the atmospheric oxygen.  Animal heat is thus produced;
and not only animal heat, but animal motion.  There is no
indistinctness about Mayer here; he grasps his subject in all its
details, and reduces to figures the concomitants of muscular action. A
bowler who imparts to an 8-lb.  ball a velocity of 30 feet, consumes
in the act one tenth of a grain of carbon.  A man weighing 150 lbs,
who lifts his own body to a height of 8 feet, consumes in the act 1
grain of carbon.  In climbing a mountain 10,000 feet high, the
consumption of the same man would be 2 oz. 4 drs.  50 grs.  of carbon.
Boussingault had determined experimentally the addition to be made to
the food of horses when actively working, and Liebig had determined
the addition to be made to the food of men.  Employing the mechanical
equivalent of heat, which he had previously calculated, Mayer proves
the additional food to be amply sufficient to cover the increased
oxidation.

But he does not content himself with showing, in a general way, that
the human body burns according to definite laws, when it performs
mechanical work.  He seeks to determine the particular portion of the
body consumed, and in doing so executes some noteworthy calculations.
The muscles of a labourer 150 lbs. in weight weigh 64 lbs; but when
perfectly desiccated they fall to 15 lbs. Were the oxidation
corresponding to that labourer's work exerted on the muscles alone,
they would be utterly consumed in 80 days.  The heart furnishes a
still more striking example.  Were the oxidation necessary to sustain
the heart's action exerted upon its own tissue, it would be utterly
consumed in 8 days.  And if we confine our attention to the two
ventricles, their action would be sufficient to consume the associated
muscular tissue in 3.5 days.  Here, in his own words, emphasised in
his own way, is Mayer's pregnant conclusion from these calculations:
'The muscle is only the apparatus by means of which the conversion of
the force is effected; but it is not the substance consumed in the
production of the mechanical effect.' He calls the blood 'the oil of
the lamp of life;' it is the slow-burning fluid whose chemical force,
in the furnace of the capillaries, is sacrificed to produce animal
motion.  This was Mayer's conclusion twenty-six years ago.  It was in
complete opposition to the scientific conclusions of his time; but
eminent investigators have since amply verified it.

Thus, in baldest outline, I have sought to give some notion of the
first half of this marvellous essay.  The second half is so
exclusively physiological that I do not wish to meddle with it.  I
will only add the illustration employed by Mayer to explain the action
of the nerves upon the muscles.  As an engineer, by the motion of his
finger in opening a valve or loosing a detent, can liberate an amount
of mechanical motion almost infinite compared with its exciting cause,
so the nerves, acting upon the muscles, can unlock an amount of
activity, wholly out of proportion to the work done by the nerves
themselves.

As regards these questions of weightiest import to the science of
physiology, Dr. Mayer, in 1845, was assuredly far in advance of all
living men.

Mayer grasped the mechanical theory of heat with commanding power,
illustrating it and applying it in the most diverse domains. He began,
as we have seen, with physical principles; he determined the numerical
relation between heat and work; he revealed the source of the energies
of the vegetable world, and showed the relationship of the heat of our
fires to solar heat. He followed the energies which were potential in
the vegetable, up to their local exhaustion in the animal.  But in
1845 a new thought was forced upon him by his calculations.  He then,
for the first time, drew attention to the astounding amount of heat
generated by gravity where the force has sufficient distance to act
through.  He proved, as I have before stated, the heat of collision of
a body falling from an infinite distance to the earth, to be
sufficient to raise the temperature of a quantity of water, equal to
the falling body in weight, 17,356°C.  He also found, in 1845, that
the gravitating force between the earth and sun was competent to
generate an amount of heat equal to that obtainable from the
combustion of 6,000 times the weight of the earth of solid coal.  With
the quickness of genius he saw that we had here a power sufficient to
produce the enormous temperature of the sun, and also to account for
the primal molten condition of our own planet.  Mayer shows the utter
inadequacy of chemical forces, as we know them, to produce or maintain
the solar temperature.  He shows that were the sun a lump of coal it
would be utterly consumed in 5,000 years.  He shows the difficulties
attending the assumption that the sun is a cooling body; for,
supposing it to possess even the high specific heat of water, its
temperature would fall 15,000' in 5,000 years.  He finally concludes
that the light and heat of the sun are maintained by the constant
impact of meteoric matter.  I never ventured an opinion as to the
truth of this theory; that is a question which may still have to be
fought out.  But I refer to it as an illustration of the force of
genius with which Mayer followed the mechanical theory of heat through
all its applications.  Whether the meteoric theory be a matter of fact
or not, with him abides the honour of proving to demonstration that
the light and heat of suns and stars may be originated and maintained
by the collisions of cold planetary matter.

It is the man who with the scantiest data could accomplish all this in
six short years, and in, the hours snatched from the duties of an
arduous profession, that the Royal Society, in 1871, crowned with its
highest honour.

Comparing this brief history with that of the Copley Medalist of 1870,
the differentiating influence of 'environment,' on two minds of
similar natural cast and endowment, comes out in an instructive
manner.  Withdrawn from mechanical appliances, Mayer fell back upon
reflection, selecting with marvellous sagacity, from existing physical
data, the single result on which could be founded a calculation of the
mechanical equivalent of heat.  In the midst of mechanical appliances,
Joule resorted to experiment, and laid the broad and firm foundation
which has secured for the mechanical theory the acceptance it now
enjoys.  A great portion of Joule's time was occupied in actual
manipulation; freed from this, Mayer had time to follow the theory
into its most abstruse and impressive applications.  With their places
reversed, however, Joule might have become Mayer, and Mayer might have
become Joule.

It does not lie within the scope of these brief articles to enter upon
the developments of the Dynamical Theory accomplished since Joule and
Mayer executed their memorable labours.

********************

XXI.  DEATH BY LIGHTNING.

PEOPLE in general imagine, when they think at all about the matter,
that an impression upon the nerves--a blow, for example, or the prick
of a pin--is felt at the moment it is inflicted.  But this is not the
case.  The seat of sensation being the brain, to it the intelligence
of any impression made upon the nerves has to be transmitted before
this impression can become manifest as consciousness.  The
transmission, moreover, requires time, and the consequence is, that a
wound inflicted on a portion of the body distant from the brain is
more tardily appreciated than one inflicted adjacent to the brain.  By
an extremely ingenious experimental arrangement, Helmholtz has
determined the velocity of this nervous transmission, and finds it to
be about eighty feet a second, or less than one-thirteenth of the
velocity of sound in air.  If therefore, a whale forty feet long were
wounded in the tail, it would not be conscious of the injury till half
a second after the wound had been inflicted. [Footnote: A most
admirable lecture on the velocity of nervous transmission has been
published by Dr. Du Bois Reymond in the 'Proceedings of the Royal
Institution' for 1866, vol. iv. p. 575.]  But this is not the only
ingredient in the delay.  There can scarcely be a doubt that to every
act of consciousness belongs a determinate molecular arrangement of
the brain--that every thought or feeling has its physical correlative
in that organ; and nothing can be more certain than that every
physical change, whether molecular or mechanical, requires time for
its accomplishment.  So that, besides the interval of transmission, a
still further time is necessary for the brain to put itself in
order--for its molecules to take up the motions or positions necessary
to the completion of consciousness.  Helmholtz considers that
one-tenth of a second is demanded for this purpose.  Thus, in the case
of the whale above supposed, we have first half a second consumed in
the transmission of the intelligence through the sensor nerves to the
head, one-tenth of a second consumed by the brain in completing the
arrangements necessary to consciousness, and, if the velocity of
transmission through the motor be the same as that through the sensor
nerves, half a second in sending a command to the tail to defend
itself.  Thus one second and a tenth would elapse before an impression
made upon its caudal nerves could be responded to by a whale forty
feet long.

Now, it is quite conceivable that an injury might be inflicted so
rapidly that within the time required by the brain to complete the
arrangements necessary to consciousness, its power of arrangement
might be destroyed.  In such a case, though the injury might be of a
nature to cause death, this would occur without pain, Death in this
case would be simply the sudden negation of life, without any
intervention of consciousness whatever.

The time required for a rifle-bullet to pass clean through a man's
head may be roughly estimated at a thousandth of a second.  Here,
therefore, we should have no room for sensation, and death would be
painless.  But there are other actions which far transcend in rapidity
that of the rifle-bullet.  A flash of lightning cleaves a cloud,
appearing and disappearing in less than a hundred-thousandth of a
second, and the velocity of electricity is such as would carry it in a
single second over a distance almost equal to that which separates the
earth and moon.  It is well known that a luminous impression once made
upon the retina endures for about one-sixth of a second, and that this
is the reason why we see a continuous band of light when a glowing
coal is caused to pass rapidly through the air.  A body illuminated by
an instantaneous flash continues to be seen for the sixth of a second
after the flash has become extinct; and if the body thus illuminated
be in motion, it appears at rest at the place where the flash falls
upon it.  When a colour-top with differently-coloured sectors is
caused to spin rapidly the colours blend together.  Such a top,
rotating in a dark room and illuminated by an electric spark, appears
motionless, each distinct colour being clearly seen.  Professor Dove
has found that a flash of lightning produces the same effect.  During
a thunderstorm he put a colour-top in exceedingly rapid motion, and
found that every flash revealed the top as a motionless object with
its colours distinct.  If illuminated solely by a flash of lightning,
the motion of all bodies on the earth's surface would, as Dove has
remarked, appear suspended.  A cannon-ball, for example, would have
its flight apparently arrested, and would seem to hang motionless in
space as long as the luminous impression which revealed the ball
remained upon the eye.

If, then, a rifle-bullet move with sufficient rapidity to destroy life
without the interposition of sensation, much more is a flash of
lightning competent to produce this effect.  Accordingly, we have
well-authenticated cases of people being struck senseless by lightning
who, on recovery, had no memory of pain.  The following circumstantial
case is described by Hemmer:

On June 30, 1788, a soldier in the neighbourhood of Mannheim, being
overtaken by rain, placed himself under a tree, beneath which a woman
had previously taken shelter.  He looked upwards to see whether the
branches were thick enough to afford the required protection, and, in
doing so, was struck by lightning, and fell senseless to the earth.
The woman at his side experienced the shock in her foot, but was not
struck down.  Some hours afterwards the man revived, but remembered
nothing about what had occurred, save the fact of his looking up at
the branches.  This was his last act of consciousness, and he passed
from the conscious to the unconscious condition without pain.  The
visible marks of a lightning stroke are usually insignificant: the
hair is sometimes burnt; slight wounds are observed; while, in some
instances, a red streak marks the track of the discharge over the
skin.

Under ordinary circumstances, the discharge from a small Leyden jar is
exceedingly unpleasant to me.  Some time ago I happened to stand in
the presence of a numerous audience, with a battery of fifteen large
Leyden jars charged beside me.  Through some awkwardness on my part, I
touched a wire leading from the battery, and the discharge went
through my body.  Life was absolutely blotted out for a very sensible
interval, without a trace of pain.  Ina second or so consciousness
returned; I vaguely discerned the audience and apparatus, and, by the
help of these external appearances, immediately concluded that I had
received the battery discharge.  The intellectual consciousness of my
position was restored with exceeding rapidity, but not so the optical
consciousness.  To prevent the audience from being alarmed, I observed
that it had often been my desire to receive accidentally such a shock,
and that my wish had at length been fulfilled.  But, while making this
remark, the appearance which my body presented to my eyes was that of
a number of separate pieces.  The arms, for example, were detached
from the trunk, and seemed suspended in the air.  In fact, memory and
the power of reasoning appeared to be complete long before the optic
nerve was restored to healthy action.  But what I wish chiefly to
dwell upon here is, the absolute painlessness of the shock; and there
cannot, I think, be a doubt that, to a person struck dead by
lightning, the passage from life to death occurs without consciousness
being in the least degree implicated.  It is an abrupt stoppage of
sensation, unaccompanied by a pang.

********************

XXII.  SCIENCE AND THE 'SPIRITS.'

THEIR refusal to investigate 'spiritual phenomena' is often urged as a
reproach against scientific men.  I here propose to give a sketch of
an attempt to apply to the 'phenomena' those methods of enquiry which
are found available in dealing with natural truth.

Some years ago, when the spirits were particularly active in this
country, Faraday was invited, or rather entreated, by one of his
friends to meet and question them.  He had, however, already made
their acquaintance, and did not wish to renew it.  I had not been so
privileged, and he therefore kindly arranged a transfer of the
invitation to me.  The spirits themselves named the time of meeting,
and I was conducted to the place at the day and hour appointed.

Absolute unbelief in the facts was by no means my condition of mind.
On the contrary, I thought it probable that some physical principle,
not evident to the spiritualists themselves, might underlie their
manifestations.  Extraordinary effects are produced by the
accumulation of small impulses.  Galileo set a heavy pendulum in
motion by the well-timed puffs of his breath.  Ellicot set one clock
going by the ticks of another, even when the two clocks were separated
by a wall.  Preconceived notions, can, moreover, vitiate, to an
extraordinary degree, the testimony of even veracious persons.  Hence
my desire to witness those extraordinary phenomena, the existence of
which seemed placed beyond a doubt by the known veracity of those who
had witnessed and described them.  The meeting took place at a private
residence in the neighbourhood of London.  My host, his intelligent
wife, and a gentleman who may be called X, were in the house when I
arrived.  I was informed that the 'medium' had not yet made her
appearance; that she was sensitive, and might resent suspicion.  It
was therefore requested that the tables and chairs should be examined
before her arrival, in order to be assured that there was no trickery
in the furniture.  This was done; and I then first learned that my
hospitable host had arranged that the séance should be a dinner-party.
This was to me an unusual form of investigation; but I accepted it, as
one of the accidents of the occasion.

The 'medium' arrived--a delicate-looking young lady, who appeared to
have suffered much from ill health.  I took her to dinner and sat
close beside her.  Facts were absent for a considerable time, a series
of very wonderful narratives supplying their place.  The duty of
belief on the testimony of witnesses was frequently insisted on.  X.
appeared to be a chosen spiritual agent, and told us many surprising
things.  He affirmed that, when he took a pen in his hand, an
influence ran from his shoulder downwards, and impelled him to write
oracular sentences.  I listened for a time, offering no observation.
'And now,' continued X, 'this power has so risen as to reveal to me
the thoughts of others.  Only this morning I told a friend what he was
thinking of, and what he intended to do during the day.' Here, I
thought, is something that can be at once tested.  I said immediately
to X: 'If you wish to win to your cause an apostle, who will proclaim
your principles to the world from the housetop, tell me what I am now
thinking of.'  X. reddened, and did not tell me my thought.

Some time previously I had visited Baron Reichenbach, in Vienna, and I
now asked the young lady who sat beside me, whether she could see any
of the curious things which he describes--the light emitted by
crystals, for example?  Here is the conversation which followed, as
extracted from my notes, written on the day following the séance.

Medium.--'Oh, yes; but I see light around all bodies.'

I--'Even in perfect darkness?'

Medium.--'Yes; I see luminous atmospheres round all people.  The
atmosphere which surrounds Mr. R. C. would fill this room with
light.'

I.--'You are aware of the effects ascribed by Baron Reichenbach to
magnets?'

Medium.--'Yes; but a magnet makes me terribly ill.'

I.--'Am I to understand that, if this room were perfectly dark, you
could tell whether it contained a magnet, without being informed of
the fact?'

Medium.--'I should know of its presence on entering the room.'

I.--'How?'

Medium.--'I should be rendered instantly ill.'

I.--'How do you feel to-day?'

Medium.--'Particularly well; I have not been so well for months.'

I.--'Then, may I ask you whether there is, at the present moment, a
magnet in my possession?'

The young lady looked at me, blushed, and stammered, 'No; I am not en
rapport with you.'

I sat at her right hand, and a left-hand pocket, within six inches of
her person, contained a magnet.

Our host here deprecated discussion, as it 'exhausted the medium.' The
wonderful narratives were resumed; but I had narratives of my own
quite as wonderful.  These spirits, indeed, seemed clumsy creations,
compared with those with which my own work had made me familiar.  I
therefore began to match the wonders related to me by other wonders. A
lady present discoursed on spiritual atmospheres, which she could see
as beautiful colours when she closed her eyes.  I professed myself
able to see similar colours, and, more than that, to be able to see
the interior of my own eyes.  The medium affirmed that she could see
actual waves of light coming from the sun.  I retorted that men of
science could tell the exact number of waves emitted in a second, and
also their exact length.  The medium spoke of the performances of the
spirits on musical instruments.  I said that such performance was
gross, in comparison with a kind of music which had been discovered
some time previously by a scientific man.  Standing at a distance of
twenty feet from a jet of gas, he could command the flame to emit a
melodious note; it would obey, and continue its song for hours.  So
loud was the music emitted by the gas-flame, that it might be heard by
an assembly of a thousand people.  These were acknowledged to be as
great marvels as any of those of spiritdom.  The spirits were then
consulted, and I was pronounced to be a first-class medium.

