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Title: Popular Scientific Recreations - in Natural Philosphy, Astronomy, Geology, Chemistry, etc., etc., etc.
Author: Tissandier, Gaston
Language: English
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[Illustration: BOILING WATER IN A PAPER CASE.]

[Illustration: DRAWING A SLIP OF PAPER FROM BENEATH A COIN.]

[Illustration: THE TALKING HEAD.]



                                POPULAR

                        SCIENTIFIC RECREATIONS

                                  IN

                    NATURAL PHILOSOPHY, ASTRONOMY,
                          GEOLOGY, CHEMISTRY,

                           ETC., ETC., ETC.


    _Translated and Enlarged from “Les Récréations Scientifiques”_

                                  OF

                          GASTON TISSANDIER.

                      (_Editor of “La Nature.”_)



                        PROFUSELY ILLUSTRATED.


                                London:

                  WARD, LOCK, AND CO., WARWICK HOUSE,
                        SALISBURY SQUARE, E.C.
                      NEW YORK: 10, BOND STREET.



PREFACE.


A learned mathematician of the seventeenth century, Ozanam by name, a
member of the Academy of Sciences and author of several distinguished
works, did not think it derogatory to his dignity to write, under the
title of “Mathematical and Physical Recreations,” a book designed for
the amusement of youth, in which science lends itself to every pastime,
even jugglery and tricks of legerdemain.

“_Jeux d’esprit_” says Ozanam, “are for all seasons and all ages; they
instruct the young, they amuse the old, they are welcomed by the rich,
and are not above the reach of the poor.”

The object of the book now presented to the reader is also to instruct
while it amuses, but we have not thought proper to make use, as Ozanam
did, of any physical feats, so called _amusing_. Such do not constitute
experiments, and are but ingenious deceptions, intended to disguise
the true mode of operation, and we have not desired to make use of or
popularise such methods. We wish, on the contrary, that every game we
describe, every pastime or amusement of which we give the exposition,
should be rigorously based on the scientific method, and looked upon
as a genuine exercise in physics, chemistry, mechanics, or natural
science. It does not appear to us desirable to teach deception, even in
play.

Science in the open air, in the fields, in the sunshine, is our first
study; we point out how, in the country, it is possible, pleasantly and
unceasingly, to occupy one’s leisure in observing nature, in capturing
insects or aquatic animals, or in noting atmospheric phenomena.

We next teach a complete course of physics without any apparatus, and
point out the methods for studying the different phenomena of heat,
light, optics, and electricity, by means of a simple water-bottle,
tumbler, stick of sealing-wax, and other ordinary objects, such as
everyone has at hand. A series of chemical experiments, performed by
means of some phials and inexpensive appliances, completes that part of
the book relating to the physical sciences.

Another kind of recreation, both intelligent and useful, consists
in collecting the ingenious inventions which are constantly being
supplied to our requirements by the applied sciences, and learning how
to use them. We have collected a number of mechanical inventions and
appliances, with which most ingenious and skilful people will wish to
supply themselves, from Edison’s electric pen, or the chromograph,
which will produce a large number of copies of a letter, drawing, etc.,
to the more complicated, but not less valuable contrivances, for making
science useful in the house.

Having described some scientific toys for the young, we have
endeavoured to point out those interesting to persons of riper years,
and have grouped together curious systems of locomotion, and ingenious
mechanical appliances, such as small steam-boats, ice-boats, swimming
apparatus, etc., under proper heads.

In addition to the foregoing subjects, we have included some of the
experimental details of Chemical Science, with illustrations. We have
added a chapter upon Aërial Navigation and Ballooning, with anecdotes
of some of our celebrated aëronauts. We have also enlarged upon Light,
Sound, Heat, Physical Geography, Mineralogy, Geology, Electrical
Appliances, the Electric Light, and most of the latest adaptations of
electricity.

It will be seen, therefore, that the present work is not only
intended for the young; everyone, it is hoped, will find in it
something interesting and also profitable, which, if not desired for
self-instruction, may at any rate be turned to account as a means of
teaching others that science, which is universal, can, when rightly
apprehended, preside even over our pleasures and amusements.

  THE EDITOR.



CONTENTS.


  CHAPTER I.—INTRODUCTORY.

        PAGE

  Science and Recreation—The Book of Nature—The Senses—Natural
  History—Natural Philosophy—Matter—Objects—Properties of Matter       1

  CHAPTER II.—OPEN-AIR SCIENCE.

  Science in the Open Air—Aphides—Evaporation by Leaves—An Aquarium—The
  Cataleptic Fowl—Needle Points and Thorns—Microscopic Aquarium—Cape
  Grisnez—Crystals—Ice on the Gas Lamps             6

  CHAPTER III.—PHYSICS.

  Physics—The Meaning of Physics—Forces of
  Nature—Gravity—Cohesion—Chemical Attraction—Centre of
  Gravity—Experiments—Automaton Tumblers      22

  CHAPTER IV.—PHYSICS (_Continued_).

  Some Properties of Solid Bodies—Inertia—Motion—Friction—The
  Pendulum—Equilibrium         35

  CHAPTER V.—GASES.

  Gases and Liquids—Pressure of the Air—Experiments            44

  CHAPTER VI.—WATER.

  About Water—Hydrostatics and Hydraulics—Law of Archimedes—The Bramah
  Press—The Syphon                      59

  CHAPTER VII.—HEAT.

  Heat—What it is—Theory of Heat—The Thermometer—Expansion by
  Heat—Ebullition and Distillation                 72

  CHAPTER VIII.—HEAT (_Continued_).

  Specific Heat—Fusion—Latent Heat—Conduction and Convection of
  Heat—Calorescence       88

  CHAPTER IX.—LIGHT.

  Light and its Sources—What is Light?—Velocity of Light—Reflection and
  Refraction—Relative Value of Lights                  93

  CHAPTER X.—LIGHT (_Continued_).

  Vision and Optical Illusions—The Eye Described—Accommodation of the
  Eye—Chromatic Aberration—Spinning Tops              102

  CHAPTER XI.—OPTICS.

  Optical Illusions—Zollner’s Designs—The Thaumatrope—Phenokistoscope—The
  Zootrope—The Praxinoscope—The Dazzling Top           116

  CHAPTER XII.—OPTICS (_Continued_).

  Optical Illusions Continued—Experiments—The Talking Head—Ghost
  Illusions       129

  CHAPTER XIII.—OPTICS (_Continued_).

  Vision—The Eye—The Stereoscope—Spectrum Analysis—The Spectroscope—The
  Telescope and Microscope—Photography—Dissolving Views—Luminous
  Paint      140

  CHAPTER XIV.—SPECTRAL ILLUSIONS.

  A Spectre Visible—Curious Illusions—Ghosts      161

  CHAPTER XV.—ACOUSTICS.

  The Ear and Hearing—Physiology of Hearing and Sound—Sound as Compared
  with Light—What is Sound?—Velocity of Sound—Conductibility—The
  Harmonograph      166

  CHAPTER XVI.—ACOUSTICS (_Continued_).

  The Topophone—The Megaphone—The Autophone—The Audiphone—The
  Telephone—The Phonograph—The Microphone      180

  CHAPTER XVII.—ACOUSTICS (_Continued_).

  The Tuning-Fork—The Syren—Sound Figures—Singing Flames      193

  CHAPTER XVIII.—ELECTRICITY.

  Derivation of Electricity—Sealing Wax Experiment—The
  Electrophorus—Leyden Jar—Positive and Negative—The
  Electroscope—Electric Machines      197

  CHAPTER XIX.

  Velocity of Electricity—Experiments—The Electric Egg—Force of the
  Electric Spark      212

  CHAPTER XX.—GALVANISM.

  Galvani’s Discovery—The Frogs Electrified—Experiments—Volta’s
  Pile—The Test—Its Usefulness—Faraday’s “Researches.”      217

  CHAPTER XXI.—MAGNETISM.

  The Loadstone—Magnetic Curves—The Magnetic Needle—The Mariner’s
  Compass—Magneto-Electricity      254

  CHAPTER XXII.—APPLIED ELECTRICITY.

  Sundry Electrical Appliances—Mr. Edison’s Inventions—The Electric
  Light—The Gyroscope—A New Electrophorus—Electric Toys      262

  CHAPTER XXIII.—AERONAUTICS.

  Pressure of Air in Bodies—Early Attempts to fly in the Air—Discovery
  of Hydrogen—The Montgolfier Balloons—First Experiments in Paris—Noted
  Ascents      293

  CHAPTER XXIV.—CHEMISTRY.

  What Chemistry is—The Elements—Metallic and Non-Metallic—Atomic
  Weight—Acids—Alkalis—Bases—Salts—Chemical Combination and Study    307

  CHAPTER XXV.—CHEMISTRY (_Continued_).

  Chemistry without a Laboratory      313

  CHAPTER XXVI.—CHEMISTRY (_Continued_).

  Chemistry and Alchemy—Chemical Combinations—The Atmospheric Air    336

  CHAPTER XXVII.—THE ELEMENTS.

  Non-Metallic Elements      348

  CHAPTER XXVIII.—NON-METALLIC ELEMENTS (_Continued_).

  Chlorine—Bromine—Iodine—Fluorine—Carbon—Sulphur—Phosphorus—Silicon—
  Boron—Tellurium—Arsenic      366

  CHAPTER XXIX.—THE METALS.

  What Metals are—Characteristics and General Properties of
  Metals—Classification—Specific Gravity—Descriptions      386

  CHAPTER XXX.—ORGANIC CHEMISTRY.

  Radicals—Acids—Bases—Neutrals      410

  CHAPTER XXXI.—MINERALOGY AND CRYSTALLOGRAPHY.

  The Minerals—Characteristics—Crystals and their Forms—Descriptions of
  Minerals      424

  CHAPTER XXXII.—NEW LOCOMOTIVE APPLIANCES.

  The Kite—The Aerophane—Ice Yachts—Sailing Trucks—Water Velocipedes  448

  CHAPTER XXXIII.—ASTRONOMY.

  Introductory—History of Astronomy—Nomenclature      466

  CHAPTER XXXIV.—ANGLES AND MEASUREMENT OF ANGLES.

  The Quadrant—Transit Instrument—Clocks—Stellar Time—Solar Time—“Mean
  Time”      474

  CHAPTER XXXV.—THE SOLAR SYSTEM.

  Gravitation—The Planets—Size and Measurement of the
  Planets—Satellites—Falling Stars—Comets—Aerolites      486

  CHAPTER XXXVI.—THE SUN.

  Motion of the Sun—The Seasons—Character of the Sun—Sun-Spots—Zodiacal
  Light      496

  CHAPTER XXXVII.—THE EARTH.

  Form of the Earth—Motion of the Globe—Rate and Manner of
  Progression—Latitude and Longitude—The Seasons      504

  CHAPTER XXXVIII.—THE MOON.

  What is it Like?—Moon Superstitions—Description of the
  Moon—Phases—Tides—Eclipses      510

  CHAPTER XXXIX.—THE STARS.

  The Planets and Asteroids      521

  CHAPTER XL.—THE FIXED STARS.

  Fixed-Stars—Magnitude of the Stars—Constellations—Descriptions
  of the Zodiacal Constellations—Northern and Southern Star
  Groups—Distance of Stars      535

  CHAPTER XLI.—THE STARS (_Continued_).

  Double and Multiple Stars—Coloured and Variable Stars—Clusters,
  Groups, and Nebulæ—The Galaxy, or Milky Way—How to Find out the
  Principal Stars      546

  CHAPTER XLII.—NEW ASTRONOMICAL APPLIANCES.

  A Celestial Indicator—Astronomical or Cosmographical Clock—A Simple
  Globe—A Solar Chronometer      557

  CHAPTER XLIII.—PHYSICAL GEOGRAPHY AND GEOLOGY.

  Geography and Geology—The Earth’s Crust—Origin of the Earth—Denudation
  and Excavation by Water—Rocks, Gravel, and Sand—Classes of Rocks    564

  CHAPTER XLIV.—GEOLOGY.

  Crust of the Earth—Geological Systems—Eozoic, Primary, Secondary,
  Tertiary, Pre-Historic Formations      573

  CHAPTER XLV.—GEOLOGY (_Continued_).

  The Mesozoic System—The Triassic, Oolitic, and Cretaceous
  Formations—The Eocene, Miocene, and Pliocene—The Glacial
  Period—Pre-Historic Man      584

  CHAPTER XLVI.—PHYSICAL GEOGRAPHY.

  Igneous Rocks—Land and Water—Springs, Wells, and Geysers—Snow and
  Ice—Their Effects      601

  CHAPTER XLVII.—THE SEA AND THE SKY.

  The Sea—Salt Water—Waves and their Effects—Under Water—The Floor of the
  Ocean      610

  CHAPTER XLVIII.—PHYSICAL GEOGRAPHY. METEOROLOGY.

  The Atmosphere—Winds and Air Currents—Wind Pressure—Storms—Rain–clouds—
  Water-Spouts—Atmospherical Phenomena      628

  CHAPTER XLIX.—PHYSICAL GEOGRAPHY. METEOROLOGY (_Continued_).

  Atmospheric Phenomena—Thunder and Lightning—Aurora Borealis—The
  Rainbow—Mock-Suns and Mock-Moons—Halos—Fata Morgana—Reflection and
  Refraction—Mirage—Spectre of the Brocken      642

  CHAPTER L.—PHYSICAL GEOGRAPHY. CLIMATOLOGY.

  Weather; Climate, and Temperature—Isothermal Lines—Isobars, Weather
  Forecasts, and Signs of the Sky      651

  CHAPTER LI.—BIOLOGY. PART I.: BOTANY.

  Plants and Animals—Structure of Plants—Flowering Plants—The Stem—The
  Leaves—Forms of Leaves      658

  CHAPTER LII.—FLOWERING PLANTS.

  Organs of Increase and Reproduction—The Flower—The Calyx—The
  Corolla—The Stamen—The Pistil      675

  CHAPTER LIII.—FLOWERING PLANTS (_Continued_).

  The Floral Axis—Inflorescence—Fruit—Seed—Nutrition of Plants—Absorbtion
  of Constituents      679

  CHAPTER LIV.—ZOOLOGY.

  Classification of Animals—Vertebrates and
  Invertebrates—Protozoa—Hydrozoa—Actinozoa      700

  CHAPTER LV.—ECHINODERMATA—ANNULOSA—ENTOZOA—INSECTA.

  Sea-Urchins—Star-Fishes—Feathery Stars—Sea-Cucumbers—Worms—Leeches—
  Rotifers—Tape Worms—Insects      712

  CHAPTER LVI.—THE ANALYSIS OF CHANCE AND MATHEMATICAL GAMES.

  Magic Squares—The Sixteen Puzzle—Solitaire—Equivalents      726

  CHAPTER LVII.—GAMES (_Continued_).

  The Magic Top—The Gyroscope and Scientific Games      740

  CHAPTER LVIII.—SCIENCE AT HOME.

  Scientific Objects for the Household      747

  CHAPTER LIX.—DOMESTIC SCIENCE.

  Science and Domestic Economy      757

  CHAPTER LX.—CURIOUS INVENTIONS.

  Some Curious Modes of Transit      770



SCIENTIFIC RECREATIONS.



CHAPTER I.—INTRODUCTORY.

 SCIENCE AND RECREATION—THE BOOK OF NATURE—THE SENSES—NATURAL
 HISTORY—NATURAL PHILOSOPHY—MATTER—OBJECTS—PROPERTIES OF MATTER.


It may at the first glance appear paradoxical to combine Science and
Recreation, but we hope to show that true scientific recreation is
anything but the dry bones of learning. To those who study science
with us, we will point out first how easy and pleasant it is to watch
the sky and the plants and Nature generally in the open air. Then we
will carry our readers along with us, and by means of illustrations
and diagrams instruct them pleasantly in the _reasons for_ things.
“How?” and “Why?” will be questions fully answered. Not only will the
usual scientific courses be touched upon, but we will show how Science
is applied to Domestic Economy. We will have Chemistry put before us
without needing a laboratory, and we will experiment in Physics without
elaborate apparatus. We will have, in short, a complete Encyclopædia of
Science free from dryness and technicalities—an amusing volume suited
to old and young who wish to find out what is going on around them in
their daily life in earth and sea and sky.

Bernard Palissy used to say that he wished “no other book than the
earth and the sky,” and that “it was given to all to read this
wonderful book.”

It is indeed by the study of the material world that discoveries are
accomplished. Let an attentive observer watch a ray of light passing
from the air into water, and he will see it deviate from the straight
line by refraction; let him seek the origin of a sound, and he will
discover that it results from a shock or a vibration. This is physical
science in its infancy. It is said that Newton was led to discover the
laws of universal gravitation by beholding an apple fall to the ground,
and that Montgolfier first dreamt of air-balloons while watching fogs
floating in the atmosphere. The idea of the inner chamber of the eye
may, in like manner, be developed in the mind of any observer, who,
seated beneath the shade of a tree, looks fixedly at the round form of
the sun through the openings in the leaves.

[Illustration: Luminous Cross seen at Havre, May 7th, 1877. Sketched
from Nature.]

Every one, of course, may not possess the ambition to make such
discoveries, but there is no one who cannot compel himself to learn to
enjoy the pleasure that can be derived from the observation of Nature.

It must not be imagined that in order to cultivate science it is
absolutely necessary to have laboratories and scientific work-rooms.
The book of which Palissy spoke is ever present; its pages are always
open, wherever we turn our eyes or direct our steps. So we may hope to
introduce all our friends to a pleasant and lasting acquaintance with
Dame Nature.

“But what _is_ Nature?” We are fond of admiring Nature, and the effects
of certain causes in the world, and we want to know why things are so.
Very well—so you shall; and as to the question “What is Nature?” we
will endeavour to answer you at once.

Nature is the united totality of all that the various Senses can
perceive. In fact, all that cannot be made by man is termed “Nature”;
_i.e._, God’s creation.

From the earliest ages man has sought to read the open leaves of the
Book of Nature, and even now, with all our attainments, we cannot grasp
all, or nearly all. One discovery only leads up to another. Cause and
Effect are followed up step by step till we lose ourselves in the
search. Every effect must have a cause. One thing depends upon another
in the world, and it does not need Divine revelation to tell us that.
Nothing happens by “mere chance.” “Chance!” said a Professor to us at
the University, “Chance!—Remember, there is no such thing in the world
as chance.”

Between our minds or consciousness and Nature are our Senses. We feel,
we see, we hear, we taste, we smell,—so it is only through the Senses
that we can come to any knowledge of the outer world. These attributes,
or Senses, act directly upon a certain “primary faculty” called
Consciousness, and thus we are enabled to understand what is going
on around us. The more this great existing faculty is educated and
trained, the more useful it will become. So if we accustom our minds to
observation of Nature, we shall find out certain causes and effects,
and discover Objects. Now an Object is a thing perceptible both to
feeling and sight, and an Object occupies space. Therefore there are
objects Artificial as well as Natural. The former are created by man
from one or more Natural products. Natural Objects are those such as
trees, rocks, plants, and animals. We may also class the heavenly
bodies, etc., as Objects, though we cannot touch them, but we can feel
their effects, and see them. The PHENOMENA of Nature include
those results which are perceptible by only one sense, as thunder;
light and sound may also be classed as Phenomena.

Take a familiar instance. A stone is a Natural Object. We take it
up, open our fingers, and it falls. The _motion_ of that object is a
Phenomenon. We know it falls because we see it fall, and it possesses
what we term _weight_; but we cannot tell _why_ it possesses weight.

[Professor Huxley says: “Stones do not fall to the ground in
consequence of a law of nature,” for a law is not a cause. “A law of
nature merely tells us what we may expect natural objects will do
under certain circumstances.”]

A cause of a Phenomenon being independent of human will is called a
_Force_, and the stone falls by the force of _Gravitation_, or that
natural law which compels every material object to approach every other
material object.

A single Force may produce a great number of Phenomena.

Nature being revealed to us by Objects, and by means of Phenomena, we
have got already two Branches of Science extending from such Roots;
viz., NATURAL HISTORY, the Science of Objects; and NATURAL
PHILOSOPHY, the Science of Phenomena.

Both of these Branches have been subdivided thus:

                      { Zoology, referring to Animals } Biology.
  Natural History     { Botany, referring to Plants   }
                      { Mineralogy } referring to Minerals, etc.
                      { Geology    }

                      { Physics. Phenomena without essential change
  Natural Philosophy  {     of the Objects.
                      { Chemistry. Phenomena with change of the Objects.
                      { Physiology. Phenomena of animated Objects.

These two great divisions comprehend, in their extended senses, all
that is known respecting the material world.

We have spoken of Objects. Objects occupy _Space_. What is Space?
Space is magnitude which can be conceived as extending in three
directions—_Length_, _Breadth_, and _Depth_. MATTER occupies
portions of Space, which is infinite. Matter, when finite, is termed
a body or object. The general properties of Matter are _Magnitude_,
_Form_, _Impenetrability_, _Inertia_, _Divisibility_, _Porosity_,
_Elasticity_, _Compressibility_, _Expansibility_.

Matter is present in Nature in three conditions. We find it as a SOLID,
a LIQUID, and a GAS. We shall explain the various properties of Solids,
Liquids, and Gases in their proper places (in Physics). To test the
actual existence of Matter in one or other of these forms our Senses
help us. We can touch a Solid, or taste it and see it. But touch is
the test. We have said that Matter possesses certain properties. We
will examine these briefly. The two which belong to all material bodies
are Impenetrability and Magnitude. You cannot, _strictly speaking_,
penetrate Matter. You can find the form of an object by touch or sight,
but you cannot penetrate it. You will think you can drive a nail or a
screw into a board, but you cannot; you only _displace_ the fibres of
the wood by the screw. Take water as a very common instance. Water is
Matter, for it occupies a certain space. Water is _impenetrable_, for
if you put your hand or foot into a basin full of it, it will overflow,
thus proving that you displace, and do not penetrate it. It is almost
impossible to compress water.

_Divisibility_ is another quality of Matter; and when we attempt to
show how far Matter can be divided, the brain refuses to grasp the
infinity. A pin’s head is a small object, but it is gigantic compared
to some animals, of which millions would occupy a space no larger
than the head of a pin. These tiny animals must contain organs and
veins, etc., and those veins are full of blood globules. Professor
Tyndall informs us that a drop of blood contains three millions of red
globules. So these infinitesimally small animals must have millions of
globules in their blood also. Thus we see to what an extent, far beyond
our Senses’ power to grasp, Matter can be divided.

But there is something even more astonishing than this. It is stated
that there are more animals in the milt of a single codfish than there
are men in the world; and that _one grain of sand is larger than four
millions of these animals_! each of which must be possessed of life
germs of an equal amount, which would grow up as it grew to maturity.
This carries us back again, and

    “Imagination’s utmost stretch
      In wonder dies away.”

Or take other interesting facts. One hundred threads of the silkworm
must be placed side by side to make up the thickness of a line (—)
about 1/25th of an inch; and metals can be drawn out to such exceeding
fineness that twelve hundred of the fine wires will occupy only the
space of one hundred silkworms’ threads, or one _millimetre_.

_Porosity_ is another attribute of Matter, for in all Matter there are
pores, or spaces, between the particles. Sometimes such openings are
plainly visible; in very “solid” bodies they are, to a great extent,
indistinguishable. But we know that the spaces exist, because we can
_compress_ the particles together.

_Inertia_ is also a general property of Matter, and the meaning of
the term is “inactivity,” or passiveness—a want of power in an object
to move, or when moving, to stop of itself. It will come to rest
apparently by itself, but the resistance of the air and the friction of
the ground, or the attraction of the earth, will really occasion the
stoppage of the object. We will speak more fully of Inertia presently.
Elasticity and Expansibility are evident in fluids and gases.

We have thus introduced our readers to some of the most evident facts
connected with Matter. The various Forces and Phenomena of attraction
will be fully considered farther on; at present we are about to show
our readers how they may first profitably study Science in the open air
for themselves, and we will give them our experience of the Book of
Nature.



CHAPTER II.

 SCIENCE IN THE OPEN AIR—APHIDES—EVAPORATION BY LEAVES—AN AQUARIUM—THE
 CATALEPTIC FOWL—NEEDLE POINTS AND THORNS—MICROSCOPIC AQUARIUM—CAPE
 GRISNEZ—CRYSTALS—ICE ON THE GAS LAMPS.


[Illustration: Fig. 1.—Ants engaged in extracting aphides from a
rose-tree (highly magnified)]

Some years ago we were staying in Normandy, not far from the town
of C——, enjoying, in the midst of most cordial hospitality, the
peacefulness of country life; and my kind hosts, with me, took great
pleasure in having what we called “a course of science in the open
air.” The recollections of that time are some of the pleasantest in
the whole course of my life, because all our leisure was intelligently
occupied. Each of us set himself to provide the subject of some curious
observation or instructive experiment; one made a collection of
insects, another studied botany. In the daytime we might have been seen
examining, under a magnifying glass, the branch of a rose-tree, from
which the ants were endeavouring to extract the aphides[1] (fig. 1).
At night we admired through the telescope the stars and planets that
were visible; or if the sky was not clear, we examined under a strong
magnifier grains of pollen from flowers, or the _infusoria_ in a drop
of stagnant water. Frequently some very insignificant object became
the occasion for some scientific discussion, which terminated with an
experimental verification.

I recollect that one day one of us remarked that after a week of dry
weather a stream of water had nearly dried up, although sheltered by
thick trees, which necessarily impeded the calorific action of the sun;
and he expressed surprise at the rapid evaporation. An agriculturist
among the company, however, drew his attention to the fact that the
roots of the trees were buried in the course of the stream, and that,
far from preventing the evaporation of the water, the leaves had
contributed to accelerate it. As the first speaker was not convinced,
the agriculturist, on our return to the house, prepared an experiment
represented in fig. 2. He placed the branch of a tree covered with
foliage in a U-shaped tube, the two branches of unequal diameter, and
filled with water. He placed the vegetable stem in the water, and
secured it to the tube by means of a cork covered with a piece of
india-rubber, and tied tightly to make it hermetically closed.

[Illustration: Fig. 2.—Experiment showing evaporation of water by
leaves.]

At the commencement of the experiment the water was level with A in
the larger branch of the tube, and level with B in the smaller, rising
by capillarity to a higher point in the more slender of the two. The
evaporation of the water caused by the leaves was so active that in a
very short time we beheld the water sink to the points C and C′.

[Illustration: Fig. 3.—Aquarium formed by means of a melon glass.]

Thus did the excellent method of seeking the cause of phenomena by
experiments often lead us to interesting results. We had among us many
children and young people who had reached the age of ardent curiosity.
We took pleasure in pointing out to them the means of studying natural
science; and we were not long before feeling convinced that our lessons
out in the fields had much greater success than those given between
the four walls of a class-room. Insects were collected, and preserved
by being carefully placed in a small bottle, into which was let fall a
drop of sulphuret of carbon;[2] the insect was immediately asphyxiated,
and we thus avoided the cruelty of passing a pin through a living
body. Having chased butterflies and insects, we next desired to study
the aquatic creatures which swarmed in the pools of the neighbourhood.
For this purpose I constructed a fishing-net fitted to an iron ring,
and firmly secured to a wooden handle. When this was plunged under the
water and drawn quickly out again, it came back full of slime. In the
midst of this muddy substance one generally succeeded in finding the
hydrophilus, tadpoles, coleoptera, many curious kinds of caddis-worms,
tritons, and sometimes frogs, completely astounded by the rapidity of
their capture. All these creatures were transported in a bottle to the
house, and I then constructed, at small expense, a glass aquarium, by
means of the bell of a melon-glass turned upside down, thus forming a
transparent receptacle of considerable size. Four wooden stakes were
then fixed in the ground, and a plank with a circular hole nailed on
the top, in which the glass bell was placed. I next scattered some
large pebbles and shells at the bottom of the vase to form a stony bed,
poured in some water, placed a few reeds and water plants among the
pebbles, and then threw a handful of water lentils on the surface;
thus a comfortable home was contrived for all the captured animals.[3]
The aquarium, when placed under the shade of a fine tree in a rustic
spot abounding with field flowers, became a favourite rendezvous, and
we often took pleasure in watching the antics of the little inmates
(fig. 3). Sometimes we beheld very sanguinary scenes; the voracious
hydrophilus would seize a poor defenceless tadpole, and rend him in
pieces for a meal without any compunction. The more robust tritons
defended themselves better, but sometimes they also succumbed in the
struggle.

[Illustration: Fig. 4.—Cage for preserving living insects.]

[Illustration: Fig. 5.—Small aquarium, with frogs’ ladder.]

The success of the aquarium was so complete that one of us resolved to
continue this museum in miniature, and one day provided himself with an
_insects palace_, which nearly made us forget the tadpoles and tritons.
It was a charming little cage, having the form of a house, covered
with a roof; wires placed at equal distances forming the sides. In it
was a large cricket beside a leaf of lettuce, which served as his food
(fig. 4). The little creature moved up and down his prison, which was
suspended from the branch of a tree, and when one approached him very
closely gave vent to his lively chirps.

[Illustration: Fig. 6.—Frog lying in wait for a fly.]

The menagerie was soon further augmented by a hitherto unthought-of
object; namely, a frogs’ ladder. It was made with much skill. A large
bottle served for the base of the structure. The ladder which was fixed
in it was composed of the twigs of very small branches, recently cut
from a tree, and undivested of their bark, which gave to the little
edifice a more picturesque and rustic appearance. The pieces of wood,
cleverly fixed into two posts, conducted the green frogs (tree-frogs)
on to a platform, whence they ascended the steps of a genuine ladder.
There they could disport themselves at pleasure, or climb up further
to a branch of birch-tree placed upright in the centre of the bottle
(fig. 5). A net with fine meshes prevented the little animals from
escaping. We gave the tree-frogs flies for their food, and sometimes
they caught them with remarkable dexterity. I have often seen a frog
when at liberty watching a fly, on which it pounces as a cat does on
a bird (fig. 6). The observations that we made on the animals of our
menagerie led us to undertake others of a very different nature; I
recollect particularly a case of catalepsy produced in a cock. I will
describe this remarkable experiment, certainly one of the most curious
we ever performed.

[Illustration: Fig. 7.—Experiment of the cataleptic cock.]

We place a cock on a table of dark colour, rest its beak on the
surface, where it is firmly held, and with a piece of chalk slowly draw
a white line in continuation from the beak, as shown in our engraving.
If the crest is thick, it is necessary to draw it back, so that the
animal may follow with his eyes the tracing of the line. When the line
has reached a length of about two feet the cock has become cataleptic.
He is absolutely motionless, his eyes are fixed, and he will remain
from thirty to sixty seconds in the same posture in which he had at
first only been held by force. His head remains resting on the table
in the position shown in fig. 7. This experiment, which we have
successfully performed on different animals, can also be accomplished
by drawing a straight line with a piece of chalk on a slate. M. Azam
declares that the same result is also produced by drawing a black line
on a table of white wood. According to M. Balbiani, German students had
formerly a great predilection for this experiment, which they always
performed with marked success. Hens do not, when operated on, fall
into a cataleptic condition so easily as cocks; but they may often be
rendered motionless by holding their heads fixed in the same position
for several minutes. The facts we have just cited come properly under
the little studied phenomena, designated by M. Braid in 1843 by the
title of _Hypnotism_. MM. Littré and Ch. Robin have given a description
of the hypnotic condition in their _Dictionnaire de Médecine_.

[Illustration: Fig. 8.—Ordinary pin and needle, seen through a
microscope (magnified 500 diameters).]

[Illustration: Fig. 9.—Thorn of a rose, and wasp’s sting through a
microscope (magnified 500 diameters).]

If any shining object, such as a lancet, or a disc of silver-paper
gummed to a plate, is placed at about the distance of a foot from the
eyes of a person, slightly above the head, and the patient regards
this object fixedly, and without interruption for twenty or thirty
minutes, he will become gradually motionless, and in a great number
of cases will fall into a condition of torpor and genuine sleep. Dr.
Braid affirms that under such circumstances he has been able to perform
surgical operations, without the patient having any consciousness of
pain. Later also, M. Azam has proved the complete insensibility to
pricking on the part of individuals whom he has rendered cataleptic by
the fixing of a brilliant object. The experiment of the cataleptic
cock was first described under the name of _Experimentum Mirabile_,
by P. Kircher, in his _Ars Magna_, published at Rome in 1646. It
evidently belongs to the class of experiments which were performed
at the Salpêtrière asylum at Paris, by M. Charcot, on patients
suffering from special disorders. It must now be evident to our
readers that our scientific occupations were sufficiently varied,
and that we easily found around us many objects of study. When the
weather was wet and cloudy we remained indoors, and devoted ourselves
to microscopical examinations. Everything that came under our hands,
insects, vegetables, etc., were worthy of observation. One day, while
engaged over a microscopical preparation, I was making use of one of
those steel points generally employed in such purposes, when happening
to pass it accidentally beneath the microscope, I was astonished to
see how rough and uneven it appeared when highly magnified. The idea
then occurred to me to have recourse to something still more pointed,
and I was thus led to make comparisons between the different objects
represented in figs. 8 and 9. It will here be seen how very coarse
is the product of our industry when compared with the product of
Nature. No. 1 of fig. 8 represents the point of a pin that has already
been used, magnified 500 diameters. The point is evidently slightly
blunted and flattened. The malleable metal has yielded a little under
the pressure necessary to make it pass through a material. No. 2 is
a little more pointed; it is a needle. This, too, will be seen to be
defective when regarded by the aid of the microscope. On the other
hand, what fineness and delicacy do the rose thorn and wasp’s sting
present when examined under the same magnifier! (See the two points in
fig. 9.)

An examination of this exact drawing has led me to make a calculation
which leads to rather curious results: at a half millimetre from the
point, the diameters of the four objects represented are in thousandths
of a millimetre respectively, 3·4; 2·2; 1·1; 0·38. The corresponding
sections in millionths of a square millimetre are: 907·92; 380·13;
95·03; 11·34; or, in round numbers, 908; 380; 95; 11.

If one bears in mind, which is much below the truth, that the pressure
exercised on the point must be proportional to the section, and
admitting that a pressure of 11 centigrams suffices to thrust in the
sting of a wasp half a millimetre, it will require more than 9 grams
of pressure to thrust in a needle to the same extent. In fact, this
latter figure is much too small, for we have not taken into account the
advantage resulting from the elongated shape of the rose thorn, which
renders it more favourable for penetration than a needle through a drop
of tallow.

It would be easy to extend observations of this kind to a number
of other objects, and the remarks I have just made on natural and
artificial points will apply incontestably to textures for example.
There is no doubt that the thread of a spider’s web would far surpass
the thread of the finest lace, and that art will always find itself
completely distanced by nature.

We amused ourselves frequently by examining the _infusoria_ which are
so easily procured by taking from some stagnant water the mucilage
adhering to the vegetation on the banks, or attached to the lower part
of water lentils. In this way we easily captured _infusoria_, which,
when placed under a strong magnifier, presented the most remarkable
spectacle that one can imagine. They are animalcules, having the form
of transparent tulips attached to a long stem. They form bunches
which expand and lengthen; then, suddenly, they are seen to contract
with such considerable rapidity that the eye can scarcely follow the
movement, and all the stems and flower-bells are folded up into the
form of a ball. Then, in another moment, the stems lengthen, and the
tulip-bells open once more. One can easily encourage the production
of _infusoria_ by constructing a small microscopic aquarium, in which
one arranges the centre in a manner favourable to the development of
the lowest organisms. It suffices to put a few leaves (a piece of
parsley answers the purpose perfectly)[4] in a small vase containing
water (fig. 10), over which a glass cover is placed, and it is then
exposed to the rays of the sun. In two or three days’ time, a drop of
this water seen under the microscope will exhibit _infusoria_. After a
certain time, too, the different species will begin to show themselves.
Microscopical observations can be made on a number of different
objects. Expose to the air some flour moistened by water, and before
long a mouldiness will form on it; it is the _penicillium glaucum_,
and when examined under a magnifier of 200 to 300 diameters, cells are
distinguishable, branching out from an organism remarkable for its
simplicity. We often amused ourselves by examining, almost at hazard,
everything that came within our reach, and sometimes we were led to
make very instructive investigations. When the sky was clear, and the
weather favourable to walking, we encouraged our young people to run
about in the fields and chase butterflies. The capture of butterflies
is accomplished, as every one knows, by means of a gauze net, with
which we provided the children, and the operation of chasing afforded
them some very salutary exercise. It sometimes happens that butterflies
abound in such numbers, that it is comparatively easy to capture them.
During the month of June 1879, a large part of Western Europe was
thronged with swarms of _Vanessa algina_ butterflies, in such numbers
that their appearance was regarded as an important event, and attracted
the lively attention of all entomologists (fig. 11). This passage of
butterflies provided the occasion for many interesting studies on the
part of naturalists.

[Illustration: Fig. 10.—Arrangement of a microscopic aquarium for
examining _infusoria_.]

[Illustration: Fig. 11.—Flight of butterflies seen near Berne, June
15th, 1879.]

[Illustration: Fig. 12.—Group of rock crystal.]

We cannot point out too strongly to our readers that the essential
condition for the student of natural science, is the possession of
that sacred fire which imparts the energy and perseverance necessary
for acquiring and enlarging collections. It is also necessary that
the investigator should furnish himself with certain indispensable
tools. For collecting plants, the botanist should be armed with a
pickaxe set in a thoroughly strong handle, a trowel, of which there
is a variety of shapes, and a knife with a sharp blade. A botanical
case must also be included, for carrying the plants. The geologist,
or mineralogist, needs no more elaborate instruments; a hammer, a
chisel, and a pickaxe with a sharp point for breaking the rocks, and a
bag for carrying the specimens, will complete his outfit. We amused
ourselves by having these instruments made by the blacksmith, sometimes
even by manufacturing them ourselves; they were simple, but solid, and
admirably adapted to the requirements of research. Often we directed
our walks to the seashore, where we liked to collect shells on the
sandy beach, or fossils among the cliffs and rocks. I recollect, in a
walk I had taken some years previously along the foot of the cliffs of
Cape Blanc-Nez, near Calais, having found an impression of an ammonite
of remarkable size, which has often excited the admiration of amateurs;
this ammonite measured no less than twelve inches in diameter. The
rocks of Cape Grisnez, not far from Boulogne, also afford the geologian
the opportunity of a number of curious investigations. In the Ardennes
and the Alps I have frequently procured some fine mineral specimens; in
the first locality crystallized pyrites, in the second, fine fragments
of rock crystal (fig. 12). I did not fail to recount these successful
expeditions to the young people who accompanied me, and their ardour
was thereby inflamed by the hope that they also should find something
valuable. It often happened when the sun was powerful, and the air
extremely calm, that my young companions and I remarked some very
beautiful effects of mirage on the beach, due to the heating of the
lower _strata_ of the atmosphere. The trees and houses appeared to
be raised above a sheet of silver, in which their reflections were
visible as in a sheet of tranquil water. It can hardly be believed how
frequently the atmosphere affords interesting spectacles which pass
unperceived before the eyes of those who know not how to observe. I
recollect having once beheld at Jersey a magnificent phenomenon of
this nature, on the 24th June, 1877, at eight o’clock in the evening:
it was a column of light which rose above the sinking sun like a
sheaf of fire. I was walking on the St. Helier pier, where there were
also many promenaders, but there were not more than two or three who
regarded with me this mighty spectacle. Columns and crosses of light
are much more frequent than is commonly supposed, but they often
pass unperceived before indifferent spectators. We will describe an
example of this phenomenon observed at Havre on the 7th May, 1877.
The sun formed the centre of the cross, which was of a yellow, golden
colour. This cross had four branches. The upper branch was much more
brilliant than the others; its height was about 15°. The lower branch
was smaller, as seen in the sketch on page 2, taken from nature by
Monsieur Albert Tissandier. The two horizontal branches were at times
scarcely visible, and merged in a streak of reddish-yellow colour,
which covered a large part of the horizon. A mass of cloud, which the
setting sun tinged with a deep violet colour, formed the foreground of
the picture. The atmosphere over the sea was very foggy. The phenomenon
did not last more than a quarter of an hour, but the conclusion of
the spectacle was signalized by an interesting circumstance. The two
horizontal branches, and the lower branch of the luminous cross,
completely disappeared, whilst the upper branch remained alone for some
minutes longer. It had now the appearance of a _vertical column_ rising
from the sun, like that which Cassini studied on the 21st May, 1672,
and that which M. Renon[5] and M. A. Guillemin observed on the 12th
July, 1876.[6] Vertical columns, which, it is well known, are extremely
rare phenomena, may therefore indicate the existence of a luminous
cross, which certain atmospheric conditions have rendered but partially
visible.

How often one sees along the roads little whirlwinds of dust raised
by the wind accomplishing a rotatory movement, thus producing the
imitation of a waterspout! How often halos encompass with a circle of
fire the sun or the stars! How often we see the rainbow develop its
iridescent beauties in the midst of a body of air traversed by bright
raindrops! And there is not one of these great natural manifestations
which may not give rise to instructive observations, and become the
object of study and research. Thus, in walks and travels alike, the
study of Science may always be exercised; and this method of study and
instruction in the open air contributes both to health of body and
of mind. As we consider the spectacles which Nature spreads before
us,—from the insect crawling on the blade of grass, to the celestial
bodies moving in the dome of the heavens,—we feel a vivifying and
salutary influence awaken in the mind. The habit of observation, too,
may be everywhere exercised—even in towns, where Nature still asserts
herself; as, for example, in displays of meteorological phenomena. We
will give an example of such.

[Illustration: Fig. 13.—Icicles on gas lamp.]

The extraordinary abundance of snow which fell in Paris for more than
ten consecutive hours, commencing on the afternoon of Wednesday,
January 22nd, 1880, will always be looked upon as memorable among
the meteorological events of the city of Paris. It was stated that
in the centre of Paris, the thickness of the snow that had fallen at
different times exceeded fourteen inches. The snow had been preceded by
a fall of small transparent icicles, of rather more than a millimetre
in diameter, some having crystalline facets. They formed on the
surface of the ground a very slippery glazed frost. On the evening
of the 22nd January, flakes of snow began to hover in the atmosphere
like voluminous masses of wool. The greater part of the gas-lamps
were ornamented by frozen stalactites, which continually attracted
the attention of passers-by. The formation of these stalactites, of
which we give a specimen (fig. 13), is easy of explanation. The snow
falling on the glass of the lamp became heated by the flame of gas,
melted, and trickled down, freezing anew into the shape of a stalactite
below the lamp, at a temperature of 0° centigrade. Not only can
meteorology be studied in towns, but certain other branches of natural
science—entomology, for example. We will quote what a young student
in science, M. A. Dubois, says on this very subject: “Coleoptera,”
he declares, “are to be met with everywhere, and I think it may be
useful to notice this fact, supporting it by examples. I desire to
prove that there are in the midst of our large towns spots that remain
unexplored, where some fine captures are to be made. Let us visit, at
certain times, the approaches to the quays, even at low tide, and we
shall be surprised to find there species which we have searched for
far and near.” This opinion is confirmed by the enumeration of several
interesting captures.

Was not the great Bacon right when he said, “For the keen observer,
nothing in Nature is mute”?

[Illustration: The cliffs of Cape Grisnez.]


FOOTNOTES:

[1] It is well known that ants, by touching the skin of aphides,
extract therefrom a secretion of viscous matter, which nourishes them.
They will frequently carry off the aphides to their habitations, and
keep them there; thus one may say they _keep a cow in their stable_.

[2] The preservation of insects, and their preparation for collections,
necessitates some precaution. Entomologists are in the habit of
spreading them out on a small board, and arranging the legs and
_antennæ_ by means of large pins. The wings should be dried by placing
them on strips of paper, which preserves them. These precautions are
indispensable if it is wished that the insects in a collection should
retain their distinctive characters. Worms and caterpillars can be
raised in pots filled with earth, if carefully covered over with muslin
or wire gauze with very fine meshes. The process of hatching may give
rise to many interesting observations.

[3] It frequently happens that in a small aquarium, constructed after
this fashion, the animals escape. This is avoided by covering the vase
with a net.

[4] The infusion of parsley has the advantage of not sensibly obscuring
the water.

[5] Detailed accounts in Vol. lxxxiii., pp. 243 and 292 of “_La
Nature_.”

[6] See “_La Nature_,” 4th year, 1876, 2nd half-year, p. 167. M. A.
Guillemin mentions, in connection with the phenomenon of July 12th,
1876, the presence of light masses of cloud of a greyish-blue colour,
similar to those perceived in the phenomena just described.



CHAPTER III.

 PHYSICS—THE MEANING OF PHYSICS—FORCES OF
 NATURE—GRAVITY—COHESION—CHEMICAL ATTRACTION—CENTRE OF
 GRAVITY—EXPERIMENTS—AUTOMATON TUMBLERS.


Having now introduced our readers to Science which they can find for
themselves in the open air, and the pursuit of which will both instruct
and amuse, we will proceed to investigate the Branch of Science called
PHYSICS.

PHYSICS may be briefly described as the Branch of Natural
Science which treats of such phenomena as are unaccompanied by any
important changes in the objects wherein such phenomena are observed.

For instance, the sounding of a bell or the falling of a stone are
physical phenomena, for the objects which cause the sound, or the fall,
undergo no change. Heat is set free when coal burns. This disengagement
of heat is a physical phenomenon; but the change during combustion
which coal undergoes is a _chemical_ phenomenon. So the objects may
be the same, but the circumstances in which they are placed, and the
forces which act upon them, may change their appearance or position.

This brings us at once to the _Forces of Nature_, which are three
in number; viz., Gravity, Cohesion, and Affinity, or Chemical
Attraction. The phenomena connected with the last-named forms the
Science of _Chemistry_. We give these three Forces these names. But
first we must see what is Force, for this is very important. Force
is a CAUSE—the cause of Motion or of Rest. This may appear
paradoxical, but a little reflection will prove it. It requires force
to set any object in motion, and this body would never stop unless some
other force or forces prevented its movement beyond a certain point.
Force is therefore the cause of a change of “state” in matter.

We have said there are three forces in nature. The first is Gravity,
or the attraction of particles at a distance from each other. We may
say that Gravity, or Gravitation, is the mutual attraction between
different portions of matter acting at all distances,—the force of
attraction being, of course, in proportion to the mass of the bodies
respectively. The greatest body is the Earth, so far as our purposes
are concerned. So the attraction of the Earth is _Gravity_, or what we
call _Weight_.

We can easily prove this. We know if we jump from a chair we shall
come to the floor; and if there were nothing between us and the actual
ground sufficient to sustain the force of the attracting power of the
earth, we should fall to the earth’s surface. In a teacup the spoon
will attract air bubbles, and large air bubbles will attract small
ones, till we find a small mass of bubbles formed in the centre of the
cup of tea. Divide this bubble, and the component parts will rush to
the sides of the cup. This form of attraction is illustrated by the
accompanying diagrams.

[Illustration: Fig. 14.]

Suppose two balls of equal magnitude, A and B (fig.
14). These being of equal magnitude, attract each other with equal
force, and will meet, if not opposed, at a point (M) half-way
between the two. But they do not meet, because the attraction of the
earth is greater than the attraction they relatively and collectively
exercise towards each other. But if the size of the balls be different,
the attraction of the greater will be more evident, as shown below,
where the points of meeting are indicated respectively (figs. 15 and
16). These experiments will illustrate the phenomena of _falling
bodies_. Gravity is the cause of this, because every object on the
surface of the earth is very much smaller than the earth itself, and
therefore all bodies _fall_ towards the centre of the earth. A certain
time is thus occupied, and we can find the _velocity_ or rapidity of a
falling body very easily. On the earth a body, if let fall, will pass
through a space sixteen feet in the first second; and as the attraction
of the earth still continues and is exercised upon a body already in
rapid motion, this rate of progress must be proportionately increased.
Just as when steam is kept up in an engine running down hill, the
velocity of the train will rapidly increase as it descends the gradient.

[Illustration: Figs. 15 and 16.]

A body falling, then, descends sixteen feet in the first second, and
for every succeeding second it assumes a greater velocity. The distance
the body travels has been calculated, and the space it passes through
has been found to _increase in proportion to the square of the time it
takes to fall_. For instance, suppose you drop a stone from the top of
a cliff to the beach, and it occupies two seconds in falling, if you
multiply 2 × 2, and the result by sixteen, you will find how high the
cliff is: in this (supposed) case it is (omitting decimals) sixty-four
feet high. The depth of a well can also be ascertained in the same way,
leaving out the effect of air resistance.

But if we go up into the air, the force of gravity will be diminished.
The attraction will be less, because we are more distant from the
centre of the earth. This decrease is scarcely, if at all, perceptible,
even on very high mountains, because their size is not great in
comparison with the mass of the earth’s surface. The rule for this is
_that gravity decreases in proportion to the square of the distance_.
So that if at a certain distance from the earth’s surface the force of
attraction be 1, if the distance be _doubled_ the attraction will be
only _one quarter_ as much as before—not one-half.

Gravity has exactly the same influence upon _all_ bodies, and the force
of the attraction is in proportion to the mass. All bodies of equal
mass will fall in the same time in a given distance. Two coins (or a
coin and a feather _in vacuo_) will fall together. But in the air the
feather will remain far behind the coin, because nearly all the atoms
of the former are resisted by the air, while in the coin only some
particles are exposed to the resistance, the _density_ of the latter
preventing the air from reaching more than a few atoms, comparatively
speaking. The theory of weight and gravitation, and experiments
relating to the falling of bodies, may be easily demonstrated with
ordinary objects that we have at hand. I take a halfpenny and a piece
of paper, which I cut in the shape of the coin, and holding them side
by side, I drop them simultaneously; the halfpenny reaches the ground
some time before the paper, a result quite in accordance with the laws
of gravitation, as one must bear in mind the presence of air, and the
different resistance it offers to two bodies differing in density. I
next place the paper disc on the upper surface of the piece of money,
and then drop them simultaneously. The two objects now reach the ground
at the same time, the paper, in contact with the halfpenny, being
preserved from the action of the air. This experiment is so well known
that we need not further discuss it; but it must be plainly evident
that it is capable of development in experiments on phenomena relating
to falling bodies.[7] When a body influenced by the action of a force
acts, in its turn, upon another, the latter reacts in an opposite
manner upon the first, and with the same intensity.

_The Attraction of Cohesion_ is the attraction of particles of bodies
to each other at very small distances apart. Cohesion has received
various names in order to express its various degrees. For instance,
we say a body is tough or brittle, or soft or hard, according to the
degrees of cohesion the particles exercise. We know if we break a glass
we destroy the cohesion; the particles cannot be reunited. Most Liquid
particles can be united, but not all. Oil will not mix with water.

The force of cohesion depends upon heat. Heat expands everything, and
the cohesion diminishes as temperature increases.

There are some objects or substances upon the earth the particles of
which adhere much more closely than others, and can only, with very
great difficulty, be separated. These are termed _Solids_. There are
other substances whose particles can easily be divided, or their
position altered. These are called _Fluids_. A third class seem to have
little or no cohesion at all. These are termed _Gases_.

Adhesion is also a form of attraction, and is cohesion existing on the
surfaces of two bodies. When a fluid adheres to a solid we say the
solid _is wet_. We turn this natural adhesion to our own purposes in
many ways,—we whitewash our walls, and paint our houses; we paste our
papers together, etc.

On the other hand, many fluids will hot adhere. Oil and water have
already been instanced. Mercury will not stick to a glass tube, nor
will the oiled glass tube retain any water. We can show the attraction
and repulsion in the following manner:—Let one glass tube be dipped
into water and another into mercury, you will see that the water will
ascend slightly at the side, owing to the attraction of the glass,
while the mercury will be higher in the centre, for it possesses no
attraction for the glass (fig. 17). If small, or what are termed
capillary (or hair) tubes, be used (fig. 18), the water will rise up in
the one tube, while in the other the mercury will remain lower than the
mercury outside the tube. (See _Capillarity_.)

[Illustration: Fig. 17.]

[Illustration: Fig. 18.]

_Chemical Attraction_ is the force by which two different bodies unite
to form a new and different body from either. This force will be fully
considered in CHEMISTRY, in a future part.

It is needless for us to dwell upon the uses of these Forces of Nature.
Gravity and Cohesion being left out of our world, we can imagine the
result. The earth and sun and planets would wander aimlessly about;
we should float away into space, and everything would fall to pieces,
while our bodies would dissolve into their component parts.

_The Balance and Centre of Gravity._—We have spoken at some length
about Gravity, and now we must say something respecting that point
called the _Centre of Gravity_, and the _Balance_, and upon the latter
we have a few remarks to make first, for a well-adjusted balance is
a most useful thing, and we will show you how to make one, and then
proceed to our illustrations of the Centre of Gravity, and explain it.

[Illustration: Fig. 19.—Torsion balance, which can easily be
constructed, capable of weighing a milligram one-tenth of full size.]

All those who cultivate experimental science are aware that it is
useful to unite with theoretical ideas that manual dexterity which is
acquired by the student accustoming himself to practical operations.
One cannot too strongly urge both chemists and physicists to exercise
themselves in the construction of the appliances they require, and
also to modify those already existing, which may be adapted to their
wants. In a large number of cases it is possible to manufacture, at
small expense, delicate instruments, capable of rendering the same
service as the most elaborate apparatus. Important scientific labours
have often been undertaken by men whose laboratories were most simple,
who, by means of skill and perseverance, knew how to do great things
with small resources. A delicate balance, for instance, indispensable
alike to chemist and physicist, can be manufactured at little cost in
different ways. A thin platinum wire and a piece of wood is all that
is needed to make a balance capable of weighing a milligram; and to
make a very sensitive hydrostatic balance, little is required besides
a glass balloon. Fig. 19 represents a small torsion balance of extreme
simplicity. A thin platinum wire is stretched horizontally through two
staples, from the wooden supports, A B, which are fixed in a
deal board. A very thin, delicate lever, C D, cut in wood,
or made with a wisp of straw, is fixed in the centre of the platinum
wire by means of a small clip, which secures it firmly. This lever
is placed in such a manner that it is raised perceptibly out of the
horizontal line. At D is fixed a paper scale, on which is put
the weight of a centigram. The lever is lowered to a certain point,
slightly twisting the platinum wire. Near the end of the lever a piece
of wood, F, is fixed, on which is marked the extreme point
of its movements. Ten equi-distant divisions are marked between these
two points, which represent the distance traversed by the lever under
the weight of the milligram. If a smaller weight than a centigram is
placed on the paper scale the lever falls, and balances itself after
a few oscillations. If it falls four divisions, it is evident that the
substance weighs four milligrams. Taking a rather thicker platinum
wire, to which a shorter lever must be adapted, one can weigh the
decigram, and so on. It would be an easy matter, also, to make, on the
same model, balances for weighing considerable weights. The platinum
wire should be replaced by iron wires of larger diameter, firmly
stretched, and the lever should be made of a piece of very resisting
wood. One can also, by adaptation, find the exact value of the most
trifling weights. By lengthening a very fine platinum wire several
yards, and adapting a long, slender lever, it will not be impossible to
ascertain the tenth of a milligram. In this latter case the balance can
be set when it is wanted.

Fig. 20 represents Nicholson’s Areometer, which any one may construct
for himself, and which, as it is here represented, constitutes another
kind of balance. A glass balloon, filled with air, is hermetically
closed with a cork, through which is passed a cylinder of wood,
surmounted by a wooden disc, D. The apparatus is terminated at
its lower end by a small tray, C, on which one can put pieces
of lead in variable quantities. It is then plunged into a glass filled
with water. The pieces of lead on the tray, C, are added by
degrees, until the stem of the areometer rises almost entirely above
the level of the water; it is next passed through a ring, which keeps
it in position, and which is fastened to the upper part of the glass by
means of four iron wires in the shape of a cross. The stem is divided
in such a way that the space comprised in each division represents the
volume of a cubic centimetre. Thus arranged, the apparatus constitutes
a balance. The object to be weighed is placed on the disc, D,
and the areometer sinks in the water, oscillates, and then remains in
equilibrium. If the stem sinks five divisions, it is evident that the
weight of the object corresponds to that of five cubic centimetres of
displaced water, or five grams.

[Illustration: Fig. 20.—Nicholson’s Areometer, contrived to serve as a
balance.]

It is obvious, therefore, from the preceding examples, that it is
not impossible to construct a weighing apparatus with ordinary and
very inexpensive objects. We can, in the same way, show that it is
possible to perform instructive experiments with no appliances at all,
or, at any rate, with common things, such as everyone has at hand.
The lamented Balard, whose loss science has had recently to deplore,
excelled in chemical experiments without a laboratory; fragments of
broken glass or earthenware were used by him for improvising retorts,
bottles and vases for forming precipitates, and carrying on many
important operations.

Scheele also operated in like manner; he knew how to make great
discoveries with the humblest appliances and most slender resources.
One cannot too earnestly endeavour to imitate such leaders, both in
teaching others and instructing oneself.

The laws relating to the weight of bodies, the centre of gravity, and
stable or unstable equilibrium, may be easily taught and demonstrated
by means of a number of very familiar objects. By putting into the
hands of a child a box of soldiers cut in elder-wood, the end of
each fixed into half a bullet, we provide him with the means of
making some easy experiments on the centre of gravity. According to
some authorities on equilibrium, it is not impossible, with a little
patience and delicacy of manipulation, to keep an egg balanced on
one of its ends. This experiment should be performed on a perfectly
horizontal surface, a marble chimney-piece, for example. If one can
succeed in keeping the egg up, it is, according to the most elementary
principles of physics, because the vertical line of the centre of
gravity passes through the point of contact between the end of the egg
and the surface on which it rests.

[Illustration: Fig. 21.—Experiment on “centre of gravity.”]

Fig. 21 reproduces a curious experiment in equilibrium, which is
performed with great facility. Two forks are stuck into a cork, and the
cork is placed on the brim of the neck of a bottle. The forks and the
cork form a whole, of which the centre of gravity is fixed over the
point of support. We can bend the bottle, empty it even, if it contains
fluid, without the little construction over its mouth being in the
least disturbed from its balance. The vertical line of the centre of
gravity passes through the point of support, and the forks oscillate
with the cork, which serves as their support, thus forming a movable
structure, but much more stable than one is inclined to suppose.
This curious experiment is often performed by conjurors, who inform
their audience, that they will undertake to empty the bottle without
disturbing the cork. If a woodcock has been served for dinner, or any
other bird with a long beak, take off the head at the extreme end of
the neck; then split a cork so that you can insert into it the neck of
the bird, which must be tightly clipped to keep it in place; two forks
are then fixed into the cork, exactly as in the preceding example, and
into the bottom of the cork a pin is inserted. This little contrivance
is next placed on a piece of money, which has been put on the opening
of the neck of the bottle, and when it is fairly balanced, we give
it a rotatory movement, by pushing one of the forks as rapidly as we
please, but as much as possible without any jerk (fig. 22). We then see
the two forks, and the cork surmounted by the woodcock’s head, turning
on the slender pivot of a pin. Nothing can be more comical than to
witness the long beak of the bird turning round and round, successively
facing all the company assembled round the table, sometimes with a
little oscillation, which gives it an almost lifelike appearance.
This rotatory movement will last some time, and wagers are often laid
as to which of the company the beak will point at when it stops. In
laboratories, wooden cylinders are often to be seen which will ascend
an inclined plane without any impulsion. This appears very surprising
at first, but astonishment ceases when we perceive that the centre of
gravity is close to the end of the cylinder, because of a piece of
lead, which has been fixed in it.

[Illustration: Fig. 22.—Another experiment on the same subject.]

[Illustration: Fig. 23.—Automatic puppets.]

Fig. 23 gives a very exact representation of a plaything which was
sold extensively on the Boulevards at Paris at the beginning of the
New Year. This little contrivance, which has been known for some time,
is one of the most charming applications of the principles relating
to the centre of gravity. With a little skill, any one may construct
it for himself. It consists of two little puppets, which turn round
axles adapted to two parallel tubes containing mercury. When we place
the little contrivance in the position of fig. 24, the mercury being
at _a_, the two dolls remain motionless, but if we lower the doll
S, so that it stands on the second step (No. 2) of the flight,
as indicated in fig. 25, the mercury descends to _b_ at the other end
of the tube; the centre of gravity is suddenly displaced; the doll
R then accomplishes a rotatory movement, as shown by the arrow
in fig. 25, and finally alights on step No. 3. The same movement is
also effected by the doll S, and so on, as many times as there
are steps. The dolls may be replaced by a hollow cylinder of cartridge
paper closed at both ends, and containing a marble; the cylinder,
when placed vertically on an inclined plane, descends in the same
way as the puppets. The laws of equilibrium and displacement of the
centre of gravity, are rigorously observed by jugglers, who achieve
many wonderful feats, generally facilitated by the rotatory motion
given to the bodies on which they operate, which brings into play the
centrifugal force. The juggler who balances on his forehead a slender
rod, on the end of which a plate turns round, would never succeed
in the experiment if the plate did not turn on its axis with great
rapidity. But by quick rotation the centre of gravity is kept near the
point of support. We need hardly remark, too, that it is the motion of
a top that tends to keep it in a vertical position.

[Illustration: Fig. 24.—First position of the puppets.]

[Illustration: Fig. 25.—Second position of the puppets.]

Many experiments in mechanical physics may occur to one’s mind. To
conclude the enumeration of those we have collected on the subject,
I will describe the method of lifting a glass bottle full of water
by means of a simple wisp of straw. The straw is bent before being
passed into the bottle of water, so that, when it is lifted, the centre
of gravity is displaced, and brought directly under the point of
suspension. Fig. 26 shows the method of operation very plainly. It is
well to have at hand several pieces of straw perfectly intact, and free
from cracks, in case the experiment does not succeed with the first
attempt.

Having now seen how this point we call the centre of gravity acts, we
may briefly explain it.

[Illustration: Fig. 26.—Lifting a bottle with a single straw.]

The centre of gravity of a body is that point in which the sum of the
forces of gravity, acting upon all the particles, may be said to be
united. We know the attraction of the earth causes bodies to have a
property we call _Weight_. This property of weight presses upon every
particle of the body, and acts upon them as parallel forces. For if a
stone be broken all the portions will equal the weight of the stone;
and if some of them be suspended, it will be seen that they hang
parallel to each other, so we may call these weights parallel forces
united in the whole stone, and equal to a single resultant. Now, to
find the centre of gravity, we must suspend the body, and it will hang
in a certain direction. Draw a line from the point of suspension, and
suspend the body again: a line drawn from that point of suspension will
pass through the same place as the former line did, and so on. That
point is the centre of gravity of that suspended body. If the form of
it be regular, like a ball or cylinder, the centre of gravity is the
same as the mathematically central point. In such forms as pyramids
it will be found near the largest mass; viz., at the bases, about
one-fourth of the distance between the apex and the centre of gravity
of the base.

When the centre of gravity of any body is supported, that body cannot
fall. So the well-known leaning towers are perfectly safe, because
their lines of direction fall within the bases. The centre of gravity
is in the centre of the leaning figure. The line of direction drawn
vertically from that point falls within the base; but if the tower were
built up higher, so that the centre of gravity were higher, then the
structure would fall, because the line of direction would fall without
the base.

We see that animals (and men) are continually altering the position of
the centre of gravity; for if a man bears a load he will lean forward,
and if he takes up a can of water in one hand he will extend the other
to preserve his balance or equilibrium.

[Illustration: Fig. 27.—Balancing a weight on a nail and key.]

The experiment shown in the accompanying illustration is apparently
very difficult, but it will be found easy enough in practice if
the hand be steady. Take a key, and by means of a crooked nail, or
“holdfast,” attach it to a bar of wood by a string tied tightly round
the bar, as in the picture. To the other extremity of the bar attach
a weight, and then drive a large-headed nail into the table. It will
be found that the key will balance, and even move upon the head of the
nail, without falling. The weight is under the table, and the centre of
gravity is exactly beneath the point of suspension.

Another simple experiment may prove amusing. Into a piece of wood
insert the points of two knives, and at the centre of the end of the
bar insert a needle between the knife handles. The wood and the knives
may then be balanced on another needle fixed in a cork at A.

[Illustration: Fig. 28.—Another experiment.]

We may conclude this chapter by summing up in a few words what the
Centre of Gravity is. We can define it as “that point in a body upon
which the body, acted on solely by the force of gravity, will balance
itself in all positions.” Such a point exists in every body, and
equally in a number of bodies fastened tightly together. The Centre of
Gravity has by some writers been denominated the _Centre of Parallel
Forces_, or the _Centre of Magnitude_, but the Centre of Gravity is the
most usual and best understood term.


FOOTNOTES:

[7] M. A. G. has written us an interesting letter on the subject of
similar experiments, which we here transcribe:—

“When a siphon of seltzer water has been opened some little time, and
the equilibrium of tension is nearly established between the escaped
gas and the dissolved gas, a vertical stream of bubbles is seen to rise
from the bottom of the apparatus, which present a very clear example
of the law of ascension of bubbles; that is to say (putting out of the
question the expansion of the bubbles in their passage upwards), it
is an inverse representation of the law of gravity affecting falling
bodies. The bubbles, in fact, detach themselves from their starting
point with perfect regularity; and as the interval varies in one
file from another, we have before us a multiplied representation of
that terrible law which Attwood’s machine made such a bugbear to the
commercial world. I believe it is possible, by counting the number
of bubbles that detach themselves in a second, in each file, and the
number which the whole stream contains at a given instant, to carry
the verification further; but I must confess that I have not done so
myself.”



CHAPTER IV.

 SOME PROPERTIES OF SOLID BODIES—INERTIA—MOTION—FRICTION—THE
 PENDULUM—EQUILIBRIUM.


Those who have followed us through the preceding pages have now, we
hope, some ideas upon Gravity and the Forces of Nature. In speaking
of Forces we said “Force was a cause of Motion.” Let us now consider
Inertia, and Motion with its accompanying opponent, Friction.

[Illustration: Fig. 29.—Shock communicated by elasticity.]

INERTIA is the passiveness of Matter. This perfect
indifference to either rest or motion makes the great distinction
between living and lifeless matter. Inertia, or _Vis Inertia_, is this
passiveness. Now, to overcome this indifference we must use force,
and when we have applied force to matter we set it in motion; that
is, we move it. When we move it we find a certain resistance which
is always proportionate to the force applied. In mechanics this is
termed _Action_, and _Reaction_, which are always equal forces acting
in opposite directions. This is Newton’s law, and may be explained by
a “weight” on a table, which presses against the table with the same
force with which the table presses against the “weight”; or when you
strike a ball, it strikes the hand with the same force.

We can communicate motion by elasticity. For instance, if we place a
number of coins upon a table touching each other and in a straight
line, and strike the last coin of the line by pushing another sharply
against it, the piece at the opposite extremity will slip out of its
place from the effect of the shock transmitted by the coin at the other
end (fig. 29).

[Illustration: Fig. 30.—Experiment to illustrate inertia.]

When two forces act upon a body at the same time, it takes a direction
intermediate. This is known as the resultant. The enormous forces
exercised by the heavenly bodies will be treated of later. We will
first consider _Inertia_.

There are several experiments relating to the subject of Inertia which
may be performed. I once witnessed one quite accidentally when taking a
walk.

[Illustration: Fig. 31.—Another experiment on the same subject.]

I was one day passing the Observatory at Paris, when I noticed a number
of people collected round a professor, who after executing several
juggling tricks, proceeded to perform the curious experiment I am about
to describe. He took a broomstick and placed it horizontally, passing
the ends through two paper rings. He then asked two children to hold
the paper rings by means of two razors, so that the rings rested on
the blade. This done, the operator took a stout stick, and, with all
his strength, struck the broomstick in the centre; it was broken into
shivers, but the paper rings were not torn in the least, or even cut
by the razors! One of my friends, M. M——, a painter, showed me how to
perform this experiment as represented in the illustration (fig. 30).
A needle is fixed at each end of the broomstick, and these needles
are made to rest on two glasses, placed on chairs; the needles alone
must be in contact with the glasses. If the broomstick is then struck
violently with another stout stick, the former will be broken, but
the glasses will remain intact. The experiment answers all the better
the more energetic the action. It is explained by the resistance of
inertia in the broomstick. The shock suddenly given, the impulse
has not time to pass on from the particles directly affected to the
adjacent particles; the former separate before the movement can be
transmitted to the glasses serving as supports.[8]

[Illustration: Fig. 32.—Extracting a “man” from a pile of draughts
without overturning the pile.]

The experiment represented in fig. 31 is of the same nature. A wooden
ball is suspended from the ceiling by a rather slender thread, and a
similar thread is attached to the lower end of the ball. If the lower
thread is pulled forcibly it will break, as shown in the illustration;
the movement communicated to it has not time to pass into the ball; if,
on the contrary, it is pulled very gradually and without any shock,
the upper thread instead will break, because in this case it supports
the weight of the ball. Motion is not imparted simultaneously to all
parts of a body, but only to the particles first exposed to a blow, for
instance. One might multiply examples of this. If a bullet be shot from
a gun, it will make a round hole in a piece of wood or glass, whilst
if thrown by the hand,—that is to say, with much less force,— it will
shiver the wood or the pane of glass to pieces. When the celerity of
the motive force is very great, the particles directly affected are
disturbed so quickly that they separate from the adjacent particles
before there is time for the movement to be communicated to the latter.

It is possible, for the same reason, to extract from a pile of money a
piece placed in the middle of the pile without overturning the others.
It suffices to move them forcibly and quickly with a flat wooden ruler.
The experiment succeeds very well also if performed with draughtsmen
piled up on the draught-board (fig. 32).

[Illustration: Fig. 33.—Calling out a sixpence from the glass.]

Fig. 33 represents another experiment which belongs to the laws of
resisting force. A sixpence is placed on a table covered with a cloth
or napkin. It is covered with a glass, turned over so that its brim
rests on two penny pieces. The problem to be solved is how to extract
the sixpence from underneath the glass without touching it, or slipping
anything beneath it. To do this it is necessary to scratch the cloth
with the nail of the forefinger; the elasticity of the material
communicates the movement to the sixpence, which slowly moves in the
direction of the finger, until it finally comes out completely from
beneath the glass.

We may give another experiment concerning Inertia. Take a strip of
paper, and upon it place a coin, on a marble chimney-piece, as in
the illustration. If, holding the paper in the left hand, you strike
it rapidly and forcibly, you will be enabled to draw away the paper
without causing the coin (say a five-shilling-piece) to fall down (fig.
34).

It is not impossible to draw away a napkin laid as a tablecloth
for one person’s dinner, without disturbing the various articles
laid upon it. A quick motion is all that is necessary, keeping the
napkin tightly extended by the hands at the same time. This latter
experiment, however, is not recommended to boys home for the holidays,
as they may unwillingly practise a feat analogous to that executed by
Humpty-Dumpty, and find equal difficulty to match the pieces.

[Illustration: Fig. 34.—Drawing a slip of paper from beneath a coin.]

We will now examine the term _Motion_. A body is said to be in motion
when it changes its position in relation to surrounding objects. To
perceive motion the surrounding objects must be relatively at rest,
for if they all hurried along at the same rate no motion would be
perceptible. This is evident, for when we stand still trees and houses
appear stationary, as do we ourselves, but we know we all are rushing
round with the earth, though our _relative_ positions are unchanged.
Hence there is no _absolute_ rest.

What are the causes of motion?—Gravity is one. The influence of
heat, which is itself caused by the motion of atoms, the effects
of electricity, etc., and finally, the power of force in men or
animals—any of these causes will produce motion. But a body at rest
cannot put itself in motion, nor can a body in motion stop itself, or
change its condition of motion.

But you may say a body will stop itself. Your ball on the ground, or
even upon ice, will eventually come to a stop. We fire a bullet, and it
will stop in time. We reply it does not stop of itself. The resistance
of the Air and Friction tend to bring the body in motion to a state of
rest. In the case of a bullet gravity brings it down.

There is no need to insist upon the resistance offered by the air
even when it is not rushing violently past to fill up a vacuum beyond
us, and called a breeze, or high wind. But we may say something of
_Friction_.

Friction is derived from the Latin _frico_, to rub, and expresses the
resistance to motion which arises from uneven surfaces. It is a passive
resistance, and depends upon the force which keeps the bodies together.
Thus a train running upon a smooth iron rail would never be able to
proceed but for friction, which gives the necessary purchase or grip to
the wheel and rail in contact.

No surface is perfectly smooth, for we must push a body upon the
smoothest surface we possess. Friction tends to resist motion always,
and is the cause of a great loss of power in mechanics, though it is
employed to stop motion by certain appliances, such as “brakes” and
“drags,” for sliding friction is greater than rolling friction. But
without friction most structures would fall to pieces, and all forward
motion would cease. So though it is an inconvenient force to overcome,
we could not do without it.

If a body is set in motion, we see that the tendency of it is to go on
for ever. Such, indeed, is the case with the stars; but so long as we
are within the influence of the earth’s attraction, we cannot expect
such a result. We know now what motion is; we must also, to understand
it perfectly, consider its direction and its velocity.

The line which indicates the way from the starting point to the end
is the _direction_ of the object in motion, and the rate it moves at
its _velocity_. The latter is calculated at so many miles an hour,
as a train; or so many feet in a second if the object be a shot, or
other very rapidly-moving body. In equal velocity the same distance
is traversed in the same time; and so if a train run a mile in a
minute, we know it will travel sixty miles in an hour, and is therefore
during that minute going at _the rate_ of sixty miles an hour. We
have already spoken of the velocity of a stone falling from a cliff
as sixteen feet in a second, and a stone thrown into the air to rise
sixteen feet will be a second in going up, and a second in descending.
But the velocity will be accelerated in the descent after the first
second of time, and retarded in the upward cast by gravity. So we have
two terms—_accelerated_ and _retarded_ velocity—used to express an
increased or decreased force of attraction.

Perpetual motion has often been sought, but never discovered, nor
will it ever be till the elixir of life has been found. It is quite
impossible to construct any machine that will work without friction;
if any work be done energy will be expended and transformed into other
energy, so the total must be diminished by so much as was employed to
transform the remainder. No body can give unlimited work, therefore
the perpetual motion theory is untenable and impossible.

The _pendulum_ is considered the nearest approach to perpetual motion.
This is so well known that no description is needed, but we may say
a few words concerning it. By the diagram, we see that if we lift
the ball to _b_, and let it fall, it will descend to _l_, and pass
it to _a_ opposite, nearly as far from _l_ as _b_ is from it. So the
oscillations will continue, each beat being less and less, till rest is
reached by the action of gravity (page 23). Were it not for friction
and the pressure of the air, the oscillations would continue for ever;
as it is, it declines by shorter swings till it remains in equilibrium.

[Illustration: Fig. 35.—The pendulum.]

The seconds’ pendulum oscillates sixty times an hour, and must be of a
certain length in certain places. In London it is 39·1393 inches, and
furnishes a certain standard of length, and by an Act of Parliament the
yard is divided into 36 parts, and 39·1393 such parts make the seconds’
pendulum in the latitude of London (_in vacuo_) in a temperature of 62°.

[Illustration: Fig. 36.—Centrifugal Force.]

But the same pendulum will not perform the same number of oscillations
in one minute in all parts of the globe. At the equator they will
be less, and at the pole more. Thus it was discovered that, as the
movements of the pendulum are dependent upon the force of gravity,
and as this force decreases the farther we get from the centre of the
earth, the equator must be farther from the earth’s centre than the
poles, and therefore the poles must be depressed. The decline of the
pendulum at the equator is also, in a measure, due to Centrifugal Force.

_Centrifugal Force_, which means “flying from the centre,” is the force
which causes an object to describe a circle with uniform velocity, and
fly away from the centre; the force that counteracts it is called the
_centripetal_ force. A very simple experiment will illustrate it.

[Illustration: Fig. 37.—Another illustration of centrifugal force.]

To represent its action, we shall have recourse to an ordinary glass
tumbler placed on a round piece of cardboard, held firmly in place by
cords. Some water is poured in the glass, and we then show that it can
be swung to and fro and round without the water being spilt, even when
the glass is upside down (fig. 36).

Another experiment on the same subject is as shown in the above
illustration, by which a napkin ring can be kept in revolution around
the forefinger, and by a continued force the ring may be even held
suspended at the tip of the finger, apparently in the air, without
support (fig. 37).


FOOTNOTES:

[8] The experiment we have just described is a very old one. M. V.
Sircoulon has told us that it was described at length in the works of
Rabelais. The following remarks are in “Pantagruel,” book II., chap.
xvii.

“Panuræ then took two glasses of the same size, filled them with water,
and put one on one stool, and the other on another, about five feet
apart, and placed the staff of a javelin about five-and-a-half feet
long across, so that the ends of the staff just touched the brim of
the glasses. That done, he took a stout piece of wood, and said to the
others: “Gentlemen, this is how we shall conquer our enemies; for in
the same way that I shall break this staff between these two glasses,
without the glasses being broken or injured, or spilling a single drop
of water, so shall we break the head of our Dipsodes, without any
injury to ourselves, and without getting wounded. But that you may not
think there is magic in it, you, Eusthenes, strike with this stick as
hard as you can in the centre.” This Eusthenes did, and the staff broke
in two pieces, without a drop of water being spilt.



CHAPTER V.

 GASES AND LIQUIDS—PRESSURE OF THE AIR—EXPERIMENTS.


We have more than once referred to the pressure of the air which exerts
a great influence upon bodies in motion, but a few experiments will
make this more obvious, and clearly demonstrate the fact. We have also
told you some of the properties of Solids, such as Weight, Inertia,
Friction, and Resistance, or Strength. Solids also, as we have seen,
occupy space, and cannot be readily compressed, nor bent to other
shapes. Now the subject of the Pressure of the Air leads us to the
other forms of Matter; namely, Gases and Liquids, which will be found
very interesting to study.

[Illustration: Fig. 38.—Blowing an egg from one glass to another.]

The force of air can very soon be shown as acting with considerable
pressure upon an egg in a glass. By blowing in a claret glass
containing a hard-boiled egg, it is possible to cause the egg to
jump out of the glass; and with practice and strength of lungs it
is not impossible to make it pass from one glass to another, as per
illustration (fig. 38).

The force of heated air ascending can also be ascertained by cutting up
a card into a spiral, and holding it above the flame of a lamp (fig.
39). The spiral, if lightly poised, will turn round rapidly.

Now let us turn to a few experiments with the air, which is composed
in two gases, Oxygen and Nitrogen, of which we shall hear more when we
come to CHEMISTRY.

[Illustration: Fig. 39.—Movement of heated air.]

[Illustration: Fig. 40.—Pressure of the air.]

It is not intended here to prosecute researches, but rather to sketch
a programme for instruction, based on amusing experiments in Physics,
performed without apparatus. The greater part of these experiments
are probably well known, and we desire to say that we merely claim to
have collected and arranged them for our descriptions. We must also
add that we have performed and verified these experiments; the reader,
therefore, can attempt them with every certainty of success. We will
suppose that we are addressing a young auditory, and commence our
course of Physics with some facts relating to the pressure of air. A
wine glass, a plate, and water, will serve for our first experiments.
Pour some water on the plate, light a piece of paper resting on a cork,
and cover the flame with the glass which I turn upside down (fig. 40).
What follows?—The water rises in the glass. Why?—Because the burning
of the paper having absorbed a part of the oxygen, and the volume
of confined gas being diminished, the pressure of the outer air has
driven back the fluid. I next fill a goblet with water up to the brim,
and cover it with a sheet of paper which touches both the edge of the
glass and the surface of the water. I turn the glass upside down, and
the sheet of paper prevents the water running out, because it is held
in place by atmospheric pressure (fig. 41). It sometimes happens that
this experiment does not succeed till after a few attempts on the part
of the operator; thus it is prudent to turn the glass over a basin, so
that, in case of failure, the water is not spilt. Having obtained a
vase and a bottle, both quite full of water, take the bottle, holding
it round the neck so that the thumb can be used as a stopper, then turn
it upside down, and pass the neck into the water in the vase. Remove
your thumb, or stopper, keeping the bottle in a vertical position, and
you will see that the water it contains does not escape, but remains in
suspension. It is atmospheric pressure which produces this phenomenon.
If, instead of water, we put milk in the bottle, or some other fluid
denser than water, we shall see that the milk also remains suspended
in the bottle, only there is a movement of the fluid in the neck of
the bottle, and on careful examination we perceive very plainly that
the milk descends to the bottom of the vase, and the water rises into
the bottle. Here, again, it is atmospheric pressure which maintains
the fluid in the bottle, but the milk descends, because fluids are
superposed according to their order of density, and the densest liquid
falls to the bottom.

This can be verified by means of the _phial of the four elements_,
which is a plain, long, and narrow bottle, containing equal volumes of
metallic mercury, salt water, alcohol, and oil. These four liquids will
lie one on the top of the other without ever mixing, even if shaken.

Another experiment as to the pressure of the air may be made (fig. 42).
Take a penny and press it against some oaken bookcase or press, rub the
coin against the wood for a few seconds, then press it, and withdraw
the fingers. The coin will continue to adhere to the wood. The reason
of this is, because by the rubbing and the pressure you have dispersed
the film of air which was between the penny and the wood, and under
those conditions the pressure of the atmospheric air was sufficient to
keep the penny in its place.

[Illustration: Fig. 41.—Pressure of the air.]

Or, again, let us now add a water-bottle and a hard-boiled egg to
our appliances; we will make use of the air-pump, and easily perform
another experiment. I light a piece of paper, and let it burn, plunging
it into a water-bottle full of air. When the paper has been burning
a few seconds I close the opening of the water-bottle by means of a
hard-boiled egg, which I have previously divested of its shell, so that
it forms a hermetic stopper. The burning of the paper has now caused a
vacuum of air in the bottle, and the egg is gradually thrust in by the
atmospheric pressure outside. Fig. 43 exhibits it slowly lengthening
and stretching out as it passes through the aperture; then it is
suddenly thrust completely into the bottle with a little explosive
sound, like that produced by striking a paper bag expanded with air.
This is atmospheric pressure demonstrated in the clearest manner, and
at little cost.

[Illustration: Fig. 42.—Coin adhering by pressure of air.]

If it is desired to pursue a little further the experiments relating
to atmospheric pressure, it will be easy enough to add to the
before-mentioned appliances a closed glass-tube and some mercury, and
one will then have the necessary elements for performing Torricelli’s
and Pascal’s experiments, and explaining the theory of the barometer
(page 52).

An amusing toy, well-known to schoolboys, called the “sucker,” may also
be made the object of many dissertations on the vacuum and the pressure
of air. It is composed of a round piece of soft leather, to the centre
of which is attached a small cord. This leather is placed on the
ground and pressed under foot, and when the cord is pulled it forms a
cupping-glass, and is only separated with difficulty from the pavement.

Atmospheric air, in common with other gases, has a tendency to fill
any space into which it may enter. The mutual attraction of particles
of air is _nil_; on the contrary, they appear to have a tendency to
fly away from each other; this property is called “repulsion.” Air
also possesses an expansive property—a tendency to press against all
the sides of any vessel in which it may be enclosed. Of course the
larger the vessel containing a given quantity of air, the less actual
pressure it will exert on the sides of the vessel. The elasticity of
air therefore decreases with increasing expansion, but it gains in
elasticity or force when compressed.

There is a law in Physics which expresses the relation between
expansion and elasticity of gases, which may be said to be as follows:—

The elasticity (of a gas) is in inverse ratio to the space it occupies,
and therefore by compressing air into a small space we can obtain a
great force, as in the air-gun and the pop-gun of our youthful days.

[Illustration: Fig. 43.—Hard boiled egg, divested of its shell, passing
through the neck of a glass bottle, under the influence of atmospheric
pressure.]

In the cut below we can illustrate the principle of the pop-gun. The
chamber full of air is closed by a cork and by an air-tight piston
(S) at _p_ and _p_. When the piston is pushed into the chamber
the air is compressed between it and the stopper, which at length flies
out forcibly with a loud report

[Illustration: Fig. 44.—The principle of the pop-gun.]

We have said that the tendency of air particles is to fly away from
each other, and were it not for the earth’s attraction the air might be
dispersed. The height of the atmosphere has been variously estimated
from a height of 45 miles to 212 miles in an attenuated form; but
perhaps 100 miles high would be a fair estimate of the height to which
our atmosphere extends.

[Illustration: Fig. 45.—Weighing the air.]

The pressure of such an enormous body of gas is very great. It has
been estimated that this pressure on the average human body amounts
to fourteen tons, but being balanced by elastic fluids in the body,
the inconvenience is not felt. The _Weight_ of Air can easily be
ascertained, though till the middle of the seventeenth century the
air was believed to be without weight. The accompanying illustration
will prove the weight of air. Take an ordinary balance; and suspend
to one side a glass globe fitted with a stop-cock. From this globe
extract the air by means of the air-pump, and weigh it. When the exact
weight is ascertained turn the stop-cock, the air will rush in, and the
globe will then pull down the balance, thus proving that air possesses
weight. The experiments of Torricelli and Otto von Guerike, however,
demonstrated that the air has weight and great pressure. Torricelli
practically invented the barometer, but Otto von Guerike, by the cups
known as _Magdeburg Hemispheres_, proved the pressure of the outward
air. This apparatus is well known, and consists of two hollow copper
hemispheres which fit very closely. By means of the air-pump which he
invented in 1650, Otto von Guerike exhausted the air from the closed
hemispheres. So long as air remained in them, there was no great
difficulty in separating them; but when it had been finally exhausted,
the pressure of the surrounding atmosphere was so great that the hollow
spheres could not be dragged asunder even by horses harnessed to rings
which had been inserted in the globes.

