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Title: Stellar Evolution and its Relations to Geological Time
Author: Croll, James
Language: English
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                           STELLAR EVOLUTION



------------------------------------------------------------------------



                          BY THE SAME AUTHOR.

                                -------

    CLIMATE AND TIME IN THEIR GEOLOGICAL RELATIONS: A Theory of
        Secular Changes of the Earth’s Climate. By JAMES CROLL,
        of H. M. Geological Survey of Scotland. With Maps and
        Illustrations. 12mo. Cloth, $2.50.

    DISCUSSIONS ON CLIMATE AND COSMOLOGY. By JAMES CROLL, LL.D.,
        F.R.S. With Chart. 12mo. Cloth, $2.00.



------------------------------------------------------------------------



                           STELLAR EVOLUTION

                          AND ITS RELATIONS TO

                            GEOLOGICAL TIME


                                   BY

                       JAMES CROLL, LL.D., F.R.S.

      AUTHOR OF ‘CLIMATE AND TIME,’ ‘CLIMATE AND COSMOLOGY,’ ETC.


                                NEW YORK
                        D. APPLETON AND COMPANY
                                  1889



------------------------------------------------------------------------



                         _Authorized Edition._



------------------------------------------------------------------------



                                PREFACE.


                             [Illustration]


There are two, and only two, conceivable sources from which the
prodigious amount of energy possessed by our sun and solar system can
possibly have been derived. Not only are these two sources radically
distinct in their essential nature, but both are admitted to be real and
not merely hypothetical sources of energy. The one source is
gravitation; the other, the source discussed in the present volume, a
source to which attention was directed some twenty years ago. A most
important distinction between these two sources is this: the amount of
energy available from the former can be accurately determined, but such
is not the case in regard to the latter. We can tell with tolerable
certainty the greatest amount of energy which gravitation could possibly
have conferred on the sun and solar system; but we have, at present, no
means of assigning a limit to the possible amount which might have been
derived from the other source. It may have been equal to that which
gravitation could afford, or it may have been twofold, fourfold, or even
tenfold that amount.

We have evidently in this case a means of determining which of the two
sources will ultimately have to be adopted as the source to which the
energy of our solar system must be referred. For if it can be proved
from the admitted facts of geology, biology, and other sciences, that
the amount of energy in the form of heat which has been radiated into
space by the sun during geological time is far greater than the amount
which could possibly have been derived from gravitation, this will
undoubtedly show that gravitation cannot account for the energy
originally possessed by our system.

The First Part of the volume is devoted to the consideration of what I
believe to be the probable origin of meteorites, comets, and nebulæ, and
of the real source from which our sun derived his energy. The facts
which support the theory here advocated, together with the light which
that theory appears to cast upon those facts, are next considered; and
it will be found, I think, that the theory has been very much
strengthened by the recent important spectroscopic researches of Mr.
Lockyer and others in reference to the constitution of nebulæ. The
Second Part of the work deals with the evidence in support of the theory
derived from the testimony of geology and biology as to the age of the
sun’s heat. The Third, and last, Part has been devoted to questions
relating to the pre-nebular condition of the universe, and the bearing
which these have on theories of stellar evolution. Several subjects
introduced in this part are only very briefly treated. These will,
however, be considered at greater length in a future volume,
“Determinism, not Force, the Foundation-stone of Evolution,” a work of a
more general and abstract character, which was commenced many years ago.


      PERTH: _January 2, 1889_.

------------------------------------------------------------------------



                               CONTENTS.


                             [Illustration]


------------------------------------------------------------------------

                                PART I.

               _THE IMPACT THEORY OF STELLAR EVOLUTION._

                                                                 PAGE


   CONSIDERATION OF THE FACTS WHICH SUPPORT THE THEORY, AND OF
     THE LIGHT WHICH THE THEORY APPEARS TO CAST UPON THE FACTS     12


      I. Probable Origin of Meteorites                             12

     II. Motion of the Stars; how of such different velocities,
     and always in straight lines                                  14

    III. Motion of the Stars not due to their mutual attractions   14

     IV. Probable Origin of Comets                                 17

      V. Nebulæ                                                    18


           1. Origin of Nebulæ                                     18

           2. How Nebulæ occupy so much space                      18

           3. Why Nebulæ are of such various shapes                19

           4. Broken fragments in a Gaseous mass of an
            excessively high temperature the First stage of a
            Nebula                                                 19

           5. The Gaseous condition the Second stage of a Nebula   24

           6. The Gaseous condition Essential to the Nebular
            Hypothesis                                             25

           7. The mass must have possessed an excessive
            temperature                                            26

           8. Gravitation could, under no possible condition,
            have generated the Amount of Heat required by the
            Nebular Hypothesis                                     27

           9. Condensation the Third and last stage of a Nebula    30

          10. How Nebulæ emit such feeble Light                    30


     VI. Binary Systems                                            32

    VII. Sudden Outbursts of Stars                                 33

   VIII. Star Clusters                                             34

     IX. Age of the Sun’s Heat: a Crucial Test                     34


                                PART II.

                   _EVIDENCE IN SUPPORT OF THE THEORY
                    FROM THE AGE OF THE SUN’S HEAT._

   TESTIMONY OF GEOLOGY AND BIOLOGY AS TO THE AGE OF THE SUN’S
     HEAT                                                          37


   Testimony of Geology: Method employed                           39

   The Average Rate of Denudation in the Past probably not much
     greater than at the Present                                   44

   How the Method has been applied                                 47

   Method as applied by Professor Haughton                         50

   Method as applied by Mr. Alfred R. Wallace                      51

   Method as applied directly                                      52

   Evidence from “faults”                                          53

   Time required to effect the foregoing amount of Denudation      62

   Age of the Earth as determined by the Date of the Glacial
     Epoch                                                         64

   Testimony of Biology                                            65


                               PART III.

                   _EVIDENCE IN SUPPORT OF THE THEORY
                   FROM THE PRE-NEBULAR CONDITION OF
                             THE UNIVERSE._

   Professor A. Winchell on the pre-nebular condition of matter    71

   Mr. Charles Morris on the pre-nebular condition of matter       75

   Sir William R. Grove on the pre-nebular condition of matter     78

   Evolution of the Chemical Elements, and its Relations to
     Stellar Evolution                                             80

   Sir Benjamin Brodie on the pre-nebular condition of matter      84

   Dr. T. Sterry Hunt on the pre-nebular condition of matter       85

   Professor Oliver Lodge on the pre-nebular condition of matter   87

   Mr. William Crookes on the pre-nebular condition of matter      90

   Professor F. W. Clarke on the pre-nebular condition of matter   98

   Dr. G. Johnstone Stoney on the pre-nebular condition of
     matter                                                        99


   THE IMPACT THEORY IN RELATION TO THE FOREGOING THEORIES OF
     THE PRE-NEBULAR CONDITION OF MATTER                          102


   The Theories do not account for the Motion of the Stars        105

   The Theories do not account for the Amount of Heat required    106

   Evolution of Matter                                            107

   Objection considered                                           109

   Can we on Scientific grounds trace back the Evolution of the
     Universe to an Absolute First condition?                     110

------------------------------------------------------------------------



                           STELLAR EVOLUTION.


                             [Illustration]



                                PART I.

               _THE IMPACT THEORY OF STELLAR EVOLUTION._


Upwards of twenty years ago[1] the theory—or, I should rather say, the
hypothesis—was advanced[2] that our sun was formed from a hot gaseous
nebula produced by the colliding of two dark stellar masses; and that,
as the stars are suns like our own, they in all likelihood had a similar
origin. The probability of this theory has been very much strengthened
by the facts, both astronomical and physical, which have accumulated
since the theory was enunciated. Before proceeding to the consideration
of these facts, and the conclusions to which they lead, it will be
necessary to give a statement of the fundamental principles of the
theory.


[Footnote 1: _Philosophical Magazine_, May 1868; _Climate and Time_,
chap. xxi.; _Quarterly Journal of Science_, July 1877; _Phil. Mag._,
July 1878; _Climate and Cosmology_, chaps. xvii. xviii. and xix.]

[Footnote 2: I prefer to use the term “theory,” with the above
understood qualification, viz. a theory in its hypothetical stage.]


In the theory here discussed the truth of the nebular hypothesis, which
begins by assuming the existence of a solar nebulous mass, is taken for
granted. The present theory deals not so much with the nebulous mass
itself as with the _formation_ of the nebula, and with those causes
which led to its formation. For convenience of reference, and to prevent
confusion, I have called it the “Impact Theory,” by which name it may be
distinguished, on the one hand, from the nebular theory, and, on the
other hand, from the meteoric theory, and all other theories which
regard gravitation as the primary source of the solar energy.

The theory starts with the assumption that the greater part of the
energy possessed by the universe exists or is stored up in the form of
the motion of stellar masses. The amount of energy which may thus be
stored up is startling to contemplate. Thus a mass equal to that of the
sun, moving with a velocity of 476 miles per second, would possess, in
virtue of that motion, energy sufficient, if converted into heat, to
maintain the present rate of the sun’s radiation for 50,000,000
years.[3] There is nothing extravagant in the assumption of such a
velocity. A comet, for example, having an orbit extending to the path of
the planet Neptune, approaching so near the sun as to almost graze his
surface in passing, would have a velocity within 86 miles of what we
have assumed. Twice this assumed velocity would give 200,000,000 years’
heat; four times the velocity would give 800,000,000 years’ heat; and so
on.


[Footnote 3: Pouillet’s estimate of the rate of solar radiation is here
taken.]


We are at perfect liberty to begin by assuming the existence of stellar
masses in motion; for we are not called upon to explain how the masses
obtained their motion, any more than we have to explain how they came to
have their existence. If the masses were created, they may as likely
have been created in motion as at rest; and if they were eternal, they
may as likely have been eternally in motion as eternally at rest.

Eternal motion is just as warrantable an assumption as eternal matter.
When we reflect that space is infinite—at least in thought—and that, for
aught we know to the contrary, bodies may be found moving throughout its
every region, we see that the amount of energy may be perfectly
illimitable.

But, illimitable as the amount of the energy may be, it could be of no
direct service while it existed simply as the motion of stellar masses.
The motion, to be available, must be transformed into heat: the motion
of translation into molecular, or some other form of motion. This can be
done in no other way than by arresting the motion of the masses. But how
is such motion to be arrested? How are bodies as large as our earth,
moving at the rate of hundreds of miles per second, to have their motion
stopped? According to the theory this is effected by _collision_: by
employing the motion of the one body to arrest that of the other.

Take the case of the formation of our sun according to the theory.
Suppose two bodies, each one-half of the mass of the sun, moving
directly towards each other with a velocity of 476 miles per second.
These bodies would, in virtue of that velocity, possess 4149 × 10^{38}
foot-pounds of energy, which is equal to 100,000,000,000 foot-pounds per
pound of the mass; and this, converted into heat by the stoppage of
their motions, would suffice to maintain, as was previously stated, the
present rate of the sun’s radiation for a period of 50,000,000 years. It
must be borne in mind that, while 476 miles per second is the velocity
at the moment of collision, more than one-half of this would be derived
from the mutual attraction of the two bodies in their approach to each
other.

Coming in collision with such a velocity, the result would inevitably be
that the two bodies would shatter each other to pieces. But, although
their onward motions would thus be stopped, it is absolutely impossible
that the whole of the energy of their motions could be at once converted
into heat; and it is equally impossible that it could be annihilated.
Physical considerations enable us to trace, though in a rough and
general way, the results which would necessarily follow. The broken
fragments, now forming one confused mass, would rebound against one
another, breaking up into smaller fragments, and flying off in all
directions. As these fragments receded from the centre of dispersion
they would strike against each other, and, by their mutual impact,
become shivered into still smaller fragments, which would in turn be
broken up into fragments yet smaller, and so on as they proceeded
outwards. This is, however, only one part of the process, and a part
which would certainly take place, though no heat were generated by the
collisions.

A far more effective means of dispersing the fragments and shattering
them to pieces would be the expansive force of the enormous amount of
incandescent gas almost instantaneously generated by the heat of
collision. The general breaking up of the two masses and the stoppage of
their motions would be the work of only a few minutes, or a few hours at
most. The heat evolved by the arrested motion would, in the first
instance, be mainly concentrated on the surface layers of the broken
blocks. The layers would be at once transformed into the gaseous
condition, thus enveloping the blocks and filling the interspaces. It is
difficult to determine what the temperature and expansive force of this
gas would at the moment be, but evidently it would be excessive; for,
were the whole of the heat of the arrested motion distributed over the
mass, it would, as has been stated, amount to 100,000,000,000
foot-pounds per pound of the mass—an amount sufficient to raise 264,000
tons of iron 1° C. Thus, if we assume the specific heat of the gas to be
equal to that of air (viz. ·2374), it would have a temperature of about
300,000,000° C. or more than 140,000 times that of the voltaic arc.

I hardly think it will be deemed extravagant to assume that at the
moment after impact the temperature of the evolved gas would be at least
as great as here stated. If we assume it to be so, it is obvious that
the broken mass would, by the expansive force of the generated gas, be
dispersed in all directions, breaking up into fragments smaller and
smaller as they knocked against one another in their progress outwards
from the centre of dispersion; and these fragments would, at the same
time, become gradually converted into the gaseous state, and gradually
come to occupy a space as large as that embraced in our solar system. In
the course of time the whole would assume the gaseous condition, and we
should then have a perfect nebula—intensely hot, but not very luminous.
As its temperature diminished, the nebulous mass would begin to
condense, and ultimately, according to the well-known nebular
hypothesis, pass through all the different phases of rings, planets, and
satellites into our solar system as it now exists.

I am glad to find that the theory, in one of its main features, has been
adopted by Sir William Thomson,[4] the highest authority we have on all
points relating to the source of the sun’s heat.


[Footnote 4: Lecture on “The Probable Origin, the Total Amount, and the
Possible Duration of the Sun’s Heat,” delivered at the Royal Institution
on January 21, 1887, and published in _Nature_ of 27th of the same
month. The lecture was afterwards published with considerable additions
and alterations in the _Proceedings of the Institution_ vol. xii. It is
from this that my quotations are taken.]


“We cannot,” says Sir William, “help asking the question, What was the
condition of the sun’s matter before it came together and became hot?
(1) It may have been two cool, solid masses, which collided with the
velocity due to their mutual gravitation; or (2), but with enormously
less of probability, it may have been two masses colliding with
velocities considerably greater than the velocities due to their mutual
gravitation.”

He adopts the first of these suppositions. “To fix the idea,” he
continues, “think of two cool, solid globes, each of the same mean
density as the earth, and of half the sun’s diameter, given at rest, or
nearly at rest, at a distance asunder equal to twice the earth’s
distance from the sun. They will fall together and collide in exactly
half a year. The collision will last for about half an hour, in the
course of which they will be transformed into a violently agitated
incandescent fluid mass flying outward from the line of the motion
before the collision, and swelling to a bulk several times greater than
the sum of the original bulks of the two globes. How far the fluid mass
will fly out all around from the line of collision it is impossible to
say. The motion is too complicated to be fully investigated by any known
mathematical method; but with sufficient patience a mathematician might
be able to calculate it with some fair approximation to the truth. The
distance reached by the extreme circular fringe of the fluid mass would
probably be much less than the distance fallen by each globe before the
collision, because the translational motion of the molecules
constituting the heat into which the whole energy of the original fall
of the globes becomes transformed in the first collision is probably
about three-fifths of the whole amount of that energy. The time of
flying out would probably be less than half a year, when the fluid mass
must begin to fall in again towards the axis. In something less than a
year after the first collision the fluid will again be in a state of
maximum crowding round the centre, and this time probably even more
violently agitated than it was immediately after the first collision;
and it will again fly outward, but this time axially towards the places
whence the two globes fell. It will again fall inwards, and after a
rapidly subsiding series of quicker and quicker oscillations it will
subside, probably in the course of two or three years, into a globular
star of about the same dimensions, heat, and brightness, as our present
sun, but differing from him in this, that it will have no rotation.”[5]


[Footnote 5: _Proceedings of the Royal Institution_, vol. xii. p. 15.]


This is precisely what I have been contending for during the past twenty
years, with the simple exception that I assume, according to his second
supposition, that the “two masses collided with velocities considerably
greater than the velocities due to mutual gravitation.” Sir William
admits, of course, my supposition to be quite a possible one, but
rejects it on the supposed ground of its improbability. His reasons for
this, stated in his own words, are as follows:

“This last supposition implies that, calling the two bodies A and B for
brevity, the motion of the centre of inertia of B relatively to A must,
when the distance between them was great, have been directed with great
exactness to pass through the centre of inertia of A; such great
exactness that the rotational momentum or moment of momentum after
collision was no more than to let the sun have his present slow rotation
when shrunk to his present dimensions. This exceedingly exact aiming of
the one body at the other, so to speak, is, on the dry theory of
probability, exceedingly improbable. On the other hand, there is
certainty that the two bodies A and B at rest in space if left to
themselves, undisturbed by other bodies and only influenced by their
mutual gravitation, shall collide with direct impact, and therefore with
no motion of their centre of inertia, and no rotational momentum of the
compound body after the collision. Thus we see that the dry probability
of collision between two neighbours of a vast number of mutually
attracting bodies widely scattered through space is much greater if the
bodies be all given at rest than if they be given moving in any random
directions and with any velocities considerable in comparison with the
velocities which they would acquire in falling from rest into
collision.”

Sir William here argues that the second supposition is far less probable
than the first, because, according to it, the motion of the one body
relatively to the other must, in order to strike, be directed with great
exactness. The result, in such a case, is that collision will rarely
occur; whereas, according to the first supposition, the two bodies
starting from a state of rest will, by their mutual gravitation,
inevitably collide. According to the second hypothesis they will
generally miss; according to the first they will always collide.

I have been led to a conclusion directly opposed to Sir William’s. The
fact, that, according to the second supposition, collisions can but
rarely occur is one reason, amongst others, why I think that supposition
to be true; and the fact that, according to the first supposition,
collisions must frequently occur is also one reason, amongst others, why
I think it very improbable that it can represent the true condition of
things.

It by no means adds anything to the probability of the first supposition
to assert that, according to it, such collisions will occur readily and
frequently. On the contrary, it would show that the supposition was the
less likely to be true. If the collisions were insufficient in
character, the fewer of them that occurred, the better; for the result
of such collisions would simply be a waste of the potential energies of
the universe. We should in this case have an innumerable host of
imperfect suns without planets, or with at most only one or two, and
these at no great distance from the luminary. There would thus be
evolved a universe without any grand planetary systems. There is still
another objection to the supposition. The same gravitating force which
makes the dark bodies liable to come into collision with each other
must, of course, make them equally liable to come into collision with
the luminous bodies, and with our sun amongst the rest. Our sun would,
accordingly, be at the mercy of any of those masses which might happen
to come within the reach of its attractive influence. It would pull the
mass towards it, and a collision would be inevitable, unless it so
happened that a transverse motion of the sun itself might enable it to
escape destruction. Even in such a case it could not by any means manage
to get rid of the entangling mass.

All this risk, in so far as gravitation is concerned, would have been
completely averted if an original projected velocity of some thirty or
forty miles per second had been conferred on the dark mass; for, in this
case, the attractive force of the sun would fail to arrest its motion,
and the mass would pass onward through space, never to return. This
simple conception of an original motion removes entirely those
objections which, we have seen, besets the supposition we have been
considering. With such a motion, not only would the risk to our solar
system be removed, but the collisions between the dark bodies themselves
would be a matter of rare occurrence; and hence the energy of the
universe would be conserved. And when a collision did happen it would be
on a grand scale, and the result would be not an imperfect sun without
planets, but an incandescent nebula, out of which, by condensation, a
complete solar system would be evolved. In fact, within the whole range
of cosmical physics, I know of nothing more impressive in its sublime
simplicity than this plan, by which the stability and perfection of the
universe is thus secured. How vast the ends—how simple the means!


CONSIDERATION OF THE FACTS WHICH SUPPORT THE THEORY, AND OF THE
    LIGHT WHICH THE THEORY APPEARS TO CAST UPON THE FACTS.