During this conversation a low knocking was heard from time to time
under the table.  These, I was told, were the spirits' knocks.  I was
informed that one knock, in answer to a question, meant 'No;' that two
knocks meant 'Not yet;' and that three knocks meant 'Yes.'

In answer to a question whether I was a medium, the response was three
brisk and vigorous knocks.  I noticed that the knocks issued from a
particular locality, and therefore requested the spirits to be good
enough to answer from another corner of the table.  They did not
comply; but I was assured that they would do it, and much more,
by-and-by.  The knocks continuing, I turned a wine-glass upside down,
and placed my ear upon it, as upon a stethoscope.  The spirits seemed
disconcerted by the act; they lost their playfulness, and did not
recover it for a considerable time.

Somewhat weary of the proceedings, I once threw myself back against my
chair and gazed listlessly out of the window.  While thus engaged, the
table was rudely pushed.  Attention was drawn to the wine, still
oscillating in the glasses, and I was asked whether that was not
convincing.  I readily granted the fact of motion, and began to feel
the delicacy of my position.  There were several pairs of arms upon
the table, and several pairs of legs under it; but how was I, without
offence, to express the conviction which I really entertained?  To
ward off the difficulty, I again turned a wine-glass upside down and
rested my ear upon it.  The rim of the glass was not level, and my
hair, on touching it, caused it to vibrate, and produce a peculiar
buzzing sound.  A perfectly candid and warm-hearted old gentleman at
the opposite side of the table, whom I may call A, drew attention to
the sound, and expressed his entire belief that it was spiritual.  I,
however, informed him that it was the moving hair acting on the glass.
The explanation was not well received; and X, in a tone of severe
pleasantry, demanded whether it was the hair that had moved the table.
The promptness of my negative probably satisfied him that my notion
was a very different one.

The superhuman power of the spirits was next dwelt upon.  The strength
of man, it was stated, was unavailing in opposition to theirs.  No
human power could prevent the table from moving when they pulled it.
During the evening this pulling of the table occurred, or rather was
attempted, three times.  Twice the table moved when my attention was
withdrawn from it; on a third occasion, I tried whether the act could
be provoked by an assumed air of inattention.  Grasping the table
firmly between my knees, I threw myself back in the chair, and waited,
with eyes fixed on vacancy, for the pull.  It came.  For some seconds
it was pull spirit, hold muscle; the muscle, however, prevailed, and
the table remained at rest. Up to the present moment, this interesting
fact is known only to the particular spirit in question and myself.

A species of mental scene-painting, with which my own pursuits had
long rendered me familiar, was employed to figure the changes and
distribution of spiritual power.  The spirits, it was alleged, were
provided with atmospheres, which combined with and interpenetrated
each other, and considerable ingenuity was shown in demonstrating the
necessity of time in effecting the adjustment of the atmospheres.  A
rearrangement of our positions was proposed and carried out; and soon
afterwards my attention was drawn to a scarcely sensible vibration on
the part of the table.  Several persons were leaning on the table at
the time, and I asked permission to touch the medium's hand.  'Oh! I
know I tremble,' was her reply.  Throwing one leg across the other, I
accidentally nipped a muscle, and produced thereby an involuntary
vibration of the free leg.  This vibration, I knew, must be
communicated to the floor, and thence to the chairs of all present.  I
therefore intentionally promoted it.  My attention was promptly drawn
to the motion; and a gentleman beside me, whose value as a witness I
was particularly desirous to test, expressed his belief that it was
out of the compass of human power to produce so strange a tremor.  'I
believe,' he added, earnestly, 'that it is entirely the spirits'
work.'  'So do I,' added, with heat, the candid and warmhearted old
gentleman A.  'Why, sir,' he continued, 'I feel them at this moment
shaking my chair.' I stopped the motion of the leg.  'Now, sir,' A.
exclaimed, 'they are gone.' I began again, and A. once more affirmed
their presence.  I could, however, notice that there were doubters
present, who did not quite know what to think of the manifestations. I
saw their perplexity; and, as there was sufficient reason to believe
that the disclosure of the secret would simply provoke anger, I kept
it to myself.

Again a period of conversation intervened, during which the spirits
became animated.  The evening was confessedly a dull one, but matters
appeared to brighten towards its close.  The spirits were requested to
spell the name by which I was known in the heavenly world.  Our host
commenced repeating the alphabet, and when he reached the letter 'P' a
knock was heard.  He began again, and the spirits knocked at the
letter 'O.'  I was puzzled, but waited for the end.  The next letter
knocked down was 'E.' I laughed, and remarked that the spirits were
going to make a poet of me.  Admonished for my levity, I was informed
that the frame of mind proper for the occasion ought to have been
superinduced by a perusal of the Bible immediately before the séance.
The spelling, however, went on, and sure enough I came out a poet. But
matters did not end here.  Our host continued his repetition of the
alphabet, and the next letter of the name proved to be '0.' Here was
manifestly an unfinished word; and the spirits were apparently in
their most communicative mood.  The knocks came from under the table,
but no person present evinced the slightest desire to look under it. I
asked whether I might go underneath; the permission was granted; so I
crept under the table.  Some tittered; but the candid old A.
exclaimed, 'He has a right to look into the very dregs of it, to
convince himself.' Having pretty well assured myself that no sound
could be produced under the table without its origin being revealed, I
requested our host to continued his questions.  He did so, but in
vain.  He adopted a tone of tender entreaty; but the 'dear spirits'
had become dumb dogs, and refused to be entreated.  I continued under
that table for at least a quarter of an hour, after which, with a
feeling of despair as regards the prospects of humanity never before
experienced, I regained my chair.  Once there, the spirits resumed
their loquacity, and dubbed me 'Poet of Science.'

This, then, is the result of an attempt made by a scientific man to
look into these spiritual phenomena.  It is not encouraging; and for
this reason.  The present promoters of spiritual phenomena divide
themselves into two classes, one of which needs no demonstration,
while the other is beyond the reach of proof.  The victims like to
believe, and they do not like to be undeceived.  Science is perfectly
powerless in the presence of this frame of mind.  It is, moreover, a
state perfectly compatible with extreme intellectual subtlety and a
capacity for devising hypotheses which only require the hardihood
engendered by strong conviction, or by callous mendacity, to render
them impregnable.  The logical feebleness of science is not
sufficiently borne in mind.  It keeps down the weed of superstition,
not by logic but by, slowly rendering the mental soil unfit for its
cultivation.  When science appeals to uniform experience, the
spiritualist will retort, 'How do you know that a uniform experience
will continue uniform?  You tell me that the sun has risen for six
thousand years: that is no proof that it will rise tomorrow; within
the next twelve hours it may be puffed out by the Almighty.'  Taking
this ground, a man may maintain the story of 'Jack and the Beanstalk'
in the face of all the science in the world.  You urge, in vain, that
science has given us all the knowledge of the universe which we now
possess, while spiritualism has added nothing to that knowledge.  The
drugged soul is beyond the reach of reason.  It is in vain that
impostors are exposed, and the special demon cast out.  He has but
slightly to change his shape, return to his house, and find it 'empty,
swept, and garnished.'

*****

Since the time when the foregoing remarks were written I have been
more than once among the spirits, at their own invitation.  They do
not improve on acquaintance.  Surely no baser delusion ever obtained
dominance over the weak mind of man.

END OF THE FIRST VOLUME.



LONDON: PRINTED BY

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**********************************************************************



FRAGMENTS OF SCIENCE:

A SERIES OF DETACHED ESSAYS, ADDRESSES, AND REVIEWS.

BY

JOHN TYNDALL, F.R.S.

LONDON: PRINTED BY

SPOTTISWOODE AND CO, NEW-STREET SQUARE

AND PARLIAMENT STREET

SIXTH EDITION.

VOL. II.

LONDON:

LONGMANS, GREEN, AND CO.

1879.

All rights reserved.

********************

In the bright sky they perceived an illuminator;
in the all-encircling firmament an embracer;
in the roar of thunder and in the violence of
the storm they felt the presence of a shouter and of furious strikers;
and out of the rain they created an Indra, or giver of rain.--MAX MULLER.


*****


I.  REFLECTIONS ON PRAYER AND NATURAL LAW.

1861.

AMID the apparent confusion and caprice of natural phenomena, which
roused emotions hostile to calm investigation, it must for ages have
seemed hopeless to seek for law or orderly relation; and before the
thought of law dawned upon the unfolding human mind these otherwise
inexplicable effects were referred to personal agency.  In the fall of
a cataract the savage saw the leap of a spirit, and the echoed
thunder-peal was to him the hammer-clang of an exasperated god.
Propitiation of these terrible powers was the consequence, and
sacrifice was offered to the demons of earth and air.

But observation tends to chasten the emotions and to check those
structural efforts of the intellect which have emotion for their base.
One by one natural phenomena came to be associated with their
proximate causes; the idea of direct personal volition mixing itself
with the economy of nature retreating more and more.  Many of us fear
this change.  Our religious feelings are dear to us, and we look with
suspicion and dislike on any philosophy, the apparent tendency of
which is to dry them up.  Probably every change from ancient savagery
to our present enlightenment has excited, in a greater or less degree,
fears of this kind.  But the fact is, that we have not yet determined
whether its present form is necessary to the life and warmth of
religious feeling.  We may err in linking the imperishable with the
transitory, and confound the living plant with the decaying pole to
which it clings.  My object, however, at present is not to argue, but
to mark a tendency.  We have ceased to propitiate the powers of
nature--ceased even to pray for things in manifest contradiction to
natural laws.  In Protestant countries, at least, I think it is
conceded that the age of miracles is past.

At an auberge near the foot of the Rhone glacier, I met, in the summer
of 1858, an athletic young priest, who, after a solid breakfast,
including a bottle of wine, informed me that he had come up to 'bless
the mountains.' This was the annual custom of the place.  Year by year
the Highest was entreated, by official intercessors, to make such
meteorological arrangements as should ensure food and shelter for the
flocks and herds of the Valaisians.  A diversion of the Rhone, or a
deepening of the river's bed, would, at the time I now mention, have
been of incalculable benefit to the inhabitants of the valley.  But
the priest would have shrunk from the idea of asking the Omnipotent to
open a new channel for the river, or to cause a portion of it to flow
over the Grimsel pass, and down the valley of Oberhasli to Brientz.
This he would have deemed a miracle, and he did not come to ask the
Creator to perform miracles, but to do something which he manifestly
thought lay quite within the bounds of the natural and non-miraculous.
A Protestant gentleman who was present at the time smiled at this
recital.  He had no faith in the priest's blessing; still, he deemed
his prayer different in kind from a request to open a new river-cut,
or to cause the water to flow up-hill.

In a similar manner the same Protestant gentleman would doubtless
smile at the honest Tyrolese priest, who, when he feared the bursting
of a glacier dam, offered the sacrifice of the Mass upon the ice as a
means of averting the calamity.  That poor man did not expect to
convert the ice into adamant, or to strengthen its texture, so as to
enable it to withstand the pressure of the water; nor did he expect
that his sacrifice would cause the stream to roll back upon its source
and relieve him, by a miracle, of its presence.  But beyond the
boundaries of his knowledge lay a region where rain was generated, he
knew not how.  He was not so presumptuous as to expect a miracle, but
he firmly believed that in yonder cloud-land matters could be so
arranged, without trespass on the miraculous, that the stream which
threatened him and his people should be caused to shrink within its
proper bounds.

Both these priests fashioned that which they did not understand to
their respective wants and wishes.  In their case imagination came
into play, uncontrolled by a knowledge of law.  A similar state of
mind was long prevalent among mechanicians.  Many of these, among whom
were to be reckoned men of consummate skill, were occupied a century
ago with the question of perpetual motion.  They aimed at constructing
a machine which should execute work without the expenditure of power;
and some of them went mad in the pursuit of this object.  The faith in
such a consummation, involving, as it did, immense personal profit to
the inventor, was extremely exciting, and every attempt to destroy
this faith was met by bitter resentment on the part of those who held
it.  Gradually, however, as men became more and more acquainted with
the true functions of machinery, the dream dissolved.  The hope of
getting work out of mere mechanical combinations disappeared: but
still there remained for the speculator a cloud-land denser than that
which filled the imagination of the Tyrolese priest, and out of which
he still hoped to evolve perpetual motion.  There was the mystic store
of chemic force, which nobody understood; there were heat and light,
electricity and magnetism, all competent to produce mechanical motion.
[Footnote: See Helmholtz: 'Wechselwirkung der Naturkräfte.']  Here,
then, was the mine in which our gem must be sought.  A modified and
more refined form of the ancient faith revived; and, for aught I know,
a remnant of sanguine designers may at the present moment be engaged
on the problem which like-minded men in former ages left unsolved.

And why should a perpetual motion, even under modern conditions, be
impossible?  The answer to this question is the statement of that
great generalisation of modern science, which is known under the name
of the Conservation of Energy.  This principle asserts that no power
can make its appearance in nature without an equivalent expenditure of
some other power; that natural agents are so related to each other as
to be mutually convertible, but that no new agency is created.  Light
runs into heat; heat into electricity; electricity into magnetism;
magnetism into mechanical force; and mechanical force again into light
and heat.  The Proteus changes, but he is ever the same; and his
changes in nature, supposing no miracle to supervene, are the
expression, not of spontaneity, but of physical necessity.  A
perpetual motion, then, is deemed impossible, because it demands the
creation of energy, whereas the principle of Conservation is--no
creation, but infinite conversion.

It is an old remark that the law which moulds a tear also rounds a
planet.  In the application of law in nature the terms great and small
are unknown.  Thus the principle referred to teaches us that the
Italian wind, gliding over the crest of the Matterhorn, is as firmly
ruled as the earth in its orbital revolution round the sun; and that
the fall of its vapour into clouds is exactly as much a matter of
necessity as the return of the seasons.  The dispersion, therefore, of
the slightest mist by the special volition of the Eternal, would be as
much a miracle as the rolling of the Rhone over the Grimsel
precipices, down the valley of Hash to Meyringen and Brientz.

It seems to me quite beyond the present power of science to
demonstrate that the Tyrolese priest, or his colleague of the Rhone
valley, asked for an 'impossibility' in praying for good weather; but
Science can demonstrate the incompleteness of the knowledge of nature
which limited their prayers to this narrow ground; and she may lessen
the number of instances in which we 'ask amiss,' by showing that we
sometimes pray for the performance of a miracle when we do not intend
it.  She does assert, for example, that without a disturbance of
natural law, quite as serious as the stoppage of an eclipse, or the
rolling of the river Niagara up the Falls, no act of humiliation,
individual or national, could call one shower from heaven, or deflect
towards us a single beam of the sun.

Those, therefore, who believe that the miraculous is still active in
nature, may, with perfect consistency, join in our periodic prayers
for fair weather and for rain: while those who hold that the age of
miracles is past, will, if they be consistent, refuse to join in these
petitions.  And these latter, if they wish to fall back upon such a
justification, may fairly urge that the latest conclusions of science
are in perfect accordance with the doctrine of the Master himself,
which manifestly was that the distribution of natural phenomena is not
affected by moral or religious causes.  'He maketh His sun to rise on
the evil and on the good, and sendeth rain on the just and on the
unjust.' Granting 'the power of Free Will in man,' so strongly claimed
by Professor Mansel in his admirable defence of the belief in
miracles, and assuming the efficacy of free prayer to produce changes
in external nature, it necessarily follows that natural laws are more
or less at the mercy of man's volition, and no conclusion founded on
the assumed permanence of those laws would be worthy of confidence.

It is a wholesome sign for England that she numbers among her clergy
men wise enough to understand all this, and courageous enough to act
up to their knowledge.  Such men do service to public character, by
encouraging a manly and intelligent conflict with the real causes of
disease and scarcity, instead of a delusive reliance on supernatural
aid.  But they have also a value beyond this local and temporary one.
They prepare the public mind for changes, which though inevitable,
could hardly, without such preparation, be wrought without violence.
Iron is strong; still, water in crystallising will shiver an iron
envelope, and the more unyielding the metal is, the worse for its
safety.  There are in the world men who would encompass philosophic
speculation by a rigid envelope, hoping thereby to restrain it, but in
reality giving it explosive force.  In England, thanks to men of the
stamp to which I have alluded, scope is gradually given to thought for
changes of aggregation, and the envelope slowly alters its form, in
accordance with the necessities of the time.

*****

The proximate origin of the foregoing slight article, and probably the
remoter origin of the next following one, was this.  Some years ago, a
day of prayer and humiliation, on account of a bad harvest, was
appointed by the proper religious authorities; but certain clergymen
of the Church of England, doubting the wisdom of the demonstration,
declined to join in the services of the day.  For this act of
nonconformity they were severely censured by some of their brethren.
Rightly or wrongly, my sympathies were on the side of these men; and,
to lend them a helping hand in their struggle against odds, I inserted
the foregoing chapter in a little book entitled 'Mountaineering in
1861.'  Some time subsequently I received from a gentleman of great
weight and distinction in the scientific world, and, I believe, of
perfect orthodoxy in the religious one, a note directing my attention
to an exceedingly thoughtful article on Prayer and Cholera in the
'Pall Mall Gazette.' My eminent correspondent deemed the article a
fair answer to the remarks made by me in 1861.  I, also, was struck by
the temper and ability of the article, but I could not deem its
arguments satisfactory, and in a short note to the editor of the 'Pall
Mall Gazette' I ventured to state so much.  This letter elicited some
very able replies, and a second leading article was also devoted to
the subject.  In answer to all, I risked the publication of a second
letter, and soon afterwards, by an extremely courteous note from the
editor, the discussion was closed.