[Illustration: Fig. 46.—Magdeburg Hemispheres.]

The _Air-Pump_ is a very useful machine, and we will now briefly
explain its action. The inventor was, as remarked above, Otto von
Guerike, of Magdeburg. The pump consists of a cylinder and piston and
rod, with two valves opening upwards—one valve being in the bottom of
the cylinder, the other in the piston. This pump is attached by a tube
to a plate with a hole in it, one extremity of the tube being fixed in
the centre of the plate, and the other at the valve at the bottom of
the cylinder. A glass shade, called the _receiver_, is placed on the
top of the plate, and of course this shade will be full of air (fig.
47).

[Illustration: Fig. 47.—The air-pump.]

When the receiver is in position, we begin to work the pump. We have
said there are two valves. So when the piston is drawn up, the cylinder
would be quite empty did not the valve at the bottom, opening upwards,
admit some air from the glass shade through the tube to enter the
cylinder. Now the lower part of the cylinder is full of air drawn from
the glass shade. When we press the piston down again, we press against
the air in it, which, being compressed, tries to escape. It cannot go
back, because the valve at the bottom of the cylinder won’t open, so
it escapes by the valve in the piston, and goes away. Thus a certain
amount of air is got rid of at each stroke of the piston. Two cylinders
and pistons can be used, and so by means of cog-wheels, etc., the air
may be rapidly exhausted from the receiver. Many experiments are made
with the assistance of the air-pump and receiver, though the air is
never _entirely_ exhausted from the glass.

The “Sprengel” air-pump is used to create an almost perfect vacuum,
by putting a vessel to be exhausted in connection with the vacuum at
the top of a tube of mercury thirty inches high. Some air will bubble
out, and the mercury will fall. By filling up again and repeating the
process, the air vessel will in time be completely exhausted. This
is done by Mr. Sprengel’s pump, and a practically perfect vacuum is
obtained, like the _Torricellian_ vacuum.

The “Torricellian vacuum” is the empty space above the column of
mercury in the barometer which we will proceed to describe. Air has a
certain weight or pressure which is sufficient to raise a column of
mercury thirty inches. We will prove this by illustration. Take a bent
tube and fill it with mercury; the liquid will stand equally high in
both arms, in consequence of the ratio of equilibrium in fluids, of
which we shall read more when we come to consider Water. So the two
columns of mercury are in equilibrium. (See A.) Now stop the
arm a with a cork, and take out half the mercury. It will remain in one
arm only. Remove the cork, and the fluid will fall in both arms, and
remain in equilibrio. If a long bent glass tube be used, the arms being
thirty-six inches high, the mercury will fall to a point _c_, which
measures 29·9 inches from the bottom. If the tube be a square inch
in bore, we have 29·9 cubic inches of mercury, weighing 14⅘ lbs.,
balancing a column of air one square inch thick and as high as the
atmosphere. So the mercury and the column of air must weigh the same.
Thus every square inch on the earth supports a weight of (nearly) 15
lbs (figs. 48 and 50).

[Illustration: Fig. 48.—Air Pressure.]

[Illustration: Fig. 49.—The Barometer.]

The barometer invented by Pascal, working on the investigations of
Torricelli, is a very simple and useful instrument. Fill a tube with
mercury from which all moisture has been expelled, and turn it over
in a dish of mercury; the mercury will rise to a certain height (30
inches), and no higher in vacuo. When the pressure of the air increases
the mercury rises a little, and falls when the pressure is removed. Air
charged with aqueous vapour is lighter than dry air, so a fall in the
mercury indicates a certain amount of water-vapour in the air, which
may condense and become rain. The action of mercury is therefore used
as a weather-glass, by which an index-point shows the movements of the
fluid, by means of a wheel over which a thread passes, sustaining a
float and a counterpoise. When the mercury rises the float goes up,
and the weight falls, and turns the wheel by means of the thread. The
wheel having a pointer on the dial tells us how the mercury moves. This
_weather-glass_ is the usual _syphon barometer_ with the float on the
surface and a weight (fig. 50).

[Illustration: Fig. 50.—Syphon barometer.]

The Syphon Barometer is a bent tube like the one already shown, with
one limb much shorter than the other.

The Aneroid Barometer, so called because it is “without moisture,”
is now in common use. In these instruments the atmospheric pressure
is held in equilibrium by an elastic metal spring or tube. A metal
box, or tube, is freed from air, and then hermetically sealed. This
box has a flexible side, the elasticity of which, and the pressure of
the air on it, keep each other in equilibrium. Upon this elastic side
the short arm of a lever is pressed, while the longer arm works an
index-point, as in the circular barometer. When pressure increases the
elastic box “gives”; when pressure diminishes it returns to its former
place, and the index moves in the opposite direction. It is necessary
to compare and “set” the aneroid with the mercurial barometer to ensure
correctness. A curved tube is sometimes used, which coils and uncoils
like a spring, according to the pressure on it.

[Illustration: Fig. 51.—The Water Barometer.]

There are other barometers, such as the Water Barometer, which can
be fixed against the side of a house, and if the water be coloured,
it will prove a useful indicator. As the name indicates, water is
used instead of mercury, but as the latter is thirteen-and-half times
heavier than water, a much longer tube is necessary; viz., one about
thirty-five feet in length. The construction is easy enough. A leaden
pipe can be fixed against the house; on the top is a funnel furnished
with a stop-cock, and placed in a vase of water. The lower part of the
tube is bent, and a glass cylinder attached, with another stop-cock—the
glass being about three feet long, and graduated. Fill the tube with
water, shut the upper stop-cock, and open the lower one. The vacuum
will be formed in the top of the tube, and the barometer will act on a
larger scale than the mercury.

The Glycerine Barometer, invented by Mr. Jordan, and in use at the
_Times_ office, registers as more than one inch movements which on the
mercurial thermometer are only one-tenth of an inch, and so are very
distinctly visible. The specific gravity of pure glycerine is less
than one-tenth that of mercury, so the mean height of the glycerine
column is twenty-seven feet at sea level. The glycerine has, however,
a tendency to absorb moisture from the air, but Mr. Jordan, by putting
some petroleum oil upon the glycerine, neutralized that tendency, and
the atmospheric pressure remains the same. A full description of this
instrument was given in the _Times_ of 25th October, 1880.

[Illustration: Fig. 52.—The principle of the diving-bell.]

The uses of the barometer are various. It is employed to calculate the
heights of mountains; for if a barometer at sea level stand at 30",
it will be lower on a mountain top, because the amount of air at an
elevation of ten thousand feet is less than at the level of the sea,
and consequently exercises less pressure, and the mercury descends.
[The pressure is on the bulb of mercury at the bottom, not on the
_top_, remember.]

The pressure of the air at the tops of mountains sometimes decreases
very much, and it is not sufficiently dense for perfect respiration, as
many people find. Some climbers suffer from bleeding at the nose, etc.,
at great altitudes. This is occasioned by the action of the heart,
which pumps with great force, and the outward pressure upon the little
veins being so much less than usual, they give way.

[Illustration: Fig. 53.—Diver under water.]

Many important instruments depend upon atmospheric pressure. The
most important of these is the pump, which will carry us to the
consideration of water and FLUIDS generally. The fire-engine
is another example, but we will now proceed to explain the diving-bell
already referred to.

Fig. 52 represents the experiment of the diving-bell, which is so
simple, and is explained below. It belongs to the same category of
experiments as those relating to the pressure of air and compression of
gas. Two or three flies have been introduced into the glass, and they
prove by their buzzing about that they are quite at their ease in the
rather confined space.

The DIVING-BELL in a crude form appears to have been used as
early as 1538. It was used by two Greeks in the presence of the Emperor
Charles V., and numerous spectators. In the year 1720 Doctor Halley
improved the diving-bell, which was a wooden box or chamber open at
the bottom. Air casks were used to keep the inmate supplied with air.
The modern diving-bell was used by Smeaton in 1788, and was made of
cast iron. It sinks by its own weight. The pressure of the air inside
is sufficient to keep the water out. Air being easily compressed, it
is always pumped in to keep the hollow iron “bell” full, and to supply
the workmen. There are inventions now in use by which the diver carries
a supply of air with him on his back, and by turning a tap can supply
himself for a long time at a distance from the place of descent,
and thus is able to dispense with the air-tube from the boat at the
surface. This apparatus was exhibited at the Crystal Palace some years
ago.

[Illustration: Fig. 54.—The Hand Fire-Engine.]


THE PUMP.

We have seen in the case of the Water Barometer that the pressure of
the air will sustain a column of water about thirty feet high. So the
distance between the lower valve and the reservoir or cistern must
not be more than thirty-two feet, practically the distance is about
twenty-five feet in pumps.

We can see by the illustration that the working is much the same as in
the air-pump. The suction pipe B is closed by the valve C, the cylinder
D and spout E are above, the piston rod F lifts the air-tight piston
in which is a valve H. When the piston is raised the valve C opens and
admits the water into the cylinder. When the piston is depressed the
valve C is closed, the water already in forces H open, and passing
through the piston, reaches the cylinder and the spout (fig. 55).

The hand fire-engine depends upon the action of compressed air, which
is so compressed by pumping water into the air chamber _a_. The tube is
closed at _g_, and the pumps _e e_ drive water into the air chamber.
At length the tap is opened, and the air drives the water out as it is
continually supplied (fig. 54).

Compressed air was also used for driving the boring machines in the
Mount Cenis tunnel. In this case also the air was compressed by water,
and then let loose, like steam, to drive a machine furnished with
boring instruments.

A pretty little toy may be made, and at the same time exemplify an
interesting fact in Physics. It is called the _ludion_, and it “lies in
a nut shell” in every sense. When the kernel has been extracted from
the shell, fasten the portions together with sealing wax, so that no
water can enter. At one end O, as in the illustration, leave
a small hole about as large as a pin’s head; fasten two threads to the
sealing wax, and to the threads a wooden doll. Let a weight be attached
to his waist. When the figure is in equilibrium, and will float, put it
into a jar of water, and tie a piece of bladder over the top. If this
covering be pressed with the finger, the doll will descend and remount
when the finger is removed. By quick successive pressure the figure may
be made to execute a _pas seul_. The reason of the movement is because
the slight cushion of air in the upper part of the vase is compressed,
and the little water thus caused to enter the nut shell makes it
heavier, and it descends with the figure (fig. 56).

[Illustration: Fig. 55.—The Pump.]

We have now seen that air is a gas, that it exercises pressure, that it
possesses weight. We know it can be applied to many useful purposes,
and that the air machines and inventions—such as the air-pump and
the “Pneumatic Despatch”—are in daily use in our laboratories, our
steam engines, our condensed milk manufactories, and in many other
industries, and for our social benefit. Compressed air is a powerful
motor for boring machinery in tunnels where steam cannot be used, even
if water could be supplied, for smoke or fire would suffocate the
workers. To air we owe our life and our happiness on earth.

Pneumatics, then, deals with the mechanical properties of elastic
fluids represented by air. A gas is an elastic fluid, and differs very
considerably, from water; for a gas will fill a large or small space
with equal convenience, like the genii which came out of the bottle and
obligingly retired into it again to please the fisherman. We have seen
that the pressure of the air is 14⅘ per square inch at a temperature
of 32°. It is not so easy to determine the pressure of air at various
times as that of water. We can always tell the pressure of a column of
water when we find the height of the column, as it is the weight of so
many cubic inches of the liquid. But the pressure of the atmosphere per
square inch at any point is equal to the weight of a vertical column
of air one inch square, reaching from that point to the limit of the
atmosphere above it. Still the density is not the same at all points,
so we have to calculate. The average pressure at sea level is 14·7
per square inch, and sustains a column of mercury 1 square inch in
thickness, 29·92, or say 30 inches high. These are the data upon which
the barometer is based, as we have seen.

[Illustration: Fig: 56.—The “Ludion.”]

In our article upon “Chemistry” we will speak more fully of the
atmosphere and of its constituents, etc.



CHAPTER VI.

 ABOUT WATER—HYDROSTATICS AND HYDRAULICS—LAW OF ARCHIMEDES—THE BRAMAH
 PRESS—THE SYPHON.


At present we will pass from Air to Water, from Pneumatics to
Hydrostatics and Hydraulics. We must remember that Hydrostatics and
Hydraulics are very different. The former treats of the weight and
pressure of liquids when they are at rest, the latter treats of them in
motion. We will now speak of the properties of Liquids, of which Water
may be taken as the most familiar example.

We have already seen that Matter exists in the form of Solids, Liquids,
and Gases, and of course Water is one form of Matter. It occupies a
certain space, is slightly compressible; it possesses weight, and
exercises force when in motion. It is a fluid, but also a liquid. There
are fluids not liquid, such as air or steam, to take equally familiar
examples. These are elastic fluids and compressible, while water is
inelastic, and termed incompressible.

The chemical composition of water will be considered hereafter, but at
present we may state that water is composed of oxygen and hydrogen, and
proportions of eight of the former to one of the latter by weight; in
volume the hydrogen is as two to one.

From these facts, as regards water, we learn that volume and weight
are very different things,—that equal volumes of various things may
have different weights, and that volume (or bulk) by no means indicates
weight Equal volumes of feathers and sand will weigh very differently.

 [The old “catch” question of the “difference in weight between a pound
 of lead and a pound of feathers” here comes to the mind. The answer
 generally given is that “feathers make the heavier ‘pound’ because
 they are weighed by avoirdupois, and lead by troy weight.” This is an
 error. They are both weighed in the same way, and pound for pound are
 the same _weight_, though different in _volume_.]

Fluids in equilibrium have all their particles at the same distance
from the centre of the earth, and although within small distances
liquids appear perfectly level (in a direct line), they must, as the
sea does, conform to the shape of the earth, though in small levels the
space is too limited to admit of any deviation from the plane at right
angle to the direction of gravity.

Liquids always fall to a perfectly level surface, and water will seek
to find its original level, whether it be in one side of a bent tube,
in a watering pot and its spout, or as a fountain. The surface of the
water will be on the same level in the arms of a bent tube, and the
fountain will rise to a height corresponding with the elevation of the
parent spring whence it issues. The waterworks companies first pump
the water to a high reservoir, and then it rises equally high in our
high-level cisterns.

As an example of the force of water, a pretty little experiment may be
easily tried, and, as many of our readers have seen in a shop in the
Strand in London, it always is attractive. A good-sized glass shade
should be procured and placed over a water tap and basin, as per the
illustration herewith. Within the glass put a number of balls of cork
or other light material. Let a stop-cock, with a small aperture, be
fixed upon the tube leading into the glass. Another tube to carry away
the water should, of course, be provided, but it may be used over
again. When the tap is properly fixed, if the pressure of the water be
sufficient, it will rush out with some force, and catching the balls as
they fall to the bottom of the glass shade bear them up as a juggler
would throw oranges from hand to hand. If coloured balls be used the
effect may be enhanced, and much variety imparted to the experiment,
which is very easy to make.

[Illustration: Fig. 57.—Water jet and balls.]

Water exercises an enormous pressure, but the pressure does not depend
upon the amount of water in the vessel. It depends upon the vessel’s
height, and the dimensions of the base. This has been proved by filling
vessels whose bases and heights are equal, but whose shapes are
different, each holding a different quantity of water. The pressure at
the bottom of each vessel is the same, and depends upon the depth of
the water. If we subject a portion of the liquid surface to certain
force, this pressure will be dispersed equally in all directions, and
from an acquaintance with this fact the Hydraulic Press was brought
into notice. If a vessel with a horizontal bottom be filled with water
to a depth of one foot, every square foot will sustain a pressure of
62·37 lbs., and each square inch of 0·433 lbs.

[Illustration: Figs. 58, 59, 60, 61.—Pressure of Water.]

We will now explain the principle of this WATER PRESS. In the small
diagram, the letters A B represent the bottom of a cylinder which has a
piston fitted in it (P). Into the opposite side a pipe is let in, which
leads from a force-pump D, which is fitted with a valve E, opening
upwards. When the piston in D is pulled up water enters through the
valve; when the piston is forced down the valve shuts, and the water
rushes into the chamber A B. The pressure pushes up the large piston
with a force multiplied as many times as the area of the small piston
is contained in the large one. So if the large one be ten times as
great as the small one, and the latter be forced down with a 10 lb.
pressure, the pressure on the large one will be 100 lbs., and so on.

[Illustration: Fig. 62.—Water Press.]

The accompanying illustration shows the form of the Hydraulic or Bramah
Press. A B C D is a strong frame, F the force-pump worked by means of
a lever fixed at G, and H is the counterprise. E is the stop-cock to
admit the water (fig. 63).

[Illustration: Fig. 63.—Bramah Press.]

The principles of hydrostatics will be easily explained. The Lectures
of M. Aimé Schuster, Professor and Librarian at Metz, have taught us in
a very simple manner the principle of Archimedes, in which it is laid
down that “a body immersed in a liquid loses a portion of its weight
equal to the weight of the liquid displaced by it.” We take a body
of as irregular form as we please; a stone, for example. A thread is
attached to the stone, and it is then placed in a glass of water full
up to the brim. The water overflows; a volume of the liquid equal to
that of the stone runs over. The glass thus partially emptied is then
dried, and placed on the scale of a balance, beneath which we suspend
the stone; equilibrium is established by placing some pieces of lead
in the other scale. We then take a vase full of water, into which we
plunge the stone suspended from the scale, supporting the vase by means
of bricks. The equilibrium is now broken; to re-establish it, it is
necessary to fill up with water the glass placed on the scale; that
is to say, we put back in the glass the weight of a volume of water
precisely equal to that of the stone.

[Illustration: Fig. 64.—Demonstration of the upward pressure of
liquids.]

If it is desired to investigate the principles relating to connected
vessels, springs of water, artesian wells, etc., two funnels, connected
by means of an india-rubber tube of certain length, will serve for the
demonstration; and by placing the first funnel at a higher level, and
pouring in water abundantly, we shall see that it overflows from the
second.

A disc of cardboard and a lamp-glass will be all that is required to
show the upward pressure of liquids. I apply to the opening of the
lamp-glass a round piece of cardboard, which I hold in place by means
of a string; the tube thus closed I plunge into a vessel filled with
water. The piece of cardboard is held by the pressure of the water
upwards. To separate it from the opening it suffices to pour some
water into the tube up to the level of the water outside (fig. 64). The
outer pressure exercised on the disc, as well as the pressure beneath,
is now equal to the weight of a body of water having for its base the
surface of the opening of the tube, its depth being the distance from
the cardboard to the level of the water.

Syringes, pumps, etc., are the effects of atmospheric pressure.
Balloons rise in the air by means of the pressure of gas; a balloon
being a body plunged in gas, is consequently submitted to the same laws
as a body plunged in water.

Boats float because of the pressure of liquid, and water spurts from
a fountain for the same reason. I recollect having read a very useful
application of the principles of fluid pressure.

[Illustration: Fig 65.—Experiment on the convexity of a meniscus.]

A horse was laden with two tubs for carrying a supply of water, and
in the bottom of the tubs a valve was fixed. When the horse entered
the stream the tubs were partly immersed; the water then exercised its
upward pressure, the valve opened, and the tubs slowly filled. When
they were nearly full the horse turned round and came out of the water;
the pressure had ceased.

Thus the action of the water first opened the valve, and then closed it.

The particular phenomena observable in the water level in narrow
spaces, as of a fine glass tube, or the level of two adjoining waves,
capillary phenomena, etc., do not need any special appliance for
demonstration, and it is the same with the convexity or concavity of
meniscuses.

Fig. 65 represents a pretty experiment in connection with these
phenomena. I take a glass, which I fill up to the brim, taking care
that the meniscus be concave, and near it I place a pile of pennies.
I then ask my young friends how many pennies can be thrown into the
glass without the water overflowing. Everyone who is not familiar with
the experiment will answer that it will only be possible to put in one
or two, whereas it is possible to put in a considerable number, even
ten or twelve. As the pennies are carefully and slowly dropped in, the
surface of the liquid will be seen to become more and more convex, and
one is surprised to what an extent this convexity increases before the
water overflows.

The common _syphon_ may be mentioned here. It consists of a bent tube
with limbs of unequal length. We give an illustration of the syphon
(fig. 66). The shorter leg being put into the mixture, the air is
exhausted from the tube at _o_, the aperture at _g_ being closed with
the finger. When the finger is removed the liquid will run out. If the
water were equally high in both legs the pressure of the atmosphere
would hold the fluid in equilibrium, but one leg being longer, the
column of water in it preponderates, and as it falls, the pressure on
the water in the vessel keeps up the supply.

[Illustration: Fig. 66.—The Syphon.]

Apropos of the syphon, we may mention a very simple application of the
principle. Cut off a strip of cloth, and arrange it so that one end
shall remain in a glass of water while the other hangs down, as in the
illustration. In a short time the water from the upper glass will have
passed through the cloth-fibres to the lower one (fig. 67).

This attribute of porous substances is called _capillarity_, and shows
itself by _capillary attraction_ in very fine pores or tubes. The same
phenomenon is exhibited in blotting paper, sugar, wood, sand, and
lamp-wicks, all of which give familiar instances of capillarity. The
cook makes use of this property by using thin paper to absorb grease
from the surface of soups.

Capillarity (referred to on page 25) is the term used to define
capillary force, and is derived from the word _capillus_, a hair; and
so very small bore tubes are called capillary tubes. We know that when
we plunge a glass tube into water the liquid will rise up in it, and
the narrower the tube the higher the water will go; moreover, the water
inside will be higher than at the outside. This is in accordance with a
well-known law of adhesion, which induces concave or convex surfaces[9]
in the liquids in the tubes, according as the tube is wetted with the
liquid or not. For instance, water, as we have said, will be higher in
the tube, and concave in form; but mercury will be depressed below the
outside level, and convex, because mercury will not adhere to glass.
When the force of cohesion to the sides of the tube is more than twice
as great as the adhesion of the particles of the liquid, it will rise
up the sides, and if the forces be reversed, the rounded appearance
will follow. This accounts for the convex appearance, or “meniscus,” in
the column of mercury in a barometer.

Amongst the complicated experiments to demonstrate molecular
attraction, the following is very simple and very pretty:—Take two
small balls of cork, and having placed them in a basin half-filled with
water, let them come close to each other. When they have approached
within a certain distance they will rush together. If you fix one of
them on the blade of your penknife, it will attract the other as a
magnet, so that you can lead it round the basin (fig. 68). But if the
balls of cork are covered with grease they will _repel_ each other,
which fact is accounted for by the form of the _menisques_, which are
convex or concave, according as they are moistened, or preserved from
action of the water by the grease.

[Illustration: Fig. 67.—An improvised syphon.]

This attribute is of great use in the animal and vegetable kingdoms.
The rising of the sap is one instance of the latter.

Experience in hydrostatics can be easily applied to amusing little
experiments. For instance, as regards the syphon, we may make an image
of _Tantalus_ as per illustration (fig. 69). A wooden figure may be cut
in a stooping posture, and placed in the centre of a wide vase, as if
about to drink. If water be poured slowly into the vase it will never
rise to the mouth of the figure, and the unhappy _Tantalus_ will remain
in expectancy. This result is obtained by the aid of a syphon hidden in
the figure, the shorter limb of which is in the chest. The longer limb
descends through a hole in the table, and carries off the water. These
vases are called _vases of Tantalus_.

The principle of the syphon may also be adapted to our domestic
filters. Charcoal, as we know, makes an excellent filter, and if we
have a block of charcoal in one of those filters,—now so common,—we can
fix a tube into it, and clear any water we may require. It sometimes
(in the country) happens that drinking-water may become turgid, and in
such a case the syphon filter will be found useful.

[Illustration: Fig. 68.—Molecular attraction.]

The old “deception” jugs have often puzzled people. We give an
illustration of one, and also a sketch of the “deceptive” portion
(figs. 70 and 71). This deception is very well managed, and will create
much amusement if a jug can be procured; they were fashionable in the
eighteenth century, and previously. A cursory inspection of these
curious utensils will lead one to vote them utterly useless. They are,
however, very quaint, and if not exactly useful are ornamental. They
are so constructed, that if an inexperienced person wish to pour out
the wine or water contained in them, the liquid will run out through
the holes cut in the jug.

To use them with safety it is necessary to put the spout A in
one’s mouth, and close the opening B with the finger, and then
by drawing in the breath, cause the water to mount to the lips by the
tube which runs around the jug. The specimens herein delineated have
been copied from some now existent in the museum of the Sèvres china
manufactory.

The _Buoyancy of Water_ is a very interesting subject, and a great
deal may be written respecting it. The swimmer will tell us that it is
easier to float in salt water than in fresh. He knows by experience
_how difficult it is to sink_ in the sea; and yet hundreds of people
are drowned in the water, which, if they permitted it to exercise its
power of buoyancy, would help to save life.

[Illustration: Fig. 69.—Vase of Tantalus.]

The sea-water holds a considerable quantity of salt in solution, and
this adds to its resistance, or floating power. It is heavier than
fresh water, and the Dead Sea is so salt that a man cannot possibly
sink in it. This means that the man’s body, bulk for bulk, is much
lighter than the water of the Dead Sea. A man will sink in fresh, or
ordinary salt water if the air in his lungs be exhausted, because
without the air he is much heavier than water, bulk for bulk. So if
anything is weighed in water, it apparently loses in weight exactly
equal to its own bulk of water.

Water is the means by which the _Specific Gravity_ of liquids or solids
is found, and by it we can determine the relative densities of matter
in proportion. Air is the standard for gases and vapours. Let us
examine this, and see what is meant by SPECIFIC GRAVITY.

We have already mentioned the difference existing between two equal
volumes of different substances, and their weight, which proves that
they may contain a different number of atoms in the same space. We also
know, from the principle of Archimedes, that _if a body be immersed in
a fluid, a portion of its weight will be sustained by the fluid equal
to the weight of the fluid displaced_.

[Illustration: Fig. 70.—Deception jugs of old pattern.]

[This theorem is easily proved by filling a bucket with water, and
moving it about in water, when it will be easy to lift; and likewise
the human body may be easily sustained in water by a finger under the
chin.]

The manner in which Archimedes discovered the displacement of liquids
is well known, but is always interesting. King Hiero, of Syracuse,
ordered a crown of gold to be made, and when it had been completed
and delivered to His Majesty, he had his doubts about the honesty of
the goldsmith, and called to Archimedes to tell him whether or not
the crown was of gold, pure and simple. Archimedes was puzzled, and
went home deep in thought. Still considering the problem he went to
the bath, and in his abstraction filled it to the brim. Stepping in he
spilt a considerable quantity of water, and at once the idea struck
him that any body put into water would displace its own weight of the
liquid. He did not wait to dress, but ran half-naked to the palace,
crying out, “Eureka, Eureka! I have found it, I have found it! “What
had he found?—He had solved the problem.

[Illustration: Fig. 71.—Section of jug.]

He got a lump of gold the same weight as the crown, and immersed it
in water. He found it weighed nineteen times as much as its own bulk
of water. But when he weighed the kings crown he found it displaced
more water than the pure gold had done, and consequently it had been
adulterated by a lighter metal. He assumed that the alloy was silver,
and by immersing lumps of silver and gold of equal weight with the
crown, and weighing the water that overflowed from each dip, he was
able to tell the king how far he had been cheated by the goldsmith.

It is by this method now that we can ascertain the specific gravity of
bodies. One cubic inch of water weighs about half an ounce (or to be
exact, 252½ grains). Take a piece of lead and weigh it in air; it
weighs, say, eleven ounces. Then weigh it in a vase of water, and it
will be only ten ounces in weight. So lead is eleven times heavier than
water, or eleven ounces of lead occupy the same space as one ounce of
water.

[Illustration: Fig. 72.—Weighing metal in water.]

[The heavier a fluid is, or the greater its density, the greater will
be the weight it will support. Therefore we can ascertain the purity
or otherwise of certain liquids by using hydrometers, etc., which will
float higher or lower in different liquids, and being gauged at the
standard of purity, we can ascertain (for instance) how much water is
in the milk when supplied from the dairy.]

[Illustration: Fig. 73.—Hydrometer.]

But to return to SPECIFIC GRAVITY, which means the
“Comparative density of any substance relatively to water,” or as
Professor Huxley says, “The weight of a volume of any liquid or solid
in proportion to the weight of the same volume of water, at a known
temperature and pressure.”

Water, therefore, is taken as the unit; so anything whose equal
volume under the same circumstances is twice as heavy as the water,
is declared to have its specific gravity 2; if three-and-a-half times
it is 3·5, and so on. We append a few examples; so we see that things
which possess a higher specific gravity than water sink, which comes to
the same thing as saying they are heavier than water, and _vice versâ_.

To find the specific gravity of any solid body proceed as above, in the
experiment of the lead. By weighing the substance in and out of water
we find the weight of the water displaced; that is, the first weight
less the second. Divide the weight in air by the remainder, and we
shall find the specific gravity of the substance.

[Illustration: Fig. 74.—Over-shot wheel of mill.]

The following is a table of specific gravities of some very different
substances, taking water as the unit.

  +----------+----------++----------+----------++------------+----------+
  |Substance.| Specific ||Substance.| Specific ||Substance.  | Specific |
  |          | Gravity. ||          | Gravity. ||            | Gravity. |
  +----------+----------++----------+----------++------------+----------+
  | Platinum |   21·5   || Iron     |   7·79   || Water      |   1·000  |
  | Gold     |   19·5   || Tin      |   7·29   || Sea Water  |   1·026  |
  | Mercury  |   13·59  || Granite  |   2·62   || Rain Water |   1·001  |
  | Lead     |   11·45  || Oak Wood |   0·77   || Ice        |    ·916  |
  | Silver   |   10·50  || Cork     |   0·24   || Ether      |   0·723  |
  | Copper   |    8·96  || Milk     |   1·032  || Alcohol    |   0·793  |
  +----------+----------++----------+----------++------------+----------+

But we have by no means exhausted the uses of water. Hydrodynamics,
which is the alternative term for hydraulics, includes the
consideration of many forms of water-wheels, most of which, as
mill-wheels, are under-shot, or over-shot accordingly as the water
passes horizontally over the floats, or acts beneath them. These wheels
are used in relation to the fall of water. If there is plenty of water
and a slight fall, the under-shot wheel is used. If there is a good
fall less water will suffice, as the weight and momentum of the falling
liquid upon the paddles will turn the wheel. Here is the Persian
water-wheel, used for irrigation (fig. 75). The Archimedian Screw,
called after its inventor, was one of the earliest modes of raising
water. It consists of a cylinder somewhat inclined, and a tube bent
like a screw within it. By turning the handle of the screw the water is
drawn up and flows out from the top.

[Illustration: Fig. 75.—Irrigation wheel in Egypt.]

The Water Ram is a machine used for raising water to a great height by
means of the momentum of falling water.

The Hydraulic Lift is familiar to us all, as it acts in our hotels, and
we need only mention these appliances here; full descriptions will be
found in Cyclopædias.

We have by no means exhausted the subject of Water in this chapter. Far
from it. But when we come to Chemistry and Physical Geography we shall
have more to tell, and our remarks as to the application of science to
Domestic Economy, in accordance with our plan, will also lead us up to
some of the uses of water. So for the present we will take our leave
of water in a liquid form, and meet it again under the application of
Heat, which subject will take us to Ice and Steam,—two very different
conditions of water.


FOOTNOTES:

[9] The curved surface of a column of liquid is termed a “meniscus,”
from the Greek word _meniskos_, meaning “a little lens.”



CHAPTER VII.

 HEAT—WHAT IT IS—THEORY OF HEAT—THE THERMOMETER—EXPANSION BY
 HEAT—EBULLITION AND DISTILLATION—LATENT HEAT—SPECIFIC HEAT.


What is Heat?—We will consider this question, and endeavour to explain
it before we speak of its effects on water and other matter.

Heat is now believed to be the effects of the rapid motion of all
the particles of a body. It is quite certain that a heated body is
no heavier than the same body before it was made “hot,” so the heat
could not have gone into it, nor does the “heat” leave it when it has
become what we call “cold,” which is a relative term. Heat is therefore
believed to be a vibratory motion, or the effects of very rapid motion
of matter.

The Science of Heat, as we may term it, is only in its infancy, or
certainly has scarcely come of age. Formerly heat was considered a
chemical agent, and was termed caloric, but now heat is found to
be motion, which affects our nerves of feeling and sight; and, as
Professor Stewart tells us, “a heated body gives a series of blows to
the medium around it; and although these blows do not affect the ear,
they affect the eye, and give us a sense of light.”

Although it is only within a comparatively few years that heat has been
really looked upon as other than matter, many ancient philosophers
regarded it as merely a _quality_ of matter. They thought it the active
principle of the universe. Epicurus declared that heat was an effluxion
of minute spherical particles possessing rapid motion, and Lucretius
maintained that the sun’s light and heat are the result of motion of
primary particles. Fire was worshipped as the active agent of the
universe, and Prometheus was fabled to have stolen fire from heaven to
vivify mankind. The views of the ancients were more or less adopted in
the Middle Ages; but John Locke recognized the theory of heat being a
motion of matter. He says: “What in our sensation is _heat_, in the
object is nothing but _motion_.”

Gradually two theories arose concerning heat;—one, the Material
theory—the theory of Caloric or Phlogiston; the other, the Kinetic
theory. Before the beginning of the present century the former theory
was generally accepted, and the development of heat was accounted for
by asserting that the friction or percussion altered the capacity
for heat of the substances acted upon. The heat was squeezed out by
the hammer, and the amount of heat in the world was regarded as a
certain quantity, which passed from one body to another, and that some
substances contained, or could “store away,” more of the material
called heat than other substances. Heat was the material of fire—the
principle of it, or _materia ignis_; and by these theories Heat,
or Caloric, was gradually adopted as a separate material agent—an
invisible and subtle matter producing certain phenomena when liberated.

So the two theories concerning heat arose at the end of the last
century. One, as we have said, is known as the Material, the other as
the Kinetic theory. The latter is the theory of motion, so called from
the Greek _kinesis_ (motion), or sometimes known as the Dynamic theory
of heat, from _dunamis_ (force); or again as Thermo-dynamics.

But any possibility of producing a new supply of heat was denied by the
materialists. They knew that some bodies possessed a greater capacity
for heat than others; but Count Rumford, at Munich, in 1797, astonished
an audience by making water boil without any fire! He had observed the
great extent to which a cannon became heated while being bored in the
gun factory, and influenced by those who maintained the material theory
of heat, paid great attention to the evolution of heat. He accordingly
endeavoured to produce heat by friction, and by means of horse power
he caused a steel borer to work upon a cylinder of metal. The shavings
were permitted to drop into a pan of water at 60° Fahrenheit. In an
hour after the commencement of the operation the temperature of the
water had risen to 107°: in another half-hour the heat of it was up
to 142°: and in two hours had measured 170°. Upon this he says: “It
is hardly necessary to add that anything which any insulated body or
system of bodies can continue to furnish without limitation cannot
possibly be a material substance, and it appears to me to be extremely
difficult, if not quite impossible, to form any distinct idea of
anything capable of being excited and communicated in these experiments
except by _motion_.”

A few years later Sir Humphrey Davy made his conclusive experiments,
and the Material theory of heat received its death-blow.

Sir Humphrey Davy—referring to the fact that water at a freezing
temperature has “more heat in it” (as it was believed) than ice at the
same temperature—said: “If I, by friction, liquify ice, a substance
will be produced which contains a far greater absolute amount of heat
than ice. In this case it cannot reasonably be affirmed that I merely
render _sensible_ heat which had been previously _insensible_ in the
frozen mass. Liquification will conclusively prove the _generation_ of
heat.

This reasoning could not be doubted. Sir Humphrey Davy made the
experiment. He rubbed together two pieces of ice in the air, and in
a vacuum surrounded by a freezing mixture. The ice became liquified,
and so the generation of heat by “mechanical means” was proved. Its
immateriality was demonstrated, but the Material theory was not even
then abandoned by its adherents.

So things continued, until in 1842-3, Doctor Julius Meyer, of
Heilbronn, and Doctor Joule, of Manchester, separately, and by
different means, arrived at the conclusion that a certain definite
amount of mechanical work corresponds to a certain definite amount
of Heat, and _vice versâ_. Thus was a great support afforded to
the Dynamic theory. This fact Doctor Joule communicated to the
_Philosophical Magazine_ in 1843, and the conclusions he came to were—

 1. “That the quantity of heat produced by the friction of bodies,
 whether solid or liquid, is always in proportion to the force expended;

 2. “That the quantity of heat capable of increasing the temperature of
 a pound of water (weighed _in vacuo_ and taken at between 55° and 60°
 Fahr.) by 1° Fahr., requires for its evolution the expenditure of a
 mechanical force represented by the fall of 772 lbs. through the space
 of one foot.”

[Illustration: Fig. 76.—Melting a piece of tin on a card.]

This is the “mechanical equivalent of heat.” The first paper written
by Mr. Joule demonstrated that the temperature of water rises when
forced through narrow tubes; and to heat it one degree, the force of
770 foot pounds was necessary, which means that the 1 lb. of water
falling 770 feet, got hotter by one degree when it reached the earth.
He subsequently arrived at the more exact conclusions quoted above.

So heat is now known to be a series of vibrations, or vibratory
motions, as sound vibrations, which we cannot hear nor see, but the
effects of which are known to us as light and heat.

In considering heat we must put aside the idea of warmth and cold, for
they are only different degrees of heat, not the absence of it.

The study of heat can be briefly undertaken without any complicated
apparatus. If we desire a proof of the great conducting power of
metals, let us place a fine piece of muslin tightly stretched over
a lump of polished metal. On the muslin we put a burning ember, and
excite combustion by blowing on it; the muslin is not burned in the
least, the heat being entirely absorbed by the metal, which draws
it through the material into itself. Fig. 76 represents a similar
experiment: it consists of melting some tin on a playing card, held
over the flame of a spirit lamp. The metal becomes completely melted
without the card being burnt. It is through a similar effect that
metals appear cold to us when we take them in our hands; by their
conductibility they remove the heat from our hands, and give us the
peculiar impression which we do not experience when in contact with
substances that are bad conductors, such as wood, woollen materials,
etc.

[Illustration: Fig. 77.—Boiling water in a paper case.]

Fig. 77 shows the method of boiling water in paper. We make a small
paper box, such as those made by school-boys, and suspend it by four
threads to a piece of wood held horizontally at a suitable height. We
fill this improvised vessel with water, and place it over the flame of
a spirit lamp. The paper is not burnt, because the water absorbs all
the heat into itself. After a few minutes the water begins to boil,
sending forth clouds of steam, but the paper remains intact. It is well
to perform this operation over a plate, in case of accident, as the
water may be spilt. We may also make use of an egg-shell as a little
vessel in which to heat the water, by resting it on a wire ring over
the flame of the spirit lamp.

[Illustration: Fig. 78.—Experiment on the regelation of ice.]

Fig. 78 shows the arrangement of a very remarkable experiment, but
little known, on the refreezing of ice. A block of ice is placed on the
edge of two iron chairs, and is encircled by a piece of wire, to which
is suspended the weight of say five pounds. The wire penetrates slowly,
and in about an hour’s time has passed completely through the lump
of ice, and the weight, with the piece of wire, falls to the ground.
What happens then to the block of ice?—You imagine, doubtless, that
it is cut in two. No such thing; it is intact, and in a single lump
as it was previous to the experiment. In proportion as the wire was
sunk through the mass, the slit has been closed again by refreezing.
Ice or snow during the winter may serve for a number of experiments
relating to heat. If we wish to demonstrate the influence of colours
on radiation, we take two pieces of cloth of the same size,—one white,
and the other black,—and place them both on the snow, if possible,
when there is a gleam of sunlight. In a short time it will be found
that the snow underneath the black cloth has melted to a much greater
extent than that beneath the white cloth, because black absorbs heat
more than white, which, on the contrary, has a tendency to reflect it.
We perceive very plainly the difference in temperature by touching the
two cloths. The white cloth feels cold in comparison with the black
cloth.

It is hardly necessary to point out experiments on the expansion of
bodies. They can be performed in a number of different ways; by placing
water in a narrow-necked bottle, and warming it over the fire, we can
ascertain the expansion of liquids under the influence of heat. We may
in this way construct a complete thermometer.

We may now consider the _Sources of Heat_, or causes of its
development, which are various, and in many cases apparent. The first
great source is the Sun, and it has been calculated that the heat
received by the earth in one year is sufficient to melt an envelope
of ice surrounding it one hundred and five feet thick. Of course the
heat at the surface of the sun is enormously greater than this, about
one-half being absorbed in the atmosphere before it reaches us at all.
In fact, it is impossible to give you an idea of the enormous heat
given out by the sun to the earth (which is a _very_ small fraction
indeed of the whole), stars, and planets, all of which give out heat.
We know that heat is stored in the earth, and that it is in a very
active condition we can perceive from the hot springs, lava, and flame
which are continually erupting from the earth in various places. These
sources of heat are beyond our control.

But apart from the extra- and intra-terrestrial sources of heat there
are mechanical causes for its generation upon our globe, such as
friction, percussion, or compression. The savage or the woodman can
procure heat and fire by rubbing a pointed stick in a grooved log. The
wooden “breaks” of a locomotive are often set on fire by friction of
the wheels, so they require grease, and the wheels on the rails will
develop heat and sparks. Our matches, and many other common instances
of the generation of heat (and fire) by friction, will occur to every
reader. Water may be heated by shaking it in a bottle, taking care to
wrap something round it to keep the warmth of the hand from the glass.
By percussion, such as hammering a nail or piece of iron, the solid
bar may be made “red-hot”; and when cannon are bored at Woolwich the
shavings of steel are too hot to hold even if soap-and-water has been
playing upon the boring-machine.

The production of heat by chemical action is termed _combustion_, and
this is the means by which all artificial heat for our daily wants is
supplied. We can also produce heat by electricity. A familiar and not
always pleasant instance of this is seen in the flash of lightning
which will fuse metals, and experiment may do the same upon a smaller
scale. These are, in brief, the Sources of Heat, and we may speak of
its effects.

We may take it for granted that no matter from what source heat is
derived, it exhibits the same phenomena in its relation to objects. One
of the most usual of these phenomena is expansion. Let us take water,
and see the effect of heat upon it.

We know that a certain weight of water under the same conditions has
always the same volume; and although the attributes of the liquid
vary under different circumstances, under the _same_ conditions its
properties are exactly the same. Now, water expands very much when
under the influence of heat, like all liquids; solids and gases also
expand upon the application of heat.

We can easily establish these statements. A metallic ring when heated
is larger than when cool. A small quantity of air in a bladder when
heated will fill the bladder, and water will boil over the vessel, or
expand into steam, and perhaps burst the boiler. So expansion is the
tendency of what we term heat.

We make use of this quality of heat in the thermometer, by which we
can measure the temperature not only of liquids or solids, but of
the atmosphere. The reading of the thermometer varies in different
countries, for the degrees are differently marked, but the construction
of the instrument is the same. It is called thermometer from two
Greek words signifying the measure of heat. It is a notable fact that
Castelli, writing in 1638, says to Ferdinand Cæsarina: “I remembered
an experiment which Signor Galileo had shown me more than thirty-five
years ago. He took a glass bottle about the size of a hen’s egg, the
neck of which was two palms long, and as narrow as a straw. Having
well heated the bulb in his hands, he placed its mouth in a vessel
containing water, and withdrawing the heat of his hand from the bulb,
the water instantly rose in the neck more than a palm above the level
of the water in the vessel.”

Here, then, we have an air-thermometer, but as it was affected by the
_pressure_ as well as the temperature of the atmosphere, it could not
be relied upon as a “measurer of heat.” Until Torricelli propounded
the principle of the barometer, this “weather-glass” of Galileo was
used, for the philosopher divided the stem into divisions, and the
air-thermometer served the purpose of our modern instruments.

The actual inventor of the thermometer is not known. It has been
attributed to Galileo, to Drebbel, and to Robert Fludd. There is little
doubt, however, that Galileo and Drebbel were both acquainted with it,
but whether either claimed the honour of the invention, whether they
discovered it independently, or together, we cannot say. Sanctorio, of
Padua, and Drebbel have also been credited with the invention. We may
add that the spirit thermometer was invented in 1655-1656. It was a
rough form of our present thermometer, and roughly graduated. But it
was hermetically closed to the air, and a great improvement on the old
“weather-glass. Edmond Halley introduced mercury as the liquid for the
instrument in 1680. Otto von Guerike first suggested the freezing point
of water as the lowest limit, and Renaldini, in 1694, proposed that the
boiling and freezing points of water should be the limit of the scale.

Let us now explain the construction and varied markings of the three
kinds of thermometers in use. By noting the differences between the
scales every reader will be able to read the records from foreign
countries noted upon the Centigrade and Réaumur instruments, which are
all based upon the theory that heat expands liquids.

[We used to hear the expression, “Heat expands, and cold contracts,”
but we trust that all our readers have now learnt that there is no
such thing as _cold_. It is only a negative term. We feel things cold
because they extract some warmth from our fingers, not because the
substances have no heat.]

Thermometers are made of very fine bore glass tubes. One end has a
bowl, or bulb, the other is at first open. By heating the bowl the air
in the tube is driven away by the open end, which is quickly dipped in
a bowl of mercury. The mercury will then occupy a certain space in the
tube; and if it be heated till the liquid boils, all the air will be
driven out by the mercurial vapour. By once again dipping the tube in
the quicksilver the glass will be filled. Then, before it cools, close
the open end of the tube, and the thermometer is so far made. Having
now caught our thermometer we must proceed to mark it, which is an easy
process. By plunging the mercury into pounded melting ice we can get
the freezing point, and boiling water will give us the boiling point.
The intermediate scale can be then indicated.

If mercury and glass expanded equally there would be no rise in the
latter. Extreme delicacy of the thermometer can be arrived at by using
a very fine tube, particularly if it be also flat.

The freezing point in Fahrenheit’s scale is 32°; in the Centigrade
it is 0°, and the boiling point 100°. This was the scale adopted
by Celsius, a Swede, and is much used. Réaumur called the freezing
point 0°, and the boiling point 80°. There is another scale, almost
obsolete,—that of Delisle, who called boiling point zero, and freezing
point 150°.

There is no difficulty in converting degrees on one scale into degrees
on the other. Fahrenheit made his zero at the greatest cold he could
get; viz., snow and salt. The freezing point of water is 32° above his
zero. Therefore 212-32 gives 180° the difference between the freezing
and boiling points of water. So 180° Fahr. corresponds to 100° Cent.,
and to 80° Réaumur, reckoning from freezing point.

[Illustration: Fig. 79.—Thermometer.]

The following tables will explain the differences:—

TABLE I.

  1° Fahr.  = 0·55° Cent., or 0·44° Réaumur.
  1° Cent.  =  ·80° Réaumur, or 1·80° Fahr.
  1° Réaumur = 1·25° Cent., or 2·25° Fahr.

TABLE II.

  |---------------------------+-------+-------+---------|
  |                           | Fahr. | Cent. | Réaumur.|
  |---------------------------+-------+-------+---------|
  | Boiling point             |  212  |  100  |   80    |
  |                           |  194  |   90  |   72    |
  |                           |  176  |   80  |   64    |
  |                           |  158  |   70  |   56    |
  |                           |  140  |   60  |   48    |
  |                           |  122  |   50  |   40    |
  |                           |  104  |   40  |   32    |
  |                           |   86  |   30  |   24    |
  |                           |   68  |   20  |   16    |
  |                           |   50  |   10  |    8    |
  | Freezing point of water   |   32  |    0  |    0    |
  |                           |   14  |  -10  |   -8    |
  |                           |   -4  |  -20  |  -16    |
  | Freezing point of Mercury |  -40  |  -40  |  -32    |
  |---------------------------+-------+-------+---------|

Alcohol is used in thermometers in very cold districts, as it does not
freeze even at a temperature of -132° Fahr.

We have now explained the way in which we can measure heat by the
expansion of mercury in a tube. We can also find out that solids and
gases expand also. Engineers always make allowances for the effects
of winter and summer weather when building bridges; in summer the
bridge gets longer, and unless due provision were made it would become
strained and weakened. So there are compensating girders, and the
structure remains safe.

The effects of expansion by heat are very great and very destructive at
times. Instances of boilers bursting will occur to every reader. It is
very important to be able to ascertain the extent to which solid bodies
will expand. Such calculations have been made, and are in daily use.

We can crack a tumbler by pouring hot water into it, or by placing it
on the “hob.” A few minutes’ consideration will assure us that the
lower particles of the glass expanded before the rest, and cracked our
tumbler. A gradual heating, particularly if the glass be thin, will
ensure safety. Thick glass will crack sooner than thin.

Again, many people at railway stations have asked us, “Why don’t they
join the rails together on this line?” We reply that if every length
of rail were tightly fixed against its neighbour, the whole railway
would be displaced. The iron expands and joins up close in hot weather.
In wet weather, also, the wooden pegs and the sleepers swell with
moisture, and get tightened up. Everyone knows how much more smoothly
a train travels in warm, wet weather. This is due to the expansion of
the iron and the swelling of the sleepers and pegs in the “chairs.”
A railway 400 miles long expands 338 yards in summer,—that is the
difference in length between the laid railroad in summer and in winter.

This can be proved. Iron expands 0·001235 of its length for every
180° Fahr. Divided by 180 it gives us the expansion for 1°, which is
0·00000686, taking the difference of winter and summer at 70° Fahr.
Multiply these together, and the result (0·00048620 of its length)
by the number of yards in 400 miles, and we find our answer 338
yards. Expansion acts in solids and most liquids by the destruction
of cohesion between the particles. Gases, however, having much less
cohesion amid the particles, will expand far more under a given heat
than either solids or liquids, and liquids expand more than solids for
the same reason, and more rapidly at a high temperature than at a low
one.

We have spoken of expansion. We may give an instance in which the
subsequent contraction of heated metal is useful. Walls sometimes
get out of the perpendicular, and require pulling together. No force
which can be conveniently applied would accomplish this so well as the
cooling force due to the potential energy of iron. Rods are passed
through the walls and braced up by nuts. The rods are then heated, and
as they cool they contract and pull the walls with them.

When glass is suddenly cooled, the inner skin, as it were, presses with
great force against the cooled surface, but as it is quite tight no
explosion can follow. But break the tail, or scratch it with a diamond,
and the strain is taken off. The glass drop crumbles with the effect
of the explosion, as in the cases of Prince Rupert’s drops, and the
Bologna flasks; the continuity is broken, and pulverization results.

But a very curious exception to the general laws of expansion is
noticed in the case of nearly freezing water. We know water expands
by heat, at first gradually, and then to an enormous extent in steam.
But when cooling water, instead of getting more and more contracted,
only contracts down to 39·2° Fahr., it then begins to expand, and at
the moment it freezes into ice it expands very much—about one-twelfth
of its volume, but according to Professor Huxley it weighs exactly the
same, and the steam produced from that given quantity of water will
weigh just exactly what the water and the ice produced by it weigh
individually. At 39·2° Fahr. water is at its maximum density, or in
other words, a vessel of a certain size will hold more water when it
is at 39° Fahr. than at any other time. Whether the water be heated or
cooled at this temperature, it _expands_ to the boiling or freezing
point when it becomes steam or ice, as the case may be.

Water, when heated, is lighter than cold water. You can prove this in
filling a bath from two taps of hot and cold water at the same time.
The cold falls to the bottom, and if you do not stir up the water when
mixed you will have a hot surface and a cold foundation. The heat
increases the volume of water, it becomes lighter, and comes uppermost.

Steam and Water and Ice are all the same things under different
conditions, although to the eye they are so different. They are alike
inasmuch as a given weight of water will weigh as much when converted
into ice or developed into steam. The half ounce of water will weigh
half an ounce as ice or as steam, but the volume or bulk will vary
greatly, as will be understood when we state that one cubic inch of
water will produce 1,700 cubic inches of steam, and 1-1/11 cubic inch
of ice; but at the same time each will yield, when decomposed, just the
same amount of oxygen and hydrogen.

Let us now consider the _Effects of Heat upon Water_. We have all seen
the vapour that hangs above a locomotive engine. We call it “steam.” It
is not pure steam, for steam is really invisible. The visible vapour is
steam on its way to become water again. On a very hot dry day we cannot
distinguish the vapour at all.

The first effect of heat upon water is to expand it; and as the heat
is applied we know that the water continues to expand and bubble up;
and at last, when the temperature is as high as 212°, we say water
“boils”—that is, at that heat it begins to pass away in vapour, and you
will find that the temperature of the steam is the same as the boiling
water. While undergoing this transformation, the water increases in
volume to 1,700 times its original bulk, although it will weigh the
same as the water. So steam has 1,700 less specific gravity than water.

It is perhaps scarcely necessary to remind our readers that water,
when heated, assumes tremendous force. Air likewise expands with great
violence, and the vessels containing either steam or air frequently
burst, with destructive effects. Solid bodies also expand when heated,
and the most useful and accurate observations have been made, so that
the temperatures at which solid bodies expand are now exactly known.
Air also expands by heat.

While speaking of Expansion by Heat, we may remark that a rapid
movement is imparted to the air by Heat. In any ordinary room the air
below is cool, while if we mount a ladder to hang up a picture, for
instance, we shall find the air quite hot near the ceiling. This is
quite in keeping with the effects of heat upon water. The hot particles
rise to the top in a vessel, and thus a motion is conveyed to the
water. So in our rooms. The heated air rushes up the chimney and causes
a draught, and this produces motion, as we have seen by fig. 39, in
which the cardboard spiral was set in motion by heated air. A balloon
will ascend, because it is filled with heated air or gas; and we all
have seen the paper balloons which will ascend if a sponge containing
spirit of wine be set on fire underneath them.

Winds are also only currents of air produced by unequal temperature in
different places. The heated air ascends, and the colder fluid rushes
in sometimes with great velocity to fill the space. “Land” and “sea”
breezes are constant; the cool air blows in from the sea during the
day, and as the land cools more rapidly at night, the breeze passes
out again. When we touch upon _Meteorology_, we will have more to say
respecting Air Currents and the various Atmospheric Phenomena.

We know that water can be made to boil by heat, but it is not perhaps
generally known that it will apparently boil by _cold_, and the
experiment may thus be made:—A flask half-full of water is maintained
at ebullition for some minutes. It is removed from the source of heat,
corked, inverted, and placed in one of the rings of a retort stand. If
cold water is poured on the upturned bottom of the flask, the fluid
will start into violent ebullition. The upper portion of the flask is
filled with steam, which maintains a certain pressure on the water.
By cooling the upper portion of the flask some of this is condensed,
and the pressure reduced. The temperature at which water boils varies
with the pressure. When it is reduced, water boils at a lower heat. By
pouring the cold water over the flask we condense the steam so that the
water is hot enough to boil at the reduced pressure. To assert that
water boils by the application of cold is a chemical sophism.

_Ebullition_ and _Evaporation_ may be now considered, and these are the
two principal modes by which liquids assume the gaseous condition. The
difference is, when water boils we term it ebullition (from the Latin
_ebullio_, I boil); evaporation means vapour given out by water not
boiling (from _evaporo_, I disperse in vapour).

There are two operations based upon the properties which bodies possess
of assuming the form of vapour under the influence of heat, which
are called _Distillation_ and _Sublimation_. These we will consider
presently.

Ebullition then means a bubbling up or boiling; and when water is
heated in an open vessel two forces oppose its conversion into vapour;
viz., its own cohesive force and atmospheric pressure. At length, at
212° Fahr., the particles of water have gained by heat a force greater
than the opposing forces; bubbles of vapour rise up from the bottom and
go off in vapour. This is _ebullition_, and at that point the tension
of the vapour is equal to the pressure of the atmosphere, for if not,
the bubbles would not form. All this time of boiling, notwithstanding
any increase of heat, the thermometer will not rise above 212° (Fahr.),
for all the heat is employed in turning the water to steam.

We have said the ebullition takes place at 212° Fahr. (or 100° C.),
but that is only at a certain level. If we ascend 600 feet high we
shall find that water will boil at a less temperature; and on the top
of a mountain (say Mont Blanc) water will boil at 185° Fahr.; so at
an elevation of three miles water boils at a temperature less by 27°
Fahr. An increase of pressure similarly will raise the boiling point
of water. The heights of mountains are often ascertained by noticing
the boiling point of water on their summits, the general rule being a
fall of one degree for every 530 feet elevation at medium altitudes. We
append a few instances taken at random:—

  +--------------------+------------------+-------------+---------------+
  |       Place.       |Height above level|  Barometer  | Boiling point |
  |                    | of the sea—Feet. | mean height.|of water, Fahr.|
  +--------------------+------------------+-------------+---------------+
  | Quito              |      9,541       |    20·75    |     194·2     |
  | Mexico             |      7,471       |    22·52    |     198·1     |
  | St. Gothard        |      6,808       |    23·07    |     199·2     |
  | Garonne (Pyrenees) |      4,738       |    24·96    |     203·0     |
  | Geneva             |      1,221       |    28·54    |     209·5     |
  | Paris (1st floor)  |        213       |    29·69    |     211·5     |
  | Sea level          |          0       |    30·00    |     212·0     |
  +--------------------+------------------+-------------+---------------+

[The difference for a degree depends upon the height, varying between
510 and 590 feet, according to the elevation reached. The approximate
height of a mountain can be found by multiplying 530 by the number
of degrees between the boiling point and 212°. In some very elevated
regions travellers have even failed to boil potatoes.]

The boiling point of liquid may be altered by mixing some substance
with it; and although such a substance as sawdust would not alter the
boiling point of water, yet if the foreign matter be dissolved in the
liquid it will alter the boiling point. Even the air dissolved in
liquids alters their boiling point, and water freed from air will not
boil till it is raised to a temperature much higher than 212° Fahr.
Water will boil at a higher temperature in a glass vessel than in
metal, because there is a greater attraction between water and glass.

We said above that an increase of pressure will raise the boiling point
of water. Under the pressure of one atmosphere—that is, when there
is a pressure of 15 lbs. on the square inch—water boils at 212°. But
under a pressure of two atmospheres, the boiling point rises to 234°,
and of four atmospheres, 294°. So we see by increasing the pressure
the water may be almost indefinitely heated, and it will not boil. We
can understand that in a very deep vessel the layer of water at the
bottom has to sustain the pressure of the water in addition to the
weight of the atmosphere above it. The pressure of thirty-four feet of
water is equal to the atmospheric pressure of 15 lbs. on the square
inch, and thus at such a distance water must be heated to 234° before
it will boil. Professor Bunsen founded his Theory of the Geysers upon
this fact, for he maintained that water falling into the earth lost
much air, and required with the super-incumbent pressure a very high
temperature to boil it. When it did boil it generated steam so suddenly
that it exploded upwards, throwing up vapour and the water with it, as
water poured into a very hot basin will do.

_Evaporation_ may now be considered, and is distinguished from
Ebullition by the production of vapour on the _surface_ of liquids, the
latter term signifying the formation of vapour in _the body_ of the
liquid. Evaporation takes place at all temperatures, and from every
liquid surface exposed to the air. We know what we call a “drying
wind.” The air in fresh layers continually passing over the wet ground,
takes up the moisture; like the east wind, for instance, which has
great capabilities of that nature. Damp air can only take up a certain
quantity, and when it contains as much water as corresponds to the
temperature it can take no more, and is “saturated with moisture”;
then evaporation ceases. Heat is a great cause of evaporation, and
the greater the surface the more rapid the process, and in a vacuum
more readily than in atmospheric air. Evaporation is resorted to
very commonly to produce coolness; for instance, the universal fan,
by increasing evaporation from a heated skin, generates a feeling
of coolness; and we know the vaporization of ether will freeze into
insensibility. When a fluid evaporates we can tell that the heat passes
away at the same time, for we cool water in porous jars, which permit
some of it to pass off in vapour, the remainder being cooled.

Sir John Leslie invented a method of freezing water by rapid
evaporation on sulphuric acid under the receiver of an air-pump, and
water has been frozen even on a _hot plate_ by these means. By pouring
sulphurous acid and water on this plate, the acid evaporates so quickly
that it produces sufficient cold to freeze the water it quitted into
solid ice.

We leave the phenomena of clouds and watery vapour in the atmosphere
for consideration on another opportunity, under the head of
_Meteorology_, _Rain_, etc.

[Illustration: Fig. 80.—Apparatus for freezing carafes of water.]

An experiment is often performed by which water is frozen in a vacuum.
By putting a saucer full of water under the receiver of an air-pump
it will first boil, and then become a solid mass of ice. It is not
difficult to understand the cause of this. The water boils as soon as
the air is removed; but in order to pass from the liquid to the gaseous
state without the assistance of exterior heat, it gives out heat to the
surroundings, and in so doing becomes ice itself. This fact Mr. Carré
has made use of in the apparatus shown above (fig. 80). A small pump
creates a vacuum in the water bottles, and ice is formed in them.

This apparatus might easily be adopted in country houses, and in places
where ice is difficult to procure in summer. The only inconvenience
attending it is the employment of sulphuric acid, of which a
considerable quantity is used to absorb the vapour from the water, as
already referred to. If proper precautions are taken, however, there
will be no danger in using the apparatus.

The mode of proceeding is as follows:—The bottle full of water is
joined to the air-pump by a tube, and after a few strokes the water
is seen in ebullition. The vapour thus disengaged traverses an
intermediate reservoir filled with sulphuric acid, which absorbs it,
and immediately condenses it, producing intense cold. In the centre of
the liquid remaining in the carafe some needles of ice will be seen,
which grow rapidly, and after a few more strokes of the pump the water
will be found transformed into a mass of ice. This is very easy of
accomplishment, and in less than a minute the carafe full of water will
be found frozen.

The problem for the truly economical formation of ice by artificial
means is one of those which have occupied chemists for a long time, but
hitherto, notwithstanding all their efforts, no satisfactory conclusion
has been arrived at. Nearly every arrangement possesses some drawback
to its complete success, which greatly increases the cost of the ice,
and causes inconvenience in its production. The usual mode in large
towns is to collect the ice, in houses constructed for the purpose,
during the winter, and this simple method is also the best, so far as
at present has been ascertained.

[Illustration: Fig. 81.—Retort and Receiver.]

In connection with vaporization we may now mention two processes
referred to just now (page 83); viz., sublimation and distillation.
The former is the means whereby we change solid bodies into vapour
and condense the vapour into proper vessels. The condensed substances
when deposited are called _sublimates_, and when we go into Chemistry
we shall hear more of them. The mode of proceeding is to place the
substance in a glass tube, and apply heat to it. Vapour will be formed,
and will condense at the cool end of the tube. The sublimate of sulphur
is called “Flowers of Sulphur,” and that of perchloride of mercury
“Corrosive Sublimate.”

Distillation is a more useful process, or, at any rate, one more
frequently employed, and is used to separate a volatile body from
substances not volatile. A distilling apparatus (_distillo_, to drop)
converts a liquid to vapour by means of heat, and then condenses it by
cold in a separate vessel.

The distilling apparatus consists of three parts,—the vessel in which
the liquid is heated (the still, or retort), the condenser, and the
receiver. The simple retort and receiver are shown in fig. 81. But when
very volatile vapours are dealt with, the arrangement shown on next
page is used (fig. 82). Then the vapour passes into the tube encased
in a larger one, the intervening space being filled with cold water
from the tap above (_c_), the warm water dropping from _g_. The vapours
are thus condensed, and drop into the bottle (or receiver) B.

[Illustration: Fig. 82.—Distilling apparatus.]

The apparatus for distilling spirits is shown below. The “still”
A is fitted into a furnace, and communicates with a worm
O in a metal cylinder filled with water, kept constantly
renewed through the tube TT′. This spirit passes through the
spiral, and being condensed, goes out into the receiver C.

[Illustration: Fig. 83.—Spirit still.]

There are even more simple apparatus for spirit distilling, but the
diagram above will show the principle of all “stills.” In former days,
in Ireland, whiskey was generally procured illegally by these means.



CHAPTER VIII.

 SPECIFIC HEAT—FUSION—LATENT HEAT—CONDUCTION AND CONVECTION OF
 HEAT—CALORESCENCE.


We have considered the effects of heat upon water, and touched upon one
or two kindred experiments. But we have some other subjects to discuss,
two in particular; viz., _Specific Heat_, and _Latent Heat_.

The specific heat of any substance is “the number of units of heat
required to raise one pound of such substance one degree.” We can
explain this farther. When heat is communicated to a body it has two or
three functions to perform. Some of it has to overcome the resistance
of the air in expanding the body, more of it expands, and the remainder
increases the temperature of the body. So some heat disappears as
heat, and is turned into energy,—“molecular potential energy,”—as it
is called, and the rest remains. Of course in objects the molecules
vary very much in weight and in their mutual attraction, and the heat
requisite to raise equal weights of different substances through
the same number of degrees of temperature will vary. This is called
capacity for heat, or specific heat. The capacity of different metals
for heat can easily be shown. The specific heat of water is very high,
because its capacity for heat is great. We can cool a hot iron in very
little water, and it takes thirty times as much heat to raise a given
weight of water a certain number of degrees, as it would to raise the
same weight of mercury to the same temperature. Water has greater
specific heat, generally speaking, than other bodies, and it is owing
to this circumstance that the climate is so affected by ocean currents.

Nearly all substances can be melted by heat, if we go far enough, or
frozen, if we could take the heat away. Solid can be made liquid, and
these liquids can be made gases and fly off in vapour. Similarly, if
we could only get heat away sufficiently from the atoms of a substance
we could freeze it. We cannot freeze alcohol, nor make ice from
air, nor can we liquify it, for we are unable to take away its heat
sufficiently. But we can turn water into steam, and into ice; or ice
into water, and then into steam. But there is one body we cannot melt
by heat, that is carbon. In the hottest fire coal will not melt, it
becomes soft. We call this melting _fusion_, and every body has its
melting point, or fusing point, which is the same at all times if the
air pressure be the same.

It is a curious fact that when a body is melting it rises to a certain
temperature (its fusing point), and then gets no hotter, no matter
whether or not the fire be increased;—all the extra heat goes to melt
the remainder of the substance. The heat only produces _changes of
state_. So this heat above fusing point disappears apparently, and is
called _Latent Heat_. This can easily be proved by melting ice. Ice
melts at 32° Fahr., or 0° Cent., and at that temperature it will remain
so long as any ice is left; but the water at 32°, into which the ice
has melted, contains a great deal of _latent_ heat, for it has melted
the ice quickly, and yet the thermometer does not show it. It is just
the same with boiling water.

When substances are fused they expand as a rule, but ice contracts;
so does antimony. On the other hand, when water solidifies it does
not contract as most things do. It expands, as many of us are aware,
by finding our water pipes burst in the winter; and the geologist
will tell us how the tiny trickling rills of water fall in between
the cracks of rocks and there freeze. In freezing the drops expand
and split the granite blocks. Type-metal expands also when it becomes
solid, and leaves us a clear type; but copper contracts, and won’t do
for moulding, so we have to stamp it when we want an impression on it.

There is no doubt that chemical combinations produce heat, as we can
see every day in house-building operations, when water is poured upon
lime; but there are also chemical combinations which produce cold.
Fahrenheit produced his greatest cold by combining snow and salt, for
in the act of combining, a great quantity of heat is swallowed up by
reason of the heat becoming latent, as it will do when solid bodies
become liquid. Such mixtures or combinations are used as _Freezing
Mixtures_ when it is necessary to produce intense cold artificially.
Sulphate of Sodium and Hydrochloric acid will also produce great cold,
and there are many other combinations equally or even more efficacious.

Heat is communicated to surrounding objects in three well-known
ways—by conduction, by radiation, and by convection. Conduction of
heat is easily understood, and is the propagation of heat through
any body, and it varies very much according to the substance through
which it passes. Some substances are better conductors of heat than
others. Silver has a far greater conductivity than gold, and copper
is a better heat-conductor than tin. Flannel is a non-conductor, or
rather a bad conductor, for no substance can be termed actually a
non-conductor. Flannel, we know, will keep ice from melting, and a
sheep’s wool or a bird’s feathers are also bad conductors of heat; so
Nature has provided these coverings to keep in the animal heat of the
body. A good conductor of heat feels cold to the touch of our fingers,
because it takes the heat from our hands. This can be tried by touching
silver, lead, marble, wood, and wool. Each in turn will feel cold and
less cold, because they respectively draw away, or conduct less and
less heat from our bodies. So our clothes are made of bad-conducting
substances. The bark of a tree is a bad conductor, and if you strip off
this clothing the tree will die.

Solids conduct heat the better the more compact they are. Air being a
bad conductor it follows that the less tightly the molecules are packed
the less conductibility there will be; and even a substance powdered
will be a worse conductor than the same substance in solid form; and
also more readily in the direction of the fibres than crossways.

Liquids do not possess great conductivity, but they, as well as gases,
are influenced by _convection_, or the transport of heat from the
bottom layers to the top (_conveho_, to carry up). We have already
mentioned that the heated particles of water rise to the top because
they expand, and so become lighter. This is convection of heat; and
by it liquids and gases, though actually bad conductors, may become
heated throughout to a uniform temperature. Of course the more easily
expansible the body is the more rapidly will convection take place—so
gases are more readily affected than liquids. Solids are not affected,
because convection of heat depends upon molecular movement or mobility,
and it is obvious that the particles of solid bodies are not mobile.
Professor Balfour Stewart says with reference to this that “were there
no gravity there would be no convection,” for the displacement of
the light warm particles by the heavier cold ones is due to gravity.
The instances of convection of heat in nature are numerous, and on a
gigantic scale. The ocean currents, trade winds, lake freezing, etc.,
while the chimney draught already referred to, is another example;
and in all these cases the particles of air or water are replaced by
convection. In the case of the lake freezing the cold particles at
the top sink, and the warmer ones ascend, until all the lake is at a
temperature of 36·2°, or say 4° above freezing. At this temperature
water assumes its maximum density, and then _expands_, as we have seen,
instead of contracting. Ice is formed, and being thus lighter than
water, floats; and so unites to cover in the water underneath, which is
never frozen solid, because the cold of the atmosphere cannot reach it
through the ice in time to solidify the whole mass.

[Illustration: Fig. 84.—Radiant heat.]

Radiant heat is the motion of heat transmitted to the ether, and
through it in the form of waves. The sun’s heat is radiant heat, and
radiation may be defined as “The communication of the motion of heat
from the articles of a heated substance to the ether.” The fire gives
out radiant heat, and so does heated metal, and it is transmitted by an
unseen medium. It is quite certain that the heat of a suspended red-hot
poker is not communicated to the air, because it will cool equally in
a vacuum. Sir Humphrey Davy proved that radiant heat could traverse a
vacuum, for by putting tin reflectors in an exhausted receiver he found
that a hot substance in the focus of one reflector caused an increase
in the heat of the other. If we put a red-hot or a hot substance in
one reflector, and tinder in the other, the latter will take fire.
The velocity of heat rays is equal to that of light, 186,000 miles in
a second, and indeed, radiant heat is identical with light. Heat is
reflected as is light, and is refracted in the same way as sound.

Some bodies allow the heat rays to pass through them, as air does, and
as rock salt will do. White clothing is preferable in summer (and also
in winter if we could only make people believe it). White garments
radiate less heat in winter, and absorb less heat in summer. An old
black kettle will boil water more quickly than a new bright one, but
the latter will keep the water hotter for the longer time when not on
the fire.

Heat, then, is movement of particles. Energy can be changed into heat,
as the savage finds when he rubs the bits of wood to produce heat and
fire. Friction causes heat, and chemical combination produces heat;
and, if “visible energy can be turned into heat, heat can be turned
back into visible energy.” For fire heats water, water expands into
steam, and steam produces motion and energy in the steam-engine.

If we heat water in Wollaston’s bulb,—the opening of which is
hermetically stopped by a piston,—the vapour will raise the piston. If
we cool the bulb we condense the steam, and the piston falls. Here we
have the principle of the steam-engine.

STEAM is the vapour of water educed by heat, and we may
give a few particulars concerning it. Its mechanical properties are
the same as those of other gases, and pure steam is colourless and
transparent—in fact, invisible. Its power when confined in boilers
and subjected to pressure is enormous, for the volume of the steam is
far greater than the water which gave rise to it. One cubic inch of
water will produce 1,700 cubic inches of steam—in other words, a cubic
inch of water produces a cubic foot of steam. When we obtain steam at
212°, we do so under the pressure of one atmosphere; but by increasing
the pressure we can raise the boiling point, and thus water at the
pressures of sixteen atmospheres will not steam till it reaches 398°.
It is thus we obtain pressure for locomotives, and other engines,
although a very small portion of the steam does work. Much the largest
portion is expended in overcoming cohesion, and one way and another,
taking into consideration defects in machinery, only about one-tenth of
the heat is employed in doing the work. The force exercised by steam
under atmospheric pressure is sufficient to raise a ton weight one foot.

To obtain very high temperatures we shall find the thermometer of no
use, for mercury boils at 662°, so an instrument called a Pyrometer
is used to ascertain the fusing point of metals. Mr. Wedgwood, the
celebrated china manufacturer, invented an instrument made of small
cylinders of clay moulded and backed, placed between two brass rods
as gauges divided into inches and tenths. But this instrument has
been long superseded by Professor Daniell’s Pyrometer, which consists
of a small bar of platina in an earthenware tube. The difference of
expansion between the platina and the tube is measured on a scale on
which one degree is equal to seven degrees of Fahrenheit. Thus the
melting temperatures of metals are ascertained.

The reflection and refraction of heat are ruled by the same laws as
the reflection and refraction of light. A convex lens will bring the
heat or light to a focus, and will act as a burning-glass if held in
the sunlight. Gunpowder has been ignited by a lens of ice, and more
than one house has been mysteriously set on fire at midday in summer
by the sun’s rays shining through a glass globe of water containing
gold fish, and falling upon some inflammable substance. Professor
Tyndall performed a series of experiments of a very interesting nature,
described in his book, “Heat considered as a Mode or Motion,” and
showed the transmutation of invisible heat rays into visible rays,
by passing a beam of electric light through an opaque solution, and
concentrating it upon a lens. The dark heat rays were thus brought to
a focus, all the light was cut off, and at the dark focus the heat was
found to be intense enough to melt copper and explode gunpowder. This
change of invisible heat into light is termed Calorescence.

It was Sir William Herschell who discovered that there were heat
rays beyond the red end of the spectrum. When light is split up into
its component rays, or decomposed, Sir William found that the heat
increased as the thermometer passed from violet to indigo, and so on
to blue, green, orange, and red, and the last were the hottest, while
beyond the spectrum there was heat even greater. A _Heat Spectrum_
was thus discovered, and by comparing, by means of the thermometer,
the various degrees of heat within certain limits, Professor Tyndall
found that the invisible Heat Spectrum is longer than the visible Light
Spectrum.



CHAPTER IX.

 LIGHT AND ITS SOURCES—WHAT IS LIGHT?—VELOCITY OF LIGHT—REFLECTION AND
 REFRACTION—RELATIVE VALUE OF LIGHTS.


The subject of Light and the science of Optics are so interesting to
all of us that some short history of light is necessary before we can
enter upon the scientific portion of the subject. The nature of the
agent (as we may term light) upon which our sight depends has employed
man’s mind from a very early period. The ancients were of opinion that
the light proceeded _from_ the eye to the object looked at. But they
discovered some of the properties of light. Ptolemy of Alexandria, who
was born A.D. 70, made some attempts to discover the law of
Refraction; and we are informed that Archimedes set the Roman fleet
on fire with burning-glasses at Syracuse. The Arabian treatise of
Alhagen, in 1100 A.D., contains a description of the eye and
its several parts; and the writer notices refraction and the effects of
magnifying glasses (or spectacles). Galen, the physician, practically
discovered the principle of the stereoscope, for he laid down the law
that our view of a solid body is made up of two pictures seen by each
eye separately.

Still the science of optics made little progress till the law
determining the path of a ray of light was made known, and the laws
of refraction discovered. Refraction means that a ray is deflected
from its straight course by its passage from one transparent medium
to another of different density. The old philosophers found out the
theory of sound, and they applied themselves to light. Newton said
light consisted of minute particles emanating from luminous bodies.
Huyghens and Euler opposed Newton’s theory of the emission of light;
and it was not till the celebrated Thomas Young, Professor at the Royal
Institution, grappled with the question that the undulating or wave
theory of light was found out. He based his investigations upon the
theory of sound waves; and we know that heat, light, and sound are most
wonderfully allied in their manner of motion by vibration. But he was
ridiculed, and his work temporarily suppressed by Mr. Brougham.

Light, then, is a vibratory motion (like sound and heat), a motion
of the atoms of our ether. But how is the motion transmitted? Sound
has its medium, air; and in a vacuum sounds will be very indistinctly
heard, if heard at all. But what is the medium of communication of
light? It is decided that light is transmitted through a medium called
_ether_, a very elastic substance surrounding us. The vibrations,
Professor Tyndall and other philosophers tell us, of the luminous
atoms are communicated to this ether, or propagated through it in
waves; these waves enter the pupil of the eye, and strike upon the
retina. The motion is thus communicated by the optic nerve to the
brain, and then arises the great primary faculty, Consciousness. We see
light, the waves of which, or ether vibrations, are transversal; air
waves or sound vibrations are longitudinal.

We have spoken of radiant heat. Light acts in the same way through the
ether; and when we consider Sound we shall learn that a certain number
of vibrations of a string give a certain sound, and the quicker the
vibration the shriller the tone. So in light. The more quickly the
waves of luminosity travel to our eye, and the faster they strike it,
the greater the difference in the _colour_, or what we call colour.
Light as we see it is composed of different colours, as visible in
the rainbow. There are seven primary colours in the sunlight, which
is white. These can be divided or “dispersed,” and the shortest rays
of the spectrum are found to be violet, the longest red. It has been
calculated that 39,000 red waves make an inch in length. Light travels
at a rate of nearly 190,000 miles a second, so if we multiply the
number of inches in that distance by the number of red waves, we shall
have millions of millions of waves entering the eye in a single second
of time. The other waves enter more rapidly still, and “the number of
shocks corresponding to the impression of violet is seven hundred and
eighty-nine millions of millions” per second! Or taking the velocity
of light at 186,000 miles in a second, it would be six hundred and
seventy-eight millions of millions (Tyndall). There may be other
colours which we cannot see because the impressions come too rapidly
upon the retina; but the violet impression has been thus accurately
determined. See page 168.

We have seen that heat is a kind of motion of particles in a body—a
vibratory motion which, instead of being apparent to the ear, is
apparent to the eye in rays of light. Thus heat, sound, and light are
all intimately connected in this way. We have also learnt that rays of
light radiate and travel with tremendous speed to our eyes, but without
any shock. There is no feeling connected with the entrance of light to
the eye any more than there is any sensation of sound when entering the
ear, except when the light is vividly and very suddenly revealed, or
when a very piercing sound is heard. Then the nerves are excited, and
a painful sensation is the result; but under ordinary circumstances we
are not physically conscious of the entrance of light or sound.

Heat and light are considered to be one and the same thing in different
degrees of intensity. The sources of light are various. The sun and
fixed stars, heat, electricity, many animals, and some plants, as well
as decaying animal matter, give out light. There are luminous and
non-luminous bodies. The moon is non-luminous, as she derives her light
from the sun, as does the earth, etc.

Light is distributed in rays. These rays are straight in all
directions. The velocity of light is almost inconceivable. It travels
at a rate of 186,500 miles a second. The latest computation with
electric light has given a rate of 187,200 miles a second; but the blue
rays in the light experimented on probably account for the difference,
for blue rays travel quicker by one per cent. than red rays. Römer
first found out the velocity of light, which comes to us from the
sun—ninety millions of miles—in eight minutes. Fizeau calculated the
velocity by means of a wheel, which was set moving with tremendous
speed by making the light pass between the teeth of the wheel and back
again.

When rays of light meet substances they are deflected, and the
phenomena under these circumstances are somewhat similar to the
phenomena of heat and sound. There are three particular conditions of
rays of light: (1) they are absorbed; (2) they are reflected; (3) they
are refracted.

Firstly. Let us see what we mean by light being absorbed; and this is
not difficult to understand, for any “black” substance shows us at once
that all the sunlight is taken in by the black object, and does not
come out again. It does not take in the light and radiate it, as it
might heat. The rose is red, because the rays of light pass through it,
and certain of them are reflected from within. So colour may be stated
to be the rays thrown out by the objects themselves—those they reject
or reflect being the “colour” of the object.

[Illustration: Fig. 85.—Angle of reflection, etc.]

Secondly. Bodies which reflect light very perfectly are known as
mirrors, and they are termed plane, concave, or convex mirrors,
according to form. A plane mirror reflects so that the reflected ray _d
i_ forms the same angle with the perpendicular as the incident ray _r
i_; in other words, the angle of incidence is always equal to the angle
of reflection, and these rays are perpendicular to the plane from which
they are reflected. The rays diverge, so that they appear to come from
a point as far behind the mirror as the luminous point is in front, and
the images reflected have the same appearance, but reversed. There is
another law, which is that “the angular velocity of a beam reflected
from a mirror is twice that of the mirror.” The Kaleidoscope, with
which we are all familiar, is based upon the fact of the multiplication
of images by two mirrors inclining towards each other.

[Illustration: Fig. 86.—Concave mirror.]

A concave mirror is seen in the accompanying diagram, and may be called
the segment of a hollow sphere—V W. The point C is the geometrical
centre, and O C the radius; F is the focus; the line passing through it
is the _optical_ axis; O being the _optical centre_. All perpendicular
rays pass through C. All rays falling in a direction parallel with the
optical axis are reflected and collected at F. Magnified images will
be produced, and if the object be placed between the mirror and the
focus, the image will appear at the back; while if the object be placed
between the geometrical centre and the focus, the image will appear to
be in front of the mirror.

We can understand these phenomena by the accompanying diagrams. Suppose
a ray A _n_ passes from one object, A B, at right angles, it will be
reflected as _n_ A C, the ray A C being reflected to F. These cannot
meet in front of the mirror, but they will if produced meet at _a_,
and the point A will be reflected there; similarly B will be reflected
at _b_, and thus a magnified image will appear behind or at the back
of the mirror’s surface. In the next diagram the second supposed case
will produce the image in the air at _a b_, and if a sheet of paper be
held so that the rays are intercepted, the image will be visible on the
sheet. In this case the perpendicular ray, A _n_, is reflected in the
same direction, and the ray, _a c_, parallel with the axis is reflected
to the focus. These rays meet at _a_ and corresponding rays at _b_,
when the image will be reproduced; viz., in front of the mirror.

[Illustration: Fig. 87.—Reflection of mirrors (I).]

[Illustration: Fig. 88.—Reflection of mirrors (II).]

The concave mirror is used in the manufacture of telescopes, which,
with other optical instruments, will be described in their proper
places. We will now look at the _Refraction_ of light.

Bodies which permit rays of light to pass through them are termed
transparent. Some possess this property more than others, and so
long as the light passes through the same medium the direction will
remain the same. But if a ray fall upon a body of a different degree
of density it cannot proceed in the same direction, and it will be
broken or _refracted_, the angle it makes being termed the angle of
refraction.

For instance, a straight stick when plunged into water appears to be
broken at the point of immersion. This appearance is caused by the rays
of light taking a different direction to our eyes. If in the diagram
(fig. 89) our eye were at _o_, and the vessel were empty, we should
not see _m_; but when water is poured into the vessel the object will
appear higher up at _n_, and all objects under water appear higher than
they really are.

[Illustration: Fig. 89.—Refraction in water.]

[Illustration: Fig. 90.—A water-bottle employed as a convergent lens.]

One may also place a piece of money at the bottom of a basin, and then
stoop down gradually, until, the edge of the basin intervening, the
coin is lost to view. If an operator then fills the basin with water,
the piece of money appears as though the bottom had been raised. The
glass lenses used by professors may be very well replaced by a round
water-bottle full of water. A candle is lighted in the darkness, and
on holding the bottle between the light and a wall which acts as a
screen, we see the reflected light turned upside down by means of
the convergent lens we have improvised (fig. 90). A balloon of glass
constitutes an excellent microscope. It must be filled with perfectly
clear, limpid water, and closed by means of a cork. A piece of wire
is then rolled round its neck, and one end is raised, and turned up
towards the focus; viz., to support the object we wish to examine,
which is magnified several diameters. If a fly, for instance, is at the
end of the wire, we find it is highly magnified when seen through the
glass balloon (fig. 91). By examining the insect through the water in
the balloon, we can distinguish every feature of its organism, thanks
to this improvised magnifier. This little apparatus may also serve to
increase the intensity of a luminous focus of feeble power, such as a
lighted candle. It is often employed in this manner by watchmakers. If
a bottle full of water is placed on a table, and exposed to the rays
of the sun, the head of a lucifer match being placed in the brightest
centre of light caused by the refracted rays, the match will not fail
to ignite. I have succeeded in this experiment even under an October
sun, and still more readily in warm weather.