                  I. _Probable Origin of Meteorites._

Recent researches establish beyond doubt that stars, nebulæ, comets and
meteorites, do not differ much from our earth in their chemical
constitution. Meteorites, it is true, differ in their physical
characteristics from ordinary rock such as is found on the earth’s
surface. But it is possible, if not probable, that the earth’s interior
mass “may,” as Sir Henry Roscoe remarks, “partake of the physical nature
of these metallic meteorites, and that if we could obtain a portion of
matter from a great depth below the earth’s surface we should find it
exactly corresponding in structure as well as in chemical composition
with a metallic meteorite, and the existence of such interior masses of
metallic iron may go far to explain the well-known magnetic condition of
our planet.”[6] I think there can be little doubt that, were our earth
broken up into small fragments, and these scattered into space, it would
probably be impossible to distinguish them from ordinary meteorites. The
two would be so like in character that one can hardly resist the
conviction that meteorites are but the fragments of sidereal masses
which have been shattered by collision. That meteorites are broken
fragments is the opinion expressed by Sir William Thomson, who says
“that he cannot but agree with the common opinion which regards
meteorites as fragments broken from larger masses, and that we cannot be
satisfied without trying to imagine what were the antecedents of those
masses.” The theory we have been considering appears to afford an
explanation of their antecedents. According to it, they are broken
fragments of two dark stellar masses which were shattered to pieces by
collision. After what has been stated concerning the production of the
gaseous nebulæ out of which our solar system was formed, it must be
regarded as highly improbable, if not impossible, that the whole of the
fragments projected outwards with such velocity should be converted into
the gaseous condition. Multitudes of the smaller fragments, especially
those towards the outer circumference of the nebulous mass, meeting with
little or no obstruction to their onward progress, would pass outwards
into space with a velocity which would carry them beyond the risk of
falling back into the nebula. They would then continue their progress in
their separated forms as meteorites. If this be their origin, then
meteorites are the offspring of sidereal masses, and not their parents,
as Mr. Lockyer concludes.


[Footnote 6: _Manchester Science Lectures_, Fifth Series, p. 31.]


These meteorites must be of vast antiquity, for if they are fragments of
the dark bodies then they must be not only older than our solar system,
but older than the nebula from which that system was formed. Some of
them, however, may have come from other systems. They are fragments
which may yet cast some light on the history of the dark bodies.

Comets, bodies which in many points seem allied to meteorites, probably
have, as we shall shortly see, a similar origin.


 II. _Motion of the Stars; how of such different velocities, and always
    in straight lines._

It will be only when the two bodies, coming from contrary directions,
collide with equal momentum that the entire motion will be stopped. But
in the case of stellar masses moving, as it were, at random in every
direction this is a condition which will but rarely occur. Accordingly,
in most cases the resulting stars will have more or less motion. In
short, the stars should, according to the theory, be moving in all
directions and with all varieties of velocity. Further, it follows that
these motions ought to be in perfectly straight lines, and not in
definite orbits of any kind. So far as observation has yet determined,
all these conditions seem to be fulfilled. Sometimes it will happen that
the two bodies will strike each other obliquely. In this case the
resulting star, both as to the direction and velocity of its motion,
will, to a large extent, be the resultant of the two concurrent forces.


    III. _Motion of the Stars not due to their Mutual Attractions._

According to the theory the absolute motion of the stars is due, not to
the influence of gravity, but to motions which originally belonged to
the two component masses out of which the star arose; motion regarding
the origin of which science can no more inform us than it can regarding
the origin of the masses themselves. There is strong presumptive
evidence that the motion of the stars is due to this cause. We know that
there are stars which have a far greater velocity than can result from
gravitation, such, for example, as the star 1830 Groombridge, which has
a velocity of 200 miles per second. Suppose, with Professor Newcomb,
that the number of stars belonging to the universe amounts to
100,000,000, and that these have, on the average, five times the mass of
the sun, and are spread out in a layer across which light requires
30,000 years to pass. Then computation shows that, unless the attractive
power of the whole were sixty-four times greater than it really is, it
could not have conferred on Groombridge the motion which it possesses,
or arrest it in its onward course.[7] We are therefore forced, as
Professor Newcomb remarks, to one of two alternatives, viz.: “Either the
bodies which compose our universe are vastly more massive and numerous
than telescopic examination seems to indicate, or 1830 Groombridge is a
runaway star, flying on a boundless course through infinite space, with
such momentum that the attraction of all the bodies of the universe can
never stop it.”


[Footnote 7: Newcomb’s _Astronomy_, p. 487, English edition, 1878.]


As regards the theory we are discussing, it is the same which
alternative is taken, for both are equally favourable. If the former,
then, according to the theory that stellar heat had its origin in
collision, it is presumptive evidence that space is occupied by dark
bodies far more numerous and massive than the luminous ones which the
telescope reveals. If the latter, viz. that the star has a velocity
which never could have been produced by attraction, “then,” as says
Professor Newcomb, “it must have been flying forward through space from
the beginning, and, having come from an infinite distance, must now be
passing through our system for the first and only time.” The probability
is, however, that the star derived its motion from the source from which
it derived its light and heat; namely, from the collision of the two
masses out of which it arose. If the star is ever to be arrested in its
onward course, it must be by collision; but such an event would be its
final end.

There are other stars, such as 61 Cygni, ε Indi, Lalande 21258, Lalande
21185, μ Cassiopeiæ, and Arcturus, possessed of motions which could not
have been derived from gravity. And there are probably many more of
which, owing to their enormous distances, the proper motions have not
been detected. α Centauri, the nearest star in the heavens, by less than
one-half, is distant twenty-one millions of millions of miles; and there
are, doubtless, many visible stars a thousand times more remote. A star
at this distance, though moving transversely to the observer at the
enormous rate of 100 miles per second, would take upwards of thirty
years to change its position so much as one second, and consequently
1,800 years to change its position one minute. In fact, we should have
to watch the star for a generation or two before we could be certain
whether it was moving or not.


                    IV. _Probable Origin of Comets._

Great difficulty has been experienced in accounting for the origin of
comets upon the nebular hypothesis. They approach the sun from all
directions, and their motions, in relation to the planets, are as often
retrograde as direct. Not only are their orbits excessively elliptical,
but they are also inclined to the ecliptic at all angles from 0° to 90°.
It is evidently impossible to account satisfactorily for the origin of
comets if we assume them all to have been evolved out of the solar
nebula, although this has been attempted by M. Faye and others. Comets
are evidently, as Laplace and Professor A. Winchell both conclude,
strangers to our system, and have come from distant regions of space. If
they belonged to the solar system they could not, says Professor
Winchell, have parabolic and hyperbolic paths. “Only a small portion of
the comets,” he remarks, “are known to move in elliptic orbits.”[8] This
assumption that they are foreigners will account for all the
peculiarities of their motions; but how are we to account for their
coming into our system? How did they manage to leave that system in
which they had their origin? If a comet have come from one of the fixed
stars trillions of miles distant, the motion by which it traversed the
intervenient space could not, possibly, have been derived from gravity.
We are therefore obliged to assume that the motion was a projected
motion. Comets, in all probability, have the same origin as meteorites.
The materials composing them, like those of the meteorites, were
probably projected from nebulæ by the expulsive force of the heat of
concussion which produced the nebulæ. Some of them, especially those
with elliptic orbits, may have possibly been projected from the solar
nebula.


[Footnote 8: _World Life._ p. 27.]


                              V. _Nebulæ._

It is a curious circumstance that the theory here advanced seems to
afford a rational explanation of almost every peculiarity of nebulæ, as
I have, on former occasions, endeavoured, at some length, to prove.[9]


[Footnote 9: _Philosophical Magazine_, July 1878; _Climate and
Cosmology_, Chap. xix.]


1. _Origin of nebulæ._—We have already seen that the theory affords a
rational account of the origin of nebulæ.

2. _How nebulæ occupy so much space._—It accounts for the enormous
_space_ occupied by nebulæ. It may be objected that, enormous as would
be the original temperature of the solar system produced by the primeval
collision, it would nevertheless be insufficient to expand the mass,
against gravity, to such an extent that it would occupy the entire space
included within the orbit of Neptune. But it will be perceived, from
what has already been stated regarding the dispersion of the materials
before they had sufficient time to assume the gaseous condition, that
this dispersion was the main cause of the gaseous nebula coming to
occupy so much space. And, to go farther back, it was the suddenness and
almost instantaneity with which the mass would receive the entire store
of energy, before it had time to assume even the molten, not to say the
gaseous, condition, which led to tremendous explosions, followed by a
wide dispersion of materials.

3. _Why nebulæ are of such varied shapes._—Although the dispersion of
the materials would be in all directions, it would, according to the law
of probability, very rarely take place uniformly in every direction.
There would generally be a greater amount of dispersion in some
directions than in others, and the materials would thus be carried along
various lines and to diverse distances; and, although gravity would tend
to bring the widely scattered materials ultimately together into one or
more spherical masses, yet, owing to the exceedingly rarified condition
of the gaseous mass, the nebulæ would change form but slowly.

4. _Broken fragments in a gaseous mass of an excessively high
temperature the first stage of a nebula._—From what has already been
shown, it will be seen that after the colliding of the two dark bodies
the first condition of the resulting nebula would be an enormous space
occupied by broken fragments of all sizes dashing against each other
with tremendous velocities, like the molecules in a perfect gas. All the
interspaces between those fragments would be entirely filled with a
gaseous mass, which, at its earliest stages at least, as in the case of
the solar nebula, would have a temperature probably more than one
hundred thousand times that of the voltaic arc. Whether such a mass
would be visible is a point which can hardly be determined, as we can
have no experience on earth of a gas at such a temperature.

That there are some of the nebulæ which appear to consist of solid
matter interspersed in a gaseous mass is shown by the researches of Mr.
Lockyer[10] and others. In fact, the theory is held by Professor
Tait[11] that nebulæ consist of clouds of stones—or meteor-swarms, as
Mr. Lockyer would term them—in an atmosphere of hydrogen, each stone of
which, moving about and coming into collision with some other, is
thereby generating heat which renders the circumambient gas
incandescent. In reference to this theory of Professor Tait, Mr. Lockyer
says that the phenomena of the spectroscope can be quite well explained
“on the assumption of a cloud of stones, providing always that you could
at the same time show reasonable cause why these clouds of stones were
‘banging about’ in an atmosphere of hydrogen.”[12] The theory, however,
does not appear to afford any rational explanation of this banging about
of the stones to and fro in all directions; for, according to it, the
only force available is gravitation, and this can produce merely a
motion of the materials towards the centre of the mass. Under these
conditions very little impinging of the stones against each other would
take place. But, according to the theory here adopted, we have an agency
incalculably more effective than gravity, one which accounts not merely
for the impact of the stones, but for their very existence as such,
inasmuch as it explains both what they are and whence they came.


[Footnote 10: _Proceedings of Royal Society_, vol. xliii. p. 117.]

[Footnote 11: _Good Words_ for 1875, p. 861.]

[Footnote 12: _Manchester Science Lectures._]


Mr. Lockyer has recently fully adopted Professor Tait’s suggestion as to
the nature and origin of nebulæ, and has endeavoured to give it further
development. He considers the nebulæ to be composed of sparse
meteorites, the collisions of which give the nebulæ their temperature
and luminosity. He divides the nebulæ into three groups, “according as
the formative action seems working towards a centre; round a centre in a
plane, or nearly so; or in one direction only.” As a result we have
globular, spheroidal, and cometic nebulæ.

_Globular nebulæ_ he accounts for in the following manner. “If we,” he
says, “for the sake of the greatest simplicity consider a swarm of
meteorites at rest, and then assume that others from without approach it
from all directions, their previous paths being deflected, the question
arises whether there will not be at some distance from the centre of the
swarm a region in which collisions will be most valid. If we can answer
this question in the affirmative, it will follow that some of the
meteorites arrested here will begin to move in almost circular orbits
round the common centre of gravity.

“The major axes of these orbits may be assumed to be not very diverse,
and we may further assume that, to begin with, one set will preponderate
over the rest. Their elliptic paths may throw the periastron passage to
a considerable distance from the common centre of gravity; and if we
assume that the meteorites with this common mean distance are moving in
all planes, and that some are direct and some retrograde, there will be
a shell in which more collisions will take place than elsewhere. _Now,
this collision surface will be practically the only thing visible, and
will present to us the exact and hitherto unexplained appearance of a
planetary nebula—a body of the same intensity of luminosity at its edge
and centre—thus putting on an almost phosphorescent appearance._

“If the collision region has any great thickness, the centre should
appear dimmer than the portion nearer the edge.

“Such a collision surface, as I use the term, is presented to us during
a meteoric display by the upper part of our atmosphere.”[13]


[Footnote 13: _Proc. of Royal Society_, vol. xliv. p. 5.]


_Spheroidal nebulæ_, he considers, are produced by the rotation of what
was at first a globular rotating swarm of meteorites.

_Cometic nebulæ_ are explained, he considers, “on the supposition that
we have either a very condensed swarm moving at a very high velocity
through a sheet of meteorites at rest, or the swarm at rest surrounded
by a sheet, all moving in the same direction.”

In an able and interesting work, which seems almost utterly unknown in
England,[14] Professor Winchell has advanced views similar to those of
Tait and Lockyer regarding the nature and origin of nebulæ. But he, in
addition, discusses the further question of the origin of those swarms.
I shall have occasion to refer to Professor Winchell’s views more fully
when we come to the consideration of the pre-nebular condition of the
universe.


[Footnote 14: _World Life, or Comparative Geology_, by Alexander
Winchell, LL.D., Professor of Geology and Palæontology in the University
of Michigan. Chicago: S. C. Griggs & Co. 1883.]


Amongst the first to advance the meteoric hypothesis of the origin and
formation of the solar system was probably the late Mr. Richard A.
Proctor. This was done in his work, “Other Worlds than Ours,” published
in 1870. “Under the continual rain of meteoric matter,” he says, “it may
be said that the earth, sun, and planets are _growing_. Now, the idea
obviously suggests itself that the whole growth of the solar system,
from its primal condition to its present state, may have been due to
processes resembling those which we now see taking place within its
bounds.” He further adds: “It seems to me that not only has this general
view of the mode in which our system has reached its present state a
greater support from what is now actually going on than the nebular
hypothesis of Laplace, but that it serves to account in a far more
satisfactory manner for the principal peculiarities of the solar system.
I might, indeed, go farther, and say that where those peculiarities seem
to oppose themselves to Laplace’s theory they give support to those I
have put forward.”[15] He then goes on to show the points wherein his
theory seems to him to offer a better explanation of those peculiarities
than that of Laplace.


[Footnote 15: _Other Worlds_, chap. ix.]


5. _The gaseous condition the second stage of a nebula._—The second
stage obviously follows as a necessary consequence from the first; for
the fragments, in the case under consideration, possess energy in the
form of motion, which, with the heat of their circumambient vapour, is
more than sufficient not only to convert the fragments into the gaseous
state, but to produce complete dissociation of the chemical elements.
The complete transformation of the first stage into the second must,
therefore, be simply a matter of time.

According to the laws of probability it may, however, sometimes happen
that the two original dark bodies will not collide with force sufficient
to confer on the broken fragments the energy required to convert them
all into the gaseous condition. The result in this case would, no doubt,
be that the untransformed fragments, drawn together by their mutual
attractions, would collide and form an imperfect star or sun, without a
planet. Such a star might continue luminous for a few thousands or
perhaps a few millions of years, as the case might be, when it would
begin to fade, and finally disappear. We have here an imperfect nebula,
resulting in an imperfect star. In short, we should have in those
stellar masses, on a grand scale, what we witness every day around us in
organic nature, viz. imperfect formations. Such occasional imperfections
give variety and add perfection to the whole. How dreary and monotonous
would nature be, were every blade of grass, every plant, every animal,
and every face we met formed after the most perfect model!

6. _The gaseous condition essential to the nebular hypothesis._—It is
found that the density of the interior planets of our solar system
compared with that of the more remote is about as five to one. The
obvious conclusion is that there is a preponderance of the metallic
elements in the interior planets and of metalloids in the exterior. It
thus becomes evident, as Mr. Lockyer has so clearly shown,[16] that when
our solar system existed in a nebulous condition the metallic or denser
elements would occupy the interior portion of the nebula and the
metalloids the exterior. Taking a section of this nebula from its centre
to its circumference, the elements would in the main be found arranged
according to their densities: the densest at the centre, and the least
dense at the circumference. If we compare the planets with their
satellites, we find the same law holding true. The satellites of
Jupiter, for example, have a density of about only one-fifth of that of
the planet, or about one twenty-fifth of that of our earth, showing that
when the planet was rotating as a nebulous mass the more dense elements
were in the central parts and the less dense at the outer rim, where the
satellites were being formed. Again, if we take the case of our globe,
we find, as Mr. Lockyer remarks, the same distribution of materials,
proving that when the earth was in the nebulous state the metallic
elements chiefly occupied the central regions, and the metalloids those
outer parts which now constitute the earth’s crust.


[Footnote 16: _Manchester Science Lectures._]


All these facts show that the _sifting_ and _sorting_ of the chemical
elements according to their densities must have taken place when our
solar system was in the condition of a nebula. But, further, it seems
impossible that this could have taken place had the materials composing
the nebula been in the solid form, even supposing that they had taken
the form of clouds of stones.

It is equally impossible that the nebula could have been in the fluid or
liquid state during this process. This is obvious, for the nebula must
then have occupied, at least, the entire space within the orbit of the
most remote planet. But our solar system in the liquid condition could
not occupy one-millionth part of that space. It is therefore evident
that the nebula must have been in the state of a gas, and a gas of
extreme tenuity.

7. _The mass must have possessed an excessive temperature._—There is
ample evidence, Mr. Lockyer thinks, to show that the temperature of the
solar nebula was as great as that of the sun at the present time. But I
think it is extremely probable that, in some of its stages, the nebula
had a very much higher temperature than that now possessed by the sun.
There must, during the sifting period, have been complete chemical
dissociation, so as to keep the metals and the metalloids uncombined,
and thus allow the elements to arrange themselves according to their
densities. The nebula hypothesis, remarks Mr. Lockyer, “is almost
worthless unless we assume very high temperatures, because, unless you
have heat enough to get perfect dissociation, you will not have that
sorting out which always seems to follow the same law.”

8. _Gravitation could, under no possible condition, have generated the
amount of heat required by the nebular hypothesis._—The nebular
hypothesis does not profess to account for the origin of nebulæ. It
starts with matter existing in space in the nebulous condition, and
explains how, by condensation, suns, planets &c. are formed out of it.
In fact, it begins at the middle of a process: it begins with this fine,
attenuated material in the process of being drawn together and condensed
under the influence of attraction, and professes to explain how, as the
process goes on, a solar system necessarily results. To simplify our
inquiry we shall confine our attention to the solar nebula, and consider
in the first place how far condensation may be regarded as a sufficient
source of heat.

A. _Condensation._—The heat which our nebula could have derived from
condensation up to the time that Neptune was detached from the mass, no
matter how far the outer circumference of the mass may have originally
extended beyond the orbit of that planet, could not have amounted to
over 1/7,000,000 of a thermal unit (772 foot-pounds) for each cubic
foot. It is perfectly obvious that this amount could not have produced
the dissociation required; and without the required dissociation Neptune
could never have been formed. Further, it is physically impossible that
the materials of which our solar system are composed could have existed
in the gaseous state in a cool condition prior to condensation. Unless
possessed of great heat, even hydrogen could not exist in stellar space
in the gaseous form; and far less could carbon, iron, platinum, &c.
Before Neptune could have been formed the whole of the materials of the
system must have possessed heat, not only sufficient to reduce them to
the gaseous state, but sufficient to produce complete dissociation. But
by no conceivable means could gravitation have conferred this amount of
heat by the time that the mass had condensed to just within the limits
of the orbit of Neptune.

B. _Solid globes colliding under the influence of gravity alone._—As we
have already seen, the view has been adopted by Sir W. Thomson that the
solar nebula may have resulted from the colliding of cold, solid globes
with the velocity due to their mutual gravitation alone. He states his
views as follows:

“Suppose, now, that 29,000,000 cold, solid globes, each of about the
same mass as the moon, and amounting in all to a total mass equal to the
sun’s, are scattered as uniformly as possible on a spherical surface of
radius equal to one hundred times the radius of the earth’s orbit, and
that they are left absolutely at rest in that position. They will all
commence falling towards the centre of the sphere, and will meet there
in 250 years, and every one of the 29,000,000 globes will then, in the
course of half an hour, be melted, and raised to a temperature of a few
hundred thousand or a million degrees Centigrade. The fluid mass thus
formed will, by this prodigious heat, be exploded outwards in vapour or
gas all round. Its boundary will reach to a distance considerably less
than one hundred times the radius of the earth’s orbit on first flying
out to its extreme limit. A diminishing series of out-and-in
oscillations will follow, and the incandescent globe, thus contracting
and expanding alternately, in the course, it may be, of 300 or 400
years, will settle to a radius of forty times the radius of the earth’s
orbit.”[17]


[Footnote 17: _Proceedings of Royal Institution_, vol. xii. p. 16.]