Though thus stopped locally, the discussion flowed in other
directions.  Sermons were preached, essays were published, articles
were written, while a copious correspondence occupied the pages of
some of the religious newspapers.  It gave me sincere pleasure to
notice that the discussion, save in a few cases where natural
coarseness had the upper hand, was conducted with a minimum of
vituperation.  The severity shown was hardly more than sufficient to
demonstrate earnestness, while gentlemanly feeling was too predominant
to permit that earnestness to contract itself to bigotry or to clothe
itself in abuse.  It was probably the memory of this discussion which
caused another excellent friend of mine to recommend to my perusal the
exceedingly able work which in the next article I have endeavoured to
review.

Mr. Mozley's book belongs to that class of writing of which Butler may
be taken as the type.  It is strong, genuine argument about difficult
matters, fairly tracing what is difficult, fairly trying to grapple,
not with what appears the gist and strong point of a question, but
with what really at bottom is the knot of it.  It is a book the
reasoning of which may not satisfy everyone... But we think it is a
book for people who wish to see a great subject handled on a scale
which befits it, and with a perception of its real elements.  It is a
book which will have attractions for those who like to see a powerful
mind applying itself, without shrinking or holding back, without trick
or reserve or show of any kind, as a wrestler closes body to body with
his antagonist, to the strength of an adverse and powerful
argument.--Times, Tuesday, June 5, 1866.

We should add, that the faults of the work are wholly on the surface
and in the arrangement; that the matter is as solid and as logical as
that of any book within recent memory, and that it abounds in striking
passages, of which we have scarcely been able even to give a sample.
No future arguer against miracles can afford to pass it
over.--SATURDAY REVIEW, September 15, 1866.

********************

II  MIRACLES AND SPECIAL PROVIDENCES.

[Footnote: Fortnightly Review, New Series, vol. i. p. 645.]

1867.

IT is my privilege to enjoy the friendship of a select number of
religious men, with whom I converse frankly upon theological subjects,
expressing without disguise the notions and opinions I entertain
regarding their tenets, and hearing in return these notions and
opinions subjected to criticism.  I have thus far found them liberal
and loving men, patient in hearing, tolerant in reply, who know how to
reconcile the duties of courtesy with the earnestness of debate.  From
one of these, nearly a year ago, I received a note, recommending
strongly to my attention the volume of 'Bampton Lectures' for 1865, in
which the question of miracles is treated by Mr. Mozley.  Previous to
receiving this note, I had in part made the acquaintance of the work
through an able and elaborate review of it in the 'Times.' The
combined effect of the letter and the review was to make the book the
companion of my summer tour in the Alps.  There, during the wet and
snowy days which were only too prevalent in 1866, and during the days
of rest interpolated between days of toil, I made myself more
thoroughly conversant with Mr. Mozley's volume.  I found it clear and
strong--an intellectual tonic, as bracing and pleasant to my mind as
the keen air of the mountains was to my body.  From time to time I
jotted down thoughts regarding it, intending afterwards to work them
up into a coherent whole.  Other duties, however, interfered with the
complete carrying out of this intention, and what I wrote last summer
I now publish, not hoping to be able, within any reasonable time, to
render my defence of scientific method more complete.

Mr. Mozley refers at the outset of his task to the movement against
miracles which of late years has taken place, and which determined his
choice of a subject.  He acquits modern science of having had any
great share in the production of this movement.  The objection against
miracles, he says, does not arise from any minute knowledge of the
laws of nature, but simply because they are opposed to that plain and
obvious order of nature which everybody sees.  The present movement
is, he thinks, to be ascribed to the greater earnestness and
penetration of the present age.  Formerly miracles were accepted
without question, because without reflection; but the exercise of the
'historic imagination' is a characteristic of our own time.  Men are
now accustomed to place before themselves vivid images of historic
facts; and when a miracle rises to view, they halt before the
astounding occurrence, and, realising it with the same clearness as if
it were now passing before their eyes, they ask themselves, 'Can this
have taken place?' In some instances the effort to answer this
question has led to a disbelief in miracles, in others to a
strengthening of belief.  The aim of Mr. Mozley's lectures is to show
that the strengthening of belief is the logical result which ought to
follow from the examination of the facts.

Attempts have been made by religious men to bring the Scripture
miracles within the scope of the order of nature, but all such
attempts are rejected by Mr. Mozley as utterly futile and wide of the
mark. Regarding miracles as a necessary accompaniment of a revelation,
their evidential value in his eyes depends entirely upon their
deviation from the order of nature.  Thus deviating, they suggest and
illustrate a power higher than nature, a 'personal will;' and they
commend the person in whom this power is vested as a messenger from on
high.  Without these credentials such a messenger would have no right
to demand belief, even were his assertions regarding his Divine
mission backed by a holy life.  Nor is it by miracles alone that the
order of nature is, or may be, disturbed.  The material universe is
also the arena of 'special providences.'  Under these two heads Mr.
Mozley distributes the total preternatural.  One form of the
preternatural may shade into the other, as one colour passes into
another in the rainbow; but, while the line which divides the
specially providential from the miraculous cannot be sharply drawn,
their distinction broadly expressed is this: that, while a special
providence can only excite surmise more or less probable, it is 'the
nature of a miracle to give proof, as distinguished from surmise, of
Divine design.'

Mr. Mozley adduces various illustrations of what he regards to be
special providences, as distinguished from miracles.  'The death of
Arius,' he says, 'was not miraculous, because the coincidence of the
death of a heresiarch taking place when it was peculiarly advantageous
to the orthodox faith ... was not such as to compel the inference
of extraordinary Divine agency; but it was a special providence,
because it carried a reasonable appearance of it.  The miracle of the
Thundering Legion was a special providence, but not a miracle, for
the same reason, because the coincidence of an instantaneous fall of
rain, in answer to prayer, carried some appearance, but not proof, of
preternatural agency.'

The eminent lecturer's remarks on this head brought to my recollection
certain narratives published in Methodist magazines, which I used to
read with avidity when a boy.  The general title of these exciting
stories, if I remember right, was 'The Providence of God asserted,'
and in them the most extraordinary escapes from peril were recounted
and ascribed to prayer, while equally wonderful instances of calamity
were adduced as illustrations of Divine retribution.  In such
magazines, or elsewhere, I found recorded the case of the celebrated
Samuel Hick, which, as it illustrates a whole class of special
providences approaching in conclusiveness to miracles, is worthy of
mention here.  It is related of this holy man that, on one occasion,
flour was lacking to make the sacramental bread.  Grain was present,
and a windmill was present, but there was no wind to grind the corn.
With faith undoubting, Samuel Hick prayed to the Lord of the winds:
the sails turned, the corn was ground, after which the wind ceased.
According to the canon of the Bampton Lecturer, this, though carrying
a strong appearance of an immediate exertion of Divine energy, lacks
by a hair's-breadth the quality of a miracle.  For the wind _might_ have
arisen, and _might_ have ceased, in the ordinary course of nature. Hence
the occurrence did not 'compel the inference of extraordinary Divine
agency.'  In like manner Mr. Mozley considers that 'the appearance of
the cross to Constantine was a miracle, or a special providence,
according to what account of it we adopt.  As only a meteoric
appearance in the shape of a cross it gave some token of preternatural
agency, but not full evidence.'

In the Catholic canton of Switzerland where I now write, and still
more among the pious Tyrolese, the mountains are dotted with shrines,
containing offerings of all kinds, in acknowledgment of special
mercies--legs, feet, arms, and hands--of gold, silver, brass, and
wood, according as worldly possessions enabled the grateful heart to
express its indebtedness.  Most of these offerings are made to the
Virgin Mary.  They are recognitions of 'special providences,' wrought
through the instrumentality of the Mother of God.  Mr. Mozley's
belief, that of the Methodist chronicler, and that of the Tyrolese
peasant, are substantially the same.  Each of them assumes that
nature, instead of flowing ever onward in the uninterrupted rhythm of
cause and effect, is mediately ruled by the free human will.  As
regards _direct_ action upon natural phenomena, man's wish and will, as
expressed in prayer, are confessedly powerless; but prayer is the
trigger which liberates the Divine power, and to this extent, if the
will be free, man, of course, commands nature.

Did the existence of this belief depend solely upon the material
benefits derived from it, it could not, in my opinion, last a decade.
As a purely objective fact, we should soon see that the distribution
of natural phenomena is unaffected by the merits or the demerits of
men; that the law of gravitation crushes the simple worshippers of
Ottery St. Mary, while singing their hymns, just as surely as if they
were engaged in a midnight brawl.  The hold of this belief upon the
human mind is not due to outward verification, but to the inner
warmth, force, and elevation with which it is commonly associated.  It
is plain, however, that these feelings may exist under the most
various forms.  They are not limited to Church of England
Protestantism--they are not even limited to Christianity.  Though less
refined, they are certainly not less strong in the heart of the
Methodist and the Tyrolese peasant than in the heart of Mr. Mozley.
Indeed, those feelings belong to the primal powers of man's nature.  A
'sceptic' may have them.  They find vent in the battle-cry of the
Moslem.  They take hue and form in the hunting-grounds of the Red
Indian; and raise all of them, as they raise the Christian, upon a
wave of victory, above the terrors of the grave.

The character, then, of a miracle, as distinguished from a special
providence, is that the former furnishes _proof_, while in the case of
the latter we have only surmise.  Dissolve the element of doubt, and
the alleged fact passes from the one class of 'the preternatural into
the other.  In other words, if a special providence could be proved to
be a special providence, it would cease to be a special providence and
become a miracle.  There is not the least cloudiness about Mr.
Mozley's meaning here.  A special providence is a doubtful miracle.
Why, then, not call it so?  The term employed by Mr. Mozley conveys no
negative suggestion, whereas the negation of certainty is the peculiar
characteristic of the thing intended to be expressed.  There is an
apparent unwillingness on the part of the lecturer to call a special
providence what his own definition makes it to be.  Instead of
speaking of it as a doubtful miracle, he calls it 'an invisible
miracle.' He speaks of the point of contact of supernatural power with
the chain of causation being so high up as to be wholly, or in part,
out of sight, whereas the essence of a special providence is the
uncertainty whether there is any contact at all, either high or low.
By the use of an incorrect term, however, a grave danger is avoided.
For the idea of doubt, if kept systematically before the mind, would
soon be fatal to the special providence, considered as a means of
edification.  The term employed, on the contrary, invites and
encourages the trust which is necessary to supplement the evidence.

This inner trust, though at first rejected by Mr. Mozley in favour of
external proof, is subsequently called upon to do momentous duty in
regard to miracles.  Whenever the evidence of the miraculous seems
incommensurate with the fact which it has to establish, or rather
when the fact is so amazing that hardly any evidence is sufficient to
establish it, Mr. Mozley invokes 'the affections.'  They must urge the
reason to accept the conclusion, from which unaided it recoils.  The
affections and emotions are eminently the court of appeal in matters
of real religion, which is an affair of the heart; but they are not, I
submit, the court in which to weigh allegations regarding the
credibility of physical facts.  These must be judged by the dry light
of the intellect alone, appeals to the affections being reserved for
cases where moral elevation, and not historic conviction, is the aim.
It is, moreover, because the result, in the case under consideration,
is deemed desirable that the affections are called upon to back it. If
undesirable, they would, with equal right, be called upon to act the
other way.  Even to the disciplined scientific mind this would be a
dangerous doctrine.  A favourite theory--the desire to establish or
avoid a certain n result--can so warp the mind as to destroy its
powers of estimating facts.  I have known men to work for years under
a fascination of this kind, unable to extricate themselves from its
fatal influence.  They had certain data, but not, as it happened,
enough.  By a process exactly analogous to that invoked by Mr. Mozley,
they supplemented the data, and went wrong.  From that hour their
intellects were so blinded to the perception of adverse phenomena that
they never reached truth.  If, then, to the disciplined scientific
mind, this incongruous mixture of proof and trust be fraught with
danger, what must it be to the indiscriminate audience which.  Mr.
Mozley addresses?  In calling upon this agency he acts the part of
Frankenstein.  It is a monster thus evoked that we see stalking
abroad, in the degrading spiritualistic phenomena of the present day.
Again, I say, where the aim is to elevate the mind, to quicken the
moral sense, to kindle the fire of religion in the soul, let the
affections by all means be invoked; but they must not be permitted to
colour our reports, or to influence our acceptance of reports of
occurrences in external nature.  Testimony as to natural facts is
worthless when wrapped in this atmosphere of the affections; the most
earnest subjective truth being thus rendered perfectly compatible with
the most astounding objective error.

There are questions in judging of which the affections or sympathies
are often our best guides, the estimation of moral goodness being one
of these.  But at this precise point, where they are really of use,
Mr. Mozley excludes the affections and demands a miracle as a
certificate of character.  He will not accept any other evidence of
the perfect goodness of Christ. 'No outward life and conduct,' he
says, 'however irreproachable, could prove His perfect sinlessness,
because goodness depends upon the inward motive, and the perfection of
the inward motive is not proved by the outward act.' But surely the
miracle is an outward act, and to pass from it to the inner motive
imposes a greater strain upon logic than that involved in our
ordinary methods of estimating men.  There is, at least, moral
congruity between the outward goodness and the inner life, but there
is no such congruity between the miracle and the life within.  The
test of moral goodness laid down by Mr. Mozley is not the test of
John, who says, 'He that doeth righteousness is righteous; 'nor is it
the test of Jesus: 'By their fruits ye shall know them: do men gather
grapes of thorns, or figs of thistles?'  But it is the test of
another: 'If thou be the Son of God, command that these stones be
made bread.' For my own part, I prefer the attitude of Fichte to that
of Mr. Mozley.  The Jesus of John,' says this noble and mighty
thinker, knows no other God than the True God, in whom we all are, and
live, and may be blessed, and out of whom there is only Death and
Nothingness.  And,' continues Fichte, 'he appeals, and rightly
appeals, in support of this truth, not to reasoning, but to the inward
practical sense of truth in man, not even knowing any other proof than
this inward testimony, "If any man will do the will of Him who sent
Me, he shall know of the doe-trine whether it be of God."'

Accepting Mr. Mozley's test, with which alone I am now dealing, it is
evident that, in the demonstration of moral goodness, the _quantity_ of
the miraculous comes into play.  Had Christ, for example, limited
himself to the conversion of water into wine, He would have fallen
short of the performance of Jannes and Jambres; for it is a smaller
thing to convert one liquid into another than to convert a dead rod
into a living serpent.  But Jannes and Jambres, we are informed, were
not good.  Hence, if Mr. Mozley's test be a true one, a point must
exist, on the one side of which miraculous power demonstrates
goodness, while on the other side it does not.  How is this 'point of
contrary flexure' to be determined?  It must lie somewhere between the
magicians and Moses, for within this space the power passed from the
diabolical to the Divine.  But how to mark the point of passage--how,
out of a purely _quantitative_ difference in the visible manifestation
of power, we are to infer a total inversion of quality--it is
extremely difficult to see.  Moses, we are informed, produced a large
reptile; Jannes and Jambres produced a small one.  I do not possess
the intellectual faculty which would enable me to infer, from those
data, either the goodness of the one or the badness of the other; and
in the highest recorded manifestations of the miraculous I am equally
at a loss.  Let us not play fast and loose with the miraculous; either
it is a demonstration of goodness in all cases or in none.  If Mr.
Mozley accepts Christ's goodness as transcendent, because He did such
works as no other man did, he ought, logically speaking, to accept the
works of those who, in His name, had cast out devils, as demonstrating
a proportionate goodness on their part.  But it is people of this
class who are consigned to ever-lasting fire prepared for the devil
and his angels.  Such zeal as that of Mr. Mozley for miracles tends, I
fear, to eat his religion up.  The logical threatens to stifles the
spiritual.  The truly religious soul needs no miraculous proof of the
goodness of Christ. The words addressed to Matthew at the receipt of
custom required no miracle to produce obedience.  It was by no stroke
of the supernatural that Jesus caused those sent to seize Him to go
backward and fall to the ground.  It was the sublime and holy
effluence from within, which needed no prodigy to commend it to the
reverence even of his foes.