[Illustration: Fig. 91.—A simple microscope formed with a glass balloon
full of water.]

In the Conservatoire des Arts in Paris a visitor will always notice
a number of people looking at the mirrors in the “optical” cabinets.
These mirrors deform and distort objects in a very curious manner, and
people find much amusement in gazing into them till they are “moved on”
by the attendants. Such experiments create great interest, and a very
excellent substitute for these may be found in a coffee-pot or even in
a large spoon, and all the grotesque appearance will be seen in the
polished surface. The least costly apparatus will sometimes produce
the most marvellous effects. Look at a soap-bubble blown from the end
of a straw. When the sphere has a very small diameter the pellicule
is colourless and transparent; but as the air enters by degrees,
pressing upon all parts of the concave surface equally, the bubble gets
bigger as the thickness decreases, and then the colours appear,—feeble
at first, but stronger and stronger as the thickness diminishes.
The study of soap-bubbles and of the effects of the light is very
interesting. Newton made the soap-bubble the object of his studies
and meditations, and it will ever hold its place amongst the curious
phenomena of the Science of Optics. But before going into all the
phases of Lights and Optics we will proceed to explain the structure of
the eye, as it is through that organ that we are enabled to appreciate
light and its marvellous effects.

[Illustration: Fig. 92.—Grotesque effects of curved surfaces.]

It is often considered an embarrassing matter to fix precisely the
value of two lights. Nothing, however, can be easier in reality, as
we will show. In comparing different lights, it is necessary to bear
in mind the amount of waste, the colour of the light, the luminous
value of the source, and the steadiness of the flame. The luminous
value of a lamp-burner is generally equalled by that of a wax candle,
and we will take as an example one of those at six to the pound. Very
precise appliances are used for this experiment when great exactness
is required; but it is easy to calculate in a simple manner the
differences in ordinary lights. Supposing we desire to test the value
of light given by a lamp and a wax candle, they must both be placed
on the table at an equal height, B and A, (fig. 93), in front of some
opaque body, A, and then a large sheet of paper must be fixed as
vertically as possible to form a screen. When B and A are lighted,
two shadows, E and F, are produced, to which it is easy to give
exactly the same intensity, by advancing or withdrawing one of the
two sources of light. The intensities of the two lights will then be
inversely proportional to the squares of the measured distances, AB
and AC. By a similar careful calculation it has been possible to draw
up a table of the relative values of various ordinary lights. We have
not included here the electric light, which has recently attracted so
much attention, because this system of lighting can hardly be said to
have yet penetrated the domain of domestic life; but when we consider
electricity, as we hope to do in a future part, we intend to study
this question fully, for there is no doubt that electricity is becoming
more and more adapted to our daily life.

[Illustration: Fig. 93.—An elementary Photometer.]

The measurement of intensity of light is called _Photometry_, and the
instruments used are _Photometers_. Bunsen’s instrument consists of a
screen of writing-paper, saturated in places with spermaceti to make it
transparent. A sperm candle is placed on one side, and the light to be
compared on the other. The lights are provided with graduated bars, and
these lights are then removed farther and farther from the screen till
the spots of grease are invisible. The relative intensities are as the
squares of the distance from the screen.

[Illustration: Fig. 94.—The soap-bubble.]

We append a table showing the comparative cost of light given by Dr.
Frankland at the Royal Institution some few years ago. The standard of
comparison was 20 sperm candles burning for 10 hours at the rate of 120
grains an hour:—

                  _s._     _d._
  Wax              7       2½
  Sperm Oil        1      10
  Paraffin         3      10
  Spermaceti       6       8
  Coal Gas         0       4½
  Paraffin Oil     0       6
  Tallow           2       8
  Cannel Gas       0       3
  Rock Oil         0       7⅔

There are many other interesting experiments connected with
Light,—Spectrum Analysis, etc., etc.,—all of which we will defer for a
time until we have examined the Eye and some effects produced upon it
by Light, illustrated by numerous diagrams in the pages next following.



CHAPTER X.

 VISION AND OPTICAL ILLUSIONS—THE EYE DESCRIBED—ACCOMMODATION OF THE
 EYE—CHROMATIC ABERRATION—SPINNING TOPS.


The eye is an optical instrument that may be compared with those
constructed by physicists themselves; the _media_ of which it is
composed have surfaces like those which enter into the construction of
optical instruments. It was Kepler who at the end of the eighteenth
century discovered the passage of light into the eye. Soon after the
discovery of the inner chamber he found that the eye realized the
conditions that Porta had combined to obtain the reflection of external
objects.

We will now briefly state that the coats of this organ are constituted
of a fibrous membrane, T (fig. 95), termed _sclerotic_, which
is opaque, except in the anterior portion of the eye, where it forms
the transparent _cornea_. The crystalline, C, enshrined
behind the cornea, is the convergent lens of the inner chamber; it is
covered with a transparent membrane, or _capsule_, and is bathed in two
fluids, the _aqueous humour_, between the crystalline humour and the
cornea, and the _vitreous body_, a gelatinous humour lodged between
the crystalline and the back of the eye. The image of exterior objects
which is produced by the passage of light through these refracting
surfaces, is received by a nervous membrane, the _retina_, B,
formed by an expansion of the optic nerve, N. We must also
mention the _choroid_, a membrane lined with a dark pigment, which
absorbs the light, and prevents interior reflections, and in front of
the crystalline lens, a curtain with an opening, H, called the
_iris_, which gives to the eyes their colour of blue, grey, or black.
The opening in the centre of the iris is called the _pupil_.

[Illustration: Fig. 95.—Structure of the eye.]

The penetration of light through the surfaces of the eye is easily
demonstrated. An object throws divergent rays on the cornea, a part
penetrates into the eye and falls upon the retina, leaving a perfectly
retained image of the object. Magendie has proved in the following
manner the truth of this mathematical deduction. The eye of a rabbit
is very similar to an albino’s; that is to say, the choroid contains
no black pigment, but a transparent matter, and when placed before a
brilliant object, the image can be seen inverted on the retina. The
experiment succeeds also with the eye of a sheep or a cow, if the
sclerotic has been lessened. The _optic centre_ of the eye is the point
where the secondary axes cross; the _optic axis_ passes through the
geometrical axis of the organ, and directs itself spontaneously towards
the point that attracts the eye.

[Illustration: Fig. 96.—Diagram of mode of vision.]

We will now point out in what distinct vision consists. A screen
placed behind a lens will only receive the image of a lighted object,
A B, if placed in a position, R R (fig. 96). If placed nearer at R´´
R´´, or further off at R´ R´, the light from the object is thrown on
the screen, and the image is confused. To prove the imperfection of
sight which is shown by the application of these theoretic rules, MM.
Boutan and d’Alméïda[10] cite the following experiment:—If the head of
a pin is placed from one to two inches from the eye, nothing will be
perceived but a confused haziness of vague outline. The distance of
distinct vision is that at which an object of small dimensions may be
placed to be plainly perceived. This distance, which averages fifteen
inches, varies with different individuals. It can be determined for
different sights by means of an apparatus constructed by Lepot. A white
thread, _a_, is stretched horizontally on a dark board (fig. 97). We
look at it by placing our eye at one end behind a little screen pierced
with an aperture, O; it then appears much reduced in length, but
either nearer or farther off it seems to enlarge and swell, having the
appearance of a white surface, becoming larger and larger in proportion
as we move away from the point at which it is seen most distinctly. In
this manner we can easily obtain a measure of the distance of distinct
vision. One of the most remarkable properties of the eye consists in
the faculty which this organ possesses of seeing different distances.
If we consider it as a dark chamber, there is but one distance at which
an object will be perfectly visible; nevertheless a metal wire, for
example, can be seen as well at a distance of seven, as ten, fifteen,
or twenty inches by good sights.

[Illustration: Fig. 97.—Experiment for sight.]

This faculty of accommodation in the eye is thus demonstrated: we place
two pins, one in front of the other, one eye only being open; we first
look at the nearest pin, which appears confused if it is near the eye,
but by an effort of will the image becomes clear. If, while preserving
the clearness of the image, we then carry our attention to the second
pin, we find that it, too, presents a confused appearance. If we make
an effort to distinguish the contour of the second pin, we at last
succeed, and the first once more appears ill-defined. It is only since
the experiments of M. Cramer and M. Helmholtz that the explanation
of this phenomenon could have been given. M. Cramer has succeeded in
determining on the living eye the curved ray of the cornea, and of the
two surfaces of the crystalline lens. In so doing he followed Samson’s
method, and observed the images thrown by a luminous object, whose
rays strike the different refracting surfaces of the eye. A candle,
L (fig. 98), is placed before the eye, O, and throws as in a convex
mirror a straight image of the flame, A (fig. 99). The other portion
of the light, which has penetrated the pupil, falls on the crystalline
lens, and produces likewise a second straight image, B. Then the light
refracted by the lens reaches the posterior surface; a portion is
reflected on a concave mirror, and gives the inverted image, C, very
small and brilliant. M. Cramer observed it through a microscope, and
studied the variations in the size of images when the eye passed from
the observation of adjacent to distant objects. He stated:—

1. That the image, A, formed on the surface of the cornea,
remains the same size in both cases; the form of the cornea therefore
remains unaltered.

[Illustration: Fig. 98.—M. Cramer’s experiment.]

[Illustration: Fig. 99.—Images in the eye.]

2. That the image, B, formed on the upper surface of the
lens, diminishes in proportion as the eye is nearer the object; the
surface therefore becoming more and more convex, as the focal distance
diminishes—a result indicated by the theory that it is possible in the
vision of near objects to receive the image on the retina.

3. That the third image, C, produced on the posterior surface
of the lens, remains nearly invariable.

We may confirm Cramer’s statements by an easy experiment. We place
ourselves in front of the eye of someone who looks in turn at two
objects placed on the same black line at unequal distances from
him, and are able to distinguish by the dimension of the images of
the candle, which object it is that he is regarding. M. Helmholtz
has carried M. Cramer’s methods to perfection, and has been able to
formulate a complete theory of all the phenomena of accommodation.
The laws of optics show that the rays emitted by a luminous point may
unite at another point by the action of the refracting surfaces of the
eye. Nevertheless, a white light being composed of rays of diverse
refrangibility, particular effects, known under the name of _chromatic
aberration_, are produced through the decomposition of light, which
we will proceed to study, under M. Helmholtz’s auspices[11]. We make
a narrow opening in a screen, and fix behind this opening a violet
glass, penetrable only by red and violet rays. We then place a light,
the red rays of which reach the eye of the observer after having passed
through the glass and the opening in the screen. If the eye is adapted
to the red rays, the violet rays will form a circle of diffusion, and
a red point encircled with a violet aureola is seen. The eye may also
be brought to a state of refraction, so that the point of convergence
of the violet rays is in front, and that of the red rays behind the
retina, the diameters of the red and violet circles of diffusion being
equal. It is then only that the luminous point appears monochromatic.
When the eye is in this state of refraction, the simple rays, whose
refrangibility is maintained between the red and the violet rays, unite
on the retina.

There is another kind of aberration of luminous rays of one colour
emitted through a hole, which generally only approach approximately to
a mathematical focus, in consequence of the properties of refracting
surfaces; it is called _aberration of sphericity_. The phenomena are as
follows:—

[Illustration: Fig. 100.]

1. We take for our object a very small luminous point (the hole
made by a pin in some black paper, through which the light passes),
and having also placed before the eye a convex glass, if we are not
near-sighted, we fix it a little beyond the point of accommodation, so
that it produces on the retina a little circle of diffusion. We then
see, instead of the luminous point, a figure representing from four
to eight irregular rays, which generally differ with both eyes, and
also with different people. We have given the result of M. Helmholtz’s
observations in fig. 100; _a_ corresponds to the right eye, and _b_ to
the left. The outer edges of the luminous parts of an image, produced
in this way by a white light, are bordered with blue; the edges towards
the centre are of a reddish yellow. The writer adds that the figure
appears to him to have greater length than breadth. If the light is
feeble, only the most brilliant parts of the figure can be seen, and
several images of the luminous point are visible, of which one is
generally more brilliant than the others. If, on the other hand, the
light is very intense,—if, for example, the direct light of the sun
passes through a small opening,—the rays mingle with each other, and
are surrounded by aureola of rays, composed of numberless extremely
fine lines, of all colours, possessing a much larger diameter, and
which we distinguish by the name of the aureola of capillary rays.

[Illustration: Fig. 101.]

The radiating form of stars, and the distant light of street-lamps
belong to the preceding phenomena. If the eye is accommodated to a
greater distance than that of the luminous point,—and for this purpose,
if the luminous point itself is distant, we place before the eye a
slightly convex lens,—we see another radiating image appear, which M.
Helmholtz represents thus (fig. 101): at _c_ as it is presented to the
right eye, and at _d_ as seen by the left.

If the pupil is covered on one side, the side opposite to the image
of diffusion disappears; that is to say, that part of the retinal
image situated on the same side as the covered half of the pupil. This
figure, then, is formed by rays which have not yet crossed the axis
of the eye. If we place the luminous point at a distance to which
the eye can accommodate itself, we see, through a moderate light, a
small, round, luminous spot, without any irregularities. If the light,
on the contrary, is intense, the image is radiated in every position
of accommodation, and we merely find that on approaching nearer, the
figure which was elongated, answering to a distant accommodation,
gradually diminishes, grows rounder, and gives place to the vertically
elongated figure, which belongs to the accommodation of a nearer point.
When we examine a slender, luminous line, we behold images developed,
which are easily foreseen, if for every point of the line we suppose
radiating images of diffusion, which encroach on each other. The
clearest portions of these images of diffusion mingle together and form
distinct lines, which show multiplied images of the luminous line. Most
persons will see two of these images; some, with the eyes in certain
positions, will see five or six.

[Illustration: Fig. 102.]

To show clearly by experiment the connection existing between double
images and radiated images from points, it is sufficient to make in a
dark sheet of paper a small rectilinear slit, and at a little distance
from one end, on a line with the slit, a small round hole, as shown
at _a_ in fig. 102. Looking at it from a distance we shall see that
the double images of the line have exactly the same distance between
them that the most brilliant parts of the starred figure of diffusion
have from the point, and that the latter are in a line with the first,
as will be seen at _b_ (fig. 102), where in the image of diffusion of
the luminous point, we only see the clearest parts of star _a_ of the
figure.

On lighted surfaces, to which the eye is not exactly accommodated,
multiplied images are often remarked through the passage from light to
darkness being made by two or three successive steps.

A series of facts which have been collected under the title of
_irradiation_, and which show that brightly-lighted surfaces
appear larger than they are in reality, and that the dark surfaces
which surround them appear diminished to a corresponding degree,
explains this by the circumstance that the luminous sensation is not
proportional to the intensity of the objective light. These phenomena
affect very various appearances, according to the form of respective
figures; they are generally seen with the greatest ease and intensity
when the eye is not exactly accommodated to the object examined, either
by the eye being too near or too far off, or by using a concave or
convex lens, which prevents the object being seen clearly. Irradiation
is not completely wanting, even when the accommodation is exact, and
we notice it clearly in very luminous objects, above all when they are
small; small circles of diffusion increase relatively the dimensions
of small objects much more than of large ones, with regard to which,
the dimensions of the small circles of diffusion which the eye
furnishes, when properly accommodated, become insensible.

[Illustration: Fig. 103.—Experiment 1.]

1. _Luminous surfaces appear larger._ We can never judge exactly of
the dimensions of a slit or small hole through which a bright light
escapes; it always appears to us larger than it really is, even with
the most exact accommodation. Similarly, the fixed stars appear in the
form of small luminous surfaces, even when we make use of a glass which
allows of perfect accommodation. If a gridiron with narrow bars—the
spaces intervening being exactly equal to the thickness of the bars—is
held over a light surface, the spaces will always appear wider than the
bars. With an inexact accommodation, these phenomena are still more
remarkable. Fig. 103 exhibits a white square on a black foundation, and
a black square on a white foundation. Although the two squares have
exactly the same dimensions, the white appears larger than the black,
unless with an intense light and an inexact accommodation.

[Illustration: Fig. 104.—Experiment 2.]

2. _Two adjacent luminous surfaces mingle together._ If we hold a fine
metallic wire between the eye and the sun, or the light of a powerful
lamp, we shall cease to see it; the lighted surfaces on all sides
of the wire in the visual range pass one into the other, and become
mingled. In objects composed of black and white squares, like those of
a draught-board (fig. 104), the angles of the white squares join by
irradiation, and separate the black squares.

3. _Straight lines appear interrupted._ If a ruler is held between
the eye and the light of a bright lamp or the sun, we perceive a very
distinct hollow on the edge of the ruler in the part corresponding to
the light. When one point of the retina is affected by a light which
undergoes periodical and regular variations, the duration of the period
being sufficiently short, there results a continuous impression, like
that which would be produced if the light given during each period
were distributed in an equal manner throughout the whole duration of
the period. To verify the truth of this law, we will make use of some
discs, such as that represented in fig. 105. The innermost circle is
half white and half black; the middle circle has two quarters, or half
its periphery, white, and the outer circle has four eighths’ white, the
rest being black. If such a disc is turned round, its entire surface
will appear grey; only it is necessary to turn it with sufficient force
to produce a continuous effect. The white may also be distributed in
other ways, and provided only that on all the circles of the disc the
proportion of the angles covered with white is the same, they will
always exhibit the same grey colour. Instead of black and white we may
make use of different colours, and obtain the same resultant colour
from all the circles, when the proportion of the angles occupied by
each of the colours in the different circles is the same.

[Illustration: Fig. 105.—Disc which appears uniformly grey by reason of
its rotation.]

If we paint on a disc a coloured star, which is detached from a
foundation of another colour (fig. 106), during the rapid rotation of
the disc the centre affects the colour of the star; the outer circle
assumes that of the background, and the intermediate parts of the disc
present the continuous series of the resultant colours. These results
are in accordance with the theory of the mixture of colours.

[Illustration: Fig. 106.—Disc with a star painted on the background of
another colour.]

Rotative discs, which are so much used in experiments in optical
physiology, were employed for the first time by Müsschenbroeck;
the most simple is the top. M. Helmholtz ordinarily uses a brass
spinning-top, which fig. 107 represents at a third the natural size.
It is set in motion by the hand, and its quickness may be increased
or moderated at will; but it cannot be made to spin quicker than six
rounds in a second; this motion will be kept up for three or four
minutes. Thus, with a feeble movement of rotation, a uniform luminous
impression can only be obtained by dividing the disc into four or six
sections, on each of which we repeat the same arrangement of colours,
light, and shade. If the number of repetitions of the design is less,
we obtain, with a bright light, a more or less shot-coloured disc.

[Illustration: Fig. 107.—M. Helmholtz’s top for studying the impression
of light on the retina.]

It is easy to place designs on the disc, even when in motion, or to
make any desired modification, by superposing on the first disc another
disc with sectors, of which we can vary the position by slightly
touching it, or even blowing on it, thus producing during the rotation
of the disc very varied modifications. If, for instance, we place on
a disc covered with blue and red sectors of equal size, a black disc,
of which the sectors are alternately filled in or empty, the disc, as
it turns round, will appear blue if the black sectors of the upper
disc exactly cover the [red] sectors of the lower disc; and it appears
red, if, on the contrary, the blue sectors are covered with the black;
while in the intermediate positions we obtain different mixtures of
red and white, and during the rotation of the disc may vary the colour
insensibly by a gentle touch. By dividing the different sectors with
broken or curved lines, instead of straight ones, we can produce an
arrangement of coloured rings of great variety and beauty. To give
the top greater speed, we set it in motion by drawing a string twined
round its stem. The simplest method, as shown in fig. 108, consists in
the employment of a handle similar to that of the German top. It is a
hollow cylinder of wood set into a handle with two circular holes; and
at right angles with these is a groove for the passage of the string.
The stem of the top is passed through the holes of the cylinder, one
end of the string is fixed in the small hole in the stem, and is rolled
round by turning the top in the hand. The part of the stem on which
the string is twisted becomes sufficiently thick for the top to remain
suspended to the handle; then holding it a little above the table, and
giving the string a powerful pull, we set the top in motion, and as
the string unrolls it falls on the table, where it will continue its
rotation for some time. The top represented in fig. 109 is constructed
so that the discs may be firmly pressed by the stem, which is necessary
in experiments for demonstrating Newton’s theory of the mingling of
colours. We make use for this purpose of a variety of discs, made of
strong paper of different sizes, having an opening in the centre and
a slit, as in fig. 110; each of the discs is covered uniformly with
a single colour; and if two or more are superposed, with their slits
placed one over the other, we obtain sectors, the size of which we
may vary at will, so that we can modify in a continuous manner the
proportions of the colours. The most perfect construction is that
of Busold’s chromatic top (fig. 111), which should only be employed
for very rapid rotations. The disc, which weighs 5 lbs., is made of
an alloy of zinc and lead, about an inch and a quarter in diameter.
The brass axis terminates at its lower end with a blunt point of
untempered steel; the cylindrical part of the axis is roughened to
encourage the adherence of the string; the axis is placed between the
clamps of a vice, and a plate is put underneath; we then pull the
string firmly with the right hand, and when the top is in motion it is
separated from the clamps. By pulling the string very powerfully it is
possible to obtain a speed of sixty turns in a second, and the movement
will be kept up for three quarters of an hour.

[Illustration: Fig. 108.—Spinning a top with coloured discs.]

[Illustration: Fig. 109.—Top for experiments demonstrating Newton’s
theory of the mingling of colours.]

[Illustration: Fig. 110.—Disc.]

[Illustration: Fig. 111.—Busold’s chromatic top.]

Besides tops, we may make use of different kinds of discs, with an
axis rotating between two clamps; they are moved either by a kind of
clock-work, or by the unrolling of a string, like the tops. Generally,
however, these contrivances have this inconvenience, that the discs
cannot be changed without stopping the instrument, and partly taking
it to pieces. On the other hand, we have the advantage of being able
to turn them on a vertical plane, so that we can conveniently carry on
our experiments before a numerous auditory, which is a more difficult
matter with tops. Montigny contrived to obtain the mingling of colours
by means of a turning prism, which he caused to throw its shadow on a
white screen. The Thaumatrope is a small rectangle of cardboard, which
is made to rotate on an axis passing through the centres of the longest
sides. We shall describe it at greater length when we come to consider
a new apparatus known under the name of the _Praxinoscope_.

More complicated contrivances have also been constructed on the same
principle, by which one may perceive the rotating disc through slits
which turn at the same time. We will now describe the construction of
some discs invented by Plateau under the name of the Phenakistoscope.
These discs are made of strong cardboard, from six to ten inches in
diameter (fig. 112), on which a certain number of figures (eight to
twelve) are placed in circles at an equal distance from each other,
presenting the successive phases of a periodical movement. This disc
is placed on another opaque circle of rather larger diameter, which
has on its margin as many openings as the first disc has figures. The
two discs are placed one on the other, and are fixed in the centre by
means of a screw at the anterior extremity of a small iron axis, the
other end being fitted into a handle. To make use of this contrivance
we place ourselves in front of the glass, towards which we turn the
disc with the figures, placing the eye so as to see the figures through
one of the holes of the large disc. Directly the apparatus begins
to turn round, the figures seen in the glass appear to execute the
particular movements which they represent in different positions. Let
us designate by means of the figures 1, 2, 3, the different openings
through which the eye successively looks, and point out by the same
numbers the figures in the radiuses thus numbered. If the experimenter
looks in the glass through opening 1, he will see first figure 1,
which appears in the glass to pass before his eyes; then the rotation
of the disc displaces opening 1, and the cardboard intervenes, until
opening 2 appears; then figure 2 takes the place of figure 1, until it
in turn disappears, and opening 3 presents figure 3 to view. If these
figures were all similar, the spectator would have but a series of
visual impressions, separate but alike, which by a sufficiently rapid
rotation mingle together in one durable impression like a perfectly
immovable object. If, on the contrary, the figures differ slightly
from each other, the luminous sensations will also mingle in a single
object, which will however appear to be modified in a continuous
manner, conformably with the differences of successive images. With a
difference of speed, we obtain a new series of phenomena. A most simple
contrivance of this kind is a top of C. B. Dancer, of Manchester (fig.
113). It will be seen that the axis carries another disc, pierced
with openings of different shapes, to the edge of which a thread is
attached. This second disc is carried along by the friction of the
axis, but its rotation is less rapid because of the great resistance
offered by the air to the piece of thread which participates in the
movement. If the lower disc has several differently-coloured sectors,
they produce a very motley appearance, which seems to move sometimes by
leaps, and sometimes by continuous motion. We must distinguish between
the phenomena of successive contrast and simultaneous contrast.

[Illustration: Fig. 112.—Rotating disc.]

[Illustration: Fig. 113.—Mr. Dancer’s top.]

Phenomena of _successive contrast_ develop what are called _accidental
images_. If we fix our eyes for a considerable time on a coloured
object, and then suddenly direct them towards a uniform white surface,
we experience the sensation of the object as it is, but it appears
coloured with a complementary tint; that is to say, it has the colour
which, superposed on the genuine tint, we obtain from pure white. Thus
a red object produces a consecutive green object. The experiment can be
tried by gazing at the sun when it is setting, and then directing one’s
eyes towards a white wall in the same direction.

Phenomena of _simultaneous contrast_ arise from the influence
exercised over each other by different shades and colours which we see
_simultaneously_. That we may be certain that we have really obtained
phenomena of this kind, the experiments must be arranged in such a
manner that accidental images are not produced, and that the part of
the retina affected by the sensation of colour does not receive, even
momentarily, a passing image.

[Illustration: Fig. 114.—Disc, which exhibits, when in rotation, a
series of concentric rings.]

The phenomena of simultaneous contrast appear with the greatest
clearness with slight differences of colour, and are therefore exactly
the contrary of phenomena of successive contrast, which are favoured
by strong oppositions of colour and light. We can, in general,
characterise phenomena of simultaneous contrast as governed by this
law, common to all perceptions of the senses: _the differences clearly
perceived appear greater than the differences equal to them, but
perceived with greater difficulty, either because they only affect
the observation in an uncertain manner, or that the memory fails to
judge of them_. A man of middle height appears small beside a tall
man, because at the moment it is forcibly impressed on us that there
are taller men than he, and we lose sight of the fact that there are
smaller. The same man of medium height appears tall beside a man of
small stature. We can easily make experiments on simultaneous contrast
with a sheet of transparent paper. We fasten together a sheet of green
and a sheet of rose-coloured paper, so as to obtain a sheet half red
and half green. On the line of separation between the two colours we
place a strip of grey paper, and cover the whole with a sheet of thin
letter-paper of the same size. The grey strip will then appear red at
the edge touching the green, and green at the edge touching the red;
the centre presenting an intermediate shade. It presents a still more
decided appearance if the grey strip is perpendicular with the line
of separation of the two colours; the piece of grey then stretching
into the green will present as deep a red as the red foundation on
the other side. If the line of grey colour exactly covers the line of
separation between the two colours, the contrasting colour is more
feeble; the edges of the grey paper then present complementary strips
of colour. Similar effects may be obtained by superposing, in gradually
diminishing layers, strips of thin paper, so as to form successive
bands of different thicknesses. If it is then lit up from behind, the
objective intensity is evidently constant through the extent of each
layer; nevertheless every strip appears darker at the edge touching
a more transparent layer, and lighter at the edge in contact with a
thicker layer. The dull tints of China ink, superposed in layers,
will produce a similar effect. The phenomena are produced by means of
rotative discs of most beautiful and delicate gradations of colour. Let
us give the sectors of the disc the form represented by fig. 114, and
make them black and white; and when in rotation we shall see a series
of concentric rings of a shade that becomes darker and darker towards
the centre. The angular surface of the dark portions is constant in
each of these rings. The intensity, therefore, of each ring is uniform
during rapid rotation; it is only between one ring and another that
the intensity varies. Each ring also appears lighter on its inner
side when it borders on a darker ring, and darker on its outer side
when in contact with a lighter ring. If the differences of intensity
in the rings are very slight, one can scarcely judge sometimes if the
inner rings are darker than the outer; the eye is only struck by the
periodical alternations of light and shade presented by the edges
of the rings. If, instead of white and black, we take two different
colours, each ring will present two colours on its two edges, although
the colour of the rest of the ring will be uniform. Each of the
constituent colours presents itself with more intensity on that edge of
the ring which borders on another ring containing a smaller quantity of
the colour. Thus, if we mix blue and yellow, and the blue predominates
in the exterior and the yellow in the interior, every ring will appear
yellow at its outer, and blue at its inner edge; and if the colours
present together very slight differences, we may fall into the illusion
which causes the differences really existing between the colours of the
different rings to disappear, leaving instead, on a uniformly coloured
background, the contrasting blue and yellow of the edges of the rings.
It is very characteristic that in these cases we do not see the mixed
colours, but seem to see the constituent colours separately, one beside
the other, and one through the other.

All the experiments we have described afford great interest to the
student; they can easily be performed by those of our readers who are
particularly interested in these little-known subjects. Any one may
construct the greater part of the appliances we have enumerated, and
others can be obtained at an optician’s. The discs in particular are
extensively manufactured, and with great success.


FOOTNOTES:

[10] _Traité de Physique_, Paris 1874.

[11] _Traité d’optique Physiologique._ French translation by MM. Javal
and Klein.



CHAPTER XI.

 OPTICAL ILLUSIONS—ZOLLNER’S DESIGNS—THE
 THAUMATROPE—PHENOKISTOSCOPE—THE ZOOTROPE—THE PRAXINOSCOPE—THE DAZZLING
 TOP.


We shall now continue the subject by describing some illusions more
curious still—those of _ocular estimation_. These illusions depend
rather on the particular properties of the figures we examine, and the
greater part of these phenomena may be placed in that category whose
law we have just formulated: _the differences clearly perceived appear
greater than the differences equal to them, but perceived with greater
difficulty_. Thus a line —— when divided appears greater than when not
divided; the direct perception of the parts makes us notice the number
of the sub-divisions, the size of which is more perceptible than when
the parts are not clearly marked off. Thus, in fig. 115, we imagine the
length _ab_ equals _bc_, although _ab_ is in reality longer than _bc_.
In an experiment consisting of dividing a line into two equal parts,
the right eye tends to increase the half on the right, and the left eye
to enlarge that on the left. To arrive at an exact estimate, we turn
over the paper and find the exact centre.

[Illustration: Fig. 115.]

[Illustration: Fig. 116.]

Illusions of this kind become more striking when the distances to be
compared run in different directions. If we look at A and B (fig. 116),
which are perfect squares, A appears greater in length than width,
whilst B, on the contrary, appears to have greater width than length.
The case is the same with angles. On looking at fig. 117, angles 1,
2, 3, 4 are straight, and should appear so when examined. But 1 and 2
appear pointed, and 3 and 4 obtuse. The illusion is still greater if we
look at the figure with the right eye. If, on the contrary, we turn it,
so that 2 and 3 are at the bottom, 1 and 2 will appear greatly pointed
to the left eye. The divided angles always appear relatively greater
than they would appear without divisions.

The same illusion is presented in a number of examples in the course of
daily life. An empty room appears smaller than a furnished room, and a
wall covered with paperhangings appears larger than a bare wall. It is
a well-known source of amusement to present someone in company with a
hat, and request him to mark on the wall its supposed height from the
ground. The height generally indicated will be a size and a half too
large.

We will relate an experience described by Bravais: “When at sea,”
he says, “at a certain distance from a coast which presents many
inequalities, if we attempt to draw the coastline as it presents
itself to the eye, we shall find on verification that the horizontal
dimensions have been correctly sketched at a certain scale, while all
the vertical angular objects have been represented on a scale twice as
large. This illusion, which is sure to occur in estimates of this kind,
can be demonstrated by numerous observations.”

M. Helmholtz has also indicated several optical illusions.

[Illustration: Fig. 117.]

[Illustration: Fig. 118.]

[Illustration: Fig. 119.]

If we examine fig. 118, the continuation of the line _a_ does not
appear to be _d_,—which it is in reality,—but _f_, which is a little
lower. This illusion is still more striking when we make the figure on
a smaller scale (fig. 119), as at B, where the two fine lines are in
continuation with each other, but do not appear to be so, and at C,
where they appear so, but are not in reality. If we draw the figures
as at A (fig. 118), leaving out the line _d_, and look at them from
a gradually increasing distance, so that they appear to diminish, it
will be found that the further off the figure is placed, the more it
seems necessary to lower the line _f_ to make it appear a continuation
of _a_. These effects are produced by irradiation; they can also be
produced by black lines on a white foundation. Near the point of the
two acute angles, the circles of diffusion of the two black lines touch
and mutually reinforce each other; consequently the retinal image of
the narrow line presents its maximum of darkness nearest to the broad
line, and appears to deviate on that side. In figures of this kind,
however, executed on a larger scale, as in fig. 118, irradiation
can scarcely be the only cause of illusion. We will continue our
exposition as a means of finding an explanation. In fig. 120, A and B
present some examples pointed out by Hering; the straight, parallel
lines, _a b_, and _c d_, appear to bend outwards at A, and inwards
at B. But the most striking example is that represented by fig. 121,
published by Zollner.

The vertical black strips of this figure are parallel with each other,
but they appear convergent and divergent, and seem constantly turned
out of a vertical position into a direction inverse to that of the
oblique lines which divide them. The separate halves of the oblique
lines are displaced respectively, like the narrow lines in fig. 119.
If the figure is turned so that the broad vertical lines present
an inclination of 45° to the horizon, the convergence appears even
more remarkable, whilst we notice less the apparent deviation of the
halves of the small lines, which are then horizontal and vertical. The
direction of the vertical and horizontal lines is less modified than
that of the oblique lines. We may look upon these latter illusions as
fresh examples of the aforesaid rule, according to which acute angles
clearly defined, but of small size, appear, as a rule, relatively
larger when we compare with obtuse or right angles which are undivided;
but if the apparent enlargement of an acute angle shows itself in such
a manner that the two sides appear to diverge, the illusions given in
figs. 118, 120, and 121, will be the result.

[Illustration: Fig. 120.—The horizontal lines, _a_, _b_, _c_, _d_,
are strictly parallel; their appearance of deviation is caused by the
oblique lines.]

In fig. 118 the narrow lines appear to turn towards the point where
they penetrate the thick line and disappear, to appear afterwards in
continuation of each other. In fig. 120 the two halves of each of the
two straight lines seem to deviate through the entire length in such
a manner that the acute angles which they form with the oblique lines
appear enlarged. The same effect is shown by the vertical lines of fig.
121.

M. Helmholtz is of opinion (figs. 120, 121) that the law of contrast
is insufficient to entirely explain the phenomena, and believes that
the effect is also caused by the movements of the eye. In fact, the
illusions almost entirely disappear, if we fix on a point of the object
in order to develop an accidental image, and when we have obtained one
very distinctly, which is quite possible with Zollner’s design (fig.
121), this image will present not the slightest trace of illusion. In
fig. 118 the displacement of the gaze will exercise no very decided
influence on the strengthening of the illusion; on the contrary, it
disappears when we turn our eyes on the narrow line, _ad_. On the other
hand, the fixing of the eyes causes the illusion to disappear with
relative facility in fig. 120, and with more difficulty in fig. 121;
it will, however, disappear equally in the latter design, if we fix it
immovably, and instead of considering it as composed of black lines on
a white background, we compel ourselves to picture it as white lines on
a black foundation; then the illusion vanishes. But if we let our eyes
wander over the illustration, the illusion will return in full force.
We can indeed succeed in completely destroying the illusion produced by
these designs by covering them with a sheet of opaque paper, on which
we rest the point of a pin. Looking fixedly at the point, we suddenly
draw away the paper, and can then judge if the gaze has been fixed and
steady according to the clearness of the accidental image which is
formed as a result of the experiment.

[Illustration: Fig. 121.—The vertical strips are parallel; they appear
convergent or divergent under the influence of the oblique lines.]

[Illustration: Fig. 122.—Observation of electric spark.]

The light of an electric spark furnishes the surest and simplest means
of counteracting the influence of movements of the eyes, as during the
momentary duration of the spark the eye cannot execute any sensible
movement. For this experiment the present writer has made use of a
wooden box, A B C D (fig. 122), blackened on the inside. Two
holes are made for the eyes on each side of the box, _f_ and _g_. The
observer looks through the openings, _f_, and in front of openings,
_g_, the objects are placed; these are pierced through with a pin,
which can be fixed by the eyes in the absence of the electric spark,
when the box is perfectly dark. The box is open, and rests on the
table, B D, to allow of changing the object. The conducting
wires of electricity are at _h_ and _i_; in the centre of the box is a
strip of cardboard, white on the side facing the spark, the light of
which it shelters from the eye of the observer and throws back again
on the object. With the electric light the illusion was completely
perceptible with fig. 118, while it disappeared altogether in fig. 120;
with fig. 121 it was not entirely absent, but when it showed itself,
it was much more feeble and doubtful than usual, though the intensity
of light was quite sufficient to allow of the form of the object being
very distinctly examined. Thus two different phenomena have to be
explained; first, the feeble illusion which is produced without the
intervention of movements of the eye; and secondly, the strengthening
of the illusion in consequence of these movements. The law of contrast
sufficiently explains the first; that which one perceives most
distinctly with indirect vision is the concordance of directions with
dimensions of the same kind. We perceive more distinctly the difference
of direction presented at their intersection by the two sides of an
acute or obtuse angle, than the deviation that exists between one of
the sides and the perpendicular which we imagine placed on the other
side, but which is not marked. By being distributed on both sides,
the apparent enlargement of the angles gives way to displacements,
and changes of direction of the sides. It is difficult to correct
the apparent displacement of the lines when they remain parallel to
their true direction; for this reason, the illusion of the figure is
relatively more inflexible. Changes of direction, on the contrary,
are recognised more easily if we examine the figure attentively, when
these changes have the effect of causing the concordance of the lines
(which accord in reality) to disappear; it is probably because of the
difference in aspect of the numerous oblique lines of figs. 120 and
121 that the concordance of these lines escapes the observer’s notice.
As regards the influence exercised by the motion of the eyes in the
apparent direction of the lines, M. Helmholtz, after discussing the
matter very thoroughly, proves the strengthening of the illusion in
Zollner’s illustration to be caused by those motions. It is not now our
intention to follow out the whole of this demonstration; it will be
sufficient to point out to the reader a fruitful force of study, with
but little known results.

The Romans were well acquainted with the influence of oblique lines. At
Pompeii, fresco paintings are to be found, in which the lines are not
parallel, so that they satisfy the eye influenced by adjacent lines.
Engravers in copper-plate have also studied the influence of _etchings_
on the parallelism of straight lines, and they calculate the effect
that they will produce on the engraving. In some ornamentations in
which these results have not been calculated, it sometimes happens that
parallel lines do not appear parallel because of the influence of other
oblique lines, and a disagreeable effect is produced. A similar result
is to be seen at the railway station at Lyons, the roof of which is
covered with inlaid work in _point de Hongrie_. The wide parallel lines
of this ceiling appear to deviate, a result produced by a series of
oblique lines formed by the planks of wood.

[Illustration: Fig. 123.—Two sides of a Thaumatrope disc.]

Having given a long account of the result of M. Helmholtz’s labours,
we will pass to the consideration of another kind of experiments,
or rather appliances, based on the illusions of vision, and the
persistence of impressions on the retina.

The Thaumatrope, to which we have already referred, is a plaything
of very ancient origin, based on the principle we have mentioned. It
consists of a cardboard disc, which we put in motion by pulling two
cords. On one side of the disc a cage, _a_, is portrayed, on the other
a bird, _b_ (fig. 123). When the little contrivance is turned round,
the two designs are seen at the same time, and form but one image—that
of a bird in its cage (fig. 124). It is of course hardly necessary to
add that the designs may be varied.

We have already referred to M. Plateau’s rotating disc (the
Phenakistoscope). Through the narrow slits we perceive in succession
representations of different positions of a certain action. The
persistence of the luminous impressions on the retina gives to the eye
the sensation of a continuous image, which seems animated by the same
movements as those portrayed in the different phases (fig. 125).

[Illustration: Fig. 124.—Appearance of the Thaumatrope in rotation.]

[Illustration: Fig. 125.—Plateau’s Phenokistoscope.]

The Zootrope (fig. 126) is a perfected specimen of this apparatus. It
is composed of a cylinder of cardboard, turning on a central axis.
The cylinder is pierced with vertical slits at regular intervals, and
through which the spectator can see the designs upon a band of paper
adapted to the interior of the apparatus in rotation. The designs are
so executed that they represent the different times of a movement
between two extremes; and in consequence of the impressions upon the
retina the successive phases are mingled, so the spectator believes he
sees, without transition, the entire movement. We give a few specimens
of the pictures for the Zootrope (fig. 127). We have here an ape
leaping over a hedge, a dancing “Punch,” a gendarme pursuing a thief,
a person holding the devil by the tail, a robber coming out of a box,
and a sportsman firing at a bird. The extremes of the movement are
right and left; the intermediary figures make the transitions, and they
are usually equal in number to the slits in the Zootrope. It is not
difficult to construct such an instrument, and better drawings could be
made than the specimens taken at random from a model. The earth might
be represented turning in space, or a fire-engine pumping water could
be given, and thus the Zootrope might be quite a vehicle of instruction
as well as of amusement. This instrument is certainly one of the most
curious in the range of optics, and never fails to excite interest. The
ingenious contrivances which have up to the present time reproduced it,
all consist in the employment of narrow slits, which besides reducing
the light to a great extent, and consequently the light and clearness
of the object, require the instrument to be set in rapid rotation,
which greatly exaggerates the rapidity of the movements represented,
and without which the intermissions of the spectacle could not unite in
a continuous sensation.

[Illustration: Fig. 126.—The Zootrope.]

[Illustration: Fig. 127.—Pictures used in the Zootrope.]

We present here an apparatus based on a very different optical
arrangement. In the _Praxinoscope_[12] (a name given by the inventor,
Mr. Reynaud, to this new apparatus), the substitution of one object for
another is accomplished without interruption in the vision, or solution
of continuity, and consequently without a sensible reduction of light;
in a word, the eye beholds _continuously_ an image which, nevertheless,
is incessantly changing before it. The result was obtained in this
manner. Having sought unsuccessfully by mechanical means to substitute
one object for another without interrupting the continuity of the
spectacle, the inventor was seized with the idea of producing this
substitution, not with the objects themselves, but with their virtual
images. He then contrived the arrangement which we will now describe.
A plane mirror, AB (fig. 129), is placed at a certain distance from
an object, CD, and the virtual image will be seen at C′D′. If we then
turn the plane mirror and object towards the point, O, letting BE and
DF be their new positions, the image will be at C″D″. _Its axis_, O,
_will not be displaced_. In the positions, AB and CD, first occupied
by the plane mirror and the object, we now place another mirror and
object. Let us imagine the eye placed at M. Half of the first object
will be seen at OD″, and half of the second at OC′. If we continue
the rotation of the instrument, we shall soon have mirror No. 2 at
TT′, and object No. 2 at SS′. At the same moment the image of object
No. 2 will be seen entirely at C‴D″. Mirror No. 2 and its object will
soon after be at BE and DF. If we then imagine another mirror and its
corresponding object at AB and CD, the same succession of phenomena
will be reproduced. This experiment therefore shows that a series of
objects placed on the perimeter of a polygon will be seen successively
at the centre, if the plane mirrors are placed on a concentric polygon,
the “apothème” of which will be less by one-half, and which will be
carried on by the same movement. In its practical form, M. Reynaud’s
apparatus consists of a polygonal or simply circular box (fig. 128),
(for the polygon may be replaced by a circle without the principle or
result being changed), in the centre of which is placed a prism of
exactly half a diameter less, the surface of which is covered with
plane mirrors. A strip of cardboard bearing a number of designs of
the same object, portrayed in different phases of action, is placed
in the interior of the circular rim of the box, so that each position
corresponds to a plate of the glass prism. A moderate movement of
rotation given to the apparatus, which is raised on a central pivot,
suffices to produce the substitution of the figures, and the animated
object is reflected on the centre of the glass prism with remarkable
brightness, clearness, and delicacy of movement. Constructed in this
manner, the _Praxinoscope_ forms an optical toy both interesting and
amusing. In the evening, a lamp placed on a support _ad hoc_, in the
centre of the apparatus, suffices to light it up very clearly, and a
number of persons may conveniently assemble round it, and witness the
effects produced.

[Illustration: Fig. 128.—M. Reynaud’s Praxinoscope.]

[Illustration: Fig. 129.]

Besides the attractions offered by the animated scenes of the
_Praxinoscope_, the apparatus may also be made the object of useful
applications in the study of optics. It permits an object, a drawing,
or a colour, to be substituted instantaneously in experiments on
secondary or subjective images, etc., on the contrast of colours or
the persistence of impressions, etc. We can also make what is called
a _synthesis of movements_ by placing before the prism a series of
diagrams of natural objects by means of photography.

M. Reynaud has already arranged an apparatus which exhibits in the
largest dimensions the animated reflection of the _Praxinoscope_, and
which lends itself to the demonstration of curious effects before
a numerous auditory. The ingenious inventor has recently contrived
also a very curious improvement in the original apparatus. In the
_Praxinoscope Theatre_ he has succeeded in producing truly ornamental
_tableaux_, as on a small Lilliputian stage, in the centre of which the
principal object moves with startling effect. To obtain this result, M.
Reynaud commences by cutting out in black paper the different figures,
the whole of which will form an object animated by the rotation
given to the _Praxinoscope_. To supply the decorations, he arranges
on the black foundation the image of an appropriate coloured design
by means of a piece of glass. It is well known that transparent glass
possesses the property of giving a reflection of the objects on the
nearest side as well as on the farthest. We may recall the applications
of this optical effect in theatres, and also in courses of physics,
under the title of _impalpable spectres_. It is also by reflection
on thin, transparent glass, that M. Reynaud produces the image of
the ornamentations in the _Praxinoscope Theatre_. The decorations
are really placed in the lid, which is held by a hook in a vertical
position, thus forming the front side of the apparatus (fig. 130). In
this side a rectangular opening is made, through which the spectator
(using both eyes) perceives at the same time the animated reflection
of the _Praxinoscope_, and the immovable image of the decorations
reflected in the transparent glass. The position of the latter and
its distance from the coloured decorations are arranged so that the
reflection is thrown behind the moving figure, which consequently
appears in strong relief against the background, the effect produced
being very striking. It is evident that to change the decorations it
is only necessary to place in succession on a slide the different
_chromos_ representing landscapes, buildings, the interior of a
circus, etc. It is easy to choose an arrangement suitable for each of
the moving figures placed in the _Praxinoscope_. By this clever and
entirely novel optical combination, the mechanism of the contrivance
is entirely lost sight of, leaving only the effect produced by the
animated figures, which fulfil their different movements on the little
stage. The _Praxinoscope Theatre_ can also be used as well in the
evening as in the daytime. By daylight, it is sufficient to place it
before a window, and in the evening the same effects may be produced,
perhaps in even a more striking manner, by simply placing a lamp on the
stand, with a small plated reflector, and a lamp-shade. The illusion
produced by this scientific plaything is very complete and curious,
and M. Reynaud cannot be too much commended for so cleverly applying
his knowledge of physics in the construction of an apparatus which is
at the same time both an optical instrument and a charming source of
amusement.

[Illustration: Fig 130.—The Praxinoscope Theatre.]

[Illustration: Fig. 131.—The Dazzling Top.]

Amongst the toys founded upon the persistency of impressions upon the
retina we may instance the “Dazzling Top” (fig. 131). This remarkable
invention is quite worthy of a place in every cabinet, and is an
ingenious specimen of a perfected Helmholtz top. It is a metallic toy
put in motion by means of a cord wound round a groove. The axis is
hollow, admits a metallic stem, and fits into a handle which is held in
the hand. The top is placed upon a little cup in an upright position,
and it is then set spinning in the usual way with the cord. The stem
and handle are then withdrawn, and as the top will continue to spin for
a long time, discs and various outline shapes can be fixed upon it, and
various objects will be shadowed thereon. Cups, bowls, candlesticks,
and jugs can be seen plainly revolving as the top carries the wire
representation in outline rapidly past the eyes. Coloured cardboard can
be worked into various patterns, and much amusement will be created
amongst children and young people.


FOOTNOTES:

[12] From _praxis_, action, and _skopein_, to show.



CHAPTER XII.

 OPTICAL ILLUSIONS CONTINUED—EXPERIMENTS—THE TALKING HEAD—GHOST
 ILLUSIONS.


The enumeration of optical illusions is so considerable that we have
no intention of describing them all, and will merely cite a few other
examples. The following facts have been communicated to us by M.
Nachet:—

[Illustration: Fig. 132.—Hexagonal appearance formed by circles joined
together.]

When examining algæ under the microscope, we notice the spaces which
separate the streaks ornamenting the silicious covering of these
various organisms, and it is explained that they are formed by hexagons
visible only when we examine the object with a powerful microscope.
“For a long time,” says M. Nachet, “I occupied myself with the
examination of the hexagonal appearance of the points constituting
the streaks. Why should these hexagons show themselves, and how could
they be other than the visible base of small pyramids piled very
closely one on the other; and if this were the case, why were not the
points of the little pyramids visible? Or, was the structure before me
analogous to that of the eyes of insects? Then the carapace would be
but a surface of perforated polygonal openings. This latter hypothesis
was attractive enough, and would have explained many things; but some
careful observations with very powerful object-glasses, quite free
from blemishes, had shown me that these hexagons had round points,
contrary to the descriptions of micrographs. These observations,
corroborated by the micrographic photographs of Lackerbauer, the
much-regretted designer, and by Colonels Woodward and Washington, left
not the slightest doubt that it was necessary to discover why the
eye persistently saw hexagons where there were circles. To elucidate
this point, it was necessary to find some means of reproducing
artificially what nature had accomplished with so much precision on
the surfaces of algæ. After many fruitless attempts, I decided on
making a trial of a stereotype plate covered with dots arranged in
quincunxes, very close together” (figs. 132 and 133). “The result was
more successful than I had hoped; the effect produced is exactly that
of the arrangement of the so-called hexagons of the most beautiful
of the algæ, the _Pleurosigma angulata_. If these stereotypes are
examined with one eye only, we shall be immediately convinced that we
have to do with hexagonal polygons.” It is useless to give any long
exposition of a figure so clearly explanatory; it is simply an effect
of the contrast and opposition of the black and white in the sensation
of the retina. This effect is particularly striking with fig. 134, a
negative photograph heliographically engraved according to fig. 133.
In this the white points seem to destroy the black spaces, and to
approach each other tangentially, and the irradiation is so intense
that the white circles appear much larger than the black of fig. 133,
although of the same diameter. There are in these facts many points
which may interest not only students of micrography, but also artists.
As to the algæ, the origin of this investigation, it remains to be
discovered if these circles which cover their silicious carapace are
the projection of small hemispheres, or the section of openings made in
the thick covering. Certain experiments, however, seem to prove that
they are hemispheres, and the theory is also confirmed by a microscopic
photograph from Lackerbauer’s collection, magnified 3,000 diameters,
in which a black central point is seen in the centre of each circle, a
certain reflection of the luminous source reproduced in the focus of
each of the small demi-spheres which constitute the ornament of the
algæ. The microscope, which has progressively shown first the streaks,
then the hexagons, and then the round points, will surely clear up the
point some day or other.

[Illustration: Fig. 133.—Another figure of the same kind.]

[Illustration: Fig. 134.—Third figure.]

Mr. Silvanus P. Thompson, Professor of Physics at University College,
Bristol, has recently presented the French Society of Physical Science
with a curious example of optical illusion, the true cause of which is
not clearly known, but which we may compare with other facts made known
some time ago, of which no precise explanation has been given. Let us
first consider in what the effect discovered by Mr. S. P. Thompson
consists, according to the description that has been given of it by
M. C. M. Gariel; the illustrations here given will also allow of our
verifying the truth of the statements.

[Illustration: Fig. 135.        Fig. 136.

Mr. Thompson’s optical illusion. Give a circular movement to these
figures, and the circles will appear to turn round.]

The first illustration consists of a series of concentric circles of
about the width of a millimetre, separated by white intervals of the
same size (fig. 135). These dimensions are not absolute; they vary with
the distance, and may even be a few inches in width if it is desired
to show the phenomenon to a rather numerous auditory. If we hold the
design in the hand, and give it a twirl by a little movement of the
wrist, the circle appears to turn round its centre, and the rotation
is in the same direction, and is equally swift; that is to say, the
circle appears to accomplish a complete turn, whilst the cardboard
really accomplishes one in the same direction. For the second effect
we draw a dark circle, in the interior of which are placed a number
of indentations at regular intervals (fig. 136). Operating in the
same manner as described above, this notched wheel appears to turn
round its centre, but this time in a different direction from the real
movement. In this, however, as in the other design, the effect is more
satisfactory if we do not look directly at it; the movements also are
particularly striking in combinations such as that represented in
fig. 137, in which the multiplicity of circles does not allow us to
fix one specially. We may add that the same effects may be obtained
with eccentric wheels, or even with other curves than circles. By
means of a photograph on glass, Mr. Thompson has been able to reflect
these designs on a screen where they were obtained on a large scale; a
circular movement was communicated to the photographic plate, so that
the design moved in a circular manner on the screen, and in this case
also there existed the illusion that every circle seemed turning round
its centre. And what is the explanation of these curious effects? Mr.
Thompson does not believe (and we share his opinion) that the faculty
possessed by the retina of preserving images during a certain time
(_persistence of impressions on the retina_) can entirely explain these
phenomena. Without desiring to formulate a decided theory, Mr. Thompson
is of opinion that we may class these facts with others which have been
known for some time, and that perhaps it is necessary to attribute to
the eye some new faculty which may explain the whole at once.

[Illustration: Fig. 137.—Another figure of Mr. Thompson’s. The
different circles appear to turn round if we give the design a rotating
movement.]

Brewster and Adams have described phenomena which are equally curious,
the principal of which we will describe, adding also some analogous
investigations due to Mr. Thompson. The result seems to be that there
exists in the eye a badly-defined purpose of nature, which in a certain
way _compensates_ (Brewster) for the real phenomenon, because it has a
contrary effect, which will continue for some time after the cessation
of the phenomena, and which gives by itself a sensation contrary to
that which the real movement would have produced. Thus, after having
fixed our eyes for two or three minutes on a rushing waterfall, if we
suddenly turn our glance on the adjacent rocks, the latter appear to
move from top to bottom. It is not a question here of the effect of
the relative movement to be observed on regarding _simultaneously_ the
falling water and the rocks; if one can succeed in abstracting oneself
to such an extent that the water appears motionless, the rocks appear
to take a contrary movement. In the phenomenon we describe there is no
simultaneous comparison; the eyes are turned _successively_ first on
the water, and then on the rocks. In a rapid river, such as the Rhine
above the fall at Schaffhausen, the stream is not equally swift in
every part, and the current is noticeably more rapid in the middle of
the river than near the banks. If we look fixedly at the centre of the
stream, and then suddenly turn our eyes towards the banks, it will
appear as though the river were flowing back towards its source. This
kind of _compensation_ does not only produce an apparent displacement,
but also changes in size. When travelling at great speed in a railway
train, the objects of the surrounding country as one flies by them
gradually appear smaller and smaller. If, when this occurs, we suddenly
remove our eyes to the interior of the railway carriage, and fix them
on immovable objects, such as the sides of the compartment, or the
faces of our travelling companions, the images on the retina will
really preserve the same size, and yet the objects will appear larger.
Such are some of the interesting facts among those discovered by Mr.
Thompson; and though we do not intend to push the inquiry further,
we think it may not be without interest to describe here another
illusion of that organ whose properties are in every way so curious and
remarkable.

[Illustration: Fig. 138.—Experiment on complementary colours.]

[Illustration: Fig. 139.—Design for experiment or the _punctum cæcum_.]

Another experiment to show the existence of impressions received by the
retina can be made with the figure above (138). If the gaze be fixed
upon the dark spot in the centre of the white figure for about half a
minute, and the eyes then directed to the ceiling, or a sheet of white
paper, the white figure will be reproduced _in black_. This result is
based upon the principle of complementary colours. A red design, for
instance, will be reproduced in green.

There is a dark spot in every human eye—that is, a spot which is
insensible to light. The eye is generally regarded as a perfect
instrument, but it is not yet so by any means. One of our great
philosophers remarked that if an instrument were sent home to him so
full of errors he would feel justified in returning it to the optician.
But the eye has its dark place, the _punctum cæcum_, and it can be
discovered by covering the left eye with the hand, and holding the
present page at arm’s length with the other. Then fix the gaze on the
small cross in the picture, and bring the book close up. At a little
distance the white ball will disappear from the page (fig. 139).

[Illustration: Fig 140.—An optical illusion.]

The illustration (fig. 140) shows us a very curious optical illusion,
and one very easy to practise. Roll up a sheet of paper, and look
through it, as through a telescope, with the right eye, _keeping both
eyes open_. Then place the left hand open palm towards you against the
roll of paper, you will then appear to be looking through a hole in
your left hand. Sometimes the effect is produced without holding up the
other hand to the roll, as shown in fig. 140.

Among optical illusions there are a great number that may be produced
by means of mirrors. The divided telescope is an example. The
apparatus, raised on a firm stand, allows of one apparently seeing an
object through a stone or other opaque object, as shown in fig. 141.
The illustration shows the arrangement of the apparatus. The observer,
looking through it, plainly perceives the object through the glass;
the image is reflected four times before reaching his eye, by means of
small mirrors concealed in the instrument.

[Illustration: Fig. 141.—A divided telescope.]

Convex or concave mirrors distort images in a singular manner, and
produce very interesting effects. _Anamorphoses_ constitute particular
objects belonging specially to the class of experiments relating to
cylindrical mirrors. They are images made according to determined
rules, but so distorted that when regarding them fixedly we can only
distinguish confused strokes. When they are seen reflected in the
curved mirrors, they present, on the contrary, a perfectly regular
appearance. Fig. 142 exhibits an Anamorphosis made by a cylindrical
mirror. It will be seen that the confused image of the horizontal paper
is reflected in the cylindrical mirror, producing the figure of a
juggler. It is easy to contrive similar designs one’s-self; and comical
mirrors may also be employed which produce particular effects of a no
less interesting kind. The next illustration is of a set of figures
which in a cylindrical mirror look like the ten of hearts (fig. 143).

[Illustration: Fig. 142.—Cylindrical Mirror and Anamorphosis.]

[Illustration: Fig. 143.—Anamorphus design for the ten of hearts.]

One of the most remarkable applications of mirrors in amusing
experiments is undoubtedly that of the severed and _talking head_. A
few years back this trick obtained considerable success in Paris and a
number of other towns. The spectators beheld a small space set apart,
in which was placed a table on three legs; on this table was a human
head, placed in cloth on a dish. The head moved its eyes and spoke;
it evidently belonged to a man whose body was completely hidden. The
spectators thought they saw an empty space beneath the table, but the
body of the individual who was really seated there was concealed by
two glasses placed at an inclination of 45° to the walls on the right
and left. The whole was arranged in such a manner that the reflection
of the walls coincided with the visible part of the wall at the back
of the room. The three walls were painted in one colour, and a subdued
light increased the illusion, the effect of which was remarkable (fig.
144).

[Illustration: Fig. 144.—The talking head.]

The spectres designed by Robin also attracted considerable public
attention within recent times. They were images formed by the medium
of transparent glass. Glass panes often produce the phenomenon of
spectres. In the evening, when it is dark out of doors, it is easy
to prove that the reflection of objects in a lighted room can be
reproduced behind the window panes by reason of the darkness outside.
If we approach the window pane, we see also the real objects outside,
a balcony, tree, etc. These real objects mingle with the reflected
image, and combine to produce very curious effects. In this way M.
Robin has contrived the effects of the theatre. He throws on the stage
the reflection of a person dressed as a Zouave, and he himself, armed
with a sabre, stabs the spectre through the body. A great number of
other singular effects have been obtained in the same manner. Pepper’s
Ghost was managed in this way.

[Illustration: Fig. 145.—The ghost effect.]

Within recent times, images produced in a similar way have been
utilized to facilitate the study of drawing. A piece of glass is fixed
vertically on a black board (fig. 146). A model to copy from is placed
on one side of the piece of glass, and is arranged so that the visual
ray passes obliquely through the glass, and we perceive the reflection
of the design very clearly on the other side. It is then very easily
reproduced with a pencil on a sheet of white paper by tracing the
outlines.

[Illustration: Fig. 146.—Drawing by reflection.]



CHAPTER XIII.

 VISION—THE EYE—THE STEREOSCOPE—SPECTRUM ANALYSIS—THE
 SPECTROSCOPE—THE TELESCOPE AND MICROSCOPE—PHOTOGRAPHY—DISSOLVING
 VIEWS—LUMINOUS PAINT.


The eye and vision are such important subjects to all of us that
we may be excused for saying something more concerning phenomena
connected with them, and the instruments we use for assisting them.
We do not propose to write a treatise upon the physiology of vision,
for we know the image in the eye is produced physically in the same
manner as the image in a camera obscura. In the eye the sides of the
box are represented by the sclerotic (see chap. x. fig. 95); the dark
inner surface has its parallel in the pigment of the choroid; the
opening in the box in the pupil of the eye; the convex lens in the
crystalline and the cornea; and the retina receives the image. But why
we see—beyond the fact that we do see—no one can explain. Science is
dumb on the subject. Thought and consciousness elude our grasp; and, as
Professor Tyndall says on this subject, “we stand face to face with the
incomprehensible.”

But there are many interesting facts connected with our vision which
may be plainly described. Some people are obliged to carry an object
(or a book) to within ten inches of the eye to see it distinctly; and
a person who does not possess convergent power of the eye will have
to move it farther off, or use convex glasses; while a “near-sighted”
person, whose eyes are too quickly convergent, will use concave glasses
to bring the object near to the eye.

There is but one small place in the retina of the eye which admits
of perfect vision. This, the most sensitive portion, is called the
“yellow spot,” and vision becomes more and more indistinct from this
point towards the circumference. This can be proved by any one; for
in reading we are obliged to carry our eyes from word to word, and
backwards and forwards along the lines of print. Another very important
element in our vision is the contraction and enlargement of the iris
around the pupil. In cases where strong light would only dazzle us
the iris expands, and the pupil is contracted to a sufficient size to
accommodate our vision. At night, or in a darkened room, the pupil is
enlarged. This change will account for our not being immediately able
to see objects when we have passed from darkness to great light, or
_vice versâ_. The iris must have time to accommodate itself to the
light.

Now, outside the small space of perfect vision above mentioned, there
is a circle of considerable extent, called the “field of vision.” In
man this field, when the eyes are fixed, subtends an angle of about
180°, because beyond that the rays cannot enter the pupil of the eye.
But in the lower animals, the fish and birds,—notably the ostrich,—the
field of vision is much more extensive, because the pupils are more
prominent, or the eyes are set more towards the sides of the head. The
ostrich can see behind him, and fish can see in any direction without
apparent limit. Man can only see indistinctly; and though he can move
his eyes rapidly, he can see distinctly but a small portion of any
object at a time, yet he sees with both eyes simultaneously a single
object, because the two lines of vision unite at a single point, and as
the two images cover each other we perceive only one image. Beyond or
within this point of meeting the vision is indistinct, but the angle of
convergence is always varied according to the distance of the object.
If we hold up a penholder in front of us, and in a line with any other
defined object,—say an ink-bottle,—we can see the penholder distinctly,
and the ink-bottle indistinctly, as _two_ images. If we then look at
the ink-bottle we shall see it single, while the penholder will appear
double, but with imperfect outlines.

[Illustration: Fig. 147.—The Stereoscope.]

[Illustration: Fig. 148.—Mode of taking photograph for Stereoscope.]

Again, if we look at a box both eyes will see it equally well, but the
right eye will see a little more on its right side, and the left eye on
the left. It is on this principle that the Stereoscope is constructed.
Sir Charles Wheatstone was the inventor, and the instrument may be
thus described:—Two pictures are taken by photography—one as the
landscape is seen by the right eye, the other as it is viewed by the
left; the points of view thus differing slightly. When both eyes are
simultaneously applied to the instrument the view is seen exactly as
it would appear to the beholder at the actual place it represents. The
views are taken singly; one side at one time, and another after, as in
the camera (fig. 148). A is the first view; B is kept dark; C is the
shutter for A. There are _Reflecting_ and _Refracting_ Stereoscopes.
In the former a mirror reflects the image into each eye; in the latter
the views are pasted on a card, side by side, and looked at through
prismatic lenses. The principles of _Binocular Vision_ have been
applied to the Microscope.

In foregoing chapters we have given many examples with diagrams of the
temporary impressions made upon the retina of the eye. It is a fact
that a wheel revolving at a great rate will appear to be standing still
when suddenly illuminated by a flash of lightning, because the eye has
not time to take in the motion in the instant of time, for the spokes
of the wheel are not moving fast enough to convey the impression of
motion in that half second to the eye; yet the perfect outline of the
wheel is distinctly visible.

Indeed, distinct vision can be exercised in a very small fraction
of a second. It was calculated by Professor Rood, and proved by
experiment, that _forty billionths_ of a second is sufficient time for
the eye to distinguish letters on a printed page. It this instance the
illuminating power was an electric spark from a Leyden Jar.

We have remarked upon the distinctness with which we can see an object
when we direct our gaze upon it, and this appears a self-evident
proposition; but have any of our readers remarked the curious fact
that when they want to see a faint and particular star in the sky
it will at once disappear when they gaze at it? The best way to see
such very faint orbs as this is to _look away from them_,—a little to
one side or the other,—and then the tiny point will become visible
again to the eye. There is also a degree of phosphorescence in the
eye, which any one who receives a blow upon that organ will readily
admit. Even a simple pressure on the closed lid will show us a circle
of light and “colours like a peacock’s tail,” as the great Newton
expressed it. There are many occasions in which light is perceived in
the eye—generally the result of muscular action; and the Irish term to
“knock fire out of my eye” is founded upon philosophical facts.

We are many of us aware of “spots” on our eyes when our digestion is
out of order, and the inability of the eye to see figures distinctly in
a faint light—within a proper seeing distance, too—has often given rise
to the “ghost.” These shadowy forms are nothing more than affections of
the eye, and, as well remarked in Brewster’s Letters on Natural Magic,
“are always _white_ because no other colour can be seen.” The light is
not sufficiently strong to enable the person to see distinctly; and as
the eye passes from side to side, and strives to take in the figure,
it naturally seems shadowy and indistinct, and appears to move as our
eyes move. “When the eye dimly descries an inanimate object whose
different parts reflect different degrees of light, its brighter parts
may enable the spectator to keep up a continued view of it; but the
disappearance and reappearance of its fainter parts, and the change of
shape which ensues, will necessarily give it the semblance of a living
form; and if it occupies a position which is unapproachable, and where
animate objects cannot find their way, the mind will soon transfer to
it a supernatural existence. In like manner a human figure shadowed
forth in a feeble twilight may undergo similar changes, and after being
distinctly seen while it is in a situation favourable for receiving
and reflecting light, it may suddenly disappear in a position before
and within the reach of the observer’s eye; and if this evanescence
takes place in a path or roadway where there is no sideway by which the
figure could escape, it is not easy for an ordinary mind to efface the
impression which it cannot fail to receive.” This will account for many
so-called “ghosts.”

Accidental colours, or ocular spectra, are, so to speak, illusions,
and differently-coloured objects will, when our gaze is turned from
them, give us different “spectra” or images. For instance, a violet
object will, when we turn to a sheet of white paper, give us a
yellow “spectrum”; orange will be blue; black and white will change
respectively; red will become a blue-green. From a very strong white
light the accidental colours will vary.

[Illustration: Fig. 149.—The Solar Spectrum.]

The Solar Spectrum is the name given to the coloured band formed by the
decomposition of a beam of light into its elementary colours, of which
there are seven. This is an easy experiment. A ray of light can be
admitted into a darkened room through a hole in the shutter, and thus
admitted will produce a white spot on the screen opposite, as at _g_ in
the diagram (fig. 149). If we interpose a prism—a triangular piece of
glass—the “drop” of a chandelier will do—we cause it to diverge from
its direct line, and it will produce a longer streak of light lower
down. This streak will exhibit the prismatic colours, or the “colours
of the rainbow”; viz., red (at the top), orange, yellow, green, blue,
indigo (blue), and violet last. These are the colours of the Solar
Spectrum. The white light is thus decomposed, and it is called mixed
light, because of the seven rays of which it is composed. These rays
can be again collected and returned to the white light by means of a
convex lens.

“White light,” said Sir Isaac Newton, “is composed of rays differently
refrangible,” and as we can obtain the colours of the rainbow from
white light, we can, by painting them on a circular plate and turning
it rapidly round, make the plate appear white. Thus we can prove that
the seven colours make “white” when intermingled. But Newton (1675) did
not arrive at the great importance of his experiment. He made a round
hole in the shutter, and found that the various colours overlapped each
other. But, in 1802, Dr. Wollaston improved on this experiment, and by
admitting the light through a tiny slit in the wood, procured an almost
perfect spectrum of “simple” colours, each one perfectly distinct and
divided by black lines.

But twelve years later, Professor Fraunhofer made a chart of these
lines, which are still known by his name. Only, instead of the 576
he discovered, there are now thousands known to us! To Fraunhofer’s
telescope Mr. Simms added a collimating lens, and so the Spectroscope
was begun; and now we use a number of prisms and almost perfect
instruments, dispersing the light through each. We have here an
illustration of a simple Spectroscope, which is much used for chemical
analysis (fig. 150).

In the spectrum we have long and short waves of light, as we have
long and short (high and low) waves in music, called notes. The long
or low notes are as the red rays, the high notes as the blue waves of
light. (Here we have another instance of the similarity between light
and sound.) But suppose we shut out the daylight and substitute an
artificial light. If we use a lamp burning alcohol with salt (chloride
of sodium), the spectrum will only consist of two yellow bands, all the
other colours being absent. With lithium we obtain only two, one orange
and one red. From this we deduce the fact that different substances
when burning produce different spectra; and although a solid may (and
platinum will) give all seven colours in its spectrum, others, as we
have seen, will only give us a few, the portion of the spectrum between
the colours being black. Others are continuous, and transversed by
“lines” or narrow spaces devoid of light; such is the spectrum of the
sun, and by careful and attentive calculation and observation we can
get an approximate idea of the matter surrounding the heavenly bodies.

[Illustration: Fig. 150.—The Spectroscope.]

We have said there are lines crossing the spectrum transversely; these
are called Fraunhofer’s lines, after the philosopher who studied
them; they were, however, discovered by Wollaston. These lines are
caused by light from the lower portion of the sun passing through the
metallic vapours surrounding the orb in a state of incandescence, such
as sodium, iron, etc. One of Fraunhofer’s lines, a black double line
known as D in the yellow portion of the spectrum, was known
to occupy the same place as a certain _luminous_ line produced by
sodium compounds in the flame of a spirit lamp. This gave rise to much
consideration, and at length Kirchkoff proved that the sodium vapour
which gives out yellow light can also absorb that light; and this fact,
viz., that every substance, which at a _certain temperature_ emits
light of a certain refrangibility, possesses at that temperature the
power to absorb that same light. So the black lines are now considered
the reversal of luminous lines due to the incandescent vapours by which
the sun is surrounded. Thus the presence of an element can be found
from black or luminous lines, so the existence of terrestrial elements
in celestial bodies has been discovered by means of preparing charts
of the lines of the terrestrial elements, and comparing them with the
lines of stella _spectra_.

We have supposed the beam of light to enter through a slit in the
shutter, and fall upon a screen or sheet. The solar spectrum shown by
the passage of the beam through a prism is roughly as below—

[Illustration: Fig. 151.—Example of the Spectrum.]

Fraunhofer substituted a telescope for the lens and the screen, and
called his instrument a SPECTROSCOPE. He then observed the lines, which
are always in the same position in the solar spectrum. The principal of
them he designated as A, B, C, D, E, F, G, H. The first three are in
the red part of the spectrum; one in the yellow, then one in the green;
F comes between green and blue, G in the indigo-blue, and H in the
violet. But these by no means exhaust the lines now visible. Year by
year the study of Spectrum Analysis has been perfected more and more,
and now we are aware of more than three thousand “lines” existing in
the solar spectrum. The spectra of the moon and planets contain similar
dark lines as are seen in the solar spectrum, but the fixed stars show
different lines. By spectrum analysis we know the various constituents
of the sun’s atmosphere, and we can fix the result of our observations
made by means of the Spectroscope in the photographic camera. By
the more recent discoveries great studies have been made in “solar
chemistry.”

What can we do with the Spectroscope, or rather, What can we _not_ do?
By Spectroscopy we can find out, and have already far advanced upon
our path of discovery, “the measure of the sun’s rotation, the speed
and direction of the fierce tornados which sweep over its surface, and
give rise to the ‘maelstroms’ we term ‘sunspots,’ and the mighty alps
of glowing gas that shoot far beyond the visible orb, ever changing
their form and size; even the temperature and pressure of the several
layers and their fluctuations are in process of being defined and
determined.” This is what science is doing for us, and when we have
actually succeeded in ascertaining the weather at various depths in
the atmosphere of the sun, we shall be able to predict our own, which
depends so much upon the sun. Last year (1880) Professor Adams, in his
address to the British Association, showed that magnetic disturbances,
identical in kind, took place at places widely apart simultaneously.
He argues that the cause of these identical disturbances must be far
removed from the earth.

“If,” he says, “we imagine the masses of iron, nickel, and magnesium
in the sun to retain even in a slight degree their magnetic power in a
gaseous state, we have a sufficient cause for all our magnetic changes.
We know that masses of metal are ever boiling up from the lower and
hotter levels of the sun’s atmosphere to the cooler upper regions,
where they must again form clouds to throw out their light and heat,
and to absorb the light and heat coming from the hotter lower regions;
then they become condensed, and are drawn back again towards the
body of the sun, so forming those remarkable dark spaces or sunspots
by their down rush to their former levels. In these vast changes we
have abundant cause for those magnetic changes which we observe at
the same instant at distant points on the surface of the earth.” So
we are indebted to the Spectroscope for many wonderful results—the
constitution of the stars, whether they are solid or gaseous, and many
other wonders.

The manner in which we have arrived at these startling conclusions is
not difficult to be understood, but some little explanation will be
necessary.

The existence of dark lines in the solar spectrum proves that certain
rays of solar light are absent, or that there is less light. When we
look through the prism we perceive the spaces or lines, and we can
produce these ourselves by interposing some substance between the
slit in the shutter before mentioned and the prism. The vapour of
sodium will answer our purpose, and we shall find a dark line in the
spectrum, the bright lines being absorbed by the vapour. We can subject
a substance to any temperature we please, and into any condition—solid,
liquid, or gaseous; we can also send the light the substance may give
out through certain media, and we can photograph the spectrum given out
under all conditions. _The distance of the source of the light makes
no difference._ So whether it be the sun, or a far-distant star, we
can tell by the light sent to us what the physical condition of the
star may be. It was discovered in 1864 that the same metallic body
may give different _spectra_; for instance, the spectrum might be a
band of light,—like the rainbow,—or a few isolated colours; or again,
certain detached lines in groups. The brightness of the spectrum lines
will change with the depth of the light-giving source, or matter which
produces it.

We have become aware by means of the Spectroscope that numerous metals
known to us on earth are in combustion in the sun, and new ones have
thus been discovered. In the immense ocean of gas surrounding the
sun there are twenty-two elements as given by Mr. Lockyer, including
iron, sodium, nickel, barium, zinc, lead, calcium, cobalt, hydrogen,
potassium, cadmium, uranium, strontium, etc. Not only is the visible
spectrum capable of minute examination, but, as in the case of the
_heat spectrum_ already mentioned when speaking of Calorescence, the
light spectrum has been traced and photographed far beyond the dark
space after the blue and violet rays, seven times longer than the
visible solar spectrum—a spectrum of light invisible to our finite
vision. Although a telescope has been invented for the examination of
these “ultra violet” rays, no human eye can see them. But—and here
science comes in—when a photographic plate is put in place of the
eye, the tiniest star can be seen and defined. Even the Spectroscope
at length fails, because light at such limits has been held to be
“too coarse-grained for our purposes”! “Light,” says a writer on this
subject “we can then no longer regard as made of smooth rays; we have
to take into account—and to our annoyance—the fact that its ‘long
levelled rules’ are rippled, and its texture, as it were, loose woven”!

Twenty years ago Professors Kirchkoff and Bunsen applied Fraunhofer’s
method to the examination of coloured flames of various substances, and
since then we have been continually investigating the subject; yet much
remains to be learnt of Spectrum Analysis, and Spectroscopy has still
much to reveal. From Newton’s time to the present our scientists have
been slowly but surely examining with the Spectroscope the composition
of _spectra_, and the Spectroscope is now the greatest assistant we
possess.

“Spectrum Analysis, then, teaches us the great fact that solids and
liquids give out continuous spectra, and vapours and gases give out
discontinuous spectra instead of an unbroken light” (Lockyer). We
have found out that the sunlight and moonlight are identical, that
the moon gives us spectrum like a reflection of the former, but has
no atmosphere, and that comets are but gases or vapours. The most
minute particles of a grain of any substance can be detected to the
millionth fraction. The 1/1000 of a grain of blood can be very readily
distinguished in a stain after years have passed. The very year of a
certain vintage of wine has been told by means of “absorption,” or
the action of different bodies on light in the spectrum. It is now
easy, “by means of the absorption of different vapours and different
substances held in solution, to determine not only what the absorbers
really are, but also to detect a minute quantity.” The application of
this theory is due to Dr. Gladstone, who used hollow prisms filled
with certain substances, and so thickened the “absorption lines.” By
these lines, or bands, with the aid of the Spectrum Microscope, most
wonderful discoveries have been made, and will continue to be made. We
will close this portion of the subject with a brief description of the
Spectroscope in principle.

The instrument consists of two telescopes arranged with two
object-glasses on a stand (fig. 150). A narrow slit is put in place
of the eye-piece of one, the arrangement admitting of the slit being
made smaller or larger by means of screws. The glass to which the slit
is attached is called the collimating lens. The light, at the end of
the slit seen from the other telescope, being separated by the prisms
between the two telescopes, will produce the spectrum. The Spectroscope
is enclosed, so that no exterior light shall interfere with the spectra
the student wishes to observe. This merely indicates the principle, not
the details, of the Spectroscope, which vary in different instruments.

We may now pass from the Spectroscope to the Telescope and the
Microscope, instruments to which we are most largely indebted for our
knowledge of our surroundings in earth, air, and water.

The word Telescope is derived from the Greek _tele_, far, and
_skopein_, to see; and the instrument is based upon the property
possessed by a convex lens or concave mirror, of converging to a focus
the rays of light falling on it from any object, and at that point
or focus forming an image of the object. The following diagram will
illustrate this. Let VW be a lens, and AB an object between the glass,
and F the focus. The ray, A_c_, is so refracted as to appear to come
from _a_. The ray from _b_ likewise appears in a similar way, and a
magnified image, _ab_, is the result (fig. 152).

The ordinary Telescope consists of an object-glass and an eye-lens,
with two intermediates to bring the object into an erect position. A
lens brings it near to us, and a magnifier enlarges it for inspection.
We will now give a short history of the Telescope and its improved
construction.

Roger Bacon was supposed to have had some knowledge of the Telescope,
for in 1551 it was written: “Great talke there is of a glass he made
at Oxford, in which men see things that were don.” But a little later,
Baptista Porta found out the power of the convex lens to bring objects
“nearer.” It was, however, according to the old tale, quite by an
accident that the Telescope was discovered about the year 1608.

[Illustration: Fig. 152.—Converging rays to a focus.]

In Middleburg, in Holland, lived a spectacle-maker named Zachary
Jansen, and his sons, when playing with the lenses in the shop,
happened to fix two of them at the proper distance, and then to look
through both. To the astonishment of the boys, they perceived an
inverted image of the church weathercock much nearer and much larger
than usual. They at once told their father what they had seen. He fixed
the glasses in a tube, and having satisfied himself that his sons were
correct, thought little more about the matter. This is the story as
told, but there is little doubt that for the first Telescope the world
was indebted either to Hans Lippersheim or Joseph Adriansz, the former
a spectacle-maker of Middleburg; and in October 1608, Lippersheim
presented to the Government three instruments, with which he “could
see things at a distance.” Jansen came after this. The report of the
invention soon spread, and Galileo, who was then in Venice, eagerly
seized upon the idea, and returning to Padua with some lenses, he
managed to construct a telescope, and began to study the heavens. This
was in 1609. _Galileo’s Tube_ became celebrated, and all the first
telescopes were made with the concave eye-lens. Rheita, a monk, made
a binocular telescope, as now used in our opera- and field-glasses
approximately.

But the prismatic colours which showed themselves in the early
telescopes were not got rid of, nor was it till 1729 that Hall, by
studying the mechanism of the eye, managed a combination of lenses
free from colour. Ten years before (in 1718) Hadley had established
the Reflector Telescope; Herschel made his celebrated forty-foot
“reflector” in 1789.

[Illustration: Fig. 153.—The Microscope.]

However, to resume. In 1747, Euler declared that it was quite possible
to construct an arrangement of lenses so as to obtain a colourless
image, but he was at first challenged by John Dollond. The latter,
however, was afterwards induced to make experiments with prisms of
crown and flint glass. He then tried lenses, and with a concave lens
of flint, and a convex lens of crown, he corrected the colours. The
question of proper curvature was finally settled, and the “Achromatic”
Telescope became an accomplished fact.

There are two classes of Telescopes—the reflecting and refracting. Lord
Rosse’s is an instance of the former. Mr. Grubb’s immense instrument in
Dublin is a refractor.

The Microscope has been also attributed to Zacharias Jansen, and
Drebbel, in 1619, possessed the instrument in London, but it was of
little or no use. The lens invented by Hall, as already mentioned,
gave an impetus to the Microscope. In the simple Microscope the
objects are seen directly through the lens or lenses acting as one.
The compound instrument is composed of two lenses (or a number formed
to do duty as two), an eye-lens, and an object-lens. Between these is
a “stop” to restrain all light, except what is necessary to view the
object distinctly. The large glass near the object bends the rays on
to the eyeglass, and a perfect magnified image is perceived. We annex
diagrams, from which the construction will be readily understood.

[Illustration: Fig. 154.—Image on the Retina.]

We have in the previous chapter mentioned the effect of light upon the
eye and its direction, and when an object is placed very near the eye
we know it cannot be distinctly seen; a magnified image is thrown upon
the retina, and the divergency of the rays prevents a clear image being
perceived. But if a small lens of a short “focal length” be placed in
front of the eye, having PQ for its focus, the rays of light
will be parallel, or very nearly so, and will as such produce “distinct
vision,” and the image will be magnified at _pq_. In the Compound
Refracting Microscope, BAB is the convex lens, near which an
object, PQ, is placed a little beyond its focal length. An
inverted image, _pq_, will then be formed. This image is produced in
the convex lens, _bab′_, and when the rays are reflected out they are
parallel, and are distinctly seen. So the eye of the observer at the
point E will see a magnified image of the object at PQ brought up to
_pq_ (fig. 155).

[Illustration: Fig. 155.—The Microscope lenses.]

Sir Isaac Newton suggested the Reflecting Microscope, and Dr. Wollaston
and Sir David Brewster improved the instrument called the “Periscopic
Microscope,” in which two hemispherical lenses were cemented together
by the plane surfaces, and having a “stop” between them to limit the
aperture. Then the “Achromatic” instrument came into use, and since
then the Microscope has gradually attained perfection.

[Illustration: Fig. 156.—Concave lens.]

We have so frequently mentioned lenses that it may be as well to
say something about them. Lenses may be spherical, double-convex,
plane-convex, plane-concave, double-concave, and concave-convex. Convex
lenses bring the parallel lines which strike them to a focus, as we see
in the “burning-glass.” The concave or hollow lens appears as in fig.
156. The rays that follow it parallel to its axis are refracted, and as
if they came from a point F in the diagram. But converging rays falling
on it emerge in a parallel direction as above, or diverge as in fig.
158.

[Illustration: Fig. 157. 1. Focus of parallel rays. 2. Focus of
divergent rays. 3. Focus of divergent rays brought forward by more
convex lens.]

The use of spectacles to long or short-sighted people is a necessity,
and the lenses used vary. The eye has usually the capacity of suiting
itself to viewing objects—its accommodation, as it is termed—near or
far. But when the forepart of the eye is curved, and cannot adapt
itself to distant objects, the person is said to be short-sighted. In
long sight the axis of the eyeball is too short, and the focus falls
beyond the retina; in short sight it is too long. In the diagrams
herewith fig. 159 shows by the dotted lines the position of the retina
in long sight, and fig. 160 in short sight, the clear lines showing in
each case the perfectly-formed eye. For long sight and old sight the
double-convex glass is used, for short sight the double-concave (fig.
162). We know the burning-glass gives us a small image of the sun as
it converges the rays to its focus. But lenses will do more than this,
and in the PHOTOGRAPHIC CAMERA we find great interest and
amusement.