The reason which he assigns for the incandescent globe settling down at
a radius forty times that of the earth’s orbit is as follows: “The
radius of a steady globular gaseous nebula of any homogeneous gas is 40
per cent. of the radius of the spherical surface from which its
ingredients must fall to their actual positions in the nebula to have
the same kinetic energy as the nebula has.”

If the solar nebula thus produced would be swelled out into a spherical
incandescent mass with a radius 40 times the radius of the earth’s
orbit, simply because the globes fell from a distance of 100 times the
radius of that orbit, then for a similar reason the mass would have a
radius of 400 times that of the earth’s orbit had the globes fallen from
a distance of 1,000 times the radius, and 400,000 times if the globes
had fallen from a distance of 1,000,000 times the radius, and two-fifths
of any conceivable distance from which they may have fallen.

Supposing all this to be _physically_ possible, which it undoubtedly is
not, still the heat generated would not be sufficient; for, whatever the
radius of the nebula might be, its entire energy, both kinetic and
potential, is simply what is obtained from gravitation, and this, as we
have seen, is insufficient.

9. _Condensation the third and last condition of a nebula._—According to
the gravitation theory, condensation is the first stage of a nebula as
well as the last; for, according to it, gravity is the force which both
collects together the scattered materials and gives them their heat.[18]
Before condensation begins there can, according to the gravitation
theory, be no such thing as a nebula properly so called. The materials
exist, of course, but they do not exist in the form of a nebula.
According to the impact theory which I here advocate, condensation
cannot begin till after the nebula has begun to lose the heat with which
it was originally endowed.


[Footnote 18: Laplace held a more accurate view of the primitive
condition of the solar nebula. He considered that, owing to intense
heat, the solar mass became expanded to the limits of the remotest
planetary orbit of our system; that, in cooling, it began slowly to
condense; and that, as condensation went on, planet after planet became
detached from the mass. Laplace, however, offered no explanation of the
manner in which the primitive nebula obtained its heat.]


10. _How nebulæ emit such feeble light._—The light of nebulæ is mainly
derived from glowing hydrogen and nitrogen in a condition of extreme
gaseous tenuity; and it is well known that these gases are exceedingly
bad radiators. The oxyhydrogen flame, although its temperature is
surpassed only by that of the voltaic arc, gives a light so feeble as to
be scarcely visible in daylight. The small luminosity of nebulæ is,
however, mainly due to a different cause. The enormous space occupied by
those bodies is not so much due to the heat which they possess as to the
fact that their materials were dispersed into space before they had time
to pass into the gaseous condition; so that, by the time that this
latter state was assumed, the space occupied was far greater than was
demanded either by the temperature or by the amount of heat which they
originally received. If we adopt the nebular hypothesis of the origin of
our solar system, we must assume that our sun’s mass, when in the
condition of a nebula, extended beyond the orbit of the planet Neptune,
and consequently filled the entire space included within that orbit.
Even supposing Neptune’s orbit to have been its outer limit, which,
obviously, was not the case, it would nevertheless have occupied
274,000,000,000 times the space it does at present. We shall assume, as
before, that 50,000,000 years’ heat was generated by the concussion. Of
course, there might have been twice or even ten times that quantity; but
it is of no importance what amount is in the meantime adopted. Enormous
as 50,000,000 years’ heat is, it yet gives, as we shall presently see,
only 32 foot-pounds of energy for each cubic foot. The amount of heat
due to concussion being equal, as before stated, to 100,000,000,000
foot-pounds for each pound of the mass, and a cubic foot of the sun at
his present density of 1·43 weighing 89 pounds, each cubic foot must
have possessed 8,900,000,000,000 foot-pounds. But when the mass was
expanded sufficiently to occupy 274,000,000,000 times its original space
(which it would do when it extended to the orbit of Neptune), the heat
possessed by each cubic foot would then amount to only 32 foot-pounds.

In point of fact it would not even amount to so much, for a quantity
equal to upwards of 20,000,000 years’ heat would necessarily be consumed
in work against gravity in the expansion of the mass, all of which
would, of course, be given back in the form of heat as the mass
contracted. During the nebulous condition, however, this quantity would
exist in an entirely different form, so that only 19 out of the 32
foot-pounds per cubic foot generated by concussion would then exist as
heat. The density of the nebula would be only 1/16,248,160 that of
hydrogen at ordinary temperature and pressure. The 19 foot-pounds of
heat in each cubic foot would thus be sufficient to maintain an
excessive temperature; for there would be in each cubic foot only
1/440,000 of a grain of matter. But, although the _temperature_ would be
excessive, the _quantity_ both of light and heat in each cubic foot
would of necessity be small. The heat being only 1/71 of a thermal unit,
the light emitted would certainly be exceedingly feeble, resembling very
much the electric light in a vacuum-tube.


                         VI. _Binary Systems._

The theory affords a rational explanation of the origin of binary stars.
Binary stars, in so far as regards their motion, follow also, of course,
as a consequence, from the gravitation theory. If two bodies come into
grazing collision, “they will,” says Sir William Thomson, “commence
revolving round their common centre of inertia in long elliptic orbits.
Tidal interaction between them will diminish the eccentricities of their
orbits, and, if continued long enough, will cause them to revolve in
circular orbits round their centre of inertia.”[19] This conclusion was
pointed out many years ago by Dr. Johnstone Stoney.


[Footnote 19: _Proceedings of Royal Institution_, vol. xii. p. 15.]


                   VII. _Sudden Outbursts of Stars._

The case of a star suddenly blazing forth and then fading away, such as
that observed by Tycho Brahe in 1572, may be accounted for by supposing
that the star had been struck by one of the dark bodies—an event not at
all impossible, or even improbable. In some cases of sudden outbursts,
such as that of Nova Cygni, for example, the phenomenon may result from
the star encountering a swarm of meteorites. The difficulty in the case
of Nova Cygni is to account for the very sudden decline of its
brilliancy. This might, however, be explained by supposing that the
outburst of luminosity was due to the destruction of the meteorites, and
not to any great increase of heat produced in the star itself. A swarm
of meteorites converted into incandescent vapour would not be long in
losing its brilliancy.

Mr. Lockyer thinks that the outburst was produced by the collision of
two swarms of meteorites, and not by the collision of the meteorites
with a previously existing star.[20]


[Footnote 20: _Proceedings of the Royal Society_, vol. xliii. p. 140.]


Amongst the millions of stars occupying stellar space catastrophes of
this sort may, according to the theory, be expected sometimes to happen,
although, like the collisions which originate stars themselves, they
must, doubtless, be events of but rare occurrence.


                         VIII. _Star Clusters._

A star cluster will result from an immensely widespread nebula breaking
up into a host of separate nuclei, each of which becomes a star. The
irregular manner in which the materials would, in many cases, be widely
distributed through space after collision would prevent a nebula from
condensing into a single mass. Subordinate centres of attraction would
be established, as was long ago shown by Sir William Herschel in his
famous memoir on the formation of stars;[21] and around these the
gaseous particles would arrange themselves and gradually condense into
separate stars, which would finally assume the condition of a cluster.


[Footnote 21: _Philosophical Transactions_ for 1811.]


              IX. _Age of the Sun’s Heat: a Crucial Test._

When we come to the question of the age of the sun’s heat, and the
length of time during which that orb has illuminated our globe, it
becomes a matter of the utmost importance which of the two theories is
to be adopted. On the age of the sun’s heat rests the whole question of
geological time. A mistake here is fundamental. If gravitation be the
only source from which the sun derived its heat, then life on the globe
cannot possibly date farther back than 20,000,000 years; for under no
possible form could gravitation have afforded, at the present rate of
radiation, sufficient heat for a longer period. It will not do to state
in a loose and general way, as has been frequently done, that the sun
may have been supplying our globe with heat at its present rate for
20,000,000 or 100,000,000 years, for gravitation could have done no such
thing; a period of 20,000,000, not 100,000,000, years is the lowest
which is admissible on that theory. Not even that length of time would
be actually available; for this period is founded on Pouillet’s estimate
of the rate of solar radiation, which has been proved by Langley to be
too small, the correct rate being 1·7 times greater. “Thus,” as says Sir
W. Thomson, “instead of Helmholtz’s 20,000,000 years, we have only
12,000,000.” But the 12,000,000 years would not in reality be available
for plant and animal life; for undoubtedly millions of years would
elapse before our globe could become adapted for either flora or fauna.
If there is no other source of heat for our system than gravitation, it
is doubtful if we can calculate on much more than half that period for
the age of life on the earth. Professor Tait probably over-estimates the
time when he affirms “that 10,000,000 years is about the utmost that can
be allowed, from the physical point of view, for all the changes that
have taken place on the earth’s surface since vegetable life of the
lowest known form was capable of existing there.”[22] And this is
certainly about all that can ever be expected from gravitation;
mathematical computation has demonstrated that it can give no more. The
other theory, founded on motion in space—a cause as real as
gravitation—labours under no such limitation. According to it, so far at
least as regards the store of energy which may have been possessed by
the sun, plant and animal life may date back, not to 10,000,000 years,
but to a period indefinitely more remote. In fact, there is as yet no
known limit to the amount of heat which this cause may have produced;
for this depended upon the velocities of the two bodies at the moment
prior to collision, and what these velocities were we have no means of
knowing. They might have been 500 miles a second, or 5,000 miles a
second, for anything which can be shown to the contrary. Of course I by
no means affirm that it is as much as 100,000,000 years since life began
on our earth; but I certainly do affirm that, in so far as a possible
source of the sun’s energy is concerned, life may have begun at a period
as remote.


[Footnote 22: _Recent Advances in Physical Sciences_, p. 175.]



                                PART II.

  _EVIDENCE IN SUPPORT OF THE THEORY FROM THE AGE OF THE SUN’S HEAT._


TESTIMONY OF GEOLOGY AND BIOLOGY AS TO THE AGE OF THE SUN’S HEAT.

The question which we have now to consider is—to which of the two
theories does geology lend its testimony? Will the length of time which,
according to the gravitation theory, can possibly be allotted satisfy
the requirements of geology? In short, are the facts of geology
reconcilable with the theory? If not, the theory must be abandoned.

Before the period when geologists felt that they were limited to time by
physical considerations, the most extravagant opinions prevailed in
regard to the length of geological epochs. So long as the physicist
continued to state in a loose and general way that the sun might have
been supplying our earth with heat at his present rate for the past
100,000,000 years, no very serious difficulty was felt; but when
geologists came to understand that ten or twenty millions of years were
all that could be granted to them, the condition of matters was entirely
altered. The belief that the mathematical physicist must be right in his
views as to the age of the sun’s heat, and that there is no possibility
of a longer period being admitted, seems at present to be leading
geologists towards the opposite extreme in regard to the length of
geological time. Attempts have been recently made to compress the
geological history of our globe into the narrow space allotted by the
physicist. The attempt is hopeless, as well as injurious to geological
science. What misleads is not the belief that gravitation could not
possibly afford a supply of heat sufficient for more than 20,000,000
years, for this is true; it is the belief that there was no other source
of heat than gravity.

We shall now consider the evidence which geology seems to afford as to
the age of the sun’s heat. Geology is quite competent to render aid on
this point, for the sun’s heat must be at least as old as life on this
globe; and the record of the rocks tells us when this life first
appeared. We require, however, to be able to measure the time which has
elapsed since these records were left. What we want is absolute time;
not relative time. Much has been done by geologists in regard to
relative time; but this can be of no service to us in our present
inquiry. Unfortunately very little trustworthy work has been done in the
way of determining the absolute length of geological periods. Happily,
however, great exactness of measure is not required. A rough
approximation to the truth will suffice for our present purpose. If it
can be shown to be more than fifteen or twenty millions of years since
life first appeared on the earth, it will as effectually prove that
gravitation alone could not have been the source from which the sun
derived his heat as if it were shown that that period was a thousand
times more remote. All we have to do is simply to assign an _inferior
limit_ to the age of life on the earth; and this can be effectually done
by means of the methods, imperfect though they be, which we have at
command. As the question of geological time is of some importance in
relation to our present inquiry, I shall consider it at some length.

_Testimony of Geology: method employed._—What has subsequently proved to
be a pretty successful method of measuring geological time suggested
itself to my mind during the summer of 1865. It then occurred to me that
we might obtain a tolerably accurate measure of absolute geological time
from the present rate of subaërial denudation, which might be
ascertained in the following way: The rate of subaërial denudation must
be equal to the rate at which materials are carried off the land into
the sea; and this is measured by the rate at which sediment is carried
down by our river systems. _Consequently, in order to determine the
present rate of subaërial denudation, we have only to ascertain the
quantity of sediment annually carried down by the river systems._ This
gives us the time required to remove any given quantity, say one foot,
off the face of the country. If we assume the rate to be pretty much the
same during past geological ages, we have a means of telling the time
that was occupied in removing any known thickness of strata. But as we
never can be perfectly certain that the rate is the same in both cases,
the results can, of course, be regarded as only approximately true.

Taking the quantity of sediment discharged into the sea annually by the
Mississippi river, as determined by Messrs. Brown and Dickson,[23] I
found that it amounted to one foot off the face of the country in 1,388
years, and that, at this rate of denudation, our continents, even if
they had an elevation of 1,000 feet, would not remain above sea-level
over 1,500,000 years.[24] This was an exaggerated estimate of the
quantity of sediment, for I shortly afterwards found that far more
accurate determinations were made by Messrs. Humphreys and Abbot,[25]
who were employed by the United States Government to report upon the
physics and hydraulics of the Mississippi. Messrs. Brown and Dickson had
estimated the quantity of sediment at 28,188,083,892 cubic feet, whereas
Messrs. Humphreys and Abbot found it to be only 6,724,000,000 cubic
feet, or less than one-fourth that amount. This gives one foot in 6,000
years as the rate of denudation.


[Footnote 23: _Proceedings of the American Association for the
Advancement of Science_ for 1848.]

[Footnote 24: _Philosophical Magazine_, February 1867. I was not aware
at this time that Mr. Alfred Tylor had previously applied the same
method to determine an entirely different point, viz.: how much the
sea-level is being raised by the sediment deposited on the sea-bottom.
Mr. Tylor’s paper, entitled “On Changes of the Sea-Level effected by
existing Physical Causes during stated Periods of Time,” appeared in the
_Phil. Mag._ for April 1853. Mr. Tylor came to the conclusion that the
sea-level was being raised, from this cause, about 3 inches in 10,000
years.]

[Footnote 25: _Report upon the Physics and Hydraulics of the
Mississippi._]


At this time Dr. Archibald Geikie took up the question and went into the
consideration of the subject in a most thorough manner; and it is mainly
through the instrumentality of his writings on the matter[26] that the
method under consideration has gained such wide-spread acceptance among
geologists. After an examination of nearly all that is known regarding
the amount of sediment carried down by rivers, he drew up the following
table, showing the number of years required by seven rivers to remove
one foot of rock from the general surface of their basins.

                      Danube           6,846 years
                      Mississippi      6,000   „
                      Nith             4,723   „
                      Ganges           2,358   „
                      Rhone            1,528   „
                      Hoang-Ho         1,464   „
                      Po                 729   „
                                       ----- -----
                                  Mean 3,378 years

This gives a mean of 3,378 years to remove one foot, or a little over
one-half the time taken by the Mississippi. This mean appears to be
generally taken as representing the average rate of subaërial denudation
of the whole earth, but it has, I fear, been rather too hastily adopted.
To estimate correctly the quantity of sediment annually discharged by a
large river is a most difficult and laborious undertaking. A perusal of
the voluminous report of Messrs. Humphreys and Abbot, extending over 690
pages, which Dr. Geikie justly styles a model of patient and exhaustive
research, will clearly show this, and at the same time prove how
skilfully and accurately the task allotted to them was performed.


[Footnote 26: _Trans. of Geol. Soc. of Glasgow_, vol. iii.; Jukes &
Geikie’s _Manual of Geology_, chap. xxv.; _Text Book of Geology_, p.
441.]


The risk of making very serious errors in computing the amount of
sediment discharged, unless proper precautions are taken, is well
illustrated in the case of the determinations made by Messrs. Brown and
Dickson, to which reference has already been made. Although their report
shows that they took great pains in order to arrive at correct
results—in fact, they computed the total annual quantity of sediment
discharged to within a cubic foot—after all, instead of being correct to
this minute quantity, they gave a total more than fourfold what it ought
to be. A somewhat similar discrepancy exists in reference to the
denudation of the basin of the Ganges. The time required to lower its
surface by one foot is, according to one estimate, 2,358 years;
according to another, 1,751; and according to a third, only 1,146 years.
The first figure is probably nearest the truth. Still, these differences
show both the difficulty of the problem and the necessity of caution in
adopting any of these results as correct.

By far the most trustworthy determinations of the whole are those of the
Mississippi by Messrs. Humphreys and Abbot, which may be relied upon as
not far from the truth. But, supposing the estimates in the foregoing
table to be perfectly correct, can we assume that their mean may be
safely taken as probably representing the average rate of denudation of
the whole earth? I would most unhesitatingly reply, Certainly not. The
Rhone and Po are full of glacier mud from the Alps; and the amount of
sediment which they carry down may give us the rate of denudation of
Switzerland, but certainly not that of the whole earth, or even of
Europe. The same may be said of the Ganges, which is charged with the
mud which it brings down from the Himalaya Mountains. The Hoang-Ho, or
Yellow River, is an exceptionally muddy river; in fact, it derives its
name from the vast quantity of yellow mud held by its waters in a state
of solution. It was probably the exceptionally muddy character of the
Po, the Rhone, the Ganges, and the Yellow River which attracted
attention, and led to observations being made of the sediment they
contain. Rivers more unsuitable than these to give us the average
denudation of the earth’s surface could not well be selected. Among the
seven rivers in the table, leaving out of account the small Scottish
stream, the Nith, with its basin of only 200 square miles, there are
only two, the Mississippi and the Danube, that drain countries which may
be regarded as in every way resembling the average condition of the
earth’s surface. I would choose the Mississippi as being superior to the
Danube, for two reasons: (1) because the rate of denudation of its basin
has been more accurately determined; and (2) because the area of its
basin not only exceeds that of the Danube as five to one, but better
fulfils the necessary conditions, as Sir Charles Lyell has so clearly
shown. “That river,” says Sir Charles, “drains a country equal to more
than half the continent of Europe, extends through twenty degrees of
latitude, and therefore through regions enjoying a great variety of
climate, and some of its tributaries descend from mountains of great
height. The Mississippi is also more likely to afford us a fair test of
ordinary denudation, because, unlike the St. Lawrence and its
tributaries, there are no great lakes in which the fluviatile sediment
is thrown down and arrested on its way to the sea.”[27] There is no
other river in the globe which to my mind better fulfils the required
conditions. It is no doubt true that the rate of denudation of the basin
of the Mississippi is probably less than that of Switzerland, Norway,
and the Himalayas, where glaciers abound, and certainly less than that
of Greenland and the Antarctic continent; but, on the other hand, this
rate is certainly much greater than that of the whole continent of
Africa, Australia, and large tracts of Asia, where the rainfall is much
smaller. One foot in 6,000 years may, therefore, I think, be safely
taken as the average rate of denudation of the whole surface of the
globe.


[Footnote 27: _Student’s Elements of Geology_, p. 91.]


_The average rate of denudation in the past probably not much greater
than in the present._—The belief has long prevailed that the rate of
denudation was much greater in past ages than it is now; but I am unable
to perceive any good grounds for concluding that such was the case at
any time since the beginning of the Palæozoic period. Various reasons
have, however, been assigned for this supposed greater rate; and to the
consideration of these I shall now very briefly refer.

It has been thought that at some remote epoch of the earth’s history,
when the moon was much nearer and the day much shorter than now, the
rate of denudation would, owing to the erosive power of the enormous
tides which would then prevail, be much greater than at the present day.
This, however, is very doubtful. There is nothing in the stratified
rocks which affords any support to the idea of great tidal waves having
swept over the land, at least since the time when life began on our
globe. Such a state of things would have destroyed all animal life. “The
Palæozoic sediments,” as Professor A. Winchell remarks, “have been
deposited, for the chief part, in quiet seas. The deep beds of
limestones and shales are spread out in sheets continent-wide, which
testify unmistakably to placid waters and slow deposition.”[28] But high
tides, not sweeping over the land, would not increase the rate of
denudation to the extent supposed. High tides silt up a river channel
more readily than they deepen it. A higher tide would probably produce a
greater destruction of sea-coast: it would tend to increase the rate of
marine denudation, but this would not materially affect the general rate
of denudation. For, as the present rate of marine denudation is to that
of subaërial denudation only as 1 to about 1,700,[29] it would take a
very large increase in the rate of marine denudation to affect sensibly
the general result. Suppose the rate of marine denudation to have been,
for example, ten times as great during the Palæozoic age as it is now
(which it certainly was not), it would only have shortened the time
required to effect a given amount of denudation of the whole earth by 9
years in 1,700, i.e. by little more than one-half per cent.