As regards the function of miracles in the founding of a religion, Mr.
Mozley institutes a comparison between the religion of Christ and that
of Mahomet; and he derides the latter as 'irrational' because it does
not profess to adduce miracles in proof of its supernatural origin.
But the religion of Mahomet, notwithstanding this drawback, has
thriven in the world, and at one time it held sway over larger
populations than Christianity itself.  The spread and influence of
Christianity are, however, brought forward by Mr. Mozley as 'a
permanent, enormous, and incalculable practical result' of Christian
miracles; and he makes use of this result to strengthen his plea for
the miraculous.  His logical warrant for this proceeding is not clear.
It is the method of science, when a phenomenon presents itself,
towards the production of which several elements may contribute, to
exclude them one by one, so as to arrive at length at the truly
effective cause.  Heat, for example, is associated with a phenomenon;
we exclude heat, but the phenomenon remains: hence, heat is not its
cause.  Magnetism is associated with a phenomenon; we exclude
magnetism, but the phenomenon remains: hence, magnetism is not its
cause.  Thus, also, when we seek the cause of a diffusion of a
religion--whether it be due to miracles, or to the spiritual force of
its founders--we exclude the miracles, and, finding the result
unchanged, we infer that miracles are not the effective cause.  This
important experiment Mahometanism has made for us.  It has lived and
spread without miracles; and to assert, in the face of this, that
Christianity has spread _because_ of miracles, is, I submit, opposed
both to the spirit of science and the common sense of mankind.

The incongruity of inferring moral goodness from miraculous power has
been dwelt upon above; in another particular also the strain put by
Mr. Mozley upon miracles is, I think, more than they can bear.  In
consistency with his principles, it is difficult to see how he is to
draw from the miracles of Christ any certain conclusion as to His
Divine nature.  He dwells very forcibly on what he calls 'the
argument from experience,' in the demolition of which he takes obvious
delight.  He destroys the argument, and repeats it, for the mere
Pleasure of again and again knocking the breath out of it. Experience,
he urges, can only deal with the past; and the moment we attempt to
project experience a hair's-breadth beyond the point it has at any
moment reached, we are condemned by reason.  It appears to me that
when he infers from Christ's miracles a Divine and altogether
superhuman energy, Mr. Mozley places himself precisely under this
condemnation.  For what is his logical ground for concluding that the
miracles of the New Testament illustrate Divine power?  May they not
be the result of expanded human power?  A miracle he defines as
something impossible to man.  But how does he know that the miracles
of the New Testament are impossible to man?  Seek as he may, he has
absolutely no reason to adduce save this--that man has never hitherto
accomplished such things.  But does the fact that man _has_ never raised
the dead prove that he _can_ never raise the dead?  'Assuredly not,'
must be Mr. Mozley's reply; 'for this would be pushing experience
beyond the limit it has now reached--which I pronounce unlawful.' Then
a period may come when man will be able to raise the dead.  If this be
conceded--and I do not see how Mr. Mozley can avoid the concession--it
destroys the necessity of inferring Christ's Divinity from His
miracles.  He, it may be contended, antedated the humanity of the
future; as a mighty tidal wave leaves high upon the beach a mark which
by-and-by becomes the general level of the ocean.  Turn the matter as
you will, no other warrant will be found for the all-important
conclusion that Christ's miracles demonstrate Divine power, than an
argument which has been stigmatised by Mr. Mozley as a 'rope of
sand'--the argument from experience.

The learned Bampton Lecturer would be in this position, even had he
seen with his own eyes every miracle recorded in the New Testament.
But he has, not seen these miracles; and his intellectual plight is
therefore worse.  He accepts these miracles on testimony.  Why does he
believe that testimony?  How does he know that it is not delusion; how
is he sure that it is not even fraud?  He will answer, that the
writing bears the marks of sobriety and truth; and that in many cases
the bearers of this message to mankind sealed it with their blood.
Granted with all my heart; but whence the value of all this?  Is it
not solely derived from the fact that men, _as we know them_, do not
sacrifice their lives in the attestation of that which they know to be
untrue?  Does not the entire value of the testimony of the Apostles
depend ultimately upon our experience of human nature?  It appears,
then, that those said to have seen the miracles, based their
inferences from what they saw on the argument from experience; and
that Mr. Mozley bases his belief in their testimony on the same
argument.  The weakness of his conclusion is quadrupled by this double
insertion of a principle of belief, to which he flatly denies
rationality.  His reasoning, in fact, cuts two ways--if it destroys
our trust in the order of nature, it far more effectually abolishes
the basis on which Mr. Mozley seeks to found the Christian religion.

*****

Over this argument from experience, which at bottom is _his_ argument,
Mr. Mozley rides rough-shod.  There is a dash of scorn in the energy
with which he tramples on it.  Probably some previous writer had made
too much of it, and thus invited his powerful assault.  Finding the
difficulty of belief in miracles to rise from their being in
contradiction to the order of nature, he sets himself to examine the
grounds of our belief in that order.  With a vigour of logic rarely
equalled, and with a confidence in its conclusions never surpassed, he
disposes of this belief in a manner calculated to startle those who,
without due examination, had come to the conclusion that the order of
nature was secure.  What we mean, he says, by our belief in the order
of nature, is the belief that the future will be like the past. There
is not, according to Mr. Mozley, the slightest rational basis for this
belief.

That any cause in nature is more permanent than its existing and known
effects, extending further, and about to produce other and more
instances besides what it has produced already, we have no evidence.
Let us imagine,' he continues, 'the occurrence of a particular
physical phenomenon for the first time.  Upon that single occurrence
we should have but the very faintest expectation of another.  If it
did occur again, once or twice, so far from counting on another
occurrence, a cessation would occur as the most natural event to us.
But let it continue one hundred times, and we should find no
hesitation in inviting persons from a distance to see it; and if it
occurred every day for years, its occurrence would be a certainty to
us, its cessation a marvel... What ground of reason can we assign for
an expectation that any part of the course of nature will be the next
moment what it has been up to this moment, i.e. for our belief in the
uniformity of nature?  None.  No demonstrative reason can be given,
for the contrary to the recurrence of a fact of nature is no
contradiction.  No probable reason can be given; for all probable
reasoning respecting the course of nature is founded _upon_ this
presumption of likeness, and therefore cannot be the foundation of it.
No reason can be given for this belief.  It is without a reason.  It
rests upon no rational grounds, and can be traced to no rational
principle.'

*****

'Everything,' Mr. Mozley, however, adds, 'depends upon this belief,
every provision we make for the future, every safeguard and caution we
employ against it, all calculation, all adjustment of means to ends,
supposes this belief; and yet this belief has no more producible
reason for it than a speculation of fancy.  It is necessary,
all-important for the purposes of life, but solely practical, and
possesses no intellectual character.

'... The proper function,' continues Mr. Mozley, 'of the inductive
principle, the argument from experience, the belief in the order of
nature--by whatever phrase we designate the same instinct--is to
operate as a practical basis for the affairs of life and the carrying
on of human society.' To sum up, the belief in the order of nature is
general, but it is 'an unintelligent impulse, of which we can give no
rational account.' It is inserted into our constitution solely to
induce us to till our fields, to raise our winter fuel, and thus to
meet the future on the perfectly gratuitous supposition that it will
be like the past.

'Thus, step by step,' says Mr. Mozley, with the emphasis of a man who
feels his position to be a strong one, 'has philosophy loosened the
connection of the order of nature with the ground of reason,
befriending in exact proportion as it has done this the principle of
miracles.' For 'this belief not having itself a foundation in reason,
the ground is gone upon which it could be maintained that miracles, as
opposed to the order of nature, are opposed to reason.' When we regard
this belief in connection with science, 'in which connection it
receives a more imposing name, and is called the inductive principle,'
the result is the same.  'The inductive principle is only this
unreasoning impulse applied to a scientifically ascertained fact...
Science has led up to the fact; but there it stops, and for converting
this fact into a law, a totally unscientific principle comes into
play, the same as that which generalises the commonest observation of
nature.'

The eloquent pleader of the cause of miracles passes over without a
word the _results_ of scientific investigation, as proving anything
rational regarding the principles or method by which such results have
been achieved.  Here, as elsewhere, he declines the test, 'By their
fruits shall ye know them.' Perhaps our best way of proceeding will be
to give one or two examples of the mode in which men of science apply
the unintelligent impulse with which Mr. Mozley credits them, and
which shall show, by illustration, the surreptitious method whereby
they climb from the region of facts to that of laws.

Before the sixteenth century it was known that water rises in a pump;
the effect being then explained by the maxim that 'Nature abhors a
vacuum.' It was not known that there was any limit to the height to
which the water would ascend, until, on one occasion, the gardeners of
Florence, while attempting to raise water to a very great elevation,
found that the column ceased at a height of thirty-two feet.  Beyond
this all the skill of the pump-maker could not get it to rise.  The
fact was brought to the notice of Galileo, and he, soured by a world
which had not treated his science over kindly, is said to have twitted
the philosophy of the time by remarking that nature evidently abhorred
a vacuum only to a height of thirty-two feet.  Galileo, however, did
not solve the problem.  It was taken up by his pupil Torricelli, to
whom, after due pondering, the thought occurred, that the water might
be forced into the tube by a pressure applied to the surface of the
liquid outside.  But where, under the actual circumstances, was such a
pressure to be found?  After much reflection, it flashed upon
Torricelli that the atmosphere might possibly exert this pressure;
that the impalpable air might possess weight, and that a column of
water thirty-two feet high might be of the exact weight necessary to
hold the pressure of the atmosphere in equilibrium.

There is much in this process of pondering and its results which it is
impossible to analyse.  It is by a kind of inspiration that we rise
from the wise and sedulous contemplation of facts to the principles on
which they depend.  The mind is, as it were, a photographic plate,
which is gradually cleansed by the effort to think rightly, and which,
when so cleansed, and not before, receives impressions from the light
of truth.  This passage from 'facts to principles is called induction;
and induction, in its highest form, is, as I have just stated, a kind
of inspiration.  But, to make it sure, the inward sight must be shown
to be in accordance with outward fact.  To prove or disprove the
induction, we must resort to deduction and experiment.

Torricelli reasoned thus: If a column of water thirty-two feet high
holds the pressure of the atmosphere in equilibrium, a shorter column
of a heavier liquid ought to do the same.  Now, mercury is thirteen
times heavier than water; hence, if my induction be correct, the
atmosphere ought to be able to sustain only thirty inches of mercury.
Here, then, is a deduction which can be immediately submitted to
experiment. Torricelli took a glass tube a yard or so in length,
closed at one end and open at the other, and filling it with mercury,
he stopped the open end with his thumb, and inverted it into a basin
filled with the liquid metal.  One can imagine the feeling with which
Torricelli removed his thumb, and the delight he experienced on
finding that his thought had forestalled a fact never before revealed
to human eyes.  The column sank, but it ceased to sink at a height of
thirty inches, leaving the Torricellian vacuum over-head.  From that
hour the theory of the pump was established.

The celebrated Pascal followed Torricelli with another deduction.  He
reasoned thus: If the mercurial column be supported by the atmosphere,
the higher we ascend in the air, the lower the column ought to sink,
for the less will be the weight of the air overhead.  He caused a
friend to ascend the Puy de Dôme, carrying with him a barometric
column; and it was found that during the ascent the column sank, and
that during the subsequent descent the column rose.

Between the time here referred to and the present, millions of
experiments have been made upon this subject.  Every village pump is
an apparatus for such experiments.  In thousands of instances,
moreover, pumps have refused to work; but on examination it has
infallibly been found that the well was dry, that the pump required
priming, or that some other defect in the apparatus accounted for the
anomalous action.  In every case of the kind the skill of the
pump-maker has been found to be the true remedy.  In no case has the
pressure of the atmosphere ceased; constancy, as regards the lifting
of pump-water, has been hitherto the demonstrated rule of nature.  So
also as regards Pascal's experiment.  His experience has been the
universal experience ever since.  Men have climbed mountains, and gone
up in balloons; but no deviation from Pascal's result has ever been
observed.  Barometers, like pumps, have refused to act; but instead of
indicating any suspension of the operations of nature, or any
interference on the part of its Author with atmospheric pressure,
examination has in every instance fixed the anomaly upon the
instruments themselves.  It is this welding, then, of rigid logic to
verifying fact that Mr. Mozley refers to an 'unreasoning impulse.'

Let us now briefly consider the case of Newton.  Before his time men
had occupied themselves with the problem of the solar system.  Kepler
had deduced, from a vast mass of observations, those general
expressions of planetary motion known as 'Kepler's laws.' It had been
observed that a magnet attracts iron; and by one of those flashes of
inspiration which reveal to the human mind the vast in the minute, the
general in the particular, it had been inferred, that the force by
which bodies fall to the earth might also be an attraction.  Newton
pondered all these things.  He looked, as was his wont, into the
darkness until it became entirely luminous.  How this light arises we
cannot explain; but, as a matter of fact, it does arise.  Let me
remark here, that this kind of pondering is a process with which the
ancients could have been but imperfectly acquainted.  They, for the
most part, found the exercise of fantasy more pleasant than careful
observation, and subsequent brooding over facts.  Hence it is, that
when those whose education has been derived from the ancients speak of
'the reason of man,' they are apt to omit from their conception of
reason one of its most important factors.

Well, Newton slowly marshalled his thoughts, or rather they came to
him while he 'intended his mind,' rising like a series of
intellectual births out of chaos.  He made this idea of attraction his
own.  But, to apply the idea to the solar system, it was necessary to
know the magnitude of the attraction, and the law of its variation
with the distance.  His conceptions first of all passed from the
action of the earth as a whole, to that of its constituent particles.
And persistent thought brought more and more clearly out the final
conclusion, that every particle of matter attracts every other
particle with a force varying inversely as the square of the distance
between the particles.

Here we have the flower and outcome of Newton's induction; and how to
verify it, or to disprove it, was the next question.  The first step
of the philosopher in this direction was to prove, mathematically,
that if this law of attraction be the true one; if the earth be
constituted of particles which obey this law; then the action of a
sphere equal to the earth in size on a body outside of it, is the same
as that which would be exerted if the whole mass of the sphere were
contracted to a point at its centre.  Practically speaking, then, the
centre of the earth is the point from which distances must be measured
to bodies attracted by the earth.

From experiments executed before his time, Newton knew the amount of
the earth's attraction at the earth's surface, or at a distance of
4,000 miles from its centre.  His object now was to measure the
attraction at a greater distance, and thus to determine the law of its
diminution.  But how was he to find a body at a sufficient distance?
He had no balloon?  and even if he had, he knew that any height to
which he could attain would be too small to enable him to solve his
problem.  What did he do?  He fixed his thoughts upon the moon;--a
body 240,000 miles, or sixty times the earth's radius, from the
earth's centre.  He virtually weighed the moon, and found that weight
to be 1/3600th of what it would be at the earth's surface.  This is
exactly what his theory required.  I will not dwell here upon the
pause of Newton after his first calculations, or speak of his
self-denial in withholding them because they did not quite agree with
the observations then at his command.  Newton's action in this matter
is the normal action of the scientific mind.  If it were otherwise--if
scientific men were not accustomed to demand verification--if they
were satisfied with the imperfect while the perfect is attainable,
their science, instead of being, as it is, a fortress of adamant,
would be a house of clay, ill-fitted to bear the buffetings of the
theologic storms to which it is periodically exposed.

Thus we see that Newton, like Torricelli, first pondered his facts,
illuminated them with persistent thought, and finally divined the
character of the force of gravitation.  But, having thus travelled
inward to the principle, he reversed his steps, carried the principle
outwards, and justified it by demonstrating its fitness to external
nature.

And here, in passing, I would notice a point which is well worthy of
attention.  Kepler had deduced his laws from observation.  As far back
as those observations extended, the planetary motions had obeyed these
laws; and neither Kepler nor Newton entertained a doubt as to their
continuing to obey them.  Year after year, as the ages rolled, they
believed that those laws would continue to illustrate themselves in
the heavens.  But this was not sufficient.  The scientific mind can
find no repose in the mere registration of sequence in nature.  The
further question intrudes itself with resistless might, Whence comes
the sequence?  What is it that binds the consequent to its antecedent
in nature?  The truly scientific intellect never can attain rest until
it reaches the _forces_ by which the observed succession is produced. It
was thus with Torricelli; it was thus with Newton; it is thus
pre-eminently with the scientific man of to-day.  In common with the
most ignorant, he shares the belief that spring will succeed winter,
that summer will succeed spring, that autumn will succeed summer, and
that winter will succeed autumn.  But he knows still further--and this
knowledge is essential to his intellectual repose--that this
succession, besides being permanent, is, under the circumstances,
_necessary_; that the gravitating force exerted between the sun and a
revolving sphere with an axis inclined to the plane of its orbit, must
produce the observed succession of the seasons.  Not until this
relation between forces and phenomena has been established, is the law
of reason rendered concentric with the law of nature; and not until
this is effected does the mind of the scientific philosopher rest in
peace.

The expectation of likeness, then, in the procession of phenomena, is
not that on which the scientific mind founds its belief in the order
of nature.  If the force be _permanent_ the phenomena are _necessary_,
whether they resemble or do not resemble anything that has gone
before.  Hence, in judging of the order of nature, our enquiries
eventually relate to the permanence of force.  From Galileo to Newton,
from Newton to our own time, eager eyes have been scanning the
heavens, and clear heads have been pondering the phenomena of the
solar system.  The same eyes and minds have been also observing,
experimenting, and reflecting on the action of gravity at the surface
of the earth.  Nothing has occurred to indicate that the operation of
the law has for a moment been suspended; nothing has ever intimated
that nature has been crossed by spontaneous action, or that a state of
things at any time existed which could not be rigorously deduced from
the preceding state.