[Illustration: Fig. 158.—Diverging rays.]

Photography (or writing by light) depends upon the property which
certain preparations possess of being blackened by exposure to light
while in contact with matter. By an achromatic arrangement of lenses
the camera gives us a representation of the desired object Fig. 163
shows the image on the plate, and figs. 164 and 165 the arrangement of
lenses.

[Illustration: Fig. 159.—Hypermetropia (long sight)..]

[Illustration: Fig. 160.—Myopia (short sight).]

[Illustration: Fig. 161.—Concave and convex lenses.]

[Illustration: Fig. 162.—Lenses for long and short sight.]

To Porta, the Neapolitan physician, whose name we have already
mentioned more than once, is due the first idea of the Photographic
Camera. He found that if light was admitted through a small aperture,
objects from which rays reached the hole would be reflected on the
wall like a picture. To this fact we are indebted for the CAMERA
OBSCURA, which receives the picture upon a plane surface by
an arrangement of lenses. In fact, Porta nearly arrived at the
Daguerreotype process. He thought he could teach people to draw by
following the focussed picture with a crayon, but he could not conquer
the aërial perspective.

So the camera languished till 1820, when Wedgwood and Sir Humphrey Davy
attempted to obtain some views with nitrate of silver, but they became
obliterated when exposed to the daylight.

[Illustration: Fig. 163.—The Camera.]

As early as 1814, however, M. Niepce had made a series of experiments
in photography, and subsequently having heard that M. Daguerre was
turning his attention to the same subject, he communicated with
him. In 1827 a paper was read before the Royal Society, and in 1829
a partnership deed was drawn up between Daguerre and Niepce for
“copying engravings by photography.” Daguerre worked hard, and at
length succeeded in obtaining a picture by a long process, to which,
perhaps, some of our readers are indebted for their likenesses forty
years ago. By means of iodine evaporated on a metal plate covered with
“gold-yellow,” and exposing the plate then in a second box to mercurial
vapour, he marked the image in the camera, and then he immersed the
plate in hyposulphate of soda, and was able to expose the image
obtained to daylight.

[Illustration:

  Fig. 164.      Arrangement of lenses      Fig. 165.]

But the mode now in use is the “collodion” process. We have all seen
the photographer pouring the iodized collodion on the plate, and
letting the superfluous liquid drain from a corner of the glass. When
it is dry the glass-plate is dipped into a solution of nitrate of
silver, and then in a few minutes the glass is ready. The focus is
then arranged, and the prepared plate conveyed in a special slide—to
keep it from the light—to the camera. When the “patient” is ready, the
covering of the lens is removed, and the light works the image into
the sensitive plate. The impression is then “brought up,” and when
developed is washed in water, and after by a solution which dissolves
all the silver from the parts not darkened by the light. Thus the
negative is obtained and printed from in the usual manner.

Instantaneous photography is now practised with great success. An
express train, or the movements of a horse at full speed, can thus
be taken in a second or less. These results are obtained by using
prepared plates, and the “emulsion process,” as it is called, succeeds
admirably. The mode of preparation is given in a late work upon the
subject, and the photographic plates may also be obtained ready for
use. Gelatine and water, mixed with bromide of ammonium, nitrate of
silver, and carbonate of ammonium, mixed with certain proportions of
water, form the “emulsion.” We need not go into all the details here.
Information can easily be obtained from published works, and as the
plates can be purchased by amateurs, they will find that the best way.

Aside from the art interest in the new plates is quite another, that
springs from the fact that it is now possible to take pictures of men,
animals, and machinery in rapid motion, thus enabling us to view them
in a way that would be impossible with the unaided eye. The first
experiments in this direction were applied to the movements of a horse
moving at full speed. The pictures, taken in series, showed that he
performed muscular actions that were not before comprehended or even
imagined. These pictures at the time attracted great attention, and
instantaneous pictures have been since taken of dancers in a ball-room,
of vessels and steam-boats in rapid movement, of all kinds of animals
in motion, and of machinery in operation. As the pictures represent the
movements at one instant of time, they give, as it were, a fixed view
of a motion, precisely as if it were suddenly arrested in full action.
In the case of animals, the motions of the nostrils are represented in
the most singular manner, and the spokes of a steam-boat’s paddle-wheel
are shown apparently perfectly still while the spray and waves appear
in active motion, or, rather, as they would look if they could be
instantly frozen. It is clear the new process and pictures will open a
wide and instructive field in art and in the study of mechanical action.

While on the subject of Photography we may mention a very ingenious
little apparatus called a SCENOGRAPH, the invention of Dr.
Candize. It is really a pocket-camera, and is so easily manipulated
that it will be found a most pleasant and useful holiday companion.
Any one may obtain good results with it, and friends of ours have had
occasion to put it in practice during a series of excursions, when it
was found to answer in every instance.

The Scenograph is something like a common Stereoscope in outward
appearance, and would, perhaps, be at first regarded as a mere toy,
did not a more intimate acquaintance prove it a great acquisition,
particularly to explorers and tourists. The tripod stand which supports
the apparatus is, when not in actual use in that capacity, a very
excellent walking-stick, in which the two other “legs” are carried. The
instrument, as will be perceived from the illustration (fig. 166),
is very handy. It produces pictures about the “cabinet” size, and the
whole is so arranged that it can be packed and carried in the pocket
with ease.

[Illustration: Fig. 166.—The Scenograph.]

Photography, as a rule, necessitates a dark room or cabinet, and
many preparations as we all know—a “messing” about with chemicals
and considerable practice before we can become proficient; so it is
not surprising that few amateurs take to it—they prefer to purchase
the pictures. But in the new apparatus of which we are speaking, the
glass plates are already prepared to receive the image. It is not at
all necessary for the operator to stain his fingers and knuckles and
nails with nitrate of silver, or any other “chemicals” whatever. He
just inserts the plate in the Scenograph, and then his apparatus being
steadily set up, he removes the covering from the lens. To develop the
image (in the dusk of the evening or by candle-light) it is necessary
to put some drops of ammonia in a saucer, breathe upon the plate so
as to soften the collodion, and hold it above the ammonia, and then,
under the influence of the vapour, the picture will appear. After this
simple operation the picture will be found fixed for a lengthened
period—practically indefinitely. Thus on the return of the pedestrian
he can reproduce, at small expense, a whole series of little pictures
faithfully representing his holiday tour. The illustration shows a
small apparatus by which on thin plates small photographs can be taken
and fixed till it is found desirable to enlarge them.

The _Photophone_, one of the most recent contributions to science,
is an instrument which, in combination with the telephone principle,
makes it possible to convey sounds by means of a ray of light, and by
means of a quivering beam “to produce articulate speech at a distance.
The success of the Photophone depends upon a rare element, selenium,
which has its “electrical resistance” affected by light. Professor
Adams demonstrated that the resistance of selenium was reduced just
in proportion as the intensity of the light which was acting upon it.
Here was the key to the Photophone as thought out by Professor Bell.
He fancied that he might by means of his telephone produce sound if he
could vary the intensity of the beam of light upon the selenium, which
he connected with his telephone and battery.

The Photophone consists of a transmitter for receiving the voice and
conveying it along the beam of light, and a receiver for taking the
light and converting it into sound—the receiver being the telephone.
There is a small mirror (silvered mica has been used) suspended freely
for vibration. A lens is used to transmit to this the beam of light,
and this beam is again reflected by another lens to the receiver, which
consists of a reflector which has a cell of selenium in its focus,
connected, as already stated, with the telephone and battery. The
speaker stands behind the mirror, and the sound of his voice against
the reverse side makes it vibrate in unison with the sounds uttered.
The movements cause a quivering in the reflected beam, and this in its
changing intensity acts on the selenium, which changes its resistance
accordingly, and through the telephone gives forth a sound!

This is the apparently complicated but really simple, and at the same
time wonderful, invention of Professor Bell. By the Photophone not
only sounds but _movements_ can be converted into sound; even the
_burning of a candle can be heard_! The Photophone is still capable of
improvement, and has not as yet arrived at its full development, for it
is stated it can be made quite independent of a battery or telephone.

There are many phenomena connected with the _Polarization of Light_.
This requires some notice at our hands. We know that a ray of ordinary
light is supposed to be caused by vibrations of the highly attenuated
medium, æther. These vibrations occur across the direction of the
ray; but when they occur only _in one plane_ the light is said to be
“polarized.” Polarization means possessing poles (like a magnet); the
polarized rays have “sides,” as Newton said, or, as explained by Dr.
Whewell, “opposite properties in opposite directions, so exactly equal
as to be capable of accurately neutralizing each other.” There are some
crystals which possess the property of “double refraction,” and thus
a ray of common light passing through such a crystal is divided into
two _polarized_ rays, taking different directions. One is refracted
according to the usual laws of refraction; the other is not, and the
planes of polarization are at right angles. It is difficult within the
limits of this chapter to explain the whole theory of Polarization. In
order to account for certain phenomena in optics, philosophers have
assumed that rays possess polarity; and polarized light is light which
has had the property of Polarization conferred upon it by reflection,
refraction, or absorption. Common light has been compared to a round
ruler, and polarized light to a flat ribbon. Huygens found out, when
engaged upon the investigation of double refraction, that the rays of
light, divided by passing through a crystal (a rhomb) of Iceland spar,
possessed certain qualities. When he passed them through a second
rhomb, he found that the brightness, relatively, of the rays depended
upon the position of the second prism, and in some positions one ray
disappeared entirely. The light had been reduced to vibrations in one
plane. In 1808, Malus, happening to direct a double refracting prism
to the windows then reflecting the sunset, found that as he turned the
prism round, the ordinary image of the window nearly disappeared in two
opposite positions; and in two other positions, at right angles, the
“extraordinary” image nearly vanished. So he found that polarization
was produced by reflection as well as by transmission. The differences
between common and polarized light have been summed up by Mr. Goddard
as follows:—

        COMMON LIGHT                         POLARIZED LIGHT

  “Is capable of reflection at         “Is capable of reflection at
  oblique angles of incidence in       oblique angles only in
  every position of the reflector.     _certain positions_ of the
                                       reflector.

  “Will pass through a bundle of       “Will only pass through such
  plates of glass in any position      glasses when they are in
  in which they may be placed.         _certain positions_.

  “Will pass through a plate of        “Will only pass in certain
  tourmaline, cut parallel to the       positions, and in others
  axis of the crystal, in every         will not pass at all.”
  position of the plate.”

The bundle of glass plates or the tourmaline plate is thus the test for
polarized light, and is termed an analyzer.

The arrangement called a “Nichol’s prism,” made by cutting a prism
of Iceland spar and uniting the halves with a cement, so that only
one polarized ray can pass through it, is termed a _Polarizer_. It
only permits one of the two rays produced by “double refraction” to
pass, and the ray (as said above) will contain none but transverse
vibrations. Polarized light will produce beautiful colours. The whole
subject is very interesting to the scientist, but rather a difficult
one for the general reader to understand.

Amongst the uses to which light has been put is that of a milk-tester.
The LACTOSCOPE will show the quantity of butter contained in
a certain quantity of milk, by diluting it till it displays a certain
degree of transparency. There is another method, by the transmission of
light.

The first test is obtained by means of a glass tube about nine inches
long, closed at one end, and containing a small porcelain rod marked
with black lines. A small quantity of milk is measured and placed in
the tube. The black lines cannot at first be seen through the tube, but
by adding water the milk is rendered transparent, and the black lines
become visible. The surface of milk in the tube, by a graduated scale
upon it, shows the percentage of butter.

[Illustration: Fig. 167.—Cut card figures.]

The second method is not so simple. A short tube of tin, blackened on
the inside, and supported upright, has an opening on one side, and
opposite this, inside the tube, is a mirror placed at an angle of 45°.
“By placing a lighted candle at a known distance opposite the opening,
its light is reflected in the mirror and thrown upward through the
tube. On top of the tube is placed a round vessel of glass or metal,
closed at the bottom by a sheet of clear glass. The vessel is closed at
the top by a cover having an opening in the centre, in which slides up
and down a small tube closed at the bottom with glass, and having an
eye-piece at the top. The milk to be tested is placed in this vessel on
the top of the tin tube, so that the light of the candle reflected from
the mirror passes upward through the milk. Then, by looking through the
sliding tube and moving it up and down, a point may be found where the
image of the candle in the mirror can be seen through the milk. This
device depends, as will be seen, on observing the light transmitted
through a film of milk, and the thickness of the film is the measure of
the value of the milk. The movable tube contains a graduated scale, and
by comparison of this with a printed table, the percentage of butter in
the milk may be ascertained.”

In concluding this chapter we give a few hints for some pleasant
relaxation for young people, which has many a time created amusement.
The experiment consists in cutting out in paper or cardboard certain
portions of a face or figure, as per the illustration herewith. Fig.
167 gives the card as cut with the scissors, and the two subsequent
faces are the result of the same held at a less or greater distance
from a screen. The illustrations (fig. 168) will assist those who wish
to amuse children by making rabbits, etc., on the wall. The shadows
will be seen perfectly thrown if the hands be carefully fixed near a
good light.

[Illustration: Fig. 168.—Hand-shadows on the wall.]

We are all so familiar with the “Magic Lantern,” and the apparatus
for dissolving views by an arrangement of lenses and manipulation of
slides, that we need do no more than refer to them.

[Illustration: Fig. 169.—Dissolving views apparatus.]

The various ghost illusions, etc., produced by indirect mirrors, have
already been referred to, the ghost being merely the reflection of an
individual seen through a sheet of glass between the spectators and
the stage. The strong light throws a reflection from a parallel mirror
lower down, and this reflected image can be made to appear amongst
the real actors who are behind the plate-glass in full view of the
audience, who are, however, ignorant of the existence of the glass
screen.

For the winter evenings one may easily procure an apparatus for
dissolving views by the oxy-hydrogen light. One, as shown in the
illustration herewith (fig. 169), will answer every purpose, and by
this double arrangement phantasmagoria may be produced, or a fairy
tale may be illustrated. The effect of gradually-approaching night
may be given to the picture by means of a special glass in the lower
lanthorn. The apparatus is exhibited by means of a Drummond light, and
is very simple, although a certain supply of gas is necessary for the
performance. But this can be easily procured by an indiarubber tube,
or in a bag supplied for the purpose. Almost any objects can be used,
photographs, etc., etc., and many very comical arrangements can be made.

We have lately been reading a curious method of obtaining light
from oyster-shells in a Trans-Atlantic magazine. We give an extract
wherewith to close this chapter. The compound is “luminous paint.”

“It has been known that certain compounds of lime and sulphur had the
property of absorbing light, and giving it out again when placed in
the dark. A simple way to do this is to expose clean oyster-shells to
a red heat for half an hour. When cold, the best pieces are picked out
and packed with alternate layers of sulphur in a crucible, and exposed
to a red heat for an hour. When cold, the mass is broken up, and the
whitest pieces are placed in a clean glass bottle. On exposing the
bottle to bright sunshine during the day, it is found that at night its
contents will give out a pale light in the dark. Such a bottle filled
more than a hundred years ago still gives out light when exposed to
the sun, proving the persistency of the property of reproducing light.
Very many experiments have been more recently made in this direction,
and the light-giving property greatly enhanced. The chemicals, ground
to a flour, may now be mixed with oils or water for paints, may be
powdered on hot glass, and glass covered with a film of clear glass,
or mixed with celluloid, papier-maché, or other plastic materials. As
a paint, it may be applied to a diver’s dress, to cards, clock dials,
sign-boards, and other surfaces exposed to sunlight during the day;
the paint gives out a pale violet light at night sufficient to enable
the objects to be readily seen in the dark. If the object covered with
the prepared paint is not exposed to the sun, or if the light fades in
the dark, a short piece of magnesium wire burned before it serves to
restore the light-giving property.”



CHAPTER XIV.

SPECTRAL ILLUSIONS.

 A SPECTRE VISIBLE—CURIOUS ILLUSIONS—GHOSTS.


We have already given numerous examples of the effects produced
by impressions on the retina by mechanical appliances. We can now
in a short chapter speak of the cause of many spectral illusions,
commonly supposed to be “ghosts” or “spirits.” That there are many
“well-authenticated ‘ghost stories’” no one can doubt who has read the
literature of the day; and we ourselves do not in any way desire to
throw any doubt upon the existence of certain so-called “ghosts.” That
appearances of some kind or another are seen by people we know. We
ourselves have seen such, but we cannot say we believe in the popular
ghost.

In ancient times mirrors were much employed by the so-called magicians,
and in our day many wonderful ghost effects have been shown at
the (late) Polytechnic Institution. Some people are believers in
table-turning and spiritualism, and mesmerists still attract large
audiences, and appear to possess extraordinary power over some
individuals. But apparitions have been seen by people eminently worthy
of credit. The experience of the learned Doctor, which appeared some
months ago in the _Athenæum_, is a case in point. This narrative is
concise and clear. The spectre was there. How did it get there? Was the
“appearance” objective or subjective? Let us give an extract from the
Reverend Doctor’s narrative, and comment upon it afterwards. We may
premise that Dr. Jessopp had gone over to Lord Orford’s (Mannington
Hall), and at eleven o’clock was busy writing in the library, and was
“the only person downstairs.” We will give this ghost story in the
Doctor’s own words. After taking up a certain volume—time about 1 a.m.:—

“I had been engaged on it about half an hour, and was beginning to
think my work was drawing to a close, when, as I was actually writing,
I saw a large white hand within a foot of my elbow. Turning my head,
there sat a figure of a somewhat large man with his back to the fire,
bending slightly over the table, and apparently examining the pile of
books that I had been at work upon.”... After describing the appearance
of the nocturnal visitor, Dr. Jessopp proceeds:—

“There he sat, and I was fascinated; afraid not of his staying, but
lest he should go. Stopping in my writing I lifted my left hand from
the paper, stretched it out to the pile of books, and moved the top
one—my arm passed in front of the figure, and it vanished.”... Shortly
after the figure appeared again, and “I was penning a sentence to
address to him, when I discovered I did not dare to speak. I was afraid
of the sound of my own voice! There he sat, and there sat I. I turned
my head and finished writing. Having finished my task, I shut the book,
and threw it on the table; it made a slight noise as it fell;—the
figure vanished.”

Now here we have a perfectly plain narrative, clear and full. A ghost
appeared; he is described distinctly. How can we account for the
apparition? In the first place, someone might have played a trick, but
that idea was put aside by Dr. Wilks, who attempted to explain the
appearances. He went fully into the question, and as it bears upon our
explanation of the reality of Spectral Illusions, we may condense his
evidence. It will of course be conceded that all the usual objects seen
by people are material, and the image of what we look at is formed upon
the retina in the manner already explained. But _all_ images upon the
retina are not immediately observed; the impression may, to a certain
extent, remain. Words are often impressed upon the brain,—words which
we in our sober senses would never think of repeating,—and yet when
we are delirious we give vent to these expressions, of whose very
nature and meaning we are perfectly unconscious. It is, according to
our reference (Dr. Wilks), “quite possible for the perceptive part of
the brain to be thrown into an active condition quite independent of
the normal stimulus conducted to it from the retina.” If, under these
circumstances, an object be viewed independently, and, as it were,
unconsciously, it is merely, we believe, a parallel to the impression
of words before noted. Sound and light are governed by the same laws.
In fevers we fancy we see all kinds of things which have no existence.
In dreams we hear noises; and many a time people dreaming have been
awakened by the report of a gun, or the ringing of a bell which had no
material origin,—the nerves were excited, the “perceptive centre” of
the brain was moved.

But if sight and hearing thus have their origin from the _brain_
and not from without, there must have been some predisposing
cause, some excitement to induce such a condition of things. “The
impressions become abnormal and subjective,—the normal condition being
objective,—the impression is received from without, and impressed upon
the eye.

Now, let us consider the “ghost”! Lately there have been many instances
brought forward of “spiritual” appearances, but we think nobody has
ever seen a “_material_” ghost; yet on the other hand none of us
have any knowledge of anything in the likeness of a ghost, or that
_has not a material basis_ which _can_ bring forward an image on the
retina! Therefore we are brought to the conclusion that apparitions
are spectres emanating from within the brain, not from any outward
manifestation, because it is within the experience of everybody that in
bad health, or disordered digestive functions, images are produced in
the brain and nerves of the eye.

These remarks have perhaps been made before in one form or other, but
as much popular interest is always awakened by the supernatural, or
what is supposed to be supernatural, we may go a little farther, and
inquire how it was that the ghost seen by Dr. Jessopp disappeared when
he raised his arm. Would any ghost be afraid of the Doctor extending
his hand? The fact no doubt occurred as related. The explanation is
that the narrator had been much impressed by a certain picture, which
a correspondent soon identified as a portrait of “Parsons, the Jesuit
Father.” The description given is that of the priest who was described
by the Doctor in one of his books. The association of ideas in the
library of a Norfolk house connected with the Walpoles, with whom
Parsons had been a leader, gave rise, during a period of “forty winks”
at midnight, to the spectre.

In the interesting letters written upon “Natural Magic” by Sir David
Brewster, the subject of Spectral Illusions is treated at some length,
and with undoubted authority. Sir David thought the subject worth
discussing with reference to the illusions or spectres mentioned by Dr.
Hibbert. Sir David Brewster gives his own experiences which occurred
while he was staying at the house of a lady in the country.

The illusions appear to have affected her ear as well as the eye.
We shall see in the next chapter how intimately sound and light are
connected, and how the eyes and ears are equally impressed, though in
a different way, by the vibration of particles. The lady referred to
was about to go upstairs to dress for dinner one afternoon, when she
heard her husband’s voice calling to her by name. She opened the door,
and nobody was outside; and when she returned for a moment to the fire
she heard the voice again calling, “Come to me; come, come away,” in a
somewhat impatient tone. She immediately went in search of her husband,
but he did not come in till half an hour afterwards, and of course said
he had not called, and told her where he had been at the time—some
distance away. This happened on the 26th December, 1830, but a more
alarming occurrence took place four days after.

About the same time in the afternoon of the 30th December, the lady
came into the drawing-room, and to her great astonishment she perceived
her husband standing with his back to the fireplace. She had seen him
go out walking a short time previously, and was naturally surprised
to find he had returned so soon. He looked at her very thoughtfully,
and made no answer. She sat down close beside him at the fire, and as
he still gazed upon her she said, “Why don’t you say something!” The
figure immediately moved away towards the window at the farther end of
the room, still gazing at her, “and it passed so close that she was
struck by the circumstance of hearing no step nor sound, nor feeling
her dress brushed against, nor even any agitation of the air.” Although
convinced this was not her husband, the lady never fancied there was
anything supernatural in the appearance of the figure. Subsequently she
was convinced that it was a spectral illusion, although she could not
see through the figure which appeared as substantial as the reality.

Were it advisable, we could multiply instances. In the _Edinburgh
Journal of Science_ these, and many more instances of spectral
illusions were narrated by the husband of the lady. She frequently
beheld deceased relatives or absent friends, and described their dress
and general appearance very minutely. On one occasion she perceived
a coach full of skeletons drive up to the door, and noticed spectral
dogs and cats (her own pets’ likenesses) in the room. There can be no
doubt upon these points; the appearances were manifest and distinct.
They were seen in the presence of other people, in solitude, and in
the society of her husband. The lady was in delicate health, and very
sensitive. The spectres appeared in daylight as well as in the dark, or
by candle-light.

Let us now, guided by what we have already written, and by Sir David
Brewster’s experience, endeavour to give a rational explanation of
these illusions. “The mind’s eye is really the body’s eye, and the
retina is the common tablet upon which both classes of impressions are
painted, and by means of which they receive their visual existence
according to the same optical laws.”

“In the healthy state of mind and body the relative intensity of the
two classes of impressions on the retina are nicely adjusted—the bodily
and mental are balanced. The latter are feeble and transient, and in
ordinary temperaments are never capable of disturbing or effacing
the direct images of visible objects.... The mind cannot perform
two different functions at the same instant, and the direction of
its attention to one of the two classes of impressions necessarily
produces the extinction of the other; but so rapid is the exercise of
mental power, that the alternate appearance and disappearance of the
two contending impressions is no more recognized than the successive
observations of external objects during the twinkling of the eyelids.”

We have before illustrated, by means of the pen and the ink-bottle, how
one object is lost sight of when the other is attentively regarded, and
a material picture or scene may be equally lost sight of, and a mental
picture take its place in the eye, when we recall places or people we
have seen or remembered.

We have all heard numerous anecdotes of what is termed
“absent-mindedness.” Some people are quite absorbed in study, and
can see or hear no one in the room when deeply occupied. We may be
satisfied then that “pictures of the mind and spectral illusions are
equally impressions upon the retina, and only differ in the degree
of vividness with which they are seen.” If we press our eyes the
phosphorescence becomes apparent, and we can make a picture of the sun
or a lamp visible for a long time to our closed eyes if we stare at
either object for a few seconds, and shut our lids. So by increasing
the sensibility of the retina we can obtain the image, and alter its
colour by pressure on the eye.

It is well known that poisons will affect sight, and belladonna
applied to the eyes will so affect them as to render the sight _nil_,
by enlargement of the “pupil.” If one is out of health there is
practically a poisoning of the system, and when we have a “bilious
headache” we see colours and stars because there is a pressure upon the
blood-vessels of the eye. The effects of a disordered stomach, induced
by drinking too much, are well known; objects are seen double, and most
ghosts may be traced to a disordered state of health of mind or body,
brought on by excitement or fatigue. We could relate a series of ghost
stories,—some in our own experience, for we have seen a ghost equally
with our neighbours,—but this is not the place for them. Although many
apparently incontrovertible assertions are made, and many spectres have
been produced to adorn a tale, we must put on record our own opinion,
that every one could be traced to mental impression or bodily affection
had we only the key to the life and circumstances of the ghost-seer.
Many celebrated conjurers will convince us almost against our reason
that our pocket-handkerchief is in the orange just cut up. They will
bring live rabbits from our coat-pockets or vests, and pigeons from our
opera-hats. These are equally illusions. We know what can be done with
mirrors. We have seen ghosts at the Polytechnic, but we must put down
all apparitions as the result of mental or bodily, even unconscious
impressions upon the retina of the eye. There are numerous illusions,
such as the Fata Morgana, the Spectre of the Brocken, etc., which
are due to a peculiar state of the atmosphere, and to the unequal
reflection and refraction of light. Those, and many other optical
phenomena, will, with phenomena of heat and sound, be treated under
METEOROLOGY, when we will consider the rainbow and the aurora,
with many other atmospheric effects.



CHAPTER XV

ACOUSTICS.

 THE EAR, AND HEARING—PHYSIOLOGY OF HEARING AND SOUND—SOUND AS COMPARED
 WITH LIGHT—WHAT IS SOUND?—VELOCITY OF SOUND—CONDUCTIBILITY—THE
 HARMONOGRAPH.


Before entering upon the science of ACOUSTICS, a short
description of the ear, and the mode in which sound is conveyed to our
brain, will be no doubt acceptable to our readers. The study of the
organs of hearing is not an easy one; although we can see the exterior
portion, the interior and delicate membranes are hidden from us in the
very hardest bone of the body—the _petrous_ bone, the temporal and
rock-like bone of the head.

[Illustration:

 Fig. 170.—1. Temple bone. 2. Outer surface of temple. 3. Upper wall
 of bony part of hearing canal. 4. Ligature holding “hammer” bone to
 roof of drum cavity. 5. Roof to drum cavity. 6. Semicircular canals.
 7. Anvil bone. 8. Hammer bone. 9. Stirrup bone. 10. Cochlea. 11.
 Drum-head cut across. 12. Isthmus of Eustachian tube. 13. Mouth of
 tube in the throat. 14. Auditory canal. 15. Lower wall of canal.
 16. Lower wall of cartilaginous part of canal. 17. Wax glands. 18.
 Lobule. 19. Upper wall of cartilaginous portion of canal. 20. Mouth of
 auditory canal. 21. Anti-tragus.]

The ear (external) is composed of the auricle, the visible ear, the
auditory canal, and the drum-head, or _membra tympani_. The tympanum,
or “drum,” is situated between the external and the internal portions
of the ear. This part is the “middle ear,” and is an air cavity, and
through it pass two nerves, one to the face, and the other to the
tongue. The internal ear is called the “labyrinth,” from its intricate
structure. We give an illustration of the auditory apparatus of man
(fig. 170).

The auricle, or exterior ear, is also represented, but we need not go
into any minute description of the parts. We will just name them (fig.
171).

Sound is the motion imparted to the auditory nerve, and we shall see
in a moment how sound is produced. The undulations enter the auditory
canal, having been taken up by the auricle; the waves or vibrations
move at the rate of 1,100 feet a second, and reach the drum-head,
which has motion imparted to it. This motion or oscillation is
imparted to other portions, and through the liquid in the labyrinth.
The impressions of the sound wave are conveyed to the nerve, and this
perception of the movement in the water of the labyrinth by the nerve
threads and the brain causes what we term “hearing.”

[Illustration: Fig. 171.—1. Pit of anti-helix. 2, 6, 10. Curved edge
of the auricle. 3. Mouth of auditory canal. 4. Tragus. 5. Lobe. 7.
Anti-helix. 8. Concha. 9. Anti-tragus.]

Let us now endeavour to explain what sound is, and how it arises. There
are some curious parallels between sound and light. When speaking of
light we mentioned some of the analogies between sound and light, and
as we proceed to consider sound, we will not lose sight of the light we
have just passed by.

Sound is the influence of air in motion upon the hearing or auditory
nerves. Light, as we have seen, is the ether in motion, the vibrations
striking the nerves of the eye.

There are musical and unmusical sounds. The former are audible when the
vibrations of the air reach our nerves at regular intervals. Unmusical
sounds, or irregular vibrations, create _noise_. Now, musical tones
bear the same relation to the ear as colours do to the eye. We must
have a certain number of vibrations of ether to give us a certain
colour (_vide_ table). “About four hundred and fifty billion impulses
in a second” give red light. The violet rays require nearly double. So
we obtain colours by the different rate of the impingement of impulses
on the retina. The eyes, as we have already learned, cannot receive
any more rapidly-recurring impressions than those producing violet,
although as proved, the spectrum is by no means exhausted, even if they
are invisible. In the consideration of Calorescence we pointed this
out. These invisible rays work great chemical changes when they get
beyond violet, and are shown to be heat. So the rays which do not reach
the velocity of red rays are also heat, which is the effect of motion.

Thus we have HEAT, LIGHT, and SOUND, all
the ascertained results of vibratory motion. The stillness of the
ether around us is known as “Darkness”; the stillness of the air is
“Silence”; the comparative absence of heat, or molecular motion of
bodies is “Cold”!

In the first part we showed how coins impart motion to each other.

  VELOCITY OF LIGHT WAVES.

  _According to_ SIR J. HERSCHEL.

  Colour of the     No. of Undulations     No. of Undulations
    Spectrum.          in an inch.            in a second.
  Extreme Red            37,640            458,000,000,000,000
  Red                    39,180            477,000,000,000,000
  Intermediate           40,720            495,000,000,000,000
  Orange                 41,610            506,000,000,000,000
  Intermediate           42,510            517,000,000,000,000
  Yellow                 44,000            535,000,000,000,000
  Intermediate           45,600            555,000,000,000,000
  Green                  47,460            577,000,000,000,000
  Intermediate           49,320            600,000,000,000,000
  Blue                   51,110            622,000,000,000,000
  Intermediate           52,910            644,000,000,000,000
  Indigo                 54,070            658,000,000,000,000
  Intermediate           55,240            672,000,000,000,000
  Violet                 57,490            699,000,000,000,000
  Extreme Violet         59,750            727,000,000,000,000

When an impulse was given the motion was carried from coin to coin,
and at length the last one in the row flew out. This is the case with
sound. The air molecules strike one upon another, and the wave of
“sound” reaches the tympanum, and thus the impression is conveyed to
the brain. We say we hear—but why we hear, in what manner the movement
of certain particles affects our consciousness, we cannot determine.

That the air is absolutely necessary to enable us to hear can readily
be proved. The experiment has frequently been made; place a bell under
the receiver of an air-pump, and we can hear it ring. But if we exhaust
the air the sound will get fainter and fainter. Similarly, as many
of us have experienced upon high mountains, sounds are less marked.
Sound diminishes in its intensity, just as heat and light do. Sound is
reflected and refracted, as are light and radiant heat. We have already
shown the effect of reflectors upon heat. Sound is caught and reflected
in the same way as light from mirrors, or as the heat waves in the
reflectors. We have what we term “sounding boards” in pulpits, and
speaking tubes will carry sound for us without loss of power. Echoes
are merely reflected sounds.

The velocity of sound is accepted as 1,100 feet in a second, which
is far inferior to the velocity of light. Fogs will retard sound,
while water will carry it. Those who have ever rowed upon a lake will
remember how easily the sound of their voices reached from boat to
boat, and Dr. Hutton says that at Chelsea, on the Thames, he heard
a person reading from a distance of a hundred and forty feet. Some
extraordinary instances could be deduced of the enormous distances
sound is said to have travelled. Guns have been heard at eighty miles
distant, and the noise of a battle between the English and Dutch, in
1672, was heard even in Wales, a distance of two hundred miles from the
scene of action.

Sound always travels with uniform velocity in the air in the same
temperature. But sound! What is the cause of it? How does it arise?
These questions can now be fully answered with reference to the
foregoing observations. Phenomena of vibration render themselves
visible by light, heat, and sound, and to arrive at some definite
ideas of sound vibrations we may compare them to waves, such as may be
produced by throwing a stone into a pond.

There are, so to speak, “standing” waves and “progressive” waves. The
former can be produced (for instance) by thrumming a fiddle-string,
and when the equilibrium of the cord is disturbed, the position of the
equilibrium is passed simultaneously by the string-waves. In water
the waves or vibrating points pass the position of equilibrium in
succession.

Waves consist of elevations and depressions alternately, and when we
obtain two “systems” of waves by throwing two stones into water, we
can observe some curious effects. It can be seen how one series of
depressions will come in contact with the other series of depressions,
and the elevations will likewise unite with the result of longer
depressions and elevations respectively; or it may very well be that
elevation will meet depression, and then the so-called “interference”
of waves will produce _points of repose_. These points are termed
_nodes_. The waves of the string proceed in the plane of its axis;
water waves extend in circles which increase in circumference.

The progression or propagation of sound may be said to begin when some
tiny globule of matter expands in the air. The air particles strike one
against the other, and so the motion is communicated to the air waves,
which in time reach the ear. But the velocity of the sound is not equal
in all substances. Air will convey it around our earth at the rate of
765 miles an hour, or 1,090 feet in a second. That is, we may accept
such rate as correct at a temperature of 32° Fahr., and at a pressure
of thirty inches, and the velocity increases almost exactly one foot
per second for each degree of temperature above 32°. Therefore on an
average, and speaking in “round numbers,” the estimate of 1,100 feet in
a second may be accepted as correct. In hydrogen gas the rate is much
higher. Through water again it is different, and still faster through
iron, glass, and wood, as will be seen in the following table:—

TAKING AIR AS 1.

  Whalebone             6⅔
  Tin                   7½
  Silver                9
  Walnut               10⅔
  Brass                10⅔
  Oak                  10⅔
  Earthen pipes        11
  Copper               12
  Pear-wood            12½
  Ebony                14⅔
  Cherry               15
  Willow               16
  Glass                16⅔
  Iron or Steel        16⅔
  Deal                 18

So there is a considerable difference in the velocities of sound
through the solid substances quoted, but these figures cannot be taken
as exact, as different samples may give different results. In wires
and bells the bodies themselves produce the sounds we hear. In wind
instruments and the voice the air is the cause of the sound.

The very deepest notes are produced by the fewest vibrations. Fourteen
or fifteen vibrations will give us a very low note, if not the very
lowest. The pipe of sixteen feet, closed at its upper end, will
produce sound waves of thirty-two feet. High notes can be formed from
vibrations up to 48,000 in a second. Beyond these limits the ear cannot
accept a musical sound.

[Illustration: Fig. 172.—The vibration of strings.]

We will explain the phenomenon of the vibration of strings by means of
the illustration. In the cut we find a string or wire, which can be
lengthened or shortened at pleasure by a movable bridge, and stretched
by weights attached to the end (fig. 172).

We can now easily perceive that the shorter and thinner the string
is, and the tighter it is, the number of vibrations will be greater
and greater. The density of it is also to be considered, and when
these conditions are in the smallest proportion then the tone will be
highest. The depth will naturally increase with the thickness, density,
and length, and with a decreasing tension. But we have strings of same
thickness stretched to different degrees of tension, and thus producing
different notes. Some strings are covered with wire to increase their
gravity, and thus to produce low notes.

When a number of separate sounds succeed each other in very rapid
course they produce a sound, but to appear as one sound to the ear
they must amount to fifteen or sixteen vibrations every second. The
particles of matter in the air form a connected system, and till they
are disturbed they remain in equilibrium; but the moment they are in
any way thrown out of this state they vibrate as the pendulum vibrates.
The particles thus strike each other, and impart a motion to the
elastic medium air, so a sound comes to us.

The intensity of sounds gets less the farther it goes from us, or the
loudness of sound is less the greater its distance. The law is, that
in an unvarying medium the loudness varies inversely as the square of
the distance. But Poisson has shown that when air-strata, differing
in density, are existing between the ear and the source of the sound,
the intensity or loudness with which it is heard depends _only_ on the
density of the air at the place the sound originated. This fact has
been substantiated by balloonists who heard a railway whistle quite
distinctly when they were nearly 20,000 feet above the ground. It
therefore follows that sound can be heard in a balloon equally well
as on the earth at certain given distances. But as the density of the
air diminishes the sound becomes fainter, as has been proved by the
bell rung in the receiver of an air-pump. The velocity of sound, to
a certain extent, depends upon its intensity, as Earnshaw sought to
prove; for he instanced a fact that in the Arctic regions, where sound
can be heard for an immense distance, in consequence of the still and
homogeneous air, the report of a cannon two miles and a half away was
heard before the loud command to “fire,” which must have preceded
the discharge. Another instance showing the difference in hearing
through mixed and homogeneous media may be referred to. In the war
with America, when the English and their foes were on opposite sides
of a stream, an American was seen to beat his drum, but no sound came
across. “A coating of soft snow and a thick atmosphere absorbed the
noise.” Glazed, or hard snow, would have a contrary effect. Reynault
also experimentally verified his theory, that sound when passing
through a space of nearly 8,000 feet lost velocity as its intensity
diminished, and in that distance between its arrival at 4,000 feet and
at 7,500 feet, the sound velocity diminished by 2·2 feet per second. He
also tried to demonstrate that sound velocity depended upon its pitch,
and that lower notes travelled with the greater speed.

_The reflection and refraction of sound_ follows the same fundamental
laws as the reflection and refraction of light. The reflection of sound
is termed an _Echo_, which is familiar to all tourists in Switzerland
and Ireland particularly. There are several very remarkable echoes in
the world: at Woodstock, and at the Sicilian cathedral of Gergenti,
where the confessions poured forth near the door to priestly ears were
heard by a man concealed behind the high altar at the opposite end.
It is curious that such a spot should have been accidentally chosen
for the Confessional. The whispering gallery in St. Paul’s is another
instance of the echo.

Echoes are produced by the reflection of sound waves from a plane or
even surface. A wall, or even a cloud, will produce echoes. Thunder
is echoed from the clouds. (The celebrated echo of “Paddy Blake,”
at Killarney, which, when you say “How do you do,” is reported to
reply, “Very well, thank you,” can scarcely be quoted as a scientific
illustration.) And the hills of Killarney contain an echo, and the
bugle sounds are beautifully repeated. In the cases of ordinary echo,
when the speaker waits for the answer, he must place himself _opposite_
the rock. If he stand at the side the echo will reply to another person
in a corresponding place on the farther side, for the voice then
strikes the rock at an angle, and the angle of reflection is the same,
as in the case of light.

But if it should happen that there are a number of reflecting surfaces
the echo will be repeated over and over again, as at the Lakes of
Killarney. The Woodstock Echo, already referred to, and mentioned
by several writers, repeats seventeen syllables by day, and twenty
by night. In Shipley there is even a greater repetition. Of course
the echo is fainter, because the waves are weaker if the reflecting
surface be flat. But, as in the case of the mirrors reflecting light, a
circular or concave surface will increase the intensity. A watch placed
in one mirror will be heard ticking in the other focus. Whispering
galleries carry sound by means of the curved surface. Sir John Herschel
mentions an echo in the Menai Suspension Bridge. The blow of a hammer
on one of the main piers will produce the sound from each of the
crossbeams supporting the roadway, and from the opposite pier 576 feet
distant, as well as many other repetitions.

Refraction of sound is caused by a wave of sound meeting another medium
of different density, just as a beam of light is refracted from water.
One sound wave imparts its motion to the new medium, and the new wave
travels in a different direction. This change is refraction. The sound
waves are refracted in different directions, according to the velocity
it can acquire in the medium. If a sound pass from water into air it
will be bent towards the perpendicular, because sound can travel faster
in water than in air. If it pass from air into water its force will
cause it to assume a less perpendicular direction, there being greater
velocity in water. The velocity in air is only 1,100 feet in a second
in our atmosphere. In water sound travels 4,700 feet in the same time.
When the wave of sound falls upon a medium parallel to the refracting
surface there is, however, no refraction—only a change of velocity, not
direction.

When sound waves are prevented from dispersing the voice can be carried
a great distance. Speaking tubes and trumpets, as well as ear trumpets,
are examples of this principle, and of the reflection of sound.

There are many very interesting experiments in connection with
Acoustics, some of which we will now impart to our readers. We shall
then find many ingenious inventions to examine,—the Audiphone,
Telephone, Megaphone, and Phonograph, which will occupy a separate
chapter. We now resume.

Amongst the experiments usually included in the course of professors
and lecturers who have a complete apparatus at their command, and
which at first appear very complicated and difficult, there are some
which can be performed with every-day articles at hand. There is no
experiment in acoustics more interesting than that of M. Lissajons,
which consists, as is well known to our scientists, of projecting upon
a table or other surface, with the aid of oxy-hydrogen light, the
vibratory curves traced by one of the prongs of a tuning-fork. We can
perform without difficulty a very similar experiment with the humble
assistance of the common knitting-needle.

Fix the flexible steel needle firmly in a cork, which will give it
sufficient support; fasten then at the upper extremity a small ball of
sealing wax, or a piece of paper about the size of a large pea. If the
cork in which the needle is fixed be held firmly in one hand, and you
cause the needle to vibrate by striking it, and then letting it sway
of itself, or with a pretty strong blow with a piece of wood, you will
perceive the little pellet of wax or paper describe an ellipse more or
less elongated, or even a circle will be described if the vibrations be
frequent. The effect is much enhanced if the experiment be performed
beneath a lamp, so that plenty of light may fall upon the vibrating
needle. In this case, the persistence of impressions upon the retina
admits of one seeing the vibrating circle in successive positions, and
we may almost fancy when the needle is struck with sufficient force,
that an elongated conical glass, like the old form of champagne glass,
is rising from the cork, as shown in the illustration annexed (fig.
173).

[Illustration: Fig. 173.—Experiment showing vibration of sound waves.]

Acoustics may be studied in the same way as other branches of physical
science. We will describe an interesting experiment, which gives a
very good idea of the transmission of sounds through solid bodies. A
silver spoon is fastened to a thread, the ends of which are thrust
into both ears, as shown in fig. 174; we then slightly swing the
spoon until we make it touch the edge of the table; the transmission
of sound is in consequence so intense that we are ready to believe
we are listening to the double diapason of an organ. This experiment
explains perfectly the transmission of spoken words by means of the
string of a telephone, another contrivance which any one may make for
himself without any trouble whatever. Two round pieces of cardboard are
fitted to two cylinders of tin-plate, as large round as a lamp-glass,
and four-and-a-half inches in length. If the two rounds of cardboard
are connected by a long string of sixteen to eighteen yards, we can
transmit sounds from one end to the other of this long cord; the
speaker pronouncing the words into the first cylinder, and the listener
placing his ear against the other. It is easy to demonstrate that sound
takes a certain time to pass from one point to another. When one sees
in the distance a carpenter driving in a stake, we find that the sound
produced by the blow of the hammer against the wood only reaches the
ear a few seconds after the contact of the two objects. We see the
flash at the firing of a gun, before hearing the sound of the report—of
course on the condition that we are at a fairly considerable distance,
as already remarked upon.

[Illustration: Fig. 174.—Conductibility of sound by solid bodies.]

[Illustration: Fig. 175.—Musical glasses.]

We can show the production of the Gamut by cutting little pieces of
wood of different sizes, which one throws on to a table; the sounds
produced vary according to the size of the different pieces. The same
effect may be obtained much better by means of goblets more or less
filled with water; they are struck with a short rod, and emit a sound
which can be modified by pouring in a greater or less quantity of
water; if the performer is gifted with a musical ear, he can obtain,
by a little arrangement, a perfect Gamut by means of seven glasses
which each give a note (fig. 175). A piece of music may be fairly
rendered in this manner, for the musical glasses frequently produce a
very pure silvery sound. We will complete the elementary principles
of acoustics by describing a very curious apparatus invented by
M. Tisley, the HARMONOGRAPH. This instrument, which we can easily
describe, is a most interesting object of study. The Harmonograph
belongs to mechanics in principle, and to the science of acoustics
in application. We will first examine the apparatus itself. It is
composed of two pendulums, A and B (fig. 176), fixed to suspensions.
Pendulum B supports a circular plate, P, on which we may place a small
sheet of paper, as shown in the illustration. This paper is fixed by
means of small brass clips. Pendulum A supports a horizontal bar, at
the extremity of which is a glass tube, T, terminating at its lower
extremity with a capillary opening; this tube is filled with aniline
ink, and just rests on the sheet of paper; the support and the tube are
balanced by a counterpoise on the right. The two pendulums, A and B,
are weighted with round pieces of lead, which can be moved at pleasure,
so that various oscillations may be obtained. The ratio between the
oscillations of the two pendulums may be exactly regulated by means of
pendulum A carrying a small additional weight, the height of which may
be regulated by means of a screw and a small windlass. If we give to
pendulum A a slight movement of oscillation, the point of tube T traces
a straight line on the paper placed in P; but if we move pendulum B,
the paper also is displaced, and the point of tube T will trace curves,
the shape of which varies with the nature of the movement of pendulum
B, the relation between the oscillations of the two pendulums, etc. If
the pendulums oscillate without any friction the curve will be clear,
and the point will pass indefinitely over the same track, but when
the oscillations diminish, the curve also diminishes in size, still
preserving its form, and tending to a point corresponding with the
position of repose of the two pendulums. The result is therefore that
the curves traced by the apparatus, of which we produce three specimens
(figs. 177, 178, 179), are traced in a continuous stroke, commencing
with the part of the greatest amplitude.

[Illustration: Fig. 176.—M. Tisley’s Harmonograph.]

By changing the relation and phases of the oscillations we obtain
curves of infinitely varied aspect. M. Tisley has a collection of more
than three thousand curves, which we have had occasion to glance over,
in which we failed to meet with two corresponding figures. The ratio
between these curves corresponds with some special class, of which
the analyst may define the general characters, but which is outside
our present subject. By giving the plate P a rotatory movement, we
obtain spiral curves of a very curious effect, but the apparatus is
more complicated. Considered from this point of view it constitutes
an interesting mechanical apparatus, showing the combination of
oscillations, and resolving certain questions of pure mechanics. From
the point of view of acoustics it constitutes a less curious object of
study. The experiments of M. Lissajons have proved that the vibrations
of diapasons are oscillations similar to, though much more rapid than
those of the pendulum. We can therefore with this apparatus reproduce
all the experiments of M. Lissajons, with this difference, that the
movements being slower are easier to study. When the ratio between the
_number of vibrations_—we purposely use the term vibration instead of
the term oscillation—is a whole number, we obtain figs. 177 and 178. If
the ratio is not exact, we obtain fig. 179, which is rather irregular
in appearance, corresponding to the distortions noticeable in M.
Lissajon’s experiments. Fig. 178 has been traced in the exact ratio
2:3; fig. 177 in the ratio 1:2; and fig. 179 corresponds to the ratio
1:2 and a small fraction, which causes the irregularity of the figure.

[Illustration: Fig. 177.—Ratio 1:2. Fig. 178.—Ratio 2:3.]

[Illustration: Fig. 179.—Ratio 1:2 and a fraction.]

[Illustration: Fig. 180. Construction of the Harmonograph. Fig. 181.]

[Illustration: Fig. 182.—Method of constructing an Harmonograph.]

[Illustration: Fig. 183.—The apparatus completed.]

In considering the harmony of figs. 177 and 178,—the first of which
corresponds to the octave, the second to the fifth, whilst fig.
179 corresponds to the disagreeable interval of the ninth,—one is
almost tempted to put a certain faith in the fundamental law of
_simple ratios_ as the basis of harmony. At first sight this appears
beyond doubt, but perhaps musicians would be hardly content with the
explanation. M. Tisley’s Harmonograph, it will be seen, is a rather
complicated apparatus; and I will now explain how it may be constructed
by means of a few pieces of wood. I endeavoured to construct as simple
an apparatus as possible, and with the commonest materials, feeling
that it is the best means of showing how it is possible for everybody
to reproduce these charming curves of musical intervals. Also I
completely excluded the employment of metals, and I constructed my
apparatus entirely with pieces of wooden rulers, and old cigar boxes.
I set to work in the following manner: on the two consecutive sides
of a drawing board I fixed four small pieces of wood (fig. 180), side
by side in twos, having at the end a small piece of tin-plate forming
a groove (fig. 181). In these grooves nails are placed which support
the pendulums. The piece of wood is placed on the corner of the table,
so that the pendulums which oscillate in two planes at right angles,
are in two planes that are sensibly parallel to the sides of the table.
The pendulums are made of a thin lath, with two small pieces of wood
fixed to them containing some very pointed nails, on which the pendulum
oscillates. Fig. 182 gives an illustration. The pendulums have a pin
fixed in vertically, which passes through a piece of wood, and by
means of a hinge connects the upper ends of the two pendulums. This
contrivance of the pin is very useful, and if care is taken to make the
hole through the hinge in the form of a double cone, as shown in fig.
182, _c_, it makes a perfect joint, which allows the piece of wood to
be freely moved.

[Illustration: Fig. 184.—Details of mechanism.]

To complete the apparatus, the heads of the two pendulums are united by
the hinge, at the bend of which a slender glass tube is fixed, which
traces the curves. The hinge is given in fig. 184, and to its two
extremities are adjusted the two pins of the pendulum (fig. 183). The
pendulums are encircled with round pieces of lead, which can be fixed
at any height by means of a screw.



CHAPTER XVI.

ACOUSTICS (_Continued_).

 THE TOPOPHONE—THE MEGAPHONE—THE AUTOPHONE—THE AUDIPHONE—THE
 TELEPHONE—THE PHONOGRAPH—THE MICROPHONE.


We propose in this chapter to give as shortly as possible a description
of the various instruments lately come into use, by means of which,
and electricity, sounds can be carried from place to place, and their
locality identified. It is only within the last few years that these
wonderful inventions have come into use, and in a measure superseded
the at one time invincible electric telegraph. The Telephone is now
in daily use in London and other places, and its novelty, if not
all its capability, has been discounted. The Phonograph has also
been frequently seen. So we will on this occasion commence with the
TOPOPHONE, a rather novel instrument.

As the name indicates, the TOPOPHONE is an apparatus for
discovering the position of a sound, from the Greek words signifying
a “place” and “sound.” The _sources_ of sound can be found by it, and
indeed this is its actual and practical use. It is claimed for this
new apparatus that it stands in the same relation to the sailor as his
old and trusty friends, the compass and sextant. These in navigation
inform the steersman as to his course, and tells him his position by
observation. The Topophone will tell him whence a sound arises, its
origin wherever it may be; and this in a fog is no mean advantage.
Suppose a ship to be approaching a dangerous coast in a fog. We are all
aware how deceptive sounds are when heard through such a medium. We
cannot tell from what precise direction the horn, whistle, or bell is
sounding. The Topophone will give us the exact spot, and we can then,
from our general knowledge of the locality, work our vessel up the
river, or into the harbour, in safety.

The Topophone was invented in 1880, by Professor Alfred Mayer, an
American, and is based upon the well-known theory of sound waves.
These, as we have already explained, exist in the air; and if the
theory of sound waves has perfected the Topophone, we can fairly say
that it has confirmed the supposed form of the sound waves. “Sound,”
says the inventor of the apparatus, “is supposed to be a particle
continually expanding in the air, composed of a wave produced by
compression, and followed by rarefaction. A continuous sound is a
series of these particles or globules spreading and expanding as the
water-rings in a pond.” This much will be at once perceived.

Now, suppose a person up to his shoulders in a pond of water, and
someone throws a stone into it. If that person extend his arms and
hands at right angles facing the sound, each hand would touch the edge
of a ripple as it came towards him across the pond. He would then be
facing the source of the ripples or waves, and look along a radius
of the circle formed by the waves. But if he please, he can move his
body so that both hands shall touch the same wave at the same time,
or he might turn away from the source, and only one hand would touch
the wave. But when both hands are actually washed by the same circular
ripple he must be facing the source of it. Any position in which his
fingers did not touch the ripple almost at the same instant, would not
be facing the source of the wave ripples. So by turning and extending
his hands, he could with his eyes shut find out whether he was or was
not facing the original source of the waves.

This applied to sound waves in the air is the whole theory of the
Topophone, which, however, depends for its usefulness upon the same
note being sounded by all horns and whistles. One note must be better
than all the others, and that note, probably C (treble), caused by
about two hundred and sixty vibrations per second, has been found most
applicable. If all whistles and horns can by law be compelled to adjust
themselves to this note, then the Topophone will be a real and lasting
benefit.

Let us now look at the apparatus itself.

It being conceded that the resonators are in the same key as the
Foghorn,—and this is necessary,—they are placed upon the deck of the
vessel. An ear-tube of indiarubber is carried from each of these
“resonators” into the cabin. These tubes unite and again separate,
ending in small pieces ready to be fitted to the ears. The apparatus
is fixed on deck, and the arrangement which supports it passes into
the cabin, and can be turned about in any direction. Of course in this
case a dial point is necessary to indicate the direction in which the
instrument is turned. If the machine be worn on the shoulders of the
officer of the watch he can move as he pleases, and wants no indicator.

The Topophone when used is so constructed, that when a horn is heard,
and when the listener is facing the sound, he can _hear nothing_! When
not facing the origin of the sound he can hear the horn very well, but
the moment the resonators receive the sound together as they face the
source, a very low murmur is heard, or perhaps no sound at all.—Why?

A certain pitch of tone is composed of vibrations or waves of equal
length. In all waves there is a hollow and a crest. One neutralizes
the other. The hollow of a sound wave meeting the crest of another
wave “interferes” to produce silence, stillness, a dead level. So in
“light”; two rays will produce darkness. We will endeavour to explain
this.

If we have two equal strings, each performing an equal number of
vibrations in a second, they will produce equal sound waves, and the
sound produced by both together will be uninterrupted, and twice as
loud as one of them. But if one string vibrate, say one hundred times,
and the other one hundred and one times in a second, they will _not_
be in unison, and one will gain upon the other string, till after it
has got to fifty vibrations it will be half a vibration ahead. At that
moment they will neutralize each other, and silence will ensue for an
appreciable time.

[Illustration: Fig. 185.—The Megaphone.]

In the case of light suppose a _red_ ray strikes the eye, and another
red ray to come upon it from somewhere else. If the difference between
its distance and the other point from the spot in the retina on which
the first ray fell, is the 258/1000 part of an inch, or exactly twice,
thrice, four times as much, etc., that distance, the light will be seen
twice as strong. But if the difference in the distances between the
points whence the light comes be only _one-half the 258/1000 part of an
inch_, or 1½, 2½, 3½, or 4½ times that distance, one light
will extinguish the other, and darkness will be the result. Now this
is precisely what happens in the case of the Topophone. To return to
our simile of water waves. If two stones be cast into a pond, and two
equal and similar waves produced, and if they reach a certain place at
the same moment, they will make one large wave. But if one followed
the other a little, so that the hollow of one coincided with the crest
of the other, and _vice versâ_, the waves would obliterate each other,
and a dead level would result. One tube of the Topophone is half a wave
length longer than the other, and when the resonators are in a line and
receive the wave at the same time, one ear hears the elevation of the
sound wave, and the other the depression,—the sound is neutralized, and
comparative, if not actual, silence results. The sailor knows in what
direction the land lies, and can calculate his distance, or anchor if
he please.

If amongst our readers there be any who wish to make for themselves an
acoustic signalling apparatus there is physically nothing to prevent
them from constructing such an instrument as that shown in the annexed
woodcut (fig. 185). It is founded upon the speaking-trumpet principle,
which is supposed to have been originated by Samuel Markland, in 1670.

Kircher, in his “_Ars magna et umbra_,” and in his “_Phonurgia_,”
mentions a kind of speaking-trumpet, or _porte voix_, of gigantic
dimensions, and called the “Horn of Alexander.” According to Kircher,
the instrument was used by Alexander the Great to summon his soldiers
from a distance of ten miles. The diameter of the circumference was
about eight feet, and Kircher conjectured that the instrument was
mounted upon three supports.

During the last century, a German professor, named Huth, made a model
of the horn, and found it answered every purpose of a speaking-trumpet
with most powerful results, but we beg leave to doubt whether the
instrument really carried the voice to any very great distance.

The Acoustic Cornet, which is the counterpart of the speaking-trumpet,
has been made in many different forms during the last two centuries,
but none of them to the present time consist of anything more intricate
than a simple tube with a mouthpiece and bell-shaped orifice.

Professor Edison, however, in his researches regarding the conveyance
of sounds, has made numerous and interesting experiments. On one
occasion, with his Megaphone he carried on a conversation at a distance
of nearly two miles, without any other assistance from instruments
except a few little cornets of cardboard. These constitute the
Megaphone, which may be regarded as a curiosity, considering the
effects produced by such simple means. The illustration (fig. 185)
represents the instrument which is (or was lately) fixed upon the
balcony of Mr. Edison’s house. At a mile-and-a-half distant from the
house, at a spot indicated by the two birds in the picture, another
instrument was fixed, and conversation was carried on with ease.

Perhaps the present opportunity will be the most convenient to speak of
the AUTOPHONE, although it is more a musical than an acoustic
instrument. Until lately Barbary organs and piano organs have been the
only means by which poor people have been able to hear any music, and
that not of a very elevated class. Besides, there is a good deal of
expense connected with the possession of an organ. But the Americans,
with a view to popularize music, have invented the AUTOPHONE,
which is simply a mechanical accordeon, manufactured by the Autophone
Company, of Ithaca, New York.

The principle of the instrument is represented in fig. 186, and is
extremely simple. An upright frame carries within it on one side a
bellows, and on the other a flexible air chamber, which serves as a
reservoir.

The upper portion contains a set of stops like an accordeon, but the
escape of the air through the small vibrating plates can only take
place by the upper surface of the frame work, upon which slides a thin
plate of Bristol board pierced with holes at convenient distances, and
set in motion by the mechanism shown in the annexed diagram (fig. 187).

[Illustration: Fig. 186.—The Autophone.]

The figure represents an axle furnished with a series of “washers,”
which, acting upon the plate, cause it to move round. It is the bellows
movement that turns the axle by the aid of two “catches,” B
and C, which work upon a toothed wheel fixed upon it.

The “catch” B moves the paper on which the tune is
“perforated,” when the bellows is empty, the other catch when it is
distended; but a counter catch, D, represented by the dotted
lines in the illustration, is so arranged that the paper cannot pass
on except the tooth of the catch D is opposite a hole pierced
upon the plate above. In the contrary case there is no movement of
the paper during the dilatation of the bellows. The effect of this
very ingenious arrangement is to give to the “musical” band of “board”
an irregular movement, but it economises it in the case of sustained
notes. The whole action of the instrument depends upon the correct
working of the bellows.

The effect, from an artistic point of view, certainly leaves something
to be desired, but the instrument is cheap, and not cumbersome, and
the slips of paper upon which the music is “cut out” can be made by
machinery, and consequently are not dear. So far, the Autophone is
fitted for popular favour and use, and may supersede the barrel organ.

[Illustration: Fig. 187.—Detail of the Autophone.]

The AUDIPHONE is an instrument to conduct sound to the ear,
to supplement it when temporary or partial deafness has occurred. Very
likely many of our readers have observed ladies carrying large black
fans on occasions,—at church, or lecture, or theatre,—and wondered
why, perhaps. Those “fans” are Audiphones. The instrument is made of
vulcanized rubber, and consists of a long flexible disc supported by
a handle. To the upper edge of the “fan” are attached cords, which
pass through a clip on the handle. If the person who wishes to hear by
means of the Audiphone will hold the fan against the upper teeth,—the
convex side of the fan outward,—he or she will hear distinctly, for
the vibrations of sound are collected and strike upon the teeth and
bones, and act upon the auditory nerves from within, precisely as the
vibrations act from without through the auricle. We need hardly add
that if the ear be injured the Audiphone will be of no use. A writer
says: “From personal observation with the Audiphone it appears to
convey the sonorous vibrations to the ear through the teeth, just as
a long wooden rod held in the teeth will convey the vibrations of the
sounding-board of a piano, though the piano is in another room and out
of hearing by the ear. In using the Audiphone during conversation there
is no movement or vibration felt by the teeth; in listening to a piano
there is a very faint sensation as if the Audiphone vibrated slightly,
while with the handle of the Audiphone resting on the sounding-board
of the piano the vibrations are so violent as to be painful to the
teeth. By closing the ears a person with even acute hearing can observe
the admirable manner in which the instrument conveys spoken words to
the ear. The Audiphone will prove to be of great value to deaf mutes,
as it enables them to hear their own voices, and thus to train them
to express words, while, before, they could only make inarticulate
sounds.”

We have a variation of this instrument which has been introduced
employing a diaphragm held in a telephone mouthpiece, and free to
vibrate under the influence of sounds. This is connected by a string
to a bit of wood that may be held in the teeth. In use the hearer
places the wood between his teeth, the string is drawn tight, and the
speaker speaks through the telephone mouthpiece, the vibrations of
the diaphragm being then conveyed to the teeth through the stretched
string. This apparatus works very successfully, and ladies use it, but
it is not so convenient for general use as the Audiphone.

[Illustration: Fig. 188.—The Telephone.]

The Telephone is now in daily use in London, and is by no means
strange to the majority of our countrymen, still some description of
it will probably be acceptable, and a glance at its history may prove
interesting.

[Illustration: Fig. 189.—The “receiving” apparatus.]

In speaking of the Telephone, we must not lose sight of the facts
before mentioned, as regards our sense of hearing, and the manner in
which the ear acts by the series of bones termed the hammer, the anvil,
and stirrup. In the process of reproduction of tone in the magnetic
instruments, the mechanism of the human ear was, to a certain extent,
imitated, and a diaphragm, by vibrations, generates and controls an
electric current.

Professor Wheatstone was the first person to employ the electric
wire for the transmission of sounds, but Professor Philip Reiss, of
Friedrichsdorf, was the first to make the experiment of producing
musical sounds at a distance. His first instrument was of a most
primitive nature; subsequently he produced an instrument of which fig.
188 is the Telephone, fig. 189 the “receiver.”

In fig. 188, it will be seen that there is an aperture on the top and
one at the side; the latter is the mouthpiece. The top aperture is
covered with a membrane which is stretched very tightly. When a person
speaks or sings into the mouthpiece his voice is at once concentrated
upon the tight membrane, which it causes to vibrate in a manner
corresponding with the vibrations of the voice. There are two binding
screws, one at each side. To the centre of the tight membrane a piece
of platinum is fixed, and this is connected with the binding screw on
one side, in which a wire from the battery is fixed. On the membrane
is a tripod, the feet of which (two) rest in metal cups, one of them
being in a mercury cup connected with the binding screw at the opposite
side to that already mentioned. The third “foot”—a platinum point—is on
the platinum in the centre of the membrane or top, and moves with it.
Every time the membrane is stretched by a vibration the platinum point
is touched, and the closed circuit is broken by the return of each
vibration.

[Illustration: Fig. 190.—Bell’s first Telephone (Transmitter).

_a._ Electro-magnet. _b._ Diaphragm. _c._ Collar. _d._ Collar and tube.
_f._ Screw. _g._ Mouthpiece. _h._ Battery. _i._ Wire from battery to
coil. _k._ Telegraph wire. _l._ Through binding screw. _m._ Pillar
holding magnet.]

The receiving instrument (fig. 189) consists of a coil enclosing an
iron rod, and fixed upon a hollow sounding box. It is founded upon a
fact discovered by Professor Henry, that iron bars when magnetized by
an electric current become a little longer, and at the interruption
of the current resume their former length. Thus in the receiver the
iron will become alternately longer and shorter in accordance with the
vibrations of the membrane in the box far away, and so the longitudinal
vibrations of the bar of iron will be communicated to the sounding box,
and become perfectly audible. This instrument, however, could only
produce the “pitch” of sound, “not different degrees of intensity, or
other qualities of tones.” It merely sang with its own little trumpet
whatever was sung into it; for all the waves were produced by an
electric current of a certain and uniform strength, and therefore the
sound waves were of the same size.

But in 1874, Mr. Elisha Gray, of Chicago, improved Reiss’ instrument,
and discovered a method by which the intensity or loudness of tones, as
well as their “pitch,” were transmitted and reproduced. In this method
he employed electrical vibrations of varying strength and rapidity,
and so was enabled to reproduce a tune. Subsequently he conceived the
notion of controlling the vibrations by means of a diaphragm, which
responded to every known sound, and by this he managed to transmit
speech in an articulate manner.

[Illustration: Fig. 191.—Bell’s Telephone (Receiver).]

In 1876, Professor Graham Bell sent a Telephone to the Centennial
Exhibition at Philadelphia. Mr. Bell, according to the report, managed
to produce a variation of strength of current in exact proportion to
the particle of air moved by the sound. A piece of iron attached to
a membrane, and moved to and fro in proximity to an electro magnet,
proved successful. The battery and wire of the electro magnet are in
circuit with the telegraph wire, and the wire of another electro magnet
at the receiving station. This second magnet has a solid bar of iron
for core, which is connected at one end, by a thick disc of iron, to
an iron tube surrounding the coil and bar. The free circular end of
the tube constitutes one pole of the electro magnet, and the adjacent
free end of the bar core the other. A thin circular iron disc held
pressed against the end of the tube by the electro-magnetic attraction,
and free to vibrate through a very small space without touching the
central pole, constitutes the sounder by which the electric effect is
reconverted into sound. The accompanying illustrations (figs. 190, 191)
show Mr. Bell’s Telephone as described.

The Telephone, subsequently simplified by Professor Bell, is shown in
the two following illustrations (figs. 192, 193). The voice strikes
against the diaphragm, and it begins to vibrate. The _sound_ is not
conveyed by the wire; the _motion_ is communicated, and the vibrations
become sound waves again. The Telephone consists of a cylindrical
magnet encircled at one end by a bobbin, on which is wound a quantity
of fine insulated copper wire. The magnet and coil are contained in a
wooden case, the ends of the coil being soldered to thick copper wire,
which traverse the “wooden envelope,” and terminate in the binding
screws. In front of the magnet is a thin circular iron plate, in which
is the mouthpiece. The drawings will explain the instrument.

[Illustration: Fig. 192.—External appearance of Bell Telephone.]

[Illustration: Fig. 193.—_a._ Bobbin of coil wire round magnet. _b._
Diaphragm. _c._ Mouthpiece. _d._ Permanent magnet. _e._ Wires to
binding screws. _f._ Both wires as one for convenience. _g._ Adjusting
screw-holding magnet.]

Mr. Edison also invented a Telephone like Gray’s, and made the
discovery, that when properly prepared, carbon would change its
resistance with pressure, and that the ratio of these changes
corresponded with the pressure. This solved the problem of the
production of speech. The carbon is placed between two plates of
platinum connected in the circuit and near the diaphragm, and the
carbon receives the pressure from it by means of the mouthpiece.

When we come to MAGNETISM and ELECTRICITY we may have
something more to say respecting the mysteries of the Telephone and
its later developments. At present we are only concerned with it as a
sound conveyer, and it answers its purpose admirably, although somewhat
liable to attract other sounds or vibrations from neighbouring wires.

The PHONOGRAPH, a mechanical invention of Mr. Edison, does not
make use of electricity, although the vibratory motion of the diaphragm
is utilized. It, in a simple form, consists of a diaphragm so arranged
as to operate upon a small stylus, placed just opposite and below the
centre, and a brass cylinder, six or eight inches long, by three or
four in diameter, mounted upon a horizontal axis, extending each way
beyond its ends for a distance about its own length.

“A spiral groove is cut in the circumference of the cylinder, from one
end to the other, each spiral of the groove being separated from its
neighbour by about one-tenth of an inch. The shaft or axis is also cut
by a screw thread corresponding to the spiral groove of the cylinder,
and works in screw bearings; consequently when the cylinder is caused
to revolve, by means of a crank that is fitted to the axis for this
purpose, it receives a forward or backward movement of about one-tenth
of an inch for every turn of the same, the direction, of course,
depending upon the way the crank is turned. The diaphragm is firmly
supported by an upright casting capable of adjustment, and so arranged
that it may be removed altogether when necessary. When in use, however,
it is clamped in a fixed position above or in front of the cylinder,
thus bringing the stylus always opposite the groove as the cylinder is
turned. A small, flat spring attached to the casting extends underneath
the diaphragm as far as its centre and carries the stylus, and between
the diaphragm and spring a small piece of india-rubber is placed
to modify the action, it having been found that better results are
obtained by this means than when the stylus is rigidly attached to the
diaphragm itself.

[Illustration: Fig. 194.—Mode of using the Telephone.]

“The action of the apparatus will now be readily understood from what
follows. The cylinder is first very smoothly covered with tin-foil, and
the diaphragm securely fastened in place by clamping its support to the
base of the instrument. When this has been properly done, the stylus
should lightly press against that part of the foil over the groove.
The crank is now turned, while, at the same time, someone speaks into
the mouthpiece of the instrument, which will cause the diaphragm to
vibrate, and as the vibrations of the latter correspond with the
movements of the air producing them, the soft and yielding foil will
become marked along the line of the groove by a series of indentations
of different depths, varying with the amplitude of the vibrations of
the diaphragm; or in other words, with the inflections or modulations
of the speaker’s voice. These inflections may therefore be looked upon
as a sort of visible speech, which, in fact, they really are. If now
the diaphragm is removed, by loosening the clamp, and the cylinder
then turned back to the starting point, we have only to replace the
diaphragm and turn in the same direction as at first, to hear repeated
all that has been spoken into the mouthpiece of the apparatus; the
stylus, by this means, being caused to traverse its former path, and
consequently, rising and falling with the depressions in the foil,
its motion is communicated to the diaphragm, and thence through the
intervening air to the ear, where the sensation of sound is produced.

[Illustration: Fig. 195.—BELL’S LONG-DISTANCE TELEPHONE

  _a._ Compound magnet.   _d._ Diaphragm.    _e._ Speaking tube.
  _f._ Telegraph wire.   _g._ Line to earth.    _b_, _c._ Small spaces.]

“As the faithful reproduction of a sound is in reality nothing more
than a reproduction of similar acoustic vibrations in a given time, it
at once becomes evident that the cylinder should be made to revolve
with absolute uniformity at all times, otherwise a difference more
or less marked between the original sound and the reproduction will
become manifest. To secure this uniformity of motion, and produce a
practically working machine for automatically recording speeches,
vocal and instrumental music, and perfectly reproducing the same, the
inventor devised an apparatus in which a plate replaces the cylinder.
This plate, which is ten inches in diameter, has a volute spiral groove
cut in its surface on both sides from its centre to within one inch of
its outer edge; an arm guided by the spiral upon the under side of the
plate carries a diaphragm and mouthpiece at its extreme end. If the
arm be placed near the centre of the plate and the latter rotated, the
motion will cause the arm to follow the spiral outward to the edge. A
spring and train of wheel-work regulated by a friction governor serves
to give uniform motion to the plate. The sheet upon which the record is
made is of tin-foil. This is fastened to a paper frame, made by cutting
a nine-inch disc from a square piece of paper of the same dimensions
as the plate. Four pins upon the plate pass through corresponding
eyelet-holes punched in the four corners of the paper, when the latter
is laid upon it, and thus secure accurate registration, while a
clamping-frame hinged to the plate fastens the foil and its paper frame
securely to the latter. The mechanism is so arranged that the plate may
be started and stopped instantly, or its motion reversed at will, thus
giving the greatest convenience to both speaker and copyist.

“The articulation and quality of the Phonograph, although not yet
perfect, is full as good as the Telephone was. The instrument, when
perfected and moved by clock-work, will undoubtedly reproduce every
condition of the human voice, including the whole world of expression
in speech and song, and will be used universally.

“The sheet of tin-foil or other plastic material receiving the
impressions of sound, will be stereotyped or electrotyped so as to be
multiplied and made durable; or the cylinder will be made of a material
plastic when used, and hardening afterward. Thin sheets of _papier
maché_, or of various substances which soften by heat, would be of this
character. Having provided thus for the durability of the Phonograph
plate, it will be very easy to make it separable from the cylinder
producing it, and attachable to a corresponding cylinder anywhere and
at any time. There will doubtless be a standard of diameter and pitch
of screw for Phonograph cylinders. Friends at a distance will then send
to each other Phonograph letters, which will talk at any time in the
friend’s voice when put upon the instrument.” (_Scribner._)

The MICROPHONE (an outcome of the Telephone) was discovered
by Professor Hughes, of London. It is an instrument which in its main
features consists of a carbon “pencil,” so suspended that one end rests
upon a carbon “die.” The instrument being connected with a Telephone
by the circuit wires, will reproduce faint sounds very distinctly.
Once a Microphone was put into a preacher’s pulpit, and joined to a
private telegraph wire which led to a gentleman’s house. The owner was
thus enabled to hear the sermon. So long as it is thus connected every
minute sound, even a fly’s footstep, will be faithfully reproduced.



CHAPTER XVII.

 THE TUNING-FORK—THE SYREN—SOUND FIGURES—SINGING FLAMES.


We cannot close the subject of Sound without some mention of the
Musical Pitch, and various instruments and experiments which have from
time to time been made to discover the pitch, sound, and vibrations,
and even to _see_ Sound. To understand the vibrations or “pitch”
of a musical note we may study the illustration, which shows us a
tuning-fork in vibration.

You will perceive that each prong of the tuning-fork beats the air
in an opposite direction at the same time, say from _a_ to _b_ (fig.
196). The prong strikes the air, and the wave thus created strikes
again outward, and the condensation thus created travels along the
back beat, rarefying the air, and both these, the rarefaction and the
condensation, move with the same rapidity one behind the other.

[Illustration: Fig. 196.]

[Illustration: Fig. 197.]

The tuning-fork of course vibrates a very great many times in a second,
every vibration generating a wave. “Pitch,” in a general sense, is the
number of vibrations per second which constitute a note. For instance,
the note A, the standard pitch consists of four hundred and thirty-five
complete vibrations per second. Concert pitch is slightly higher,
for there are a few more vibrations in the second. The lowest sound
pitch is forty vibrations, the highest forty thousand. “Pitch” may be
determined by an instrument termed the “Syren,” or by a tooth-wheeled
apparatus.

The SYREN was invented by Cagniard de Latour. It consists of
a metal cylinder, a tube passes through the bottom, and through the
tube air is blown into the cylinder. On the top a number of holes are
drilled, while just over the cylinder top, almost in contact with it,
is a metallic disc, which rotates upon a vertical axis. The disc is
perforated with holes equal in number to those in the cylinder top, but
the holes are not perpendicular, they slope in opposite directions. So
when the air is forced through the holes in the top of the cylinder it
impinges upon one side of the holes in the rotating disc, and blows it
round.

The disc in one revolution will therefore open and shut as many holes
as there are in the disc and cylinder, and the air blown in will escape
in so many puffs—the number of puffs in a given time depending upon the
rapidity of rotation. There is an arrangement to show the number of
turns. By these rotations a sound is produced which rises in pitch as
the revolutions are increased in number.

[Illustration:

  Fig. 198.      Sound Figures.      Fig. 199.]

To determine the pitch of a certain sound we must find the number of
times the plate revolves in that time, then we shall have the number
of vibrations per second required to produce the note we desire.
The arrangement working in a notched wheel tells us the number of
rotations of the disc. Successive, and rapidly-successive puffs or
beats are heard as the rotation increases, and at length the two
sounds will disappear, and merge into one, which is perhaps that of
the tuning-fork, whose note you require to find the “pitch” of. By
maintaining this rate for a minute or less, and setting the gear to
tell the revolutions, the number will be found marked on the dial of
the apparatus. So by multiplying the number of revolutions of the disc
by the number of the holes, and dividing the product by the number of
seconds during which the disc was in connection with the recording
gear, we shall have the number of vibrations per second necessary to
produce the pitch corresponding to the given sound. The above is the
description of one form of Syren; there are others, which, however, we
need not detail.

We have seen that there are certain _nodal points_, or resting-places,
in vibrations, and this can easily be shown upon a fiddle-string, from
which paper discs will fall off except on the nodal point, showing
that there is no vibration there. The same experiment may be made by
means of plates, which will give us what are termed Chladni’s figures.
Suppose we strew a glass-plate with fine sand, and stroke the edge with
a fiddle-bow. The vibrations of the plates will make certain patterns,
and cast the sand upon those points of repose to form nodal lines in
various directions. The plates must, of course, be held or fastened,
and a variety of _sound figures_ may be produced. (_See_ figs. 198 and
199.)

The relation between the number of segments on the plate and the pitch
of the note, can be ascertained by using a circular plate clamped
in the centre. “If the finger on the plate and the fiddle-bow are
one-eighth of the circumference apart, the fundamental note will be
produced. If one-sixteenth apart, the higher octave will be heard.”

Sensitive flames will detect air vibrations, and flames can also be
made to sing. Sensitive flames were discovered by Mr. Barrett, who
noticed the effect a shrill note had upon a gas flame from a tapering
jet. The flame was a very long one (fourteen inches), and when the
sound was produced it shortened at once, while the upper part expanded
like a fan; the same effects, in a less marked degree, were observable
when the shrill sound was prolonged from a distance of forty feet.
Professor Tyndall was immediately interested in this discovery, and in
January 1867 he lectured upon it at the Royal Institution.

If any one wish to try the experiment, a piece of glass tubing should
be obtained, and let the mouth be tapered down to a small orifice
one-sixteenth of an inch in diameter. Then when the highest pressure
is on for the evening, light the gas and sound a shrill whistle. The
flame will sink down and spread out. The illuminating power may thus be
increased, and many experiments may be made. For instance, if a person
be in the room and try to read, he will probably not be able to do so
at a little distance; but if his friend whistle to the gas it will so
expand itself as to enable him to read, so long as the whistle lasts.

A very ingenious burglar-detector was made upon the principle of the
sensitive flame, which expands at a noise and heats a welded plate of
gold, silver, and platinum. The plate swerves aside, the metals being
unequally affected by heat, and as it is connected with a battery,
rings a bell by electricity. A small high flame has been made sensitive
to the chinking of coin, or even to the ticking of a watch. We will now
give some explanation, derived partly from Professor Tyndall, of the
cause of sensitive flames.

A sensitive flame is one just on the point of “roaring,” and about
to change its aspect. “It stands,” says Tyndall, “on the edge of
a precipice. The proper sound pushes it over.... We bring it to
the verge of falling, and the sonorous pulses precipitate what was
already imminent.” The flame is in a state of vibration, so sounds
being vibrations, practically increase the pressure; and the flame
acknowledges the pressure thus invisibly applied by air waves.

SINGING FLAMES are produced by burning hydrogen in a tube;
a musical note is thus produced in the same way as the air causes a
note in an organ pipe. Faraday attributed the sound to rapid vibration
caused by successive explosions of the burning gas. The Gas Harmonicon
has been made on this principle. The air, being heated in the glass
tube, ascends, and the flame is thus permitted to come up more forcibly
in the tube; so violent agitation results when the air tries to get
into the opening above. The size of the flame and its position in the
tube will give a certain note which will be the same note as the air
would emit if in a pipe, for the vibrations give the sound.

Sir Charles Wheatstone has shown by experiment how sound can be
transmitted by placing a rod on a musical-box, and carrying the rod
through the ceiling. When a guitar or violin was placed upon the rod,
the sounds of the musical-box were distinctly heard in the upper room.
A _Phantom Band_ can be made by connecting certain instruments with
others being played on under the stage. Every one will then appear to
play by itself.



CHAPTER XVIII

ELECTRICITY.

 DERIVATION OF ELECTRICITY—SEALING-WAX EXPERIMENT—THE
 ELECTROPHORUS—LEYDEN JAR—POSITIVE AND NEGATIVE—THE
 ELECTROSCOPE—ELECTRIC MACHINES.


We have now briefly and of course imperfectly reviewed the phenomena of
Vibration, as exemplified in what we term Heat, Light, and Sound. We
now come to a most mysterious servant of mankind, as mysterious as any
Djinn of romance; viz., ELECTRICITY.

The term Electricity is derived from the Greek word _electron_,
meaning “amber”; because from amber the properties of what we call
“Electricity” were first discovered. Six hundred years before the
Christian Era, Thales wrote concerning the attraction which amber,
when rubbed, possessed for light and dry bodies. But it is to an
Englishman named Gilbert that we owe the word “Electricity,” which he
derived from the Greek, and in his works (about 1600 A.D.)
he discusses the force of the so-called “fluid.” Otto von Guerike, of
“air-pump” celebrity, and many other philosophers after him, continued
the investigation of the subject. At the beginning of the last century
great attention was paid to the Electric Machine. The Leyden Jar was,
as its name denotes, discovered by Muschenbrock, of Leyden, (though
the honour was disputed). Franklin made the first lightning conductor
in 1760. Volta and Galvani, to whose invention we owe “Voltaic
Electricity” and “Galvanism,” and Faraday in more modern times gave a
great impetus to electrical science. The great part that electricity
has been playing in the domestic history of the world since Faraday’s
lamented death, is probably known to the youngest of our readers. What
the future of this agent may be we can only guess, but even now we may
regard electricity as only in its infancy.

There are few scientific studies more attractive to the general reader
than electricity, and few admit of more popular demonstration. The
success of the late electrical exhibition in Paris, and its successor
in London at the present time, are proofs of the interest taken in this
great and mysterious agent whose origin we are in ignorance of, and of
whose nature and powers we are daily discovering more and more, and
finding there is still an immense field for its application.

Some fundamental facts regarding electricity may very easily be studied
with the assistance of every-day objects at hand. Amber was the first
substance to show attraction when rubbed, but Gilbert found out that
glass and sealing-wax, etc., possessed like properties with amber.

If we rub a stick of sealing-wax with a piece of cloth, we shall see
that it will attract some small fragments of paper placed near it.
Nothing is easier than to construct a small pendulum to show with
perfect clearness the phenomenon of electric attraction. A piece of
iron is fixed on a wooden pedestal, and supports a thread of silk, to
the end of which is fastened a little ball cut out of a piece of cork.
The stick of sealing-wax after being rubbed with the cloth will attract
the ball as shown in fig. 200.

[Illustration: Fig. 200.—Sealing-wax attracting a piece of cork.]

By means of a piece of paper we can produce a spark. I take a large,
strong sheet of drawing paper, heat it very thoroughly, and lay it on
a wooden table. I rub it with a perfectly dry hand, or with a piece
of woollen material until it adheres to the table. That done, I place
a bunch of keys in the centre of the sheet of paper, which I raise,
lifting it by two corners. If at this moment any one touches the bunch
of keys with his finger, a bright spark will be elicited. The metal is
charged with the electricity developed on the paper; if the weather
is dry, and the paper thoroughly heated several times, the spark may
attain nearly an inch in length.

We can easily construct other electrical apparatus. For instance, an
“Electrophorus,” or instrument for obtaining electricity by means of
induction, or a Leyden jar, can both be made at home. Let us proceed to
construct the former, of which we give an illustration (fig. 201).

We take a lacquered tea-tray about a foot long, and cut out a sheet of
thick wrapping paper, so that it will lie over all the level portion
of the tray. At each side of this sheet of paper we fix two bands of
paper, as in the illustration (fig. 201), so as to serve as handles.
The tea-tray should be placed upon two tumblers to support it and
to insulate it, glass being a “non-conductor.” (We will speak of
conductors and non-conductors presently.) We have now our Electrophorus
made ready for action; let us proceed to see how it will act.

[Illustration: Fig. 201—Simple Electrophorus.]

First, rub the thick packing paper over a hot fire or a stove, and the
friction must be continued for some time, until the paper has become
thoroughly dry, and as hot as possible without charring. When this has
been accomplished, place it quickly upon a wooden table, and rub it
rapidly and energetically with a clothes’ brush, dry and hard as can
be obtained. Place the paper upon the tray; touch the tray with the
knuckle, and draw away the paper by the handles fixed to it (_see_
fig. 201); a spark will result. Then if the paper be replaced upon the
tray, and the hand again presented, the same result will follow. This
experiment may be repeated five or six times, at least, with success.