[Footnote 28: _World Life_, p. 265.]

[Footnote 29: See _Climate and Time_, p. 337.]


Again, it is assumed that the greater rate of terrestrial rotation in
the early ages would produce certain influences which would in turn
bring about a greater amount of denudation. The rate of rotation has
been slowly decreasing for ages, and in Palæozoic times it must, of
course, have been greater than at present. A more rapid rotation would
increase the velocity of the trade and anti-trade winds, and would thus
tend to augment the action of those meteorological agents chiefly
effective in the work of subaërial denudation. Here again the testimony
of geology is negative. We have no geological grounds to conclude that
the winds of Palæozoic times were stronger than those at the present
day. The heat was no doubt greater, and perhaps there was more rain;
but, on the other hand, there would be less frost, snow, ice, and other
denuding agents.

There is one cause which would, perhaps, be more effective than any of
the foregoing: viz. the periodic occurrence of glacial epochs. When a
country is buried under ice, the erosion of the surface is great. But it
must be borne in mind that the influence of rain, rivers, and other
denuding agents now in operation would then, in the glaciated regions,
be almost _nil_. Besides, the greater part of the materials ground off
the rocks would be left on the land; and, when the ice disappeared, it
would be found in the form of a thick mantle of boulder clay—a mantle
which would protect the rocky surface of the country for thousands and
tens of thousands of years from further denudation. This is shown by the
fine striæ on the rocky surface, made perhaps more than 50,000 years
ago, remaining under the boulder clay as perfect as the day on which
they were engraved. But, more than all this, a very considerable part of
the 1 foot presently being removed off the country in 6,000 years
consists of the loose materials belonging to the glacial epoch, such as
sands, gravels, and boulder clay, which are being swept off the surface
by rain and river action. Were it not for this, the present rate of
subaërial denudation would not be so high as it actually is. Taking all
things into consideration, it is, I think, obvious that the average rate
of denudation since the beginning of Palæozoic times was probably not
much greater than at the present day.

_How the method has been applied._—Having determined what appears to be
the probable average rate of subaërial denudation, we may now proceed to
consider the way in which this rate has been applied to measure past
geological time. There are two ways in which it may be applied for this
purpose. It may (1) be applied directly: knowing the thickness of strata
which may have been removed by denudation, we can easily tell, from that
rate, the time it required to effect their removal. If we have evidence,
for example, that at some epoch 1,000 feet of stratified rock were
carried away, then, on the assumption that the rate of denudation was
the same at that epoch as now, we have 1,000 × 6,000 = 6,000,000 years
as the required time. (2) It may be applied indirectly: knowing the
thickness of the strata, we may estimate the time required for their
formation. This is the way in which it has usually been applied, but, as
we shall see, it is the less satisfactory way of the two.

Dr. A. Geikie gives the land area of the globe as 52,000,000 square
miles, and that of water as 144,712,000 square miles.[30] We may thus
take the proportion of land to water roughly as 1 to 3; about
one-quarter of the earth’s surface being land, and three-quarters water.
One foot, therefore, removed off the surface of the land would cover the
whole globe with a layer 3 inches thick, or the entire sea-bottom with a
layer 4 inches thick.


[Footnote 30: _Physical Geography_, p. 103.]


If we knew the total quantity of stratified rock on the globe, we could
easily tell the time that would be required for its formation. Most
geologists would, I believe, be inclined to admit that, if spread
uniformly over the entire globe, it would form a layer of at least 1,000
feet in thickness. In such a case the time required for its deposition
would be as follows:

                 1,000 × 6,000 × 4 = 24,000,000 years.

This would not, however, represent the age of the stratified rocks. It
would only represent the time required to deposit the rocks which we
have assumed to be now in existence. The greater mass of sedimentary
rocks has been formed out of previously existing sedimentary rocks, and
these again out of sedimentary rocks still older. The oldest known
sedimentary rocks are the Laurentian; but these are believed by
geologists to have been formed from still older sedimentary rocks. It is
therefore evident that the materials composing our stratified beds must
have passed through many cycles of destruction and re-formation. The
materials of some of the recent formations, for example, may have passed
through denudation and deposition a dozen of times over.[31] The time
required to have deposited at a given rate the present existing mass of
sedimentary rocks is probably but a small fraction of the time required
to have deposited at the same rate the total mass that has actually been
formed. Few geologists, I think, who will duly reflect on the subject,
will deem it too much to say that the present existing stratified rocks
have on an average passed at least thrice through the cycle of
destruction and re-formation. If this be admitted, then the 1,000 feet
of stratified rock represent, not a period of 24,000,000 years, but a
period three times as great, viz. 72,000,000 years.


[Footnote 31: It is this destruction of the stratified rocks which makes
it so difficult to detect the marks of former glacial epochs, and which
has led to such prevailing misconceptions regarding the evidence which
we ought to expect of those epochs. See paper read before the Geological
Society, “On Prevailing Misconceptions regarding the Evidence which we
ought to expect of former Glacial Periods,” January 23, 1889.]


It is impossible to tell from geological data the actual age of the
stratified rocks; but this is not required. What we require is, as
already stated, not their _actual_ age, but an _inferior limit_ to that
age.

_Method as applied by Professor Haughton._—Professor Haughton estimates
the mass of the stratified rocks down to the time of the Miocene
Tertiary period as being 177,200 feet in thickness, and covering an area
equal to that of the sea. The present rate of subaërial denudation he
considers to be equal to one foot removed off the surface of the land in
3,090 years. If the proportion of land to water be taken as 52 to 145,
it thus requires 8,616 years to deposit one foot of sediment over the
bed of the ocean, and consequently this is the rate at which strata are
at present being formed. This would give 8,616 × 177,200 = 1,526,750,000
years for the age of the stratified rocks. But he assumes the rate of
denudation to have been _ten times_ greater in geological time than at
present. This consequently reduces the age of the rocks to 152,675,000
years. By adding one-third for the time which has elapsed since the
Miocene Tertiary period he gets 200,000,000 years as a minimum length of
geological time.[32]


[Footnote 32: _Physical Geography_, p. 94.]


The validity of this result rests upon what appear to me to be two very
doubtful assumptions. It is assumed in his calculations that the total
amount of strata formed during past ages (not the amount presently
remaining) was equal to a mass 177,200 feet in thickness, covering the
entire area of the ocean. This is certainly doubtful. It may have been
as great, for anything that can be proved to the contrary; but we have
no evidence that it was so. Certainly there is no evidence that the rate
of subaërial denudation during past ages was ever ten times as great as
it is now. But how is a length of 200,000,000 years to be reconciled
with the age of the sun’s heat? The stratified rocks may be as old as
this, but assuredly they are not if gravitation was the only source from
which the sun derived his energy.

_Method as applied by Mr. Alfred R. Wallace._—Mr. Wallace adopts
Professor Haughton’s estimate of 177,200 feet for the maximum thickness
of the sedimentary rocks. But, instead of supposing, like Professor
Haughton, the products of denudation to be uniformly spread over the
entire sea-bottom, he supposes them spread over a belt of merely 30
miles broad, extending along the entire coast-line of the globe, which
he estimates at 100,000 miles. This gives an area of 3,000,000 square
miles on which the denuded matter of the whole land area of 57,000,000
square miles is deposited. These two areas are to one another as 1 to
19, and thus it follows that deposition goes on 19 times as fast as
denudation. The rate of denudation he takes as one foot removed off the
surface of the land in 3,000 years, so that the rate of deposition would
be about one foot in 158 years, and consequently the time required to
deposit the 177,200 feet of rock would be

                   177,200 × 158 = 27,997,600 years.

This is a period double what the gravitation theory of the source of the
sun’s energy can afford. And if the rate of denudation be taken at one
foot in 6,000 years, which is, as we have seen, probably nearer the
truth, then this would make the age of the stratified rocks 56,000,000
years.

There seems to be a little ambiguity about Mr. Wallace’s result. Do the
177,200 feet represent the quantity of rock which presently exists, or
do they represent the total quantity which has been formed during all
past ages? If the former, then the 28,000,000 years are but a fraction
of the time which must have been required; for, as we have been shown,
the materials composing the stratified rocks have, on an average, been
deposited at least three or four times over. If, on the other hand, the
thickness is meant to represent the total quantity of rock which has
been formed during the whole of past geological time, then the question
arises, by what means could this quantity possibly be ascertained? In
other words, how was the relation between the present quantity and the
total quantity ascertained? But in either case the result is wholly
irreconcilable with the gravitation theory of the source of the sun’s
heat.

_Method as applied directly._—We have seen that it is impossible to
determine the actual age of the earth from the stratified rocks, even if
we knew with perfect accuracy their present total amount. We have also
seen that from the rate of deposition we cannot fix with any degree of
certainty a minimum value for the age of these rocks. We can, however,
by means of the first or direct application of the method, assign with
tolerable accuracy, as was shown on a former occasion,[33] a minimum age
to the earth. We can be far more certain of the time which must have
been required to remove by denudation, say, a thousand feet of rock than
we can possibly be of the time required to have deposited a thousand
feet of sediment. The thousand feet of sediment may, under certain
conditions, have been deposited in a hundred years, while under other
conditions they may have required a million of years. In fact, nothing
can be more uncertain than the rate of deposition: it depends upon such
a multitude of circumstances. At the mouth of a great river, for
example, a foot of sediment may be deposited in a single day, whereas in
some places, as in mid-ocean, it may require a million of years to
deposit the same amount. But in reference to subaërial denudation no
such uncertainty exists.


[Footnote 33: _Quart. Journ. of Science_, July 1877; _Climate and
Cosmology_, chap. xvii.]


The utter inadequacy of a period of 20,000,000 years for the age of our
earth is demonstrable from the enormous thickness of rock which is known
to have been removed off certain areas by denudation. I shall now
briefly refer to a few of the many facts which might be adduced on this
point.

_Evidence from “faults.”_—One plain and obvious method of showing the
great extent to which the general surface of the country has been
lowered by denudation is furnished, as is well known, by the way in
which the inequalities of surface produced by faults or dislocations
have been effaced. It is quite common to meet with faults where the
strata on the one side have been depressed several hundreds—and in some
cases thousands—of feet below those on the other; but we seldom find any
indications of such on the surface, the inequalities on the surface
having been all removed by denudation. Now, in order to effect this, a
mass of rock must have been removed equal in thickness to the extent of
the dislocation. The following are a few examples of large faults:

The great Irwell fault, described by Professor Hull,[34] which stretches
from the Mersey west of Stockport to the north of Bolton, has a throw of
upwards of 3,000 feet.

Some remarkable faults have been found by Professor Ramsay in North
Wales. For example, near Snowdon, and about a mile E.S.E. of Beddgelert,
there is a fault with a downthrow of 5,000 feet; and in the Berwyn
Hills, between Bryn-mawr and Post-gwyn, there is one of 5,000 feet. In
the Aran Range there is a great fault, designated the Bala fault, with a
downthrow of 7,000 feet. Again, between Aran Mowddwy and Careg Aderyn
the displacement of the strata amounts to no less than from 10,000 to
11,000 feet.[35] Here we have evidence that a mass of rock, varying from
one to two miles in vertical thickness, must have been denuded in many
places from the surface of the country in North Wales.

The fault which passes along the east side of the Pentlands is estimated
to have a throw of upwards of 3,000 feet.[36] Along the flank of the
Grampians a great fault runs from the North Sea at Stonehaven to the
estuary of the Clyde, throwing the Old Red Sandstone on end sometimes
for a distance of two miles from the line of dislocation. The amount of
the displacement, Dr. A. Geikie[37] concludes, must in some places be
not less than 5,000 feet, as indicated by the position of occasional
outliers of conglomerate on the Highland side of the fault.

The great fault crossing Scotland from near Dunbar to the Ayrshire
coast, which separates the Silurians of the South of Scotland from the
Old Red Sandstone and Carboniferous tracts of the North, has been found
by Mr. B. N. Peach, of the Geological Survey,[38] to have in some places
a throw of fully 15,000 feet. This great dislocation is older than the
Carboniferous period, as is shown by the entire absence of any Old Red
Sandstone on the south side of the fault, and by the occurrence of the
Carboniferous Limestone and Coal-measures lying directly on the Silurian
rocks. We obtain here some idea of the enormous amount of denudation
which must have taken place during a comparatively limited geological
epoch. So vast a thickness of Old Red Sandstone could not, as Mr. Peach
remarks, “have ended originally where the fault now is, but must have
swept southwards over the Lower Silurian uplands. Yet these thousands of
feet of sandstones, conglomerates, lavas, and tuffs were so completely
removed from the south side of the fault previous to the deposition of
the Carboniferous Limestone series and the Coal-measures, that not a
fragment of them is anywhere to be seen between these latter formations
and the old Silurian floor.”[39] This enormous thickness of nearly three
miles of Old Red Sandstone must have been carried away during the period
which intervened between the deposition of the lower members of the
Lower Old Red Sandstone and the accumulation of the Carboniferous
Limestone.

Near Tipperary, in the south of Ireland, there is a dislocation of the
strata of not less than 4,000 feet,[40] which brings down the
Coal-measures against the Silurian rocks. Here 1,000 feet of Old Red
Sandstone, 3,000 feet of Carboniferous Limestone, and 800 feet of
Coal-measures have been removed by denudation off the Silurian rocks.
Not only has this immense thickness of beds been carried away, but the
Silurian itself on which they rested has been eaten down in some places
into deep valleys several hundreds of feet below the surface on which
the Old Red Sandstone rested.


[Footnote 34: _Mem. Geol. Survey of Lancashire_, 1862.]

[Footnote 35: _Mem. Geol. Survey of Great Britain_, vol. iii.]

[Footnote 36: _Memoir_ to Sheet 32, Geol. Survey Map of Scotland.]

[Footnote 37: _Nature_, vol. xiii. p. 390.]

[Footnote 38: _Explanation_ to Sheet 15, Geol. Survey Map of Scotland.]

[Footnote 39: I have been informed by Mr. Peach that since the above was
written additional light has been cast on this immense fault. It has
been found, he says, that the fault consists of two sub-parallel
branches, the more southerly of which has the effect of bringing the
rocks of the Upper Silurian age against the Lower Silurian beds. The
northern branch brings the upper division of the Lower Old Red
Sandstones, in turn, against the Upper Silurian rocks. This, Mr. Peach
remarks, does not in the least invalidate the reasoning as to the amount
of material removed by denudation from this region in the time
specified. In fact, it shows, he says, that a greater amount must have
been removed than was at first suspected.]

[Footnote 40: Jukes’s and Geikie’s _Manual of Geology_, p. 441.]


Turning to the American continent, we find the amount of rock removed to
be even still greater. In the Valley of Thessolon, to the north of Lake
Huron, there is a dislocation of the strata to the extent of 9,000
feet.[41]

In front of the Chilowee Mountains there is a vertical displacement of
the strata of more than 10,000 feet.[42] Professor H. D. Rogers found in
the Appalachian coal-fields faults ranging from 5,000 feet to more than
10,000 feet of displacement.

In the Nova Scotia coal-fields one or two miles in thickness of strata
have been removed in some places.[43]

A great fracture runs along the axis of the Sierra Nevada for 300 miles,
accompanied by a dislocation of 3,000 to 10,000 feet.[44]

The anticlinal of the Park Range of the Rocky Mountains was cleft down
the axis, and the eastern half depressed 10,000 feet. And Mr. J. P.
Lesley gives an account of a fault in the Appalachians of not less than
20,000 feet, bringing the upper Devonian strata on the one side opposite
to the lowest Cambrian on the other.[45]

A fault with a vertical displacement of 20,000 feet was found in the
Uinta Mountains.[46]

In the Aqui range of mountains, Utah, there is a fault determined by Mr.
S. F. Emmons to be at least 10,000 feet.[47]

The Grand Cañon of Colorado, in some places 4,000, 5,000, and 6,000 feet
in depth, is cut, says Professor A. Winchell, in a plateau which has
itself been lowered by erosion to the extent of 10,000 feet; and this
plateau occupies an area of 13,000 to 15,000 square miles.[48]

The Grand “Wash Fault,” Colorado, has a downthrow to the west of 6,000
feet. The “Hurricane Fault,” close to it, has displaced the strata to
the extent of over 12,000 feet.[49]

In the Valley of East Tennessee, Appalachian Mountains, it has been
shown by Mr. J. P. Lesley that as much as 35,000 feet of rock have been
removed by denudation. But this being from an anticlinal arch, it does
not, of course, afford any measure of the extent of the denudation of
the surrounding country. Major J. W. Powell, Director of the U.S. Geol.
Survey, found that under a similar condition as much as three and a half
miles of strata have been removed by denudation from the top of
anticlinal beds in the Uinta Mountains.[50]

Probably the most enormous displacement of strata which has yet been
found is that of the “Wahsatch Fault,” Utah. This fault is about 100
miles in length, crossing the fortieth parallel of latitude from north
to south, with a downthrow to the west of not less than 40,000 feet. So
clear is the evidence regarding this fault that Mr. Clarence King says
“that there can be no doubt of the quantitative correctness of my
reading of this tremendous dislocation.”[51]

There are other modes than the foregoing by means of which geologists
are enabled to measure the thickness of strata which may have been
removed in places off the present surface of the country. Into the
details of these I need not here enter; but I may give a few examples of
the enormous extent to which the country, in some places, has been found
to have been lowered by denudation.

Dr. A. Geikie has shown[52] that the Pentlands must at one time have
been covered with Carboniferous rocks, upwards of a mile in thickness,
which have all been removed by denudation.

In the Bristol coal-fields, between the river Avon and the Mendips, Sir
Andrew C. Ramsay has shown[53] that about 9,000 feet of Carboniferous
strata have been removed by denudation from the present surface.

Between Bendrick Rock and Garth Hill, South Glamorganshire, a mass of
Carboniferous and Old Red Sandstone, of upwards of 9,000 feet, has been
removed. At the Vale of Towy, Caermarthenshire, about 6,000 feet of
Silurian and 5,000 feet of Old Red Sandstone—in all about 11,000
vertical feet—have been swept away. Between Llandovery and Aberaeron a
mass of about 12,000 vertical feet of the Silurian series has been
removed by denudation. Between Ebwy and the Forest of Dean, a distance
of upwards of 20 miles, a thickness of rock varying from 5,000 to 10,000
feet has been abstracted.

Prof. Hull found[54] on the northern flanks of the Pendle Range,
Lancashire, the Permian beds resting on the denuded edges of the
Millstone Grit, and these were again observed resting on the Upper
Coal-measures south of the Wigan coal-field. Now from the known
thickness of the Carboniferous series in this part of Lancashire he was
enabled to calculate approximately the quantity of Carboniferous strata
which must have been carried away between the period of the Millstone
Grit and the deposition of the Permian beds, and found that it actually
amounted to no less than 9,900 feet. He also found in the Vale of
Clitheroe, and at the base of the Pendle Range, that the Coal-measures,
the whole of the Millstone Grit, the Yoredale series, and part of the
Carboniferous Limestone, amounting in all to nearly 20,000 feet, had
been swept away—an amount of denudation which, as Prof. Hull remarks,
cannot fail to impress us with some idea of the prodigious lapse of time
necessary for its accomplishment.


[Footnote 41: _Geology of Canada_, 1863, p. 61.]

[Footnote 42: Safford’s _Geology of Tennessee_, p. 309.]

[Footnote 43: Lyell’s _Student’s Manual_, chap. xxiii.]

[Footnote 44: _Geological Studies_, by Prof. A. Winchell, p. 165.]

[Footnote 45: _Geological Studies_, pp. 93, 163.]

[Footnote 46: Powell’s _Geology of the Uinta Mountains_.]

[Footnote 47: _Geological Exploration of the Fortieth Parallel_, vol.
ii. p. 456.]

[Footnote 48: _Geological Studies_, p. 92; see also Dutton’s _Tertiary
History of the Cañon District_.]

[Footnote 49: _Tertiary History of the Cañon District_, pp. 20, 113;
_Second Annual Report, U.S. Geol. Survey_, p. 125.]