Given the distribution of matter, and the forces in operation, in the
time of Galileo, the competent mathematician of that day could predict
what is now occurring in our own.  We calculate eclipses in advance,
and find our calculations true to the second.  We determine the dates
of those that have occurred in the early times of history, and find
calculation and history in harmony.  Anomalies and perturbations in
the planets have been over and over again observed; but these, instead
of demonstrating any inconstancy on the part of natural law, have
invariably been reduced to consequences of that law.  Instead of
referring the perturbations of Uranus to any interference on the part
of the Author of nature with the law of gravitation, the question
which the astronomer proposed to himself was, 'How, in accordance with
this law, can the perturbation be produced?' Guided by a principle, he
was enabled to fix the point of space in which, if a mass of matter
were placed, the observed perturbations would follow.  We know the
result.  The practical astronomer turned his telescope towards the
region which the intellect of the theoretic astronomer had already
explored, and the Planet now named Neptune was found in its predicted
Place.  A very respectable outcome, it will be admitted, of an impulse
which 'rests upon no rational grounds, and can be traced to no
rational principle;' which possesses 'no intellectual character;'
which 'philosophy' has uprooted from 'the ground of reason,' and fixed
in that 'large irrational department' discovered for it, by Mr.
Mozley, in the hitherto unexplored wilderness of the human mind.

The proper function of the inductive principle, or the belief in the
order of nature, says Mr. Mozley, is 'to act as a practical basis for
the affairs of life, and the carrying on of human society.' But what,
it may be asked, has the planet Neptune, or the belts of Jupiter, or
the whiteness about the poles of Mars, to do with the affairs of
society?  How is society affected by the fact that the sun's
atmosphere contains sodium, or that the nebula of Orion contains
hydrogen gas?  Nineteen-twentieths of the force employed in the
exercise of the inductive principle, which, reiterates Mr. Mozley, is
'purely practical,' have been expended upon subjects as unpractical as
these.  What practical interest has society in the fact that the spots
on the sun have a decennial period, and that when a magnet is closely
watched for half a century, it is found to perform small motions which
synchronise with the appearance and disappearance of the solar spots?
And yet, I doubt not, Sir Edward Sabine would deem a life of
intellectual toil amply rewarded by being privileged to solve, at its
close, these infinitesimal motions.  The inductive principle is
founded in man's desire to know--a desire arising from his position
among phenomena which are reducible to order by his intellect: The
material universe is the complement of the intellect; and, without the
study of its laws, reason could never have awakened to the higher
forms of self-consciousness at all.  It is the Non-ego through and by
which the Ego is endowed with self-discernment. We hold it to be an
exercise of reason to explore the meaning of a universe to which we
stand in this relation, and the work we have accomplished is the
proper commentary on the methods we have pursued.

Before these methods were adopted the unbridled imagination roamed
through nature, putting in the place of law the figments of
superstitious dread.  For thousands of years witchcraft, and magic,
and miracles, and special providences, and Mr. Mozley's 'distinctive
reason of man,' had the world to themselves.  They made worse than
nothing of it--_worse_, I say, because they let and hindered those who
might have made something of it.  Hence it is, that during a single
lifetime of this era of 'unintelligent impulse,' the progress in
knowledge is all but infinite as compared with that of the ages which
preceded ours.

The believers in magic and miracles of a couple of centuries ago had
all the strength of Mr. Mozley's present logic on their side.  They
had done for themselves what he rejoices in having so effectually done
for us--cleared the ground of the belief in the order of nature, and
declared magic, miracles, and witchcraft to be matters for 'ordinary
evidence' to decide.  'The principle of miracles' thus 'befriended'
had free scope, and we know the result.  Lacking that rock-barrier of
natural knowledge which we now possess, keen jurists and cultivated
men were hurried on to deeds, the bare recital of which makes the
blood run cold.  Skilled in all the rules of human evidence, and
versed in all the arts of cross-examination, these men, nevertheless,
went systematically astray, and committed the deadliest wrongs against
humanity.  And why?  Because they could not put Nature into the
witness-box, and question her--of her voiceless 'testimony' they knew
nothing.  In all cases between man and man, their judgment was to be
relied on; but in all cases between man and nature, they were blind
leaders of the blind. [Footnote: 'In 1664 two women were hung in
Suffolk, under a sentence of Sir Matthew Hale, who took the
opportunity of declaring that the reality of witchcraft was
unquestionable; "for first, the Scriptures had affirmed so much; and
secondly, the wisdom of all nations had provided laws against such
persons, which is an argument of their confidence of such a crime."
Sir Thomas Browne, who was a great physician as well as a great
writer, was called as a witness, and swore "that he was clearly of
opinion that the persons were bewitched."--Lecky's History of
Rationalism, vol. i. p. 120.]

Mr. Mozley concedes that it would be no great result if miracles were
only accepted by the ignorant and superstitious, 'because it is easy
to satisfy those who do not enquire.'  But he does consider it 'a
great result' that they have been accepted by the educated.  In what
sense educated?  Like those statesmen, jurists, and church dignitaries
whose education was unable to save them from the frightful errors
glanced at above?  Not even in this sense; for the great mass of Mr.
Mozley's educated people had no legal training, and must have been
absolutely defenceless against delusions which could set even that
training at naught.  Like nine-tenths of our clergy at the present
day, they were versed in the literature of Greece, Rome, and Judea;
but as regards a knowledge of nature, which is here the one thing
needful, they were 'noble savages,' and nothing more.  In the case of
miracles, then, it behoves us to understand the weight of the
negative, before we assign a value to the positive; to comprehend the
depositions of nature, before we attempt to measure, with them, the
evidence of men.  We have only to open our eyes to see what honest and
even intellectual men and women are capable of, as to judging
evidence, in this nineteenth century of the Christian era, and in
latitude fifty-two degrees north.  The experience thus gained ought, I
imagine, to influence our opinion regarding the testimony of people
inhabiting a sunnier clime, with a richer imagination, and without a
particle of that restraint which the discoveries of physical science
have imposed upon mankind.

*****

Having thus submitted Mr. Mozley's views to the examination which they
challenged at the hands of a student of nature, I am unwilling to quit
his book without expressing my admiration of his genius, and my
respect for his character.  Though barely known to him personally, his
recent death affected me as that of a friend.  With regard to the
style of his book, I heartily subscribe to the description with which
the 'Times' winds up its able and appreciative review.  It is marked
throughout with the most serious and earnest conviction, but is
without a single word from first to last of asperity or insinuation
against opponents; and this not from any deficiency of feeling as to
the importance of the issue, but from a deliberate and resolutely
maintained self-control, and from an over-ruling, ever-present sense
of the duty, on themes like these, of a more than judicial calmness.'

[To the argument regarding the quantity of the miraculous, introduced
at page 17, Mr. Mozley has done me the honour of publishing a Reply in
the seventh volume of the 'Contemporary Review.'--J.  T.]

ADDITIONAL REMARKS ON MIRACLES.

AMONG the scraps of manuscript, written at the time when Mr. Mozley's
work occupied my attention, I find the following reflections:

With regard to the influence of modern science which Mr. Mozley rates
so low, one obvious effect of it is to enhance the magnitude of many
of the recorded miracles, and to increase proportionably the
difficulties of belief.  The ancients knew but little of the vastness
of the universe.  The Rev. Mr. Kirkman, for example, has shown what
inadequate notions the Jews entertained regarding the 'firmament of
heaven;' and Sir George Airy refers to the case of a Greek philosopher
who was persecuted for hazarding the assertion, then deemed monstrous,
that the sun might be as large as the whole country of Greece.  The
concerns of a universe, regarded from this point of view, were much
more commensurate with man and his concerns than those of the universe
which science now reveals to us; and hence that to suit man's
purposes, or that in compliance with his prayers, changes should occur
in the order of the universe, was more easy of belief in the ancient
world than it can be now.  In the very magnitude which it assigns to
natural phenomena, science has augmented the distance between them and
man, and increased the popular belief in their orderly progression.

As a natural consequence the demand for evidence is more exacting than
it used to be, whenever it is affirmed that the order of nature has
been disturbed.  Let us take as an illustration the miracle by which
the victory of Joshua over the Amorites was rendered complete.  In
this case the sun is reported to have stood still for 'about a whole
day' upon Gibeon, and the moon in the valley of Ajalon.  An Englishman
of average education at the present day would naturally demand a
greater amount of evidence to prove that this occurrence took place,
than would have satisfied an Israelite in the age succeeding that of
Joshua.  For to the one, the miracle probably consisted in the
stoppage of a fiery ball less than a yard in diameter, while to the
other it would be the stoppage of an orb fourteen hundred thousand
times the earth in size.  And even accepting the interpretation that
Joshua dealt with what was apparent merely, but that what really
occurred was the suspension of the earth's rotation, I think the right
to exercise a greater reserve in accepting the miracle, and to demand
stronger evidence in support of it than that which would have
satisfied an ancient Israelite, will still be conceded to a man of
science.

There is a scientific as well as an historic imagination; and when, by
the exercise of the former, the stoppage of the earth's rotation is
clearly realised, the event assumes proportions so vast, in comparison
with the result to be obtained by it, that belief reels under the
reflection.  The energy here involved is equal to that of six
trillions of horses working for the whole of the time employed by
Joshua in the destruction of his foes.  The amount of power thus
expended would be sufficient to supply every individual of an army a
thousand times the strength of that of Joshua, with a thousand times
the fighting power of each of Joshua's soldiers, not for the few hours
necessary to the extinction of a handful of Amorites, but for millions
of years.  All this wonder is silently passed over by the sacred
historian, manifestly because he knew nothing about it.  Whether,
therefore, we consider the miracle as purely evidential, or as a
practical means of vengeance, the same lavish squandering of energy
stares us in the face.  If evidential, the energy was wasted, because
the Israelites knew nothing of its amount; if simply destructive, then
the ratio of the quantity lost to the quantity employed, may be
inferred from the foregoing figures.

To other miracles similar remarks apply.  Transferring our thoughts
from this little sand-grain of an earth to the immeasurable heavens,
where countless worlds with freights of life probably revolve unseen,
the very suns which warm them being barely visible across abysmal
space; reflecting that beyond these sparks of solar fire, suns
innumerable may burn, whose light can never stir the optic nerve at
all; and bringing these reflections face to face with the idea of the
Builder and Sustainer of it all showing Himself in a burning bush,
exhibiting His hinder parts, or behaving in other familiar ways
ascribed to Him in the Jewish Scriptures, the incongruity must appear.
Did this credulous prattle of the ancients about miracles stand alone;
were it not associated with words of imperishable wisdom, and with
examples of moral grandeur unmatched elsewhere in the history of the
human race, both the miracles and their 'evidences' would have long
since ceased to be the transmitted inheritance of intelligent men.
Influenced by the thoughts which this universe inspires, well may we
exclaim in David's spirit, if not in David's words: 'When I consider
the heavens, the work of thy fingers, the moon, and the stars, which
thou hast ordained; what is man that thou shouldst be mindful of him,
or the son of man that thou shouldst so regard him?'

If you ask me who is to limit the outgoings of Almighty power, my
answer is, Not I.  If you should urge that if the Builder and Maker of
this universe chose to stop the rotation of the earth, or to take the
form of a burning bush, there is nothing to prevent Him from doing so,
I am not prepared to contradict you.  I neither agree with you nor
differ from you, for it is a subject of which I know nothing.  But I
observe that in such questions regarding Almighty power, your
enquiries relate, not to that power as it is actually displayed in the
universe, but to the power of your own imagination.  Your question is,
not has the Omnipotent done so and so?  or is it in the least degree
likely that the Omnipotent should do so and so?  but, is my
imagination competent to picture a Being able and willing to do so and
so?  I am not prepared to deny your competence.  To the human mind
belongs the faculty of enlarging and diminishing, of distorting and
combining, indefinitely the objects revealed by the senses.  It can
imagine a mouse as large as an elephant, an elephant as large as a
mountain, and a mountain as high as the stars.  It can separate
congruities and unite incongruities.  We see a fish and we see a woman
we can drop one half of each, and unite in idea the other two halves
to a mermaid.  We see a horse and we see a man; we are able to drop
one half of each, and unite the other two halves to a centaur.  Thus
also the pictorial representations of the Deity, the bodies and wings
of cherubs and seraphs, the hoofs, horns, and tail of the Evil One,
the joys of the blessed, and the torments of the damned, have been
elaborated from materials furnished to the imagination by the senses.
It behoves you and me to take care that our notions of the Power which
rules the universe are not mere fanciful or ignorant enlargements of
human power.  The capabilities of what you call your reason are not
denied.  By the exercise of the faculty here adverted to, you can
picture to yourself a Being able and willing to do any and every
conceivable thing.  You are right in saying that in opposition to this
Power science is of no avail--that it is 'a weapon of air.' The man of
science, however, while accepting the figure, would probably reverse
its application, thinking it is not science which is here the thing of
air, but that unsubstantial pageant of the imagination to which the
solidity of science is opposed.



********************

        Prayer as a means to effect a private end is theft and
        meanness.--EMERSON.

*****


III  ON PRAYER AS A FORM OF PHYSICAL ENERGY.

THE Editor of the 'Contemporary Review' is liberal enough to grant me
space for some remarks upon a subject, which, though my relation to it
was simply that of a vehicle of transmission, has brought down upon me
a considerable amount of animadversion.

It may be interesting to some of my readers if I glance at a few cases
illustrative of the history of the human mind, in relation to this and
kindred questions.  In the fourth century the belief in Antipodes was
deemed unscriptural and heretical.  The pious Lactantius was as angry
with the people who held this notion as my censors are now with me,
and quite as unsparing in his denunciations of their 'Monstrosities.'
Lactantius was irritated because, in his mind, by education and habit,
cosmogony and religion were indissolubly associated, and, therefore,
simultaneously disturbed.  In the early part of the seventeenth
century the notion that the earth was fixed, and that the sun and
stars revolved round it daily, was interwoven with religious feeling,
the separation then attempted by Galileo rousing the animosity and
kindling the persecution of the Church.  Men still living can remember
the indignation excited by the first revelations of geology regarding
the age of the earth, the association between chronology and religion
being for the time indissoluble.  In our day, however, the
best-informed theologians are prepared to admit that our views of the
Universe and its Author are not impaired, but improved, by the
abandonment of the Mosaic account of the Creation. Look, finally, at
the excitement caused by the publication of the 'Origin of Species;'
and compare it with the calm attendant on the appearance of the far
more outspoken, and, from the old point of view, more impious,
'Descent of Man.'

Thus religion survives-after the removal of what had been long
considered essential to it.  In our day the Antipodes are accepted;
the fixity of the earth is given up; the period of Creation and the
reputed age of the world are alike dissipated; Evolution is looked
upon without terror; and other changes have occurred in the same
direction too numerous to be dwelt upon here.  In fact, from the
earliest times to the present, religion has been undergoing a process
of purification, freeing itself slowly and painfully from the physical
errors which the active but uninformed intellect mingled with the
aspirations of the soul.  Some of us think that a final act of
purification is needed, while others oppose this notion with the
confidence and the warmth of ancient times.  The bone of contention at
present is _the physical value of prayer_.  It is not my wish to excite
surprise, much less to draw forth protest, by the employment of this
phrase.  I would simply ask any intelligent person to look the problem
honestly in the face, and then to say whether, in the estimation of
the great body of those who sincerely resort to it, prayer does not,
at all events upon special occasions, invoke a Power which checks and
augments the descent of rain, which changes the force and direction of
winds, which affects the growth of corn and the health of men and
cattle a Power, in short, which, when appealed to under pressing
circumstances, produces the precise effects caused by physical energy
in the ordinary course of things.  To any person who deals sincerely
with the subject, and refuses to blur his moral vision by intellectual
subtleties, this, I think, will appear a true statement of the case.

It is under this aspect alone that the scientific student, so far as I
represent him, has any wish to meddle with prayer.  Forced upon his
attention as a form of physical energy, or as the equivalent of such
energy, he claims the right of subjecting it to those methods of
examination from which all our present knowledge of the physical
universe is derived.  And if his researches lead him to a conclusion
adverse to its claims--if his enquiries rivet him still closer to the
philosophy implied in the words, 'He maketh His sun to shine on the
evil and on the good, and sendeth rain upon the just and upon the
unjust'--he contends only for the displacement of prayer, not for its
extinction.  He simply says, physical nature is not its legitimate
domain.