We have in this tea-tray and its paper covering a real electric
machine. How can we manage to provide a Leyden jar to contain our
electricity? Nothing is more easy. Let us take a tumbler and partly
fill it with shot; insert into the glass a tea-spoon, and if all the
articles are quite dry we shall possess a Leyden jar.

To charge the jar we have thus provided we must work the Electrophorus
we have already described. While one person lifts off the paper as
directed, another must hold the glass filled with shot close to the
edge of the tray, and touch the corner with the tea-spoon; the spark
will then enter the “jar” or tumbler. We can thus charge the jar as we
please, and by presenting the finger as in the illustration (fig. 202),
we shall obtain a discharge from it.

[Illustration: Fig. 202.—A Leyden jar.]

Mr. Louis Figuier, in his “_Merveilles de la Science_,” relates that
Wollaston, meeting one of his friends one evening in the streets of
London, drew from his pocket a copper thimble, and proceeded to turn it
into a microscopic pile.[13]

In order to do this he removed the bottom of the thimble, flattened
it with a stone, so as to bring the two internal surfaces about on a
line with each other, then placed between the copper surfaces a small
strip of zinc, which was not in contact with the copper, owing to the
interposition of a little sealing-wax. He then placed it in a glass
cup, previously filled with the contents of a small phial of water,
acidulated with sulphuric acid. He next wound round the strip of zinc
and its copper covering a piece of platinum wire, the wire becoming
red through the electricity developed in the pile. The dimensions of
this platinum wire were extremely small; it was only 30/1000 of an inch
in diameter, and 1/30 of an inch in length. By reason of its small
dimensions it could not only be reddened, but fused by the little
battery.

Thus Wollaston’s friend, who was a witness of the experiment, was
able to light a tinder at the red wire. In this little battery of
Wollaston’s the copper enveloped the strip of zinc in every part;
that is to say, the negative element was on a higher surface than the
positive metal.

[Illustration: Fig. 203.-A simple compass.]

After considering Electricity, it is not impossible to approach the
study of Magnetism, and even to construct a mariner’s compass. We
shall find the method of doing so by borrowing an interesting passage
from the “_Magasin Pittoresque_.” Let us take a small cork and pass
through it an ordinary knitting-needle (fig. 203), which we have
already magnetized by placing it N.S., rubbing it gently, and always in
the same direction, with one of those little iron magnets with which
children amuse themselves. After the needle has been passed through
the cork, we also fix into it a sewing-needle, or rather a pin, the
point of which rests in one of the little holes in the upper part of
the thimble. In order to balance the magnetised needle, we thrust a
match into both sides of the cork, as shown in the illustration, and
fasten to the ends of each a ball of wax. Thus the needle, the balls,
and the pin are all balanced at once, so that the contrivance has the
appearance of the illustration.

As it is very important that with such a sensitive instrument any
agitation of the air should be avoided, the thimble must be placed at
the bottom of a common earthen pan, B D T, which should be
covered over with a piece of glass, V. To graduate the compass
a circle is described on a piece of paper. On this dial we trace the
divisions sufficiently close only at the north extremity of the needle,
and the paper is fixed underneath, as in fig. 203. Then we fix a piece
of wax at the end of the match pointing N., opposite the
northern extremity of the needle inside the basin. In this way we have
a very useful and inexpensive compass.

We may also magnetize a fine sewing-needle, and grease it by rubbing
it with a little suet. It is then capable of floating on the surface
of water running in the direction of the north pole. We might go on
multiplying indefinitely examples of physical experiments without
apparatus, but we have probably already given a sufficient number to
aid our readers in imagining others.

We have now in a simple manner shown how we can easily produce
electricity. We may understand that electrical phenomena are
produced—(1) friction between different bodies; (2) by placing bodies
which differ in contact; (3) by the transition of bodies from one
condition to another; (4) by chemical changes; (5) by animals. The two
first, and the fourth, are the most usual causes.

We know that certain substances when rubbed with silk or wool acquire
the property of attracting other substances. But in the case of a
rod of glass or stick of wax, the attraction will only be perceived
when the rubbing has been applied. But metal will behave differently.
Any part of the metal rod will continue to attract. So metals are
CONDUCTORS of electricity; while glass, wax, silk, amber,
sulphur, etc., are _bad_, or NON-CONDUCTORS. Metals are the
best conductors we have, but trees, plants, liquids, and the bodies of
animals, including men, are all good conductors of electricity. Dry air
is a bad conductor.

[Illustration: Fig. 204.—Attraction and repulsion.]

There are two kinds of electricity, known as positive and negative
(_plus_ or _minus_), vitreous or resinous. We saw in fig. 200 that we
can attract a small ball of pith or cork by a piece of sealing-wax
rubbed with flannel. If we then present a glass rod rubbed with silk
to it, it will be equally attracted, but will be at once repelled; and
after being so repelled, if we put the wax to it, it will be attracted
to the sealing-wax again. So wax at first attracts then repels the
ball, and so does glass, but either will attract the ball if presented
alternately (fig. 204). The reason for this is as follows:—

When we have rubbed the glass with silk, we charge it with positive
electricity, and when the rod touches the ball, the latter imbibes that
electricity, and flies away from the glass rod. The sealing-wax imparts
negative electricity in the same way.

The law is, that bodies charged with the same kind of electricity
_repel_ each other, and those containing the opposite kinds _attract_
each other. Positive _repels_ positive; negative _repels_ negative.
But positive _attracts_ negative, and negative _attracts_ positive.
Opposite electricities unite, and so neutralize each other that no
effect is perceived; but it must be borne in mind that all bodies
possess both electricities in some quantity, greater or less. By
rubbing we separate these electricities, the rubber becoming negative,
the rub_bee_ positive. The friction of glass supplies positive
electricity, and sealing-wax supplies negative electricity, or we can
obtain the same effect by rubbing either with certain material.

The manner in which a body is electrified depends upon its nature
and condition; but we may accept as a general axiom,—but by no means
as a law,—that when two bodies are rubbed together, that which gets
the hotter in the process takes the negative kind of electricity. In
the following list the substances have been so arranged that each is
negatively electrified by those preceding, and positively by those
succeeding it. 1, cat’s skin; 2, glass; 3, woollen stuffs; 4, feathers;
5, wood; 6, paper; 7, silk; 8, shellac; 9, rough glass. We append a
list of conducting and non-conducting bodies in their order:—

CONDUCTORS.

  Metals.
  Lime, coal, or coke.
  Saline mixtures.
  Pure water.
  Vegetable tissues.
  Animal tissues.
  Hot air.
  Steam.
  Rarefied air.

NON-CONDUCTORS.

  Ice.
  India-rubber.
  Marble.
  Porcelain.
  Resin.
  Dry gases.
  Paper.
  Wool.
  Silk.
  Shell-lac.
  Diamond.
  Glass.
  Wax.
  Sulphur.

[Illustration: Fig. 205.—Positive and negative.]

[Illustration: Fig. 206.—The Battery.]

It should be observed that the degree of value as a conductor or
non-conductor depends somewhat upon the atmosphere. For instance, glass
is an excellent insulator, or non-conductor, when dry, but when wet
it changes to a conductor. So insulators are at times covered with a
solution of shell-lac, or fat, to keep away moisture. We may reasonably
conclude that bodies which are good and bad conductors are good and
bad conductors of electricity. Water is a good conductor, air is a
bad one; were it otherwise, electricity would escape from the ground
into the air; as it is, the air manages in some degree to retain the
electricity at the surface of bodies, for it is on the surface that we
find the electric “fluid.”

We have mentioned _electrical induction_ in a former experiment with
the tea-tray. We will now explain it more fully, as a consideration
of it will bring us to the _electric spark_, or lightning, with the
account of the discovery of the _Conductor_ and the _Electrical
Machine_.

Let us look at the illustration next below. A B is a cylinder supported
on a glass rod, and at each extremity is a small pith ball, _a_ and
_b_. The cylinder is in a neutral condition, as is evidenced at first
by the pellets being in a vertical position. But suppose we bring a
ball, C, towards the cylinder. C is charged with positive electricity,
which attracts the negative to itself, and so repels the positive away
at the opposite side. So the pellet at one side will be attracted to C,
and the other will fly in an opposite direction.

[Illustration: Fig. 207.—Electrical induction.]

Let us take another illustration. Here we have a horizontal metal rod,
_cc′_, insulated on a glass stand. Two balls of cork are attached
at both ends of the rod by metallic wires. Hold a rod of resin, _r_,
which has been made negatively electrical, and apply it to one pair of
the cork balls. The positive electricity will be attracted at _c′_ and
the negative repulsed, and fly away at _c_. If we remove the resin the
equilibrium will be again established, and the balls will fall to a
vertical position.

[Illustration: Fig. 208.—Induction.]

We can also by drawing off the negative electricity by the finger at
_c_, while the resin rod is still held to the other side, _c′_, fill
the whole of the metal rod with the positive electricity when the
finger and the resin have been removed respectively first and last. The
balls will then fly in opposite directions again, in consequence of the
repulsion exercised by the positive poles.

The “Electrophore,” or “_Electrophorus_,” we have already learned
to make for ourselves, as also the Leyden Jar. But we give cuts of
them. The former is very simple, and can be made by mixing two parts
of shell-lac and one of turpentine, and pouring the mixture upon a
metal plate. If this be rubbed with a cat’s skin when dry, and a metal
cover with a glass handle be placed upon it, it will be found that the
positive and negative electricity are collected on the lower and upper
surfaces of the plate respectively, and can be drawn away with a spark
as before, and made use of.

[Illustration: Fig. 209.—Electrophorus.]

The Leyden Jar requires a little more detailed description, as it is
to it we are indebted for our Battery. It is a common glass bottle or
jar, coated both inside and out with tinfoil nearly as high as the
shoulder, _a_ _a_. The mouth should be firmly closed with a bung of
wood, _g_ _g_; a hole should be bored in the bung, through which a
brass rod is tightly pushed. The rod, too, is topped by a brass knob,
and a brass chain is attached to the other extremity. The interior of
the tinfoil receives positive electricity, and the exterior negative
when the jar is charged from the “Electrophorus.” To discharge the jar
and create a _shock_ it is necessary to put one hand on the outside,
and the other on the knob of the jar. A brilliant spark and a severe
shock will result if the jar has been fully charged. It is as well to
be cautious when trying this experiment. The effect of the shock may be
felt by any number of persons joining hands, if one at one end of the
row, and one at the other end, touch the knob and the outside of the
jar simultaneously.

[Illustration: Fig. 210.—The Leyden Jar.]

This electric discharge is lightning in miniature, and it is to
Benjamin Franklin that the world is indebted for the discovery. The
philosopher was greatly interested in the science of Electricity and,
having retired from business, he devoted himself to the consideration
of thunderstorms. He wrote a treatise to show that points drew off
electricity, and that electricity and lightning were similar. He urged
that metallic rods might be attached to ships and buildings, so that
during thunderstorms, or at other times, the electricity might be
harmlessly carried into the ground. This suggestion he made without
being able to explain _why_ points did carry off electricity without
a spark. The reason is because there is no place to store it; it runs
away at once, without having time to collect, as in a ball.

Franklin made one or two experiments before his renowned kite-flying
arrangement, which convinced him that electricity was by no means
an agent to be played with. He endeavoured to kill a turkey by
electricity, but by incautious handling of the jars in which the
“fluid” was stored, he discharged them, and describes the result: “The
flash was very great, and the crack was as loud as a pistol; yet my
senses being instantly gone, I neither saw the one nor heard the other,
nor did I feel the stroke on my hand, though I afterwards found it
raised a round swelling where the fire entered as big as half a pistol
bullet.” On a subsequent occasion he was again struck senseless while
endeavouring to administer a shock to a paralytic patient.

It was not until June 1752 that Franklin made the experiment with
the kite, which resulted in such great discoveries. He made his kite
of a silk pocket-handkerchief, and he fixed a pointed rod upon the
upright portion of the frame at the top; the string ended in a foot
or so of silk, which was held by the philosopher, and to the end of
the hempen portion of the string a large key was tied. For some time,
notwithstanding the approach of most unmistakable thunder-clouds, his
patience was tried. But at last the strands of the hempen string began
to bristle up, and soon after, when Franklin applied his knuckle to
the key, a spark was obtained. The great discovery was made. Franklin
subsequently obtained lightning in his own house, and performed several
experiments with it.

[Illustration: Fig. 211.—The Electroscope.]

The Electroscope (fig. 211) is an instrument by which we can ascertain
whether electricity is present or not, and the nature of it. If we
bring an object unelectrified close to the ball or knob on the top of
the glass shade, the two needles, or strips of gold-leaf, which are
often used, will remain still. But if the body has been electrified
it will communicate the electricity to the rod inside, and attract to
itself the fluid of opposite quality; the same kind of electricity then
is in action in the gold-leaf or needles, and they fly apart—repel each
other. Supposing that positive electricity were first communicated, we
can cause the contraction of the leaves or wires by applying a negative
kind, which, meeting the positive, neutralizes it, and the wires
collapse.

If the electricity with which the instrument is charged be positive, by
approaching the baton to the ball, A, we shall see the wires
diverge more than before, and they will finally be discharged by the
knobs within. If the electricity be contrary to that in the baton, the
wires will approach each other, but by gradually withdrawing the baton
they will again separate, and even to a greater distance than before.

The Electric Machine is shown in the illustration (fig. 212). It
consists of a large plate of glass fixed upon a glass stand, between
wooden supports. The handle is of glass; two pairs of rubbers are
fastened above and below; the plate is turned between them, and becomes
“positively” electrified. The rubbers are covered with leather and
stuffed with horsehair, DD, and press very tightly against the
glass, so that the friction is constant. The rubbers are covered with
an amalgam made of mercury, zinc, and tin, two parts of the first to
one each of the others. A chain (of metal) connects the machine with
the ground. The conductors, PP, are united by a cross-piece,
Q, and sustained upon glass supports. At the end of the
conductors are two curved rods, CC, which are provided with
points to take the electricity from the plate, but do not touch it.

[Illustration: Fig. 212.—The Electric Machine.]

The electricity is thus stored in the insulated conductors as the
machine is turned. The negative portion is carried into the ground by
the chain from the rubbers, while the positive electricity is retained.
The longer we turn the more we shall obtain, and the quantity is
measured by an electric pendulum on one of the conductors, which flies
out by degrees as the charge increases, and indicates its power by
means of a needle it works upon an ivory index.

It is not difficult to make an electric machine out of a glass bottle.
This will furnish the glass cylinder. If a stick be run through it (for
which purpose a hole must be drilled in the bottom of the bottle), a
handle can be fixed, and the bottle mounted on a stand. A wash-leather
cushion, stuffed, can be so arranged that it will press against the
bottle as it is turned; a piece of silk should be permitted to hang
from the cushion frame over the glass. A conductor may be made from a
piece of wood neatly rounded and smoothed, and coated with tinfoil.
The ends should be rounded like “knobs.” Stick pins in to collect the
electricity (and it will be readily obtained). The cushion should of
course be well smeared with amalgam. From this, as well as from the
glass-plate machine, the “positive” electricity can be drawn off and
stored in a Leyden jar, and then discharged by the “discharging rod,”
which is represented on the cut. It may have one or two handles, and
one knob is placed outside the jar, the other near the ball surmounting
it. The glass being a non-conductor saves the operator, and some long
sparks and loud reports may be obtained.

[Illustration: Fig. 213.—Cylinder machine.]

The Electric Machine is always assumed to give off _positive_
electricity.

Sir William Armstrong’s Electric Machine is a mode of obtaining
electricity by moist steam. The design is Armstrong’s, and Professor
Faraday subsequently went into all the conditions to produce the
“fluid” by the friction of steam. The machine was something like a
small boiler supported on glass legs. A row of nozzles was fixed upon
the escape pipe so as to create a great velocity and friction in
the escaping steam. Round the nozzles was a box of cold water, for
that fluid was found necessary for the production of electricity as
demonstrated by Professor Faraday. The steam rushed against a row of
points attached to the prime conductor of an electric machine, and the
electricity of the steam was thus given off to the conductor. There are
many other forms of electric machines, but it will serve no purpose to
detail them.

[Illustration: Fig. 214.—Discharging rod.]

The _Electric Battery_ (_see_ fig. 206) is formed by a collection of
Leyden jars. The inside and outside coatings are connected in a box
divided into partitions lined with tinfoil. The rods of the jars are
also connected, as in the illustration, by brass rods, and when this
battery is charged people should be careful how they handle it, for a
shock may be produced which would cause serious injury, if not death.
The battery can be charged from the machine by a chain fastened to the
central ball, while a second chain connects the exterior of the box and
all the outside of the jars, by means of the handle, to the ground.

When electricity is at rest it is termed “static electricity,” and
when in motion “dynamic” electricity. The latter treats of electric
currents which can be sent through wires or chains. We can keep this
current moving by means of a machine, and the battery called a Voltaic
battery, from Volta. We will describe it presently. Electric currents
can be measured, for they may be of different strengths according
to the battery, and they are measured by the GALVANOMETER.
Electricity can therefore be transferred and carried by the conducting
substances, and much heat will be engendered as the “electric fluid”
passes along a wire. Lightning frequently fuses bell-wires as it
passes, and when we touch upon Galvanism or Dynamical Electricity we
shall hear more about it.

By the Electric Machine we can obtain some very powerful currents
of electricity; we can produce many pleasing effects, and perform a
number of experiments, such as making balls or figures of pith dance,
and several other easy and entertaining tricks, which will be found in
books more specially devoted to the entertainment of young people.

[Illustration: Fig. 215.—Leyden Jar.]

We have now given some explanation of the manner in which electrical
phenomena can be produced,—viz., by the Electric Machine and by the
Leyden Jar,—but we must not expect to find any electricity inside any
charged body. It has been proved that all the electricity is upon
the surface of bodies, even if in varying quantity, and that equal
quantities of electricity are always produced when bodies are excited
by friction, but the _kinds_ are different. The rubbing body is of one
kind, the body rubbed another, and consequently the forces neutralize
each other. The two forces or kinds of electricity we have seen repel
or attract each other, and we can imagine the farther they are apart
the less will be the force, and the _rate of diminution of force_,
according to distance, is ascertained by an ingenious apparatus called
a “Torsion” Electromoter, which was constructed by Coulomb, and was
frequently used by Faraday.

Perhaps some people may not be aware of the term “torsion.” It means
twisting, and “the torsion of a thread suspended vertically is the
force tending to twist the lower extremity when the upper end is turned
through an angle.” This instrument is really an Electromoter, and is
not considered suited to beginners, and it is scarcely accurate in its
workings. We need not therefore describe it in detail. There are some
excellent Electromoters, the Elliott being, we believe, the best for
use. A full and detailed description of the Quadrant Electromoter will
be found in Mr. Gordon’s treatise on Electricity.

_Recapitulation of foregoing Chapter._ So far, we have seen there is
electricity in everything, although some bodies are termed conductors
and others non-conductors; though, as in applying the terms heat and
cold, we must remember that no body is entirely devoid of electricity,
and no body is therefore an absolute _non_-conductor any more than any
object is absolutely devoid of heat. Faraday, indeed, was of opinion
that “conduction and insulation are only extreme degrees of one common
condition”; they are identical both in principle and action, except
that in _conduction_ an effect common to both is raised to the highest
degree, and in the case of insulation it occurs in an almost insensible
quantity.

We have also read of positive and negative electricities, and we must
not fancy there is any particular reason for this distinction. It was
Du Fay, whom we have mentioned, who gave the names “vitreous” and
“resinous” to the two kinds, as one was developed by rubbing glass, and
the other by rubbing resin. But, as shown by our experiments, either
kind of electricity can be excited in glass or sealing-wax, and both
kinds are produced at once. You cannot get “positive” without negative
electricity. “Positive” is the term applied to the kind produced by
rubbing glass with silk or wool; “negative” is the term applied to
the kind developed by rubbing sealing-wax, but the kind developed by
friction depends on the rubbing substance and certain conditions.

Bodies charged with the same electricity repel; if charged with
different kinds they attract each other. The more readily displaced
particles when bodies are rubbed become negatively electrified as a
rule.

Similar electricities repel each other with a force inversely
proportional to the squares of the distance between their centres,
as established by Coulomb. So if the space between any two similarly
electrified bodies be reduced by say _one-half_, the force of the
repulsion will be increased _four_ times. The rule for attraction is
similar; so when two bodies are charged with opposite electricities,
and the distance between them is increased, the attractive force is
diminished in proportion as the square of the distance between them.
Many confirmations of this theory were made by the late friend of
our boyhood, Sir W. Snow Harris, and published in the _Philosophical
Transactions_.

The following full list of conductors and non-conductors (copied from
Professor Noad’s Text-book of Electricity, and compared with De La
Rive’s Treatise) may be useful:—

  CONDUCTING BODIES IN ORDER OF
  CONDUCTING POWER.

  All the metals.
  Well-burnt charcoal.
  Plumbago.
  Concentrated acids.
  Powdered charcoal.
  Dilute acids.
  Saline solutions.

  INSULATORS IN THE INVERSE ORDER OF
  INSULATING POWER.

  Dry metallic oxides.
  Oils (heavier the better).
  Vegetable ashes.
  Transparent dry crystals.
  Ice below 13° Fahr.
  Phosphorus.
  Lime.

  CONDUCTING BODIES IN ORDER OF
  CONDUCTING POWER.

  Metallic ores.
  Animal fluids.
  Sea-water.
  Spring-water.
  Rain-water.
  Ice above 13° Fahr.
  Snow.
  Living vegetables.
  Living animals.
  Flame smoke.
  Steam.
  Salts soluble in water.
  Rarefied air.
  Vapour of alcohol.
  Vapour of ether.
  Moist earth and stones.
  Powdered glass.
  Flowers of sulphur.

  INSULATORS IN THE INVERSE ORDER OF
  INSULATING POWER.

  Dry chalk.
  Native carbonate of baryta.
  Lycopodium.
  Caoutchouc.
  Camphor.
  Silicious and argillaceous stones.
  Dry marble.
  Porcelain.
  Dry vegetables.
  Baked wood.
  Leather.
  Parchment.
  Dry paper.
  Hair.
  Wool.
  Dyed silk.
  Bleached silk.
  Raw silk.

The following may be added to the Insulators, viz.:—

  Transparent precious stones.
  Diamond.
  Mica.
  All vitrefactions.
  Glass.
  Jet.
  Wax.
  Sulphur.
  Resins.
  Amber.
  Gutta-percha.
  Shell-lac (or gum-lac).
  Ebonite.

There are, as we know, two kinds of electricity, the _static_ and
_dynamic_; and when the latter state is _instantaneous_, it is
referred to as the “electric discharge,” which occurs when opposite
electricities seek each other, and the bodies return to a state of
equilibrium or neutralization. “These bodies, if _insulated_, obtain no
more electricity after the spark has passed; but if there be a constant
source of negative electricity supplying one, and a constant source of
negative electricity supplying the other, there will be a succession of
sparks; and if they communicate by a conductor there will be, through
this conductor, an uninterrupted neutralization of a continual reunion
of the two electricities, and this is what is termed the continuous
dynamic state or electric current” (De la Rive).


FOOTNOTES:

[13] This interesting experiment, which we have exactly verified, was
described to us by Professor Waldner, and M. A. Keppler.



CHAPTER XIX.

 VELOCITY OF ELECTRICITY—EXPERIMENTS—THE ELECTRIC EGG—FORCE OF THE
 ELECTRIC SPARK.


We are now acquainted with many facts concerning electricity, and have
seen that electrical phenomena can be produced by the Electric Machine
and the Leyden Jar. (An insulating stool—a stool with glass legs—is
a very desirable adjunct for those who wish to experiment with the
machine). Glass is a great insulator, or non-conductor, as a Russian
philosopher found to his cost. He had an iron lightning-conductor from
his house into his room, the end not connected with the earth but with
a glass. One day the lightning came down the rod and reached the glass;
had a communication been made with the earth by a chain, or directly,
no mischief would have ensued. As it happened, however, the current was
checked by the glass, and immediately darted towards him; it struck
him in the head, and killed the poor man on the spot. If no insulating
stool were used, the body charged would be discharged upon contact with
the ground.

The velocity of electricity is very great, and experiments have
frequently been made. Wheatstone undertook to ascertain the speed
of the electric fluid, and the instrument he employed he called a
“Chronoscope.” He caused a mirror to revolve with enormous velocity,
and measured the speed by the vibrations of air, which produced a
certain note by the same motive power. (We know already that certain
notes are produced by a certain number of vibrations per second.)
Wheatstone’s Chronoscope consisted of this mirror, in front of which
was placed a circular block of wood, in which, in a row, were set six
wires carrying small knobs; round these and over the wood he put an
insulating varnish. A Leyden jar was connected outside with the first
knob; between the second and third a quarter of a mile of copper wire
was coiled, and a like length of wire between the fourth and fifth; the
inside of the Leyden jar was then connected with the last knob, and the
spark passed; it ran from knob to knob over the long coils of wire. If
all the flashes over the wire and knobs occurred simultaneously the
mirror would show them side by side; if not, as the mirror turned a
trifle, the difference would be observed. The mirror did show a slight
retardation in the passage of the flash, and from certain measurements
and calculations Wheatstone estimated the velocity of the spark to be
288,000 miles a second. This rate will carry electricity round the
earth in about a twelfth of a second, a rate Puck never dreamed of when
he promised to “put a girdle round the earth in forty minutes.”

But it appeared from investigations subsequently made that it was not
possible to express the velocity of electricity with any certainty,
and a number of experiments were made as per the following table, with
very different results. Sir William Thomson and Faraday endeavoured
to account for these stupendous discrepancies, and the principles of
retardation of electricity were established. The differences are shown
below:—

                                   Nature of  Velocity per
                                    Wire.       Second.
  Wheatstone’s Experiment in 1834   Copper      288,000
  Gonella and Fizeau               {Copper      111,834
                                   {Iron         62,130
  Mitchell                          Iron         28,331
  Walker                            Iron         18,639
  Gould                             Iron         15,830
  Astronomers in Greenwich          Copper        7,600
    “            Brussels           Copper        2,700
  Result in Atlantic Cable, 1857    Copper        1,430
    “               “       1858    Copper        3,000

To account for the comparatively low velocities of the cables, Faraday
proved that they act very much as a Leyden jar acts; that is, it takes
time to fill, as it were, and to discharge them, the wire coating of
the cable in air acting like the outer coating of the jar or the water
in the case of an immersed cable, and the retardation observed is
owing to resistance of conduction, and depends upon the way in which
the electrical impulses traverse the wire. “There is a long, gradual
swell, and still more gradual subsidence of the electric current, and
the length of time that elapses between the initial impulse and the
attainment of maximum strength, is proportional to the square of the
length of the line.”

The duration of the electric spark has been calculated at the 1/24000
part of a second, but Professor Tyndall regards this as the longest
or nearly the longest time it is perceptible; the _shortest_ time
is almost inconceivable. The brightest portion of a spark has been
ascertained to last only forty-six millionths of a second, and certain
experiments were made to ascertain the actual duration with various
numbers of Leyden jars. It was discovered by Messrs. Lucas and Cuzin,
by an application of the Vernier, with batteries consisting variously
of two to eight jars, and obtained the following results[14]—

                         Duration in
  No. of Jars.     millionths of a Second.
      2                      26
      4                      41
      6                      45
      8                      47

“So,” adds the writer, “the duration (of spark) increases in proportion
with the number of the jars. It increases also with the striking
distance, but is independent of the diameter of the balls or globes
between which the spark strikes.”

Many examples might be given of the _spark discharge_ of the electric
current. This form is seen in the blue line extending between the knob
of a machine and the hand, and the duration of a spark with a jar
charged with an induction coil is stated by Professor Rood to vary, but
the brightest portion with a jar of 114 square inches only existed for
the 175th _billionth_ of a second, and with a smaller surface was much
shorter. Such a spark may be conducted to a plate of gunpowder and will
not ignite it, because the time of the duration of the “fire” is not
sufficiently long; the powder will be scattered, but not ignited. If,
however, a partly non-conducting medium be interposed between the jar
and the powder, so that the spark be retarded a little, the gunpowder
will be fired.

While speaking of electric discharge we may remark upon the beautiful
effect of lightning. These discharges are sometimes miles long, and by
the return stroke from the cloud may kill a person a long way from the
actual discharge. This phenomenon was illustrated by Viscount Mahon in
1779, in a very interesting book on the principles of electricity.

There are different ways in which the electric discharge shows itself.
We have spoken about the spark discharge which, however, is found to
present very different appearances in varying conditions. Professor
Faraday proved that the colour of the electric sparks showed in air,
when obtained with brass balls, the intense light and blue colour so
familiar to all. In nitrogen they are even bluer. In oxygen again the
sparks are much _brighter_ than in air, but not so _brilliant_. In
hydrogen they become crimson, but the sound is almost inaudible because
of the physical character of the gas. In carbonic acid gas they are
almost the same as in atmospheric air, only more irregular. In dry
muriatic gas they are nearly white and very bright. In coal gas the
colours vary—sometimes being green and sometimes red. Occasionally the
same spark will be red and green at different extremities, and even
_black_ portions have been observed. The density and pressure of the
atmosphere has been proved to exercise considerable influence upon the
spark discharge.

The “Brush” discharge is shown in “a series of intermittent discharges
which appear continuous.” This discharge assumes the shape of a fan.
“It is accompanied with a low chattering sound,” which is the result of
the separate and continuous discharges, and Faraday also demonstrated
that its effects varied according to the medium in which it was
exhibited.

The effect of the air pressure on electricity may be observed in the
following way:—

If we pass a spark through rarefied air by an apparatus known as the
Electric Egg, we may obtain many curious effects. The “egg” consists
of a glass globe, through which enter two rods with a knob upon each
inside end. The upper rod is moveable, and held in its place by a “cap”
like the lower rod. There is a stop-cock in the lower cap, so that the
egg may be fastened to any plate or stand. When the egg is filled with
air, the electric spark passed into the glass globe has the usual
appearance, but as the air is gradually rarefied by an air-pump, the
spark assumes beautiful forms and colours. As the exhaustion continues,
however, we shall find the spark decreasing in brilliancy, and finally
the spark will cease to be visible. It thus is shown that the colour
of the spark depends upon the gaseous medium and on the material of
the conductors, and when the electric spark is faint this medium can
be observed, for nitrogen will produce a blue tinge and carbonic acid
a green; hydrogen gives us a red, as already remarked. By multiplying
the number of eggs and plates of glass, and placing discs of tinfoil
in various shapes at certain distances, many beautiful figures may be
observed when the spark is set free.

Professor Tyndall at the Royal Institution showed a very pretty
experiment. He took a funnel with a very fine bore, and permitted
sand to flow from it as it will in the hour-glass. When he permitted
the electric current to come in contact with the sand, however, it,
instead of falling vertically to the table, spread out fan-like, each
grain repelling and being repelled by its neighbour with an effect very
beautiful to see. Luminous effects have frequently been produced by
passing an electric spark through various bodies. For instance, a lump
of sugar can be made quite brilliant in the dark by passing electricity
through it, and there are other substances similarly affected. Even
eggs and some fruits are thus made phosphorescent. The illumination of
the “diamond” covered Leyden jar is familiar to all who ever attended a
lecture on Electricity.

The various effects of the electric discharge need not here be
described. We have witnessed the results of lightning, but even in our
laboratory many pretty little experiments can be made, such as the
perforation of a card by the electric discharge. The chemical effects
are various. Decomposition of water is effected by electricity, and the
discharge can also be, and has been utilized for military purposes,
such as employed by Professor Abel in his fuse, and in his apparatus
for firing mines. Experiments at Chatham and elsewhere have been very
successful in the application of electricity to modern warfare.

We will illustrate one or two of these. A thick card should be placed,
as in the illustration (fig. 216), between two insulated points, and
to the lower portion of the apparatus a chain be attached, held in the
hand, and wound round the Leyden jar. If then the knob of the jar and
the knob above the upper point be brought together, the spark will pass
through the card.

[Illustration: Fig. 216.—Experiment with card.]

In the same manner a glass may be perforated if the current be
stronger. Of course the whole apparatus—and particularly the plate—must
be quite dry, and it will be better to put a drop of oil under the
upper needle point so as to prevent the electricity spreading over the
glass. The glass will be found pierced by the electric spark when the
Leyden jar is brought into requisition (fig. 217).

It was at one time contended that the sudden expansion of the air by
the electric spark caused heat to be generated, and a thermometer was
invented by Kinnersley to show this. The illustration (fig. 218) will
explain it. When the electric spark passed between the balls in the
large tube the water rose in the smaller. But the immediate return of
the water to its level showed that the disturbance was only mechanical,
and not owing to heat.

[Illustration: Fig. 217.—Experiment with glass.]

[Illustration: Fig. 218.—Kinnersley’s thermometer.]

We may now pass from the “frictional” to the other kind—viz.,
“dynamical” electricity, and we shall begin with the consideration of
GALVANI’S experiments and the VOLTAIC PILE.

[Illustration: Electric condenser.]


FOOTNOTES:

[14] GANOT: _Eléments de Physique_.



CHAPTER XX.

GALVANISM.

 GALVANI’S DISCOVERY—THE FROGS ELECTRIFIED—EXPERIMENTS—VOLTA’S PILE—THE
 TEST—ITS USEFULNESS—FARADAY’S “RESEARCHES.”


Galvanism owes its origin to the researches of Galvani, the celebrated
professor of Bologna, and we are indebted to what was a mere “accident”
for our knowledge of this science.

Before Galvani’s time there had been many instances adduced of animal
electricity. The Rev. F. Lunn, in his article upon Electricity,[15]
mentions the fact that fire streamed from the head of Servius Tullius
when about seven years of age, and Virgil we know refers to flame
emitted by the hair of Ascanius—

  “Lambere flamma comas, et circum tempora pasci”;

and if any one will comb his or her hair with an ebonite comb in the
dark, with what is sometimes called an “india-rubber comb,” the hair
will give out sufficient light to enable the operator to see himself
in a looking-glass. In olden days it is related that a lady when
touched with a linen cloth emitted sparks, and the same phenomenon
was observable when a bookseller at Pisa removed his under-garment
or vest (De Castro). We are all aware of the electricity of the cat
and of certain fishes (_see_ Electricity of Animals in sequel), and
“torpedos.” Galvani had of course a knowledge of this property, and had
occupied himself for some time making experiments upon the electricity
in animals. He was not in his laboratory that day when the great
discovery was made by means of the edible frog.

Galvani’s wife was just then in a very delicate state of health,
and in accordance with usage had been ordered soup made from frogs.
It is related that some of these animals, ready skinned, were lying
upon the laboratory table, for the Professor had been just previously
investigating the question of what he opined was “animal” electricity;
that is, he fancied that muscular motion depended upon that subtle
force.

The electric machine was in action, and one of the attendants happening
to approach or touch one of the frogs, the man as well as Madame
Galvani observed that the limbs were violently agitated. Galvani was
at once informed of this, and he made repeated experiments, which
showed him that the convulsive movements only took place when a spark
was drawn from the prime conductor of the electric machine, and while
the nerve was touched by a conductor. Galvani then suspended a number
of frogs to a railing by metal hooks, with a view to experiment upon
them with atmospherical electricity. But the frogs’ limbs were again
agitated when no electricity was apparent, and Galvani after some
consideration came to the conclusion that the movement was owing to the
position the animals assumed with reference to the metallic bodies.
Thus when muscle and nerve were in contact with metallic bodies and
connected by metal, the movements of the limbs were observable, and
the greater the surface contact the greater was the convulsion. The
philosopher next tried various metals, and discovered that the most
powerful combination was zinc and silver.

Galvani, in 1791, published his discovery and his theory that the body
acted as a Leyden jar, different parts being in a different state of
electricity. No sooner were his deductions published than all Europe
was in a ferment, and philosophers of all nations were discussing
it. Fowler, Valli, Robison, Wells, Humboldt, etc., all were deeply
interested, but none of them appear to have arrived at so correct
conclusions as did Volta, the physician of Pavia. “Wherever frogs were
to be found,” says Du Bois Reymond, “and where two different kinds of
metal could be procured, everybody was anxious to see the mangled limbs
of frogs brought to life in this wonderful way. Physiologists believed
that at length they should realize their visions of a vital power, and
physicians thought no cure was impossible.”

But notwithstanding the popular theory, Volta, in his letters to
Carallo, while giving a full and clear account of the discovery made
by Galvani and his own experiments, attacked and finally defeated the
Professor. Volta quite upset Galvani’s Leyden-jar theory; Volta says
that it was by accident that Mr. Galvani discovered the phenomenon, and
by which he was more astonished than he ought to have been. Volta’s
letters will be found in the _Philosophical Transactions_ of the Royal
Society (in French), and he attributes the effect to the metals which
produced a small amount of electricity. He found that the nerve was
acted upon on even parts of a muscle laid upon two different metals,
and if those were united, a contraction took place.

“Many experiments were made in all parts of Europe,” says Doctor Roget,
and “an opinion had been very prevalent that the real source of the
power developed existed in the muscle and nerve which formed part of
the circuit, and that the metals which composed the other part acted
merely as the conductors by which that agency was transferred from the
one to the other of these animal structures. But the discoveries of
Volta dispelled the error, by proving that the sources of power were
derived from the galvanic properties of the metals themselves when
combined with certain fluids,” and decided that this principle was
electricity. From this the “general fact” was deduced—viz., “that when
a certain portion of a nerve which is distributed to any muscle is made
part of a galvanic circuit, convulsions, generally of a violent and
convulsive kind, are produced in that muscle.”

Volta at length made the discovery that when two metals were brought
together electricity was developed, and by uniting a disc of copper
and one of zinc, and subjecting them to the test of an Electroscope,
he found positive and negative electricity developed in the zinc and
copper respectively; so Volta came to the conclusion that each metal
parted with electricity, and one became all “positive” and the other
all “negative.” But when he came further to consider the possibility
of building up a “pile” of these metal discs sufficiently strong to
produce electric effects, he found that if his theory were correct he
would lose from one side of the metal all he would gain from the other,
and therefore he could never obtain more than the slight effect he had
originally produced.

This was at first a difficulty apparently impossible to remove. It
was so self-evident that the discs of metal, if placed in a pile in a
series of pairs, would continually exercise like effects to the first
pair of discs, that Volta was puzzled, and for some time he could
not arrive at any reasonable solution. At last it struck him that
if he placed between the discs some slow-conducting substance, the
electricity would not pass from disc to disc, and the force developed
or set in motion would be more powerful.

He made the experiment. The result was the Voltaic pile made in 1800,
of which we give an illustration (fig. 219). A communication on the
subject of Electricity by contact, written by M. Volta, is to be found
in the _Philosophical Proceedings_ for the year 1800.

Volta constructed the pile which bears his name, on the assumption
that “every two heterogeneous bodies form a galvanic circle or arc in
which electricity is generated.” The “pile” consisted of a number of
discs of zinc and copper separated by discs of card soaked in water.
This combination of metals separated by a bad conductor, developed
considerable electricity, the “positive” going to the zinc at the top,
and the “negative” turning to the opposite end. By touching the zinc
and copper extremities simultaneously with wetted fingers we shall
experience a shock. “I don’t need your frog,” Volta said, when he had
proved his theory; “give me two metals and a moist rag, and I will
produce your animal electricity. Your frog is nothing but a moist
conductor, and in this respect it is inferior to my wet rag!”

[Illustration: Fig. 219.—Voltaic Pile.]

After this discovery the theory of animal electricity died away for
many years, till in 1825, Nobili, and afterwards Matteucci, proved the
existence of galvanic currents in muscles.

After Volta had succeeded in obtaining a shock from his “pile,”
he proceeded to the construction of another instrument, or rather
apparatus, which he denominated “Couronne des tasses” (fig. 220). It
consisted of a series of small glasses containing water or a saline
solution. He then procured a number of “metallic arcs,” partly composed
of zinc and partly of copper; these were inserted into the glasses, so
that every glass contained the zinc of one and the copper of another
arc, not in contact, but one at the right hand the other at the left.
The electro motion, supposed to be the primary cause of the galvanic
action, was thus produced as well as from the “pile.” The principle was
just the same in both apparatus, the metals being divided by the water
in one case, and by a wet card or cloth in the other.

Volta, in 1800, addressed to the Royal Society his celebrated letter
upon electricity excited by contact of conducting substances, and then
the English philosophers proceeded to make further experiments. It
was Fabroni of Florence who had just before suggested that chemical
action was really the cause of the phenomena exhibited. Sir Humphrey
Davy warmly advocated this theory, and made numerous experiments with
the view to establish it. Nicholson, Carlisle, and Cruickshank also
paid great attention to the subject. Volta, although he had laid the
foundation, did not venture to build upon it. Messrs. Nicholson and
Carlisle found the two kinds of electricity in the pile, the zinc
being positive and the silver negative. They also found that the
water was decomposed both in the circuit and in the body of the pile.
Subsequently Cruickshank confirmed Nicholson’s observations, and made
use of what is termed the “trough” apparatus. He found that hydrogen
was emitted from the silver or upper end, and oxygen from the other.

[Illustration: Fig. 220.—Volta’s couronne des tasses.]

These discoveries opened up a wide field. “The power of the pile
in decomposing chemical substances was now established.” Dr. Henry
employed galvanism for analysis, and Sir Humphrey Davy invented new
combinations of substances. He formed a pile of charcoal and zinc,
and found out that a pile could consist of only one metal, different
fluids being applied to the opposite surfaces separated by water, and
one fluid “capable of oxidating the metal, the other of preventing the
effect of oxidation.” Soon after a pile was made of charcoal.

In 1806, Sir H. Davy gave the results of his researches to the world
upon the electro-chemical action of bodies. In the course of his
experiments he found out the chemical constituents of the alkalies, and
a surprising number of new things were brought to light, and chemical
science received a most astonishing ally. Sir W. S. Harris says: “A
series of new substances were speedily discovered, the existence of
which had never before been imagined. Oxygen, chlorine, and acids
were all dragged, as it were, to the positive pole, while metals,
inflammable bodies, alkalies, and earths became determined to the
negative pole of the (galvanic) battery. When wires connected with each
extremity of the new battery were tipped with prepared and well-pointed
charcoal, and the points brought near each other, then a most intense
and pure evolution of light followed, which on separating the points
extended to a gorgeous arc.” So the elements of all bodies were
separated and the composition of their compounds closely investigated.

Michael Faraday threw himself _con amore_ into the question. He
set about to classify the pile phenomena, and arranged them with
appropriate terms, and in a series of papers, between the years 1830
and 1840 (_see_ his “Experimental Researches”), he explained the
chemical effects of voltaic electricity and electro-magnetic induction.
He showed that the electricities obtainable from the voltaic pile and
the electrical machine are essentially the same in their action. He
proved that the theory held respecting the necessity for the presence
of water in electro-chemical composition was erroneous, and that
many other fluids and compounds were equally effective. We have not
space at our disposal to include a digest of his various lectures and
papers. He calculated that as much electricity is employed in holding
the gases oxygen and hydrogen together in a grain of water, “as is
present in a discharge of lightning.” When water is decomposed by
the electric current, the force which determines the oxygen and acid
matter held in solution to the positive, while the hydrogen passes to
the negative pole, is not in the poles, but in the body decomposed, he
says. “The poles,” writes Faraday, “are merely the surfaces or doors
by which the electricity enters into or passes out of the decomposing
substance. They limit the extent of that substance in the course of the
electric current, being its termination in that direction. Hence the
elements evolved passed so far and no farther.” Faraday named the poles
“electrodes”—the way (in or out) of electricity.

A very simple voltaic pile may be constructed with “gold-leaf” paper.
Take two sheets of the gold paper and paste them back to back,
and two of silver paper; cut them into discs about the size of a
five-shilling-piece (or even of half-a-crown), and place them one on
the top of the other, so as the gold and silver may be alternate; press
the discs together slightly when a good many layers have been piled up,
and introduce them into a glass tube; close the ends of the tubes with
corks through which wires are passed from the discs top and bottom. It
will be found that the ends are charged with opposite electricities.
This is the _Zamboni_ pile, or the dry pile, which was constructed of
hundreds of paper discs “tinned on one side, and covered with binoxide
of manganese on the other,” put into a tube, and closed with brass
stoppers. The electricity will last a long time in a dry pile.

[Illustration: Fig. 221.—The Galvanic Pile.]

In the accompanying illustration of the Galvanic Pile a disc of copper
is at the bottom and a disc of zinc at the top. The latter, P,
is the positive pole; the former, N, the negative. When the
wires are united the current is closed, and no sign of disturbance can
be detected, although the action, of course, is proceeding within the
pile. The opposite kinds of electricity neutralize each other, and if a
continuous supply were not kept up the electricity would disappear; but
as it is, a powerful current is obtained, and if the wire be divided a
spark will be observed.

[Illustration: Fig. 222.—Bunsen Battery.]

There are many forms of galvanic batteries. The Trough Battery or
Cruickshank has been mentioned. There is Wollaston’s Pile, Bunsen’s
Battery, Grove’s Battery, and Daniell’s, called the “Constant” Battery.
In this last a porous earthenware cell is placed within a cylinder
of copper; in the cell a rod of zinc is inserted, the cell being
filled with diluted sulphuric acid,—one part of acid to ten parts of
water,—and in the outer cylinder is a solution of sulphate of copper.

[Illustration: Fig. 223.—Daniell’s Battery.]

[Illustration: Fig. 224.—Grove’s Battery.]

The cut above illustrates Daniell’s Battery (fig. 223) with connectors.

In _Bunsen’s Battery_ (or the Zinc-Carbon Battery), which is very like
the “Daniell” arrangement, as will be seen from the plates (figs. 222,
223), the porous cell has a prism of carbon immersed in it, and is
apparently a modification of the powerful “Grove” Battery (fig. 224).
This consists of slips of platinum, _h_, placed in porous cells, _g_,
each cell being surrounded by a glass cylinder. The outer (glass) cells
are filled, or nearly filled, with diluted sulphuric acid; nitric
acid is used in the porous cells, and a platinum plate inserted. The
chemical action of the Grove cell is thus explained by Professor
Stewart: “The zinc dissolves in the dilute sulphuric acid, and during
this process hydrogen gas is given off. But this hydrogen does not
rise up in the shape of bubbles; it finds its way into the porous
vessel which contains the strong nitric acid. It there decomposes the
acid, taking some oxygen to itself, so as to become water (hydrogen
and oxygen forming water), and thereby turning the nitric into nitrous
acid, which shows its presence by strong orange-coloured fumes.” By
this decomposition of the nitric acid the polarization of the platinum
(due to hydrogen) is avoided. The porous cell, while keeping the
liquids apart, does not interfere with the chemical action.

A great number of cells are used in the Grove Battery; perhaps even a
hundred may be employed.

_Smee’s Battery_ consists of a plate of platinized silver, S, with a
bar of wood to prevent contact with the zinc on each side, Z. These are
immersed in a glass jar, A, which contains dilute sulphuric acid. The
current is obtained by metallic communication with the binding-screws
on the top. This battery has much the same general arrangement as
Wollaston’s—the position of the plates being, however, reversed; in the
latter there are two negative plates to one positive. In Smee’s Battery
there are two positive (zinc) plates to one negative plate.

[Illustration: Fig. 225.—Smee’s Cell.]

[Illustration: Fig. 226.—Smee’s Battery.]

It will now be understood how an electric current is produced; the
electricity passing through the cells, etc., to wires, confers certain
properties upon the wires, and we can ascertain the effect of the
current by means of a _Galvanometer_, an instrument used to detect the
strength and direction of electric currents. The current will evolve
heat and light; it will excite muscular action, and will decompose
substances into their constituent elements. The deflection of the
magnetic needle by the electric current is considered the best evidence
of its power; it is on this that the Galvanometer is based.

We can perform a few simple experiments with the current. Suppose, for
instance, that a piece of fine wire be fixed between the pole wires of
the battery; it will be heated “white hot.” Or if two carbon points
be approached in a glass of water, as in the illustration (fig. 227),
they will emit a brilliant light in the fluid from the _voltaic arc_
which has given us the electric light. The current is the passage of
electricity along the wire, and continues until the working power or
“potential” of one conductor is equal to that of the other. When they
become equal of course the action ceases, as there is equilibrium. But
when an apparatus like the galvanic battery is brought to bear so that
the force of electricity from one conductor is made always greater than
that of the other conductor, we have a continuous flow while the action
of the battery goes on. One view of the principle is thus expressed by
Professor Gordon:[16]

“If two metals be placed near together, but not in contact, in a liquid
which acts chemically more upon one than upon the other, the metals
become charged, so that the one least acted on is of higher potential
than the one most acted on. The difference of potential produced
depends only upon the nature of the metals and of the liquid, and not
on the size or position of the plates. As soon as the difference of
potential has reached its constant value the chemical action ceases.

[Illustration: Fig. 227.—The Voltaic arc.]

“If now the metals are connected by a wire outside the liquid the
difference of potential begins to diminish, and an electric current
flows through the wire. As soon as the difference of potential becomes
less than the maximum for the metals and liquid, chemical action
recommences and brings it up to the maximum; and thus if no disturbing
cause interferes the current will continue until the metal most acted
on is entirely dissolved.”

The metal most acted on is considered the “generating plate,” and is
“positive.” The other attacked less is “negative,” and is known as
the “collecting plate,” and the zinc is the positive plate. Sir W.
Thomson has shown that the electrical movement in the galvanic circuit
is entirely due to the electrical difference produced at the surfaces
of contact of the dissimilar metals. The electro-motive force obtained
is not the same with all metals. We have mentioned that some are
electro-positive and some electro-negative, and it is with reference
to each other that the metals are considered to be endowed with these
properties respectively. It all depends how the metals are arranged or
coupled. With reference to their behaviour in this respect scientists
have arranged them in a series, as follows:—

  1.  Zinc.
  2.  Cadmium.
  3.  Tin.
  4.  Lead.
  5.  Iron.
  6.  Nickel.
  7.  Bismuth.
  8.  Antimony.
  9.  Copper.
  10. Silver.
  11. Gold.
  12. Platinum.
  13. Graphite.

Each metal in the list is arranged so that it is electro-positive to
any one below, and electro-negative to any one above it.

There is another curious fact which should be mentioned. In associating
these metals it has been found that when two are brought into contact
the electro-motive force becomes greater the more distant they are in
the series given above; in other words, the force between any two is
equal to the sum of the forces between those intervening between those
two. So when zinc is used with copper its force is not so great as when
used with platinum.

It was Herr G. S. Ohm who laid down the law that the strength of
the electric current is equal to the electro-motive force divided
by the resistance, for he proved that the “resistance was inversely
proportional to the strength of a current.”

There are two other laws respecting currents; viz.,—

(1.) Parallel currents in the same direction attract each other.

(2.) Parallel currents in opposite directions repel each other.

[Illustration: Fig. 228. Chemical action of electricity. Fig. 229.]

Upon these two hang all the varied phenomena of electro-dynamics. That
chemical action develops electricity we can perceive with the aid of
the two cuts (figs. 228 and 229). If the wires be attached to the
collecting-plate of a condenser of electricity and the metal plate of a
cell, as shown in the figure (fig. 228), the electricity on the plate
will be negative. If the operation be reversed, and the plate be put in
connection with the acid, and the metal with the earth, the instrument
will be charged with positive electricity. In the other case, when two
cups are used, united by a magnet so that the solutions (one acid and
the other alkaline) can by capillary attraction unite upon the binding
of the magnet, and we place the wires as in fig. 229, the charge on
the plate will be positive if it be in connection with the acid, and
negative if in communication with the alkaline solution. Every time
there is chemical action between two bodies in contact electricity
is produced—positive on one negative on the other, and that is the
fundamental principle of the voltaic pile.

The decomposition of water can also be effected by means of the
electric current. If two tubes or vessels be placed in a vase of water,
and the wires from the battery be inserted in them respectively, the
oxygen will go to the platinum or positive pole wire, and the hydrogen
to the zinc or negative pole. This decomposition or “splitting up” of
components was termed ELECTROLYSIS by Faraday, who gave a
series of names to the action and the actors in these phenomena (fig.
230).

Any liquid body, such as the water we have just decomposed for
instance, Faraday termed an _electrolyte_; the surfaces where the
current enters or leaves the body were called _electrodes_—the “ways,”
from _odos_, a “way”; the entry is the _anode_; the leaving point
the _katode_, from _ana_, “up,” and _kata_, “down.” The electrolyte
is divided into two portions, “ions” (“movers”), which move towards
the electrodes, which are positive and negative. In the case of the
decomposition of water the hydrogen goes to the negative electrode, the
oxygen to the positive.

[Illustration: Fig. 230.—Decomposition of water.]

There are a few observations to be made respecting electrolysis. One
rule is, that it “never takes place unless the electrolyte is in a
liquid state.” The liquid state is essential. It is also observed
that the components go to the different electrodes; such elements as
go to the negative electrode are termed electro-positive, the others
electro-negative; or, as Faraday termed them, “anions” or “kations:”
The chemical power or electrolytic action of the current is the same at
all parts of the circuit; the quantity of the substance decomposed is
in exact proportion to the strength of the current. Faraday measured
the strength of the electric current, and invented for the purpose an
instrument called the Voltameter. We have mentioned the Galvanometer
more than once, and will proceed to describe it. There are several
forms of this instrument: the Tangent, the Marine, and the Reflecting
Galvanometers, and the Astatic, or “Multiplier.” In the first-named the
direction of the current is determined by Ampère’s rule, which is as
follows:—

“Imagine an observer placed in the wire so that the current shall pass
through him from his feet to his head; let him turn his face to the
needle: its north pole is always deflected to his left side.”

The _“Tangent” Galvanometer_ consists of a vertical circle like an
upright ring, across which is a support in the centre holding a
copper wire, through which the electric current passes. On this point
(where the wire is) a needle is very lightly supported, and when the
instrument is to be used it is placed so that the plane of the circle
is parallel to the line in which the needle points. The current passes,
and the needle is deviated. By noting which side the north end of the
needle goes the _direction_ of the current is ascertained, and the
length of the needle being small in comparison with the diameter of the
circle through which the current passes, the _strength_ of the current
in the vertical circle is in proportion to the _tangent_ of the angle
through which the needle turns. Hence the term “Tangent” Galvanometer.

The “Reflecting” instrument is the invention of Sir William Thomson, in
which a mirror is attached to the needle, and reflects a small focus of
light upon a scale. The movements, however minute, are easily read. Sir
W. Thomson’s Galvanometers are extremely sensitive. We need not mention
any other varieties, as full descriptions can easily be obtained. We
only need to indicate the mode of working.

[Illustration: Fig. 231.—Galvanometer.]

The accompanying illustration (fig. 231) shows an Astatic Galvanometer
which may be used in two ways—either to measure strength of current,
or to find out a current; in the latter case it would be termed a
Galvanoscope. It is a compound needle instrument, and consists of two
needles placed in parallel directions with opposite poles above each
other in a coil. The wire coil is wound round a bobbin, and the astatic
needle is placed therein and suspended freely, as in the illustration,
by a cocoon thread. The upper needle moves upon a scale, O
O, and the instrument is enclosed in a glass shade. The screw,
V, communicates with the upper needle, and fixes it at zero
point when ready for use. The wires are fastened to the binding-screws,
and the current is then sent. The needle is deflected accordingly, and
the number of degrees on the scale can be read off.

The uses of the galvanic current are many. Amongst them Electroplating
is perhaps the most generally useful, though Electrotyping is also a
very important process in art. A visitor to Birmingham may view the
process carried on there by some enterprising firms, who have succeeded
wonderfully in producing electro-plate. The principle is very simple
and easy to understand, but the greatest care and watchfulness are
required on the part of the men employed. The principle, as we have
said, is simple, and consists in the fact that if a plate of metal be
suspended and attached to the positive pole of a galvanic battery and
immersed in a solution of the same metal, the conducting substance hung
opposite at the negative pole becomes coated with the metal immersed in
the solution.

[Illustration: Fig. 232.—Trough for electro-deposition.]

[Illustration: Fig. 233.—Plates immersed.]

Suppose we take a plate of silver, and immerse it in cyanide of
silver dissolved in cyanide of potassium; a coating of silver will
be deposited upon the nickel spoon or other article suspended at the
other pole. But to make the coating adhere the spoons, forks, etc.,
are prepared for the bath by cleansing in caustic potash to remove
grease, and washed in nitric acid to remove all traces of oxide, then
are scoured with sand. Next, a thin coating of mercury is put on by
immersion in solution of nitrate of mercury. Finally, they are hung
in the bath. A metal rod is hung across the bath (fig. 232), and the
plate is immersed. If the rod to which the articles are suspended be
attached to the zinc or negative pole, and the plate of silver to the
positive pole of the battery, decomposition begins, and the silver
begins to attach itself to the suspended objects. If it be desirable
to give the plated articles a thick coating, they are retained for a
long time in the bath, which is of some non-conducting material. The
dull appearance is easily removed by brushing and burnishing, and then
the “Electro-plate” is ready for the warehouse. The gilding process is
performed in the same manner, a gold plate being substituted for the
silver.

[Illustration: Fig. 234.—Medico-galvanic Battery.]

[Illustration: Fig. 235.—Battery in case.]

ELECTROTYPING may be briefly explained as follows:—Take two vessels, A
and B, and in one, A, put some dilute sulphuric acid and two plates,
one of zinc, Z, the other of copper, D, but be sure they are not
touching each other; each of these plates must have a piece of wire
fastened, by soldering to their upper parts. In the vessel, B, put some
solution of sulphate of copper and a small quantity of dilute sulphuric
acid, and attach another copper plate to the wire which comes from the
copper plate in the acid; this second copper plate is to be immersed in
the solution of sulphate of copper, and to the wire from the zinc plate
is to be fixed the object to be coated. If a medallion or other object
in plaster, it should be soaked in very hot wax and then brushed over
with blacklead until the surface is perfectly blackened and bright;
the wire should be bound all round the margin and soldered (as it
were) with melted wax to the medallion, taking care that this wax also
is well coated with blacklead. If the object be now immersed in the
sulphate of copper solution and kept at a short distance from the plate
(it must not touch it), a coating of copper will soon cover the surface
and form a perfect cast, which, when of sufficient thickness, may be
removed by filing the edge all round. If instead of the plaster cast
a copper coin or other copper object be used, the blackleading is not
required, but the surface must be first made clean and bright.

Many uses are made of the galvanic current by medical men. If the
circuit of the pile is closed and we take a wire in each hand and
break contact, a concussion will be felt in the joints of the arm and
fingers, and a certain contraction of the muscles. The currents of
electricity cause the shocks, and by a peculiar arrangement by which
the circuit can be closed or broken at pleasure, a series of shocks can
be sent through the body when it forms the connection between the poles
of the battery. We give illustrations of a medico-galvanic machine.
In fig. 235 there are two batteries, A and B, with cells, C D. Each
battery consists of a central plate of platinized silver separated
from the zinc plates by a piece of wood, E and F; the binding-screws
are fastened to the silver plates, and G H retain the zinc plates; I
is a copper band connecting the zinc plate of one battery with the
silver plate of the other. At Z and opposite are wires leading to the
coil machine. The quantity and intensity of the current are regulated
respectively by the indicator, O, and the wires, Q. There is a point, R
S, for the breaking of the contact; P N are screws retaining the wires
which lead to the handles, U V, grasped by the patients.

[Illustration: Fig. 236. Fig. 237. Fig. 238.

Horse-shoe magnets.]

The electric current is employed in many diseases, and is of great use
in some cases, but the further consideration of it with reference to
its medical applications does not fall within the scope of our present
work. We will now pass on to one of the most useful applications of the
electric force, the Telegraph, and in dealing with it we must make a
few remarks upon magnetism. First, let us make an experiment or two,
and see the reciprocal action between electricity and magnetism.

(1.) If we take a piece of iron of the form of a horse-shoe (fig. 236),
and wind around it copper wire, and pass through the wire an electric
current from our battery, the iron will exhibit strong magnetic
properties, which it will lose when the current is interrupted. The
conducting wires are insulated with silk, and the current will then
travel in one direction.

(2.) If we cover the ends of a non-magnetic piece of iron with coils
of wire, and rotate the magnet, A B, so as to cause the poles to
approach each end of the iron alternately, an electric current will be
established in the wire.

(3.) Referring to the first experiment, if we bring a needle in contact
with the iron horse-shoe, while the current is passing through the wire
we shall find that the needle has become a magnet; _i.e._, that it will
point due north and south when suspended.

We will now see what a Magnet is, and why it has obtained this name.

[Illustration: Fig. 239.—Magnetic attraction.]

In Magnesia, in Lydia, in olden times was found a stone of peculiar
attributes, which had the property of attracting small portions of
iron. The Chinese were acquainted with it, and nowadays it is found in
many places. In our childhood we have all read of it in the story of
“Sinbad the Sailor.” Popularly it is known as the loadstone; chemists
call it magnetic oxide of iron (F_{2}O_{3}). This stone is a _natural
magnet_. In Sweden it exists in great quantities as “magnetic iron,”
for it has a great affinity for that metal.

If we rub a piece of steel upon the loadstone we convert the former
into a magnet—an artificial magnet as it is called, and the _magnetic
needle_ so useful to us in our compasses and in the working of one form
of the electric telegraph is thus obtained. Let us see how this needle
acts.

[Illustration: Fig. 240.—Simple touch.]

[Illustration: Fig. 241.—Double touch.]

Take a magnetic needle and dust upon it some iron filings. You will
observe that the filings will be attracted to both ends of the magnet,
but the centre will remain uncovered. The ends of a magnet are termed
“poles,” the centre the equator. So one end is north and the other
south, and we might perhaps imagine that the same characteristics would
abide in the bar when it is cut in two. But we find that as when a worm
is divided, each portion gets a new head or tail, and makes a perfect
worm, so in the magnet each divided half becomes a perfect magnet with
separate poles, one of which always points to the north.

The poles of the magnet display the same phenomena as regards
attraction and repulsion, as do the opposite kinds of electricity. If
we suspend a magnet and bring the north pole of another to the north
pole of the suspended magnet, the latter will turn away; but if we
apply the north pole of one to the south pole of the other they will be
attracted just as opposite electricities attract each other.

MAGNETIZATION is the term applied to the making of artificial
magnets, which act is accomplished by bringing the needle in contact
with other magnets, or sometimes by means of the electric current. If
we carefully stroke the needle with the magnet, always in the same
direction, lifting the magnet and beginning afresh every time, we shall
magnetize the needle, but with a different polarity from the pole it
was rubbed with. A magnet rubbing its _north_ pole against a needle
will make the latter’s point _south_, and _vice versâ_.

Now that we have seen how the “magnetic needle” is arrived at, we
can proceed to explain the electric telegraph. The term telegraph is
derived from the Greek words _tele_, “far,” and _graphein_, “to write,”
and now includes all modes of signalling. Signalling, or telegraphing,
is of very ancient origin; the Roman generals spelt words by fire.
The beacons fired on the hills, the “Fiery Cross,” and other ancient
modes are well known. The semaphore and flags have long been and are
still used as modes of signalling, while the flashing of the heliograph
“telegraphs” to a distant camp.

The Semaphore was invented by Chappé, and was really the first
practical system of telegraphy. It was adopted in 1794, but before
this, in 1753, a letter appeared in the _Scots Magazine_, by Charles
Marshall, suggesting that signals should be given by means of electric
wires, equal in number to the letters of the alphabet. Soon afterwards
Lesage, of Geneva, made an electric telegraph to be worked by
frictional electricity, and many ingenious attempts were subsequently
made to utilize electricity for signalling purposes, but without any
permanent success; indeed, the British government were quite content
with their semaphores, for they wrote that “telegraphs of any kind are
now wholly unnecessary, and no other than the one now in use will be
adopted”!

The _Electric Telegraph_ has had considerable antiquity claimed for
it, but it is pretty certain that the discovery made by Doctor Watson,
in 1747, that electricity would pass through wires, and that the earth
would complete the circuit, gave the first impetus to the Electric
Telegraph. Doctor Watson was enabled to transmit shocks across the
Thames, and made experiments at Shooters Hill. Franklin did likewise
across the Schuykill in 1748, and De Luc performed the same experiments
on the Lake of Geneva. Both Lesage and Lomond caused pith balls to
diverge at distant points, and in 1794 Reizen made use of the electric
spark for transmitting signals, and made strips of foil show out
certain letters when the spark passed. He had a wire and a return wire
for each letter of the alphabet.

These were all slow advances, and subsequently many learned men in
Europe sought to improve upon the ideas then promulgated. We read
of telegraphs constructed at Madrid by Salvá and Betancourt in 1797
and 1798, one extending for more than twenty miles. The first-named
gentleman finally proposed to substitute the Voltaic pile for the usual
machine, and Ronalds and Dyar in England and New York respectively
employed frictional electricity with some success. The latter sent
charges of frictional electricity through a wire, and they were
recorded by being made to pass through litmus paper. The distances
between the discharges were intended to indicate the letters of the
alphabet, but even if the experiment was fairly tried it failed, for
little was heard of the result.

After the invention of Volta’s pile, which Salvá wished to adopt,
Sömmering began his experiments. He used thirty-five wires, set up
vertically at the bottom of a glass reservoir of water, and terminating
in gold points. These wires ended in the opposite direction in brass
plates attached to a bar of wood. At one end the points and at the
other the plates bore the same letters respectively; hydrogen at one
gold point, and oxygen at another, and two different letters were
indicated when the current was sent through any two plates. This
arrangement was afterwards improved upon, and only two wires retained.

It was not until electro-magnetism had been developed, however, that
Œrstead found out the power of electricity to deflect the magnetised
needle, and in 1820, Scheweigger added a “multiplier.” Then came
Arago into the field with his discovery, that a “wire carrying a
current could magnetise a steel rod.” Ampère substituted a helix for
a straight wire, and Sturgeon used soft iron for steel, and developed
the electro-magnet. Daniell’s battery, and Faraday’s discoveries of
magneto-electricity and the induction coil were the means of putting
a constant supply of electricity at the service of the telegraph and
so on, till 1830 brought out a more practical method introduced by
Schilling.

In that year Baron Schilling made a telegraph, and exhibited it in 1832
at Bonn. This invention, with five vertical needles, was shown to Mr.
Cooke in 1836. But in 1834, Gauss and Weber had succeeded in sending
signals by means of a voltaic current acting upon a magnetised needle,
and this apparatus was really the first practical electric telegraph
in use, and it was much improved by Professor Steinheil of Munich.
They employed a magnetic-electro machine, and caused a bar to move in
certain directions to indicate certain letters of the alphabet. This
was really of value, but Steinheil, the pupil of Gauss, assisted by his
government, employed only a single wire, and made the earth complete
the circuit for him instead of having a return wire as his predecessors
had. This telegraph was perfected by a series of bells, which gave
different tones for different letters, and he also caused the needle to
make certain tracings as it moved upon a paper slip, something like the
Morse pattern, which was completed in 1837.

Professor Morse, in 1832, conceived the idea of an electric telegraph
but his claim was disputed by a Doctor Jackson, who was on the same
vessel when the subject was discussed. We need not enter into the
details of the controversy. Mr. Morse won the day, and patented his
invention.

“It was once a popular fallacy in England and elsewhere that Messrs.
Cooke and Wheatstone were the original inventors of the electric
telegraph. The electric telegraph had, properly speaking, no
inventor.... Messrs. Cooke and Wheatstone were, however, the first who
established a telegraph for practical purposes comparatively on a large
scale, and in which the public were more nearly concerned.... Therefore
it was that the names of these enterprising and talented inventors
came to the public ear, while those of Ampère and Steinheil remained
comparatively unknown.[17] The telegraph, as used in Great Britain, was
the result of the co-operation of Professors Cooke and Wheatstone.

Mr. Cooke, in 1836, having seen the needle telegraph when in
Heidelberg, made certain designs, and soon entered into partnership
with Professor Wheatstone for the application of electric telegraphs
to railways. Their apparatus with five needles and five wires was put
up on the London and North-Western (then London and Birmingham) and
Great Western lines, but proved too expensive. The instrument was
subsequently modified, and is used on the English railways still.

[Illustration: Fig. 242.—Cell.]

We may now proceed to look at the Wheatstone needle telegraph and see
the method of working it. We know already that when a pair of metallic
plates are immersed in a fluid which acts chemically more rapidly on
the one than the other, and a wire connects the upper parts of these
plates, this wonderful agency is set in motion, and circulates from
the one plate to the other (fig. 242). This arrangement may be best
shown by using one plate of zinc and the other of copper, and a dilute
solution of sulphuric acid for the liquid; this, however, produces by
far too little of the agent to be used on a telegraphic line, there are
therefore combinations of such pairs of plates, so arranged that the
power of one pair shall be added to the next in such a way that at the
end of the series (called a “battery”) there shall be a great increase
of the power accumulated; this arrangement is shown in fig. 244. Now
(if the power be sufficient) it does not signify what length of wire
there may be between the two ends of this arrangement or “battery”;
whether they be connected by a few feet or many hundred miles, the
electricity passes instantaneously from one end to the other; and
furthermore, it has been found in practice, that this electrical
influence can be transmitted through the earth in one direction if
sent by a wire in the other; for instance, if a wire from one end of
the battery be carried on from London to Liverpool, instead of having
another from Liverpool to London, to connect the two ends of the
battery, it is found to answer the same purpose if the end of the wire
at Liverpool be fastened to a plate of metal buried beneath the surface
of the earth, and the other end of the battery at London furnished with
a similar plate, also buried. In this arrangement, the electricity
will pass beneath the surface of the earth from Liverpool to London,
and through the wire from London to Liverpool, thus completing
the circuit. The end from which the electricity passes is called
the “positive electrode,” that to which it returns, the “negative
electrode.” Fig. 245 will show this arrangement.

[Illustration: Fig. 243.—The Needle Telegraph.]

[Illustration: Fig. 244.—The Battery.]

[Illustration: Fig. 245.—The circuit.]

[Illustration: Fig. 246. Fig. 247. Fig. 248.

Magnetic needles.]

If a bar-magnet be suspended on a pivot so that it may turn freely,
it will (as is well known) turn with one end to the north, which is
owing to a current of natural electricity passing round the earth in
the direction of east and west, the magnet crossing the current at
a right angle; and if a coil of wire coated with silk (to keep one
part of the coil from another) be placed round, above, and below the
long axis of a bar of steel, as shown at fig. 246, and a current of
electricity passed through the wire, the steel becomes a magnet, and
will take a direction similar to the natural magnet, more or less, at
right angles to this coil, as in fig. 247, according to the intensity
of the current; and the instant this electrical current is stopped, it
will resume its former direction. This fact has been made use of to
form the principal feature of all English telegraphs; such a needle is
mounted in an upright position, and instead of its tendency to turn
to the north, a tendency to maintain the upright position is given
to it by having one of the arms of the magnet a little heavier than
the other; such a magnet having a coil of wire surrounding it. When
the electric current passes through the coil, it will turn out of the
upright position to either one side or the other, according to the
direction of the current, from its tendency to assume a position at
an angle thereto (fig. 248); if the current be stopped even for an
instant, then the needle, or magnet, will again assume its upright
position. The pivot of this magnet is brought forward, and has on its
front part another needle, which turns with it; this is visible on the
outside of the apparatus, and is looked at to ascertain the movement of
the one within. There is also an arrangement called a “commutator,” so
contrived, that by moving a handle to the right or left, a connection
shall be made with either end of the battery, and thereby cause the
direction of the current and needle to be changed at pleasure; also
by moving the handle into an upright position the current shall be
stopped; and finally, by a third movement, a bell shall be rung. Now,
as has already been explained, when the current goes in one direction,
the magnetic needle is deflected in that direction; and when the
current is reversed the position of the needle is also reversed, and
when the current is cut off the needle will resume its perpendicular
position. If two such needles and two such handles be at each station,
when the handles at one station are moved, the needles at the other
station will take on a similar movement; and when the handles at that
station are moved, the needles at the first station will be moved to
correspond. This constitutes the system of communication kept up by
the electric telegraphs in England; but it remains to be shown how all
the letters of the alphabet and numerals can be represented by the
movements of the two handles. These handles can be placed in eight
positions (besides the upright one) by a single movement of each hand,
as may be seen in fig. 249; and these eight signals if repeated, or
made twice in rapid succession will make eight more, and by being
repeated three times will constitute a third eight, making twenty-four;
finally, by a rapid motion right and left, they may be caused to
signify a fourth eight, or thirty-two signals, which are found to be
sufficient for every purpose, and by practice may be both produced and
read off with facility. Before a message is about to be delivered the
commutator is so placed as to ring a bell, which is done by the same
arrangement as in a common alarm-clock, but the action is set in motion
by a peculiar contrivance, which depends upon the property a bar of
soft iron has of becoming magnetic when a wire is wound round it and a
current of electricity passed through this wire; this magnetic property
exists only as long as the current passes, and stops the instant it is
cut off. The catch of the alarm is disengaged by the movement of a bar
of iron being drawn to the magnet while the current passes, and forced
back again by a spring when it is stopped, thus setting in action the
mechanism of the alarm; or in some cases there is a simple contrivance
for causing a rapid flow and stoppage of the electricity, so that the
bar is alternately attracted by the magnet and released by the spring,
and this motion rings the bell as long as it is continued. The bell
is always rung to give notice that a message is about to be sent, and
at the station where it rings, the bell at the former station is rung
in return, to show that they are prepared to receive the message:
which is then spelt, letter after letter, by moving the handles into
the proper positions, and as it is being sent, the eye is kept on
the dials, certain single signs are made and recognised, which will
communicate any reply from the station to which the message is being
sent, such as “repeat,” or “not understood.” The wires which convey
the electricity are made of galvanised iron (iron coated with zinc),
and as they must be kept from all communication with the earth by some
substance incapable of conducting it, they are therefore stretched
between wooden poles (fig. 250), and rest upon sockets or supports of
glass or glazed earthenware, which are both substances incapable of
conducting the electricity to the earth (fig. 251), and in order that
these may be quite dry, an inverted cup of metal, glass, or earthenware
is placed over it, or the whole is blown or moulded in one piece. If
the support for the wires were not kept from the rain, the wet would
form a conducting surface, and allow the electricity to escape into the
earth.

[Illustration: Fig. 249.—Handles and needles of telegraph.]

[Illustration: Fig. 250.—Telegraph wires.]

[Illustration: Fig. 251.—Insulator.]

The Telegraph Alphabet, in the two-needle instrument, now not generally
used in England, is given below.

                         TWO-NEEDLE ALPHABET.

  Movements of            |                      |
           First Needle.  |   Of Second Needle.  |Of both Needles
                          |                      |   together.
                          |                      |
  A is signified by moving|                      |
      needle once to right|                      |
  B   “  “  “ once to left|                      |
  C                       |By moving needle      |
                          |         once to right|
  D                       |    “   “ once to left|
  E                       |                      |Moving to right.
  F                       |                      |Moving to left.
  G         twice to right|                      |
  H           “    to left|                      |
  I (or J)                |        Twice to right|
  K                       |          “       left|
  L                       |                      |Twice to right.
  M                       |                      |Twice to left.
  N One to right,         |                      |
             one to left  |                      |
  O One to left,          |                      |
            one to right  |                      |
  P                       |One to right,         |
                          |         one to left  |
  O                       |One to left,          |
                          |        one to right  |
  R                       |                      |One to right,
                          |                      |         one to left.
  S                       |                      |One to left,
                          |                      |        one to right.
  T Two to right,         |                      |
             one to left  |                      |
  U One to right,         |                      |
             two to left  |                      |
  V                       |Two to right,         |
                          |         one to left  |
  W X                     |One to right,         |
                          |         two to left  |
  Y                       |                      |Two to right,
                          |                      |         one to left.
  Z                       |                      |One to right,
                          |                      |         two to left.

In the single needle instrument the letters are indicated by right and
left vibrations, from A one right, to B one right and left, and so on,
increasing to Z. This mode is now generally used.

The manner in which the current passes is shown by the following
illustrations (figs. 252, 253).

[Illustration: Fig 252.—Passage of the current (1).]

[Illustration: Fig. 253.—Passage of the current (2).]

For the sake of clearness, the diagram has been drawn with simple
lines only. In the real needle-machine the construction is much more
complicated; perspective drawings of it may be seen in Lardner’s
“Electric Telegraph,” and numerous other works. In fig. 1, B is a
single cell of a battery containing a plate of copper, C, and a plate
of zinc, Z, immersed in sulphuric acid and water. H is the handle of
the instrument, turning from left to right, and _vice versâ_, like the
handle of a door, consisting of two pieces of brass insulated from
each other by being inserted in an axis of ivory. To the ends of the
two pieces of brass are fixed the wires, CW and ZW, leading from the
copper and zinc ends of the cell respectively. Fixed on each side of
the handle are two plates of metal, which may be called LP, the left
plate, and RP, the right plate. They are connected with the needle
wire, NW, which passes before and behind the magnetized needle, N,
suspended perpendicularly on its axis, with its north pole upwards.
As long as the wires, CW and ZW, remain insulated from each other,
no current passes from the cell; but as soon as the handle, H, is
turned, so that the copper end touches the left plate (fig. 2), and
the zinc end touches the right, communication is established between
the plates of the cell, and the current commencing at the copper
passes along CW, the top half of H, into LP, along N W, travelling
_up before_ and _down behind_ the needle, causing it to deflect to
the observer’s left, according to the rule given above. Reaching RP,
it passes downwards to the cell to Z, and so on to C, continuing its
travels and keeping the needle deflected as long as the handle remains
in contact with the plates. If it is required to deflect the needle to
the right, the handle is turned to the right, bringing its copper end
in contact with the right plate, causing the current to travel in the
opposite direction. By following the current from the copper to the
zinc, as indicated by the arrows in fig. 3, it will be seen that it now
travels _up behind_ and _down before_ the needle, deflecting it to the
observer’s right. Thus, by causing a current of voltaic electricity to
pass alternately _up before_ and _down behind_, and _up behind_ and
_down before_, the needle is moved to the left or the right at will.
The way in which the current is made to act on a distant needle is now
simple. The following figure (fig. 253) shows the arrangement. The
left portion of the figure represents an instrument at London, that
on the right an instrument at York. The needle-wire, instead of being
continued directly to the zinc plate of the battery, passes away from
the needle over poles to York, where it joins an instrument similar in
all respects to that at London. It passes similarly before and behind a
magnetized needle, joining the right plate of the instrument. As long
as the two plates are unconnected, no current can pass. The current is
therefore completed by a contrivance which may be represented by the
semicircular piece of metal, K. In practice the two plates or springs,
which, when not in use, are always pressed against the connector, K,
which is a cross-piece on the top of the handle, keeping the London
needle in circuit with the York battery, and _vice versâ_. As soon
as London uses his handle, it presses the spring-plate, and puts
his needle out of the York circuit, the current he sets up sending
York needle to the right or left, as the case maybe. The second wire
connecting the left York plate with the right London plate is, it will
be seen, not carried along like the first wire. Use is made in this
case of the conducting power of the earth itself, plates from the wires
being buried many feet below the surface at London and York. When
London wishes to speak to York, he first signifies his intention of so
doing by ringing York’s alarum. This he effects by sending a current
through an electro-magnet placed above York’s instrument. The armature
is attracted, and frees the detent of the alarum, setting it ringing
until York signals ready. London then stops the bell, and commences his
message. By following the direction of the current, when the handle is
turned to the left, as in fig. 4 [within fig. 253], it will be
readily seen how this is effected: commencing with London’s copper, LC,
it passes up before and down behind London’s needle, flowing along the
wire between the two cities to York’s needle, up before and down behind
which it travels, sending it also to the left. It then passes to York’s
right plate, through the connector to the left plate, and so on to
earth at York, coming to the surface again at London, passing through
London’s right plate and through the lower part of the handle to the
zinc of the battery. The reverse current may be easily followed. Any
number of instruments with similar needles may be interposed along the
course of the wire. When the operator wishes to speak to any particular
one, he rings all the bells for attention, and then signals Derby or
Nottingham, as the case may be. They all then throw their instruments
out of current except the one required. The mode by which the needle
movements are converted into language is simple. A is signalled by
causing the needle to vibrate once to the right, B once right and once
left, C once right and twice left; and so on, as arranged.

The following is the alphabet (with numbers) once in use on the
South-Eastern Railway for the double-needle instrument. The table is
taken from Mr. Walker’s translation of De la Rive’s work on Electricity
and Magnetism.

  A is signified by two movements of left needle to left.
  B      ”       by three   ”           ”            ”
  C (and fig. 1) by two     ”           ”        right first, then left.
  D (and fig. 2) by two     ”           ”        left first, then right.
  E (and fig. 3) by one     ”           ”        to the right.
  F              by two     ”           ”              ”
  G              by three   ”           ”              ”
  H (and fig. 4) by one     ”       right needle to the left.
  I              by two     ”           ”              ”
  J (same as G).
  K              by three   ”           ”              ”
  L (and fig. 5) by two     ”           ”        right and left.
  M (and fig. 6) by two     ”           ”        left and right.
  N (and fig. 7) by one     ”           ”        to the right.
  O              by two     ”           ”              ”
  P              by three   ”           ”              ”
  Q (same as K).
  R (and fig. 8) by one parallel movement of lower points, both needles
                                                             to the left.
  S              by two     ”           ”              ”
  T              by three   ”           ”              ”
  U (and fig. 9) by two     ”           ”        first right, then left.
  V (and fig. 0) by two     ”           ”        first left, then right.
  W              by one movement of both needles (lower points) to right.
  X              by two     ”           ”              ”           ”
  Y              by three   ”           ”              ”           ”
  Z (same as S) or specially.

The Morse system of telegraphy was first brought out in 1844, and was
worked by means of a Voltaic battery, an electro-magnet being used at
the receiving station. This magnet attracted an “armature,” and by
it dots or lines are marked on a moving paper band by a point at the
other end of the wire, on the register in which the paper is carried
by rollers which move out by clockwork. The lever being “tapped” down
in fast or slow pressures will give a corresponding series of dots or
lines (according as the pressure is long or short) upon the moving
strip of paper at the receiving station. Three taps will give C, one
tap and a pause will make A. The dots are “taps” on the key, the lines
brief “rests” on it, as will be seen from the alphabet below, which is
given as a specimen.

  MORSE ALPHABET.

  A .-
  B -...
  C .. .
  D -..
  E .
  F .-.
  G --.
  H ....
  I ..
  J -.-.
  K -.-
  L -
  M --
  N -.
  O . .
  P .....
  Q ..-.
  R . ..
  S ...
  T -
  U ..-
  V ...-
  W .--
  X .-..
  Y .. ..
  Z ... .

  NUMBERS.

  1 .--.
  2 ..-..
  3 ...-.
  4 ....-
  5 ---
  6 ......
  7 --..
  8 -....
  9 -..-
  0 ___

The various stops are also indicated in the same manner by combinations
of dots and lines.

The Atlantic telegraph cables and similar enclosed wires between other
countries are too well known to need detailed description. There
is a great variety of telegraphic instruments. The dial, and other
arrangements, are very common, and the Wheatstone Key instrument is
supplied to private firms as being the most handy. It requires but
a very short apprenticeship, and any person who is handy can easily
learn to work it in a few minutes. The apparatus consists of a dial
upon which the letters of the alphabet are printed, each letter being
supplied with a key or stop. A pointer is placed in the centre, as in
the wheel barometer, and there is a handle beneath. In front, upon a
sloping board, is another dial plate and pointer; thus we have the
receiver and transmitter before us in a very small space.

[Illustration: Fig. 254.—Receiver. Dial Telegraph. Fig.
255.—Manipulator.]

When it is necessary to work the instrument a bell is rung by turning
the handle rapidly. To speak by the instrument it is necessary to keep
turning the handle with the right hand while the fingers of the left
are employed in pressing down in as rapid succession as practice will
permit the keys corresponding to the letters on the dial while the
handle is kept turning. When a word is completed the operator must stop
at the + at the top, and then begin again, stopping after each word.
When all is said, a couple of rapid turns of the dial will signify that
you have ended.

There are many other systems of telegraph, but all are dependent upon
the same principles. The accompanying illustrations (figs. 254, 255)
show a dial telegraph of a simple kind, which almost explains itself.

The first figure is the receiver, on which is a pointer fixed to a
dial-plate having the letters of the alphabet inscribed around it.
When the manipulator is being worked the dart points to the letters
in succession of the words used, and they are separately spelt. The
manipulator (fig. 255), by closing and opening the circuit, works the
needle.

In the manipulator we have a wheel with an index point fixed above
it. In this wheel are thirteen teeth, with the openings between them
filled with ivory. The axis of the wheel is in contact with the wire
from the positive pole, _p_, and a spring attached to the wire or by
the binding-screw, _t_, presses against the circumference of the wheel,
and completes the circuit. When the wheel is placed so that the arrow
point is above the +, the needle of the receiver is also at +. By
turning the wheel to bring the needle to A, the spring on the
circumference is passed from an ivory “tooth” to a “metal” one; the
circuit is closed, the point of the receiver also turns to A,
and so on through the word by successive closing and breaking of the
circuit.