[Footnote 50: Powell’s _Geology of Uinta Mountains_.]

[Footnote 51: _Geological Exploration of the Fortieth Parallel_, vol. i.
p. 745.]

[Footnote 52: _Memoir_ to Sheet 32, Geol. Survey of Scotland.]

[Footnote 53: _Denudation of South Wales._ _Memoirs of Geol. Survey_,
vol. i.]

[Footnote 54: _Quart. Journ. Geol. Soc._ vol. xxiv. p. 323.]


It may be observed that, enormous as is the amount of denudation
indicated by the foregoing figures, these figures do not represent in
most cases the actual thickness of rock removed from the surface. We are
necessitated to conclude that a mass of rock equal to the thickness
stated must have been removed, but we are in most cases left in
uncertainty as to the total thickness which has actually been carried
away. It cannot be imagined that these great disruptions occurred first
when the surface became subject to denuding agencies, or that denudation
ceased to operate precisely when the inequality was smoothed away.
Moreover, during the time the surface on one side of the fault was being
reduced, some amount of denudation must also have been in progress on
the other and lower side. In the case of a fault, for example, with a
displacement of, say, one mile, where no indication of it is seen at the
surface of the ground, we know that on one side of the fault a thickness
of rock equal to one mile must have been denuded, but we do not know how
much more than that may have been removed. For anything which we know to
the contrary, hundreds of feet of rock may have been removed before the
dislocation took place, and as many more hundreds after all indications
of dislocation had been effaced at the surface.

But it must be observed that the total quantity of rock which has been
removed from the _present_ surface of the land is evidently small in
proportion to the total quantity removed during the past history of our
globe. For those thousands and thousands of feet of rock which have been
denuded were formed out of the waste of previously existing rocks, just
as these had been formed out of the waste of yet older rock-masses. In
short, as a general rule, the rocks of one epoch have been formed out of
those of preceding periods, and go themselves to form those of
subsequent epochs.

In many of the cases of enormous denudation to which we have referred,
the erosion has been effected during a limited geological epoch. We
have, for example, seen that upwards of a mile in thickness of
Carboniferous rock has been denuded in the area of the Pentlands. But
the Pentlands themselves, it can be proved, existed as hills, in much
their present form, before the Carboniferous rocks were laid down over
them; and as they are of Lower Old Red Sandstone age, and have been
formed by denudation, they must consequently have been carved out of the
solid rock between the period of the Old Red Sandstone and the beginning
of the Carboniferous age. This affords us some conception of the immense
lapse of time represented by the Middle and Upper Old Red Sandstone
periods.

Again, in the case of the great fault separating the Silurians of the
south of Scotland from the Old Red Sandstone tracts lying to the north,
a thickness of the latter strata of probably more than a mile, as we
have seen, must have been removed from the ground to the south of the
fault before the commencement of the Carboniferous period. And again, in
the case of the Lancashire coal-fields, to which reference has been
made, nearly two miles in thickness of strata had been removed in the
interval which elapsed between the Millstone Grit and the Permian
periods.

_Time required to effect the foregoing amount of denudation._—To lower
the country one mile by denudation would therefore require, according to
the rate which we have already established, about 15,000,000 years; but
we have seen that a thickness of rock more than equal to that must have
been swept away since the Carboniferous period; and even during the
Carboniferous period itself more than a mile in thickness of strata in
many places was removed. Again, there can be no doubt whatever that the
amount of rock removed during the Old Red Sandstone period was much
greater than one mile; for we know perfectly well that over large tracts
of country nearly a mile in thickness of rock was carried away _between
the period of the Lower Old Red Sandstone and the Carboniferous epoch_.
Further, all geological facts go to show that the time represented by
the Lower Old Red Sandstone itself must have been enormous.

Now, three miles of rock removed since the commencement of the Old Red
Sandstone period (which, doubtless, is an under-estimate) would give us
45,000,000 years.

Again, going farther back, we find the lapse of time represented by the
Silurian period to be even more striking than that of the Old Red
Sandstone. The unconformities in the Silurian series indicate that many
thousands of feet of these strata were denuded before overlying members
of the same great formations were deposited. And again, this immense
formation was formed in the ocean by the slow denudation of pre-existing
Cambrian continents, just as these had been built up out of the ruins of
the still prior Laurentian land. And even here we do not reach the end
of the series, for the Laurentians themselves resulted from the
denudation, not of the primary rocks of the globe, but of previously
existing sedimentary and probably igneous rocks, of which, perhaps, no
recognisable portion now remains.

It is the opinion of Mr. Darwin, and also of Mr. Wallace, that the
geological time which elapsed anterior to the Cambrian period was as
long as the whole interval from that period to the present day. This is
an opinion which I suppose is supported by most geologists. But, to err
on the safe side, I shall assume that the time which had elapsed prior
to the Old Red Sandstone was not greater than the time which has elapsed
since that period. Even on this assumption we have at least 90,000,000
years as a minimum duration of geological time.

_Age of the earth as determined by the date of the glacial
epoch._—Professor A. Winchell, by a most careful examination of the
probable relative lengths of geological periods, arrived at the
conclusion that the time which elapsed since the beginning of the
_glacial epoch_ is to the time which has elapsed since the
solidification of the earth’s surface as 1 to 250.[55] According to the
eccentricity theory of the cause of the glacial epoch, that epoch began
240,000 years ago; consequently this makes the time since solidification
took place 60,000,000 years, a period which agrees roughly with that
deduced from denudation, and is so far presumptive evidence of the truth
of that theory of the cause of the glacial cold.


[Footnote 55: _World Life_, p. 369.]


_Testimony of Biology_.—The time required for the variation and
modification of organic forms has, Mr. Alfred R. Wallace states, been
generally considered to require an even longer series of ages than might
satisfy the demands of physical geology. This is a point, however, on
which I am not qualified to venture an opinion. I shall simply refer to
the views held by our highest authorities on the subject.

Referring to Professor Huxley’s anniversary address to the Geological
Society in 1870, where he shows that almost all the higher forms of life
must have existed during the Palæozoic period, Mr. Wallace says: “Thus,
from the fact that almost the whole of the Tertiary period has been
required to convert the ancestral Orohippus into the true horse, he,
Professor Huxley, believes that, in order to have time for the much
greater change of the ancestral ungulata into the two great odd-toed and
even-toed divisions (of which change there is no trace even among the
earliest Eocene mammals), we should require a large portion, if not the
whole, of the Mesozoic or Secondary period. Another case is furnished by
the bats and whales, both of which strange modifications of the
mammalian type occur perfectly developed in the Eocene formation. What
countless ages back must we, then, go for the origin of these groups,
the whales from some ancestral carnivorous animal, and the bats from the
insectivora! And even then we have to seek for the common origin of
carnivora, insectivora, ungulata, and marsupials at a far earlier
period; so that, on the lowest estimate, we must place the origin of the
mammalia very far back in Palæozoic times.”[56]

“If the very small differences,” says Professor Huxley,[57] “which are
observable between the _Crocodilia_ of the older Mesozoic formations and
those of the present day furnish any sort of approximation towards an
estimate of the average rate of change among the _Sauropsida_, it is
almost appalling to reflect how far back in Palæozoic times we must go
before we can hope to arrive at that common stock from which the
_Crocodilia_, _Lacertilia_, _Ornithoscelida_, and _Plesiosauria_, which
had attained so great a development in the Triassic epoch, must have
been derived.

“The _Amphibia_ and _Pisces_ tell the same story. There is not a single
class of vertebrated animals which, when it first appears, is
represented by analogues of the lowest known members of the same class.
Therefore, if there is any truth in the doctrine of evolution, every
class must be vastly older than the first record of its appearance upon
the surface of the globe. But if considerations of this kind compel us
to place the origin of vertebrated animals at a period sufficiently
distant from the Upper Silurian, in which the first Elasmobranchs and
Ganoids occur, to allow of the evolution of such fishes as these from a
vertebrate as simple as the _Amphioxus_, I can only repeat that it is
appalling to speculate upon the extent to which that origin must have
preceded the epoch of the first recorded appearance of vertebrate life.”

“If the theory be true,” says Mr. Darwin, “it is indisputable that
before the lowest Cambrian stratum was deposited long periods elapsed—as
long as, or probably far longer than, the whole interval from the
Cambrian age to the present day; and that during these vast periods the
world swarmed with living creatures.”[58]

In referring to the abundant and well-developed fauna of the Cambrian
period, Sir Andrew C. Ramsay remarks:[59] “In this earliest known varied
life we find no evidence of its having lived near the beginning of the
Zoological series. In a broad sense, compared with what must have gone
before, both biologically and physically, all the phenomena connected
with this old period seem, to my mind, to be quite of a recent
description; and the climates of seas and lands were of the very same
kind as those that the world enjoys at the present day—one proof of
which, in my opinion, is the existence of great glacial boulder beds in
the Lower Silurian strata of Wigtonshire, west of Loch Ryan.”

Professor Haeckel remarks that “Darwin’s theory, as well as that of
Lyell, renders the assumption of immense periods absolutely necessary.
If the theory of development be true at all, there must certainly have
elapsed immense periods, utterly inconceivable to us.”

In reference to the foregoing, Mr. Wallace says:[60] “These opinions,
and the facts on which they are founded, are so weighty that we can
hardly doubt that, if the time since the Cambrian epoch is correctly
estimated at 200,000,000 of years,[61] the date of the commencement of
life on the earth cannot be much less than 500,000,000; while it may not
improbably have been longer, because the reaction of the organism under
changes of the environment is believed to have been less active in low
and simple than in high and complex forms of life, and thus the
processes of organic development may for countless ages have been
excessively slow.”


[Footnote 56: _Island Life_, p. 204.]

[Footnote 57: _Quart. Journ. of Geol. Soc._ vol. xxvi. p. 53.]

[Footnote 58: _Origin of Species_, p. 286.]

[Footnote 59: _Proceedings of the Royal Society_, No. 152, 1874, p.
342.]

[Footnote 60: _Island Life_, p. 205.]

[Footnote 61: Of course, Mr. Wallace does not believe that it is
actually 200,000,000 years since the Cambrian period.]


I think it must now be perfectly evident that the facts both of geology
and of biology are utterly irreconcilable with the theory that the sun’s
heat was derived from the condensation of its mass by gravitation; and
that the mistake in regard to geological time has been committed by the
physicist, and not by the geologist. The grounds upon which the
geologists and the biologists found the conclusion that it is more than
20 or 30 millions of years since life began on the earth are far more
certain and reliable than the grounds upon which the physicist concludes
that the period must be less. The only real ground that the physicist
has is that according to the theory which he holds of the origin of the
sun’s heat a longer period is not possible. This might be considered
good evidence were no other theory possible; but there is another
theory, which accords with all the facts, and consequently has a strong
presumption in its favour.



                               PART III.

 _EVIDENCE IN SUPPORT OF THE THEORY FROM THE PRE-NEBULAR CONDITION OF THE
    UNIVERSE._


The nebular hypothesis, strictly speaking, is one simply intended to
account for the origin of our solar system. “It is,” as remarks
Professor A. Winchell, “primarily a genetic explanation of the phenomena
of the solar system; and accessorily a co-ordination, in a common
conception, of the principal phenomena in the stellar and nebular
firmament, as far as human vision has been able to penetrate.”[62] The
theory starts with the assumption that all the materials composing the
solar system once existed in a state of extreme tenuity and diffusion,
filling far more than the entire space included within the orbit of the
most remote planet. It begins with this diffused nebulous mass tending
slowly, under the influence of gravitation, towards a state of
aggregation. Beyond this point the received nebular hypothesis does not
extend.


[Footnote 62: _World Life_, p. 196.]


It will be observed that the theory here begins in the middle of a
process. It begins with the assumption of a mass in the act of
condensing under the influence of gravity. It offers no explanation of
the origin of the mass, or how it came to be in this attenuated state,
or in what condition it existed before the materials began to draw
together. These are, however, inquiries which naturally force themselves
on our attention. If the nebular theory be a true theory of the origin
of the solar system, then this nebulous mass must have had an antecedent
history, and we cannot help feeling the instinctive desire of tracing
the chain of causation farther back. The mind presses towards an
absolute beginning. It is the goal to which it aspires, and no amount of
failure will ever deter it from renewing its efforts. Of recent years a
considerable amount of attention has been devoted to inquiries in this
direction; nearly all of which, it is true, has necessarily been of a
speculative and hypothetical character. But hypothesis, as Mr. Locyker
remarks, is the life-blood of investigation.

The nebular hypothesis is so highly probable as to have gained almost
universal acceptance. In fact, it contains very little of a hypothetical
nature. It is, as Mr. Mill says, “an example of legitimate reasoning
from a present effect to its past cause, according to the known laws of
that cause.” Like the hypothesis of a luminiferous ether, if it is not a
true theory, one would almost think that it deserves to be so.

There seems no reason why inquiries should stop at the point where
Laplace began. The same line of reasoning may yet carry us back into the
pre-nebular region, and perhaps with as great a degree of certainty as
it has done in the nebular; though, no doubt, the farther back we
proceed, the more difficult probably will the inquiry become. But, be
all this as it may, there can be little doubt that the path of
investigation is a legitimate one, and also one which is worthy of being
traced out.

I shall now briefly refer to some of the leading views which have been
expressed in regard to the pre-nebular history of the universe, and
shall afterwards consider the additional light which the theory
discussed in this volume seems to cast on the subject.

The commonly received opinion is that the nebulæ were formed from
ordinary matter existing in a high state of division, and widely
diffused through space. The “cosmical dust,” as it is called, was the
universal “world-stuff,” out of which all things were supposed to be
formed. It is held that in receding backwards in pre-nebular times, the
smaller, more simple, and elementary the materials were. Out of this
primitive cosmical dust, or world-stuff, by aggregation, the materials
became successively larger and more complex. The theory of the origin of
nebulæ, on this principle, has been clearly stated by Professor
Winchell, and I here give a brief outline of his views on the subject.

_Professor A. Winchell on the pre-nebular condition of matter._—This
cosmical dust, or world-stuff, he considers to be scattered
promiscuously through boundless space. It is cold and non-luminous, and
is acted upon by forces of attraction and probably of repulsion. The
material particles, either as atoms or less probably as molecules, are
drawn by mutual attraction into groups and swarms. Any central
attractive force, as of a sun or planet, by causing the particles to
move in converging lines, would cause them to become approximated and
ultimately aggregated. Thus both mutual attractions and centric
movements would tend to produce aggregations dispersed through space.
But in the presence of two or more attractive centres, as in the present
constitution of the Cosmos, it is impossible that any mass shall fall
directly upon its centre of attraction. Hence motions of rotation will
be established in the mass, and also orbital motions of masses about
each other. In addition to the mutual attraction of the molecules, the
convergence of their paths towards centres of attraction must also tend
to the formation of masses and swarms of masses and particles. “We have
then,” he says, “to picture indefinite space as pervaded by swarms of
masses and particles of dark matter. Each mass or particle may,
nevertheless, be separated by thousands of miles. It is manifest,
therefore, that each mass or particle will eventually dispose itself,
under the fixed action of the forces of matter, in some definite order.
It is manifest also, from what has been said, that each swarm will have
a progressive motion along a path having the essential character of an
orbit around some dominant centre of attraction. If, as seems to be the
fact, an ethereal medium, or any condition of interplanetary matter,
exists in space, it opposes the movements of these swarms by opposing
the motion of each constituent mass. But the smaller masses—the
particles and molecules—would feel this resistance to the greatest
extent. They would therefore fall behind the heavier masses, and would
be most deflected toward the attracting centre. The smallest particles
would be driven farthest to the rear, and dispersed farthest from the
orbit of the train, along the side turned toward the principal
attraction. The swarm would present an elongated form, in which the
larger and heavier masses would move foremost, and nearest the line of
the orbit—that is, near the exterior skirt of the area covered by the
general swarm—while the smaller ones would follow, in graduated
succession, in a long train which would present a fan-like expansion
lying mostly on the inside of the path of the principal masses.”

“This, it may be conceived, is the mode of aggregation of these cosmical
matters in the depths of space. Of course the attractions which control
them are feeble; their movements are slow, the resistances are
relatively inconsiderable, and the elongation of the swarm is
correspondingly inconspicuous. What I have described is a tendency which
would be present. Sometimes the controlling attraction would be only
another cosmical swarm. The two swarms would revolve similarly about
their common centre of gravity, while prolonged resistances would cause
their slow approximation and final coalescence at the common centre of
gravity. Sometimes the controlling attraction would be exerted by a
distant sun, around which it would slowly move, continually gathering up
additions of matter from the wide fields of space.”

“In most cases all controlling attraction would be feebly felt. These
clouds of cosmical dust would float practically poised in the midst of
space, and would gradually grow by the continued accession of new
matter. Some of them would become aggregates of large dimensions, and
their attraction would be distinctly felt by other aggregates. There
would be a tendency of such aggregates to approach each other. They
might possibly approach along a straight line; but more probably some
third aggregation, or some distant sun, would deflect them into orbits
about their common centre of gravity, in which, by prolonged collisions
of cosmical matter, they are brought to ultimate coalescence with each
other. Or some other attractive disturbance affords such a resultant of
actions as may bring them more directly together. When these larger
aggregations of world-stuff come together, the result is an aggregation
approaching the dimensions of the Herschellian nebulæ.”[63]


[Footnote 63: _World Life_, p. 72.]


In regard to the origin of the heat of the nebulæ, I am glad to find
that Professor Winchell, to a certain extent, adopts the views which I
have so long entertained on the subject. “The thought,” he says, “must
already have suggested itself to the reader that the process of
conglomeration affords an explanation of the intense heat which
vaporises its substance, and causes it to yield a spectrum of bright
lines. As the sudden compression of a portion of atmospheric air yields
heat sufficient to ignite tinder, or fuse and volatilise a descending
meteor-mass, so the precipitation of one planet upon another would
liberate sufficient heat to reduce them both to a state of fusion, or
even of vapour. Still more must the intensest heat be generated by the
impact of two nebulous masses, one or both of which together may embrace
more matter than all our planets and the sun combined—as much even as
the matter of our entire visible firmament of stars. One experiences a
distinct feeling of relief in the discovery of such a possible means of
ignition of nebulæ.”

_Mr. Charles Morris on the pre-nebular condition of matter._—Others
again suppose matter to be present everywhere throughout space. This
view has been ingeniously advocated by Mr. Charles Morris in an article
on “The Matter of Space,” which appeared in _Nature_, February 8, 1883.
The hypothesis of an ether specially distinct from matter he considers
to be a gratuitous assumption, and one of the last surviving relics of
eighteenth century science, and, unless it can be proved that highly
disintegrated matter is positively incapable of conveying light
vibrations, there is no warrant for assigning this duty to a distinct
form of substance. But that matter exists in outer space in the same
conditions as in planetary atmospheres he thinks is improbable. Its duty
as a conveyer of radiant vibrations seems to require a far greater
tensity, and its disintegration is probably extreme. Assuming matter
throughout the universe—here as condensed spheres, and there in outer
space as highly rarified substance—the atmospheric envelopes of the
spheres, he considers, will gradually shade off into the excessively
rare matter of mid-space. Matter may exist in countless conditions as to
simplicity and complexity, &c., but the base particle he assumes to be
the same under all conditions. In the spheres there is matter ranging
from the simplest elementary gases, through the mineral compounds of the
solid surface, to the highly compounded organic molecules. In outer
space the variation is in the opposite direction; the matter existing
there in a highly disintegrated condition.

Every particle he considers to possess a certain amount of motor energy
in the form of heat. As the total amount of this energy in the universe
remains unchanged, a particle can only lose energy by transferring it to
others. This heat energy acts, of course, in opposition to gravity: it
tends to repel the particles from each other, while gravity, on the
other hand, tends to draw them together. The former acts as a
centrifugal, the latter as a centripetal energy. If the heat momentum of
the particles be insufficient to constitute a centrifugal energy equal
to the centripetal energy of gravitation, then the material contents of
space will be drawn into the attracting spheres as atmospheric
substance, and outer space, in this case, will be left destitute of
matter. If, on the contrary, the centrifugal energy of the particles be
sufficient to resist gravitation, then the particles will remain free,
and space will continue to be occupied with matter. As has been stated,
the sum of motor energy in the universe remaining unchanged, the
aggregation of atmospheric substance around any planet resulting from
the loss of motor energy must cause an increase of motor energy in the
particles outside.