This conclusion, moreover, must be based on pure physical evidence,
and not on any inherent, unreasonableness in the act of prayer.  The
theory that the system of nature is under the control of a Being who
changes phenomena in compliance with the prayers of men, is, in my
opinion, a perfectly legitimate one.  It may of course be rendered
futile by being associated `with conceptions which contradict it; but
such conceptions form no necessary part of the theory.  It is a matter
of experience that an earthly father, who is at the same time both
wise and tender, listens to the requests of his children, and, if they
do not ask amiss, takes pleasure in granting their requests.  We know
also that this compliance extends to the alteration, within certain
limits, of the current of events on earth.  With this suggestion
offered by experience, it is no departure from scientific method to
place behind natural phenomena a Universal Father, who, in answer to
the prayers of His children, alters the currents of those phenomena.
Thus far Theology and Science go hand in hand.  The conception of an
aether, for example, trembling with the waves of light, is suggested
by the ordinary phenomena of wave-motion in water and in air; and in
like manner the conception of personal volition in nature is suggested
by the ordinary action of man upon earth.  I therefore urge no
_impossibilities_, though I am constantly charged with doing so.  I do
not even urge inconsistency, but, on the contrary, frankly admit that
the theologian has as good a right to place his conception at the root
of phenomena as I have to place mine.

But without _verification_ a theoretic conception is a mere figment of
the intellect, and I am sorry to find us parting company at this
point.  The region of theory, both in science and theology, lies
behind the world of the senses, but the verification of theory occurs
in the sensible world.  To check the theory we have simply to compare
the deductions from it with the facts of observation.  If the
deductions be in accordance with the facts, we accept the theory: if
in opposition, the theory is given up.  A single experiment is
frequently devised, by which the theory must stand or fall.  Of this
character was the determination of the velocity of light in liquids,
as a crucial test of the Emission Theory.  According to it, light
travelled faster in water than in air; according to the Undulatory
Theory, it travelled faster in air than in water.  An experiment
suggested by Arago, and executed by Fizeau and Foucault, was
conclusive against Newton's theory.

But while science cheerfully submits to this ordeal, it seems
impossible to devise a mode of verification of their theories which
does not rouse resentment in theological minds.  Is it that, while the
pleasure of the scientific man culminates in the demonstrated harmony
between theory and fact, the highest pleasure of the religious man has
been already tasted in the very act of praying, prior to verification,
any further effort in this direction being a mere disturbance of his
peace?  Or is it that we have before us a residue of that mysticism of
the middle ages, so admirably described by Whewell--that 'practice of
referring things and events not to clear and distinct notions, not to
general rules capable of direct verification, but to notions vague,
distant, and vast, which we cannot bring into contact with facts; as
when we connect natural events with moral and historic causes.'
'Thus,' he continues, 'the character of mysticism is that it refers
particulars, not to generalisations, homogeneous and immediate, but to
such as are heterogeneous and remote; to which we must add, that the
process of this reference is not a calm act of the intellect, but is
accompanied with a glow of enthusiastic feeling.'

Every feature here depicted, and some more questionable ones, have
shown themselves of late; most conspicuously, I regret to say, in the
leaders' of a weekly journal of considerable influence, and one, on
many grounds, entitled to the respect of thoughtful men.  In the
correspondence, however, published by the same journal, are to be
found two or three letters well calculated to correct the temporary
flightiness of the journal itself.

It is not my habit of mind to think otherwise than solemnly of the
feeling which prompts prayer.  It is a power which I should like to
see guided, not extinguished--devoted to practicable objects instead
of wasted upon air.  In some form or other, not yet evident, it may,
as alleged, be necessary to man's highest culture.  Certain it is
that, while I rank many persons who resort to prayer low in the scale
of being--natural foolishness, bigotry, and intolerance being in their
case intensified by the notion that they have access to the ear of
God--I regard others who employ it, as forming part of the very cream
of the earth.  The faith that adds to the folly and ferocity of the
one is turned to enduring sweetness, holiness, abounding charity, and
self-sacrifice by the other.  Religion, in fact, varies with the
nature upon which it falls.  Often unreasonable, if not contemptible,
prayer, in its purer forms, hints at disciplines which few of us can
neglect without moral loss.  But no good can come of giving it a
delusive value, by claiming for it a power in physical nature.  It may
strengthen the heart to meet life's losses, and thus indirectly
promote physical well-being, as the digging of Aesop's orchard brought
a treasure of fertility greater than the golden treasure sought.  Such
indirect issues we all admit; but it would be simply dishonest to
affirm that it is such issues that are always in view.  Here, for the
present, I must end.  I ask no space to reply to those railers who
make such free use of the terms insolence, outrage, profanity, and
blasphemy.  They obviously lack the sobriety of mind necessary to give
accuracy to their statements, or to render their charges worthy of
serious refutation.



********************

IV.  VITALITY.

THE origin, growth, and energies of living things are subjects which
have always engaged the attention of thinking men.  To account for
them it was usual to assume a special agent, free to a great extent
from the limitations observed among the powers of inorganic nature.
This agent was called _vital force_; and, under its influence, plants
and animals were supposed to collect their materials and to assume
determinate forms.  Within the last few years, however, our ideas of
vital processes have undergone profound modifications; and the
interest, and even disquietude, which the change has excited are amply
evidenced by the discussions and protests which are now common,
regarding the phenomena of vitality.  In tracing these phenomena
through all their modifications, the most advanced philosophers of the
present day declare that they ultimately arrive at a single source of
power, from which all vital energy is derived; and the disquieting
circumstance is that this source is not the direct fiat of a
supernatural agent, but a reservoir of what, if we do not accept the
creed of Zoroaster, must be regarded as inorganic force.  In short, it
is considered as proved that all the energy which we derive from
plants and animals is drawn from the sun.

A few years ago, when the sun was affirmed to be the source of life,
nine out of ten of those who are alarmed by the form which this
assertion has latterly assumed would have assented, in a general way,
to its correctness.  Their assent, however, was more poetic than
scientific, and they were by no means prepared to see a rigid
mechanical signification attached to their words.  This, however, is
the peculiarity of modern conclusions: that there is no creative
energy whatever in the vegetable or animal organism, but that all the
power which we obtain from the muscles of man and animals, as much as
that which we develop by the combustion of wood or coal, has been
produced at the sun's expense.  The sun is so much the colder that we
may have our fires; he is also so much the colder that we may have our
horse-racing and Alpine climbing.  It is, for example, certain that
the sun has been chilled to an extent capable of being accurately
expressed in numbers, in order to furnish the power which lifted this
year a certain number of tourists from the vale of Chamouni to the
summit of Mont Blanc.

To most minds, however, the energy of light and heat presents itself
as a thing totally distinct from ordinary mechanical energy.  Either
of them can nevertheless be derived from the other.  Wood can be
raised by friction to the temperature of ignition; while by properly
striking a piece of iron a skilful blacksmith can cause it to glow.
Thus, by the rude agency of his hammer, he generates light and heat.
This action, if carried far enough, would produce the light and heat
of the sun.  In fact, the sun's light and heat have actually been
referred to the fall of meteoric matter upon his surface; and whether
the sun is thus supported or not, it is perfectly certain that he
might be thus supported.  Whether, moreover, the whilom molten
condition of our planet was, as supposed by eminent men, due to the
collision of cosmic masses or not, it is perfectly certain that the
molten condition might be thus brought about.

If, then, solar light and heat can be produced by the impact of dead
matter, and if from the light and heat thus produced we can derive the
energies which we have been accustomed to call _vital_, it indubitably
follows that vital energy may have a proximately mechanical origin.

In what sense, then, is the sun to be regarded as the origin of the
energy derivable from plants and animals?  Let us try to give an
intelligible answer to this question.  Water may be raised from the
sea-level to a high elevation, and then permitted to descend.  In
descending it may be made to assume various forms--to fall in
cascades, to spurt in fountains, to boil in eddies, or to flow
tranquilly along a uniform bed.  It may, moreover, be caused to set
complex machinery in motion, to turn millstones, throw shuttles, work
saws and hammers, and drive piles.  But every form of power here
indicated would be derived from the original power expended in raising
the water to the height from which it fell.  There is no energy
_generated_ by the machinery: the work performed by the water in
descending is merely the parcelling out and distribution of the work
expended in raising it.  In precisely this sense is all the energy of
plants and animals the parcelling out and distribution of a power
originally exerted by the sun.  In the case of the water, the source
of the power consists in the forcible separation of a quantity of the
liquid from a low level of the earth's surface, and its elevation to a
higher position, the power thus expended being returned by the water
in its descent.  In the case of vital phenomena, the source of power
consists in the forcible separation of the atoms of compound
substances by the sun.  We name the force which draws the water
earthward 'gravity,' and that which draws atoms together 'chemical
affinity'; but these different names must not mislead us regarding the
qualitative identity of the two forces.  They are both _attractions_;
and, to the intellect, the falling of carbon atoms against oxygen
atoms is not more difficult of conception than the falling of water to
the earth.

The building up of the vegetable, then, is effected by the sun,
through the reduction of chemical compounds.  The phenomena of animal
life are more or less complicated reversals of these processes of
reduction.  We eat the vegetable, and we breathe the oxygen of the
air; and in our bodies the oxygen, which had been lifted from the
carbon and hydrogen by the action of the sun, again falls towards
them, producing animal heat and developing animal forms.  Through the
most complicated phenomena of vitality this law runs: the vegetable
is produced while a weight rises, the animal is produced while a
weight falls.  But the question is not exhausted here.  The water
employed in our first illustration generates all the motion displayed
in its descent, but the _form_ of the motion depends on the character of
the machinery interposed in the path of the water.  In a similar way,
the primary action of the sun's rays is qualified by the atoms and
molecules among which their energy is distributed.  Molecular forces
determine the form which the solar energy will assume.  In the
separation of the carbon and oxygen this energy may be so conditioned
as to result in one case in the formation of a cabbage, and in another
case in the formation of an oak.  So also, as regards the reunion of
the carbon and the oxygen, the molecular machinery through which the
combining energy acts may, in one case, weave the texture of a frog,
while in another it may weave the texture of a man.

The matter of the animal body is that of inorganic nature.  There is
no substance in the animal tissues which is not primarily derived from
the rocks, the water, and the air.  Are the forces of organic matter,
then, different in kind from those of inorganic matter?  The
philosophy of the present day negatives the question.  It is the
compounding, in the organic world, of forces belonging equally to the
inorganic, that constitutes the mystery and the miracle of vitality.
Every portion of every animal body may be reduced to purely inorganic
matter.  A perfect reversal of this process of reduction would carry
us from the inorganic to the organic; and such a reversal is at least
conceivable.  The tendency, indeed, of modern science is to break down
the wall of partition between organic and inorganic, and to reduce
both to the operation of forces which are the same in kind, but which
are differently compounded.

Consider the question of personal identity, in relation to that of
molecular form.  Thirty-four years ago, Mayer of Heilbronn, with that
power of genius which breathes large meanings into scanty facts,
pointed out that the blood was 6 the oil of the lamp of life,' the
combustion of which sustains muscular action.  The muscles are the
machinery by which the dynamic power of the blood is brought into
play.  Thus the blood is consumed.  But the whole body, though more
slowly than the blood, wastes also, so that after a certain number of
years it is entirely renewed.  How is the sense of personal identity
maintained across this flight of molecules?  To man, as we know him,
matter is necessary to consciousness; but the matter of any period may
be all changed, while consciousness exhibits no solution of
continuity.  Like changing sentinels, the oxygen, hydrogen, and carbon
that depart, seem to whisper their secret to their comrades that
arrive, and thus, while the Non-ego shifts, the Ego remains the same.
Constancy of form in the grouping of the molecules, and not constancy
of the molecules themselves, is the correlative of this constancy of
perception.  Life is a wave which in no two consecutive moments of its
existence is composed of the same particles.

Supposing, then, the molecules of the human body, instead of replacing
others, and thus renewing a pre-existing form, to be gathered first
hand from nature and put together in the same relative positions as
those which they occupy in the body.  Supposing them to have the
selfsame forces and distribution of forces, the selfsame motions and
distribution of motions--would this organised concourse of molecules
stand before us as a sentient thinking being?  There seems no valid
reason to believe that it would not.  Or, supposing a planet carved
from the sun, set spinning round an axis, and revolving round the sun
at a distance from him equal to that of our earth, would one of the
consequences of its refrigeration be the development of organic forms?
I lean to the affirmative.  _Structural_ forces are certainly in the
mass, whether or not those forces reach to the extent of forming a
plant or an animal.  In an amorphous drop of water lie latent all the
marvels of crystalline force; and who will set limits to the possible
play of molecules in a cooling planet?  If these statements startle,
it is because matter has been defined and maligned by philosophers and
theologians, who were equally unaware that it is, at bottom,
essentially mystical and transcendental.

Questions such as these derive their present interest in great part
from their audacity, which is sure, in due time, to disappear.  And
the sooner the public dread is abolished with reference to such
questions the better for the cause of truth.  As regards knowledge,
physical science is polar.  In one sense it knows, or is destined to
know, everything.  In another sense it knows nothing.  Science
understands much of this intermediate phase of things that we call
nature, of which it is the product; but science knows nothing of the
origin or destiny of nature.  Who or what made the sun, and gave his
rays their alleged power?  Who or what made and bestowed upon the
ultimate particles of matter their wondrous power of varied
interaction?  Science does not know: the mystery, though pushed back,
remains unaltered.  To many of us who feel that there are more things
in heaven and earth than are dreamt of in the present philosophy of
science, but who have been also taught, by baffled efforts, how vain
is the attempt to grapple with the Inscrutable, the ultimate frame of
mind is that of Goethe:

    Who dares to name His name,
    Or belief in Him proclaim,
    Veiled in mystery as He is, the All-enfolder?
    Gleams across the mind His light,
    Feels the lifted soul His might,
    Dare it then deny His reign, the All-upholder?



********************

  As I rode through the Schwarzwald, I said to myself: That little fire
  which glows star-like across the dark-growing moor, where the sooty
  smith bends over his anvil, and thou hopest to replace thy lost
  horse-shoe,--is it a detached, separated speck, cut off from the whole
  Universe; or indissolubly joined to the whole?  Thou fool, that
  smithy-fire was primarily kindled at the Sun; is fed by air that
  circulates from before Noah's Deluge, from beyond the Dogstar;
  therein, with Iron Force, and Coal Force, and the far stranger Force
  of Man, are cunning affinities and battles and victories of Force
  brought about; it is a little ganglion, or nervous centre, in the
  great vital system of Immensity.  Call it, if thou wilt, an
  unconscious Altar, kindled on the bosom of the All... Detached,
  separated!  I say there is no such separation: nothing hitherto was
  ever stranded, cast aside; but all, were it only a withered leaf,
  works together with all; is borne forward on the bottomless, shoreless
  flood of action, and lives through perpetual metamorphoses.--CARLYLE.

*****


V.  MATTER AND FORCE.

[Footnote: A Lecture delivered to the working men of Dundee, September
5, 1867, with additions.]

It is the custom of the Professors in the Royal School of Mines in
London to give courses of evening lectures every year to working men.
The lecture-room holds 600 people; and tickets to this amount are
disposed of as quickly as they can be handed to those who apply for
them.  So desirous are the working men of London to attend these
lectures, that the persons who fail to obtain tickets always bear a
large proportion to those who succeed.  Indeed, if the lecture-room
could hold 2,000 instead of 600, I do not doubt that every one of its
benches would be occupied on these occasions.  It is, moreover, worthy
of remark that the lectures are but rarely of a character which could
help the working man in his daily pursuits.  The information acquired
is hardly ever of a nature which admits of being turned into money. It
is, therefore, a pure desire for knowledge, as a thing good in itself,
and without regard to its practical application, which animates the
hearers of these lectures.

It is also my privilege to lecture to another audience in London,
composed in part of the aristocracy of rank, while the audience just
referred to is composed wholly of the aristocracy of labour.  As
regards attention and courtesy to the lecturer, neither of these
audiences has anything to learn of the other; neither can claim
superiority over the other.  It would not, perhaps, be quite correct
to take those persons who flock to the School of Mines as average
samples of their class; they are probably picked men--the aristocracy
of labour, as I have just called them.  At all events, their conduct
demonstrates that the essential qualities of what we in England
understand by a gentleman are confined to no class; and they have
often raised in my mind the wish that the gentlemen of all classes,
artisans as well as lords, could, by some process of selection, be
sifted from the general mass of the community, and caused to know each
other better.

When pressed some months ago by the Council of the British Association
to give an evening lecture to the working men of Dundee, my experience
of the working men of London naturally rose to my mind; and, though
heavily weighted with other duties, I could not bring myself to
decline the request of the Council.  Hitherto, the evening discourses
of the Association have been delivered before its members and
associates alone.  But after the meeting at Nottingham, last year,
where the working men, at their own request, were addressed by our
late President, Mr. Grove, and by my excellent friend, Professor
Huxley, the idea arose of incorporating with all subsequent meetings
of the Association an address to the working men of the town in which
the meeting is held.  A resolution to that effect was sent to the
Committee of Recommendations; the Committee supported the resolution;
the Council of the Association ratified the decision of the Committee;
and here I am to carry out to the best of my ability their united
wishes.