As there are a great many other applications of electricity of
which we have to treat,—the Electric Light, and Mr. Edison’s other
inventions,—our space will not permit a much more detailed account of
the telegraph, but there are some incidents connected with its progress
which it would be as well to mention.

[Illustration: Fig. 256.—Electric cable.]

[Illustration: Fig. 257.—Ocean cable.]

Alexander Bain, about 1840, attempted to produce a printing telegraph,
and in 1846 he actually accomplished a registering apparatus, which was
an application of the principles of Dyar and Davy. But although Bain’s
system was good, Morse had the advantage of possession in the United
States, where it was tried, and Bain went out of fashion. Bain’s system
was, in fact, the present chemical “automatic” telegraph, which has
been perfected for rapid transmission.

Bakewell’s instrument, which has been improved upon by later
electricians, is termed the fac-simile telegraph. The message to be
sent is written with a pen which has been dipped in varnish (for ink),
and the characters are inscribed upon prepared tinfoil. The message is
then put upon a cylinder covered with prepared paper, and has a pointer
attached. There is a precisely similar cylinder at the receiving
station. When the cylinders are simultaneously set going, the point at
one will trace a spiral line as the first (transmitting) point passes
round its cylinder. However, as the latter “stylus” meets the varnish
letters a break occurs, and these spaces are exactly reproduced as
blanks at the other end, and the form of the letters can be seen.
Coselli, in his adaptation, caused dark letters to be registered on
a white ground, and thus simplified matters. Since then we have had
printing telegraphs, and dials, and writing machines, one of which will
be described presently.

Submarine telegraphs were, it is said, first suggested by Salvá in
1797, and Wheatstone, in 1840, declared that it was quite possible to
connect England and France by wire. Morse and Calt experimented with
submarine cables in America, and Lieutenant Siemens first applied
gutta-percha to the wires as an insulator in the Prussian line of
telegraph across the Rhine. The English laid a wire between Dover and
Calais, which was broken, but successfully relaid. And so on, till in
1857 the great project of the Atlantic cable was broached. We give
illustrations of the cables; the circumstances connected with the
laying of which, and the enthusiasm over the successful accomplishment
of the task, must be in the memory of all. The readings of the messages
were shown by delicate galvanometers, the beam of light being reflected
from a mirror. This cable was lost, and in 1862 Mr. Field came over to
urge the importance of the submarine cable between this country and
America. The cable was shipped on the _Great Eastern_ in 1865, and
was 2,186 miles long. It consisted of seven copper wires twisted, and
covered with gutta-percha. The outside coating consists of ten iron
wires surrounded by manilla yarn. But this cable broke, and a third was
made and laid in 1866. The old cable was then recovered and spliced.
There are some two hundred cables now in existence, the last being the
Cape cable, laid when the Boer War was engaging our attention. The
transmitting apparatus of Mr. Varley and Sir W. Thomson has greatly
accelerated the rapidity of messages, and Thomson’s syphon recorder
farther increased the speed.

The following description of a new system is from _Scribner’s Magazine_
for 1880:—

“NEW TELEGRAPHIC SYSTEM.

“A new system of sending and receiving electrical impulses over an
insulated wire has recently been brought into successful operation,
that seems to promise not only a radical change in the present methods
of telegraphing, but a material gain in the speed and cost of sending
messages by wire. It is founded on a union of the so-called “automatic”
and “chemical” systems of telegraphy. The first of these employs a
strip of paper having, by some mechanical means, a series of small
holes punched in it, the design being to pass the perforated strip
under a needle, or stylus, in electrical connection with the line. This
stylus, on passing over the paper, opens the circuit, but in passing
one of the holes, drops through and closes it,—this alternate making
and breaking of the circuit transmitting the message. The chemical
telegraph records any electrical impulses sent over a line by staining
a strip of prepared paper passing under it. This is founded on the fact
that electricity has the power of decomposing certain chemicals, and
if paper is soaked in these chemicals and submitted to the action of
electricity, it will be discoloured wherever the current passes. While
both of these systems have been used, neither has been able to compete
with the more simple Morse key and sounder, and it has remained for
the new system to bring them to a position where they may come into
general use. The new system is a modification and combination of the
automatic and chemical systems, the transmitting being performed by
means of a perforated strip of paper, and the receiving of the message
being recorded by the discolouration of chemically prepared paper. The
process is entirely mechanical and chemical, the telegraph operator
having no direct control over the message, either by sight, sound, or
touch. The written message is sent to the operating-room, and given
to the person using the perforating machine. This consists of a small
key-board, with black and white keys, each marked with a letter or
sign, and an ingenious system of levers, operated by the keys, for
punching small holes in a ribbon of paper moving past the side of the
machine. The machine stands upon a small table, and under it is a
treadle for giving motion to the feeding apparatus for supplying the
paper to the machine. The operator moves the treadle with his feet, and
at the same time touches each key to spell out the message. In a very
few seconds the message is imprinted on the ribbon in the form of a
double row of small perforations, each group of two holes representing
a dash, and each single hole a dot, as in the Morse alphabet. Each
letter is separated from the next by a longer dash, and each word by a
still longer dash, and each sentence by a dash of indefinite length.
This spacing of the letters is performed automatically, the spacing of
words and sentences is performed by the operator. The perforated slip
containing the message is then sent to the transmitting machine. This
consists essentially of a metallic wheel, divided into two sections by
means of a thin insulation of hard rubber. One section of the wheel is
connected with the positive pole of the battery, and the other section
with the negative pole. A pair of fine metallic brushes, both of which
are connected directly with the line, are suspended above the wheel,
and are arranged so as to press lightly upon the latter, when desired.
When resting on the wheel the circuit is closed, and when raised above
it the circuit is broken. The perforated strip is, by a simple piece
of mechanism, made to pass over the face of this wheel and under the
brushes. While the paper is passing, both brushes are raised from the
wheel, and slide over the paper, and the circuit is broken. On passing
a hole, one of the brushes drops through and closes the circuit for
an instant. On passing two or more holes, arranged in a series close
together, the brush closes the circuit for a shorter or longer time,
according to the number of holes, and as the perforations on the
paper are arranged in two rows, alternating from one to the other,
the brushes are used alternately, and the polarity of the current is
continually changed with every impulse sent over the line. No special
skill is required in sending a message, as the operator has only to put
the perforated strip in the machine and turn a hand-crank, to cause it
to pass rapidly under the brushes, and with a little practice, a young
girl can send messages at the rate of one thousand words a minute, with
absolute precision. The receiving apparatus consists essentially of a
simple piece of mechanism for causing a strip of chemically prepared
paper to pass rapidly under two small needles that are connected with
the line. As the paper passes the needles, the electricity sent over
the line from the transmitting machine seeks the earth through the wet
paper and the machine, and in passing discolours the paper, each stain
representing a dot or dash, and the message is printed on the paper in
a double row of marks at the same speed with which it was dispatched.
In practice, a Morse key and sounder is placed at each end of the
line, and on sending a message the transmitting operator calls the
receiving station, and when the operator at the distant end replies,
both turn the cranks in their machine swiftly, and the message is
sent and received at an average speed of one thousand words a minute.
The message received is given to a person using a type-writer, and
at once translated into print and sent out by the messenger boy. It
is found in practice that two operators, one at each end of a single
wire of indefinite length, can keep fifteen operators fully employed
in preparing the messages, and fifteen girls busy in translating and
printing the messages for delivery. The system is of American origin.”

Of the hundred and one uses to which electric wires are now
appropriated—of the alarms, fire-calls, clocks, etc.—we need not speak.
We must pass on to the Writing Machine (fig. 258) before we make
mention of Mr. Edison’s inventions.

The Writing Machine is as remarkable for the simplicity of its
mechanism as for the facility and ease with which it can be used. It
was invented by Remington, the American, whose name is so universally
known in connection with a repeating rifle. He makes these writing
machines in his own factory, where he associates them with rifles and
sewing machines—implements for war and peace.

The appearance of the Writing Machine may be easily perceived from the
illustration (fig. 258), which is drawn to scale one-fourth of the
actual size. It comprises a key-board, upon which there are forty-four
keys or stops, including numbers from 2 to 9, the _i_ and _o_ of the
alphabet serving for numbers 1 and 0, and all the letters of the
alphabet arranged in the manner most convenient for manipulation. There
are also the various accents and stops, with note of interrogation,
etc. The flat ruler at the base of the key-board is struck when it is
necessary to separate one word from another.

In the interior of the apparatus every letter is attached to a small
hammer, and corresponds to the pressure bestowed upon the notes, which
are disposed in a circle. If A, for example, be touched upon the
key-board, the hammer will bring A to the centre of the circle, and so
every letter of the word will be, by such action, brought to the centre
of the circle in succession. The paper upon which the letter is printed
is wound upon a cylinder mounted upon a slide, as seen in the upper
portion of the illustration.

When the letter is pressed down on the key-board the corresponding
hammer strikes against the cylinder, between which and the hammer is
a ribbon prepared with a special ink. The letter being in relief like
ordinary type is impressed upon the paper. The slide upon which the
paper is mounted is so arranged as to move from right to left exactly
a letter-breadth after each impression. Thus as every hammer strikes
at the same spot a regular succession of letters are printed off, the
paper moving with regularity. When the line is filled—that is, when
the paper has moved across the cylinder—a bell rings, and a handle is
moved by the operator who is thus warned. The lever moved brings the
slide back again, and a new plain surface is ready to commence upon the
cylinder moving upwards at the same time, and displaying the printed
line.

[Illustration: Fig. 258.—Remington’s Writing Machine.]

In operating both hands may be employed, but between each word care
should be taken to press down the flat board at the base of the
key-board, which has the effect of leaving a space upon the paper.
Immediately the sound of the warning bell is heard the lever at the
right-hand side must be lowered. The word can be finished in the line
following if it be not concluded, the hyphen button being pressed to
indicate the continuation.

The paper used must not exceed the width of the cylinder, but it may
be of any less width, and a post-card or any small sheet of paper
may be substituted. If the width be thus limited the length may be
indefinite, and a very long line of paper may be used if desirable.
The cylinder being made of gutta-percha offers a soft surface to the
impression of the hammer, and causes the letter to assume greater
distinctness.

The inked ribbon which passes underneath the paper is so arranged that
no two successive letters strike it on the same place. It moves from
an ink reservoir on the right to another on the opposite side, and it
can be made to return beneath the paper, thus keeping up the supply.
The impression being made in copying ink, the message or letter when
finished can easily be reproduced in an ordinary press. The characters
are all “capitals.”

At first it may be found a slow means of writing, and the manipulator
may imagine he can do better without it. But if the author be certain
of what he intends to say, after a little practice at the instrument,
and when he becomes accustomed to the positions of the various letters,
etc., the rate at which words can be printed off will far exceed that
at which even rapid writers can work. A young English lady after some
days’ practice was able to write as many as ninety words a minute
with this machine—a rate more than double the average writing rate of
penmanship. When such a rate or an approximation to it can be attained,
those who are quick in their ideas will find the machine a great saving
of time, and for any one afflicted with “writer’s cramp” the gain must
be enormous. We need not insist upon the advantages the adaptation of
the apparatus would confer upon editors and readers of MSS. too often
badly written, and to compositors the invention is a great boon.

Finally, the working of the machine could be entrusted to the blind,
and by teaching them the form of letters which could be raised upon
the key-board, those so sadly afflicted could write with facility.
Some methods for teaching the blind to manipulate and to read from the
impressions of the hammers on the paper have already been tried with
success.

The Electric Pen, an invention of the fertile brain of Mr. Edison, is
shown in fig. 259. The “writing” consists of a series of little holes
close together, made by a fine steel point like a put-crayon. This
point is thrust in and out with great rapidity, and passes quickly over
the paper. If the characters cannot be formed so quickly as with an
ordinary pen, the writing is very distinct.

The alternative movement is given to the pen by an electric motor at
once simple and ingenious, which is placed on the top of the penholder.
The general appearance of the apparatus will be understood from the cut
on next page.

The point is the termination of a wire which traverses the penholder,
and the upper extremity of which catches on the motor by an eccentric.
This eccentric has three teeth or cogs, and it makes sixty revolutions
a second, thus producing one hundred and eighty beats in that time.
The axle carries a plate of soft iron, which acts like the armature
of an electro-magnet, before which it turns with great rapidity, the
current being interrupted twice in every revolution by the commutator.
The current which moves this little apparatus is furnished by a pile
of two elements in bichromate of potash, according to Mr. Edison’s
arrangement, which is considered very successful. Carbon and zinc are
employed, and when ready for action the battery assumes the appearance
of the cells in the illustration. When the operator wishes to
discontinue writing, he simply raises the stem which has the electrodes
attached to it, and the elements are thus preserved for a future time.

Under these circumstances the battery could be made to last several
days without any renewal of the liquid, and the plates will last for
weeks. Thus a very simple arrangement is at our disposal. Let us see
what use can be made of it.

[Illustration: Fig. 259.—Edison’s Electric Pen.]

When we use the electric pen we obtain a great number of small holes
close to each other. Such hand-writing is not easy to decipher by mere
inspection like ordinary writing. By holding it up to the light it is
more easy to read, but in both instances reading is not easy, nor does
it come by nature as Dogberry declares. But if we consider the paper as
a “negative,” we may obtain a number of positive proofs or copies of
the writing. To obtain these successfully we must use a press, as shown
in the accompanying illustration (fig. 260).

The writing, or negative, is placed upon the cover to the left, where
it is firmly fastened. Upon the body of the press a sheet of white
paper is placed, and when the lid is shut down the negative comes
in contact with the paper. By means of a roller, represented in the
box, the writing is blackened,—the ink penetrates into all the holes
which are upon the paper,—and after the manner of a stencil plate the
impression will be found upon the paper when the cover is removed.
The writing will have a curious effect, but practice will speedily
remove all deficiencies. The same negative will serve for a great many
impressions, quite a thousand having been taken from one. By people
accustomed to such work as many as six proofs a minute may be obtained.
Of course a little practice will be necessary in this, as in every
other case, before a correct or rapid result can be obtained, but there
is no difficulty in the practice.

There are two or three other applications of electricity which we must
refer to; such as the electric stamp, of which we give an illustration,
and a curious method of stopping a horse by electricity. The electric
stamp might be very advantageously employed in our post offices to
obliterate the “Queen’s Head.” The description, with illustration of
this apparatus, is annexed. (_See_ fig. 261.)

[Illustration: Fig. 260.—Duplicating press.]

At the lower end of the apparatus is a thin platinum wire, so arranged
as to form either a design or an initial; by this the postage stamp
can be defaced. The stamp being put in communication with the pile,
the circuit is closed by the pressure of the finger, as shown in the
illustration. The platinum grows heated and carbonises the paper, and
thus proves itself an ineffaceable stamp.

This apparatus may easily be used, not only by the post office
authorities, but by every one who is obliged to deface a certain
number of stamps every day, and wishes to do so rapidly and without
possibility of error.

An ingenious, if scarcely necessary arrangement for conquering
restive horses, and frightening them into submission, is shown in
the illustration (fig. 262). Many means have been tried to stop or
conquer a restive horse, but the most efficacious has been designed by
M. Defoy; and the director of the Paris General Omnibus Company has
experimented successfully, as we are informed, with the arrangement we
are about to describe. A small magneto-electric machine is contained
in a box beside the driver, within easy and convenient reach of his
hand. The reins contain a wire, one end of which terminates in the
horse’s bit, and the other in the electro-magnetic apparatus. When
the electro-magnet is put in action an electric current is generated,
which gives the horse a shock in the mouth, and so astonishes him that
he suddenly stops in his course. If the operator have the humanity
and good sense to unite kindness to the abrupt application of the
electricity,—which in our opinion should be only used as a last
resource,—no doubt some excellent results may be obtained even with
vicious animals.

[Illustration: Fig. 261.—The electric stamp.]

M. Bella, the Director of the Omnibus Company, has reported that the
apparatus was tried in his presence and found very successful, and
quite easy of application; and that even the most unruly animals have
been subjected by it. On one occasion a most restive animal was thus
treated on the way to the forge. He had a tremendous objection to be
shod, and made no secret of his dislike. But a gentle application of
the electric current put quite an opposite complexion upon the matter,
and after a few minutes the animal permitted himself to be patted and
caressed, and even allowed the smith to feel his legs and inspect his
feet without making any objection whatever. His shoes were taken off,
and the horse was re-shod without any of the dangerous demonstrations
hitherto indulged in by the animal.

We may quote another instance of the efficacy of this method, which is
reported from Paris by M. Camille.

[Illustration: Fig. 262.—Electric apparatus for checking vicious
horses.]

“Many experiments have been made upon horses which had been most
difficult to shoe, and in each case we have succeeded when the electric
apparatus has been put in requisition. One horse, in particular,
nothing could subdue. He kicked and bit and jumped about in such
a manner as to render all approach impossible. We had recourse at
length to M. Defoy’s apparatus, and after the first application, and
without any great difficulty, we were able to raise the animal’s feet;
but after a second lesson we were permitted to shoe him without his
offering the slightest resistance. He was completely subdued.”

M. Defoy recently made the experiment with a very dangerous animal,
which he stopped instantaneously in full gallop (_see_ fig. 262).
It may be remarked that the application of the current is not
sufficiently strong to stop the horse too suddenly. It merely causes
a very unpleasant sensation—he is not stupefied nor galvanized by
the electricity. The narrator has felt the shock applied without
inconvenience, and the conclusion arrived at is, that this method
of employing electricity is far superior to the violent and inhuman
treatment so often employed to break horses, which renders them
subsequently sulky and vindictive.

M. Defoy has completed an electric bit and an _electric stick_ quite
as ingenious as the electric rein. The _modus operandi_ is simple and
effective, the wires being insulated by leather, and terminating at the
extremities of the stick. The current is induced, as before, by a small
magneto-electric machine.

[Illustration: Electric Time Ball.]


FOOTNOTES:

[15] _Encly. Metrop._

[16] “Treatise on Electricity and Magnetism.”

[17] Sabine: “The Electric Telegraph.”



CHAPTER XXI.

MAGNETISM.

 THE LOADSTONE—MAGNETIC CURVES—THE MAGNETIC NEEDLE—THE MARINER’S
 COMPASS—MAGNETO-ELECTRICITY.


We have already mentioned some of the properties of the loadstone
or magnet; but as we are now about to enter more fully into the
considerations of its attributes and of the compass, etc., we will add
some further interesting particulars.

Ancient writers (Pliny, Homer, and Aristotle) mentioned the existence
of the magnet, and Humboldt refers to the knowledge of it possessed by
the ancients. Pliny says “the magnet-stone is found in Cantabria,” and
we have heard of the loadstones that are supposed to support Mahomet’s
coffin at Medina. The origin of this fable was (probably) owing to the
order given by Ptolemy to his architect, Dinochares. Ptolemy wished
the roof of a temple at Alexandria to be roofed with the magnet-stone,
so that the own image of his sister, Arsinoe, should remain suspended
therein. But the death of the king and his architect prevented the
project from being carried out.

The name “magnet” is said to have been derived from a shepherd named
MAGNES, who, when tending his flock on Mount Ida, found that
his iron crook was attracted to a certain stone; and six hundred
years before the Christian era Thales wrote respecting amber and the
magnet; and because they attracted various substances, he supposed they
possessed life and power. They were the germs of the science now so
developed in their applications, and whose full powers we are scarcely
yet acquainted with.

We may remark that other bodies besides iron and steel are capable of
magnetization; nickel and cobalt have the like property. Magnetism,
properly so called, treats of certain bodies known as MAGNETS,
describes the properties they possess, and the influence of magnetic
force upon other substances. Electricity and magnetism are always
associated, but practically the force is the same, electricity being
the current or motive power, so to speak; and it is to Faraday that the
world is indebted for the discovery of magneto-electricity.

Epinus’ theory of magnetism was that all bodies possessed a substance
he termed magnetic fluid, the particles of which repelled each other.
But while supposed to repel each other, they attracted particles in
other bodies. Thus they attracted iron. Coulomb asserted that there
were two fluids—a north and south fluid. Ampère’s theory was that
magnetic bodies are made up of molecules, round which currents are
always circulating in all directions when non-magnetized, but when
magnetized the currents all flow in the same direction. The space
through which a magnet diffuses its influence was called by Faraday
the _Magnetic Field_. The lines of _magnetic force_ will be understood
from the accompanying illustration (fig. 263). If we cover a magnet
with a paper and scatter iron filings over it, we shall see the manner
in which the filings arrange themselves. They radiate in curves from
the poles of the magnet, and are dependent upon its form. If it be a
straight bar magnet, evenly magnetized, they will turn inward in oval
curves.

[Illustration: Fig. 263.—Lines of magnetic force.]

[Illustration: Fig. 264.—Magnetization.]

The manner of magnetization has already been mentioned, but here we
will give further illustrations of the method of _magnetization_. Four
magnets are used, two being placed with their opposite poles apart,
and upon them is placed the bar of which a magnet is to be made. Two
other magnets separated by a piece of wood are then brought near, and
subsequently drawn from the centre to the ends of the bar. This is the
_separated touch_ system; the _double touch_ of Mitchell is completed
by moving the upper two magnets from end to end backwards and forwards,
and finally lifting them away from the centre.

A magnet, then, is a bar of steel endowed with certain properties, such
as attracting iron, etc.; and electro-magnetism is the term applied to
the production of magnetism by means of electricity, the medium being
the electro-magnet. To understand the science it will be necessary to
mention Ampère’s theory of magnetism.

It was Œrsted who observed that when a magnet is placed within reach of
an electric current and free to move, it sets itself at right angles
to the direction of the current; and Ampère defined the law already
referred to when treating of the electric current,—viz., “that if a
person be imagined as placed in the wire so that the current shall pass
through him from feet to head, if he turn his face toward the magnetic
needle the north pole will always be deflected to his left-hand side.”
When the current is passed above the needle from south to north poles,
the deflection is to the west; when from north to south the deflection
is east. When the current is below the needle the contrary is the
case. Ampère decided that currents circulating in the same direction
attracted each other, and when running in opposite ways they repelled
each other. He supposed currents to circulate within all magnetic
substances, and then—that is, when the body is magnetized—these
currents flow in planes parallel to each other, and the material which
offers the least resistance to the circulation of these currents
becomes the most magnetic.

The earth being supposed to be an immense magnet has currents
circulating through it in a direction from east to west; and having the
property or power of turning a magnetized bar in a direction similar
to that in which the bar would be turned by a magnet, the earth is
considered a magnetic mass. This influence is due to what is called
“terrestrial magnetism.” If we suspend a bar by a thread it will point
in no particular direction, but may be turned towards any side we
please. But when once the needle is magnetized it will point north and
south; or, as we say (but not correctly), the north pole of the magnet
points to the north of the globe. It is really the south pole that
points to the north, and the north pole of the magnet points _south_,
as can be proved by suspending the bar over another magnetized bar. So
if the earth be considered a magnet our English terms are inconsistent
with our theories. Continental writers are more correct.

The line of the magnetic needle’s direction, which differs in different
places, is called the magnetic meridian, and the amount of its
divergence from the astronomical meridian is termed its declination or
variation. When the amount of this variation is known it is allowed
for, and the needle can be considered as pointing due north and south.

But the needle does not assume a position perfectly parallel to the
horizon. It dips down in different hemispheres. As we approach the
north pole the dip or inclination will become greater, and the same
effect is observable at the south pole. Again, there are certain places
on the earth where the attraction is so evenly balanced that the needle
is perfectly horizontal. The line uniting these places is the magnetic
equator. This does not coincide with the earth’s equator any more than
the magnetic poles coincide with the geographical poles of the earth.

The declination of the needle varies from the meridian of Greenwich
at different times. If we travel to the west the variation increases
westerly, and is greatest in the Atlantic. It then decreases; the
needle points due north in North America. Going still forward the
variation becomes easterly, increases, and decreases to nothing in
Eastern Russia. Thence the variations are westerly. Columbus discovered
the variation of the compass needle in September 1493. In places where
the needle is due north and south the lines drawn through them are
termed lines of no variation.

The variation, however, is not always the same in the same place. In
the year 1580, in London, for instance, the variation was 11° 11´ E.
A little more than one hundred years later London was on the line of
no variation, and now the tendency is westerly. On the other hand,
there are places where there is no deviation, and Sir John Herschell
says that West India property has been saved from litigation in
consequence of the invariability of magnetic declination there, for all
surveys were made by the compass. Lines of equal variation are called
isogonial; those of equal dip or inclination, isoclinical; and those of
equal intensity, isodynamical.

As we have said, the magnetic elements are not always the same, and the
variations of the compass are daily and annually observed with certain
instruments. What are termed secular variations take place at long
intervals, as the following table will show:—

  In 1576 the angle of declination in England was 11° 15´ East.
     1622           ”                   ”          6° 12´  ”
     1660           ”                   ”          0°  ”   ”
     1730           ”                   ”         13°     West.
     1760           ”                   ”         19° 30´  ”
     1800           ”                   ”         24° 36´  ”
     1818           ”                   ”         24° 41´  ”
     1850           ”                   ”         22° 29´  ”
     1870           ”                   ”         19° 55´  ”
     1873           ”                   ”         19° 58´  ”
     1879           ”                   ”         19°  7´  ”

In the year 1818 therefore the maximum declination was reached in
London. In Paris the maximum was arrived at in 1814, and was 22° 34´.
The rate of decrease is about 8´ a year, but varies in different
periods, as may be seen. The discovery of the fact that an annual
variation took place in the angle of declination, is attributable to
Cassini, and the diurnal variation was discovered by Graham in 1722.
From 8 o’clock in the morning, when the needle is pointing a little
to the east of its “mean position,” it turns towards the west until 1
p.m. It then returns towards the east again, and passing westerly again
between midnight and three o’clock a.m., settles down till eight a.m.,
when it begins afresh. This variation does not apply to all places.

Magnetic inclination is besides subject to changes. There are also
variations of magnetic force which occur at very irregular periods,
and cannot be said to follow any laws. These disturbances are called
Magnetic Storms, of which the Aurora Borealis is one result.

Professor Faraday in his memorable experiments divided a long list
of different substances into para-magnetic and dia-magnetic bodies.
He classed them under these two heads, according as they took up a
certain position parallel or perpendicular to the axial or equatorial
line. This definition of “dia-magnetic” was “a body through which the
lines of magnetic force are passing, and which does not by their action
assume the usual magnetic state of iron or loadstone.” He concluded
that all bodies were magnetic, and by suspending a great number of
various substances he found they placed themselves axially,—that is,
lying between the poles of the magnet, or equatorially,—viz., at right
angles to that line. If the magnets be suspended at each side the same
bodies will assume a position with their longest diameters between the
poles, while others will be repelled by the magnets even if the poles
be reversed. So those bodies which are attracted and lie in the axial
line are termed para-magnetic; those repelled into the equatorial line
are termed dia-magnetic. In the “Proceedings of the Royal Society for
1846,” Faraday’s account of the various experiments can be studied in
detail. We can only give a brief _resumé_ of them here; and he showed
that the motions displayed by dia-magnetic bodies in a magnetic field
are all reducible to one simple law—viz., that the particles of the
dia-magnetic tend to move into the positions of the weakest magnetic
force.

He experimented upon a large number of bodies and gases; he tested
crystals, metals, liquids, and solids, and proved in whatever state a
body might be in the effect was the same; whether simple or compound,
it made no difference. Of course in a compound the preponderance of the
dia-magnetic or para-magnetic property would influence the result, and
the medium in which the body operated on was placed, was a condition
in the experiment. He proved that if a body be suspended in a medium
or surrounded by a medium whose power either way is stronger than the
body, that body is para-magnetic or dia-magnetic, according as it is
surrounded by a medium whose power is weaker or stronger than the
body itself. The arrangement of the bodies is as follows, from the
para-magnetic to the dia-magnetic, bismuth being the most dia-magnetic
of all:—

  PARA-MAGNETIC METALS.

  Iron.
  Nickel.
  Cobalt.
  Manganese.
  Chromium.
  Cerium.
  Titanium.
  Palladium.
  Platinum.
  Osmium.

  DIA-MAGNETIC.

  Bismuth.
  Antimony.
  Zinc.
  Tin.
  Cadmium.
  Sodium.
  Mercury.
  Lead.
  Silver.
  Copper.
  Gold.
  Arsenic.
  Uranium.
  Rhodium.
  Iridium, etc.

Common air was also discovered to have a magnetic action, and hot air
is more dia-magnetic than cold. Oxygen is as para-magnetic in the air
as iron is on the earth, and this, it was considered, may give rise to
magnetic storms, and account for the declination of the needle.

We may now proceed to consider the Mariner’s Compass. The compass, or
the mariner’s compass, is so common that it is scarcely necessary to
give a long description of it. Its history is unknown. The Chinese
seem to have been aware of its usefulness long before the western
nations adopted it. It was about the time of the Crusades that it was
brought into western prominence, but was not generally known till the
thirteenth century. Chinese writings ascribe to the compass a great
antiquity; they maintain that it was discovered two thousand five
hundred years before the Christian Era, and used for travelling on
land. But, according to other accounts, it was not used at sea till the
year 300 A.D.

The Chinese put the south first when speaking of the points of the
compass, and in the Chinese empire and Thibet west goes likewise before
east. So the imperial edifices in China face the south, and the needle,
in their expression, points south and north—not as we say, north and
south. The antiquity of the compass may be inferred from the recorded
fact in Chinese chronicles, written in the second century before the
Christian Era, that nine hundred years previously to the date of the
chronicle the Emperor gave magnetic cars to certain ambassadors to
guide them home in safety. These cars were fitted with a magnetic
needle which communicated with a figure. Its outstretched hand and
finger followed the compass-direction, and pointed out the way.

The Chinese subsequently (in the twelfth century) suspended the needle
by a thread, and it is said their philosophers at that time noticed
the variation of the needle. But Columbus first, in 1492, and Cabot,
in 1540, certainly remarked it in Europe. It is to Marco Polo that we
are indebted for the direct introduction of the needle into Europe,
although it probably had been in use in the Levant previously, for we
have seen a quotation by an Arab writer, who, in 1242, described the
needle as being used at that time on his voyage from Tripoli in Syria
to Alexandria, two years previously.

Friar Bacon possessed a loadstone, and there are many instances in
which it is referred to in ancient writings. The inventor of the
compass we cannot trace, but no doubt exists as to its being of Chinese
origin.

The ordinary compass is shown in the illustration herewith (fig. 265).
It consists of a magnetized needle, suspended freely, and fixed to a
circular card, which is divided and subdivided into thirty-two points,
as in the cut. This compass is suspended upon gimbals to keep it in
an upright position when the vessel rolls or plunges. The gimbals
are concentric rings, the compass being fastened to the inner one,
and keeps its position in all weathers. It is then enclosed in the
binnacle, a glass receptacle. The card moves with the needle which
points north. There is a dark line (lubber line) which indicates the
ship’s course, and when sailing the steersman must keep that line
opposite the compass direction-point which indicates the course. At
night a lamp is lighted in the binnacle, and the card being transparent
and the points opaque they are easily seen.

The magnetism of iron ships has a tendency to disturb the needle, and
many suggestions have been made and discussed with a view to obviate
this. To put the compass at the mast-head was one, to surround the
compass with “counter-irritants” another. But the usual way is to
“swing the ship,” and so adjust the compass. Swinging the ship means
turning her round point by point, and marking the deflection of the
needle with reference to a certain object. The amount of deflection at
each point is read and noted, and subsequently taken into consideration
when sailing.

The Azimuth Compass is a mariner’s compass fitted with brass uprights
slit through the centre, through which the heavenly bodies may be
seen. These are the _sights_. The card is divided into _degrees_ and
_quarters_. A fine wire is fixed upon one of the sights, and in the
other slit is a prism to reflect the divisions of the card to the eye.
The object—the azimuth distance of which it is desirable to know—is
looked at through the slit, and bisected by the wire. The divisions of
the scale are at the same time reflected, and the number read gives the
azimuth distance required.

[Illustration: Fig. 265.—Compass.]

The compass has led us away slightly from our consideration of the
electro-magnet, but we will now examine it and its effects as briefly
as possible.

An electro-magnet is formed by wrapping a copper wire round a piece
of soft iron shaped like a horse-shoe; the wire should be insulated
with silk. If the wire be wound round the iron in the same direction,
and a current be merely sent through the coil, it will be found that
the horse-shoe iron is highly magnetic, but if the current be stopped
the power is lost. Such magnets will carry weights much heavier than
themselves, and by careful consideration of certain laws, and with
reference to the number of coils and the strength of the current, these
magnets will sustain a weight some thousands of times greater than
their own weight.

[Illustration: Fig. 266.—Electro-Magnet.]

If we cover a non-magnetic piece of iron with a wire coil, and taking
a magnet turn it rapidly beneath the wire-bound iron, so that the
magnetic poles approach each other alternately, an electrical current
will be generated in the wire. The electro-magnetic machine is thus
made; but although strong currents may be generated as a source of
motive power it is a failure.

To Faraday our knowledge of magneto-electricity is due. “He knew” (says
Professor Tyndall in his interesting work, “Faraday as a Discoverer”)
“that under ordinary circumstances the presence of an electrified body
was sufficient to excite by induction an unelectrified body. He knew
that the wire which carried an electric current was an electrified
body, and still all attempts had failed to make it excite in other
wires a state similar to its own.”

But while he was making his experiments on the induction of electric
currents he noticed that at the time the current was passing from the
battery through the coils of wire that no motion was perceptible in
the galvanometer. But when the circuit was opened, and when it was
closed, there was a slight motion of the needle in the galvanometer,
but in different directions. After consideration the philosopher came
to the conclusion “that a battery current through one wire induced a
similar current through the other, but for an instant only.”

Œrsted had already demonstrated that all magnetic effects were
attributable to the attraction and repulsion of electric currents;
and founding his views upon the theory of Ampère, Faraday came to the
conclusion that electricity could be produced from magnetism, or that
the electric current could be obtained from magnets. This he succeeded
in doing. By inserting a steel magnet about half its length into a
coil of wire, Faraday induced a current to pass through the wire
in two directions. Thus he proceeded to solve all the mysteries of
magneto-electricity, and stated that to produce currents it was only
necessary to “cut appropriately the lines of magnetic force.”

The application of the magnet to the machines for electric lighting
will be shown further on. Very powerful currents are obtained by the
induction coil; but the currents would not be of practical service were
it not for the apparatus called a Commutator, or key, which reverses
the connection of the bobbins, and turns the current at every half
revolution. Just as if a current were being sent across and back over
a table, and when the current has reached the end, an instantaneous
_wheel round_, or pivoting of the table, _sends the current on_, in
continuation (but on the table all the time), because of the sudden
change of its position. The back rush being on the table, the movement
of the latter really makes the line continuous, and by quickly breaking
and reversing the current in the commutator, the effect is gained in
the machine.

[Illustration: Electro-magnets and bobbin, etc. (Clarke’s machine)]



CHAPTER XXII.

 SUNDRY ELECTRICAL APPLIANCES—MR. EDISON’S INVENTIONS—THE ELECTRIC
 LIGHT—THE GYROSCOPE—A NEW ELECTROPHORUS—ELECTRIC TOYS.


THE ELECTRO-MOTOGRAPH—although perhaps even yet scarcely
developed—has already proved a very useful invention. The idea of it
first occurred to Mr. Edison in 1873, when he was prosecuting some
researches in chemical telegraphy. “One day,” says Mr. Fox, in his
account of the invention, “as he sat pondering over his work, he
happened to take in hand the metallic point through which, as it rested
upon the paper, the current was wont to pass. When again he closed the
circuit to let the current through the paper, he held the metallic
point loosely, and unintentionally allowed it to rest upon the paper.
Every time he moved the metallic strip on the paper the latter became
wonderfully smooth. Edison was determined to find the reason of this,
and he decided that the electricity very much lessened the friction
of the metal on the paper. He made many experiments, and brought the
subject before the Royal Society in 1874, but nothing came of the idea
till 1876, when Edison was perfecting his musical telephone.

“The new appliance is, in fact, the same invention revived and now
perfected by the original inventor, and brought to complete practical
success under the title of the ‘electro-motograph.’ The action of the
‘electro-motograph’ depends on the fact, discovered during former
experiments, and employed imperfectly in the musical telephone, that
the friction of moving bodies varies in greater or less degree with
their electrical condition. In the electro-motograph a cylinder made
of prepared chalk, and saturated with a strong solution of caustic
alkali, is set upon supports, so that it can be turned upon its axis.
A strip of metal fastened to the mica diaphragm rests on the cylinder,
and is pressed so firmly by its spring upon the cylinder that when
it is turned by means of the handle the friction of the strip on the
cylinder tends to pull the diaphragm out of shape, causing it to bulge
inward as long as the cylinder is in motion. If now, while this motion
of the cylinder is maintained, an electric current passes through the
strip of metal, and then through the chalk cylinder to earth, the
amount of this friction is varied or it is destroyed altogether, and
the strip slides freely on the cylinder. This was the basis of the
former invention. The release from friction by a change in electric
condition in the first instrument failed simply from ignorance of some
slight matters of detail, that in the electro-motograph are corrected
and made practical. In the musical telephone the releasing of the
frictional resistance by electric action caused the sounding-board
of a guitar to vibrate, and thus set up sonorous vibrations. In the
electro-motograph the mica disc takes the place of the guitar, and,
by the improved construction of the apparatus, intricate and complex
vibrations, such as are produced in speaking, are reproduced in their
original or even in greater volume. When the apparatus is at rest
the diaphragm is motionless, and electric currents shot through the
apparatus produce no effect. In the same manner the mere turning of
the cylinder without electric action produces no effect, except to
pull the diaphragm slightly out of shape. If while the cylinder is
being turned an electric impulse arrives, the pull on the diaphragm,
caused by the friction of the strip on the cylinder, is more or less
released, and the diaphragm is free to vibrate or spring back into its
original condition. If now, the electric impulses follow one another in
regular order in correspondence with the sonorous vibrations imparted
to the transmitting telephone, the alternate slipping and catching of
the metal strip on the cylinder will follow in the same order, and
thus the diaphragm will be made to vibrate in unison with the original
vibrations, and thus reproduce the original words. As the mica disc is
much larger than the disc of the transmitting instrument, the amplitude
of its swing may be much greater, and consequently it will repeat the
words with greater power. The electro-motograph is practically an
apparatus for transforming electric action received from a distance
into mechanical work. The amount of electric action has nothing to do
with the amount of the mechanical work performed, because the movement
of the cylinder is controlled by power independently of the electric
action, the electricity merely releasing this power by destroying the
friction in greater or less degree. The electric action set up by the
sonorous vibrations at the transmitting end of the line may be very
slight, while the mechanical action at the distant end may be powerful,
and in this manner the amplitude of the vibrations may be increased to
an indefinite extent, and a whisper may reappear as a loud shout.

“The electro-motograph is not only a solution of the telephone,
making it capable of sounds of every quality and pitch and in greatly
increased volume, but by this conversion of electrical action into
mechanical work at a distance makes it possible to unite the telephone
and phonograph. Telephonic messages by the electro-motograph may
be impressed upon a self-acting (clock-work) phonograph, the same
current starting and stopping the phonograph after the manner of the
stock-reporting machines, and afterward the phonograph may be made to
repeat the message impressed upon it.”

[The above extract, which explains the principle fully, has been
taken from a long article on the subject which formerly appeared in
_Scribner’s Magazine_.]

The uses to which the electro-motograph may be applied are various.
It can produce mechanical motion even at a distance, and is useful to
lessen friction by machinery; and in this way its service to railways
and other locomotive systems may be estimated. It is a great help to
telegraphy by increasing the speed of transmission, and can ascertain
the beatings of the heart of the apparently dead. It amplifies sound in
a much greater degree than the microphone, by which even a fly can be
heard moving. In fact, the limit of the usefulness of this wonderful
machine has not been reached.

Another very ingenious apparatus has been developed by Professor Bell.
This is for the purpose of ascertaining the position of bullets in the
body. The following is condensed from the _Times_:—

“Two conductors are used, and the ball completes the circuit. Professor
Bell inserts a fine needle in the suspected region. It is connected by
wire with one of the binding screws of a telephone, which the surgeon
holds to his ear; the other binding screw being connected with a
metallic mass applied to the skin. When the needle point touches the
ball, an electric couple is formed, and the current generates the sound
in the telephone. The surgeon may then use his knife with confidence,
guided by the needle. He may make several insertions of the needle if
necessary without danger, and any pain may be obviated by etherization.
This simple method (which should prove useful on the field of battle)
was tried with success with a lead ball introduced into a piece of
beef. Contact of the needle with bone had no effect, but a very
distinct sound was heard each time the ball was reached. A modification
consists in inserting a vibrator in the circuit; this gives a musical
note in the telephone at each contact of ball and needle. Again, if the
circuit include a battery, the telephone sounds may be heard by several
persons at once. A sound is heard, in this case, whenever the needle
enters the skin; but, on reaching the ball, it is much intensified,
owing to lessened resistance. A galvanometer may be used in place of
the telephone.”

Mr. C. Vernon Boys has exhibited and described a very ingenious
new integrating machine of his invention, and its application as a
measurer of the electric energy in the circuit of an electric lamp
or a dynamo-electric motor. Mr. Boys’ mechanical integrator belongs
to the class termed tangent machines, and consists essentially of a
small disc or wheel running along the surface of a drum or cylinder.
When the wheel runs straight along the drum parallel to its axis
there is no rotation of the latter, but when the wheel is inclined
to the axis the drum rotates, and the integral is represented by the
amount of rotation. Continuous action is secured in giving the drum
a reciprocating motion along its axis, so that when the wheel has
travelled to one end of the cylinder it can travel back again. The
new integrator is especially adapted for measuring forces which are
either delicate or variable. It is applied by causing the varying
force to be measured to vary in a corresponding manner the inclination
of the wheel to the axis of the rotating cylinder. In this way it
can be used to find the work done by a fluid pressure reciprocating
engine, or the energy transmitted by a shaft or belt from one part of
a factory to another. By making the wheel very small and light, the
strength of an electric current may be continuously measured, if the
disc is inclined by means of the needle of a galvanometer in circuit.
Mr. Boys has constructed on the same principle an electric energy
meter, which integrates the product of the strength of current and
the difference of potential between two points with respect to time.
In it the current is passed through a pair of concentric solenoids or
coils of wire, and in the annular space between these is hung a third
solenoid, the upper half of which is wound in the opposite direction
to its lower half. By the use of what Mr. Boys calls “induction traps”
of soft iron, the magnetic force is confined to a small portion of
the suspended solenoid, and by this means the attracting force of
the fixed solenoids upon it is independent of position. The middle
solenoid is hung from the end of a balance beam, and its motion is
retarded by a counterweight, which admits of regulating the meter to
give standard measure as a clock gives standard time. The motion of the
beam is caused to incline the integrating wheel, and the rotation of
the cylinder gives the energy expended in foot-pounds by means of an
indicator or diagram, as the case may be. The object in giving an equal
number of turns in opposite directions to the suspended solenoid is to
render the instrument insensible to external magnetic forces.

We have, in a former portion of this work, explained the construction
of the telephone and phonograph with other inventions to make sounds
audible at a distance, so we need not repeat the explanations
here. A brief reference to them will, however, be found in this
chapter, in which the electro-magnet and the methods of lighting by
magneto-electric machines are treated of. We will proceed to give some
particulars concerning the electric light before considering the means
by which it is produced, as such an arrangement is more convenient.

The light is very easily produced by uniting and then separating the
terminals of a strong battery. The passage of the electric current
induces intense heat and a most brilliant light. But if this were
continued the wires would melt, and therefore some non-fusible
substance is placed at the ends of the wires, which will be at once a
conductor and infusible. Now in gas-carbon (the deposited substance
found in gas retorts) we have a substance suited to these conditions.
The carbon is heated to an intense brightness, and particles of it are
passed across the arc of flame almost in a state of fusion. Combustion
does not actually take place, because it has been proved that the wires
will give out light under water, and in the vacuum of an air-pump the
light is even increased, so that had the oxygen of the air any part in
the production of the light it would not remain unaffected under these
conditions. The heat arising from this Voltaic arc is intense, and
even platinum may be fused with the assistance of the gas carbon. The
carbon points are of course liable to be worn away, and one side more
than another. The positive pole is generally more concave than the
other, for it sheds its particles in a greater degree, and is the more
intensely heated. The electric light first appeared in public at the
opera in Paris in 1836, to illustrate a sunrise, but it was not till
1843 that it was experimented upon in the open air. We need not trace
it farther at present, for a full account of its origin, rise, and
progress is published in a small shilling volume by Messrs. Ward, Lock,
& Co. We will proceed to the methods of bringing out the light.

[Illustration: Fig. 267.—The Maxim light.]

[Illustration: Fig. 268.—Mechanism of Maxim’s lamp.]

There are various lamps, many of which required a regulator in
consequence of the wearing away of the carbon points, as already
explained. We append two illustrations of the Maxim lamp, the invention
of Hiram Maxim, of New York. In both cuts the letters refer to the same
portions.

In the first illustration (fig. 267), A and B are the positive and
negative carbon-holders respectively, and the carbon points are
controlled by an armature, which is, in its turn, adjusted by the
screw, D. When it happens that the magnetic force is reduced
the spring acts and permits the points to approach again, and the light
is rekindled; the carbons are then locked till required to move. The
second illustration (fig. 268) shows a section of the lamp with the
wheel arrangements for controlling the advance of the carbon points as
they waste away.

[Illustration: Fig. 269.—Wallace lamp]

[Illustration: Fig. 270.—Houston lamp.]

In the “Brush” light, which is in use in London, and is fitted for
large spaces, the carbon points are held by a regulator side by side,
and they last eight hours without renewal. The power is generated
by an electro-dynamic engine. We give illustrations of the lamps of
Wallace and Houston (figs. 269, 270). The current is conveyed through
_b_ and the magnet, _m_. The armature, _a_, separates the electrodes,
and the weakened current is restored by _b_, and the light continues.
The pillar, _p_, is hollow, with a wire running through it. The
positive electrode is supported by J, the negative by C; V is a button
which comes in contact with the lever, T, when the carbon points are
exhausted, and cuts the lamp out of the circuit by passing it direct
through mercury cups.

The Jablochkoff candle and chandelier are also represented (figs.
271, 272). The candles consist of carbons connected at the top, but
otherwise insulated, and fixed in a socket. They do not last very
long without renewal. The exhibition at the Crystal Palace will be
essentially an Electric Light Exhibition, and all the latest forms can
be studied there. The great attraction will doubtless be, as at Paris,
the varied and numerous inventions of Mr. Edison. The early career of
that American “magician” is now tolerably well known; his tremendous
energy and application are fully appreciated. With only a few months
schooling all his life he has taken a foremost place in the scientific
world. In ten years he has invented the phonograph, the electric pen,
a system of fast telegraphy, the electro-motograph, the telephone,
a tasimeter, and other useful applications of electricity, besides
solving the problem of electric light for domestic purposes.

Mr. Edison’s electric light[18] requires something more than a passing
notice, and we will therefore endeavour to give a sketch of the general
subject. Now that the electric light has been made available for
domestic purposes, and the very simple lamp (consisting of an exhausted
glass globe, two platinum wires, and a piece of charred paper) can be
obtained, people will no doubt soon largely adopt electric lighting
in their houses. The light has found a success at the theatre, in the
streets, and in the train; there is no reason why it should not be
adopted generally, being more economical and more healthy than gas.

[Illustration: Fig. 271. Electric candle.]

[Illustration: Fig. 272.—Chandelier.]

If we sever an electric wire, and bring the ends, tipped with carbon,
into juxtaposition, we obtain a brilliant light. This is the Voltaic
arc we have already mentioned, produced by the incandescence of
finely-divided matter; it was the first method of illuminating by
electricity, and was discovered by Sir Humphrey Davy, who obtained a
very brilliant light, but at great expense—about a guinea a minute! But
the Daniell and Grove batteries and generators, and modern improvements
in 1860, brought the use of the electric light into prominence. Faraday
lighted a lighthouse with its assistance.

But when the GRAMME GENERATOR was invented the needed impetus
was applied. The Jablochkoff candles followed, and now we have the
electric light in full operation. So far we have sketched the history
of illumination by the Voltaic arc, and descriptions of the various
apparatus will be found at the end of this chapter. But the method of
lighting with an incandescent solid was introduced in 1845 by Starr
and Peabody, who took out a patent for the use of platinum. Later on
Drs. Draper and Despretz made experiments with platinum and carbon.
The latter gentleman sealed the carbon in an exhausted globe, and then
introduced nitrogen in place of the air. But the method died out and
was forgotten, and in 1873 a medal was actually given by the Academy of
St. Petersburg for the “discovery” to Messrs. Sawyer and Mann.

In 1878 Paris was lighted with the electric candles of Jablochkoff.
This application of electricity stirred up our transatlantic cousins,
and Mr. Edison was requested—backed up by many influential persons—to
make the investigation whether the light could be produced for domestic
purposes. The celebrated electrician undertook the commission, and
certainly came unprejudiced to the encounter, for he had not at that
time even seen an electric light.

He perceived at once that “permanence in the lamp and the subdivision
of the light” were the two desiderata. He put the Voltaic arc aside
as unsuitable, and addressed himself to the problem of obtaining the
desired results from an incandescent solid. The subdivision of the
light is really an important point, and a comparison between divided
and undivided burners is in favour of the more diffused light in a
number of burners. This subdivision Edison worked hard to secure, and,
as it is said of him, “With a steadfast faith in the fulness of nature,
a profound conviction that if a new substance were demanded for the
carrying out of some beneficial project, that substance need only be
sought for, he set to work.”

Mr. Edison found difficulties in his way. One was the apparent
impossibility of illuminating by means of an incandescent solid, for
even platinum will melt at a heat too low for use. But this apparent
impossibility was overcome by the inventor’s genius. He, after many
trials, found that if he raised the platinum to a white heat _in a
vacuum_ he would practically obtain _a new metal_ which would sustain
the required heat.

[Illustration: Fig. 273.—Edison’s platinum lamp.]

“In making an electric lamp without a regulator,” says Mr. Upton,
“two things are essential,—great resistance in the wire, and a small
radiating surface. Mr. Edison sought to combine these two essential
conditions by using a considerable quantity of insulated platinum
wire wound like thread on a spool.” This platinum, as shown in the
accompanying cut (fig. 273), was suspended in a glass bulb _in vacuo_,
the air contained in it being expelled by electricity, heating it,
and suddenly cooling the platinum, and squeezing out the air by the
process. But, after all, the great difficulty of the inventor was to
insulate his wires so perfectly that they would not meet and become a
conductor. For, to perfect his lamp, this non-conducting principle was
a necessity, otherwise the current would flow across instead of going
all along the wires. He had previously made many uses of carbon, which
we know is infusible. He tried lampblack tar, but it contained air, and
would not do.

Thread answered his purpose, but was too fragile and uneven in texture.
It suddenly occurred to him that paper—_charred paper_—cut into a
thread-like form would satisfy all his conditions.

The problem was solved—the lamp was a fact. But how can paper, so
easily burned, answer? We will endeavour to explain. “A piece of
charred paper, cut into horse-shoe shape, so delicate that it looked
like a fine wire firmly clamped to the two ends of the conducting and
discharging wires, so as to form part of the electric circuit, proved
to be the long-sought combination.”

We will now explain the construction of this little lamp, which is
shown in the illustration (fig. 274) one-half of its actual size. The
illuminating is equal to ten or twelve ordinary gas jets.

[Illustration: Fig. 274.—Edison’s electric lamp.]

The manner in which the paper is prepared is, like many other very
important inventions, extremely simple, and, we may add, almost
costless. Cardboard will furnish us with the loops, and these
“horseshoes” are placed in layers in an iron box with tissue paper
between each. The box is then hermetically sealed, and made red hot.
The carbonized paper remains till all the air has been got rid of,
and although it will burn freely to ashes in atmospheric air, _in
the vacuum prepared for it_ it is never consumed. That is the plain
fact—the secret of the Edison lamp.

A vacuum can now be produced almost perfect. It is of course impossible
to extract every tiny particle of air from the globes, but by the
Sprengel pump, in which mercury is employed, excellent vacuums are
obtained. Several very curious phenomena have been observed in
these vacuums, and the Royal Society has been engaged upon their
consideration. Another advantage of the vacuum, as applied by Edison,
is that little or no heat is generated. The electricity is all, or very
nearly all, converted into light. Thus the glass globes remain almost
unheated, and are unbroken.

The electric current passes along the wire, W, and at a
certain place marked B, the copper is soldered to a platinum
wire, which enters at C, and so by platinum clamps into the
horse-shoe, L. The return wire is similarly arranged; the
carbon is enclosed in a glass bulb, GG, and all the air is
extracted by the pumps; the end is then sealed up by melting it at
F.

The world is now in possession of a lamp for household use, and we are
surprised that it is not more extensively adopted in England. There
are some Swan lamps used in parts of the British Museum, and when we
have explained the application of the light, and the uses to which the
motive power can be applied, we shall, we believe, convince the most
conservative gas bill advocate that Edison’s lamp is cheaper, safer,
and far better in illuminating power than gas, if the success of the
electric lamp can be assured.

We need not dwell upon the construction of the “pumping station,” for
that is virtually what the magneto-electric generator is. Several of
these stations can be established in various parts of the city, and
each station will supply a district with electricity. The wires are
laid in a tight box along the street, beneath the footpath, or other
convenient position, and we are informed that the frost rather improves
their electrical condition. Here is one advantage over gas.

From the main wires smaller ones enter the houses, and are carried
through a “meter” containing a safety valve. There are two wires—a
distributing wire and a waste—coloured, one red and the other green,
which communicate respectively with the main supply and return wires
to the “pumping station” or generator. The electricity is admitted
between carbon points and flows round a magnet, the armature of which
is held above it by a spring. If too much force be put on and any
danger incurred, the magnet will attract the armature, and the current
will cease. A snap connected by a small wire will then be closed by the
electricity, and melting from the heat will cut off all the current.
In ordinary circumstances the electricity passes through regulators
(wire wound on spools) and on to a copper plate, “through a solution
of copper salt.” Thus for every unit of current a certain quantity of
copper is deposited. A certain standard amount represents five cubic
feet, and the bills, based on the accumulation of copper, are made out
like gas bills.

When the lamp is required a small handle is turned, and is instantly
lighted; the reverse motion cuts off the current. “By touching a knob
in the bedroom the whole house can be simultaneously lighted up” if
desirable. No matches are necessary, as the lamps light themselves.

By adding a small electro-motor to the furniture of the house, and
turning a handle, the sewing-machine can be worked by electricity, or
lathes turned; and any business operations, such as lifting by cranes,
etc., can be easily carried on.

The Swan electric lamps, which, with Mr. Edison’s, were exhibited in
Paris, and will be found at Sydenham, give about twelve candle-power
light. Edison’s lamps are made in two sizes, and vary accordingly.
The Swan lamps give a very soft light, and are as easily manipulated
as Edison’s. The Siemens system of lighting was also well seen in
Paris, and the Faure storage system enables our trains to be lighted
instantaneously by simply turning a handle. A full description of the
_Faure_ battery was given in the _Times_ by Sir William Thomson, and
in his address to the British Association at York in September last.
He pointed out that in the accumulators of M. Faure,—which can be seen
at 446, West Strand, London,—by means of a large battery it is quite
easy to draw off electricity and to apply it as Edison proposed to do,
in lighting our houses and do any little service. The electricity thus
stored would be always ready for use, and would be supplied and paid
for. It can be applied to any purpose, and locomotion by its means
will ere long become more general. In Paris Dr. Siemens exhibited
his electric tramway. This was an improvement upon the first Berlin
tramway, for in it the horses frequently received shocks which they
resented. In the later application the current comes from the generator
by metal rods carried above the heads of the passengers alongside the
line. Little rollers upon these are united with an electric machine in
the tram-car. The current is sent along the wires, and reconverted into
mechanical energy in the second machine, turns the wheels of the cars.
In this way, as the car proceeds, the rollers overhead or alongside the
track are kept moving by the car, and the connection is never broken.

But this is a digression. The electric light as applied to lighthouses
was also exhibited, and any reader desirous to obtain full information
upon the subject of lights and lighthouses will find it in a very
pleasantly-written work by Mr. Thomas Stevenson, in which the various
systems of lighting by electricity and otherwise are fully recounted,
the conclusion being in favour of electricity, which is employed and
has been used for years in France and in some English beacons. If its
penetrative power can be finally established,—for some authorities
maintain that the electric is more easily absorbed by fog than other
light,—there is no doubt about its being universally adopted.

It is very interesting to watch the uses to which the electric light
is being put. The latest experiment has been made by an Austrian,
Doctor Mikerliez. Almost incredible as it may seem, the interior of the
human stomach can now be illuminated by means of a wonderful little
instrument called the Gastroscope, which is said to be actually in
use and to have been favourably reported upon by the medical faculty
of Vienna. There is at the end of a jointed flexible tube (which can
be passed down the gullet) a miniature lamp, far more marvellous and
mysterious than that of Aladdin, in which a strip of platinum is fixed
and connected with fine wires conducting the electricity from a small
battery. When contact is made, and the “light turned on,” the cavernous
interior of the stomach is lit up. Still more extraordinary is the
fact that the tube can be made to revolve, and the light reflected
from the walls of the stomach and directed to the eye of the observer.
There is necessarily a bend in the instrument, so that the light has
literally to turn a corner before it reaches the surgeon’s eye; here
the inventor’s skill and thorough knowledge of the laws of optics are
brought into requisition. The reflected rays of light fall upon a sort
of window situated a little above the lantern, and by means of prisms
and a series of lenses, the light is twisted and turned about until it
arrives at the eye-piece. No sensation of heat is to be feared, the
little lamp being kept constantly cool by a reservoir of water.

Several contrivances have been invented within the last few years for
examining the interior of the body, but they are very costly; the
Gastroscope is likely to render great service to medical science.

The term “magneto-electric machine” is given to a collection of parts
of mechanism intended to create or gather together induced electric
currents. The invention of the magneto-electric machine was by no
means a sudden inspiration, but the gradual result of a series of
experiments and discoveries, the first of which, dating from 1820,
may be said to be Œrsted’s observation, that a magnetised needle is
deflected by the approach of an electric current as well as by that
of a magnet, clearly proving that magnetism and electricity have some
relation to one another. In the same year Arago discovered that a coil
of insulated wire wound round a core of soft iron, converts it into
a powerful magnet (_i.e._, an electro-magnet) when a current passes
through the coil. It was in 1830, however, that our countryman Faraday
proved the creation of a current by the action of a magnet on a coil of
wire, and his experiment proved shortly as follows:—If a coil of wire
be wound on a hollow core, and a permanent bar magnet be introduced
into the hollow core, whilst introducing it a current may be proved
(by a galvanometer), to be induced in the coil flowing in a certain
direction, A B, which ceases as soon as the magnet is at rest in the
centre of coil. On the withdrawal of the magnet a second current is
induced flowing in the opposite direction, B A. Therefore it is clear
that if a magnet be incessantly approached to and withdrawn from a coil
of wire a constant succession of currents will be produced, and if a
charged coil (_i.e._, a coil connected with the poles of a voltaic
battery) take the place of the magnet a precisely similar result
will be obtained. Now it will have been noticed that two opposite
currents are constantly being formed, and as the object is to obtain a
continuous flow of electricity in _one given direction_, or, in fact,
divert or reverse the current instantly on its formation to make it
practically the same current, for this purpose a commutator is used,
and as for most purposes a commutator is one of the essentials of a
magneto-electric machine, we will here give a description thereof.
(_See_ fig. 275.) The machine is composed of a cylinder, consisting of
two metallic conducting halves, separated by a non-conducting layer.
Whilst it is at rest the alternating currents, from being connected
with the halves by the current, will pass to the two contact springs,
and thence through the circuit. Now if (as is the case) the current
is constantly changing, as has been noticed, the inverse current will
at the first change pass through the same channels, but in another
direction; but if at the instant of the reversal of the current the
cylinder be revolved, the current flowing the reverse way will be
guided through other channels respectively, instead of the original
channels, and the direction of the current being changed at the same
moment as the current itself, the two inversions neutralize themselves,
and one constant current is produced. In a magneto-electric machine
the commutator revolves identically with the magnet or armature, and
the point at which sparks are being constantly produced is where
the contact is being continually broken and made by the passage of
the friction springs from over the non-conducting layer. The first
machine formed on the basis of Faraday’s experiments was Pixii’s. It
was composed of two uprights and a cross bar, to which is attached,
hanging poles downwards, an electro-magnet; underneath this, the
poles upwards, revolves a magnet. The commutator is fixed on the same
axle and revolves with the permanent magnet. Saxton, and subsequently
Clarke, made the obvious improvement of making the magnet less cumbrous
and fixed, and causing the bobbins of the electro-magnet to revolve
before or rather beside its poles; the commutator was fixed at the end
of the axle on which the revolving bobbins (or armature) are fixed.
Niaudat formed a compound Clarke machine, by setting two horse-shoe
magnets a short distance apart. The armature revolves between them,
and consists of twelve coils set between two plates; the coils are set
alternately and connected,—_i.e._, the poles of the electro-magnets
are set beside one another,—N. to S., S. to N., and so on, so that the
N. pole receding produces a current; _but_ the N. pole receding makes
the S. pole approach, and produces another current, A B; in fact, a
continuation of the same, for the _approach_ of a N. pole naturally
produces the same current as the recession of a S. pole; then as the
S. pole in turn recedes it produces an inverse current, B A, which is
in turn kept up by the approach of the next N. pole, and so on. Each
coil is attached to a radiating metal bar, which conveys the current to
be redirected to the commutator, which is affixed to the axle of the
revolving armature as in Clarke’s machine. In 1854 Siemens completed
his machine, the chief peculiarity of which was its cylindrical bobbin;
the core is grooved deeply, parallel with its axis, and the wire is
wound on cylindrically and covered with plates of brass; one end of
the coil is fixed to the metal axis, the other to an insulated ferule
at the end of the axis, where is also situate the commutator. This
armature revolves between the poles by which it is closely embraced.
One of the most celebrated of the magneto-electrical machines is that
known as the “Alliance,” invented by Nollet, and perfected by Van
Malderen. It is composed of four or six bronze discs, revolving on an
axle, round the external circumference of each of which are set sixteen
bobbins. This rotating compound armature revolves between four to six
sets of horse-shoe magnets, which, being fixed radially to the centre,
present in each set sixteen poles to the sixteen bobbins. It will be
readily understood that this immense quantity of poles and bobbins
produces a highly concentrated current, the ends of which proceed from
the axle and an insulated ferule at its extremity.

[Illustration: Fig. 275.—The Wallace Machine.]

In 1869 Mr. Holmes perfected his machine, which differs from all
previous ones (except Pixii’s), in that the electro-magnets revolve in
front of the coils instead of _vice versâ_; and besides magnetising
his electro-magnets with part of the self-produced electricity, his
bobbins are so disposed as to be able to keep several independent
lights going at once. The Wylde machine consists, as it were, of two
Siemens machines, one on the top of the other, the lower and larger
of which is worked by an electro-magnet, which is magnetised by the
action of the upper or smaller one, consisting in the ordinary way of
a permanent magnet apparatus, which is termed “the exciting machine.”
The longitudinal bobbin revolved between these permanent poles produces
alternating currents, which are commutated (or redirected), and
pass to work the larger and lower electro-magnet, which is composed
of two large sheets of iron connected by a plate (on which stands
the exciter). Its poles are two masses of iron separated by a layer
of copper, and in this armature revolves the larger longitudinal
bobbin. This lower machine is called the generator. Both bobbins
are simultaneously revolved, and an intense current of electricity
is thereby generated. Almost simultaneously with this one Mr. Ladd
invented his machine, which is distinguished from all hitherto
described by being composed of two parallel _bar_ electro-magnets,
between the extremities of which are placed two Siemens armatures, one
smaller than the other; both being revolved, the smaller excites the
electro-magnets, and the larger generates the electricity required.
The wire is wound round the magnets so that the N. and S. poles face
each other at each end. The chief advantage of the Ladd machine is the
conversion of dynamic force into electricity, there always being just
sufficient magnetism in an iron bar (by induction from terrestrial
magnetism and other causes) to produce a very feeble current in the
Siemens bobbin, and the bobbin taking it up and returning it to the
electro-magnet, and the electro-magnet at once giving it back to the
bobbin, the current gradually increases till the maximum is reached.
And when we take into consideration this modicum of utilisable
terrestrial magnetism, we may truly say in the words of M. Hippolyte
Fontaine, “The mind is lost in contemplation of the succession of
discoveries completing one another, and showing that with apparatus of
small dimensions an infinite source of electricity could be produced
if matter could withstand infinite velocities.” The Lontin machine,
which supplied the current for the electric light which used to make
night bright outside the Gaiety, is also composed of two parts, one
dividing, the other generating the electricity produced. The principle
of the dividing machine is somewhat similar to the alliance, excepting
that a number of electro-magnets arranged radially round a core,
revolve close to a corresponding number of bobbins fixed inside an iron
cylinder, outside which is the collecting and dividing apparatus. The
Maxim machine is constructed on the principle of sets of coils rotating
between powerful electro-magnets. The Wallace machine was invented by
the inventor of the Wallace-Farmer lamp. It consists of two horse-shoe
electro-magnets placed side by side, the opposing poles facing each
other. Each magnet has a rotating armature of twenty-five bobbins,
on which the wire is wound quadruply, and the current generated by
these coils is conducted away, passing through and exciting the
electro-magnets, thus utilizing the residual and terrestrial magnetism
before mentioned in connection with the Ladd machine; otherwise it
partakes of the nature of the Niaudet machine.

[Illustration: Fig. 276.—The Gramme Machine.]

We now come to what is perhaps the most perfect magneto-electric
machine, which was first constructed by M. Gramme, a Parisian, in 1872,
and differs in principle and construction from all those hitherto
noticed. Its essential characteristic is a soft iron ring, round which
is coiled one single continuous wire (_i.e._, the two ends are joined).
Round the exterior surface of the wire coil a band is bared, and on
this bared part two friction springs act. If the ring and coil be
placed before the poles of a magnet, the ring will have two poles, S.
and N., induced opposite the opposing poles N. and S. of the magnet;
and if the ring revolve the poles will remain stationary, and as the
coil revolves each coil of the wire will pass this induced pole, and
as naturally half the coil will be inducted with one current, the
other half (acted on by the other pole) will be charged with another
or opposite current, which two kinds of electricity are carried away
by the friction springs before mentioned. In the machine, as actually
constructed, the soft iron ring is composed like the magnet or wire
bundle of an induction coil, and the coils are set upon it side by
side. Inside the ring are radially set insulated pieces, to each of
which is attached the issuing end of one and the entering end of
another bobbin; these answer the same purpose as the denudation of the
external layer of wire. These pieces are bent so as to come out of the
centre of the ring at right angles, and lay side by side (insulated)
round a small cylinder. These, as they revolve, are touched by friction
springs, which draw off the electricity induced in the coils in one
continuous current. No sparks are produced at the contact of the
friction springs, and there is no tendency to become heated. To obviate
the inconvenience of the secondary or inverted current produced by
the stopping of the machine, the inventor has contrived a circuit
breaker on the principle of the electro-magnet, the magnets holding
the circuit breaker in contact so long as the machine is working; but
the decrease of velocity lessening the attractive power of the magnet,
the circuit breaker opens by its own weight (or a counter-weight), and
all danger of a reverse current is obviated. Experimental machines are
manufactured by Bréquet & Cie (Paris), composed of Jamin’s magnets, and
turned with a handle, and produce a force of eight Bunsen cells.

A great revolution, or rather the beginning of a new era in the history
of electricity, may be said to have commenced with the perfection of M.
Faure’s accumulators. These are troughs containing eleven lead plates,
each coated with oxide of lead and wrapped in felt, the fluid being
dilute sulphuric acid. The application of them to the electric light is
one of their most valuable features; at the depôt in the Strand, where
they may be seen at work, there are thirty such elements, each weighing
about 50 lbs. It takes a two-horse-power engine working an Edison or
Gramme machine six to eight hours to charge them, and when charged they
will keep almost any number of lamps of sixteen-candle-power going some
eight hours. They are used on the Brighton and South Coast Railway,
and seem peculiarly adapted to lighting by incandescence, by Swan, or
Edison’s lamp. The elements fully charged may be carried any distance
without losing their electric power. And the stored force may be used
for charging the accumulators themselves afresh from the machine. These
accumulators may be seen any day at 446, Strand, and are well worth a
visit.

The Gyroscope, though now an instrument common and familiar to all
students, is none the less the subject of a problem, the solution
of which is still to seek. It has indeed been entitled the paradox
of mechanics; for though it depends on gravitation, gravitation yet
appears indifferent to it. In order to render the operation of the
Gyroscope as continuous as possible, so as to facilitate the profound
study of its working, and also to unite another influence with those of
the ordinary Gyroscope, producing phenomena of which this instrument
affords us the spectacle, a learned American has employed electricity
as a motive power.

[Illustration: Fig. 277.—The Gyroscope.]

The Gyroscope, shown in fig. 277, has a large, heavy pedestal, with a
pointed column, which supports the instrument itself. The frame, of
which the electro-magnets form a part, is connected with a rod, having
at one end a hollow cavity which rests on the point of the vertical
column. One of the extremities of the magnetic spool is attached to
this cavity, the other end communicating with the bar which unites
the two magnets. Over this bar is a spring which breaks the current,
supported by an insulator in hard india-rubber; it is adjusted so that
it touches a small cylinder on the axis of the wheel twice during every
rotation of the latter. The wheel’s plane of rotation is at right
angles with the magnets, and it carries an armature of soft iron, which
rotates close to the magnet without touching it. The armature is so
placed in relation to the surface of contact with the cylinder that
breaks the current, that twice during each rotation, as the armature
approaches the magnets, it is attracted; but immediately afterwards,
as the armature comes directly in front of the magnets, the current
is broken, and the acquired impulsion is sufficient to move the wheel
until the armature comes again under the influence of the magnet. The
spring which interrupts the current is connected with a thin copper
wire, which stretches back as far as the point of the column, entwining
it several times to render it flexible, finally bending down and
plunging into some mercury enclosed in a round vulcanite cup placed on
the column near the pedestal. The pedestal also bears two small stakes
for receiving the wires of the battery, one connected with the column,
and the other communicating by a small wire with the mercury contained
in the vulcanite vessel. The magnets, the wheel, and all the connected
parts can move in any direction round the point of the column. When
two large Bunsen cells, or four small ones, are connected with the
Gyroscope, the wheel turns with great rapidity, and allowing the
magnets to operate, it not only sustains itself, but also the magnets
and the other objects which are between it and the point of the column
in opposition to the laws of gravitation. The wheel, besides turning
rapidly round its axis, also effects a slow rotation round the column
in the direction of the movement experienced by the _lower part_ of the
wheel. By placing the arm and the counterpoise of the machine as shown
in fig. 277, so that the wheel and the magnets balance exactly on the
pointed column, the whole machine rests stationary; but if we give the
preponderance to the wheel and the magnets, the apparatus begins to
rotate in a direction contrary to or following that of the _upper part_
of the wheel.

The Gyroscope exemplifies very clearly the persistence with which a
body undergoing a movement of rotation maintains itself in the plane of
its rotation in spite of gravitation. It shows also the result of the
combined action of two forces tending to produce rotations round two
axes which are separate, but situated in the same plane. The rotation
of the wheel round its axis, produced, in the present instance, by
the electro-magnet, and the tendency of the wheel to fall or turn in
a vertical plane, parallel to its axis, produce, as a result, the
rotation of the entire instrument round a new axis which coincides with
the column.


PEIFFER’S ELECTROPHORUS.

It will now perhaps interest our readers to describe a charming little
plaything which is a great favourite with children, and which has the
incontestable merit of early initiating them into all the principal
phenomena of the statics of electricity, and teaching them the science
of physics in an amusing form.

It is a small electrophorus invented by M. J. Peiffer, and reduced
to such a point of simplicity, that it consists merely of a thin
plate of ebonite, about the size of a large sheet of letter paper.
The tinned wooden disc of the electrophorus which is found described
in most treatises on physics, is replaced by a small sheet of tin,
about the size of a playing-card, fastened on to the surface of the
ebonite. The ebonite electrophorus produces electricity with remarkable
facility. It must be placed flat on a wooden table, and thoroughly
rubbed with the hand; if it is then lifted, and the sheet of tin
lightly touched, a spark is elicited from ¼ inch to ½ inch in
length. The electrophorus is completed by the addition of a number
of small accessories in the shape of small dolls made of elder-wood,
which exhibit in a very amusing manner the phenomena of attraction and
repulsion. After the board has been charged with electricity, place the
three little figures on the sheet of tin, and lift up the apparatus,
so as to isolate it from any support. You will then see one little doll
extending its arms, another with its silky hair standing on end, and a
third, lighter than the others, leaping like a clown, and displacing as
he does so the two small balls of elder-wood which have been placed on
each side of him. We have given an illustration of the three figures
performing at once (fig. 278), but they are generally used separately.
M. Peiffer has indeed collected every known accessory for an electric
machine, such as Geissler’s tube, the electric carillon, etc. These
different experiments are all reduced to their simplest form, and, with
their appliances, are all contained in a cardboard box. They are placed
beside the electrophorus, which thus takes the place of an unwieldy
electric machine. M. J. Peiffer accompanies this little portable
cabinet with an exhaustive pamphlet, which is a valuable guide to the
young physicist in studying the first principles of electricity.

[Illustration: Fig. 278.—M. J. Peiffer’s electrophorus with dolls.]

“It is easy to discover in the education of children,” says M.
Peiffer in his preface, “how to turn their budding faculties to the
best account. Would you utilize them in a satisfactory manner?—Then
put in their hands playthings which, in an attractive form, serve to
familiarize them at an early age with those sciences, a knowledge of
which will be at a later period absolutely indispensable to them; and
they will be much more amused than with ordinary commonplace toys.”

These are sensible words, in which we heartily concur. Yes! Science
properly taught, and properly understood, can indeed be brought within
the range of children; it should give a lasting interest to all
amusements, and form a part of the culture of the youthful mind, as at
a later period it will contribute to the perfect development of the
grown man.


MAGIC FISH.

An ingenious physicist, M. de. Combettes, who is a civil engineer at
Paris, has devoted himself to constructing a number of playthings and
scientific appliances for young people, among which we will describe
the very curious one represented in fig. 279. A jar is filled with
water, holding in suspension some fish made of tin, similar to those
which children put in water and attract with a magnet. In this case,
however, the mechanism is hidden, and the operator can turn the fish
first in one direction and then in the other at pleasure. The secret of
this experiment is easily explained by examining the illustration (fig.
279). In the wooden stand which supports the jar there is concealed
a small electro-magnet which acts on the soft iron contained in the
floating fish. When the current passes the small magnet turns round and
attracts the little fish swimming in the water. This gyratory movement
can be changed at pleasure by means of a regulator.

[Illustration: Fig. 279.—Experiment of magic fish set in motion by
electricity.]

[Illustration: Fig. 280.—An electric toy.]

We will give an illustration of a few electric toys which M. Trouvé
has found for us. In the picture (fig. 281) we see three different
objects,—a rabbit beating a small bell, a representation of a bird with
outstretched wings, and a pin surmounted by a skull. All these are
capable of having movement imparted to them by means of electricity,
although made and intended for ordinary use in the form of scarf-pin or
other ornament.

Let us take the “death’s head” pin first. It is in gold, and enamelled
with diamond eyes and articulated jaws. The rabbit is also gold, and
carries two small drumsticks, with which he can play a tiny bell. This
device also can be worn as a scarf-pin.

[Illustration: Fig. 281.—Magic toys.]

A conducting wire leads from the pin into the waistcoat pocket, where a
small “pile,” about the size of a cigarette, is hidden away. If any one
particularly admires the scarf-pin, all you have to do is to insinuate
your fingers into your pocket, and you will, by contact, cause the
electric current to act upon the pin in your scarf. The death’s head
will at once begin to roll its eyes and grind its teeth, while the
rabbit, under similar circumstances, will begin to play its bell with
the greatest energy.

The handsome diamond bird represented in the centre of the illustration
belongs to Madame de Metternich. When any lady wears it in her hair,
she can, by the concealed wire, make it flap its jewelled wings, and by
so doing cause much surprise amongst the spectators.

We will now endeavour to give a description of the manner in which
these toys play their parts in company with the “hermetic-pile” which
M. Trouvé has applied to many specialities that he has supplied to
doctors, who use them largely.

This pile is formed by a “couple” of carbon and zinc hermetically
enclosed in an ebonite box. The carbon and zinc only occupy one-half
of the case. The liquid occupies the other. The sketch (fig. 280) on
preceding page will explain the apparatus.

So long as the case is in its normal position the elements are not
immersed in the solution, and consequently no electricity is developed.
But as soon as the figure is placed in a horizontal or leaning position
the force is generated; on readjusting the box the electric current is
cut off, and all development ceases. Many curious electrical toys can
be seen in Paris. Dolls are made to talk, and many other wonders for
children can be easily procured.


ANIMAL AND ATMOSPHERIC ELECTRICITY.

Before concluding the subject of electricity we must devote a few pages
to the consideration of the electric influence possessed by certain
fishes, and to some of the phenomena of the atmosphere, especially
thunderstorms. We have seen how Galvani experimented upon the limbs of
frogs, and maintained that they possessed electricity; he attributed
the current in the muscles to that cause. This theory Volta denied,
but subsequently Nobili, in 1827, proved the existence of a current
in the frog by means of a Galvanometer. This was conclusive, and the
experiment was performed in the following manner:—He filled two vessels
with salt and water, and into one he dipped the crucal muscles of a
frog, and in the other the lumbar nerves were immersed. By putting
these vessels in communication with his improved Galvanometer, which
was extremely sensitive, he perceived a current passing from the feet
towards the head of the animal.

It is, however, to Matteucci and Du Bois Reymond that the investigation
of the phenomena of the _courant propre_ are due. The former formed a
“pile” of the thighs of frogs, and by placing the interior and exterior
muscles in contact he formed a current from the inside to the outside
muscles. This current is supposed to be occasioned by certain chemical
changes which are continually taking place, and it continues longer in
the case of a cold-blooded animal than in a warm-blooded one. There
are many interesting papers on this subject included amongst the
“Philosophical Transactions”; and the “Physical Phenomena of Living
Beings” is fully treated in Matteucci’s lectures on that subject. In
the “Transactions” for 1848 and subsequent years, other experiments
may be perused, but space will not permit us to dilate upon them. The
fact has been established, and we are told that muscles and nerves,
as well as certain glands of the body, possess certain electrical
properties.

The electricity of fishes, and the power possessed by the torpedo—whose
name is now chiefly known in connection with warlike appliances—and the
gymnotus, have been known for a very long time. This fish, popularly
known as the electric eel, inhabits the warm fresh-water lakes of
Africa, Asia, and America. A specimen was exhibited at the Polytechnic
some years ago. This was the fish experimented on at the Adelaide
Gallery by Professor Faraday, who clearly demonstrated the fact that
the electricity of the animal and the common electricity are identical.
Numerous experiments were made, and the circuit shock and even sparks
were obtained from the gymnotus. In fact, the gymnotus is a natural
electric machine. The force of the shock given by the electric eel is
very great, for Faraday has put on record that a single discharge of
the eel is equal to fifteen Leyden jars charged as highly as possible.
Its power does not even end there, for having shocked people to that
extent, it was capable of a second and occasionally of a third shock of
less violence.

[Illustration: Fig. 282.—Electric eel.]

The manner in which the gymnotus acts is from a regular battery in
the head, the sides of which are filled with a fluid. These cells are
something like a honeycomb in appearance. The shock is quite voluntary
on the part of the fish. Sometimes it will kill its prey, on other
occasions it is merely numbed. Professor Faraday on one occasion placed
a live fish in the tub with the gymnotus, which curled itself so as to
enclose the unsuspecting one. In a second the prey was struck dead, and
floated on the water. The gymnotus immediately devoured it, and went in
quest of more. Another, but an injured fish, was then introduced, but
the electric eel took no trouble about this one. It did not trouble to
give it a shock, seeing it was disabled, it merely swallowed without
killing it. It is also on record that on one occasion an electric eel
had stunned a fish which, before he began to eat it, gave signs of
returning animation; the eel immediately gave it another shock and
killed it.

[Illustration: Fig. 283.—Large gymnotus.]

There were some other curious peculiarities connected with the electric
eel. It appears to be quite capable of discriminating between animate
and inanimate touch. For instance, when touched with a glass rod it
at first gave signs of electricity, and discharged a shock at the
attacking party. But on subsequent occasions, when touched with metal
rods or glass, the fish declined to “shock”; nevertheless the Professor
succeeded the moment he touched the animal with his hands.

The torpedo is something like the well-known skate; it is sometimes
called the electric ray, and is common enough in the Bay of Biscay and
in the Mediterranean Sea. It sometimes pays England a visit, or is
caught by fishermen and brought in. We have seen one at Plymouth, and a
very ugly-looking fish it was. Its electric power is considerable.

[Illustration: Fig. 284.—Ray torpedo.

_c_, brain; _m_ _e_, spinal chord; _o_, eye; _e_, electric organs; _b_,
gills; _np_, _nl_, nerves; _n_, spinal nerve.]

There is yet another fish known as the malapterurus; one species is
called the thunder-fish. Professor Wilson has written a paper upon the
electric fishes as applied to the remedy of disease, and considers them
the “earliest electric machines ever known.”

Humboldt relates that the South-American Indians capture the gymnotus
by driving horses into ponds which the electric eels are known to
inhabit. The result is that the fish deliver shock after shock upon the
unfortunate quadrupeds. Mules and horses have frequently been killed by
these powerful eels, and even Faraday experienced a very great shock
when he touched the head and tail of the captive gymnotus with either
hand.

The malapterurus to which we have referred is an inhabitant of the
African rivers, chiefly in the Nile and Senegal. Such a fish has been
known with others for some hundreds of years; its electrical powers are
not great. There are one or two other species of fish which possess
electrical qualities, but none apparently to the same extent as the
torpedo and the gymnotus.

[Illustration: Fig. 285.—The Malapterurus.]

The electricity of plants also is in some cases very marked. Flashes
have been seen to come from some flowers in hot and dry weather.
Currents of electricity have been detected, and Wartmann investigated
the subject closely. He says the currents in flowers are feeble, but in
succulent fruits and some kinds of grain they are very marked. These
currents depend upon the season, and are greatest in the spring,
when the plant is bathed in sap. These experiences were confirmed
by Bequerel in 1850, and he concludes that the rank vegetation in
some parts of the world must exercise considerable influence on the
electric phenomena of the atmosphere. M. Buff has more recently made
experiments in this direction, and he examined plants and trees, and
even mushrooms. M. de la Rive, after carefully summing up the various
theories, comes to the conclusion that it is to chemical reactions that
the traces of electricity are due.

The subject of atmospherical electricity properly belongs to
meteorology, and under that heading we will treat of it more fully. But
lightning is so identified with electricity, and being the most common
form observable to all, we will say something about thunderstorms and
the electric discharges accompanying them.

[Illustration: Fig. 286.—Benjamin Franklin.]

Before Franklin’s ever-memorable experiment with his kite established
the identity of lightning and electricity, the resemblance between the
two discharges had been frequently noticed. The Etruscans pretended
to bring down lightning from heaven, and Tullus Hostilius, when
experimenting or performing certain “ceremonies,” was killed by the
electric discharge he desired to attract. But after all, we cannot
attribute any knowledge of electric science to the ancients, although
they were, of course, familiar with electric phenomena.

It is to Dr. Wall that testimony points as the first person who
remarked the analogy between the electric spark and lightning. This was
in 1708. Grey and other philosophers supported the theory, but could
not establish it. To Franklin, who in June 1752 actually brought down
the lightning by his kite and a key, is the actual discovery due. We
have already detailed the circumstances (page 206) and need not repeat
the account of the experiment.

Of course the American philosopher found numerous imitators, not always
with impunity. Professor Richmann was killed by the spirit he was
invoking; Lemounier and Beccaria confirmed the theory that the air was
full of electricity; while Du Saussure, from his investigations on the
Alps, and Volta from the invention of the pile, are most famous in the
history of electricity. They applied themselves with much success to
the investigation of the electric condition of the atmosphere, of which
the disturbances called thunderstorms are the result.

The amount of electricity varies in the atmosphere at different times
in the day and night. Towards midday and midnight the development is
generally greatest, and this fact will account for the prevalence of
storms during our hours of rest. Again, different kinds of clouds have
different degrees of electricity, and of different kinds. Under certain
conditions these clouds will give forth lightning, and a storm will
begin. The more clouds the more globules, and therefore in summer,
while there is more production of vapour from solutions of salts, etc.,
we are more likely to have the storms. We are most of us familiar
with the mass of the “thunder cloud” rising in the distance, light at
the upper part, very dark below, and throwing out tentacles like the
octopus, coming up sometimes—frequently, indeed—“against the wind,”
impelled by an upper current, or following the course of a river,
which is not unusual. Below, there is perhaps an army of thin dark
clouds. The nature and height of clouds have also a great deal to do
with the phenomena displayed. In general, storm-clouds are positively
electrified.