The theory seems to dispense with the necessity for assuming a
luminiferous ether, for the functions attributed to the ether may, it is
thought, be performed by the particles themselves; a view which has been
advocated by Euler, Grove, and others. The origin of nebulæ, according
to the theory, is accounted for as follows:

“The nebular hypothesis,” says Mr. Morris, “holds that the matter now
concentrated into suns and planets was once more widely disseminated, so
that the substance of each sphere occupied a very considerable extent of
space. It even declares that the matter of the solar system was a
nebulous cloud, extending far beyond the present limits of that system.
From this original condition the existing condition of the spheres has
arisen through a continued concentration of matter. But this
concentration was constantly opposed by the heat energy of the
particles, or, in other words, by their centrifugal momentum. This
momentum could only be got rid of by a redistribution of motor energy.
If, for illustration, the average momentum of the particles of the
nebulæ was just equivalent to their gravitative energy, then a portion
of this energy must radiate or be conducted outwards ere the internal
particles could be held prisoners by gravitation. The loss of momentum
inwardly must be correlated with an increase of momentum outwardly.

“This is a necessary consequence of the heat relations of matter. As
substance condenses, its capacity for heat decreases and its temperature
rises, hence a difference of temperature must constantly have arisen
between the denser and the rarer portions of the nebulous mass, and
equality of temperature could be restored only by heat radiation. This
radiation still continues, and must continue until condensation ceases
and the temperatures of the spheres and space become equalised; but this
is equivalent to declaring that as the particles of the spheres decrease
in heat momentum those of interspheral space increase, and if originally
the centrifugal and centripetal energies of matter approached equality
they must become unequal, centripetal energy becoming in excess in
spheral matter, centrifugal energy in the matter of space. Thus, as a
portion of the widely distributed nebulous matter lost its heat, and
became permanently fixed in place by gravitative attraction, another
portion gained heat, became still more independent of gravity, and
assumed a state of greater nebulous diffusion than originally. The
condensing spheres only denuded space of a portion of the matter which
it formerly held, and left the remainder more thinly distributed than
before. The spheres, in their concentration, have emitted, and are
emitting, a vast energy of motion. This motor energy yet exists in space
as a motion of the particles of matter, which, therefore, press upon
each other, or seek to extend their limits, with increasing vigour, so
that the elasticity of interspheral matter is constantly increasing.”

_Sir William R. Grove on the pre-nebular condition of matter._—Amongst
the first to advocate the view that ordinary matter is everywhere
present in space was Sir William R. Grove. In a lecture delivered at the
London Institution as far back as January 1842, he stated that it
appeared to him that heat and light, according to the undulatory theory,
were the result of the vibrations of ordinary matter itself, and not
that of a distinct ethereal fluid. Twenty years afterwards, referring to
the views he then advanced, he says: “Although this theory has been
considered defective by a philosopher of high repute, I cannot see the
force of the arguments by which it has been assailed; and, therefore,
for the present, though with diffidence, I still adhere to it.”[64]


[Footnote 64: _Correlation of Physical Forces_, p. 164 (fifth edition),
1867.]


He adduces a great many facts and forcible arguments in support of his
position. He says that “there appears no reason why the atmosphere of
the different planets should not be, with reference to each other, in a
state of equilibrium. Ether, or the highly attenuated matter existing in
the interplanetary space, being an expansion of some or all of these
atmospheres, or of the more volatile portions of them, would thus
furnish matter for the transmission of the modes of motion which we call
light, heat, &c.” It is assumed in the theory, of course, that matter
must form a universal planum.

Sir William Grove favours the idea that the universe is illimitable in
extent, a view held by many eminent thinkers.


EVOLUTION OF THE CHEMICAL ELEMENTS, AND ITS RELATIONS TO STELLAR
    EVOLUTION.

We come now to the consideration of a subject which has a most important
bearing on the question of stellar evolution, viz. the genesis and
dissociation of the chemical elements. The evolution of one element from
another is, it is true, as yet but a mere hypothesis, but it is an
hypothesis well supported by a host of facts and considerations, and
held by a large number of our leading chemists and physicists. “The
demonstrated unity of force,” says Professor F. W. Clarke,[65] “leads us
by analogy to expect a similar unity of matter; and the many strange and
hitherto unexplained relations between the different elements tend to
encourage our expectations.” The hypothesis throws much light on some
obscure points in stellar evolution. In regard to this, Professor Clarke
justly remarks that “it is plain that the nebular hypothesis would be
doubled in importance, and our views of the universe greatly expanded,
if it could be shown that an evolution of complex from simple forms of
matter accompanied the development of planets from the nebulæ. Evolution
could look for no grander triumph.” In fact, it is difficult to
understand how our sun and the stars could have been evolved from nebulæ
without assuming an evolution of the chemical elements. The true nebulæ
show the presence of only two elements, nitrogen and hydrogen, but our
sun contains more than a dozen of distinct elements, and the planets
more than three times that number. How, then, could all these have
arisen out of nebulæ composed simply of nitrogen and hydrogen? The
matter is plain if we assume an evolution of the elements.


[Footnote 65: _Popular Science Monthly_ for January 1873.]


The stars have been classed into four groups, which, as Professor Clarke
has remarked, indicate different stages in the process of evolution. The
first class, containing white stars like Sirius, show the predominance
of hydrogen and a scarcity of the metallic elements. In the second class
the metallic elements become more numerous and the hydrogen less
distinct; while in the third class hydrogen is difficult to detect.[66]
This seems to show a gradual development of the chemical elements as the
star cools and grows older. I shall now give a brief account of the
views expressed on the subject by some of our leading physicists and
chemists.


[Footnote 66: See also on this point Mr. Lockyer’s “Bakerian Lecture,”
_Proc. Roy. Soc._ No. 266, p. 21.]


It will be observed, in reference to the theories we have just
considered, that the process of evolution is supposed to take place from
the smaller to the larger aggregates of matter. Beginning with an
extreme condition of tenuity, by aggregation, the materials become
successively larger and more complex. In passing backwards in the
process we find the aggregates becoming less and less till they reach
the “cosmical dust,” or “fire-mist,” out of which the primitive nebulæ
were supposed to be formed. Receding still farther back, we have the
universal atmosphere from which the fire-mist is assumed to have been
derived.

This universal atmosphere, though in a state of extreme tenuity, is, as
we shall see, supposed by some to be in a more elemental form than
anything revealed to us in the laboratory. The suggestion of the
dissociation of the chemical elements and its application to stellar
physics was, I think, first advanced by Sir Benjamin Brodie in 1866, and
more fully in 1867. In the latter year views similar were considered
more fully by Dr. T. Sterry Hunt. The question of the dissociation of
elements has been ably discussed by Mr. Lockyer in his various writings.
It has been suggested by Mr. Lockyer that the coincidence of rays
emitted by different chemical elements when subjected to very high
temperatures affords evidence of a common element in the composition of
the metals producing the coincident rays. Mr. Lockyer states that many
trains of thought suggested by solar and stellar physics point to the
hypothesis that the _elements themselves, or at all events some of them,
are compound bodies_.[67] This view was also put forward by Professor
Graham, who says “that it is conceivable that the various kinds of
matter now recognised in different elementary substances may possess one
and the same element or atomic molecule existing in different conditions
of mobility. The essential unity of matter,” he adds, “is an hypothesis
in harmony with the equal action of gravity upon all bodies.” Similar
views have been advocated by M. Dumas, who based the suggestion of the
composite nature of the elementary atoms on certain relations of atomic
weights. The composite nature of the chemical elements has also been
maintained by Henri Sainte-Claire Deville, and also by Berthelot, who
held that the atoms of the elements are the same, and distinguished only
by their modes of motion. Professor Schuster, in a paper read before the
British Association in 1880, supports the view of the dissociation of
the chemical elements.


[Footnote 67: _Proc. Roy. Soc._ vol. xxviii. p. 160.]


That all the purely physical sciences will one day be brought under a
few general laws and principles, and the whole of the recognised
chemical elements will be resolved into one or two material elements, is
a conclusion towards which physical science seems at present slowly
tending. There is certainly something fascinating in this view of the
unity of nature. There is in this idea more than a purely physical
interest attached to it. It has, as I hope to show in a future work, an
important bearing on questions relating to the foundations of the true
theory of evolution.

The question of the unity of the chemical elements is one, however, yet
in a hypothetical condition. Professors Liveing and Dewar, who have
given attention to this subject, say: “The supposition that the
different elements may be resolved into simpler constituents, or into a
single one, has long been a favourite speculation with chemists; but,
however probable this hypothesis may appear _à priori_, it must be
acknowledged that the facts derived from the most powerful method of
analytical investigation yet devised give it scant support.”[68]


[Footnote 68: _Proc. Roy. Soc._ vol. xxxii. p. 230.]


_Sir Benjamin Brodie on the pre-nebular condition of matter._—There are,
considers Sir Benjamin Brodie, very forcible reasons which lead us to
suspect that chemical substances are really composed of a primitive
system of elemental bodies, analogous in their general nature to our
present elements: that some of those bodies which we speak of as
elements may be compounds. These ideal elements assumed by him, he says,
“though now revealed to us by the numerical properties of chemical
equations only as _implicit and dependent existences_, we cannot but
surmise may sometimes become, or may in the past have been, _isolated
and independent existences_”—as, for instance, in the case of the sun,
where the temperature is excessive. “We may,” he further adds, “consider
that in remote ages the temperature of matter was much higher than it is
now, and that these other things [ideal elements] existed then in the
state of perfect gases—separate existences—uncombined.”[69] He then
refers to certain observations of Mr. Huggins and Dr. Miller on the
spectra of nebulæ where one of the lines of nitrogen was found alone;
and that this suggested to them that the line might have been produced
by one of the elements of nitrogen; and that nitrogen may therefore be
compound. He mentions as a significant fact that a large proportion of
the class of elements which he has termed “composite elements” has not
been found in the sun, they having probably been decomposed by the
intense heat.


[Footnote 69: _Ideal Chemistry_, p. 56.]


_Dr. T. Sterry Hunt on the pre-nebular condition of matter._—A year
after the foregoing views regarding chemical dissociation had been
advanced by Sir Benjamin Brodie, Dr. T. Sterry Hunt, in a lecture on
“The Chemistry of the Primeval Earth,” delivered at the Royal
Institution (May 31, 1867), put forward, apparently quite independently,
opinions on dissociation similar to those of Brodie. In this lecture he
says: “I considered the chemistry of nebulæ, sun, and stars in the
combined light of spectroscopic analysis and Deville’s researches on
dissociation, and concluded with the generalisation that the breaking-up
of compounds, or dissociation of elements, by intense heat is a
principle of universal application, so that we may suppose that all the
elements which make up the sun, or our planet, would, when so intensely
heated as to be in the gaseous condition which all matter is capable of
assuming, remain uncombined, that is to say, would exist together in the
state of chemical elements, whose further dissociation in stellar or
nebulous masses may even give us evidence of matter still more elemental
than that revealed in the experiments of the laboratory, where we can
only conjecture the compound nature of many of the so-called elementary
substances.”[70] And in his address at the grave of Priestley he
referred to the suggestion of Lavoisier that hydrogen, nitrogen, and
oxygen, with heat and light, might be regarded as simpler forms of
matter from which all others are derived. This suggestion was considered
in connection with the fact that the nebulæ, which we conceive to be
condensing into suns and planets, have hitherto shown evidences only of
the presence of the first two of these elements, which, as is well
known, make up a large part of the gaseous envelope of our planet, in
the forms of air and aqueous vapour. With this he connected the
hypothesis advanced by Grove, “that our atmosphere and ocean are but
portions of the universal medium which, in an attenuated form, fills the
interstellary spaces;[71] and further suggested as a legitimate and
plausible speculation that these same nebulæ and their resulting worlds
_may be evolved by a process of chemical condensation from this
universal atmosphere_, to which they would sustain a relation somewhat
analogous to that of clouds and rain to the aqueous vapour around us.”


[Footnote 70: _American Journal of Science_, vol. xxiii. p. 124.]

[Footnote 71: “Our atmosphere,” says Dr. Hunt, “is not terrestrial, but
cosmical, being a universal medium diffused throughout all space, but
condensed around the various centres of attraction in amount
proportional to their mass and temperature, the waters of the ocean
themselves belonging to this universal atmosphere.” (_Nature_, August
29, 1878, p. 475.) Similar views have been advocated by Mr. Mattieu
Williams, who says “that the gaseous ocean, in which we are immersed, is
but a portion of the infinite atmosphere that fills the whole solidity
of space; that links together all the elements of the universe, and
diffuses among them their heat and light, and all the other physical and
vital forces which heat and light are capable of generating.” (_Fuel of
the Sun_, p. 5.) In 1854 Sir William Thomson suggested the idea that the
luminiferous ether was probably a continuation of our atmosphere, though
I do not think he continues to hold that opinion. The first to advance
this idea was, undoubtedly, Newton, who assumed interplanetary space to
be universally filled with an ethereal medium “much of the same
constitution as air, but far rarer, subtler, and more elastic.”]


_Professor Oliver Lodge on the pre-nebular condition of matter._—Some
have gone still farther back and supposed that the material universe may
have arisen out of the luminiferous ether—the hypothetical medium which
is assumed to pervade all space. The universal world-stuff scattered
through boundless space may in an extreme state of attenuation be, says
Professor Winchell, the ethereal medium, and out of this semi-spiritual
substance may have germinated the molecules of common matter. “It is
certainly possible,” he says, “to conceive these cosmical atoms as a
rising-out of some transformation of the ethereal medium; but we know
too little of the nature of ether to ground a scientific inference of
this kind.”[72]


[Footnote 72: _World Life_, p. 533.]


The ethereal origin of matter has been advocated by M. Saigey, Dr.
Macvicar, and others. In a lecture by Professor Oliver Lodge, delivered
at the London Institution in December 1882, he also advocates the
ethereal origin of matter. “As far as we know,” to state his views in
his own words, “this ether appears to be a perfectly homogeneous,
incompressible, continuous body, incapable of being resolved into simple
elements or atoms; it is, in fact, continuous, not molecular. There is
no other body of which we can say this, and hence the properties of
ether must be somewhat different from those of ordinary matter.” ...
“One naturally asks, is there any such clear distinction to be drawn
between ether and matter as we have hitherto tacitly assumed? May they
not be different modifications, or even manifestations, of the same
thing?” He then adopts Sir William Thomson’s theory of vortex atoms,
into the details of which I need not here enter. In conclusion, says
Professor Lodge, “I have now endeavoured to introduce you to the
simplest conception of the material universe which has yet occurred to
man—the conception that it is of one universal substance, perfectly
homogeneous and continuous, and simple in structure, extending to the
farthest limits of space of which we have any knowledge, existing
equally everywhere: some portions either at rest or in simple
irrotational motion, transmitting the undulations which we call light;
other portions in rotational motion—in vortices, that is—and
differentiated permanently from the rest of the medium by reason of this
motion.

“These whirling portions constitute what we call matter; their motion
gives them rigidity, and of them our bodies and all other material
bodies with which we are acquainted are built up.

“One continuous substance filling all space, which can vibrate as light;
which can be sheared into positive and negative electricity; which in
whirls constitutes matter; and which transmits by continuity, and not by
impact, every action and reaction of which matter is capable. This is
the modern view of ether and its functions.”[73]


[Footnote 73: _Nature_, February 1, 1883, p. 330.]


There is this objection to Professor Lodge’s theory: it is purely
hypothetical. The vortex atoms are not only hypothetical, but the
substance out of which these atoms are assumed to be formed is also
itself hypothetical. We have no certain evidence that such a medium as
is thus supposed exists, or that a medium possessing the qualities
attributed to it could exist. In fact, we have here one hypothesis built
upon another.

The vortex theory appears to me to be beset by a difficulty of another
kind, viz. that of reconciling it with the First Law of Motion.
According to that law no body possessing inertia can deviate from the
straight line unless forced to do so. A planet will not move round the
sun unless it be constantly acted upon by a force deflecting it from the
straight path. A grindstone will not rotate on its axis unless its
particles are held together by a force preventing them from flying off
at a tangent to the curve in which they are moving. Centrifugal force
must always be balanced by centripetal force. The difficulty is to
understand what force counterbalances the centrifugal force of the
rotating material of the vortex-atom. It is not because the centrifugal
tendency of the rotating material is controlled by the exterior
incompressible fluid, for it offers no resistance whatever to the
passage of the atom through it—in short, in so far as the motion of the
atom is concerned, this fluid is a perfect void. Now, if this fluid can
offer no resistance to the passage of the atom as a whole, how then does
it manage to offer such enormous resistance to the materials composing
the atom, so as to continually deflect them from the straight path and
compel them to move in a curve? The centrifugal force of these
vortex-atoms must be enormous, for on it is assumed to depend the
hardness or resistance of matter to pressure. Now the centripetal force
which balances this centrifugal force must be equally enormous. If,
then, this perfect fluid outside the vortex-atom can exert this enormous
force on the revolving material without being itself possessed of
vortex-motion, there does not seem to be any necessity for vortex-motion
in order to produce resistance. In short, how is the existence of the
atom possible under the physical conditions assumed in the theory? How
this may be, like the space of four dimensions, may be expressed in
mathematical language, but like it, I fear, it is unthinkable as a
physical conception.

_Mr. William Crookes on the pre-nebular condition of matter._—In his
opening address before the Chemical Section of the British Association
in 1886, Mr. William Crookes entered at considerable length into the
question of the genesis and evolution of the chemical elements. I shall
here give a brief statement of his views as embodied in his important
address, and this I shall endeavour to do as nearly as possible in Mr.
Crookes’s own words.

“We ask,” says Mr. Crookes, “whether the chemical elements may not have
been evolved from a few antecedent forms of matter—or possibly from only
one such—just as it is now held that all the innumerable variations of
plants and animals have been developed from fewer and earlier forms of
organic life: built up, as Dr. Gladstone remarks, from one another
according to some general plan. This building up, or evolution, is above
all things not fortuitous: the variation and development which we
recognise in the universe run along certain fixed lines which have been
preconceived and foreordained. To the careless and hasty eye design and
evolution seem antagonistic; the more careful inquirer sees that
evolution, steadily proceeding along an ascending scale of excellence,
is the strongest argument in favour of a preconceived plan.”

Now, as in the organic world, so in the inorganic, it seems natural to
view the chemical elements not as primordial, but as the gradual outcome
of a process of development, possibly even of a struggle for existence.
But this evolution of the elements must have taken place at a period so
remote as to be difficult to grasp by the imagination, when our earth,
or rather the matter of which it consists, was in a state very different
from its present condition. The epoch of elemental development, remarks
Mr. Crookes, is decidedly over, and it may be observed that in the
opinion of not a few biologists the epoch of organic development is
verging upon its close.

Is there then, in the first place, any direct evidence of the
transmutation of any supposed “element” of our existing list into
another, or of its resolution into anything simpler? To this question
Mr. Crookes answers in the negative.

We find ourselves thus driven to indirect evidence—to that which we may
glean from the mutual relations of the elementary bodies. First, we may
consider the conclusion arrived at by Herschel, and pursued by
Clerk-Maxwell, that atoms bear the impress of manufactured articles. “A
manufactured article may well be supposed to involve a manufacturer. But
it does something more: it implies certainly a raw material, and
probably, though not certainly, the existence of by-products, residues,
paraleipomena. What or where is here the raw material? Can we detect any
form of matter which bears to the chemical elements a relation like that
of a raw material to the finished product, like that, say, of coal-tar
to alizarin? Or can we recognise any elementary bodies which seem like
waste or refuse? Or are all the elements, according to the common view,
co-equals? To these questions no direct answers are forthcoming.”

_Argument from Prout’s Law._—The bearing of the hypothesis of Prout in
relation to the evolution of the elements is first considered by Mr.
Crookes. If that hypothesis were demonstrated it would show that the
accepted elements are not co-equal, but have been formed by a process of
expansion or evolution. According to this hypothesis the atomic weights
of the elements are multiples by a series of whole numbers of the atomic
weight of hydrogen. It is true that accurate determinations of the
atomic weights of different elements do not by any means harmonise with
the values which Prout’s Law requires; nevertheless the agreement in so
many cases is so close that one can scarcely regard the coincidence as
accidental.