*****

Whether it be a consequence of long-continued development, or an
endowment conferred once for all on man at his creation, we find him
here gifted with a mind curious to know the causes of things, and
surrounded by objects which excite its questionings, and raise the
desire for an explanation.  It is related of a young Prince of one of
the Pacific Islands, that when he first saw himself in a
looking-glass, he ran round the glass to see who was standing at the
back.  And thus it is with the general human intellect, as regards the
phenomena of the external world.  It wishes to get behind and learn
the causes and connections of these phenomena.  What is the sun, what
is the earth, what should we see if we came to the edge of the earth
and looked over?  What is the meaning of thunder and lightning, of
hail, rain, storm, and snow?  Such questions presented themselves to
early men, and by and by it was discovered that this desire for
knowledge was not implanted in vain.  After many trials it became
evident that man's capacities were, so to speak, the complement of
nature's facts, and that, within certain limits, the secret of the
universe was open to the human understanding.  It was found that the
mind of man had the power of penetrating far beyond the boundaries of
his five senses; that the things which are seen in the material world
depend for their action upon things unseen; in short, that besides the
phenomena which address the senses, there are laws and principles and
processes which do not address the senses at all, but which must be,
and can be, spiritually discerned.

To the subjects which require this discernment belong the phenomena of
molecular force.  But to trace the genesis of the notions now
entertained upon this subject, we have to go a long way back.  In the
drawing of a bow, the darting of a javelin, the throwing of a
stone--in the lifting of burdens, and in personal combats, even savage
man became acquainted with the operation of _force_.  Ages of
discipline, moreover, taught him foresight.  He laid by at the proper
season stores of food, thus obtaining time to look about him, and to
become an observer and enquirer.  Two things which he noticed must
have profoundly stirred his curiosity.  He found that a kind of resin
dropped from a certain tree possessed, when rubbed, the power of
drawing light bodies to itself, and of causing them to cling to it;
and he also found that a particular stone exerted a similar power over
a particular kind of metal.  I allude, of course, to electrified
amber, and to the load-stone, or natural magnet, and its power to
attract particles of iron.  Previous experience of his own muscles had
enabled our early enquirer to distinguish between a push and a pull.
Augmented experience showed him that in the case of the magnet and the
amber, pulls and pushes--attractions and repulsions--were also
exerted; and, by a kind of poetic transfer, be applied to things
external to himself, conceptions derived from himself.  The magnet and
the rubbed amber were credited with pushing and pulling, or, in other
words, with exerting force.

In the time of the great Lord Bacon the margin of these pushes and
pulls was vastly extended by Dr. Gilbert, a man probably of firmer
scientific fibre, and of finer insight, than Bacon himself.  Gilbert
proved that a multitude of other bodies, when rubbed, exerted the
power which, thousands of years previously, had been observed in
amber.  In this way the notion of attraction and repulsion in external
nature was rendered familiar.  It was a matter of experience that
bodies, between which no visible link or connection existed, possessed
the power of acting upon each other; and the action came to be
technically called 'action at a distance.'

But out of experience in science there grows something finer than mere
experience.  Experience furnishes the soil for plants of higher
growth; and this observation of action at a distance provided material
for speculation upon the largest of problems.  Bodies were observed to
fall to the earth.  Why should they do so?  The earth was proved to
revolve round the sun; and the moon to revolve round the earth.  Why
should they do so?  What prevents them from flying straight off into
space?  Supposing it were ascertained that from a part of the earth's
rocky crust a firmly fixed and tightly stretched chain started towards
the sun, we might be inclined to conclude that the earth is held in
its orbit by the chain--that the sun twirls the earth around him, as a
boy twirls round his head a bullet at the end of a string.  But why
should the chain be needed?  It is a fact of experience that bodies
can attract each other at a distance, without the intervention of any
chain.  Why should not the sun and earth so attract each other?  and
why should not the fall of bodies from a height be the result of their
attraction by the earth?  Here then we reach one of those higher
speculations which grow out of the fruitful soil of observation.
Having started with the savage, and his sensations of muscular force,
we pass on to the observation of force exerted between a magnet and
rubbed amber and the bodies which they attract, rising, by an unbroken
growth of ideas, to a conception of the force by which sun and planets
are held together.

This idea of attraction between sun and planets had become familiar in
the time of Newton.  He set himself to examine the attraction; and
here, as elsewhere, we find the speculative mind falling back for its
materials upon experience.  It had been observed, in the case of
magnetic and electric bodies, that the nearer they were brought
together the stronger was the force exerted between them; while, by
increasing the distance, the force diminished until it became
insensible.  Hence the inference that the assumed pull between the
earth and the sun would be influenced by their distance asunder.
Guesses had been made as to the exact manner in which the force varied
with the distance; but Newton supplemented the guess by the severe
test of experiment and calculation.  Comparing the pull of the earth
upon a body close to its surface, with its pull upon the moon, 240,000
miles away, Newton rigidly established the law of variation with the
distance.  But on his way to this result Newton found room for other
conceptions, some of which, indeed, constituted the necessary
stepping-stones to his result.  The one which here concerns us is,
that not only does the sun attract the earth, and the earth attract
the sun, as wholes, but every particle of the sun attracts every
particle of the earth, and the reverse.  His conclusion was, that the
attraction of the masses was simply the sum of the attractions of
their constituent particles.

This result seems so obvious that you will perhaps wonder at my
dwelling upon it; but it really marks a turning point in our notions
of force.  You have probably heard of certain philosophers of the
ancient world named Democritus, Epicurus, and Lucretius.  These men
adopted, developed, and diffused the doctrine of atoms and molecules,
which found its consummation at the hands of the illustrious John
Dalton.  But the Greek and Roman philosophers I have named, and their
followers, up to the time of Newton, pictured their atoms as falling
and flying through space, hitting each other, and clinging together by
imaginary hooks and claws.  They missed the central idea that atoms
and molecules could come together, not by being fortuitously knocked
Against each other, but by their own mutual attractions.  This is one
of the great steps taken by Newton.  He familiarised the world with
the conception of _molecular force_.

Newton, you know, was preceded by a grand fellow named John Kepler--a
true working man--who, by analysing the astronomical observations of
his master, Tycho Brahe, had actually found that the planets moved as
they are now known to move.  Kepler knew as much about the motion of
the planets as Newton did; in fact, Kepler taught Newton and the world
generally the facts of planetary motion.  But this was not enough. The
question arose--Why should the facts be so?  This was the great
question for Newton, and it was the solution of it which renders his
name and fame immortal.  Starting from the principle that every
particle of matter in the solar system attracts every other particle
by a force which varies as the inverse square of the distance between
the particles, he proved that the Planetary motions must be what
observation makes them to be.  He showed that the moon fell towards
the earth, and that the planets fell towards the sun, through the
operation of the same force that pulls an apple from its tree.  This
all-pervading force, which forms the solder of the material universe,
and the conception of which was necessary to Newton's intellectual
peace, is called the force of gravitation.

Gravitation is a purely attractive force, but in electricity and
magnetism, repulsion had been always seen to accompany attraction.
Electricity and magnetism are double or _polar forces_.  In the case of
magnetism, experience soon pushed the mind beyond the bounds of
experience, compelling it to conclude that the polarity of the magnet
was resident in its molecules.  I hold a magnetised strip of steel by
its centre, and find that one half of the strip attracts, and the
other half repels, the north end of a magnetic needle.  I break the
strip in the middle, find that this half, which a moment ago attracted
throughout its entire length the north pole of a magnetic needle, is
now divided into two new halves, one of which wholly attracts, and the
other of which wholly repels, the north pole of the needle.  The half
proves to be as perfect a magnet as the whole.  You may break this
half and go on till further breaking becomes impossible through the
very smallness of the fragments; the smallest fragment is found
endowed with two poles, and is, therefore, a perfect magnet.  But you
cannot stop here: you _imagine_ where you cannot _experiment_; and reach
the conclusion entertained by all scientific men, that the magnet
which you see and feel is an assemblage of molecular magnets which you
cannot see and feel, but which, as before stated, must be
intellectually discerned.

Magnetism then is a polar force; and experience hints that a force of
this kind may exert a certain structural power.  It is known, for
example, that iron filings strewn round a magnet arrange themselves in
definite lines, called, by some, 'magnetic curves,' and, by others,
'lines of magnetic force.' Over two magnets now before me is spread a
sheet of paper.  Scattering iron filings over the paper, polar force
comes into play, and every particle of the iron responds to that
force.  We have a kind of architectural effort--if I may use the
term--exerted on the part of the iron filings.  Here then is a fact of
experience which, as you will see immediately, furnishes further
material for the mind to operate upon, rendering it possible to attain
intellectual clearness and repose, while speculating upon apparently
remote phenomena.

The magnetic force has here acted upon particles visible to the eye.
But, as already stated, there are numerous processes in nature which
entirely elude the eye of the body, and must be figured by the eye of
the mind.  The processes of chemistry are examples of these.  Long
thinking and experimenting has led philosophers to conclude that
matter is composed of atoms from which, whether separate or in
combination, the whole material world is built up.  The air we
breathe, for example, as mainly a mechanical mixture of the atoms of
oxygen and nitrogen.  The water we drink is also composed of oxygen
and hydrogen.  But it differs from the air in this particular, that in
water the oxygen and hydrogen are not mechanically mixed, but
chemically combined.  The atoms of oxygen and those of hydrogen exert
enormous attractions on each other, so that when brought into
sufficient proximity they rush together with an almost incredible
force to form a chemical compound.  But powerful as is the force with
which these atoms lock themselves together, we have the means of
tearing them asunder, and the agent by which we accomplish this may
here receive a few moments' attention.

Into a vessel containing acidulated water I dip two strips of metal,
the one being zinc and the other platinum, not permitting them to
touch each other in the liquid.  I connect the two upper ends of the
strips by a piece of copper wire.  The wire is now the channel of
what, for want of a better name, we call an 6 electric current.' What
the inner change of the wire is we do not know, but we do know that a
change has occurred, by the external effects produced by the wire. Let
me show you one or two of these effects.  Before you is a series of
ten vessels, each with its pair of metals, and I wish to get the added
force of all ten.  The arrangement is called a voltaic battery. I
plunge a piece of copper wire among these iron filings; they refuse to
cling to it.  I employ the selfsame wire to connect the two ends of
the battery, and subject it to the same test. The iron filings now
crowd round the wire and cling to it.  I interrupt the current, and
the filings immediately fall; the power of attraction continues only
so long as the wire connects the two ends of the battery.

Here is a piece of similar wire, overspun with cotton, to prevent the
contact of its various parts, and formed into a coil.  I make the coil
part of the wire which connects the two ends of the voltaic battery.
By the attractive force with which it has become suddenly endowed, it
now empties this tool-box of its iron nails.  I twist a covered copper
wire round this common poker; connecting the wire with the two ends of
the voltaic battery, the poker is instantly transformed into a strong
magnet.  Two flat spirals are here suspended facing each other, about
six inches apart.  Sending a current through both spirals, they clash
suddenly together; reversing what is called the direction of the
current in one of the spirals, they fly asunder.  All these effects
are due to the power which we name an electric current, and which we
figure as flowing through the wire when the voltaic circuit is
complete.

By the same agent we tear asunder the locked atoms of a chemical
compound.  Into this small cell, containing water, dip two thin wires.
A magnified image of the cell is thrown upon the screen before you,
and you see plainly the images of the wires.  From a small battery I
send an electric current from wire to wire.  Bubbles of gas rise
immediately from each of them, and these are the two gases of which
the water is composed.  The oxygen is always liberated on the one
wire, the hydrogen on the other.  The gases may be collected either
separately or mixed.  I place upon my hand a soap bubble filled with
the mixture of both gases.  Applying a taper to the bubble, a loud
explosion is heard.  The atoms have rushed together with detonation,
and without injury to my hand, and the water from which they were
extracted is the result of their re-union.

*****

One consequence of the rushing together of the atoms is the
development of heat.  What is this heat?  Here are two ivory balls
suspended from the same point of support by two short strings.  I draw
them thus apart and then liberate them.  They clash together, but, by
virtue of their elasticity, they quickly recoil, and a sharp vibratory
rattle succeeds their collision.  This experiment will enable you to
figure to your mind a pair of clashing atoms.  We have in the first
place, a motion of the one atom towards the other--a motion of
translation, as it is usually called--then a recoil, and afterwards a
motion of vibration.  To this vibratory motion we give the name of
heat.  Thus, three things are to be kept before the mind--first, the
atoms themselves; secondly, the force with which they attract each
other; and thirdly, the motion consequent upon the exertion of that
force.  This motion must be figured first as a motion of translation,
and then as a motion of vibration, to which latter we give the name of
heat.  For some time after the act of combination this motion is so
violent as to prevent the molecules from coming together, the water
being maintained in a state of vapour.  But as the vapour cools, or in
other words loses its motion, the molecules coalesce to form a liquid.

And now we approach a new and wonderful display of force.  As long as
the substance remains in a liquid or vaporous condition, the play of
this force is altogether masked and bidden.  But as the heat is
gradually withdrawn, the molecules prepare for new arrangements and
combinations.  Solid crystals of water are at length formed, to which
we give the familiar name of ice.  Looking at these beautiful edifices
and their internal structure, the pondering mind has forced upon it
the question, How are they built up?  We have obtained clear
conceptions of polar force; and we infer from our broken magnet that
polar force may be resident in the molecules or smallest particles of
matter, and that by the play of this force structural arrangement is
possible.  What, in relation to our present question, is the natural
action of a mind furnished with this knowledge?  It is compelled to
transcend experience, and endow the atoms and molecules of which
crystals are built with definite poles whence issue attractions and
repulsions.  In virtue of these forces some poles are drawn together,
while some retreat from each other; atom is added to atom, and
molecule to molecule, not boisterously or fortuitously, but silently
and symmetrically, and in accordance with laws more rigid than those
which guide a human builder when he places his materials together.
Imagine the bricks and stones of this town of Dundee endowed with
structural power.

Imagine them attracting and repelling, and arranging themselves into
streets and houses and Kinnaird Halls--would not that be wonderful?
Hardly less wonderful is the play of force by which the molecules of
water build themselves into the sheets of ice which every winter roof
your ponds and lakes.

If I could show you the actual progress of this molecular
architecture, its beauty would delight and astonish you.  A reversal
of the process of crystallisation may be actually shown.  The
molecules of a piece of ice may be taken asunder before your eyes; and
from the manner in which they separate, you may to some extent infer
the manner in which they go together.  When a beam is sent from our
electric lamp through a plate of glass, a portion of the beam is
intercepted, and the glass is warmed by the portion thus retained
within it.  When the beam is sent through a plate of ice, a portion of
the beam is also absorbed; but instead of warming the ice, the
intercepted heat melts it internally.  It is to the delicate silent
action of this beam within the ice that I now wish to direct your
attention.  Upon the screen is thrown a magnified image of the slab of
ice: the light of the beam passes freely through the ice without
melting it, and enables us to form the image; but the heat is in great
part intercepted, and that heat now applies itself to the work of
internal liquefaction.  Selecting certain points for attack, round
about those points the beam works silently, undoing the crystalline
architecture, and reducing to the freedom of liquidity molecules which
had been previously locked in a solid embrace.  The liquefied spaces
are rendered visible by strong illumination.  Observe those
six-petaled flowers breaking out over the white surface, and expanding
in size as the action of the beam continues.  These flowers are
liquefied ice.  Under the action of the heat the molecules of the
crystals fall asunder, so as to leave behind them these exquisite
forms.  We have here a process of demolition which clearly reveals the
reverse process of construction.  In this fashion, and in strict
accordance with this hexangular type, every ice molecule takes its
place upon our ponds and lakes during the frosts of winter.  To use
the language of an American poet, 'the atoms march in tune,' moving to
the music of law, which thus renders the commonest substance in nature
a miracle of beauty.

It is the function of science, not as some think to divest this
universe of its wonder and mystery, but, as in the case before us, to
point out the wonder and the mystery of common things.  Those
fern-like forms, which on a frosty morning overspread your
windowpanes, illustrate the action of the same force.  Breathe upon
such a pane before the fires are lighted, and reduce the solid
crystalline film to the liquid condition; then watch its subsequent
resolidification.  You will see it all the better if you look at it
through a common magnifying glass.  After you have ceased breathing,
the film, abandoned to the action of its own forces, appears for a
moment to be alive.  Lines of motion run through it; molecule closes
with molecule, until finally the whole film passes from the state of
liquidity, through this state of motion, to its final crystalline
repose.

I can show you something similar.  Over a piece of perfectly clean
glass I pour a little water in which certain crystals have been
dissolved.  A film of the solution clings to the glass.  By means of a
microscope and a lamp, an image of the plate of glass is thrown upon
the screen.  The beam of the lamp, besides illuminating the glass,
also heats it; evaporation sets in, and at a certain moment, when the
solution has become supersaturated, splendid branches of crystal shoot
out over the screen.  A dozen square feet of surface are now covered
by those beautiful forms.  With another solution we obtain crystalline
spears, feathered right and left by other spears.  From distant nuclei
in the middle of the field of view the spears shoot with magical
rapidity in all directions.  The film of water on a window-pane on a
frosty morning exhibits effects quite as wonderful as these.  Latent
in these formless solutions, latent in every drop of water, lies this
marvellous structural power, which only requires the withdrawal of
opposing forces to bring it into action.