[Illustration: Fig. 287.—Cirrus cloud.]

Clouds are good conductors of electricity, and yet they may be so
insulated by the dry air surrounding them that they will accumulate
it; and when thus charged, if they encounter other clouds charged with
opposite electricity, the opposing masses will attract each other until
a discharge takes place. This is what we term lightning, and under
such conditions electricity, though very dazzling, is harmless. It is
when the cloud comes near to the earth, and a discharge is released,
that lightning is so dangerous to persons who remain in the fields.
Sometimes the discharge comes from the earth to meet that from the
cloud. Sheep are frequently killed by ground lightning, and once, at
Malvern, we had an escape from an upward stroke. The back-stroke from
a cloud is also dangerous. It may happen that the cloud has discharged
itself upon the earth many miles away, but a return discharge takes
place at the other end, and if that end be near the earth the
consequences may be serious. As a rule, the return stroke is not so
violent as the first discharge.

The colour of lightning varies very much. We have the white, the blue,
the violet, and red. The colour depends upon the distance and intensity
of the lightning, and the more there is of it the whiter the light. We
can illustrate the varied hues of the electric “fluid” by passing a
spark through the receiver of an air-pump. If the air be rarefied, or
there be a vacuum, we shall perceive a blue or violet light. Therefore
we may conclude that the blue and violet flashes have birth in high
strata of the atmosphere.

[Illustration: Fig. 288.—Cumulus cloud.]

We have all heard how dangerous it is to stand under a tree during a
thunderstorm, or rather, we should say, when the storm is approaching
us nearly. The tree is a conductor, and the lightning having no better
one at hand will pass through the tree on its way to the earth, and if
we are standing against the tree we shall be included in the course,
and die from the shock to the nerves while the lightning is passing
through us. The best position in a thunderstorm, if we are in the
neighbourhood of trees, is to sit or lie down on the ground some little
distance from the base of the nearest tree. If the tree be sixty feet
high suppose, and we sit fifty feet or less from the trunk, we shall
be pretty safe, because the lightning will reach the tree top before
it can reach us. We are protected by it as by a conductor, bad though
it be. Standing up in a boat during a storm is not wise. Lightning has
an affinity for water, and besides, if no higher objects are near, our
body will act as a conductor. Bed is the safest place, as blankets are
non-conductors. Cellars are not the safest by any means. Lightning may,
and it frequently does, strike the house and descend to the basement.
If the air be very full of electricity, and a flash be near, a person
running away may conduct the lightning to himself by creating a vacuum
into which the flash may dart.

[Illustration: Fig. 289.—Nimbus, or rain cloud.]

Arago classified lightning into three kinds: zig-zag, globular, and
sheet. The first we call forked lightning, and frequently this kind
branches out at the end, so that although there may be only one flash,
it may strike out in two or three directions at the same time. This may
be accounted for by the unequal power of the air strata to conduct the
electricity. The forked flashes are of very great length, extending
frequently for miles, and the bifurcations also are often miles apart.
The duration of the flash is less than the thousandth part of a second;
so instantaneous is it that no motion can be perceived even in a most
rapidly-moving wheel, as proved by Professor Wheatstone. We sometimes
fancy that the flash lasts longer, but the impression received by the
eye quite accounts for the apparently prolonged sight of the lightning.

Sheet lightning, the faint flashes frequently seen upon the horizon,
are quite harmless. Sheet lightning is that which is seen reflected
behind clouds or from far-distant storms. It is sometimes very
beautiful. Ball, or globular lightning, is dangerous, and globes of
fire have been seen to descend, and striking the ground, bound onwards
for some distance. The descent of these forms of electric discharge
has given rise to the popular notion of “thunderbolts.” The “Mariner’s
Lights,” or St. Elmo’s fire, is frequently observed in ships. It is
usually regarded as a fortunate occurrence. It was noticed by Columbus.
One voyager describes the phenomena as follows:—“The sky was suddenly
covered with thick clouds.... There were more than thirty of St. Elmo’s
fires on the ship. One of them occupied the vane of the mainmast. I
sent a sailor to fetch it. When he was aloft he heard a noise like that
which is made when moist gunpowder is burned. I ordered him to take off
the vane. He had scarcely executed this order, when the fire quitted
it and placed itself at the apex of the mainmast, whence it could not
possibly be removed.”

[Illustration: Fig. 290.—Thunderstorm.]

There have been occasions when the manes and tails of horses, and
even the ears of human beings, have shown a phosphorescent light
which emitted a hissing noise. Alpine travellers have noticed similar
phenomena; and Professor Forbes, when crossing the Theodule Pass
into Italy, heard the hissing sound in his alpenstock. The tips of
rocks and grass points were all hissing too. The party were in the
midst of an electric cloud. When the Professor turned the point of
his alpenstock upwards, a vivid flash was emitted, but no thunder
followed. They descended as quickly as possible from such a dangerous
neighbourhood.

[Illustration: Fig. 291.—St. Elmo’s fire.]

It is observable that the properties of lightning and of the electric
spark are identical—the faint crackle of the latter being magnified
into the loud rolling of the thunder. The disturbance of the atmosphere
is the cause of the loud reverberations, and echoes produced from
clouds tend to intensify and prolong the peal. The sound rises and
falls, and varies accordingly as the cloud is near or far. A smart
sharp report or rattle denotes the nearness of the lightning, while the
gradual swelling and subsidence, followed, mayhap, by an increasing
volume of sound, which in its turn dies away, tells us that the danger
is not imminent. The cause o£ this loud rolling, unless it proceeds
from echoes from different clouds, has not been satisfactorily
explained. Sound travels less quickly than light, and therefore we
only hear the thunder some seconds after we have perceived the flash.
It is therefore conceivable that we may hear the last reverberations
and its echoes first, and the sound of the first disturbance with its
echoes last of all. Thus there will be distinct sounds. Firstly, the
actual noise we call thunder from the air strata _nearest_ to us;
secondly, the echoes of that disturbance from the clouds, of course
fainter; then we hear the sound caused by the tearing asunder of the
air particles farthest off, and again the echoes of that disturbance.
This theory will, we think, account for the swelling peals of thunder,
and the successive loud and fainter reverberations. At any rate, in the
absence of any other theory, we submit it for consideration. The sound
of thunder is seldom or never heard at a distance greater than fifteen
miles.

Lightning conductors are such every-day objects that no description is
necessary; but the reason the lightning runs along it harmlessly is
because the galvanized iron rod is the best conductor in the immediate
neighbourhood. Where there is not a good conductor lightning will
accept the next best, and so on, any conductor being better than none.
The point of the rod cannot contain any electricity, there being no
room for it, and the “fluid,” as it is termed, runs down to the ground,
to terminate, when possible, in water or charcoal. A great deal of
electricity is no doubt carried away from the air by the numerous
conductors without any spark passing. Until Sir W. Snow Harris brought
forward his lightning conductors for ships, the loss was great at sea.
But now we rarely hear of any vessel being disabled by lightning. We
owe to Franklin the idea of the lightning conductor.

According to observations made by Mr. Crosse, the following statement
shows the tendency of the atmosphere, in certain conditions, to
thunderstorms. We may accept the deduction of M. Peltier that grey and
slate-colour clouds are charged with negative, and white, rose-colour,
and orange clouds with positive electricity. The order of arrangement
in the following table places the most likely source of thunderstorms
first, and the least likely source at the end, with regular rotation of
intermediate probabilities intervening:—

  1. Regular thunder clouds.
  2. Driving fog with small rain.
  3. Fall of snow, or hailstorm.
  4. Smart shower on a hot day.
  5. Smart shower on a cold day.
  6. Hot weather after wet days.
  7. Wet weather after dry days.
  8. Clear frosty weather.
  9. Clear warm weather.
  10. Cloudy days.
  11. “Mackerel” sky.
  12. Sultry weather and hazy clouds.
  13. Cold damp night.
  14. Cold, dry north-east winds.

We have thus briefly touched upon some of the atmospherical phenomena
directly attributable to electricity. In our articles upon Meteorology
we will consider the aurora and many other interesting facts concerning
the atmosphere, and the effects of sound, heat, and light upon the air.

[Illustration: Fig. 292.—Lightning conductor.]


FOOTNOTES:

[18] We are indebted for many facts respecting Mr. Edison’s light in
this chapter to a paper by Mr. Upton.



CHAPTER XXIII.

AERONAUTICS.

 PRESSURE OF AIR IN BODIES—EARLY ATTEMPTS TO FLY IN THE AIR—DISCOVERY
 OF HYDROGEN—THE MONTGOLFIER BALLOONS—FIRST EXPERIMENTS IN PARIS—NOTED
 ASCENTS.


In the first part of this volume we entered into the circumstances of
air pressure, and in the Chemistry section we shall be told about the
atmosphere and its constituents. We know that the air around us is
composed principally of two gases, oxygen and nitrogen, with aqueous
vapour and some carbonic acid. An enormous quantity of carbonic acid
is produced every day, and were it not for the action of vegetation
the amount produced would speedily set all animal life at rest. But
our friends, the plants, decompose the carbonic acid by assimilating
the carbon and setting free the oxygen which animals consume. Thus our
atmosphere keeps its balance, so to speak. Nothing is lost in nature.

We have illustrated the pressure of the atmosphere by the Magdeburg
hemispheres, and we know that the higher we ascend the pressure is
lessened. The weight of the atmosphere is 15 lbs. to the square inch
at sea level. This we have seen in the barometer. Now pressure is
equal. Any body immersed in a liquid suffers pressure, and we remember
Archimedes and the crown. It displaced a certain amount of water when
immersed. A body in air displaces it just the same. Therefore when any
body is heavier than the air, it will fall just as a stone will fall in
water. If it be of equal weight, it will remain balanced in the air,
if lighter it will rise, till it attains a height where the weight of
the atmosphere and its own are equal; there it will remain till the
conditions are altered. Now we will readily understand why balloons
float in the air, and why clouds ascend and descend in the atmosphere.

In the following pages we propose to consider the question of
ballooning, and the possibility of flying. We all have been anxious
concerning the unfortunate balloonist who was lost in the Channel, so
some details concerning the science generally, with the experiences of
skilled aeronauts, will guide us in our selection of material. We will
first give a history of the efforts made by the ancients to fly, and
this ambition to soar above the earth has not yet died out.

From a very early period man appears to have been desirous to study the
art of flying. The old myths of Dædalus and Icarus show us this, and it
is not to be wondered at. When the graceful flight of birds is noticed,
we feel envious almost that we cannot rise from the earth and sail away
at our pleasure over land and sea. Any one who has watched the flight
of the storks around and above Strasburg will feel desirous to emulate
that long, swift-sailing flight without apparent motion of wing, and
envy the accuracy with which the bird hits the point aimed at on the
chimney, however small. It is small wonder that some heathens of old
time looked upon birds as deities.

The earliest flying machine that we can trace is that invented by
Archytas, of Tarentum, B.C. 400. The historian of the “Brazen
Age” tells us how the geometrician, Archytas, made a wooden pigeon
which was able to sustain itself in the air for a few minutes, but
it came down to the ground after a short time, notwithstanding the
mysterious “aura spirit” with which it was supposed to be endowed. The
capability of flying has for centuries been regarded as supernatural.
Putting angels aside, demons are depicted with wings like bats’ wings,
while witches, etc., possessed the faculty of flying up chimneys upon
broomsticks. We even read in childish lore of an old woman who “went up
in a basket” (perhaps a balloon-car), and attained a most astonishing
altitude-an elevation no less than “seventy times as high as the moon!”

But to descend to history. It is undoubtedly true that in the time of
Nero, Simon Magus attempted to fly from one house to another by means
of some mechanical contrivance, and failing, killed himself. Roger
Bacon, the “admirable doctor,” to whom the invention of gunpowder
is generally attributed, had distinct notions of flying by means of
machines, and “hollow globes,” and “liquid fire.” But he did not
succeed, nor did many successive attempts succeed any better in
subsequent years. Bishop Wilkins treated of the art of flying, but
most, if not all who discussed the subject appear to have been indebted
to Roger Bacon for the idea.

When the nature and pressure of the atmosphere by Torricelli’s
experiments became better known, Father Lana, a Jesuit priest,
constructed a flying machine or balloon of curious shape. He proposed
to fix four copper globes, very thin, and about twenty feet in
diameter, and to these he fastened a boat or car, looking very much
like a basin. His idea was to empty his great copper globes, and that
their buoyancy would then bear the weight of the traveller. But he
overlooked or was ignorant of the effect of the atmospheric pressure,
which would have speedily crushed the thin copper globes when empty.
Lana’s suggestion was made in 1670, the barometer had been discovered
in 1643.

There were some fairly successful experiments made in flying in 1678
and in 1709. The former attempt was made by Besmir, a locksmith of
Sable, who raised himself by means of wings up to the top of a house by
leaps, and then succeeded in passing from one house to another lower
down by supporting himself in the air for a time. He started from an
elevated position, and came down by degrees. Dante, a mathematician,
also tried to fly, but without great success. He broke his thigh on one
occasion. Laurence de Gusman claimed an invention for flying in 1709,
and petitioned for a “patent,” which was granted by the king’s letter.
The machine appears to have borne some resemblance to a bird.

It was not till 1782, however, that the true art of aerial navigation
was discovered. The knowledge of hydrogen gas possessed by Cavendish
in 1766 no doubt led up to it, and in the year following its discovery
Professor Black, lecturing in Edinburgh, stated that it was much
lighter than the atmosphere, and that any vessel filled with the
gas would rise in the air. We now come to the invention of the
BALLOON (so called from its shape being similar to a vessel
used in the laboratory) by the Brothers Montgolfier.

[Illustration: Fig. 293.—Montgolfier balloon.]

Stephen and James Montgolfier were paper-makers, and carried on their
business at Annonay, near Lyons, but it was partly by accident that
the great discovery was made. They had no knowledge of the buoyancy
of hydrogen gas. They took their idea of the balloon (inflated) from
noticing an ascending column of smoke. It occurred to Stephen that
if a paper bag were filled with smoke it would ascend into the air.
A large bag was made and some paper burnt beneath it in a room. When
the smoke had filled the bag it was released, and immediately ascended
to the ceiling. Here was the germ of the Montgolfier or heated air
balloon. The experiment was repeated in the open air with even greater
success, and a trial upon a larger scale was immediately determined
upon. A story is related of Mongolfier when prosecuting his researches,
that a widow whose husband had belonged to the printing firm with whom
Montgolfier was then connected in business, saw the smoke issuing from
the room in which the little balloon was being filled. She entered, and
was astonished to see the difficulty experienced by the experimenter
in filling the balloon. It swerved aside, and increased the trouble
he had to keep it above the chafing dish. Montgolfier was greatly
troubled, and seeing his disappointment, the widow said, “Why don’t you
fasten the balloon to the chafing dish?” This had not occurred to the
experimenter, and the idea was a valuable one. That was the secret of
success.

The Montgolfier Brothers determined to exhibit their successful
experiment, and accordingly on the 5th of June, 1783, a great concourse
assembled to see the wonderful sight. A large canvas or linen balloon
was made and suspended over a fire of chopped straw. The heated air
quickly filled the balloon, which rose high in the air, and descended
more than a mile away. This balloon contained 22,000 cubic feet of
heated air, which is lighter than cold air, and of course rising
carried the globe with it. As soon as the air began to cool the balloon
ceased to rise, and as it got colder descended.

Here was the actual discovery of the science of Aerostatics. The
intelligence of the success achieved soon spread from France to other
countries. Paris, however, was in advance, and the Brothers Robert
applied hydrogen gas to a balloon which was sent up from the Champ de
Mars in August 1783. There was some trouble experienced in filling
it, but when the balloon was at length released it realized all
expectations by remaining in the air nearly an hour. When at length it
fell it met with a worse fate than it deserved, for the ignorant and
superstitious peasantry at once destroyed it. After this Montgolfier
exhibited his experiment next time at Versailles in the presence of
the Court. The first aerial travellers appeared on this occasion—viz.,
a sheep, a cock, and a duck, which were secured in the car. They all
descended in safety, and this success encouraged M. Pilatre de Rozier
to make an attempt in a “fire balloon.” He went up first in a captive
balloon, and at length he and a friend, the Marquis d’Arlandes,
ascended from the Bois de Boulogne. The trip was a decided success, and
the possibility of navigating the air was fully demonstrated.

Soon after this,—viz., in December 1783,—an Italian Count, named
Zambeccari, made an ascent in London, and came down safely at Petworth.
MM. Charles and Robert ascended from Paris in December, and in February
a balloon crossed the English Channel. We must pass over some time
and come to the ascents of Lunardi, which caused great excitement in
London. His balloon was a very large one, and was inflated, or rather
partially so, at the Artillery ground. Some delay occurred, and fearing
a riot, M. Lunardi proposed to go up alone with the partially-filled
balloon. A Mr. Biggin who had intended to ascend was left behind. The
Prince of Wales was present, with thousands of spectators. Lunardi
cast off and ascended rapidly, causing great admiration from the whole
metropolis. Judge and jury, sovereign and ministers, all turned out
to gaze at the balloon; a guilty prisoner was acquitted hurriedly, so
that no time was lost in discussion, and one lady died of excitement.
Lunardi was regarded as a hero, and made many other ascents. He died in
1806.

In those earlier days one or two fatal accidents happened. Count
Zambeccari and a companion were in a balloon which caught fire, and
both occupants of the car leaped from it as they were descending. The
Count was killed on the spot, and his companion was much injured.
Pilatre de Rozier made an attempt to cross the channel to England in
1785; he had reached three thousand feet when the balloon caught fire,
and the unfortunate traveller was precipitated to the ground. His
associate only survived him a few minutes.

[Illustration: Fig. 294.—MM. Charles’ and Roberts’ balloon.]

[Illustration: Fig. 295.—Blanchard’s balloon.]

It is to the celebrated English aeronaut, Mr. Green, that the
substitution of carburetted hydrogen or street gas for hydrogen is due,
and since his ascent in 1821 no other means of inflation have been
used. A great many quite successful and a few unsuccessful ascents
have been made for pleasure or profit. Mr. Green, in the _Nassau_
balloon, passed over to Nassau, a distance of five hundred miles, in
eighteen hours. This exploit was the cause of the name being bestowed
upon the balloon. The _Giant_ of M. Nadar was exhibited in England,
and it was an enormous one, being an hundred feet high, and nearly as
wide in the widest part. But even this machine was outdone by the
Godard “Montgolfier” balloon, which was one hundred and seventeen feet
high, and carried a stove. We give illustrations of these celebrated
balloons, and will now pass on to the more scientific portion of the
subject and the ascents of Mr. Glaisher and other aeronauts for the
purpose of making meteorological observations, and the use of balloons
for purposes of observation in war.

It appears that the first ascent for scientific investigation was made
in the year 1803. The aeronauts were Messrs. Robertson and Lhoest.
They ascended from Hamburg and came down at Hanover, and made meantime
several experiments with reference to the electrical condition of the
atmosphere, its influence upon a magnetic needle, and some experiments
with regard to acoustics and heat. The report was presented to the
St. Petersburg Academy, and contains the result of their interesting
observations. The travellers ascertained that at the elevation to which
they attained,—viz., 25,500 feet,—the temperature was on that July
day fifty degrees colder, falling to 19·6°, while on the earth the
thermometer had shown 68°. They ascertained that glass and wax did not
become electric when rubbed, that the Voltaic battery lost much of its
power, that the oscillation of a “dipping needle” increased as they
mounted into the air, while sound was certainly less easily transmitted
at that elevation, and struck them as less powerful in tone. The heat
experiment was not a success, owing to the breaking of the thermometer.
They wished to find the temperature of boiling water at that elevation,
but when the experiment was about to be made Robertson accidentally
plunged the instrument _into the fire_ instead of into the water. So
the question was not settled.

The effect upon the aeronauts was a sensation of sleepiness, and two
birds died. The muscular powers of the voyagers also appear to have
been much affected, and similar sensations may be experienced by
travellers on high mountains who find their breath very short and a
disinclination to exertion oppress them.

MM. Biot and Gay-Lussac made a very interesting ascent in 1804. We will
detail their experiences at some length, for the coolness displayed
and the value of the observations made are remarkable in the history
of scientific ballooning. They started, at 10 o’clock a.m. on the 23rd
of August, and when the balloon had carried them up to an altitude of
8,600 feet they commenced their experiments. They had some animals in
the car with them, a bee amongst the number, and the insect was let go
first. It flew away swiftly, not at all inconvenienced apparently. The
sun was very hot at 56° Fahr. Their pulses were beating very fast, but
no inconvenience was felt.

When 11,000 feet had been reached a linnet was permitted to go at
large, but after a little time the bird returned to the balloon. It
remained perched for a few minutes, and then dashed downwards at a
tremendous pace. A pigeon was then liberated. It also appeared very
uncertain, and wheeled around in circles for a time. At last it gained
confidence, and descended, and disappeared in the clouds beneath.
They made other experiments, but descended without having obtained as
accurate results as had been anticipated.

[Illustration: Fig. 296.—The Nassau balloon.]

[Illustration: Fig. 297.—The “Giant” balloon of M. Nadar.]

On the next occasion, however, every care was taken, and on the 15th
of September the important ascent was made by Gay-Lussac alone. He
fixed hanging ropes to the balloon with the view to check the rotating
movements, and having provided himself with all necessary apparatus
and two vacuum flasks to bring down some of the upper air, the young
man started. The barometer marked 30·66", the thermometer 82° (Fahr.).
At an elevation of 12,680 feet Lussac perceived that the variation of
the compass was the same as on land. Two hundred feet higher up he
ascertained that a key held in the magnetic direction repelled with
the lower, and attracted with its upper extremity the north pole of
a needle. This experiment was repeated with the same result at an
elevation of 20,000 feet, which shows how the earth exercises its
magnetic influence. The temperature of the air was found to decrease
in proportion as the ascent up to 12,000 feet, where the reading was
47·3°. It then increased up to 14,000 feet by 6°, and then regularly
diminished again as the balloon rose, till at the greatest elevation
reached, 23,000 feet, there was a difference of 67° in the temperature
on the earth, for at the maximum height attained the thermometer stood
at 14·9°.

But the most important fact ascertained, and one which set many
theories at rest, was the composition of the atmosphere in those high
altitudes. We mentioned that Gay-Lussac took up two empty flasks from
which the air had been taken. The vacuum was almost perfect. When
the aeronaut had reached 21,460 feet he opened one flask, and it was
quickly filled; he secured it carefully; and when at his highest
point,—four miles and a half above the sea-level,—he opened the
other flask. The barometer stood at 12.95 inches, and the cold was
very great. The voyager felt benumbed, and experienced difficulty of
breathing; his throat was parched and dry. So Lussac determined to
return, he could go no higher. He dropped gently near Rouen, and soon
reached Paris. As soon as possible the air in the flasks was submitted
to very delicate tests, and to the satisfaction of the scientists
engaged it was found to be in exactly the same proportions as that
collected near the earth—two hundred and fifteen parts of oxygen to
every thousand of atmospheric air.

Messrs. Banal and Bixio, in 1850, also made some observations, and
found the temperature very variable. At 23,000 feet they found the
thermometer at _minus_ 38·2° Fahr., which was much below the cold
experienced by Gay-Lussac. We may still conclude that the various
currents of the atmosphere cause considerable variation, and that it is
impossible to lay down anything respecting the degrees of heat and cold
likely to be found at certain elevations. We quote Arago’s observations
upon this ascent:—

“This discovery” (the ice particles found in the air) “explains how
these minute crystals may become the nucleus of large hailstones,
for they may condense round them the aqueous vapour contained in the
portion of the atmosphere where they exist. They go far to prove the
truth of Mariotte’s theory, according to which these crystals of ice
suspended in the air are the cause of parahelia—or mock-suns and
mock-moons. Moreover, the great extent of so cold a cloud explains very
satisfactorily the sudden changes of temperature which occur in our
climates.”

M. Flammarion gives in his “Voyages” some very interesting and amusing
particulars, as well as many valuable scientific observations. During
one ascent he remarked that the shadow of the balloon was _white_, and
at another time dark. When white the surface upon which it fell looked
more luminous than any other part of the country! The phenomenon was
an _anthelion_. The absolute silence impressed the voyager very much.
He adds, “The silence was so oppressive that we cannot help asking
ourselves are we still alive! We appear to appertain no longer to the
world below.” M. Flammarion’s observations on the colour of what we
term the sky are worth quoting—not because they are novel, but because
they put so very clearly before us the appearance we call the “blue
vault.” He says,—speaking of the non-existence of the “celestial
vault,”—“The air reflects the blue rays of the solar spectrum from
every side. The white light of the sun contains every colour, and the
air allows all tints to pass through it except the blue. This causes
us to suppose the atmosphere is blue. But the air has no such colour,
and the tint in question is merely owing to the reflection of light.
Planetary space is absolutely black; the higher we rise the thinner the
layer of atmosphere that separates us from it, and the darker the sky
appears.”

[Illustration: Fig. 298.—The “Eagle” of M. Godard.]

Some beautiful effects may be witnessed at night from a balloon, and
considering the few accidents there have been in proportion to the
number of ascents, we do not wonder at balloon voyages being undertaken
for mere pleasure. When science has to be advanced there can be no
objection made, for then experience goes hand-in-hand with caution.
It is only the ignorant who are rash; the student of Nature learns to
respect her, and to attend to her admonitions and warnings in time.
The details of the ascents of famous aeronauts give us a great deal
of pleasant and profitable reading. The phenomena of the sky and
clouds, and of the heavens, are all studied with great advantage from
a balloon, or “aerostat,” as it is the fashion to call it. The grand
phenomena of “Ulloa’s circles,” or _anthelia_, which represent the
balloon in air, and surrounded by a kind of glory, or aureola, like
those represented behind saintly heads, appear, as the name denotes,
opposite to the sun.

The various experiments made to ascertain the intensity of sounds have
resulted in the conclusion that they can be heard at great distances.
For instance, the steam whistle is distinctly audible 10,000 feet up in
the air, and human voices are heard at an altitude of 5,000 feet. A
man’s voice alone will penetrate more than 3,000 feet into the air; and
at that elevation the croaking of frogs is quite distinguishable. This
shows that sound ascends with ease, but it meets with great resistance
in its downward course, for the aeronaut cannot make himself audible
to a listener on the earth at a greater distance than 300 or 400 feet,
though the latter can be distinctly heard at an elevation of 1,600
feet. The diminution of temperature noted by M. Flammarion is stated
to be 1° Fahr. for every 345 feet on a fine day. On a cloudy day the
mean decrease was 1° for every 354 feet of altitude. The temperature
of clouds is higher than the air surrounding them, and the decrease is
more rapid near the surface, less rapid as the balloon ascends. We may
add that at high elevations the cork from a water-bottle will pop out
as if from a champagne flask.

We have hitherto referred more to M. Flammarion and other French
aeronauts, but we must not be considered in any way oblivious of
our countrymen, Messrs. Glaisher, Green, and Coxwell, nor of the
American,—one of the most experienced of aerial voyagers,—Mr. Wise. The
scientific observations made by the French voyagers confirmed generally
Mr. Glaisher’s experiments. This noted air-traveller made twenty-eight
ascents in the cause of science, and his experiences related in
“Travels in the Air,” and in the “Reports” of the British Association,
are both useful and entertaining. For “Sensational ballooning” one
wishes to go no farther than his account of his experience with Mr.
Coxwell, when (on the 5th of September, 1862) he attained the greatest
elevation ever reached, viz., seven miles, or thirty-seven thousand
feet.

We condense this exciting narrative for the benefit of those who have
not seen it already.

The ascent was made from Wolverhampton. At 1.39 p.m., the balloon was
four miles high, the temperature was 8°, and by the time the fifth
mile had been reached the mercury was below zero, and up to this time
observations had been made without discomfort, though Mr. Coxwell,
having exerted himself as aeronaut, found some difficulty in breathing.
About 2 o’clock, the balloon still ascending, Mr. Glaisher could not
see the mercury in the thermometer, and Mr. Coxwell had just then
ascended into the ring above the car to release the valve line which
had become twisted. Mr. Glaisher was able to note the barometer,
however, and found it marked 10 inches, and was rapidly decreasing. It
fell to 9¾ inches, and this indicated an elevation of 29,000 feet!
But the idea was to ascend as high as possible, so the upward direction
was maintained. “Shortly afterwards,” writes Mr. Glaisher, “I laid my
arm upon the table possessed of its full vigour, and on being desirous
of using it I found it powerless,—it must have lost power momentarily.
I tried to move the other arm, and found it powerless also. I then
tried to shake myself, and succeeded in shaking my body. I seemed to
have no limbs. I then looked at the barometer, and whilst doing so my
head fell on my left shoulder.”

Mr. Glaisher subsequently quite lost consciousness, and “black
darkness” came. While powerless he heard Mr. Coxwell speaking, and then
the words, “Do try, now do.” Then sight slowly returned, and rousing
himself, Mr. Glaisher said, “I have been insensible.” Mr. Coxwell
replied, “You have, and I, too, very nearly.” Mr. Coxwell’s hands were
black, and his companion had to pour brandy upon them. Mr. Coxwell’s
situation was a perilous one. He had lost the use of his hands,
which were frozen, and had to hang by his arms to the ring and drop
into the car. He then perceived his friend was insensible, and found
insensibility coming on himself. There was only one course to pursue—to
pull the valve line and let the gas escape, so as to descend. But his
hands were powerless! As a last resource he gripped the line with his
teeth, and bending down his head, after many attempts succeeded in
opening the valve and letting the gas escape. The descent was easily
made, and accomplished in safety.

[Illustration: Fig. 299.—A descending balloon.]

Some pigeons were taken up on this occasion, and were set free at
different altitudes. The first, at three miles, “dropped as a piece of
paper”; the second, at four miles, “flew vigorously round and round,
apparently taking a dip each time”; a third, a little later, “fell like
a stone.” On descending a fourth was thrown out at four miles, and
after flying in a circle, “alighted on the top of the balloon.” Of the
remaining pair one was dead when the ground was gained, and the other
recovered.

The observations noted are too numerous to be included here. Some,
we have seen, were confirmed by subsequent aeronauts, and as we have
mentioned them in former pages we need not repeat them. The results
differed very much under different conditions, and it is almost
impossible to decide upon any law. The direction of the wind in the
higher and lower regions sometimes differed, sometimes was the same,
and so on. The “Reports” of the British Association (1862-1866) will
furnish full particulars of all Mr. Glaisher’s experiments.

We have scarcely space left to mention the parachutes or umbrella-like
balloons which have occasionally been used. Its invention is traced
to very early times; but Garnerin was the first who descended in a
parachute, in 1797, and continued to do so in safety on many subsequent
occasions. The parachute was suspended to a balloon, and at a certain
elevation the voyager let go and came down in safety. He ascended once
from London, and let go when 8,000 feet up. The parachute did not
expand as usual, and fell at a tremendous rate. At length it opened
out, and the occupier of the car came down forcibly, it is true, but
safely. The form of the parachute is not unlike an umbrella opened,
with cords attaching the car to the extremities of the “ribs,” the top
of the basket car being fastened to the “stick” of the umbrella.

Mr. Robert Cocking invented a novel kind of parachute, but when he
attempted to descend by it from Mr. Green’s balloon it collapsed, and
the unfortunate voyager was dashed to pieces. His remains were found
near Lee, in Kent. Mr. Hampton did better on Garnerin’s principle, and
made several descents in safety and without injury.

The problem of flying in the air has attracted the notice of the
Aeronautical Society, established in 1873, but so far without leading
to practical results, though many daring and ingenious suggestions have
been put forth in the “Reports.”

It is not within our province to do more than refer to the uses of
the balloon for scientific purposes, but we may mention the services
it was employed upon during the French war, 1870-71. The investment
of Paris by the German army necessitated aerial communication, for no
other means were available. Balloon manufactories were established,
and a great number were made, and carried millions of letters to the
provinces. Carrier-pigeons were used to carry the return messages to
the city, and photography was applied to bring the correspondence into
the smallest legible compass. The many adventures of the aeronauts are
within the recollection of all. A few of the balloons never reappeared;
some were carried into Norway, and beyond the French frontier in other
directions. The average capacity of these balloons was 70,000 cubic
feet.

Of course it will be understood how balloons are enabled to navigate
the air. The envelope being partly filled with coal-gas-heated air and
hydrogen, is much lighter than the surrounding atmosphere, and rises
to a height according as the density of the air strata diminishes. The
density is less as we ascend, and the buoyant force also is lessened
in proportion. So when the weight of the balloon and its occupants is
the same as the power of buoyancy, it will come to a stand, and go no
higher. It can also be understood that as the pressure of the outside
becomes less, the expansive force of the gas within becomes greater.
We know that gas is very compressible, and yet a little gas will fill
a large space. Therefore, as the balloon rises, it retains its rounded
form, and appears full even at great altitudes; but if the upper
part were quite filled before it left the ground, the balloon would
inevitably burst at a certain elevation when the external pressure of
the air would be removed, unless an escape were provided. This escape
is arranged for by a valve at the top of the balloon, and the lower
part above the car is also left open very often, so that the gas can
escape of itself. When a rapid descent is necessary, the top valve is
opened by means of a rope, and the balloon sinks by its own weight. Mr.
Glaisher advises for great ascensions a balloon of a capacity of 90,000
cubic feet, and only filled one-third of that capacity with gas. Six
hundred pounds of ballast should be taken.

[Illustration: Fig. 300.—Filling a balloon.]

A very small quantity of ballast thrown away will make a great
difference; a handful will raise the balloon many feet, and a chicken
bone cast out occasions a rise of thirty yards. The ballast is carried
in small bags, and consists of dry sand, which speedily dissipates in
the air as it falls. By throwing out ballast the aeronaut can ascend
to a great height—in fact, as high as he can go, the limit apparently
for human existence being about seven miles, when cold and rarefied air
will speedily put an end to existence.

It is a curious fact, that however rapidly the balloon may be
travelling through the air, the occupants are not sensible of the
motion. This, in part, arises from the impossibility of comparing it
with other objects. We pass nothing stationary which would indicate
the pace at which we travel. But the absence of oscillation is also
remarkable; even a glass of water may be filled brim-full, and to
such a level that the water is above the rim of the glass, and yet
not a drop will fall. This experiment was made by M. Flammarion. When
the aeronaut has ascended some distance the earth loses its flat
appearance, and appears as concave as the firmament above. Guide ropes
are usually attached to balloons, and as they rest upon the ground
they relieve the balloon of the amount of weight the length trailing
would cause. They thus act as a kind of substitute for ballast as the
balloon is descending. Most of the danger of aerial travelling lies in
the descent; and though in fine weather the aeronaut can calculate to a
nicety where he will descend, on a windy day, he must cast a grapnel,
which catches with an ugly jerk, and the balloon bounds and strains at
her moorings.

Although many attempts have been made to guide balloons through the
air, no successful apparatus has ever been completed for use. Paddles,
sails, fans, and screws have all been tried, but have failed to achieve
the desired end. Whether man will ever be able to fly we cannot of
course say. In the present advancing state of science it may not be
impossible ere long to supply human beings with an apparatus worked
by electricity, perhaps, which will enable them to mount into the air
and sustain themselves. But even the bird cannot always fly without
previous momentum. A rook will run before it rises, and many other
birds have to “get up steam,” as it were, before they can soar in the
atmosphere. Eagles and such heavy birds find it very difficult to rise
from the ground. We know that vultures when gorged cannot move at all,
or certainly cannot fly away; and eagles take up their positions on
high rocks, so that they may launch down on their prey, and avoid the
difficulty of rising from the ground. They swoop down with tremendous
momentum and carry off their booty, but often lose their lives from
the initial difficulty of soaring immediately. We fear man’s weight
will militate against his ever becoming a flying animal. When we obtain
a knowledge of the atmospheric currents we shall no doubt be able to
navigate our balloons; but until then—and the information is as yet
very limited, and the currents themselves very variable—we must be
content to rise and fall in the air, and travel at the will of the wind
in the upper regions of the atmosphere.



CHAPTER XXIV.

CHEMISTRY.

_INTRODUCTION._

 WHAT CHEMISTRY IS—THE ELEMENTS—METALLIC AND NON-METALLIC—ATOMIC
 WEIGHT—ACIDS—ALKALIS—BASES—SALTS—CHEMICAL COMBINATION AND STUDY.


Chemistry is the science of phenomena which are attended by a change
of the objects which produce them. We know that when a candle burns,
or when wood is burned, or even a piece of metal becomes what we term
“rusty,” that certain chemical changes take place. There is a change
by what is termed chemical action. Rust on iron is not iron; it is
oxide of iron. The oxygen of the air causes it. So we endeavour, by
Chemistry, to find out the nature of various bodies, their changes, and
the results.

[Illustration: Fig. 301.—The Laboratory.]

In nature we have simple and compound bodies. The former are called
ELEMENTS. We must not confuse these elements with the so-called
elements—earth, air, fire, and water. These are really compound bodies.
An element is a substance or a gas which is not composed of more than
one constituent; _it is itself_—a compound of perfectly identical
particles. Every “compound” body, therefore, must be made up of some of
the elements, of which there are sixty-five. These bodies are divided
into non-metallic and metallic elements, and all bodies are composed
of them, or are these bodies themselves. The list is as follows. The
non-metallic elements are “metalloids.” We have omitted fractions from
the weights, on which chemists differ.

 TABLE OF ELEMENTS WITH THEIR CHEMICAL SYMBOLS AND COMBINING WEIGHTS.

  Non-Metallic                 Atomic or
    Elements.        Symbols.  Combining          Derivation of Name.
                                Weights.

  Oxygen               O         16        Gr. Oxus, acid; gennaō, to make.
  Hydrogen }         { H          1        Gr. Udor, water; gennaō, to make.
  Nitrogen } Gaseous { N         14        Gr. Natron, nitre; gennaō, to make.
  Chlorine }         {Cl         35        Gr. Chloros, green.
  Iodine      }      { I        127        Gr. Ioeides, violet.
  Fluorine    }      { F         19        Fluor spar, the mineral.
  Carbon      }      { C         12        Lat. Carbo, coal.
  Sulphur     }      { S         32        Lat. Sulphurium.
  Phosphorus  }      { P         31        Gr. Phos, light; pherein, to carry.
  Arsenic*    } Solid{ As        75        Gr. Arsenicon, potent.
  Silicon     }      { Si        28        Gr. Silex, flint.
  Boron       }      { B         11        Gr. Borax, Arab., baraga, to shine.
  Selenium    }      { Se        79        Gr. Selene, the moon.
  Tellurium   }      { Te       129        Lat. Tellus, the earth.
  Bromine       Fluid            80        Gr. Bromos, offensive smell.

                                METALS.

  Name.            Symbols.   Atomic or   Derivation.
                              Combining
                               Weights.

  Aluminium            Al        27        Lat. Alumen, alum.
  Antimony (Stibium)   Sb       122        Gr. Anti, against; minos, one.
  [Arsenic]            As        75        (Not known.)
  Barium               Ba       137        Gr. Barsù, heavy.
  Bismuth              Bi       210        Ger. Weissmuth, white matter.
  Cadmium              Cd       112        Gr. Cadmeia, calamite.
  Cæsium               Cs       133        Lat. Cæsius, sky-blue.
  Calcium              Ca        40        Lat. Calx, lime.
  Cerium               Ce       141        The planet Ceres.
  Chromium             Cr        52        Gr. Chroma, colour.
  Cobalt               Co        58        Ger. Kobald, a sprite.
  Copper               Cu        63        Lat. Cuprum (Cyprium), Cyprus.
  Didymium             D        147        Gr. Didumos, twins.
  Erbium               E         —         Ytterby in Sweden.
  Gallium              Ga        70        (Not known.)
  Glucinum             Gl         9        Gr. Glukos, sweet.
  Gold                 Au       197        From Hebrew, to shine (doubtful).
  Indium               In       113        Indigo colour.
  Iridium              Ir       198        Gr. Iris, rainbow.
  Iron                 Fe        56        Hebrew, to meet (doubtful).
  Lanthanum            La       139        Gr. Lanthanein, to lie hid.
  Lead                 Pb       207        (Plumbum) malubodos (galena).
  Lithium              Li         7        Gr. Lithos, stone.
  Magnesium            Mg        24        Magnesia, Asia Minor.
  Manganese            Mn        55        Mangana, E. I. (or Magnesia).
  Mercury             Hg        200        Heathen deity (quick).
  Molybdenum          Mo         96        Gr. Molybdena, lead ore, like lead.
  Nickel              Ni         58        Ger. Kupfer nikel, false copper.
  Niobium (Columbium) Nb         94        Columbite.
  Osmium              Os        199        Osme, an odour.
  Palladium           Pl        106        Pallas, Minerva.
  Platinum            Pt        197        Spanish, platina, little silver.
  Potassium (Kalium)  K          39        Potash.
  Rhodium             Rh        104        Gr. Roda, rose.
  Rubidium            Rb         85        Lat. Rubidus, red.
  Ruthenium           Ru        104        (Not known.)
  Silver (Argentum)   Ag        108        Hebrew, money.
  Sodium (Natrium)    Na         23        Salsoda plant.
  Strontium           Sr         87        Strontian, N.B.
  Tantalum            Ta        182        Tantalite mineral.
  Terbium             Tr         —         (Not known.)
  Thallium            Tl        204        Gr. Thallos, green twig.
  Thorium             Th        230        Thor, deity.
  Tin (Stannum)       Sn        118        (Not known.)
  Titanium            Ti         50        Titans.
  Tungsten (Wolfram)  W         184        Swedish.
  Uranium             U         240        Urania.
  Vanadium            V          51        Vanadis, a goddess in Sweden, etc.
  Yttrium             Y          93        (Not known.)
  Zinc                Zn         65        Ger. Zinken, nails.
  Zirconium           Zr         89        Ger. Zircon, four-cornered (Ceylon).

 *Arsenic is sometimes considered a non-metallic and sometimes a
 metallic substance.

The term “combining weight” requires a little explanation. We are
aware that water, for instance, is made up of oxygen and hydrogen in
certain proportions. This we will prove by-and-by. The proportions are
in eighteen grains or parts of water, sixteen parts (by weight) of
oxygen, and two parts (by weight) of hydrogen. These are the weights
or proportions in which oxygen and hydrogen combine to form water,
and such weights are always the same in these proportions. Chemical
combination always occurs for certain substances in certain proportions
which never vary in those compounds, and if we wish to extract oxygen
from an oxide we must take the aggregate amount of the combining
weights of the oxide, and we shall find the proportion of oxygen; for
the compound always weighs the same as the sum of the elements that
compose it. To return to the illustration of water. The molecule of
water is made up of one atom of oxygen and two atoms of hydrogen. One
atom of the former weighs sixteen times the atom of the latter. The
weights given in the foregoing table are _atomic_ weights, and the law
of their proportions is called the Atomic Theory.

An _atom_ in chemistry is usually considered the smallest quantity of
matter that exists, and is indivisible. A _molecule_ is supposed to
contain two or more atoms, and is the smallest portion of a compound
body. The standard atom is hydrogen, which is put down as 1, because
we find that when one part by weight of hydrogen is put in combination,
it must have many more parts _by weight_ of others to form a compound.
Two grains of hydrogen, combining with sixteen of oxygen, makes
eighteen of water, as we have already seen.

Take an example so plainly given by Professor Roscoe, remembering that
the numbers in our table represent the fixed weight or proportion by
weight in which the simple body combines. The red oxide of mercury
contains sixteen parts by weight of oxygen to two hundred parts by
weight of mercury (we see the same numbers in the table); these
combined make two hundred and sixteen parts of oxide. So to obtain 16
lbs. of oxygen we must get 216 lbs. of the powder. It is the same all
through, and it will be found by experiment that if any more parts
than these fixed proportions be taken to form a compound, some of that
element used in excess will remain free. Lime is made up of calcium and
oxygen. We find calcium combining weight is forty, oxygen sixteen. Lime
is oxide of calcium in these proportions (by weight).

When we wish to express the number of atoms in a compound we write the
number underneath when more than one; thus water is H_{2}O. Sulphuric
acid H_{2}SO_{4}. As we proceed we will give the various formulæ when
considering the chief elements.

In chemistry we have acids, alkalis, and salts, with metallic oxides,
termed _bases_, or bodies, that when combined with _acids_ form
_salts_. Alkalis are bases.

ACIDS are compounds which possess an acid taste, impart red
colour to vegetable blues, but lose their qualities when combined with
bases. Hydrogen is present in all acids. There are insoluble acids.
Silicic acid, for instance, is not soluble in water, has no sour taste,
and will not redden the test litmus paper. On the other hand, there are
substances (not acids) which possess the characteristics of acids, and
most acids have only one or two of these characteristics.

Thus it has come to pass that the term “acid” has in a measure dropped
out from scientific nomenclature, and salt of hydrogen has been
substituted by chemists. For popular exposition, however, the term is
retained.

ALKALIS are bases distinguished by an alkaline taste. The
derivation is from Arabic, _al-kali_. They are characterized by certain
properties, and they change vegetable blues to green, and will restore
the blue to a substance which has been reddened by acid. They are
soluble in water, and the solutions are caustic in their effects.
Potash, soda, and ammonia are alkalis, or chemically, the oxides of
potassium, sodium, ammonium, lithium, and cæsium are all alkalis.
Potash is sometimes called “caustic” potash. There are alkaline earths,
such as oxides of barium, strontium, etc. _Bases_ may be defined as the
converse of acids.

Acids and alkalis are then evidently opposite in character, and yet
they readily combine, and in chemistry we shall find that unlike bodies
are very fond of combining (just as opposite electricities attract
each other), and the body made by this combination differs in its
properties from its constituents.

SALTS are composed of acids and bases, and are considered
neutral compounds, but there are other bodies not salts, which likewise
come under that definition—sugar, for instance. As a rule, when acids
and alkalis combine _salts_ are found.

Chemical phenomena are divided into two groups, called _inorganic_ and
_organic_, comprising the simple and compound aspects of the subject,
the elementary substances being in the first, and the chemistry of
animals or vegetables, or organic substances, in the latter. In the
inorganic section we shall become acquainted with the elements and
their combinations so often seen as _minerals_ in nature. Chemical
_preparations_ are artificially prepared. To consider these elements we
must have certain appliances, and indeed a laboratory is needed. Heat,
as we have already seen, plays a great part in developing substances,
and by means of heat we can do a great deal in the way of chemical
decomposition. It expands, and thus diminishes cohesion; it counteracts
the chemical attraction. Light and electricity also decompose chemical
combinations. But before proceeding it will be as well to notice a few
facts showing how Nature has balanced all things.

The earth, and its surrounding envelope, the atmosphere, consist of a
number of elements, which in myriad combinations give us everything we
possess,—the air we breathe, the water we drink, the fire that warms
us, are all made up of certain elements or gases. Water, hydrogen and
oxygen; air, oxygen and nitrogen. Fire is combustion evolving light and
heat. Chemical union always evolves heat, and when such union proceeds
very rapidly fire is the result.

In all these combinations we shall find when we study chemistry that
not a particle or atom of matter is ever lost. It may change or combine
or be “given off,” but the matter in some shape or way exists still.
We may burn things, and rid ourselves, as we think, of them. We do rid
ourselves of the compounds, the elements remain somewhere. We only
alter the _condition_. During combustion, as in a candle or a fire,
the simple bodies assume gaseous or other forms, such as carbon, but
they do not escape far. True they pass beyond our ken, but nature is so
nicely balanced that there is a place for everything, and everything
is in its place under certain conditions which never alter. We cannot
_destroy_ and we cannot _create_. We may prepare a combination, and
science has even succeeded in producing a form like the diamond—a
crystal of carbon which looks like that most beautiful of all crystals,
but we cannot make a diamond after all. We can only separate the
chemical compounds. We can turn diamonds into charcoal it is true,
but we cannot create “natural” products. We can take a particle of
an element and hide it, or let it pass beyond our ken, and remain
incapable of detection, but the particle is there all the time, and
when we retrace our steps we shall find it as it was before.

This view of chemistry carries it as a science beyond the mere holiday
amusement we frequently take it to be. It is a grand study, a study
for a lifetime. Nature is always willing, like a kind, good mother as
she is, to render us up her secrets if we inquire respectfully and
lovingly. The more we inquire the more we shall find we have to learn.
In these and the following pages we can only tell you a few things,
but no one need be turned away because he does not find all he wants.
We never do get all we want in life, and there are many first-rate
men—scientists—who would give “half their kingdom” for a certain bit
of knowledge concerning some natural phenomena. There are numerous
excellent treatises on chemistry, and exhaustive as they are, at
present they do not tell us all. Let these popular pages lead us to the
study of nature, and we shall find our labour far from onerous and full
of interest, daily increasing to the end, when we shall know no more of
earth, or chemistry.

As a preliminary we will put our workshop aside, and show you something
of _Chemistry without a Laboratory_.



CHAPTER XXV.

CHEMISTRY WITHOUT A LABORATORY.


We have already pointed out the possibility of going through a course
of physics without any special apparatus, we shall now endeavour to
show our readers the method of performing some experiments in chemistry
without a laboratory, or at any rate with only a few simple and
inexpensive appliances. The preparation of gases, such as hydrogen,
carbonic acid, and oxygen, is very easily accomplished, but we shall
here point out principally a series of experiments that are not so much
known. We will commence, for example, by describing an interesting
experiment which often occurs in a course of chemistry. Ammoniacal
gas combined with the elements of water is analogous to a metallic
oxide which includes a metallic root, _ammonium_. This hypothetically
composed metal may be in a manner perceived, since it is possible to
amalgamate it with mercury by operating in the following manner:—We
take a porcelain mortar, in which we pour a quantity of mercury, and
then cut some thin strips of sodium, which are thrown into the mercury.
By stirring it about with the pestle a loud cracking is produced,
accompanied by a flame, which bears evidence to the union of the
mercury and the sodium, and the formation of an amalgam of sodium. If
this amalgam of sodium is put into a slender glass tube containing a
concentrated solution of hydrochlorate of ammonia in water, we see
the ammonia expand in an extraordinary manner, and spout out from the
end of the tube, which is now too small to contain it, in the form
of a metallic substance (fig. 302). In this case, the ammonium, the
radical which exists in the ammoniacal salts, becomes amalgamated with
the mercury, driving out the sodium with which it had previously been
combined; the ammonium thus united with the mercury becomes decomposed
in ammoniacal gas and hydrogen, the mercury assuming its ordinary form.
Phosphate of ammonia is very valuable from its property of rendering
the lightest materials, such as gauze or muslin, incombustible. Dip a
piece of muslin in a solution of phosphate of ammonia, and dry it in
contact with the air; that done, you will find it is impossible to set
fire to the material; it will get charred, but you cannot make it burn.
It is to be wished that this useful precaution were oftener taken in
the matter of ball-dresses, which have so frequently been the cause of
serious accidents. There is no danger whatever with a dress that has
been soaked in phosphate of ammonia, which is very inexpensive, and
easily procured.

[Illustration: Fig. 302.—Experiment with ammonium.]

For preparing cool drinks in the summer ammoniacal salts are very
useful; some _nitrate of ammonia_ mixed with its weight in water,
produces a considerable lowering of the temperature, and is very useful
for making ice. _Volatile alkali_, which is so useful an application
for stings from insects, is a solution of ammoniacal gas in water, and
_sal-volatile_, which has such a refreshing and reanimating odour, is
a carbonate of ammonia. We often see in chemists’ shops large glass
jars, the insides of which are covered with beautiful transparent white
crystals, which are formed over a red powder placed at the bottom of
the vase. These crystals are the result of a combination of cyanogen
and iodine. Nothing is easier than the preparation of _iodide of
cyanogen_, a very volatile body, which possesses a strong tendency to
assume a definite crystalline form. We pound in a mortar a mixture of
50 grams of cyanide of mercury, and 100 grams of iodine; under the
action of the pestle the powder, which was at first a brownish colour,
assumes a shade of bright vermilion red. The cyanogen combines with
the iodine, and transforms itself into fumes with great rapidity. If
the powder is placed at the bottom of a stoppered glass jar, the fumes
of the iodide of cyanogen immediately condense, thereby producing
beautiful crystals which often attain considerable size (fig. 303).
Cyanogen forms with sulphur a remarkable substance, _sulpho-cyanogen_,
the properties of which we cannot describe without exceeding the
limits of our present treatise; we shall therefore confine ourselves
to pointing out one of its combinations, which is well known at the
present day, owing to its singular properties. This is sulpho-cyanide
of mercury, with which small combustible cones are made, generally
designated by the pompous title of _Pharaoh’s serpents_. For making
these, some sulpho-cyanide of potassium is poured into a solution of
nitrate acid on mercury, which forms a precipitate of sulpho-cyanide
of mercury. This is a white, combustible powder, which after passing
through a filter, should be transformed into a stiff pulp by means of
water containing a solution of gum. The pulp is afterwards mixed with
a small quantity of nitrate of potash, and fashioned into cones or
cylinders of about an inch and a quarter in length, which should be
thoroughly dried. The egg thus obtained can be hatched by the simple
application of a lighted match, and gives rise to the phenomenon.
The sulpho-cyanide slowly expands, the cylinder increases in length,
and changes to a yellowish substance, which dilates and extends to a
length of twenty or five-and-twenty inches. It has the appearance of a
genuine serpent, which has just started into existence, and stretches
out its tortuous coils, endeavouring to escape from its narrow prison
(fig. 304). The residue is composed partly of cyanide of mercury and of
para-cyanogen; it constitutes a very poisonous substance, which should
be immediately thrown away or burned. It can be easily powdered into
dust in the fingers. During the decomposition of the sulpho-cyanide of
mercury, quantities of sulphurous acid are thrown off, and it is to be
regretted that Pharaoh’s serpent should herald his appearance by such a
disagreeable, suffocating odour.

[Illustration: Fig. 303.—Iodide of cyanogen.]

[Illustration: Fig. 304.—Pharaoh’s serpent.]

After these few preliminary experiments, we will endeavour to show
the interest afforded by the study of chemistry in relation to the
commonest substances of every-day life. We will first consider the
nature of a few pinches of salt. We know that kitchen salt, or sea
salt, is white or greyish, according to its degree of purity; that it
has a peculiar flavour, is soluble in water, and makes a peculiar
crackling when thrown in the fire. But though its principal physical
properties may be familiar enough, many people are entirely ignorant of
its chemical nature and elementary composition. Kitchen salt contains
a metal, combined with a gas possessing a very suffocating odour; the
metal is _sodium_, the gas is _chlorine_. The scientific name for the
substance is _chloride of sodium_ (salt).[19] The metal contained in
common salt in no way resembles ordinary metals; it is white like
silver, but tarnishes immediately in contact with air, and unites with
oxygen, thus transforming itself into _oxide of sodium_. To preserve
this singular metal it is necessary to protect it from the action of
the atmosphere, and to keep it in a bottle containing oil of naptha.
Sodium is soft, and it is possible with a pair of scissors to cut it
like a ball of soft bread that has been kneaded in the hand. It is
lighter than water, and when placed in a basin of water floats on the
top like a piece of cork; only it is disturbed, and takes the form of
a small brilliant sphere; great effervescence is also produced as it
floats along, for it reduces the water to a common temperature by its
contact. By degrees the small metallic ball disappears from view, after
blazing into flame (fig. 305).

[Illustration: Fig. 305.—Combustion of sodium in water.]

This remarkable experiment is very easy to carry out, and sodium is now
easily procured at any shop where chemicals are sold. The combustion
of sodium in water can be explained in a very simple manner. Water, as
we know, is composed of hydrogen and oxygen. Sodium, by reason of its
great affinity for the latter gas, combines with it, and forms a very
soluble oxide; the hydrogen is released and thrown off, as we shall
perceive by placing a lighted match in the jar, when the combustible
gas ignites.

Oxide of sodium has a great affinity for water; it combines with it,
and absorbs it in great quantities. It is a solid, white substance,
which burns and cauterizes the skin; it is also _alkaline_, and brings
back the blue colour to litmus paper that has been reddened by acids.

Sodium combines easily also with chlorine. If plunged into a jar
containing this gas it is transformed into a substance, which is sea
salt. If the chlorine is in excess a part of the gas remains free, for
simple substances do not mingle in undetermined ratios; they combine,
on the contrary, in very definite proportions, and 35·5 gr. of dry
chlorine always unites with the same quantity of soda equal to 23
grams. A gram of kitchen salt is formed, therefore, of 0·606 gr. of
chlorine, and 0·394 gr. of sodium. Besides sea salt, there are a number
of different salts which may be made the object of curious experiments.
We know that caustic soda, or oxide of sodium, is an alkaline product
possessing very powerful properties; it burns the skin, and destroys
organic substances.

Sulphuric acid is endowed with no less powerful properties; if a little
is dropped on the hand it produces great pain and a sense of burning; a
piece of wood plunged into this acid is almost immediately carbonized.
If we mix forty-nine grams of sulphuric acid and thirty-one grams of
caustic soda a very intense reaction is produced, accompanied by a
considerable elevation of temperature; after it has cooled we have
a substance which can be handled with impunity; the acid and alkali
have combined, and their properties have been reciprocally destroyed.
They have now originated a _salt_ which is _sulphate of soda_. This
substance exercises no influence on litmus paper, and resembles in no
way the substances from which it originated.

There are an infinite number of salts which result in like manner
from the combination of an acid with an alkali or _base_. Some, such
as sulphate of copper, or chromate of potash, are coloured; others,
like sulphate of soda, are colourless. The last-mentioned salt, with a
number of others, will take a crystalline form; if dissolved in boiling
water, and the solution left to stand, we shall perceive a deposit of
transparent prisms of very remarkable appearance. This was discovered
by Glauber, and was formerly called _Glauber’s salts_.

[Illustration: Fig. 306.—Preparation of a solution saturated with
sulphate of soda.]

Sulphate of soda is very soluble in water, and at a temperature of
thirty-three (Centigr.) water can dissolve it in the greatest degree.
If we pour a layer of oil on a solution saturated with Glauber’s
salts, and let it stand, it will not produce crystals; but if we
thrust a glass rod through the oil into contact with the solution,
crystallization will be instantaneous. This singular phenomenon becomes
even more striking when we put the warm concentrated solution into a
slender glass tube, A B, which we close after having driven out the
air by the bubbling of the liquid (fig. 306). When the tube has been
closed, the crystals of sulphate of soda will not form, even with the
temperature at zero; nevertheless the salts, being less soluble cold
than hot, are found in the fluid in a proportion ten times larger than
they would contain under ordinary conditions. If the end of the tube be
broken the salt will crystallize immediately. We will describe another
experiment, but little known and very remarkable, which exhibits in a
striking manner the process of instantaneous crystallizations. Let one
hundred and fifty parts of hyposulphite of soda be dissolved in fifteen
parts of water, and the solution slowly poured into a test-glass,
previously warmed by means of boiling water, until the vessel is about
half-full. One hundred parts of acetate of soda is then dissolved in
fifteen parts of water, and poured slowly into the first solution,
so that they form two layers perfectly distinct from each other. The
two solutions are then covered with a little boiling water, which,
however is not represented in our illustration. After it has been left
to stand and cool slowly, we have two solutions of hyposulphite of
soda and acetate of soda superposed on each other. A thread, at the
end of which is fixed a small crystal of hyposulphite of soda, is then
lowered into the test-glass; the crystal passes through the solution of
acetate without disturbing it, but it has scarcely reached the lower
solution of hyposulphite than the salt crystallizes instantaneously.
(_See_ the test-glass on the left of fig. 307.) We then lower into the
upper solution a crystal of acetate of soda, suspended from another
thread. This salt then crystallizes also. (_See_ experiment glass on
the right of fig. 307.) This very successful experiment is one of the
most remarkable belonging to the subject of instantaneous crystals.
The successive appearance of crystals of hyposulphite of soda, which
take the form of large, rhomboidal prisms, terminating at the two
extremities with an oblique surface, and the crystals of acetate of
soda, which have the appearance of rhomboidal, oblique prisms, cannot
fail to strike the attention and excite the interest of those who are
not initiated into these kinds of experiments.

[Illustration: Fig. 307.—Experiment of instantaneous crystallization.]

Another remarkable instantaneous crystallization is that of alum. If we
leave standing a solution of this salt it gradually cools, at the same
time becoming limpid and clear. When it is perfectly cold, if we plunge
into it a small octahedral crystal of alum suspended from a thread, we
perceive that crystallization instantly commences on the surface of the
small crystal; it rapidly and perceptibly increases in size, until it
nearly fills the whole jar.


COMMON METALS AND PRECIOUS METALS.

How many invalids have swallowed _magnesia_ without suspecting that
this powder contains a metal nearly as white as silver, and is
malleable, and capable of burning with so intense a light that it
rivals even the electric light in brilliancy! If any of our readers
desire to prepare magnesium themselves it can be done in the following
manner:—Some white magnesia must be obtained from the chemist, and
after having been calcined, must be submitted to the influence of
hydrochloric acid and hydrochlorate of ammonia. A clear solution will
thus be obtained, which by means of evaporation under the influence
of heat, furnishes a double chloride, hydrated and crystallised. This
chloride, if heated to redness in an earthenware crucible, leaves as
a residue a nacreous product, composed of micaceous, white scales,
chloride of anhydrous magnesium.

[Illustration: Fig. 308.—Group of alum crystals.]

If six hundred grams of this chloride of magnesium are mixed with one
hundred grams of chloride of sodium, or kitchen salt, and the same
quantity of fluoride of calcium and metallic sodium in small fragments,
and the mixture is put into an earthenware crucible made red-hot, and
heated for a quarter of an hour under a closed lid, we shall find on
pouring out the fluid on to a handful of earth, that we have obtained
instead of scoria, forty-five grams of metallic magnesium. The metal
thus obtained is impure, and to remove all foreign substances it
must be heated in a charcoal tube, through which passes a current of
hydrogen.

Magnesium is now produced in great abundance, and is very inexpensive.
It is a metal endowed with a great affinity for oxygen, and it is
only necessary to thrust it into the flame of a candle to produce
combustion; it burns with a brightness that the eye can scarcely
tolerate, and is transformed into a white powder—oxide of magnesium,
or magnesia. Combustion is still more active in oxygen, and powder
of magnesium placed in a jar filled with this gas produces a perfect
shower of fire of very beautiful effect. To give an idea of the
lighting power of magnesium, we may add that a wire of this metal,
which is 29/100 of a millimetre in diameter, produces by combustion a
light equal to that of seventy-four candles.

[Illustration: Fig. 309.—Calcined alum.]

[Illustration: Fig. 310.—Preparation of metallic iron.]

The humble earth of the fields—the clay which is used in our potteries,
also contains aluminium, that brilliant metal which is as malleable
as silver, and unspoilable as gold. When clay is submitted to the
influence of sulphuric acid and chloride of potassium, we obtain alum,
which is a sulphate of alumina and potash. Alum is a colourless salt,
which crystallizes on the surface of water in beautiful octahedrons of
striking regularity. Fig. 308 represents a group of alum crystals. This
salt is much used in the colouring of fabrics; it is also used for the
sizing of papers, and the clarification of tallow. Doctors also use
it as an astringent and caustic substance. When alum is submitted to
the action of heat in an earthenware crucible, it loses the water of
crystallization which it contains, and expands in a singular manner,
overflowing from the jar in which it is calcined (fig. 309).

Iron, the most important of common metals, rapidly unites with oxygen,
and, as we know, when a piece of this metal is exposed to the influence
of damp air, it becomes covered with a reddish substance. In the
well-known experiment of the formation of rust, the iron gradually
oxidises without its temperature rising, but this combination of iron
with oxygen is effected much more rapidly under the influence of heat.
If, for example, we redden at the fire a nail attached to a wire, and
give it a movement of rotation as of a sling, we see flashing out
from the metal a thousand bright sparks due to the combination of
iron with oxygen, and the formation of an oxide. Particles of iron
burn spontaneously in contact with air, and this property for many
centuries has been utilized in striking a tinder-box; that is to say,
in separating, by striking a flint, small particles of iron, which
ignite under the influence of the heat produced by the friction. We can
prepare iron in such atoms that it ignites at an ordinary temperature
by simple contact with the air. To bring it to this state of extreme
tenuity, we reduce its oxalate by hydrogen. We prepare an apparatus for
hydrogen as shown in fig. 310, and the gas produced at A is
passed through a desiccative tube, B, and finally reaches a
glass receptacle, C, in which some oxalate of iron is placed.
The latter salt, under the combined influence of hydrogen and heat,
is reduced to metallic iron, which assumes the appearance of a fine
black powder. When the experiment is completed the glass vessel is
closed, and the iron, thus protected from contact with the air, can be
preserved indefinitely; but if it is exposed to the air by breaking
off the end of the receptacle (fig. 311), it ignites immediately,
producing a shower of fire of very beautiful effect. Iron thus prepared
is known under the name of _pyrophoric iron_. Iron is acted upon in
a very powerful manner by most acids. If some nitric acid is poured
on iron nails, a stream of red, nitrous vapour is let loose, and the
oxidised iron is dissolved in the liquid to the condition of nitrate
of iron. This experiment is very easy to perform, and it gives an idea
of the energy of certain chemical actions. We have endeavoured to
represent its appearance in fig. 312. Fuming nitric acid does not act
on iron, and prevents it being attacked by ordinary nitric acid. This
property has given rise to a very remarkable experiment on passive
iron. It consists in placing some nails in a glass, into which some
fuming nitric acid is poured, which produces no result; the fuming acid
is then taken out, and is replaced by ordinary nitric acid, which no
longer acts on the iron rendered passive by the smoking acid. After
this, if the nails are touched by a piece of iron, which has not
undergone the action of nitric acid, they are immediately acted upon,
and a giving off of nitrous vapour is manifested with great energy.
Lead is a very soft metal, and can even be scratched by the nails. It
is also extremely pliable, and so entirely devoid of elasticity that
when bent it has no tendency whatever to return to its primitive form.
Lead is heavy, and has a density represented by 11·4; that is to say,
the weight of a quart of water being one kilogram, that of the same
volume of lead is 11·400 k.

[Illustration: Fig. 311.—Pyrophoric iron.]

Fig. 313 represents cylindrical bars of the best known metals, all
weighing the same, showing their comparative density.

[Illustration: Fig. 312.—Iron and nitric acid.]

Lead, like tin, is capable of taking a beautiful crystalline form
when placed in solution by a metal that is less oxydisable. The
crystallization of lead, represented in fig. 314, is designated by the
name of the _Tree of Saturn_. This is how the experiment is produced:
Thirty grams of acetate of lead are dissolved in a quart of water, and
the solution is poured into a vase of a spherical shape. A stopper for
this vase is made out of a piece of zinc, to which five or six separate
brass wires are attached; these are plunged into the fluid, and we
see the wires become immediately covered with brilliant crystallized
spangles of lead, which continue increasing in size.

The alchemists, who were familiar with this experiment, believed that
it consisted in a transformation of copper into lead, while in reality
it only consists in the substitution of one metal for another. The
copper is dissolved in the liquid, and is replaced by the lead, but no
metamorphosis is brought about. We may vary at will the form of the
vase or the arrangement of the wire; thus it is easy to form letters,
numbers, or figures, by the crystallization of brilliant spangles.

  [Illustration: Platinum Density 21.50
  Gold D. 19.25
  Mercury D. 13.56
  Lead D. 11.35
  Silver D. 10.47
  Bismuth D. 9.82
  Copper D. 8.78
  Nickel D. 8.27
  Tin D. 7.29
  Iron D. 7.20
  Zinc D. 6.86
  Aluminium D. 2.56
  Magnesium D. 1.43
  Sodium D. 0.97
  Fig. 313.—Representation of bars of metal, all of the same weight.]

Copper, when it is pure, has a characteristic red colour, which
prevents it being confounded with any other metal; it dissolves easily
in nitric acid, and with considerable effervescence, giving off vapour
very abundantly. This property has been put to good use in engraving
with aqua fortis. A copper plate is covered with a layer of varnish,
and when it is dry some strokes are made on it by means of a graver;
if nitric acid is poured on the plate when thus prepared, the copper
is only acted on in the parts that have been exposed by the point of
steel. By afterwards removing the varnish, we have an engraved plate,
which will serve for printing purposes.

[Illustration: Fig. 314.—Tree of Saturn.]

Among experiments that may be attempted with common metals, we may
mention that in which salts of tin are employed. Tin has a great
tendency to assume a crystalline form, and it will be easy to show
this property by an interesting experiment. A concentrated solution
of proto-chloride of tin, prepared by dissolving some metallic tin in
hydrochloric acid, is placed in a test glass; then a rod of tin is
introduced, as shown in fig. 315. Some water is next slowly poured on
the rod, so that it gradually trickles down, and prevents the mingling
of the proto-chloride of tin. The vessel is then left to stand,
and we soon see brilliant crystals starting out from the rod. This
crystallization is not effected in the water; it is explained by an
electric influence, into the details of which we cannot enter without
overstepping our limits; it is known as “Jupiter’s Tree.” It is well
known that alchemists, with their strange system of nomenclature,
believed there was a certain mysterious relation between the seven
metals then known and the seven planets; each metal was dedicated to
a planet; tin was called Jupiter; silver, Luna; gold, Sol; lead,
Saturn; iron, Mars; quicksilver, Mercury; and copper, Venus. The
crystallization of tin may be recognised also by rubbing a piece of
this metal with hydrochloric acid; the fragments thus rubbed off
exhibit specimens of branching crystals similar to the hoar-frost which
we see in severe winter weather. If we bend a rod of tin in our hands
the crystals break, with a peculiar rustling sound.

When speaking of precious metals, we may call to mind that the
alchemists considered gold as the king of metals, and the other
valuable ones as noble metals. This definition is erroneous, if we
look upon the useful as the most precious; for, in that case, iron and
copper would be placed in the first rank. If gold were found abundantly
on the surface of the soil, and iron was extremely rare, we should seek
most eagerly for this useful metal, and should despise the former,
with which we can neither make a ploughshare nor any other implement
of industry. Nevertheless, the scarcity of gold, its beautiful yellow
colour, and its unalterability when in contact with air, combine to
place it in the first rank in the list of precious metals. Gold is very
heavy; its density is represented by the figure 19·5. It is the most
malleable and the most ductile of metals, and can be reduced by beating
to such thin sheets that ten thousand can be laid, one over the other,
to obtain the thickness of a millimetre. With a grain of gold a thread
may be manufactured extending a league in length, and so fine that it
resembles a spider’s web. When gold is beaten into thin sheets it is
no longer opaque; if it is fastened, by means of a solution of gum, on
to a sheet of glass, the light passes right through it, and presents
a very perceptible green shade. Gold is sometimes found scattered in
sand, in a condition of impalpable dust, and, in certain localities,
in irregular lumps of varying size, called nuggets. Gold is the least
alterable of the metals, and can be exposed, indefinitely, to the
contact of humid atmosphere without oxidizing. It is not acted on by
the most powerful acids, and only dissolves in a mixture of nitric acid
and hydrochloric acid. We can prove that gold resists the influence of
acids by the following operation:—

Some gold-leaf is placed in two small phials, the first containing
hydrochloric acid, and the second nitric acid. The two vessels are
warmed on the stove, and whatever the duration of the ebullition of
the acids, the gold-leaf remains intact, and completely resists their
action. If we then empty the contents of one phial into the other,
the hydrochloric and nitric acids are mixed, and we see the gold-leaf
immediately disappear, easily dissolved by the action of the liquid
(_aqua regia_). Gold also changes when in contact with mercury; this is
proved by suspending some gold-leaf above the surface of this liquid
(fig. 316); it quickly changes, and unites with the fumes of the
mercury, becoming of a greyish colour.

Silver is more easily affected than gold, and though so white when
fused, tarnishes rapidly in contact with air. It does not oxidize, but
sulphurizes under the influence of hydro-sulphuric emanations. Silver
does not combine directly with the oxygen of the atmosphere; but under
certain conditions it can dissolve great quantities of this gas. If it
is fused in a small bone cupel, in contact with the air, and left to
cool quickly, it expands in a remarkable manner, and gives off oxygen.

[Illustration: Fig. 315.—Jupiter’s Tree.]

Nitric acid dissolves silver very easily, by causing the formation
of abundant fumes. When the solution evaporates, we perceive white
crystals forming, which are nitrate of silver. This fused nitrate of
silver takes the name of _lunar caustic_, and is employed in medicine.
Nitrate of silver is very poisonous; it possesses the singular property
of turning black under the action of the sun’s rays, and is used in
many curious operations in photography. It is also employed in the
manufacture of dyes for the hair; it is applied to white hair with
gall-nut, and under the influence of the light it turns black, and
gives the hair a very dark shade. Salts of silver in solution with
water have the property of forming a precipitate under the influence of
chlorides, such as sea salt. If a few grains of common salt are thrown
into a solution of nitrate of silver, it forms an abundant precipitate
of chloride of silver, which blackens in the light. This precipitate,
insoluble in nitric acid, dissolves very easily in ammonia.

Platinum, which is the last of the precious metals that we have to
consider, is a greyish-white colour, and like gold is only affected by
a mixture of nitric acid and hydrochloric acid. It is the heaviest of
all the ordinary metals; its density is 21·50. It is very malleable
and ductile, and can be beaten into very thin sheets, and into wires as
slender as wires of gold. Platinum wires have even been made so fine
that the eye can scarcely perceive them; these are known as Wollaston’s
invisible wires. Platinum resists the action of the most intense fire,
and we can only fuse it by means of a blow-pipe and hydro-oxide gas.
Its inalterability and the resistance it opposes to fire render it very
valuable for use in the laboratory. Small crucibles are made of it,
which are used by chemists to calcine their precipitates in analytical
operations, or to bring about reactions under the influence of a high
temperature. Platinum may be reduced to very small particles; it then
takes the form of a black powder. In this pulverulent condition it
absorbs gases with great rapidity, to such an extent that a cubic
centimetre can condense seven hundred and fifty times its own volume
of hydrogen gas. It also condenses oxygen, and in a number of cases
acts as a powerful agent. Platinum is also obtained in porous masses
(“spongy platinum”), which produce phenomena of oxidation.

[Illustration: Fig. 316.—Gold-leaf exposed to the fumes of mercury.]

A very ingenious little lamp may be constructed which lights of
itself without the help of a flame. It contains a bell of glass,
which is filled with hydrogen gas, produced by the action exercised
by a foundation of zinc on acidulated water. If the knob on the upper
part of the apparatus is pressed, the hydrogen escapes, and comes in
contact with a piece of spongy platinum, which, acting by oxidation,
becomes ignited. The flame produced sets fire to a small oil lamp,
which is opposite the jet of gas. This very ingenious lamp is known
under the name of Gay-Lussac’s lamp. Platinum can also produce, by mere
contact, a great number of chemical reactions. Place in a test glass an
explosive mixture formed of two volumes of hydrogen and one volume of
oxygen; in this gas plunge a small piece of spongy platinum, and the
combination of the two bodies will be instantly brought about, making
a violent explosion. Make a small spiral of platinum red-hot in the
flame of a lamp, having suspended it to a card; then plunge it quickly
into a glass containing ether, and you will see the metallic spiral
remain red for some time, while in the air it would cool immediately.
This phenomenon is due to the action of oxidation which the platinum
exercises over the fumes of ether. This curious experiment is known
under the name of _the lamp without a flame_. This remarkable oxidizing
power of platinum, which has not yet been explained, was formerly
designated by the title of _catalytic action_. But a phrase is not a
theory, and it is always preferable to avow one’s ignorance than to
simulate an apparent knowledge. Science is powerful enough to be able
to express her doubts and uncertainties boldly. In observing nature we
find an experience of this, and often meet with facts which may be put
to profit, and become useful in application; nevertheless it is often
the case that the why and the wherefore will for a long time escape the
most penetrating eye and lucid intelligence. It is true the admirable
applications of science strike us with the importance of their results,
and the wonderful inventions they originate; but if they turn to
account the observed facts of nature, what do they teach us as to the
first cause of all things, the _wherefore_ of nature?—Almost nothing.
We must humbly confess our powerlessness, and say with d’Alembert: “The
encyclopædia is very abundant, but what of that if it discourses of
what we do not understand?”

[Illustration: Fig. 317.—Discolouration of periwinkles by sulphuric
acid.]


ARTIFICIAL COLOURING OF FLOWERS.

In a course of chemistry, the action exercised by sulphurous acid
on coloured vegetable matter is proved by exposing violets to the
influence of this gas, which whitens them instantaneously. Sulphurous
acid, by its dis-oxidating properties, destroys the colour of many
flowers, such as roses, periwinkles, etc. The experiment succeeds very
readily by means of the little apparatus which we give in fig. 317.
We dissolve in a small vessel some sulphur, which ignites in contact
with air, and gives rise, by its combination with oxygen, to sulphurous
acid; the capsule is covered with a conical chimney, made out of a thin
sheet of copper, and at the opening at the top the flowers that are to
be discoloured are placed. The action is very rapid, and a few seconds
only are necessary to render roses, periwinkles, and violets absolutely
white.

[Illustration: Fig. 318.—Experiment for turning columbines a green
colour with ammoniacal ether.]

M. Filpol, a distinguished _savant_, has exhibited to the members of
the Scientific Association, Paris, the results which he obtained by
subjecting flowers to the influence of a mixture of sulphuric ether
and some drops of ammonia; he has shown that, under the influence of
this liquid, a great number of violets or roses turn a deep green.
We have recently made on this subject a series of experiments which
we will here describe, and which may be easily attempted by those of
our readers who are interested in the question. Some common ether is
poured into a glass, and to it is added a small quantity of liquid
ammonia (about one-tenth of the volume). The flowers with which it
is desired to experiment are then plunged into the fluid (fig. 318).
A number of flowers, whose natural colour is red or violet, take
instantaneously a bright green tint; these are red geranium, violet,
periwinkle, lilac, red and pink roses, wall-flower, thyme, small
blue campanula, fumeter, myosotis, and heliotrope. Other flowers,
whose colours are not of the same shade, take different tints when
in contact with ammoniacal ether. The upper petal of the violet
sweet-pea becomes dark blue, whilst the lower petal turns a bright
green colour. The streaked carnation becomes brown and bright green.
White flowers generally turn yellow, such as the white poppy, the
variegated snow-dragon, which becomes yellow and dark violet, the
white rose, which takes a straw colour, white columbine, camomile,
syringa, white daisy, potatoe blossom, white julian, honeysuckle, and
white foxglove, which in contact with ammoniacal ether assume more or
less deep shades of yellow. White snap-dragon becomes yellow and dark
orange. Red geranium turns blue in a very remarkable fashion; with
the monkey-flower the ammoniacal ether only affects the red spots,
which turn a brownish green; red snap-dragon turns a beautiful brown;
valerian takes a shade of grey; and the red corn-poppy assumes a dark
violet. Yellow flowers are not changed by ammoniacal ether; buttercups,
marigolds, and yellow snap-dragon preserve their natural colour. Leaves
of a red colour are instantly turned green when placed in contact with
ammoniacal ether. The action of this liquid is so rapid that it is
easy to procure green spots by pouring here and there a drop of the
solution. In like manner violet flowers, such as periwinkles, can be
spotted with white, even without gathering them. We will complete our
remarks on this subject with a description of experiments performed
by M. Gabba in Italy by means of ammonia acting on flowers. M. Gabba
simply used a plate, in which he poured a certain quantity of solution
of ammonia. He placed on the plate a funnel turned upside down, in the
tube of which he arranged the flowers on which he wished to experiment.
He then found that under the influence of the ammonia the blue, violet,
and purple flowers became a beautiful green, red flowers black, and
white yellow, etc.

The most singular changes of colour are shown by flowers which are
composed of different tints, their red streaks turning green, the
white yellow, etc. Another curious example is that of red and white
fuchsias, which, through the action of ammonia, turn yellow, blue, and
green. When flowers have been subjected to these changes of colour,
and afterwards plunged into pure water, they preserve their new tint
for several hours, after which they gradually return to their natural
colour. Another interesting observation, due to M. Gabba, is that
asters, which are naturally inodorous, acquire an agreeable aromatic
odour under the influence of ammonia. Asters of a violet colour become
red when wetted with nitric acid mixed with water. On the other hand,
if these same flowers are enclosed in a wooden box, where they are
exposed to the fumes of hydrochloric acid, they become, in six hours’
time, a beautiful red colour, which they preserve when placed in a dry,
shady place, after having been properly dried. Hydrochloric acid has
the effect of making flowers red that have been rendered green by the
action of ammonia, and also alters their appearance very sensibly. We
may also mention, in conclusion, that ammonia, combined with ether,
acts much more promptly than when employed alone.


PHOSPHORESCENCE.

Artificial flowers are frequently to be seen prepared in a particular
manner, which have the property of becoming phosphorescent in darkness,
when they have been exposed to the action of a ray of light, solar or
electric. These curious chemical objects are connected with some very
interesting phenomena and remarkable experiments but little known at
the present time, to which we will now draw the reader’s attention.

The faculty possessed by certain bodies of emitting light when placed
in certain conditions, is much more general than is usually supposed.

M. Edmond Becquerel, to whom we owe a remarkable work on this subject,
divides the phenomena of phosphorescence into five distinct classes:

1. _Phosphorescence through elevation of temperature._ Among the
substances which exhibit this phenomenon in a high degree we may
mention certain diamonds, coloured varieties of fluoride of calcium,
some minerals; and sulphur, known under the name of artificial
phosphorus, when it has previously been exposed to the action of the
light.

2. _Phosphorescence through mechanical action._ This is to be observed
when we rub certain bodies together, or against a hard substance. If we
rub together two quartz crystals in the dark, we perceive red sparks;
and when pounding chalk or sugar, there is also an emission of sparks.

3. _Phosphorescence through electricity._ This is manifested by the
light accompanying disengagement of electricity, and when gases and
rarefied vapours transmit electric discharges.

4. _Spontaneous Phosphorescence_ is observed, as every one knows,
in connection with several kinds of living creatures,—glow-worms,
noctilucids, etc., and similar phosphorescent effects are produced also
with organic substances, animal or vegetable, before putrefaction sets
in. It is manifested also at the flowering time of certain plants, etc.

5. _Phosphorescence through insolation and the action of light._ “It
consists,” says M. Edmond Becquerel, “in exposing for some instants to
the action of the sun, or to that of rays emanating from a powerful
luminous source, certain mineral or organic substances, which
immediately become luminous, and shine in the dark with a light, the
colour and brilliancy of which depend on their nature and physical
character; the light gradually diminishes in intensity during a period
varying from some seconds to several hours. When these substances are
exposed anew to the action of light, the same effect is reproduced.
The intensity of the light emitted after insolation is always much less
than that of the incidental light.” These phenomena appear to have been
first observed with precious stones; then, in 1604, in calcined Bologna
stone, and later, in a diamond by Boyle, in 1663; in 1675 it was
noticed in Baudoin phosphorus (residuum of the calcination of nitrate
of lime), and more recently still in connection with other substances
which we will mention. The substances most powerfully influenced by
the action of light are sulphates of calcium and barium, sulphate of
strontium, certain kinds of diamonds, and that variety of fluoride of
calcium, which has received the name of _chlorophane_.

[Illustration: Fig. 319.—Artificial flower coated with phosphorescent
powder, exposed to the light of magnesium wire.]

Phosphorescent sulphate of calcium is prepared by calcining in an
earthenware crucible a mixture of flowers of sulphur and carbonate
of lime. But the preparation only succeeds with carbonate of lime of
a particular character. That obtained from the calcination of oyster
shells produces very good results. Three parts of this substance is
mixed with one part of flowers of sulphur, and is made red-hot in a
crucible covered in from contact with the air. The substance thus
obtained gives, after its insolation, a yellow light in the dark. The
shells of oysters, however, are not always pure, and the result is
sometimes not very satisfactory; it is therefore better to make use of
some substance whose composition is more to be relied on.

“When we desire to prepare a phosphorescent sulphate with lime,
or carbonate of lime,” says M. E. Becquerel, “the most suitable
proportions are those which in a hundred parts of the substance are
composed of eighty to a hundred of flowers of sulphur in the first
case, and forty-eight to a hundred in the second, that is, when we
employ the quantity of sulphur which will be necessary for burning with
carbonate of lime to produce a monosulphate.[20] It is necessary to
have regard to the elevation of the temperature in the preparation. By
using lime procured from arragonite, and reducing the temperature below
five hundred degrees for a sufficient time for the reaction between the
sulphur and lime to take place, the excess of sulphur is eliminated,
and we have a feebly luminous mass, of a bluish tint; if this mass is
raised to a temperature of eight hundred or nine hundred degrees, it
will exhibit a very bright light.”