The atomic weights have been recalculated with extreme care by Professor
F. W. Clarke, of Cincinnati, and he says that “none of the seeming
exceptions are inexplicable. In short, admitting half-multiples as
legitimate, it is more probable that the few apparent exceptions are due
to undetected constant errors than that the great number of close
agreements should be merely accidental.” In reference to this suggestion
of Professor Clarke, Mr. Crookes thinks that it places the matter upon
an entirely new basis. For, suppose the unit atom to be not hydrogen,
but some element of still lower atomic weight, say _helium_, an element
supposed by many authorities to exist in the sun and other stellar
bodies—an element whose spectrum consists of a single ray, and whose
vapour possesses no absorbent power, which indicates a remarkable
simplicity of molecular constitution. Granting that helium exists, all
analogy points, says Mr. Crookes, to its atomic weight being below that
of hydrogen; and here, then, we have the very element with atomic weight
half that of hydrogen required by Professor Clarke as the basis of
Prout’s Law.

_Argument from the earth’s crust._—The probable compound nature of the
chemical elements, Mr. Crookes thinks, is better shown by a
consideration of certain peculiarities in their occurrence in the
earth’s crust. “We do not,” he says, “find them evenly distributed
throughout the globe. Nor are they associated in accordance with their
specific gravities: the lighter elements placed on or near the surface,
and the heavier ones following serially deeper and deeper. Neither can
we trace any distinct relation between local climate and mineral
distribution. And by no means can we say that elements are always or
chiefly associated in nature in the order of their so-called chemical
affinities: those which have a strong tendency to form with each other
definite chemical combinations being found together, whilst those which
have little or no such tendency exist apart. We certainly find calcium
as carbonate and sulphate, sodium as chloride, silver and lead as
sulphides; but why do we find certain groups of elements, with little
affinity for each other, yet existing in juxtaposition or commixture?”

As instances of such grouping he mentions nickel and cobalt; the two
groups of platinum metals; and the so-called “rare earths,” existing in
gadolinite, samarskite, &c. Why, then, are these elements so closely
associated? What agency has brought them together? It cannot be
considered that nickel and cobalt have been deposited in admixture by
organic agency; nor yet the groups of iridium, osmium, and platinum;
ruthenium, rhodium, and palladium.

These features, Mr. Crookes thinks, seem to point to their formation
severally from some common material placed in conditions in each case
nearly identical.

_Argument from the compound radicals._—A strong argument in favour of
the compound nature of the elements, Mr. Crookes thinks, is derived from
a consideration of their analogy to the compound radicals, or
pseudo-elements as they might be called. It may be fairly held that if a
body known to be compound is found behaving as an element, this fact
lends plausibility to the supposition that the elements are not
absolutely simple. From a comparison of the physical properties of
inorganic with those of organic compounds, Dr. Carnelley concluded that
the elements, as a whole, are analogous to the hydrocarbon radicals.
This conclusion, if true, he added, should lead to the further inference
that the so-called elements are not truly elementary, but are made up of
at least two absolute elements, which he named provisionally A and B.

In Dr. Carnelley’s scheme all the chemical elements save hydrogen are
supposed to be composed of two simpler elements, A = 12 and B = 2. Of
these he regards A as a tetrad identical with carbon, and B as a monad
of negative weight; perhaps the ethereal fluid of space. His three
primary elements are, therefore, carbon, hydrogen, and the ether.

_Argument from polymerisation._—The polymeristic theory of the genesis
of the chemical elements propounded by Dr. Mills falls next to be
considered.

It has been suggested by Dr. E. J. Mills that the pristine matter was
once in an intensely heated condition, and that it has reached its
present state by a process of free cooling, and that the elements, as we
now have them, are the result of successive polymerisations. Chemical
substances in cooling naturally increase in density, and we sometimes
observe that as the density increases there are critical points
corresponding to the formation of new and well-defined substances. The
bodies thus formed are known as polymers. From a study of the
classification of the elements Mr. Mills is of opinion that the only
known polymers of the primitive matter are arsenic, antimony, and
perhaps erbium and osmium.

_Argument from the Periodic Law._—Lastly a scheme of the origin of the
elements, suggested to Mr. Crookes by consideration of Professor
Reynolds’s method of illustrating the periodic law of Newlands, is
discussed.

It was pointed out by Newlands that atomicity and other properties of
some of the chemical elements depend on the order in which their atomic
weights succeeded one another; and when this law was extended by
Professor Mendelejeff to all elements it was apparent that a
mathematical relation exists between the elements. This far-reaching law
has been fruitful of results. Referring to Professor Reynolds’s diagram
illustrating the law, Mr. Crookes says: “The more I study the
arrangement of this zigzag curve, the more I am convinced that he who
grasps the key will be permitted to unlock some of the deepest mysteries
of creation. Let us imagine if it is possible to get a glimpse of a few
of the secrets here hidden. Let us picture the very beginnings of time,
before geological ages, before the earth was thrown off from the central
nucleus of molten fluid, before even the sun himself had consolidated
from the original _protyle_.[74] Let us still imagine that at this
primal stage all was in an ultra-gaseous state, at a temperature
inconceivably hotter than anything now existing in the visible universe;
so high, indeed, that the chemical atoms could not yet have been formed,
being still far above their dissociation-point. In so far as _protyle_
is capable of radiating or reflecting light, this vast sea of
incandescent mist, to an astronomer in a distant star, might have
appeared as a nebula, showing in the spectroscope a few isolated lines,
forecasts of hydrogen, carbon, and nitrogen spectra.


[Footnote 74: _Protyle_ is the term adopted by Mr. Crookes to designate
the original primal matter existing before the evolution of the chemical
elements, and out of which they were evolved. Protyle in chemistry is
analogous to _protoplasm_ in biology, with this difference, however,
that protyle is as yet hypothetical, whereas protoplasm is known to be
real.]


“But in course of time some process akin to cooling, probably internal,
reduces the temperature of the cosmic _protyle_ to a point at which the
first step in granulation takes place; matter as we know it comes into
existence, and atoms are formed. As soon as an atom is formed out of
_protyle_ it is a store of energy, potential (from its tendency to
coalesce with other atoms by gravitation or chemically) and kinetic
(from its internal motions). To obtain this energy, the neighbouring
_protyle_ must be refrigerated by it, and thereby the subsequent
formation of other atoms will be accelerated. But with atomic matter the
various forms of energy which require matter to render them evident
begin to act; and, amongst others, that form of energy which has for one
of its factors what we now call _atomic weight_. Let us assume that the
elementary _protyle_ contains within itself the potentiality of every
possible combining proportion or atomic weight. Let it be granted that
the whole of our known elements were not at this epoch simultaneously
created. The easiest formed element, the one most nearly allied to the
_protyle_ in simplicity, is first born. Hydrogen—or shall we say
helium?—of all the known elements the one of simplest structure and
lowest atomic weight, is the first to come into being. For some time
hydrogen would be the only form of matter (as we know it) in existence,
and between hydrogen and the next formed element there would be a
considerable gap in time, during the latter part of which the element
next in order of simplicity would be slowly approaching its birth-point:
pending this period we may suppose that the evolutionary process, which
soon was to determine the birth of a new element, would also determine
its atomic weight, its affinities, and its chemical position.”

_Professor F. W. Clarke on the pre-nebular condition of matter._—Views
on elemental evolution almost similar to those of Mr. Crookes’s have
been advocated by Professor Clarke. Spectroscopic phenomena, says
Professor Clarke, are quite in harmony with the idea that all matter is
at bottom one, our supposed atoms being really various aggregations of
the same fundamental unit.

“Everybody knows that the nebular hypothesis, as it is to-day, draws its
strongest support from spectroscopic facts. There shine the nebulæ in
the heavens, and the spectroscope tells us what they really are, namely,
vast clouds of incandescent gas, mainly, if not entirely, hydrogen and
nitrogen. If we attempt to trace the chain of evolution through which
our planet is supposed to have grown, we shall find the sky is full of
intermediate forms. The nebulæ themselves appear to be in various stages
of development; the fixed stars or suns differ widely in chemical
constitution and in temperature; our earth is most complex of all. There
are no ‘missing links’ such as the zoologist longs to discover when he
tries to explain the origin of species. First, we have a nebula
containing little more than hydrogen, then a very hot star with calcium,
magnesium, and one or two other metals added; next comes a cooler sun in
which free hydrogen is missing, but whose chemical complexity is much
increased; at last we reach the true planets with their multitudes of
material forms. Could there well be a more straightforward story? Could
the unity of creation receive a much more ringing emphasis? We see the
evolution of planets from nebulæ still going on, and parallel with it an
evolution of higher from lower kinds of matter.

“Just here, perhaps, is the key to the whole subject. If the elements
are all in essence one, how could their many forms originate save by a
process of evolution upward? How could their numerous relations with
each other, and their regular serial arrangements into groups, be better
explained? In this, as in other problems, the hypothesis of evolution is
the simplest, most natural, and best in accordance with facts.”[75]


[Footnote 75: _Popular Science Monthly_ for February 1876. See also the
January number for 1873.]


_Dr. G. Johnstone Stoney on the pre-nebular condition of
matter._—Further evidence that all the chemical elements were probably
evolved from one common source, is furnished by Dr. G. Johnstone
Stoney’s “Logarithmic Law of Atomic Weights,” a theory recently advanced
in a communication to the Royal Society.[76] A cardinal feature of this
investigation is that in it atomic weights are represented by volumes,
not by lines. A succession of spheres are taken whose volumes are
proportional to the atomic weights, and which may be called _the atomic
spheres_. When the radii of these spheres are plotted down on a diagram
as ordinates, and a series of integers as abscissas, the general form of
the logarithmic curve becomes apparent; and close scrutiny has shown
that either the logarithmic curve, or some curve lying very close to it,
expresses the real law of nature.


[Footnote 76: _Proc. Roy. Soc._ for April 19, 1888, p. 115.]


If, as seems probable, the logarithmic law is the law of nature, there
appear to be three elements lighter than hydrogen, which Dr. Stoney has
termed infra-fluorine, infra-oxygen, and infra-nitrogen. And there are,
at all events, six missing elements between hydrogen and lithium.

Dr. Stoney’s investigation is based on the fact that if the atomic
weights of the chemical elements be arranged in order of magnitude,
periodic laws come to light, viz.: those discovered by Newlands,
Mendelejeff, and Meyer. From this it follows that there must be some law
connecting the atomic weights with the successive terms of a numerical
series—either alone or along with other variables.

“This law,” says Dr. Stoney, “may be obtained in one of its graphical
forms by plotting down a series of integers as abscissas, and the
successive atomic weights as ordinates. In this way it furnishes a
diagram which has somewhat the shape of a hurling-stick, consisting of a
short curved portion succeeded by a long and nearly straight portion.
But as this diagram cannot be directly identified with any known curve,
it does not suffice for the determination of the law.

“The diagram, however, assumes a form which can be interpreted when we
use the cube roots of the atomic weights for its ordinates, instead of
the atomic weights themselves. This is equivalent to taking volumes
instead of lines to represent the atomic weights. When this is done, the
ends of the ordinates are found to lie near a regular and gradual curve,
from which they deviate to the right and left by displacements that are
small and appear to follow periodic laws which have been in part traced.
The central curve is found on examination to be either a logarithmic
curve or some curve lying exceedingly close to it. If the curve be in
reality the logarithmic curve, it furnishes us the law that:

“The cube root of the n^{th} atomic weight = κ log (n q) + a small
periodic correction; where κ and q are constants, the values of which
are furnished by the observations.

“Either this logarithmic law, or a law that lies exceedingly close to
it, must be the law of nature.”

Referring to this theory, Professor Reynolds says: “It certainly
introduced points of extraordinary importance, though perhaps at present
they could not all quite realise its fullest import. There were several
points of some little difficulty to be grappled with, but it clearly
pointed to the conclusion that we were fast approaching the time when
physicists—both chemical and physicists proper—are combining to evolve
out of the scientific work lying on the borderland most important and
startling facts.”

The bearing which Dr. Stoney’s conclusions, like those of Mr. Crookes,
have on the primitive condition of the material universe is obvious.

Dr. Stoney, like Mr. Crookes, considers that the chemical elements are
subject to decay. That they are not only generated but destroyed—that
they are subject not only to evolution but dissolution. He believes that
the generative process probably takes place only at, or beyond, the
confines of the universe, and the destructive process at the centres of
overgrown stars, which is the position of lowest potential. Dr. Stoney
thinks that this extinction of the chemical elements in the centre of a
star is a cause which limits its size and prevents its overgrowth.


THE IMPACT THEORY IN RELATION TO THE FOREGOING THEORIES OF THE
    PRE-NEBULAR CONDITION OF MATTER.

In all these theories, as has already been observed, the primitive
condition of the universe was that of matter in a state of extreme
tenuity, while by aggregation the materials became successively larger
and larger until they assumed the magnitude of suns and planets. For
example, according to the meteoric theory, meteorites are formed out of
“cosmical dust,” “fire-mist,” or condensed vapour, and then suns and
planets are formed by aggregation from these meteorites. Facts seem,
however, to point to the very reverse as being the true course of
events.

Meteorites are undoubtedly the fragments of larger masses. It looks more
likely that they are, as has already been stated, fragments of stellar
masses which have been shattered to pieces by collision, and that this
“cosmical dust,” from which the meteorites are alleged to have been
formed, are simply the dust arising out of the destruction of the
masses. After the two bodies had collided and been shattered to pieces,
some of the fragments would undoubtedly be projected with a velocity
that would carry them beyond the attractive power of the general mass,
and thus they would escape being volatilised. These fragments would
continue their wanderings through space as meteorites.

I cannot but think that the number, as well as the importance, of these
wanderers has been greatly over-estimated. Mr. Lockyer states that Dr.
Schmidt, of Athens, found that the mean hourly number of luminous
meteors visible on a clear moonless night by one observer was fourteen.
Certainly no such quantity is visible in this country. In Scotland, at
least, one may often watch night after night under the most favourable
conditions without having the good fortune to see a single meteor.

It is, of course, true that the immediately prior condition of a sun or
a planet was that of matter in an extremely attenuated or dissociated
state. This is essential to the nebular, as well as to the meteoric,
hypothesis. But it is not with the immediately prior condition that we
are at present concerned, but with the primitive, or pre-nebular,
condition. Take, for example, the case of the solar nebula, out of which
our sun and planets were formed. Was this nebulous mass formed from
matter in a state of extreme tenuity, scattered through space and
collected together by gravity? Or did it result from two solid globes
shattered to pieces by collision, which were then converted into the
nebulous condition by the heat generated from the collision? It is no
doubt true that the analogies of nature would, at first sight, be apt to
lead us to the conclusion that the former theory was the more likely of
the two, as the larger is generally made by aggregation from the
smaller. But a little consideration will show that, in the present case,
the weight of this analogy is more apparent than real. The impact theory
does not rest upon a purely hypothetical basis. The cause to which it
appeals has a real existence. The point of uncertainty is whether the
cause actually produces the effect which is attributed to it. We know
from observation that there are stellar masses, some of them probably
larger than our sun, moving through space with enormous velocities in
all directions.[77] According to the ordinary laws of chance, collision
at times would be an inevitable result, and when such an event did take
place the destruction of the colliding bodies, and their consequent
transformation into a nebulous mass, would, at least in many cases, be a
_necessary_ result. In fact, we have, in the case of these vast stellar
masses, what we know occurs among the invisible molecules of a gas. So
far as mere analogy is concerned, the impact theory is just about as
probable as the other.


[Footnote 77: The dark stellar masses which escape observation may be as
numerous as those that are visible.]


From what has been stated it would follow that in most cases the stellar
masses have been formed out of the destruction of pre-existing masses,
like the geological formations out of the destruction of prior
formations.

_The theories do not account for the motion of the stars._—According to
all the foregoing theories aggregation and condensation are produced by
gravity. The materials dispersed throughout space are drawn together by
their mutual attraction, and aggregated round a centre of gravity.
Gravitation, although it imparts motion to the materials, can impart no
motion of translation to the mass itself. Gravitation cannot, therefore,
be the cause of the motion of translation of the mass. The stars are not
supposed to be gravitating towards, or around, a great centre of
attraction, for they are found moving in straight lines in all
directions, which could not be the case if gravity were the cause of
their motion. To what cause is their motion, therefore, to be
attributed? A meteorite or other small body might be ejected from any
system, by the explosive force of heat or some other cause, with a
velocity which might carry it into boundless space; but such could not
be the case in regard to a body of the magnitude of a star. No one for a
moment could suppose that 1830 Groombridge, for example, moving at the
rate of 200 miles a second, is an eject from any system.

According to the impact theory the whole is plain; for this 200 miles
per second is simply a part of the untransformed motion of translation
which the materials composing the star had from the beginning. In other
words, the matter and the motion were eternal, or, what is more
probable, as will afterwards be seen, co-existed from creation—not,
however, as molecular motion, but as motion of the mass.

_The theories do not account for the amount of heat required._—It has
been shown that, although the materials of our solar system had fallen
together from an infinite distance, it could not have generated heat
sufficient to have formed a gaseous nebula extending to the distance of
the planet Neptune. Gravitation alone could not, therefore, have been
the source from which the nebula obtained its heat. The solar nebula,
however, must originally have extended far beyond the orbit of Neptune.

But supposing it could be demonstrated that the heat thus generated was
sufficient to have formed a nebula extending to even twice the distance
of Neptune, this would not remove the fatal objection to the gravitation
theory of the origin of the solar nebula. For the facts, both of geology
and of biology, equally show that the sun has been radiating his heat at
the present rate for more than twice the length of time that it could
possibly have done had gravitation been the source from which the energy
was derived. This objection is alike fatal to the meteoric theory as it
is to all other theories which attribute the origin and source of the
heat to gravitation.

_Evolution of matter._—Our inquiries into stellar evolution do not,
however, begin with the consideration of a gaseous nebula, or with
swarms of meteorites. There was a pre-nebular evolution. The researches
of Prout, Newlands, Mendelejeff, Meyer, Dumas, Clarke, Lockyer, Crookes,
Brodie, Hunt, Graham, Deville, Berthelot, Stoney, Reynolds, Carnelley,
Mills, and others, clearly show, I think, that the very matter forming
this nebulous mass passed through a long anterior process of evolution.
And not only the matter, but the very elements themselves constituting
the matter, were evolved out of some prior condition of substance.

I have already given at some length the views which have been advanced
by several of our leading physicists and chemists on the evolution of
the chemical elements, and on some of the bearings which these views
have on stellar evolution. I shall now briefly refer to a point on which
I venture to think the theory discussed in this volume seems to cast
some additional light.

If the elements were evolved out of a common source, there is, in order
to this, one necessary condition, viz. an excessively high temperature;
for the temperature must be above the point of the dissociation of all
the chemical elements. “In the primal stage of the universe,” says Mr.
Crookes, “before matter, as we now find it, was formed from the protyle,
all was in an ultra-gaseous state, at a temperature inconceivably hotter
than anything now existing in the visible universe; so high, indeed,
that the chemical atoms could not yet have been formed, being still far
above their dissociation point.”

What, then, produced this excessive temperature in this supposed
ultra-gaseous protyle? It could not have resulted from condensation by
gravity. In condensation the heat increases as the condensation
proceeds, because it is the condensation which produces the heat. But
here the reverse must have been the case, for the ultra-gaseous mass was
much hotter than the sun which was afterwards formed out of it. It was,
according to Mr. Crookes, when this gaseous mass cooled down, so as to
permit of its becoming converted into solid matter, that condensation
into a sun could take place. Besides, was it not the excessive heat
which produced the assumed ultra-gaseous condition?

There is another difficulty besetting the theory that the primitive heat
was derived from condensation by gravitation. Supposing we should assume
it possible that the protyle could exist in this ultra-gaseous state
without possessing temperature, and that it obtained its heat from
condensation by gravity, then the fact of condensation taking place
shows that the gas was not in a state of equilibrium. But the gas could
not have remained stationary for a single moment without beginning to
condense while in a condition of unstable equilibrium. We must therefore
conclude that the gas must have been in some other condition than the
gaseous state prior to condensation.

The impact theory seems to remove all these difficulties. It is just as
likely _à priori_, if not more so, that the primitive form of the
protyle should have been that of large cold masses moving through space
in all directions, with excessive velocities, as that it should have
been that of a gaseous mass in a state of unstable equilibrium. If we
assume the former condition, then the colliding of these masses would
account not only for the ultra-gaseous state, but also for its
inconceivably high temperature. Besides, in this case we are not called
upon to account for any other antecedent state of the masses before
collision, for they may have existed from the beginning of creation in
the form of masses in motion through space.

Had space and time permitted, it might have been shown that there are
other obscure points on which the theory seems to shed additional light.
I shall now, in conclusion, refer to a point wherein the theory differs
radically from that of all other theories of stellar evolution. But
before doing so I may briefly refer to an objection which has been
frequently urged against the theory.