The clear liquid now held up before you is a solution of nitrate of
silver--a compound of silver and nitric acid.  When an electric
current is sent through this liquid the silver is severed from the
acid, as the hydrogen was separated from the oxygen in a former
experiment; and I would ask you to observe how the metal behaves when
its molecules are thus successively set free.  The image of the cell,
and of the two wires which dip into the liquid of the cell, are now
clearly shown upon the screen.  Let us close the circuit, and send the
current through the liquid.  From one of the wires a beautiful silver
tree commences immediately to sprout.  Branches of the metal are
thrown out, and umbrageous foliage loads the branches.  You have here
a growth, apparently as wonderful as that of any vegetable, perfected
in a minute before your eyes.  Substituting for the nitrate of silver
acetate of lead, which is a compound of lead and acetic acid, the
electric current severs the lead from the acid, and you see the metal
slowly branching into exquisite metallic ferns, the fronds of which,
as they become too heavy, break from their roots and fall to the
bottom of the cell.

These experiments show that the common matter of our earth--'brute
matter,' as Dr. Young, in his _Night Thoughts_, is pleased to call
it--when its atoms and molecules are permitted to bring their forces
into free play, arranges itself, under the operation of these forces,
into forms which rival in beauty those of the vegetable world.  And
what is the vegetable world itself, but the result of the complex play
of these molecular forces?  Here, as elsewhere throughout nature, if
matter moves it is force that moves it, and if a certain structure,
vegetable or mineral, is produced, it is through the operation of the
forces exerted between the atoms and molecules.

The solid matter of which our lead and silver trees were formed was,
in the first instance, disguised in a transparent liquid; the solid
matter of which our woods and forests are composed is also, for the
most part disguised in a transparent gas, which is mixed in small
quantities with the air of our atmosphere.  This gas is formed by the
union of carbon and oxygen, and is called carbonic acid gas.  The
carbonic acid of the air being subjected to an action somewhat
analogous to that of the electric current in the case of our lead and
silver solutions, has its carbon liberated and deposited as woody
fibre.  The watery vapour of the air is subjected to similar action;
its hydrogen is liberated from its oxygen, and lies down side by side
with the carbon in the tissues of the tree.  The oxygen in both cases
is permitted to wander away into the atmosphere.  But what is it in
nature that plays the part of the electric current in our experiments,
tearing asunder the locked atoms of carbon, oxygen, and hydrogen?  The
rays of the sun.  The leaves of plants which absorb both the carbonic
acid and the aqueous vapour of the air, answer to the cells in which
our decompositions took place.  And just as the molecular attractions
of the silver and the lead found expression in those beautiful
branching forms seen in our experiments, so do the molecular
attractions of the liberated carbon and hydrogen find expression in
the architecture of grasses, plants, and trees.

In the fall of a cataract and the rush of the wind we have examples of
mechanical power.  In the combinations of chemistry and in the
formation of crystals and vegetables we have examples of molecular
power.  You have learned how the atoms of oxygen and hydrogen rush
together to form water.  I have not thought it necessary to dwell upon
the mighty mechanical energy of their act of combination; but it may
be said, in passing, that the clashing together of 1 lb.  of hydrogen
and 8 lbs. of oxygen to form 9 lbs. of aqueous vapour, is greater than
the shock of a weight of 1,000 tons falling from a height of 20 feet
against the earth.  Now, in order that the atoms of oxygen and
hydrogen should rise by their mutual attractions to the velocity
corresponding to this enormous mechanical effect, a certain distance
must exist between the particles.  It is in rushing over this that the
velocity is attained.

*****

This idea of distance between the attracting atoms is of the highest
importance in our conception of the system of the world.  For the
matter of the world may be classified under two distinct heads: atoms
and molecules which have already combined and thus satisfied their
mutual attractions, and atoms and molecules which have not yet
combined, and whose mutual attractions are, therefore, unsatisfied.
Now, as regards motive power, we are entirely dependent on atoms and
molecules of the latter kind.  Their attractions can produce motion,
because sufficient distance intervenes between the attracting atoms,
and it is this atomic motion that we utilise in our machines.  Thus we
can get power out of oxygen and hydrogen by the act of their union;
but once they are combined, and once the vibratory motion consequent
on their combination has been expended, no further power can be got
out of their mutual attraction.  As dynamic agents they are dead.  The
materials of the earth's crust consist for the most part of substances
whose atoms have already closed in chemical union--whose mutual
attractions are satisfied.  Granite, for instance, is a widely
diffused substance; but granite consists, in great part, of silicon,
oxygen, potassium, calcium, and aluminum, whose atoms united long ago,
and are therefore dead.  Limestone is composed of carbon, oxygen, and
a metal called calcium, the atoms of which have already closed in
chemical union, and are therefore finally at rest. In this way we
might go over nearly the whole of the materials of the earth's crust,
and satisfy ourselves that though they were sources of power in ages
past, and long before any creature appeared on the earth capable of
turning their power to account, they are sources of power no longer.
And here we might halt for a moment to remark on that tendency, so
prevalent in the world, to regard everything as made for human use.
Those who entertain this notion, hold, I think, an overweening opinion
of their own importance in the system of nature.  Flowers bloomed
before men saw them, and the quantity of power wasted before man could
utilise it is all but infinite compared with what now remains.  We are
truly heirs of all the ages; but as honest men it behoves us to learn
the extent of our inheritance, and as brave ones not to whimper if it
should prove less than we had supposed.  The healthy attitude of mind
with reference to this subject is that of the poet, who, when asked
whence came the rhodora, joyfully acknowledged his brotherhood with
the flower:

    Why thou wert there, O rival of the rose!
    I never thought to ask, I never knew,
    But in my simple ignorance supposed
    The self-same power that brought me there brought you.

                                     Emerson.

A few exceptions to the general state of union of the molecules of the
earth's crust--vast in relation to us, but trivial in comparison to
the total store of which they are the residue--still remain.  They
constitute our main sources of motive power.  By far the most
important of these are our beds of coal.  Distance still intervenes
between the atoms of carbon and those of atmospheric oxygen, across
which the atoms may be urged by their mutual attractions; and we can
utilise the motion thus produced.  Once the carbon and the oxygen have
rushed together, so as to form carbonic acid, their mutual attractions
are satisfied; and, while they continue in this condition, as dynamic
agents they are dead.  Our woods and forests are also sources of
mechanical energy, because they have the power of uniting with the
atmospheric oxygen.  Passing from plants to animals, we find that the
source of motive power just referred to is also the source of muscular
power.  A horse can perform work, and so can a man; but this work is
at bottom the molecular work of the transmuted food and the oxygen of
the air.  We inhale this vital gas, and bring it into sufficiently
close proximity with the carbon and the hydrogen of the body.  These
unite in obedience to their mutual, attractions; and their motion
towards each other, properly turned to account by the wonderful
mechanism of the body, becomes muscular motion.

One fundamental thought pervades all these statements: there is one
tap root from which they all spring.  This is the ancient maxim that
out of nothing nothing comes; that neither in the organic world nor in
the inorganic is power produced without the expenditure of power; that
neither in the plant nor in the animal is there a creation of force or
motion.  Trees grow, and so do men and horses; and here we have new
power incessantly introduced upon the earth.  But its source, as I
have already stated, is the sun.  It is the sun that separates the
carbon from the oxygen of the carbonic acid, and thus enables them to
recombine.  Whether they recombine in the furnace of the steam-engine
or in the animal body, the origin of the power they produce is the
same.  In this sense we are all 'souls of fire and children of the
sun.' But, as remarked by Helmholtz, we must be content to share our
celestial pedigree with the meanest of living things.

Some estimable persons, here present, very possibly shrink from
accepting these statements; they may be frightened by their apparent
tendency towards what is called materialism--a word which, to many
minds, expresses something very dreadful.  But it ought to be known
and avowed that the physical philosopher, as such, must be a pure
materialist. His enquiries deal with matter and force, and with them
alone.  And whatever be the forms which matter and force assume,
whether in the organic world or the inorganic, whether in the
coal-beds and forests of the earth, or in the brains and muscles of
men, the physical philosopher will make good his right to investigate
them.  It is perfectly vain to attempt to stop enquiry in this
direction.  Depend upon it, if a chemist by bringing the proper
materials together, in a retort or crucible, could make a baby, he
would do it.  There is no law, moral or physical, forbidding him to do
it.  At the present moment there are, no doubt, persons experimenting
on the possibility of producing what we call life out of inorganic
materials.  Let them pursue their studies in peace; it is only by such
trials that they will learn the limits of their own powers and the
operation of the laws of matter and force.

But while thus making the largest demand for freedom of
investigation--while I consider science to be alike powerful as an
instrument of intellectual culture and as a ministrant to the material
wants of men; if you ask me whether it has solved, or is likely in our
day to solve, the problem of this universe, I must shake my head in
doubt.  You remember the first Napoleon's question, when the savants
who accompanied him to Egypt discussed in his presence the origin of
the universe, and solved it to their own apparent satisfaction.  He
looked aloft to the starry heavens, and said, 'It is all very well,
gentlemen; but who made these?'  That question still remains
unanswered, and science makes no attempt to answer it.  As far as I
can see, there is no quality in the human intellect which is fit to be
applied to the solution of the problem.  It entirely transcends us.
The mind of man may be compared to a musical instrument with a certain
range of notes, beyond which in both directions we have an infinitude
of silence.  The phenomena of matter and force lie within our
intellectual range, and as far as they reach we will at all hazards
push our enquiries.  But behind, and above, and around all, the real
mystery of this universe lies unsolved, and, as far as we are
concerned, is incapable of solution.  Fashion this mystery as you
will, with that I have nothing to do.  But let your conception of it
not be an unworthy one.  Invest that conception with your highest and
holiest thought, but be careful of pretending to know more about it
than is given to man to know.  Be careful, above all things, of
professing to see in the phenomena of the material world the evidences
of Divine pleasure or displeasure.  Doubt those who would deduce from
the fall of the tower of Siloam the anger of the Lord against those
who were crushed.  Doubt equally those who pretend to see in cholera,
cattle-plague, and bad harvests, evidences of Divine anger.  Doubt
those spiritual guides who in Scotland have lately propounded the
monstrous theory that the depreciation of railway scrip is a
consequence of railway travelling on Sundays.  Let them not, as far as
you are concerned, libel the system of nature with their ignorant
hypotheses.  Looking from the solitudes of thought into this highest
of questions, and seeing the puerile attempts often made to solve it,
well might the mightiest of living Scotchmen--that strong and earnest
soul, who has made every soul of like nature in these islands his
debtor--well, I say, might your noble old Carlyle scornfully retort on
such interpreters of the ways of God to men:

    The Builder of this universe was wise,
    He formed all souls, all systems, planets, particles;
    The plan he formed his worlds and Aeons by,
    Was--Heavens!--was thy small nine-and-thirty articles!



********************


  Here, indeed, we arrive at the barrier which needs to be perpetually
  pointed out; alike to those who seek materialistic explanations of
  mental phenomena, and to those who are alarmed lest such explanations
  may be found.  The last class prove by their fear almost as much as
  the first prove by their hope, that they believe Mind may possibly be
  interpreted in terms of Matter; whereas many whom they vituperate as
  materialists are profoundly convinced that there is not the remotest
  possibility of so interpreting them.

                                         HERBERT SPENCER.

====================



VI.  SCIENTIFIC MATERIALISM.

[Footnote: President's Address to the Mathematical and Physical
Section of the British Association at Norwich.]

1868.

THE celebrated Fichte, in his lectures on the 'Vocation of the
Scholar,' insisted on a culture which should be not one-sided, but
all-sided.  The scholar's intellect was to expand spherically, and not
in a single direction only.  In one direction, however, Fichte
required that the scholar should apply himself directly to nature,
become a creator of knowledge, and thus repay, by original labours of
his own, the immense debt he owed to the labours of others.  It was
these which enabled him to supplement the knowledge derived from his
own researches, so as to render his culture rounded and not one-sided.

As regards science, Fichte's idea is to some extent illustrated by the
constitution and labours of the British Association.  We have here a
body of men engaged in the pursuit of Natural Knowledge, but variously
engaged.  While sympathising with each of its departments, and
supplementing his culture by knowledge drawn from all of them, each
student amongst us selects one subject for the exercise of his own
original faculty--one line, along which he may carry the light of his
private intelligence a little way into the darkness by which all
knowledge is surrounded.  Thus, the geologist deals with the rocks;
the biologist with the conditions and phenomena of life; the
astronomer with stellar masses and motions; the mathematician with the
relations of space and number; the chemist pursues his atoms; while
the physical investigator has his own large field in optical, thermal,
electrical, acoustical, and other phenomena.  The British Association
then, as a whole, faces physical nature on all sides, and pushes
knowledge centrifugally outwards, the sum of its labours constituting
what Fichte might call the sphere of natural knowledge.  In the
meetings of the Association it is found necessary to resolve this
sphere into its component parts, which take concrete form under the
respective letters of our Sections.

Mathematics and Physics have been long accustomed to coalesce, and
here they form a single section.  No matter how subtle a natural
phenomenon may be, whether we observe it in the region of sense, or
follow it into that of imagination, it is in the long run reducible to
mechanical laws.  But the mechanical data once guessed or given,
mathematics are all-powerful as an instrument of deduction.  The
command of Geometry over the relations of space, and the far-reaching
power which Analysis confers, are potent both As means of physical
discovery, and of reaping the entire fruits of discovery.  Indeed,
without mathematics, expressed or implied, our knowledge of physical
science would be both friable and incomplete.

Side by side with the mathematical method we have the method of
experiment.  Here from a starting-point furnished by his own
researches or those of others, the investigator proceeds by combining
intuition and verication.  He ponders the knowledge he possesses, and
tries to push it further; he guesses, and checks his guess; he
conjectures, and confirms or explodes his conjecture.  These guesses
and conjectures are by no means leaps in the dark; for knowledge once
gained casts a faint light beyond its own immediate boundaries.  There
is no discovery so limited as not to illuminate something beyond
itself.  The force of intellectual penetration into this penumbral
region which surrounds actual knowledge is not, as some seem to think,
dependent upon method, but upon the genius of the investigator.  There
is, however, no genius so gifted as not to need control and
verification.  The profoundest minds know best that Nature's ways are
not at all times their ways, and that the brightest flashes in the
world of thought are incomplete until they have been proved to have
their counterparts in the world of fact.  Thus the vocation of the
true experimentalist may be defined as the continued exercise of
spiritual insight, and its incessant correction and realisation.  His
experiments constitute a body, of which his purified intuitions are,
as it were, the soul.

Partly through mathematical and partly through experimental research,
physical science has, of late years, assumed a momentous position in
the world.  Both in a material and in an intellectual point of view it
has produced, and it is destined to produce, immense changes--vast
social ameliorations, and vast alterations in the popular conception
of the origin, rule, and governance of natural things.  By science, in
the physical world, miracles are wrought, while philosophy is
forsaking its ancient metaphysical channels, and pursuing others which
have been opened, or indicated by, scientific research.  This must
become more and more the case as philosophical writers become more
deeply imbued with the methods of science, better acquainted with the
facts which scientific men have established, and with the great
theories which they have elaborated.

If you look at the face of a watch, you see the hour and minute-hands,
and possibly also a second-hand, moving over the graduated dial.  Why
do these hands move?  and why are their relative motions such as they
are observed to be?  These questions cannot be answered without
opening the watch, mastering its various parts, and ascertaining their
relationship to each other.  When this is done, we find that the
observed motion of the hands follows of necessity from the inner
mechanism of the watch when acted upon by the force invested in the
spring.  The motion of the hands may be called a phenomenon of art,
but the case is similar with the phenomena of nature.  These also have
their inner mechanism and their store of force to set that mechanism
going.  The ultimate problem of physical science is to reveal this
mechanism, to discern this store, and to show that from the combined
action of both, the phenomena of which they constitute the basis,
must, of necessity, flow.

I thought an attempt to give you even a brief and sketchy illustration
of the manner in which scientific thinkers regard this problem, would
not be uninteresting to you on the present occasion; more especially
as it will give me occasion to say a word or two on the tendencies and
limits of modern science; to point out the region which men of science
claim as their own, and where it is futile to oppose their advance;
and also to define, if possible, the bourne between this and that
other region, to which the questionings and yearnings of the
scientific intellect are directed in vain.

But here your tolerance will be needed.  It was the American Emerson,
I think, who said that it is hardly possible to state any truth
strongly, without apparent injustice to some other truth.  Truth is
often of a dual character, taking the form of a magnet with two poles;
and many of the differences which agitate the thinking part of mankind
are to be traced to the exclusiveness with which partisan reasoners
dwell upon one half of the duality, in forgetfulness of the other. The
proper course appears to be to state both halves strongly, and allow
each its fair share in the formation of the resultant conviction.  But
this waiting for the statement of the two sides of a question implies
patience.  It implies a resolution to suppress indignation, if the
statement of the one half should clash with our convictions; and to
repress equally undue elation, if the half-statement should happen to
chime in with our views.  It implies a determination to wait calmly
for the statement of the whole, before we pronounce