_Objection considered._—The objection to which I refer is this, that,
had the nebulæ been produced by impact in the way implied in the theory,
then we ought to have had some historical record of such an event. I can
perceive no force in such an objection. Our historical records, I
presume, do not extend much farther back than about 3,000 years, and we
have no evidence to conclude that a new nebula makes its appearance in
the visible firmament with such frequency; and supposing it did, we have
no grounds for assuming that its production by impact in the way
supposed by the theory would attract general notice. It is doubtful if
the nebula produced would, in the first instance, be actually visible. I
have shown that the temperature of the nebula could not have been less
than about 300,000,000° C., and it is very doubtful if the gaseous mass
enveloping all that was solid in the nebula would, at such a
temperature, be self-luminous. The probability is that all the chemical
elements composing it would be in a state of utter dissociation, and
converted back into the original protyle from which they were derived,
again to be slowly reconverted into their former atomic condition as the
temperature fell.

_Can we on scientific grounds trace back the evolution of the universe
to an absolute first condition?_—As has been repeatedly stated, all
inquiries into the evolutionary history of the stellar universe begin in
the middle of a process. Evolution is a process. The changes that now
occur arose out of preceding changes, and these, preceding changes out
of changes still prior, and so on indefinitely back into the unknown
past. This chain of causation—this succession of change—of consequent
and antecedent—could not in this manner have extended back to infinity,
or else the present stage of the universe’s evolution ought to have been
reached infinite ages ago. The evolution of things must therefore have
had a beginning in time. Professor Winchell, in his final generalisation
to his work, “World Life,” has stated this matter so clearly and
forcibly that I cannot do better than here quote his words on the
subject.

“We have not,” says Professor Winchell, “the slightest scientific
grounds for assuming that matter existed in a certain condition from all
eternity, and only began undergoing its changes a few millions or
billions of years ago. The essential activity of the powers ascribed to
it forbids the thought. For all that we know—and, indeed, as the
_conclusion_ from all that we know—primal matter began its progressive
changes on the morning of its existence. As, therefore, the series of
changes is demonstrably finite, the lifetime of matter itself is
necessarily finite. There is no real refuge from this conclusion; for,
if we suppose the beginning of the present cycle to have been only a
restitution of an older order effected by the operations of natural
causes, and suppose—what science is unable to comprehend—that older
order to be a similar re-inauguration, and so on indefinitely through
the past, we only postpone the predication of an absolute beginning,
since, by all the admissions of modern scientific philosophy, it is a
necessity of nature to run down.”

These are consequences which necessarily follow from every theory of
stellar evolution which has hitherto been advanced. The impact theory,
however, completely removes the difficulty, for according to it the
evolutionary process can, on purely scientific grounds, be traced back
to an absolute beginning in time. If huge solid masses moving through
space were the original condition of the universe, then, in so far as
either philosophy or science can demonstrate to the contrary, it might
have been in this condition from all eternity. We are therefore not
called upon to account for this primitive condition of things. Now it is
evident, unless a collision should take place, the universe would remain
in this condition for ever: without a collision there could be no
change, no work performed, and absolutely no loss or gain of energy, and
therefore no process of evolution. The first collision would be the
absolute commencement of evolution—the beginning of the process of the
development of the universe. Evolution would, in this case, have its
absolute beginning in time, and consequently was not eternal. If, on the
other hand, we assume, what is far more in harmony with physics,
metaphysics, and common sense, that the universe was created in time, we
are still led to the same result as to an absolute commencement of
evolution. In both cases we reach a point beyond which there can be no
legitimate inquiry; no further question which the scientists can
reasonably ask.

We have no grounds to conclude that there is anything eternal, except
God, Time, and Space. But if time and space be subjective, as Kant
supposes, and not modes pertaining to the existence of things in
themselves, then God alone was uncreated, and _of_ Him and _to_ Him are
all things.

------------------------------------------------------------------------

------------------------------------------------------------------------



                                 INDEX.


 Aqui Range, Utah, fault in, 57

 Arcturus, motion of, 16

 Atmosphere, universal, 82

    „    Dr. Hunt on, 86

    „    Mr. Mattieu Williams on, 86

 Atomic weights, logarithmic law of, 100

 Atoms, according to Herschel and Clerk-Maxwell, manufactured articles,
    92


 Binary systems, 32

    „    Dr. Johnstone Stoney on, 33

    „    Sir W. Thomson on, 33

 Biology, testimony of, as to age of sun’s heat, 65

 Brodie, Sir B., on the pre-nebular condition of matter, 84

 Brown and Dickson on sediment of the Mississippi, 40


 Carnelley, Dr., argument from compound radicals, 94

 μ Cassiopeiæ, motion of, 16

 α Centauri, distance of, 16

 Chemical elements, evolution of, 80

 Clarke, Prof. F. W., on atomic weights, 93

    „    on evolution of the chemical elements, 80, 89

    „    on the pre-nebular condition of matter, 98

 Comets, according to Laplace, strangers to our system, 17

    „    according to Prof. A. Winchell, strangers to our system, 17

    „    M. Faye on origin of, 17

    „    probable origin of, 17

 Compound radicals, argument from, 95

 Condensation in relation to nebulæ, 27

    „    the last condition of a nebula, 30

 Cosmical dust and “fire-mist,” 81, 102

 Crookes, Mr. W., on the pre-nebular condition of matter, 90-98

    „    on _protyle_, 96

 61 Cygni, motion of, 16


 Darwin, Mr. Charles, on geological time, 67

 Denudation, age of the globe as represented by, 63, 64

    „    average rate of whole globe, 44

    „    evidence from faults as to rate of, 53

    „    Dr. A. Geikie on rate of, 41

    „    glacial epochs in relation to, 46, 47

    „    in Colorado, 58

    „    in past ages not much greater than at present, 44

    „    method employed to estimate its rate, 39, 47

    „    Mr. A. R. Wallace’s method of estimating its rate, 51

    „    of Bristol coal-fields, 59

    „    of Mississippi basin, Sir Charles Lyell on, 44

    „    of Pendle Range, 60

    „    of Pentlands, 59

    „    of river basins, 41

    „    of South of Scotland, 55

    „    of Wales, 59

    „    Prof. Haughton’s method of estimating its rate, 50

    „    Rotation of the earth in relation to, 46

    „    the direct method of estimating its rate, 52

    „    time required to effect the amount of, 63

 Dewar and Liveing on dissociation of chemical elements, 83

 Dissociation of chemical elements, Dr. T. Sterry Hunt on, 82, 85

    „    of chemical elements, M. Berthelot on, 83

    „    of chemical elements, M. Deville on, 83

    „    of chemical elements, Mr. Lockyer on, 82

    „    of chemical elements, Profs. Liveing and Dewar on, 83

    „    of chemical elements, Prof. Schuster on, 83

    „    of chemical elements, Sir B. Brodie on, 82, 84

    „    Dumas, M., on essential unity of matter, 83


 Earth’s crust, argument from, 93

    „    rotation, its influence on denudation, 46

 Emmons, Mr. S. F., on a fault in Aqui Range, 58

 Energy existing as motion of stellar masses, 3

    „    transformed by collision, 3

 Evolution, can it be traced back to a first condition? 110

    „    evidence of, from the grouping of the stars, 81

    „    from smaller to larger aggregates of matter, 81

    „    of matter, 107

    „    of the chemical elements, 80, 107


 Faults, evidence of rate of denudation from, 53

    „    examples of, 54-60

    „    “Grand Wash,” Colorado, 58

    „    in East Tennessee, 58

    „    in Strathmore, 55

 Faye, M., on origin of comets, 17


 Gaseous condition essential to the nebular hypothesis, 25

    „    state, second condition of a nebula, 24

 Geikie, Dr. A., on area of the globe, 48

    „    on denudation of the Pentlands, 59

    „    on examples of enormous faults, 55

    „    on rate of denudation, 41

 Geological epochs of past ages, misconceptions regarding, 49

 Geological time, Mr. A. R. Wallace on, 65

    „    time, Mr. Charles Darwin on, 66

    „    time, Prof. Haeckel on, 67

    „    time, Prof. Huxley on, 65, 66

    „    time, Sir Andrew C. Ramsay on, 67

 Geology, testimony of, in regard to age of sun’s heat, 39

 Glacial epoch, age of the earth as determined by, 64

    „    epochs, influence on denudation, 46

 Gravitation does not account for the heat required, 106

    „    does not account for motion of the stars, 105

    „    insufficient to account for heat of nebulæ, 27

 Groombridge 1830, motion of, 15

    „    not an eject, 106

    „    Prof. Newcomb on motion of, 15

 Grove, Sir W. R., on the pre-nebular condition of matter, 78


 Haeckel, Prof, on geological time, 67

 Haughton, Prof., method of estimating rate of denudation, 50

 Heat, age of the sun’s, 37

 Helmholtz on age of sun’s heat, 35

 Huggins, Mr., and Dr. Miller on spectra of nebulæ with one nitrogen
    line, 84

 Hull, Prof., on denudation of Pendle Range, 60

    „    on examples of enormous faults, 54

 Humphreys and Abbot on sediment of the Mississippi, 40

 Hunt, Dr. T., on the pre-nebular condition of matter, 85

    „    on universal atmosphere, 86

 Huxley, Prof., on geological time, 65, 66

 Hypothesis, value of, 70


 “Impact Theory,” why so called, 2

    „    in relation to theories of pre-nebular condition of matter, 102

    „    removes difficulties regarding origin of heat, 108, 109

 ε Indi, motion of, 16


 King, Mr. Clarence, on the Wahsatch Fault, 59


 Lalande 21185, motion of, 16

    „    21258, motion of, 16

 Laplace, M., on the heat of the solar nebula, 30

 Lavoisier, M., on simpler forms of matter, 86

 Lesley, Mr. J. P., on a fault in the Appalachians, 57

    „    on fault in East Tennessee, 58

 Liveing and Dewar on dissociation of chemical elements, 83

 Lockyer, Mr., on arrangement of the planets according to density, 25

    „    nebulæ with solid matter in a gaseous mass, 20

 Lockyer, Mr., on essential condition of solar nebulæ, 25

    „    on hypothesis, 70

    „    on number of meteorites, 103

    „    on outburst of stars, 33

    „    on “sorting” of the chemical elements, 25

 Lodge, Prof. O., on ethereal origin of matter, 87

    „    on the pre-nebular condition of matter, 87

    „    on vortex atoms, 88

 Logarithmic law of atomic weights, 100

 Lyell, Sir Charles, on denudation of the Mississippi basin, 44


 Macvicar, Dr., on ethereal origin of matter, 87

 Matter not probably eternal, 112

 Mendelejeff, Prof., on Periodic Law, 96

 Meteorites, number greatly exaggerated, 103

    „    probable origin of, 12

    „    Sir H. Roscoe on constitution of, 12

    „    Sir W. Thomson on, 12

 Mill, Mr. J. S., on hypothesis, 70

 Miller, Dr., and Mr. Huggins on spectra of nebulæ with one nitrogen
    line, 84

 Mills, Dr., on Polymerisation, 95

 Morris, Mr. Charles, on the pre-nebular condition of matter, 75


 Nebulæ, broken fragments in a gaseous mass, 19

    „    cometic, 22

    „    condensation insufficient to account for heat of, 27

 Nebulæ condensation, last condition of, 30

    „    first condition of, 19

    „    gaseous state, second condition of, 24

    „       „    globular, 21

    „    heat of, not due to gravitation, 27

    „    how they occupy so much space, 18

    „    how origin of by impact might not have been observed, 110

    „    must possess an excessive temperature, 26

    „    Mr. Lockyer on, 20-22

    „    origin of, 18

    „    Prof. A. Winchell on meteoric origin of, 22

    „    Prof. Tait on, 20

    „    Sir W. Thomson on origin of, 6, 28

    „    spheroidal, 22

    „    why of such various shapes, 19

 Nebular hypothesis, gaseous condition essential to, 2, 5

 Newcomb, Prof., on motion of 1830 Groombridge, 15

 Newlands on Periodic Law, 96

 Nova Cygni, on sudden outburst of, 33


 Objection considered, 109


 Palæozoic times, winds probably not higher than at present, 46

 Peach, Mr. B. N., on examples of enormous faults, 55

    „    on denudation of the south of Scotland, 55

 Periodic Law, argument from, 96

    „    Prof. Mendelejeff on 96

 Periodic Law, Newlands on, 96

    „    Prof. Reynolds on, 96

 Planets, on their arrangement according to density, 25

 Polymerisation, argument from, 95

    „    Dr. Mills on, 95

 Pouillet, on rate of solar radiation, 2, 35

 Powell, Major J. W., on denudation of Uinta Mountains, 58

 Pre-nebular condition of matter, Dr. G. Johnstone Stoney on, 99

    „    condition of matter, Dr. T. Sterry Hunt on, 85

    „    condition of matter, Mr. Charles Morris on, 75

    „    condition of matter, Mr. W. Crookes on, 90

    „    condition of matter, Prof. A. Winchell on, 71

    „    condition of matter, Prof. F. W. Clarke on, 98

    „    condition of matter, Prof. Lodge on, 87

    „    condition of matter, Sir B. Brodie on, 84

    „    condition of matter, Sir W. R. Grove on, 78

    „    evolution, 107

 Proctor, R. A., on meteoric origin of solar system, 23

 _Protyle_, the primal matter, 96

 Prout’s Law, argument from, 92


 Ramsay, Sir Andrew C, on denudation of Bristol coal-fields, 59

    „    on denudation of Wales, 59

    „    on geological time, 67

 Reynolds, Prof., on Periodic Law 96

 Rogers, Prof. H. D., on a great fault in the Appalachian coal-fields,
    57

 Roscoe, Sir H., on constitution of meteorites, 12

 Rotation, supposed influence on denudation, 46


 Saigey, M., on ethereal origin of matter, 87

 Schmidt, Dr., on number of meteorites, 103

 Solar nebula, M. Laplace on heat of, 30

    „    Mr. Lockyer on condition essential to, 25

    „    Sir W. Thomson on, 6, 28

 Solar radiation, rate of, according to Pouillet and Langley, 35

 Solar system, Mr. R. A. Proctor on meteoric origin of, 23

 Star clusters, 34

 Stars, evidence of evolution from their grouping, 81

    „    how origin of by impact might not have been observed, 110

    „    in four groups, 81

    „    motion not accounted for by gravitation, 105

    „    motion not due to their mutual attractions, 14

    „    motion of, how in straight lines, 14

    „    sudden outbursts of, 33

 Stoney, Dr. G. Johnstone, on the pre-nebular condition of matter, 99

 Subaërial denudation, method of estimating rate of, 39, 47

 Sun, age of heat of, 34

 Sun’s heat, age of, according to Geology, 37

    „    age of, a crucial test, 34, 37

 Sun’s heat, age of, according to Thomson and Tait, 35

    „    age of, as determined by Biology, 64

    „    age of, as determined by Geology, 39


 Tait, Prof., nebulæ with solid matter in a gaseous mass, 20

    „    on age of sun’s heat, 35

 Temperature excessive, essential to nebulæ, 26

    „    produced by collision, 5

 Thomson, Sir W., on age of sun’s heat, 35

    „    on meteorites, 12

    „    on origin of solar nebula, 28

    „    on solar nebula, 6

    „    suggestion by, 86

 Tides, supposed influence on denudation, 45

 Tycho Brahe, on sudden outburst of a star, 33

 Tylor, Alfred, on the denudation of Mississippi Basin, 40


 Uinta Mountains, denudation of, 58

    „    fault in, 57


 Vortex atoms, Prof. Lodge on, 88


 “Wahsatch Fault,” Utah, immense dislocation, 58

 Wallace, Mr. A. R., on geological time, 65, 68

    „    method of estimating rate of denudation, 57

 Williams, Mr. Mattieu, on universal atmosphere, 86

 Winchell, Prof. A., on age of the earth, 64

    „    on comets strangers to our system, 17

    „    on denudation of Colorado plateau, 58

    „    on deposition of Palæozoic sediment, 45

    „    on ethereal medium, 87

    „    on meteoric origin of nebulæ, 22

    „    on nebular hypothesis, 69

    „    on the pre-nebular condition of matter, 71

------------------------------------------------------------------------



                  _D. APPLETON & CO.’S PUBLICATIONS._

DISCUSSIONS ON CLIMATE AND COSMOLOGY. By JAMES CROLL, LL. D., F. R.
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CONTENTS: Misapprehensions regarding the Physical Theory of Secular
Changes of Climate.—The Ice of Greenland and the Antarctic Continent not
due to Elevation of the Land.—Mr. Alfred R. Wallace’s Modification of
the Physical Theory or Secular Changes of Climate.—The Physical Cause of
Mild Polar Climates.—Interglacial Periods and Distribution of Flora and
Fauna in Arctic Regions.—Temperature of Space and its Bearing on
Terrestrial Physics.—Probable Origin and Age of the Sun’s Heat, etc.,
etc.

CLIMATE AND TIME IN THEIR GEOLOGICAL RELATIONS: A THEORY OF SECULAR
    CHANGES OF THE EARTH’S CLIMATE. By JAMES CROLL, of H. M.
    Geological Survey of Scotland. With Maps and Illustrations.
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physical agencies, and that these changes, in as far as the past
climatic condition of the globe is concerned, fully meet the demand of
the geologist.”—_From the Preface._

OTHER WORLDS THAN OURS: THE PLURALITY OF WORLDS, STUDIED UNDER THE
    LIGHT OF RECENT SCIENTIFIC RESEARCHES. By RICHARD ANTHONY
    PROCTOR. With Illustrations, some colored. 12mo. Cloth, $2.50.

LIGHT SCIENCE FOR LEISURE HOURS. A Series of Familiar Essays on
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THE MOON: HER MOTIONS, ASPECT, SCENERY, AND PHYSICAL CONDITIONS.
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THE EXPANSE OF HEAVEN. A Series of Essays on the Wonders of the
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PHYSICAL EXPRESSION: Its Modes and Principles. By FRANCIS WARNER, M.
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of inner processes; but a little observation will convince one that the
posture assumed by the body—the poise of the head and the position of
the hands—as well as the many alternations of color and of general
nutrition, are just as striking evidences of the course of thought. The
subject thus developed by the author becomes quite extensive, and is
exceedingly interesting. The work is fully up to the standard maintained
in ‘The International Scientific Series.’”—_Science._

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Warner’s book will be of benefit are actors and artists. The art of
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head, expression in the face, expression in the eyes, and in that on art
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COMMON SENSE OF THE EXACT SCIENCES. By the late WILLIAM KINGDON
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treating number, space, quantity, position, and motion, respectively.
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the principles underlying each. The whole volume is written in a
masterful, scholarly manner, and the theories are illustrated by one
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JELLY-FISH, STAR-FISH, AND SEA-URCHINS. Being a Research on
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monograph which leaves nothing to be said about jelly-fish, star-fish,
and sea-urchins. Every one who has studied the lowest forms of life on
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exquisite delicacy of their structure and their nice adaptation to their
place in nature. Mr. Romanes brings out the subtile beauties of the
rudimentary organisms, and shows the resemblances they bear to the
higher types of creation. His explanations are made more clear by a
large number of illustrations. While the book is well adapted for
popular reading, it is of special value to working physiologists.”—_New
York Journal of Commerce._

“A most admirable treatise on primitive nervous systems. The
subject-matter is full of original investigations and experiments
upon the animals mentioned as types of the lowest nervous
developments.”—_Boston Commercial Bulletin._

          New York: D. APPLETON & CO., 1, 3, & 5 Bond Street.

------------------------------------------------------------------------

                          Transcriber’s note:

Front matter, ‘By’ changed from small caps to letter case, “By JAMES
CROLL, LL.D., F.R.S. With”

Page 2, comma inserted after ‘Mag.,’ “Phil. Mag., July 1878;”

Page 25, full stop inserted after ‘Lectures,’ “Manchester Science
Lectures.”

Page 37, heading ‘Testimony...’ changed to small caps.

Page 41, ‘years’ inserted, “Mean        3,378   years”

Page 54, full stop inserted after ‘Mem.,’ “Mem. Geol. Survey of
Lancashire”

Page 59, full stop inserted after ‘vol. i.,’ “Memoirs of Geol. Survey,
vol. i.”

Page 113, ‘radicles’ changed to ‘radicals,’ “argument from compound
radicals”

Page 115, ditto inserted, “„ on examples of enormous faults”

Page 116, ditto inserted, “„ globular”

Page 117, page entries reversed, “of solar radiation, 2, 35”

Page 117, ‘geologica’ changed to ‘geological,’ “on geological time”

Page 118, ditto inserted, “„ fault in, 57”





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