Sulphate of calcium possesses different phosphorescent properties
according to the nature of the salt which has served to produce the
carbonate of lime employed. If we transform marble into nitrate of
lime, by dissolving it in water and nitric acid, and form a precipitate
with carbonate of ammonium, and use the carbonate of lime thus obtained
in the preparation of sulphate of calcium, we have a product which
gives a phosphorescence of a violet-red colour. If the carbonate
of lime used is obtained from chloride of calcium precipitated by
carbonate of ammonia, the phosphorescence is yellow. If we submit
carbonate of lime, prepared with lime water and carbonic acid, to the
influence of sulphur, we obtain a sulphur giving a phosphorescent
light of very pure violet. Carbonate of lime obtained by forming a
precipitate of crystallized chloride of calcium with different alkaline
carbonates also gives satisfactory results.

Luminous sulphates of strontium may be obtained, like those of calcium,
by the action of sulphur on strontia or the carbonate of this base, by
the reduction of sulphates of strontia with charcoal. Blue and green
shades are the most common. Sulphates of barium also present very
remarkable phenomena of phosphorescence; but to obtain very luminous
intensity a higher temperature is needed than with the other substances
mentioned, and we have the same result when we reduce native sulphate
of baryta with charcoal; that is to say, when the reaction takes
place which produces the phosphorus formerly known as _phosphorus of
Bologna_. Preparations obtained from baryta have a phosphorescence
varying from orange-red to green.

The preparation of such substances as we have just enumerated afford an
easy explanation of the method of manufacturing the luminous flowers
which we described at the commencement of this chapter. We obtain
some artificial flowers, cover them with some liquid gum, sprinkle
with phosphorescent sulphur, and let them dry. The pulverulent matter
then adheres to them securely, and it is only necessary to expose the
flowers thus prepared to the light of the sun, or the rays emanating
from magnesium wire in a state of combustion (fig. 319), to produce
immediate phosphorescent effects. If taken into a dark room (fig. 320)
they shine with great brilliancy, and give off very exquisite coloured
rays. Phosphorescent sulphates are used also in tracing names or
designs on a paper surface, etc., and it can easily be conceived that
such experiments may be infinitely varied according to the pleasure of
the experimenter.

[Illustration: Fig. 320.—Phosphorescent flower emitting light in a dark
room.]

But let us ask ourselves if these substances are not capable of
being put to more serious uses, and of being classed among useful
products. To this we can reply very decidedly in the affirmative. With
phosphorescent matter we can obtain luminous faces for clocks placed
in dark, obscure spots, and it is not impossible to use it for making
sign-boards for shops, or numbers of houses, which can be lit up at
night. Professor Norton even goes so far as to propose in the “Journal
of the Franklin Institute,” not only coating the walls of rooms with
these phosphorescent substances, but also the fronts of houses, when he
considers it would be possible to do away entirely with street lights,
the house-fronts absorbing sufficient light during the day to remain
luminous the whole of the night.


CHEMISTRY APPLIED TO SLEIGHT OF HAND.

While physics has provided the species of entertainment called “sleight
of hand” with a number of interesting effects, chemistry has only
offered it very feeble contributions. Robert Houdin formerly made use
of electricity to move the hands of his magic clock, and the electric
magnet in making an iron box so heavy instantaneously that no one
could lift it. Robin has made use of optics to produce the curious
spectacle of the decapitated man, spectres, etc. Those persons who are
fond of this kind of amusement may, however, borrow from chemistry
some original experiments, which can be easily undertaken, and I will
conclude this chapter by describing a juggling feat which I have seen
recently executed before a numerous audience by a very clever conjuror.

[Illustration: Fig. 321.—Amusing experiment in chemistry.]

The operator took a glass that was perfectly transparent, and placed
it on a table, announcing that he should cover the glass with a
saucer, and then, retiring to some distance, would fill it with the
smoke from a cigarette. And this he carried out exactly, standing
smoking his cigarette in the background, while the glass, as though
by enchantment, slowly filled with the fumes of the smoke. This trick
is easily accomplished. It is only necessary to pour previously into
the glass two or three drops of hydrochloric acid, and to moisten the
bottom of the saucer with a few drops of ammonia. These two liquids
are unperceived by the spectators, but as soon as the saucer is placed
over the glass, they unite in forming white fumes of hydrochlorate of
ammonia, which bear a complete resemblance to the smoke of tobacco.

This experiment excited the greatest astonishment among the spectators
present on the occasion, but understanding something of chemistry
myself, I easily guessed at the solution of the mystery. The same
result is obtained in a course of chemistry in a more simple manner,
and without any attempt at trickery, by placing the opening of a
bottle of ammonia against the opening of another bottle containing
hydrochloric acid.


FOOTNOTES:

[19] It is the same with a number of other common products, such as
clay, sandstone, etc., the composition of which chemistry has revealed.
Argil, or clay, slate, and schist all contain a metal—_aluminium_,
which has become most valuable for industrial purposes. Stones
for building are composed of a metal combined with carbon and
oxygen—_calcium_; sandstone is composed of _silicium_, a metallic body
united with oxygen; and sulphate of magnesia, which enters into the
composition of a purgative drink, also contains a metal—_magnesium_.

[20] These substances must be finely powdered and thoroughly mixed.



CHAPTER XXVI.

 CHEMISTRY AND ALCHEMY—CHEMICAL COMBINATIONS—THE ATMOSPHERIC AIR.


We have in the foregoing pages given some experiments, and considered
several of the metals, but there are numerous very interesting subjects
still remaining; indeed, the number is so great that we can only
pick and choose. All people are desirous to hear something of the
atmosphere, of water, and the earth; and as we proceed to speak of
crystals and minerals, and so on to geology, we shall learn a good
deal respecting our globe—its conformation and constituents. But the
atmospheric air must be treated of first. This will lead us to speak of
oxygen and nitrogen. Water will serve to introduce hydrogen with a few
experiments, and thus we shall have covered a good deal of ground on
our way towards various other elements in daily use and appreciation.
Now let us begin with a few words concerning CHEMISTRY itself.

At the very outset we are obliged to grope in the dark after the origin
of this fascinating science. Shem, or “Chem,” the son of Noah, has
been credited with its introduction, and, at any rate, magicians were
in Egypt in the time of Moses, and the lawgiver is stated by ancient
writers to have gained his knowledge from the Egyptians. But we need
not pursue that line of argument. In more modern times the search for
the Philosopher’s Stone and the Elixir of Life, which respectively
turned everything to gold, and bestowed long life upon the fortunate
finder, occupied many people, who in their researches no doubt
discovered the germs of the popular science of Chemistry in Alchemy,
while the pursuit took a firm hold of the popular imagination for
centuries; and even now chemistry is the most favoured science, because
of its adaptability to all minds, for it holds plain and simple truths
for our every-day experience to confirm, while it leads us step by step
into the infinite, pleasing us with experiments as we proceed.

Alchemy was practised by numerous quacks in ancient times and the
Middle Ages, but all its professors were not quacks. Astrology and
alchemy were associated by the Arabians. Geber was a philosopher who
devoted himself entirely to alchemy, and who lived in the year 730
A.D. He fancied gold would cure all disease, and he did
actually discover corrosive sublimate, nitric acid, and nitrate of
silver. To give even a list of the noted alchemists and magicians would
fill too much space. Raymond Sully, Paracelsus, Friar Bacon, Albertus
Magnus, Thomas Aquinas, Flamel, Bernard of Treves, Doctor Dee, with his
assistant Kelly, and in later times Jean Delisle, and Joseph Balsamo
(Cagliostro), who was one of the most notorious persons in Europe about
one hundred years ago (1765-1789), are names taken at random; and with
the older philosophers chemistry was an all-absorbing occupation—not
for gold, but knowledge.

The revelation was slow. On the temperature of bodies the old arts
of healing were based—for chemistry and medicine were allies. The
elements, we read, existed on the supposition “that bodies were hot
or cold, dry or moist”; and on this distinction for a long time “was
based the practice of medicine.” The doctrine of the “three principles”
of existence superseded this,—the principles being salt, mercury, and
sulphur. Metals had been regarded as living bodies, gases as souls or
spirits. The idea remained that the _form_ of the substance gave it its
character. Acid was pointed; sweet things were round.

Chemistry, then, has had a great deal to contend against. From the
time of the Egyptians and Chinese, who were evidently acquainted with
various processes,—dyeing, etc.,—the science filtered through the
alchemists to Beecher and Stahl, and then the principle of affinity—a
disposition to combine—was promulgated, supplemented in 1674 by
Mayow, by the theory of divorce or analysis. He concluded that where
union could be effected, separation was equally possible. In 1718 the
first “Table of Affinities” was produced. Affinity had been shown
to be _elective_, for Mayow pointed out that fixed salts chose one
acid rather than another. Richter and Dalton made great advances.
Before them Hales, Black, Priestley, Scheele, Lavoisier, and numerous
others penetrated the mysteries of the science whose history has been
pleasantly written by more than one author who we have not been able
to consult, and have no space to do more than indicate. In later days
Faraday, De la Rive, Roscoe, and many others have rendered chemistry
much more popular, while they have added to its treasures. The story
of the progress of chemistry would fill a large volume, and we have
regretfully to put aside the introduction and pass on.

Before proceeding to investigate the elements, a few words concerning
the general terms used in chemistry will be beneficial to the reader.
If we look at the list of the elements, pp. 308-9, we shall see
various terminations. Some are apparently named from places, some from
their characteristics. Metals lately discovered by the spectroscope
(and recently) end in _ium_; some end in “ine,” some in “on.” As far
as possible in late years a certain system of nomenclature has been
adhered to, but the old popular names have not been interfered with.

When elements combine together in certain proportions of each they
receive certain names. The following table will explain the terms used;
for instance, we find that—

  Compounds of Oxygen   are termed Oxides, as oxide of copper.
     ”         Hydrogen      ”     Hydrides, as hydride of potassium.
     ”         Chlorine      ”     Chlorides, as chloride of sodium.
     ”         Nitrogen      ”     Nitrides, as nitride of boron.
     ”         Bromine       ”     Bromides, as bromide of potassium.
     ”         Iodine        ”     Iodides, as iodide of potassium.
     ”         Sulphur       ”     Sulphides, or}
                                     Sulphurets,} as sulphuret of lead.
     ”         Selenium      ”     Selenides, as selenide of mercury.
     ”         Carbon        ”     Carbides, or} as carbide of nitrogen,
                                     Carburets,} and so on.

The above examples refer to the union in single proportion of each, and
are called Binary Compounds. When more than one atom of each element
exists in different proportions we have different terms to express
such union. If one atom of oxygen be in the compound it is called
a “monoxide” or “protoxide”; two atoms of oxygen in combination is
termed “dioxide” or “binoxide”; three, “trioxide,” or “tritoxide”;
four is the “tetroxide” or “per-oxide,” etc. When more than one
atom, but not two atoms is involved, we speak of the _sesqui_-oxide
(one-and-a-half),—“oxide” being interchangeable for “sulphide” or
“chloride,” according to the element.

There are other distinctions adopted when metals form two series of
combinations, such as _ous_ and _ic_, which apply, as will be seen, to
acids. Sulphur_ic_ and sulphur_ous_ acids, nitr_ic_ and nitr_ous_ acid
are familiar examples. In these cases we shall find that in the acids
ending in “ous” oxygen is present in less quantity than in the acids
ending in _ic_. The symbolic form will prove this directly, the number
of atoms of oxygen being written below,

  Sulphurous Acid = H_{2} SO_{3}.     Nitrous Acid = HNO_{2}.
  Sulphuric Acid = H_{2}SO_{4}.       Nitric Acid = HNO_{3}.

Whenever a stronger compound of oxygen is discovered than that
denominated by _ic_, chemists adopt the plan of dubbing it the _per_
(ὑπέρ over), as per-chloric acid, which possesses four atoms of oxygen
(HClO_{4}), chloric acid being HClO_{3}. The opposite Greek term,
ὑπὸ (_hupo_, below), is used for an acid with less than two atoms
of oxygen, and in books is written “hypo”-chlorous (for instance).
Care has been taken to distinguish between the higher and lower; for
“_hyper_” is used in English to denote excess, as hyper-critical; and
_hypo_ might to a reader unacquainted with the derivation convey just
the opposite meaning to what is intended.

While speaking of these terminations we may show how these distinctive
endings are carried out. We shall find, if we pursue the subject, that
when we have a salt of any acid ending in _ic_ the salt terminates in
“_ate_.” Similarly the salts of acids ending in _ous_, end in “_ite_.”
To continue the same example we have—

  Sulphur_ous_ Acid, which forms salts called Sulph_ites_.
  Sulphur_ic_ Acid,       ”           ”       Sulph_ates_.

Besides these are sulph_ides_, which are results of the unions or
compounds of elementary bodies. Sulph_ites_ are more complicated
unions of the compounds. Sulphates are the salts formed by the union
of sulphuric acid with bases. Sulphides or sulphurets are compounds in
which sulphur forms the electro-negative element, and sulphites are
salts formed by the union of sulphur_ous_ acids with bases, or by their
action upon them.

[Illustration: Fig. 322.—Combinations of elements.]

The symbolical nomenclature of the chemist is worse than Greek to the
uninitiated. We frequently see in so-called popular chemical books a
number of hieroglyphics and combinations of letters with figures very
difficult to decipher, much less to interpret. These symbols take the
place of the names of the chemical compounds. Thus water is made up of
oxygen and hydrogen in certain proportions; that is, two of hydrogen to
one of oxygen. The symbolic reading is simple, H_{2}O, = the oxide of
hydrogen. Potassium again mingles with oxygen. Potassium is K in our
list; KO is oxide of potassium (potash). Let us look into this a little
closer.

The union of one particle of a simple body with a particle of another
simple body can be easily understood; but, as we have seen, it is
possible to have substances consisting of four or five different
particles, though the greater number of chemical combinations consist
of two or three dissimilar ones. In the diagram (fig. 322) we have some
possible combinations.

In these combinations we may have one particle of _a_ in combination
with one, two, three, four, or five of _b_, and many particles of _a_
can unite with various molecules of _b_. Suppose we have oxygen and
sulphur compounds as follows:—

Thus there are three different compounds of these two elements—SO,
SO_{2}, SO_{3} (without water).

[Illustration: (1) (2) (3)

Fig. 323.-(1) Hydrosulphurous Acid. (2) Sulphurous Acid. (3) Sulphuric
Acid.]

A compound body may combine with another compound body, and this
makes a complicated compound. Suppose we have a mixture of sulphuric
acid and potash. We have a sulphate of potassium (K_{2}SO_{4}) and
combinations of these combinations may likewise be formed. We must
read these symbols by the light of the combining weights given in the
table, and then we shall find the weight of oxygen or other elements
in combination. Thus when we see a certain symbol (Hg.S for instance),
we understand that they form a compound including so many parts of
mercury and so many of sulphur, which is known as vermilion. Hg.O is
oxide of mercury, and by reference to the table of Atomic Weights, we
find mercury is Hg., and its combining weight is 200; while oxygen is
O, and its weight is 16. Thus we see at once how much of each element
is contained in oxide of mercury, and this proportion never varies;
there must be 200 of one and 16 of the other, by weight, to produce the
oxide. So if the oxygen has to be separated from it, the sum of 216
parts must be taken to procure the 16 parts of oxygen. When we see, as
above, O_{2} or O_{3}, we know that the weight must be calculated twice
or three times, O being 16; O_{2} is therefore 32 parts by weight. So
when we have found what the compounds consist of, we can write them
symbolically with ease.


COMPOSITION OF THE ATMOSPHERIC AIR.

We have already communicated a variety of facts concerning the air.
We have seen that it possesses pressure and weight. We call the
gaseous envelope of the earth the atmosphere, and we are justified in
concluding that other planets possess an atmosphere also, though of a
different nature to ours. We have seen how easy it is to weigh the air,
but we may repeat the experiment. (_See_ illustration, fig. 45, page
50.) We shall find that a perfectly empty glass globe will balance the
weights in the scale-pan; admit the air, and the glass globe will sink.
So air possesses weight. We have mentioned the Magdeburg hemispheres,
the barometer, the air-pump, and the height and the pressure of the
atmosphere have been indicated. The density of the atmosphere decreases
as we ascend; for the first seven miles the density diminishes
one-fourth that of the air at the sea-level, and so on for every
succeeding seven.

In consequence of the equal, if enormous, pressure exercised in every
direction, we do not perceive the inconvenience, but if the air were
removed from inside of a drum, the parchment would quickly collapse.
We feel the air when we move rapidly. We breathe the air, and that
statement brings us to consider the composition of the atmosphere,
which, _chemically_ speaking, _may vary a little_ (as compared with
the whole mass) in consequence of changes which are continually taking
place, but to all intents and purposes the air is composed as follows,
in 100 parts:

  Nitrogen          79 parts.
  Oxygen            20   ”
  Carbonic Acid    .04   ”

with minute quantities of other ingredients, such as ammonia, iodine,
carbonetted hydrogen, hydrochloric acid, sulphuretted hydrogen, nitric
acid, carbonic oxide, and dust particles, as visible in the sunbeams,
added.

The true composition of the atmosphere was not known till Lavoisier
demonstrated that it consisted of two gases, one of which was the vital
fluid, or oxygen, discovered by Priestley. To the other gas Lavoisier
gave the name of Azote,—an enemy of life,—because it caused death if
inhaled alone. The carbonic acid in the air varies very much, and in
close, heated, and crowded rooms increases to a large quantity, which
causes lassitude and headache.

We can easily prove the existence of carbonic acid gas as exhaled
from the lungs. Suppose we take a glass and fill it partly with clear
lime-water; breathe through a glass tube into the water in the glass,
and very quickly you will perceive that the lime-water is becoming
cloudy and turbid. This cloudiness is due to the presence of chalk,
which has been produced by the action of the carbonic acid gas in the
lime-water. This is a well known and always interesting experiment,
because it leads up to the vital question of our existence, and the
functions of breathing and living.

A popular writer once wrote a book entitled, “Is Life Worth Living?”
and a witty commentator replied to the implied question by saying, “It
depends upon the _liver_.” This was felt to be true by many people who
suffer, but the scientific man will go farther, and tell you it depends
upon the air you breathe, and on the carbonic acid you can raise to
create heat,—animal heat,—which is so essential to our well-being.
We are always burning; a furnace is within us, never ceasing to burn
without visible combustion. We are generating heat by means of the
blood. We know that we inhale air into the lungs, and probably are
aware that the air so received parts with the oxygen to renew the
blood. The nitrogen dilutes the oxygen, for if we inhaled a less-mixed
air we should either be burnt up or become lunatics, as light-headed
as when inhaling “laughing-gas.” This beautifully graduated mixture is
taken into our bodies, the oxygen renews the blood and gives it its
bright red colour; the carbon which exists in all our bodies is cold
and dead when not so vivified by oxygen. The carbonic acid given off
produces heat, and our bodies are warm. But when the action ceases we
become cold, we die away, and cease to live. Man’s life exemplifies
a taper burning; the carbon waste is consumed as the wax is, and
when the candle burns away—it dies! It is a beautiful study, full of
suggestiveness to all who care to study the great facts of Nature,
which works by the same means in all matter. We will refer to plants
presently, after having proved by experiment the existence of nitrogen
in the air.

Rutherford experimented very cruelly upon a bird, which he placed
beneath a glass shade, and there let it remain in the carbonic acid
exhaled from its lungs, till the oxygen being at length all consumed
by the bird, it died. When the atmosphere had been chemically purified
by a solution of caustic potash, another bird was introduced, but
though it lived for some time, it did not exist so long as the first.
Again the air was deprived of the carbonic acid, and a third bird was
introduced. The experiment was thus repeated, till at length a bird
was placed beneath the receiver, and it perished at once. This is at
once a cruel and clumsy method of making an experiment, which can be
more pleasantly and satisfactorily practised by burning some substance
in the air beneath the glass. Phosphorus, having a great affinity for
oxygen, is usually chosen. The experiment can be performed as follows
with a taper, but the phosphorus is a better exponent.

Let us take a shallow basin with some water in it, a cork or small
plate floating upon the water, and in the plate a piece of phosphorus.
We must be careful how we handle phosphorus, for it has a habit, well
known, but sometimes forgotten by amateur chemists, of suddenly taking
fire. Light this piece of phosphorus,—a small piece will do if the jar
be of average “shade” size,—and place the glass over it, as in the
illustration (fig. 325). The smoke will quickly spread in the jar, and
the entry of air being prevented, because the jar is resting under
water, phosphoric acid will be formed, and the oxygen thereby consumed.
The water, meanwhile, will rise in the jar, the pressure of the air
being removed. The burning phosphorus will soon go out, and when the
glass is cool, you will be able to ascertain what is inside the jar.
Put a lighted taper underneath, and it will go out. The taper would
not go out before the phosphorus was burnt in the glass, and so now we
perceive we have azote in the receptacle—that is, nitrogen. The other,
the constituent of our atmosphere, carbonic acid, as we have seen, is
very injurious to the life of animals, and as every animal breathes
it out into the air, what becomes of it? Where does all this enormous
volume of carbonic acid, the quantities of this poison which are daily
and nightly exhaled, where do they all go to? We may be sure nature has
provided for the safe disposal of it all. Not only because we live and
move about still,—and of course that is a proof,—but because nature
always has a compensating law. Remember _nothing is wasted_; not even
the refuse, poisonous air we get rid of from our lungs. Where does it
go?

[Illustration: 324.—Rutherford’s experiment.]

It goes to nourish the plants and trees and vegetables that we delight
to look upon and to eat the fruit of. Thus the vegetable world forms a
link between the animals and the minerals. Vegetables obtain food, so
to speak, and nourishment from water, ammonia, and carbonic acid, all
compound bodies, but inorganic.

Water consists of oxygen and hydrogen, carbonic acid of carbon and
oxygen, and ammonia of hydrogen and nitrogen. Water and ammonia are
present in the air; so are oxygen and nitrogen. Water falls in the
form of rain, dew, etc. So in the atmosphere around us we find nearly
every necessary for plant-life; and in the ground, which supplies some
metallic oxides for their use, we find the remainder. From the air,
then, the plant derives its life.

[Illustration: Fig. 325—Drawing the oxygen from air by combustion.]

The vegetable kingdom in turn gives all animals their food. This you
will see at a glance is true. Certainly animals live on animals. Man
and wilder animals live on the beasts of the field in a measure, but
those beasts derive their nourishment from vegetables—the vegetable
kingdom. So we live on the vegetable kingdom, and it separates the
carbonic acid from the air, and absorbs it. What we do not want it
takes. What we want it gives. Vegetables give out oxygen, and we
consume it gladly. We throw away carbonic acid, and the plants take
it greedily; and thus the atmosphere is retained pure for our use. We
can, if desirable, prove that plants absorb carbonic acid and give
out oxygen by placing leaves of a plant in water, holding the acid in
solution, and let the sun shine upon them. Before long we shall find
that the carbonic acid has disappeared, and that oxygen has come into
the water.

Carbonic acid is sufficiently heavy to be poured from one vessel to
another; and if we have obtained some in a glass, we can extinguish a
taper by pouring the invisible gas on to the lighted taper, when it
will be immediately extinguished.

From the foregoing observations it will be perceived how very desirable
it is that ventilation should be attended to. People close up windows
and doors and fireplaces, and go to bed and sleep. In the morning
they complain of headache and lassitude; they wonder what is the
matter, and why the children are not well. Simply because they have
been rebreathing the carbonic acid. Go into a closed railway carriage
which is nearly filled (and it is astonishing to us how people can be
so foolish as to close every outlet), and you will recoil in disgust.
These travellers shut the ventilators and windows “because of the
cold.” A very small aperture will ventilate a railway carriage; but a
close carriage is sickening and enervating, as these kind of travellers
find out by the time they reach their journey’s end. Air was given us
to breathe at night as well as by day; and though from man’s acts or
omissions there may be circumstances in which “night” air may affect
the health, we maintain that air is no more injurious naturally than
“day” air. Colder it may be, but any air at night is “night” air, in
or out of doors at night; and we are certain that night air in itself
never hurt any healthy person. It is not nature’s plan to destroy, but
to save. If a person delicate in constitution gets hot, and comes out
into a colder atmosphere, and defy nature in that way, he (or she)
must take the consequences. But air and _ventilation_ (not draught)
are necessaries of health, and to say they injure is to accuse nature
falsely. There are many impurities in the air in cities, and in country
places sometimes, but such impurities are owing to man’s acts and
omissions. With average sanitary arrangements and appliances in a
neighbourhood no one need be afraid to breathe fresh air night or day;
and while many invalids have, we believe, been retarded in recovery
from being kept in a close room, hundreds will be benefited by plenty
of fresh air. We should not so insist upon these plain and simple
truths were there not so many individuals who think it beneficial to
close up every avenue by which air can enter, and who then feel ill and
out of spirits, blaming everything but their own short-sightedness for
the effect of their own acts. An inch or two of a window may be open
at night in a room, as the chimney register should be always fully up
in bedrooms. When there are fires the draught supplies fresh air to
the room with sufficient rapidity. But many seaside journeys might be
avoided if fresh air were insisted on at home.

There is another and an important constituent of the atmosphere called
OZONE, which is very superior oxygen, or oxygen in what is
termed the “Allotropic” state, and is distantly related to electricity,
inasmuch as it can be produced by an electrical discharge. This partly
accounts for the freshness in the air after a thunderstorm, for we
are all conscious that the storm has “cleared the air.” The fresh,
crisp ozone in the atmosphere is evident. Ozone differs from oxygen
in possessing taste and smell, and it is heavier by one-half than the
oxygen gas. There is a good deal of ozone in the sea breeze, and we
can, though not infallibly, detect its presence by test-paper prepared
with iodide of potassium, which, when ozone is present, will turn
blue. We have still something to learn about ozone, which may be
considered as “condensed oxygen.”

[Illustration: Fig. 326. Development of gas by combustion. Fig. 327.]

We have frequently mentioned “combustion,” and as under ordinary
circumstances such effects cannot take place without atmospheric air,
we will consider it. Combustion is chemical action accompanied by
light and heat. Chemical union is always attended by the development
of heat, not always by light, because the union varies in intensity
and quickness. But when a candle is burning we can study all the
interesting phenomena of combustion. We have already spoken of HEAT
and LIGHT, so we need only refer the readers to those subjects in the
former parts of this volume. Heat is referable to chemical action,
and varies according to the energy of union. Heat is always present,
remember, in a greater or less degree; and when visible combustion
takes place we see light. Invisible combustion goes on in our bodies,
and we feel heat; when we get cold we feed the fire by eating, or blow
it by exercise and air in our lungs.

[Illustration: Fig. 328.—Gas evolved from flame.]

We shall speak, however, of combustion now as it affects us in daily
life; our fires, our candles, gas, etc., and under these ordinary
circumstances hydrogen and carbon are present. (We shall hear more
about carbon presently.) These unite with the oxygen to form water and
carbonic acid; the water being visible as we first put the cold shade
upon the lighted lamp, and the carbonic acid renders the air impure.

In the case of a common candle, or lamp, combustion takes place in the
same way. The wick is the intermediary. The oil mounts in the lamp
wick, where it is converted into a gas by heat; it then “takes fire,”
and gives us light and heat. The candle-flame is just the same with
one exception: the burning material is solid, not liquid, though the
difference is only apparent, for the wax is melted and goes up as gas.
The burning part of the wick has a centre where there is no combustion,
and contains carbon. We can prove this by placing a bent tube, as in
the illustration (fig. 326), one end in the unburning part of the
flame. We shall soon see a dark vapour come over into the receiver.
This is combustible, for if we raise the tube without the glass we can
light the gas (fig. 327). If we insert the end of the tube into the
brilliant portion of the flame we shall perceive a black vapour, which
will extinguish the combustion, for it is a mixture of carbonic acid
gas and aqueous vapour, in which (fig. 328) particles of carbon are
floating.

[Illustration: Fig. 329.—Davy’s safety lamp.]

[Illustration: Fig. 330.—Davy lamp (section).]

When we proceed to light our lamps to read or to write by, we find some
difficulty in making the wick burn at first. We present to it a lighted
taper, and it has no immediate effect. Here we have oil and cotton,
two things which would speedily set a warehouse in flames from top to
bottom, but we cannot even ignite them, try all we can. Why?—Because we
must first obtain a gas, oil will not burn liquid; it must be heated
to a gaseous point before it will burn, as all combustion depends upon
that,—so flames mount high in air. Now in a candle-flame, as will be
seen in the diagram (fig. 331.), there are three portions,—the inner
dark core, which consists of unburnt gas; the outer flame, which gives
light; and the outside rim of perfect combustion non-luminous. In the
centre, A, there is no heat. If we place a piece of gauze wire over the
flame at a little distance the flame will not penetrate it. It will
remain underneath, because the wire, being of metal, quickly absorbs
the heat, and consequently there is no flame. This idea led to the
invention of the “safety” lamp by Sir Humphrey Davy, which, although it
is not infallible, is the only lamp in general use in mines (figs. 329,
330).

[Illustration: Fig. 331.—Construction of a candle flame.]

Mines must have light, but there is a gas in mines, a “marsh” gas,
which becomes very explosive when it mixes with oxygen. Of course the
gas will be harmless till it meets oxygen, but, in its efforts to meet,
it explodes the moment the union takes place; instead of burning slowly
like a candle it goes off all at once. This gas, called “fire damp,” is
carburetted hydrogen, and when it explodes it develops into carbonic
acid gas, which suffocates the miners.

[Illustration: Fig. 332.—Pouring carbonic acid on a lighted taper.]



CHAPTER XXVII.

NON-METALLIC ELEMENTS.


OXYGEN—SYMBOL =O=; ATOMIC WEIGHT 16.

Oxygen is certainly the most abundant element in nature. It exists all
around us, and the animal and vegetable worlds are dependent upon it.
It constitutes in combination about one-half of the crust of the earth,
and composes eight-ninths of its weight of water. It is a gas without
taste or colour. Oxygen was discovered by Priestley and Scheele, in
1774, independently of each other.

[Illustration: Fig. 333.—Oxygen from oxide of mercury.]

Oxygen can be procured from the oxides of the metals, particularly from
gold, silver, and platinum. The noble metals are reducible from their
oxides by heat, and this fact assists us at once. If we heat chlorate
of potash, mixed with binoxide of manganese, in a retort in a furnace,
the gas will be given off. There are many other ways of obtaining
oxygen, and we illustrate two (figs. 333, 335).

The red oxide of mercury will very readily evolve oxygen, and if we
heat a small quantity of the compound in a retort as per illustration
(fig. 333) we shall get the gas. In a basin of water we place a tube
test-glass, and the gas from the retort will pass over and collect in
the test tube, driving out the water.

The other method mentioned above,—viz., by heating chlorate of potash,
etc., in a furnace, is shown in the following illustration. Oxygen, as
we have said, is a colourless and inodorous gas, and for a long time
it could not be obtained in any other form; but lately both oxygen and
hydrogen have been liquified under tremendous pressure at a very low
temperature. Oxygen causes any red-hot substance plunged into it to
burn brightly; a match will readily inflame if a spark be remaining,
while phosphorus is exceedingly brilliant, and these appearances, with
many others equally striking, are caused by the affinity for those
substances possessed by the gas. Combustion is merely oxidation, just
as the process of rusting is, only in the latter case the action is so
slow that no sensible heat is produced. But when an aggregate of slowly
oxidising masses are heaped together, heat is generated, and at length
bursts into flame. This phenomenon is called “spontaneous combustion.”
Cases have been known in which the gases developed in the human body by
the abuse of alcoholic drinks have ended fatally; in like manner the
body being completely charred. (Combustion must not be confounded with
_ignition_, as in the electric light.) Oxygen then, we see, is a great
supporter of combustion, though not a combustible itself as coal is.
When the chemical union of oxygen with another substance is very rapid
an explosion takes place.

[Illustration: Fig. 334.—Showing retort placed in furnace.]

[Illustration: Fig. 335.—The generation of oxygen from oxide of
manganese and potash.]

Oxidation occurs in various ways. Besides those already mentioned,
all verdigris produced on copper, all decays of whatever kind,
disintegration, and respiration, are the effects of oxygen. The
following experiment for the extraction of oxygen directly from the air
was made by M. Boussingault, who passed the gas upon a substance at a
certain temperature, and released it at a higher. The illustration on
page 351 will show the way in which the experiment was performed.

Boussingault permitted a thin stream of water to flow into a large
empty flask, and by this water the air was gradually driven out into
a flask containing chloride of calcium and sulphuric acid, which
effectually dried it. This dry air then passed into a large tube inside
the reverberatory furnace, in which tube were pieces of caustic baryta.
Heated to a dull redness this absorbs oxygen, and when the heat is
increased to a bright red the superabundant gas is given off. Thus the
oxygen was permitted to pass from the furnace-tube into the receiving
glass, and so pure oxygen was obtained from the air which had been in
the glass bottle at first (fig. 338).

[Illustration: Fig. 336.—Phosphorus burning in oxygen.]


HYDROGEN—SYMBOL =H=; ATOMIC WEIGHT 1.

Hydrogen is abundant in nature, but never free. United with oxygen it
forms water, hence its name, “water-former.” It is to Parcelcus that
its discovery is due, for he found that oil of vitriol in contact with
iron disengaged a gas which was a constituent of water. This gas was
subsequently found to be inflammable, but it is to Cavendish that the
real explanation of hydrogen is owing. He explained his views in 1766.

Hydrogen is obtained in the manner illustrated in the cut, by means of
a furnace, as in fig. 339, or by the bottle method, as per fig. 340.
The first method is less convenient than the second. A gun-barrel or
fire-proof tube is passed through the furnace, and filled with iron
nails or filings; a delivery tube is at the farther end, and a flask
of water boiling at the other. The oxygen combines with the iron in
the tube, and the hydrogen passes over. The second method is easily
arranged. A flask, as in the cut, is provided, and in it some zinc
shavings are put. Diluted sulphuric acid is then poured upon the metal.
Sulphate of zinc is formed in the flask, and the hydrogen passes off.

Hydrogen being the lightest of all known bodies, its weight is put
as 1, and thus we are relatively with it enabled to write down the
weights of all the other elements. Hydrogen is fourteen-and-a-half
times lighter than atmospheric air, and would do admirably for the
inflation of balloons were it not so expensive to procure in such large
quantities as would be necessary. Ordinary coal gas, however, contains
a great deal of hydrogen, and answers the same purpose.

[Illustration: Fig. 337.—Magnesium wire burning in oxygen.]

A very pretty experiment may be made with a bladder full of hydrogen
gas. If a tube be fitted to the bladder already provided with a
stop-cock, and a basin of ordinary soap-suds be at hand, by dipping the
end of the tube in the solution and gently expressing the gas, bubbles
will be formed which are of exceeding lightness (fig. 341). They can
also be fired with a taper.

Another experiment may be made with hydrogen as follows:—If we permit
the gas to escape from the flask, and light it, as in the illustration,
and put a glass over it, we shall obtain a musical note, higher or
lower, according to the length, breadth, and thickness of the open
glass-tube (fig. 342). If a number of different tubes be employed, we
can obtain a musical instrument—a gas harmonium.

[Illustration: Fig. 338.—Extraction of oxygen from air.]

Hydrogen burns with a blue flame, and is very inflammable. Even water
sprinkled upon a fire will increase its fierceness, because the
hydrogen burns with great heat, and the oxygen is liberated. Being
very light, H can be transferred from one vessel to another if both
be held upside down. Some mixtures of H and O are very explosive. The
oxyhydrogen blow-pipe is used with a mixture of O and H, which is
forcibly blown through a tube and then ignited. The flame thus produced
has a most intense heating-power.

A very easy method of producing hydrogen is to put a piece of sodium
into an inverted cylinder full of water, standing in a basin of water.
The sodium liberates the hydrogen by removing the oxygen from the
liquid.


WATER—SYMBOL =H_{2}O=; ATOMIC WEIGHT 18.

At page 59 of this volume we said something about water, and remarked
(as we have since perceived by experiment) that “water is composed of
oxygen and hydrogen in proportions, by weight, of eight of the former
to one of the latter gas; in volume, hydrogen is two to one”; and we
saw that “volume and weight were very different things.” This we will
do well to bear in mind, and that, to quote Professor Roscoe, “Water is
always made up of sixteen parts of oxygen to two parts of hydrogen by
weight”; sixteen and two being eighteen, the combining weight of water
is eighteen.

[Illustration: Fig. 339.—Preparation of hydrogen with furnace.]

[Illustration: Fig. 340.—Apparatus for generating hydrogen by flask.]

We can prove by the Eudiometer that hydrogen when burnt with oxygen
forms water; and here we must remark that water is not a mere
mechanical mixture of gases, as air is. Water is the product of
chemical combination, and as we have before said, is really an oxide
of hydrogen, and therefore combustion, or electricity, must be called
to our assistance before we can form water, which is the result of an
explosion, the mixture meeting with an ignited body—the aqueous vapour
being expanded by heat.

The ancients supposed water to be a simple body, but Lavoisier and
Cavendish demonstrated its true character. Pure water, at ordinary
temperatures, is devoid of taste and smell, and is a transparent,
nearly colourless, liquid. When viewed in masses it is blue, as
visible in a marked degree in the Rhone and Rhine, at Geneva, and
Bâle respectively. Its specific gravity is 1, and it is taken as the
standard for Sp. Gravity, as hydrogen is taken as the standard for
Atomic Weight. The uses of water and the very important part it plays
in the arrangements of nature as a mechanical agent, geology can
attest, and meteorology confirm. It composes the greater portions of
animals and plants; without water the world would be a desert—a dead
planet.

[Illustration: Fig. 341.—Blowing bubbles with hydrogen gas.]

We sometimes speak of “pure” spring water, but such a fluid absolutely
pure can scarcely be obtained; and though we can filter water there
will always remain some foreign substance or substances in solution.
It is well known that the action of water wears away and rounds off
hard rocks, and this power of disintegration is supplemented by its
strength as a solvent, which is very great. Rain-water is purest in
the country as it falls from the clouds. In smoky towns it becomes
sooty and dirty. It is owing to the solvent properties of water,
therefore, that we have such difficulty in obtaining a pure supply.
There is _hard_ water and _soft_ water. The former is derived from the
calcareous formations, and contains lime, like the Kent water. This can
be ascertained by noticing the incrustations of the vessels wherein the
water is boiled. But water rising from hard rocks, such as granite, can
do little to disintegrate them at the moment, and therefore the water
rises purer. Springs from a great depth are warm, and are known as
“thermal springs”; and when they come in contact with carbonic acid and
some salts in their passage to the surface, they are known as “mineral
waters.” These waters hold in solution salts of lime and magnesia, or
carbonates of soda with those of lime and magnesia; salts of iron, and
compounds of iodine and bromine are found in the natural mineral waters
also, as well as sulphurous impregnations, instances of which will
occur to every reader.

[Illustration: Fig. 342.—Experiment with hydrogen.]

[Illustration: Fig. 343.—The composition of water.]

We mentioned the Eudiometer just now, and we give an illustration of
it. This instrument is used to ascertain the proportions in which the
elements of water are composed by _synthesis_, or a putting together of
the constituents of a body to make it up. This is distinguished from
_analysis_, which means separating the compound body into its elements,
as we do when we pass the electric current through water.

The Eudiometer consists of a stout glass tube sealed hermetically at
one end; two platinum wires are pushed in through the glass just before
the end is sealed. The tube is now filled with mercury, and inverted
in a bowl of the same metal. Hydrogen, and then oxygen, are admitted
through the mercury in the recognised proportions of two to one. By
the time the mercury is somewhat more than half displaced, the tube
should be held upon a sheet of india-rubber at the bottom of the
vessel to keep the metal in the tube, for when the necessary explosion
takes place the mercury might also be driven out. A spark from the
electrophorus or from a Leyden jar may now be passed through the gases
in the tube. The explosion occurs, and water is formed inside. If the
mercury be again admitted it will rise nearly to the very top of the
tube, driving the bubble up. Thus we find we have formed water from the
two gases.

The decomposition of water is easily affected by electricity, and if
a little sulphuric acid be added to the water, the experiment will
be thereby facilitated. Two wires from a battery should be inserted
through a glass filled with the water, and into two test tubes also
filled. The wires terminate in platinum strips, and are fastened at the
other end to the positive and negative poles of the galvanic battery.
The gases will collect in the test tubes, and will be found in proper
proportions when the current passes.

[Illustration: Fig. 344.—The Eudiometer.]

[Illustration: Fig. 345.—Decomposition of water.]

So much for water in its liquid state. The solid condition of water
(ice) is equally interesting. When we apply heat to water, we get a
vapour called “steam”; when we cool water to 32° Fahr., we get a solid
mass which weighs just the same as the liquid we have congealed, or the
steam we have raised from an equal amount of water. But water expands
while in the process of solidification, just as it does when it becomes
gaseous, and as we have remarked before, our water-pipes bear full
testimony to this scientific fact. When ice forms it has a tendency
to crystallize, and some of these ice crystals are, as we see, very
beautiful. Snow is only water in a nearly solid form, and the crystals
are extremely elegant, appearing more like flowers than congealed
water, in tiny six-pointed ice crystals. Many philosophers of late
years have written concerning these tiny crystals, which, in common
with all crystals, have their own certain form, from which they never
depart. Snowflakes are regular six-sided prisms grouped around a centre
forming angles of 60° and 120°. There are a number of forms, as will
be seen from the accompanying illustrations, and at least ninety-six
varieties have been observed. One snowflake, apparently so like all
other flakes that fall, can thus be viewed with much interest, and yet,
while so very various, snowflakes never get away from their proper
hexagonal structure. It has been remarked that snowflakes falling at
the same time have generally the same form.

Of the latent heat of ice, etc., we have already spoken in our article
upon Heat, and therefore it will be sufficient to state that the latent
heat of water is 79 thermal units, because when passing from the liquid
to the solid state a certain amount of water absorbs sufficient heat to
raise an equal quantity of the liquid 79°. This can be proved by taking
a measured quantity (say a pint) of water at 79° and adding ice of the
same weight to the water. The mixture will be found to be at zero.
Therefore the ice has absorbed or rendered latent 79° of heat which the
water possessed. If we melt ice until only a trace of it is left, we
shall still find the water as cold as the ice was; all the latent heat
is employed in melting the ice. So it will take as much heat to bring a
pound of ice at zero to a pound of water at zero, as it would to raise
79 pounds of water 1°. The same law applies to steam.

[Illustration: Fig. 346.—Snow crystals.]

Water can be distilled in small quantities by an apparatus, as figured
in the illustration, and by these means we get rid of all impurities
which are inseparable from the liquid otherwise. When it is desirable
to distil large quantities of water a larger apparatus is used, called
an “Alembic.” The principle is simply to convert the liquid by heat
into vapour, then cool it, by condensation, in another vessel.

[Illustration: Fig. 347.—Distilling water.]

The evaporation of water, with its effects upon our globe, belong more
to the study of Meteorology.

[Illustration: Fig. 348.—Distillation.]

Rain-water is the purest, as we have said, because it goes through the
process of distillation by nature. The sun takes it up, by evaporation,
into the air, where it is condensed, and falls as rain-water. Water
containing carbonate of lime will petrify or harden, as in stalactite
caverns. The carbonic acid escapes from the dripping water, the
carbonate in solution is deposited as a stalactite, and finally forms
pillars in the cave. Sea-water contains many salts; its composition is
as follows, according to Dr. Schwertzer, of Brighton:—

  Water                                      964·74372 grains.
  Chloride of sodium (salt)                   28·05948    ”
  Chloride of potassium                        0·76552    ”
  Chloride of magnesium                        3·66658    ”
  Bromide of magnesium                         0·02929    ”
  Sulphate of magnesia                         2·29578    ”
  Sulphate of lime                             0·40662    ”
  Carbonate of lime                            0·03301    ”
    (With traces of iodine and ammonia).    --—--—----
                                            1000·00000 grains.

[Illustration: Fig. 349.—Stalactite Cavern.]

There is much more oxygen in water than in air, as can be ascertained
by analysis of these compounds. This great proportion in favour of
water enables fish to breathe by passing the water through the gills.
Marine animals (not fishes), like the whale,—which is a warm-blooded
creature, and therefore not suited to exist without air,—are obliged
to come to the surface to breathe. The density of salt water is much
greater than that of fresh water, and therefore swimming and flotation
is easier in the sea than in a river. We shall have more to say of
water by-and-by.


NITROGEN—SYMBOL =N=; ATOMIC WEIGHT 14.

We have already made some reference to this gas when speaking of the
atmosphere and its constituents, of which nitrogen is the principal.
From its life-destroying properties it is called “azote” by French
chemists, and when we wish to obtain a supply of nitrogen all we have
to do is to take away the oxygen from the air by burning phosphorus
on water under a glass. Nitrogen is not found frequently in solid
portions of the globe. It is abundant in animals. It is without colour
or smell, and can be breathed in air without danger. It is heavy and
sluggish; but if we put a taper into a jar of nitrogen it will go out,
and animals die in the gas for want of oxygen, as nitrogen alone cannot
support life.

[Illustration: Fig. 350.—Obtaining nitrogen.]

The affinity of nitrogen for other substances is not great, but it
gives rise to five compounds, which are as below, in the order they are
combined with oxygen:—

  Nitrous oxide (“laughing gas”)  (Monoxide)   N_{2}O.
  Nitric oxide                    Dioxide      N_{2}O_{2}.
  Nitrous acid                    Trioxide     N_{2}O_{3}.
  Nitric peroxide                 Tetroxide    N_{2}O_{4}.
  Nitric acid                     Pentoxide    N_{2}O_{5}.

These compounds are usually taken as representative examples of
combining weight, and as explanatory of the symbolic nomenclature of
chemistry, as they advance in such regular proportions of oxygen with
nitrogen. The combining weight of nitrogen is 14, and when two parts
combine with five of oxygen it makes nitric acid, and we put it down
as N_{2}O_{5}; on adding water, HNO_{3}, as we can see by eliminating
the constituents and putting in the proportions. Actually it is
H_{2}N_{2}O_{6}, or, by division, HNO_{3}.

Nitrogen plays a very important part in nature, particularly in the
vegetable kingdom. Nitric acid has been known for centuries. Geber,
the alchemist, was acquainted with a substance called “nitric,”
which he found would yield a dissolvent under certain circumstances.
He called it “dissolving fluid.” At the end of the twelfth century
Albert Magnus investigated the properties of this acid, and in 1235
Raymond Lully prepared nitre with clay, and gave the liquid the
name of “aqua-fortis.” But till 1849 nitric acid was only known as
a hydrate,—that is, in combination with water,—but now we have the
anhydrous acid.

[Illustration: Fig. 351.—Apparatus for obtaining nitrogen by using
metal to absorb the oxygen of the air.]

Oxygen and nitrogen combine under the influence of electricity, as
shown by Cavendish, who passed a current through an atmospheric
mixture of oxygen and nitrogen, in a tube terminating in a solution of
potash, lime, and soda. Every time the spark passed, the volume of gas
diminished, and nitric acid was formed, as it is in thunderstorms, when
it does not remain free, but unites with ammonia, and forms a highly
useful salt, which promotes vegetable growth. Here is another instance
of the usefulness of thunderstorms, and of the grand provisions of
nature for our benefit. Nitric acid is obtained by distilling nitre
with sulphuric acid. The liquid is, when pure, colourless, and is a
powerful oxidizer. It dissolves most metals, and destroys vegetable and
animal substances. By an addition of a little sulphuric acid the water
is taken from the nitric acid, and a very powerful form of it is the
result. The acid is of great use in medicine, and as an application
to bites of rabid animals or serpents. It converts cotton waste into
“gun-cotton” by a very simple process of steeping, washing, and
pressing. From the hydraulic press it comes in discs like “quoits,”
which will burn harmlessly and smoulder away, but if detonated
they explode with great violence. As a rule, when damp, it is not
dangerous, but it can be fired even when wet. It will explode at a less
temperature than gunpowder, and, moreover, yields no smoke, nor does
it foul a gun. Gun-cotton, when dissolved in ether, gives us collodion
for photographic purposes.

[Illustration: Fig. 352.—Nitric acid obtained from nitre and sulphuric
acid.]

In speaking farther of the compounds of nitrogen with oxygen, we will
limit ourselves to the monoxide, or laughing gas. This is now used
as an anæsthetic in dentistry, etc., and is quite successful, as a
rule. People afflicted with heart disease should not use it without
advice, however. When inhaled into the lungs it makes the subject very
hilarious, and the effect is rather noisy. It is obtained from the
nitrate of ammonia, which, on the application of heat, decomposes into
nitrous oxide and vapour. Warm water should be used for the trough. The
gas is a powerful supporter of combustion.

[Illustration: Fig. 353.—Cavendish’s experiment.]

Binoxide of nitrogen is of importance in the manufacture of sulphuric
acid.

Nitrogen combines with hydrogen, forming various compounds. These
are the “amines,” also ammonia, and ammonium. Ammonia possesses the
properties of a base. Its name is derived from Jupiter Ammon, near
whose temple it was prepared, from camels’ dung. But bodies containing
nitrogen give off ammonia in course of distilling, and hartshorn is
the term applied to horn-cuttings, which yield ammonia, which is a
colourless gas of strong odour and taste now obtained from gas-works.

[Illustration: Fig. 354.—Experiment to obtain nitric acid.]

[Illustration: Fig. 355.—Apparatus for obtaining laughing-gas.]

[Illustration: Fig. 356.—Inhaling laughing gas.]

[Illustration: Fig. 357.—Generation of ammonia.]

To obtain ammonia heat equal parts of chloride of ammonia (sal
ammoniac) and quick-lime powdered (_see_ fig. 357). The gas must be
collected over mercury, because it is very soluble in water. Ammonia
is useful to restore tipsy people and fainting ladies. A solution of
ammonia is used for cauteries. Ammoniacal gas is remarkable for its
solubility in water. To prepare the solution the gas is forced through
a series of flasks. The tubes carrying the gas should be continued to
the bottoms of the flasks, else the solution, being lighter than water,
the upper portion alone would be saturated. The tubes carrying away
the solution are raised a little, so that the renewal is continually
proceeding. The gas liquifies under a pressure of six atmospheres,
at a temperature of 10° Cent. This experiment can be artificially
performed by heating chloride of silver saturated with ammonia, and
the silver will part with the gas at a temperature of 40° C. The gas
will then condense in a liquid form in the tube. The experiment may
be facilitated by placing the other extremity of the tube in snow and
salt, and by the liquid we can obtain intense cold. This experiment
has been made use of by M. Carré in his refrigerator (which was
described in the Physics’ section), by which he freezes water. We may,
however, just refer to the process. Whenever the condition of a body
is changed from that of liquid to a gas, the temperature is greatly
lowered, because the heat becomes “latent.” The latest freezing machine
consists of an apparatus as shown in the illustrations herewith (figs.
359 and 360). The machine is of wrought iron, and contains, when
ready for action, a saturated solution of ammonia at zero. This is
in communication with another and an air-tight vessel, of which the
centre is hollow. The first process is to heat the solution, and the
gas escapes into the second “vase,” which is surrounded by cold water,
and quite unable to escape. A tremendous pressure is soon obtained, and
this, added to the cold water, before long liquifies the ammonia, and
when the temperature indicates 130° the hot vessel is suddenly cooled
by being put into the water. The gas is thus suddenly converted into
a liquid, the water in the second hollow vase is taken out, and the
bottle to be frozen is put into the cavity. The cold is so great, in
consequence of the transformation of the liquid ammonia into a gas,
that it freezes the water in any vessel put into the receiver. The
ammonia can be reconverted into liquid and back again, so no loss is
occasioned by the process, which is rapid and simple. This is how great
blocks of ice are produced in water-bottles.

[Illustration: Fig. 358.—Liquefaction of ammonia.]

[Illustration: Fig. 359.—Carré’s refrigerator (first action).]

The one important point upon which care is necessary is the raising of
the temperature. If it be elevated beyond 130° C., the pressure will be
too great, and an explosion will occur.

The abundant formation of ammonia from decaying animal matter is
evident to everyone, and depends upon the presence of moisture to a
great extent. Chloride of ammonia is called sal-ammoniac, and the
carbonate of ammonia crystallizes from the alkaline liquid produced by
the distillation of certain animal matter. The compounds of ammonia are
easily recognized by a certain sharp taste. They are highly valuable
remedial agents, acting particularly upon the cutaneous system, and
when taken internally, produce the effect of powerful sudorifics.
Their _volatility_, and the facility with which they are expelled from
other substances, render them of great importance in chemistry, and
peculiarly fit them for the purposes of many chemical analyses. The
ammonia compounds display a remarkable analogy to the corresponding
combinations of potash and soda. The compounds of ammonia are highly
important in their relation to the vegetable kingdom. It may be assumed
that all the _nitrogen_ of plants is derived from the ammonia which
they absorb from the soil, and from the surrounding atmosphere.

[Illustration: Fig. 360.—Carré’s refrigerator (second action).]

The similarity of ammonia to the metallic oxides has led to the
conjecture that all its combinations contain a _compound_ metallic
body, which has received the name _ammonium_ (NH_{4}); but no one has
yet succeeded in its preparation, although by peculiar processes it may
be obtained in the form of an amalgam.

Ammonias, in which one or more atoms of hydrogen are replaced by basic
radicals, are termed _Amides_, or _Amines_.



CHAPTER XXVIII.

NON-METALLIC ELEMENTS (_continued_).

 CHLORINE—BROMINE—IODINE—FLUORINE—CARBON—SULPHUR—PHOSPHORUS—SILICON—
 BORON—TELLURIUM—ARSENIC.


Chlorine (Cl.) is usually found with sodium in the mineral kingdom, and
this chloride of sodium is our common salt. Chlorine can be obtained by
heating hydrochloric acid with binoxide of manganese. (Atomic weight
35·5.)

[Illustration: Fig. 361.—Generation of chlorine.]

Chlorine possesses a greenish-yellow colour, hence its name “Chloros,”
green. It should be handled carefully, for it is highly injurious and
suffocating. It possesses a great affinity for other substances, and
attacks the metals. For hydrogen it has a great affection, and when
hydrogen is combined with any other substances chlorine immediately
attacks them, and in time destroys them. But even this destructive
and apparently objectionable quality makes chlorine very valuable;
for if we carry the idea to its conclusion, we shall find that it
also destroys offensive and putrid matter, and purifies the atmosphere
very much. Most colouring matters include hydrogen, and therefore they
are destroyed by chlorine, which is a great “bleacher” as well as a
purifier. If we dip any vegetable dyes into a jar of chlorine, they
will become white if the dyed substances are damp.

Hydrochloric acid is known as muriatic acid and spirits of salt. It is
obtained when salt is treated with sulphuric acid and the gas comes
off into water. Equal parts of the acid and the salt are put into a
flask as in the cut (fig. 362), and diluted with water. The mixture is
then heated. The gas is condensed in the bottles half-full of water.
The result gives sulphate of soda and hydrochloric acid. This acid is
procured in soda manufactories, and with nitric acid is called “aqua
regia,” a solvent for gold. When chlorine and hydrogen are mixed
in equal proportions they explode in sunlight. In the dark or by
candle-light they are harmless. Dry chlorine gas can be obtained by
interposing a glass filled with some chloride of calcium. The gas being
heavier than air (about 2½ times), displaces it in the flask, and
when it is filled another can be placed in position. This mode causes a
little waste of gas, which should not be breathed.

[Illustration: Fig. 362.—Production of hydrochloric acid.]

Chlorine possesses a great affinity for certain bodies. If the gas be
thrown upon phosphorus, the latter will burn brilliantly. Arsenic,
tin, and antimony when powdered and poured from a shoot into a vase of
chlorine will burst into brilliant sparks, and other metals will glow
when introduced to this gas. Chlorine forms many unstable combinations
with oxygen. Its combination with hydrogen has already been referred to.

BROMINE is a rare element. (Symbol Br. Atomic weight 80.)

It is deep brownish red, very volatile, and of a peculiar odour.
Bromine unites with the elementary bodies, and forms some oxygen
compounds. It resembles chlorine in its properties, and is used in
medicine and in photography. It is found in saline springs and in salt
water, combined with soda and magnesium. The presence of bromine may
easily be detected in the strong smell of seaweed. Its combinations
with metals are termed bromides. It is a powerful poison.

IODINE is another relative of chlorine. It is found in
seaweed, which by burning is reduced to _kelp_. When iodine is heated
a beautiful violet vapour comes off, and this characteristic has given
it its name (“iodes,” violet). Iodine was discovered by Courtois, of
Paris, and in 1813, Gay Lussac made it a special study. It is solid
at ordinary temperatures, and assumes crystallized forms in plates
of metallic lustre. It is an excellent remedy in “goitre” and such
affections. (Symbol I. Atomic weight 127.)

FLUORINE is very difficult to prepare. Fluor spar is a compound of
fluorine and calcium. This element is gaseous, and combines so rapidly
that it is very difficult to obtain in a free state. Etching on glass
is accomplished by means of hydrofluoric acid, for fluorine has a
great affinity for silicic acid, which is contained in glass. The
glass is covered with wax, and the design is traced with a needle. The
acid attacks the glass and leaves the wax, so the design is eaten in.
(Symbol F. Atomic weight 19.)

[Illustration: Fig. 363.—Apparatus for obtaining dry chlorine gas.]

Chlorine, fluorine, bromine, and iodine are termed “Halogens”
(producers of salts). They appear, as we have seen, in a gaseous,
liquid, and solid form respectively.

CARBON is the most, or one of the most, largely diffused elements in
nature, and claims more than a passing notice at our hands, though
even that must be brief. We may put down carbon next to oxygen as the
most important element in the world. The forms assumed by carbon are
very variable, and pervade nature in all its phases. We have carbon in
crystals, in the animal and vegetable kingdoms, and amongst the chief
minerals a solid, odourless, tasteless, infusible, and almost insoluble
body. In various combinations carbon meets us at every turn; united
with oxygen it forms carbonic acid, which we exhale for the plants
to imbibe. We have it in coal, with hydrogen and oxygen. We have it
building up animal tissues, and it is never absent in two out of the
three great divisions of nature—the plants and the animals (Symbol C;
Atomic W. 12).

[Illustration: Fig. 364.—Facets of a brilliant.]

[Illustration: Fig. 365.—Facets of a rose diamond.]

We have carbon in three different and well-known conditions; as
the diamond, as graphite, or black-lead, and as charcoal. The
properties of the diamond are well known, and we shall, when we get
to Crystallography, learn the forms of diamond or crystals of carbon.
At present we give an illustration or two, reserving all explanation
for the present. Diamond cutting is a matter of some difficulty, and
it requires skill to cut in the proper direction. Diamonds are found
in India, Brazil, and at the Cape of Good Hope, in alluvial soil.
The identity of diamond and charcoal was discovered accidentally.
An experiment to fuse a few small diamonds resulted in their
disappearance, and when the residue was examined it was found that the
diamonds had been burned, that they had combined with oxygen and formed
carbonic acid, just as when coal burns. The diamond is the hardest of
all substances, the most valuable of gems, and the purest condition in
which carbon appears.

GRAPHITE (Plumbago) is termed “black-lead,” and is the next purest form
of carbon. It crystallizes and belongs to the primitive formations. In
Cumberland it is dug up and used to make pencils; the operations can be
seen at Keswick. It has other uses of a domestic character.

Charcoal is the third form of carbon, and as it possesses no definite
form, is said to be _amorphous_. Charcoal is prepared in air-tight
ovens, so that no oxygen can enter and burn the wood thus treated.
Coke is the result of the same process applied to coal. The gas
manufactories are the chief depôts for this article, and it is used in
locomotive engines. The various smokeless coals and prepared fuels,
however, are frequently substituted.

[Illustration: Fig. 366. —Coke ovens.]

Coke ovens were formerly much resorted to by the railway companies, who
found the ordinary coal too smoky for locomotive purposes, and apt to
give rise to complaints by passengers and residents near the line.

The origin of wood charcoal we have seen. All vegetable substances
contain carbon. When we burn wood, in the absence of air as far as
possible, oxygen and hydrogen are expelled. The wood is piled in layers
as in the illustration (fig. 368), covered over with turf and mould,
with occasional apertures for air. This mass is ignited, the oxygen
and hydrogen are driven off, and carbon remains. (Animal charcoal
is obtained from calcining bones). Wood charcoal attracts vapours,
and water, if impure, can be purified by charcoal, and any impure or
tainted animal matter can be rendered inoffensive by reason of charcoal
absorbing the gases, while the process of decay goes on just the same.
Housekeepers should therefore not always decide that meat is good
because it is not offensive to the olfactory nerves. Charcoal will
remove the aroma, but the meat may be nevertheless bad. The use of
charcoal in filters is acknowledged universally, and as a constituent
of gunpowder it is important.

[Illustration: Fig. 367.—Charcoal burning.]

Carbon is not easily affected by the atmospheric air, or in the
earth; so in many instances wood is charred before being driven into
the ground; and casks for water are prepared so. Soot is carbon in
a pulverised condition, and Indian ink is manufactured with its
assistance.

[Illustration: Fig. 368.—Wood piles of charcoal burners.]

The preparation of wood charcoal gives occupation to men who are
frequently wild and untutored, but the results of their labour are very
beneficial. Care should be taken not to sleep in a room with a charcoal
stove burning, unless there is ample vent for the carbonic acid gas,
for it will cause suffocation. Lampblack is obtained by holding a plate
over the flame of some resinous substance, which deposits the black
upon it. There is a special apparatus for this purpose.

[Illustration: Fig. 369.—Seltzer-water manufactory.]

Carbon combines with oxygen to make carbonic acid gas, as we have
already mentioned, and in other proportions to form a more deadly
compound than the other. The former is the dioxide (CO_{2}), the
latter the monoxide, or carbonic oxide (CO). The dioxide is the
more important, being held in the atmosphere, and combined with lime
in chalk. All sparkling beverages contain carbonic acid, to which
their effervescence is due. The soda and other mineral waters owe
their sparkle to this gas. Soda-water consists of a weak solution of
carbonate of soda and the acid. There is a vessel holding chalk and
water, and another containing some sulphuric acid. When the sulphuric
acid is permitted to unite with the chalk and water, carbonic acid
is liberated. A boy turning a wheel forces the gas into the water in
the bottles, or the water and carbonate of soda is drawn off thus
impregnated into bottles and corked down, in the manner so familiar to
all. The bottles are made of the shape depicted, so that the bubble
of air shall be at the top when the bottle lies down. If it be not
kept so, the air will eventually escape, no matter how tightly the
cork be put in. The ordinary “soda-water” contains scarcely any soda.
It is merely water, chalk, and carbonic acid. The “Gazogene” is made
useful for small quantities of soda-water, and is arranged in the
following manner. The appearance of it is familiar to all. It consists
of a double vessel, into the upper part of which a solution of any
kind—wine and water, or even plain water—is put, to be saturated with
carbonic acid, or “aerated,” and into the lower one some carbonate of
soda and tartaric acid. A tube leads from this lower to the top of the
upper vessel, which screws on and off. By shaking the apparatus when
thus charged and screwed together, some of the liquid descends through
the tube into the lower vessel and moistens the soda and acid, which
therefore act on each other, and cause carbonic acid to be disengaged;
this, rising up through the tube (which is perforated with small holes
at the upper part), disperses itself through the liquid in small
bubbles, and causes sufficient pressure to enable the liquid to absorb
it, which therefore effervesces when drawn off by the tap.

[Illustration: Fig. 370.—Gazogene.]

[Illustration: Fig. 371.—Soda-water apparatus.]

Carbonic acid can be liquified, and then it is colourless. In a solid
form it resembles snow, and if pressed with the fingers it will blister
them. Being very heavy the gas can be poured into a vase, and if there
be a light in the receptacle the flame will be immediately extinguished.

That even the gas introduced into seltzer-water is capable of
destroying life, the following experiment will prove. Let us place
a bird within a glass case as in the illustration (fig. 373), and
connect the glass with a bottle of seltzer-water or a siphon. As soon
as the liquid enters, the carbonic acid will ascend, and this, if
continued for a long time, would suffocate the bird, which soon begins
to develop an appearance of restlessness.

[Illustration: Fig. 372.—Pouring out the carbonic acid gas.]

[Illustration: Fig. 373.—Experiment with carbonic acid.]

We have already remarked upon the important part taken by this gas
in nature, so we need only mention its existence in pits and caves.
There are many places in which the vapour is so strong as to render
the localities uninhabitable. In the Middle Ages the vapours were
attributed to the presence of evil spirits, who were supposed to
extinguish miners’ lamps, and suffocate people who ventured into the
caves. In the Grotto Del Cane there is still an example, and certain
caves of Montrouge are often filled with the gas. A lighted taper
held in the hand will, by its extinction, give the necessary warning.
Oxygen and carbon are condensed in carbonic acid, for the gas contains
a volume of oxygen equal to its own. If we fill a glass globe, as per
illustration (fig. 374), with pure oxygen, and in the globe insert two
carbon points, through which we pass a current of electricity, we shall
find, after the experiment, that if the stop-cock be opened, there is
no escape of gas, and yet the mercury does not rise in the tube, so the
oxygen absorbed has been replaced by an equal volume of carbonic acid.

The other combination of carbon with oxygen is the carbonic oxide (CO),
and when a small quantity of oxygen is burnt with it it gives a blue
flame, as on the top of the fire in our ordinary grates. This gas is
present in lime kilns, and is a very deadly one. We must now pass
rapidly through the compounds of carbon with hydrogen, merely referring
to coal for a moment as we go on.

Coal, of which we shall learn more in Mineralogy and Geology, is
a combination, mechanical or otherwise, and is the result of the
decomposition of vegetable matter in remote ages,—the so-called
“forests,” which were more like the jungles than the woods of the
present day. Moss and fern played prominent parts in this great
transformation, as we can see in the Irish peat-bogs, where the first
steps to the coal measures are taken.

[Illustration: Fig. 374.—Experiment showing that carbonic acid contains
oxygen and carbon.]

The compounds of carbon with hydrogen are important. There is the
“light” carburetted hydrogen (CH_{4}), which is usually known as
fire-damp in coal mines. It is highly inflammable and dangerous. The
safety-lamp invented by Davy is a great protection against it, for as
the gas enters it is cooled by the wire, and burns within harmlessly.
The explosion warns the miner. “Heavy” carburetted hydrogen possesses
double the quantity of carbon (C_{2}H_{4}). It is also explosive when
mixed with oxygen.

[Illustration: Fig. 375.—Temperature reduced by contact with wire.]

The most useful compound is coal-gas, and though its principal function
appears to be in some manner superseded by electricity, “gas” is still
too important to be put aside. It can easily be obtained by putting
small fragments of coal in the bowl of a tobacco-pipe, closing the
bowl with clay, and putting it in the fire. Before long the gas will
issue from the stem of the pipe, and may either be lighted or collected
in a bladder. For the use of the “million,” however, gas is prepared
upon a very large scale, and is divided into three processes—its
“formation,” “purification,” and its “collection” for distribution to
consumers. The first process is carried on by means of retorts shown
in the illustration (fig. 376). The first portion of the next figure
is a section of a furnace, the other part shows two furnaces from the
front. The following is the mode employed. The coal is put into retorts
fitted to the furnace, so that they are surrounded by the flames, and
terminating in a horizontal tube called the hydraulic main, E, which is
in its turn connected with a pit or opening for the reception of the
tar and ammoniacal liquor, etc., which condenses from the gas. It then
passes up and down a series of tubes in water, called a “condenser,”
and in this are reservoirs or receptacles for any tar and ammonia
that remain. But sulphur is still present, so the gas is carried to
the purifying apparatus (D in fig. 378), which consists of a large
cylindrical vessel air-tight, with an inverted funnel, nearly filled
with a mixture of lime and water. The gas bubbles in, and the sulphur
unites with the lime, while the gas rises to the top (trays of lime
are used when the gas enters from the bottom). The Gasometer, a large
vessel closed at the top and open below, dips into a large trough of
circular shape. The gasometer is balanced by weights and chains, and
may be raised (_See_ fig. 379). When quite empty the top rests upon
the ground, and when the gas enters it is raised to the top of the
frame which supports it. We have now our Gasometer full. When the time
comes to fill the pipes for lighting purposes, some of the weights are
removed, the Gasometer falls down slowly, and forces the gas through
the tubes into the main supply to be distributed. About four cubic feet
of gas is obtained from every pound of coal. When gas and air become
mixed, the mixture is very explosive. In a house where an escape of
gas is detected let the windows be opened at the top, and no light
introduced for several minutes.

[Illustration: Fig. 376.—Retorts.]

[Illustration:
  Fig. 377.—Section.      Front view.]

[Illustration:
  Fig. 378.—Condenser.      Purifier.      Gasometer.]

[Illustration: Fig. 379.—Gasometer.]

It has been calculated that one ton of good coal produces the
following:—

      1 Chaldron of coke              weighing  1,494 lbs.
     12 Gallons of tar                   ”        135 lbs.
     12 Gallons of ammoniacal liquor     ”        100 lbs.
  5,900 Cubic feet of gas                ”        291 lbs.
        Loss (water)                     ”        220 lbs.
                                                ----------
                                          Total 2,240 lbs.
                                                ==========

[Illustration: Fig. 380.—Gasometer]

We can thus estimate the profits of our gas companies at leisure. The
analysis of gas made by Professor Bunsen is as under, in 100 parts.

  Hydrogen                  45·58
  Marsh gas                 34·90
  Carbonic oxide             6·64
  Olefiant gas               4·08
  Butyline                   2·38
  Sulphide of hydrogen       0·29
  Nitrogen                   2·46
  Carbonic acid              3·67
                           ------
                           100·00
                           ======

Gas, therefore, is very injurious, for it rapidly vitiates the
atmosphere it burns in, and is very trying to the eyes, as well as
destructive to gilt ornaments.

Tar is familiar to all readers, and though unpleasant to handle or
to smell, it produces the beautiful aniline dyes. Tar pills are very
efficacious for some blood disorders, and will remove pimples, etc.,
from the face, and cure “boils” effectually. If a dose of five be
taken first, in a day or two four, and so on, no second remedy need be
applied. We have known cases finally cured, and no recurrence of boils
ever ensued after this simple remedy.

[Illustration: Fig. 381.—Tar manufactory]

Tar is one of the results left in the distillation both of wood and
coal: in places where wood is plentiful and tar in request, it is
produced by burning the wood for that purpose; and in some of the pits
in which charcoal is produced, an arrangement is made to collect the
tar also. Coal-tar and wood-tar are different in some respects, and
are both distilled to procure the napthas which bear their respective
names. From wood-tar creosote is also extracted, and it is this
substance which gives the peculiar tarry flavours to provisions, such
as ham, bacon, or herrings, cured or preserved by being smoked over
wood fires. Tar is used as a sort of paint for covering wood-work and
cordage when much exposed to wet, which it resists better than anything
else at the same price; but the tar chiefly used for these purposes is
that produced by burning fir or deal wood and condensing the tar in a
pit below the stack of wood; it is called Stockholm tar, as it comes
chiefly from that place.

Carbon only combines with nitrogen under peculiar circumstances. This
indirect combination is termed _cyanogen_ (CN). It was discovered
by Gay-Lussac, and is used for the production of Prussian blue.
Hydrocyanide of potassium (Prussic acid) is prepared by heating cyanide
of potassium with sulphuric acid. It is a deadly poison, and found
in peach-stones. Free cyanogen is a gas. The bisulphide of carbon is
a colourless, transparent liquid. It will easily dissolve sulphur and
phosphorus and several resins. When phosphorus is dissolved in it, it
makes a very dangerous “fire,” and one difficult to extinguish. We must
now leave carbon and its combinations, and come to sulphur.

[Illustration: Fig. 382.—Sulphur furnace.]

SULPHUR is found in a native state in Sicily and many other localities
which are volcanic. It is a yellow, solid body, and as it is never
perfectly free from earthy matter, it must be purified before it can
be used. It possesses neither taste nor smell, and is insoluble in
water. Sulphur is purified in a retort, C D, which communicates with
a brick chamber, A. The retort is placed over a furnace, K, and the
vapour passes into the chimney through the tube, D, where it condenses
into fine powder called “flowers of sulphur” (brimstone). A valve
permits the heated air to pass off, while no exterior air can pass in,
for explosions would take place were the heated vapour to meet the
atmospheric air. The danger is avoided by putting an air reservoir
outside the chimney which is heated by the furnace. The sulphur is
drawn out through the aperture, _r_, when deposited on the floor of
the chamber. The sulphur is cast into cylinders and sold. Sulphur is
soluble in bisulphide of carbon, and is used as a medical agent.

The compounds of sulphurs with oxygen form an interesting series. There
are two anhydrous oxides (anhydrides),—viz., sulphurous and sulphuric
anhydride (SO_{2} and SO_{3}). There are two notable acids formed
by the combination with water, sulphurous and sulphuric, and some
others, which, as in the case of nitrogen, form a series of multiple
proportions, the oxygen being present in an increasing regularity of
progression, as follows:—

  Name of Acid.                             Chemical Formula.

  Hypo-sulphurous acid                      H_{2}SO_{2}
  Sulphurous acid                           H_{2}SO_{3}
  Sulphuric acid                            H_{2}SO_{4}
  Thio-sulphuric, or hypo-sulphuric acid    H_{2}S_{2}O_{3}
  Dithionic acid                            H_{2}S_{2}O_{6}
  Trithionic acid                           H_{2}S_{3}O_{6}
  Tetrathionic acid                         H_{2}S_{4}O_{6}
  Pentathionic acid                         H_{2}S_{5}O_{6}

The last four are termed “polythionic,” because the proportions of
sulphur vary with constant proportions of the other constituents.

[Illustration: Fig. 383.—Liquefaction of sulphuric acid.]

The sulphurous anhydride mentioned above is produced when we burn
sulphur in the air, or in oxygen; it may be obtained in other ways.
It is a colourless gas, and when subjected to pressure may be
liquified, and crystallized at very low temperature. It was formerly
called sulphuric acid. It is a powerful “reducing agent,” and a good
antiseptic. It dissolves in water, and forms the H_{2}SO_{3}, now known
as sulphurous acid.

[Illustration: Fig. 384.—Retorts and receivers for acid.]

Sulphuric acid is a most dangerous agent in wicked or inexperienced
hands, and amateurs should be very careful when dealing with it. It
takes the water from the moist air, and from vegetable and animal
substances. It carbonizes and destroys all animal tissues. Its
discovery is due to Basil Valentine, in 1440. He distilled sulphate
of iron, or green vitriol, and the result was “oil of vitriol.” It
is still manufactured in this way in the Hartz district, and the
acid passes by retorts into receivers. The earthen retorts, A, are
arranged in the furnace as in the illustration, and the receivers,
B, containing a little sulphuric acid, are firmly fixed to them. The
oily brown product fumes in the air, and is called “fuming sulphuric
acid,” or Nordhausen acid. Sulphuric acid is very much used in chemical
manufactures, and the prices of many necessaries, such as soap, soda,
calico, stearin, paper, etc., are in close relationship with the cost
and production of sulphur, which also plays an important part in the
making of gunpowder. The manufacture of the acid is carried on in
platinum stills.

[Illustration: Fig. 385.—Experiment to show the existence of gases in
solution.]

Sulphuretted hydrogen, or the hydric sulphide (H_{2}S), is a colourless
and horribly-smelling gas, and arises from putrefying vegetable and
animal matter which contains sulphur. The odour of rotten eggs is due
to this gas, which is very dangerous when breathed in a pure state in
drains, etc. It can be made by treating a sulphide with sulphuric acid.
It is capable of precipitating the metals when in solution, and so
by its aid we can discover the metallic ingredient if it be present.
The gas is soluble in water, and makes its presence known in certain
sulphur springs. The colour imparted to egg-spoons and fish-knives
and forks sometimes is due to the presence of metallic sulphides. The
solution is called hydro-sulphuric acid.

PHOSPHORUS occurs in very small quantities, though in the form of
phosphates we are acquainted with it pretty generally, and as such it
is absorbed by plants, and is useful in agricultural operations. In
our organization—in the brain, the nerves, flesh, and particularly in
bones—phosphorus is present, and likewise in all animals. Nevertheless
it is highly poisonous. It is usually obtained from the calcined bones
of mammalia by obtaining _phosphoric acid_ by means of acting upon the
bone-ash with sulphuric acid. Phosphorus when pure is colourless,
nearly transparent, soft, and easily cut. It has a strong affinity for
oxygen. It evolves white vapour in atmospheric air, and is luminous;
to this element is attributable the luminosity of bones of decaying
animal matter. It should be kept in water, and handled—or indeed not
handled—but grasped with a proper instrument.

Phosphorus is much used in the manufacture of lucifer matches, and we
are all aware of the ghastly appearance and ghostly presentment it
gives when rubbed upon the face and hands in the dark. In the ripples
of the waves and under the counter of ships at sea, the phosphorescence
of the ocean is very marked. In Calais harbour we have frequently
noticed it of a very brilliant appearance as the mail steamer slowly
came to her moorings. This appearance is due to the presence of
phosphorus in the tiny animalculæ of the sea. It is also observable in
the female glow-worm, and the “fire-fly.” Phosphorus was discovered by
Brandt in 1669.

[Illustration: Fig. 386.—Manufacture of sulphuric acid.]

It forms two compounds with oxygen-phosphorous acid, H_{2}PO_{4}, and
phosphoric acid, H_{3}PO_{4}. The compound with hydrogen is well marked
as phosphuretted hydrogen, and is a product of animal and vegetable
decomposition. It may frequently be observed in stagnant pools, for
when emitted it becomes luminous by contact with atmospheric air. There
is a very pretty but not altogether safe experiment to be performed
when phosphuretted hydrogen has been prepared in the following manner.
Heat small pieces of phosphorus with milk of lime or a solution of
caustic potash; or make a paste of quick-lime and phosphorus, and
put into the flask with some quick-lime powdered. Fix a tube to the
neck, and let the other end be inserted in a basin of water. (_See_
illustration, fig. 388.) Apply heat; the phosphuretted hydrogen will
be given off, and will emerge from the water in the basin in luminous
rings of a very beautiful appearance. The greatest care should be
taken in the performance of this very simple experiment. _No water
must on any account come in contact with the mixture in the flask._ If
even a drop or two find its way in through the bent tube a tremendous
explosion will result, and then the fire generated will surely prove
disastrous. The experiment can be performed in a cheaper and less
dangerous fashion by dropping phosphate of lime into the basin. We
strongly recommend the latter course to the student unless he has
had some practice in the handling of these inflammable substances,
and learnt caution by experience. The effect when the experiment is
properly performed is very good, the smoke rising in a succession of
coloured rings.

[Illustration: Fig. 387.—(Phosphuretted hydrogen and marsh gas)
Will-o’-the-Wisp.]

SILICON is not found in a free state in nature, but, combined with
oxygen, as _Silica_ it constitutes the major portion of our earth,
and even occurs in wheat stalks and bones of animals. As flint or
quartz (see _Mineralogy_) it is very plentiful, and in its purest
form is known as rock crystal, and approaches the form of carbon
known as diamond. When separated from oxygen, silicon is a powder of
greyish-brown appearance, and when heated in an atmosphere of oxygen
forms silicic “acid” again, which, however, is not acid to the taste,
and is also termed “silica,” or “silex.” It is fused with great
difficulty, but enters into the manufacture of glass in the form of
sand. The chemical composition of glass is mixed silicate of potassium
or sodium, with silicates of calcium, lead, etc. Ordinary window-glass
is a mixture of silicates of sodium and calcium; crown glass contains
calcium and silicate of potassium. Crystal glass is a mixture of the
same silicate and lead. Flint glass is of a heavier composition. Glass
can be coloured by copper to a gold tinge, blue by cobalt, green
by chromium, etc. Glass made on a large scale is composed of the
following materials, according to the kind of glass that is required.

Flint glass (“crystal”) is very heavy and moderately soft, very white
and bright. It is essentially a table-glass, and was used in the
construction of the Crystal Palace. Its composition is—pure white
sea-sand, 52 parts, potash 14 parts, oxide of lead, 34 parts = 100.

  Plate Glass.                Crown Glass.          Green (Bottle) Glass.

  Pure white sand  55 parts.  Fine sand  63 parts.  Sea sand  80 parts.
  Soda             35  ”      Chalk       7  ”      Salt      10  ”
  Nitre             8  ”      Soda       30  ”      Lime      10  ”
  Lime              2  ”
                  ---                   ---                  ---
                  100  ”                100  ”               100  ”

The ingredients to be made into glass (of whatever kind it may be)
are thoroughly mixed together and thrown from time to time into large
crucibles placed in a circle, A A (fig. 389), in a furnace resting on
buttresses, B B, and heated to whiteness by means of a fire in the
centre, C, blown by a blowing machine, the tube of which is seen at D.
This furnace is shown in prospective in fig. 390. The ingredients melt
and sink down into a clear fluid, throwing up a scum, which is removed.
This clear glass in the fused state is kept at a white heat till all
air-bubbles have disappeared; the heat is then lowered to a bright
redness, when the glass assumes a consistence and ductility suitable to
the purposes of the “blower.”

[Illustration: Fig. 388.—Experiment with phosphuretted hydrogen.]

Glass blowing requires great care and dexterity, and is done by
twirling a hollow rod of iron on one end of which is a globe of
melted glass, the workman blowing into the other end all the time. By
reheating and twirling a sheet of glass is produced. Plate glass is
formed by pouring the molten glass upon a table with raised edges. When
cold it is ground with emery powder, and then polished by machinery.

[Illustration: Fig. 389.—Crucibles.]

Many glass articles are cast, or “struck-up,” by compression in moulds,
and are made to resemble cut-glass, but they are much inferior in
appearance. The best are first blown, and afterwards cut and polished.
Of whatever kind of glass the article may be, it is so brittle that the
slightest blow would break it, a bad quality which is got rid of by a
process called “annealing,” that is, placing it while quite hot on the
floor of an oven, which is allowed to cool very gradually. This slow
cooling takes off the brittleness, consequently articles of glass well
annealed are very much tougher than others, and will scarcely break in
boiling water.

[Illustration: Fig. 390.—Plate-glass casting—bringing out the pot.]

The kind generally used for ornamental cutting is flint-glass.
Decanters and wine-glasses are therefore made of it; it is very bright,
white, and easily cut. The cutting is performed by means of wheels of
different sizes and materials, turned by a treadle, as in a common
lathe, or by steam power; some wheels are made of fine sandstone, some
of iron, others of tin or copper; the edges of some are square,or
round, or sharp. They are used with sand and water, or emery and water,
stone wheels with water only.

[Illustration: Fig. 391.—Glass furnace. (_See_ also fig. 390 for
detail.)]

[Illustration: Fig. 392.—Glass-cutting.]

In a soluble form silicic acid is found in springs, and thus
enters into the composition of most plants and grasses, while the
shells and scales of “infusoria” consist of silica. As silicate
of alumina,—_i.e._, clay,—it plays a very important _rôle_ in our
porcelain and pottery works.

       *       *       *       *       *

BORON is found in volcanic districts, in lakes as boracic acid, in
combination with oxygen. It is a brownish-green, insoluble powder, in
a free state, but as boracic acid it is white. It is used to colour
fireworks with the beautiful green tints we see. Soda and boracic acid
combine to make borax (or biborate of soda). Another and inferior
quality of this combination is _tinkal_, found in Thibet. Borax is much
used in art and manufactures, and in glazing porcelain. (Symbol B,
Atomic Weight 11).

SELENIUM is a very rare element. It was found by Berzilius in a
sulphuric-acid factory. It is not found in a free state in nature. It
closely resembles sulphur in its properties. Its union with hydrogen
produces a gas, seleniuretted hydrogen, which is even more offensive
than sulphuretted hydrogen. (Symbol Se, Atomic Weight 79).

TELLURIUM is also a rare substance generally found in combination with
gold and silver. It is like bismuth, and is lustrous in appearance.
Telluretted hydrogen is horrible as a gas. Tellurium, like selenium,
sulphur, and oxygen, combines with two atoms of hydrogen. (Symbol Te,
Atomic Weight 129).

[Illustration: Fig. 393.—Casting plate-glass.]

ARSENIC, like tellurium, possesses many attributes of a metal, and on
the other hand has some resemblance to phosphorus. Arsenic is sometimes
found free, but usually combined with metals, and is reduced from
the ores by roasting; and uniting with oxygen in the air, is known
as “white arsenic.” The brilliant greens on papers, etc., contain
arsenic, and are poisonous on that account. Arsenic and hydrogen unite
(as do sulphur and hydrogen, etc.), and produce a foetid gas of a most
deadly quality. This element also unites with sulphur. If poured into
a glass containing chlorine it will sparkle and scintillate as in the
illustration (fig. 395). (Symbol As, Atomic Weight 75).

Before closing this division, and passing on to a brief review of
the METALS, we would call attention to a few facts connected with
the metalloids we have been considering. Some, we have seen, unite
with hydrogen only, as chlorine; some with two atoms of hydrogen, as
oxygen, sulphur, etc., and some with three, as nitrogen and phosphorus;
some again with four, as carbon and silicon. It has been impossible
in the pages we have been able to devote to the Metalloids to do more
than mention each briefly and incompletely, but the student will find
sufficient, we trust, to interest him, and to induce him to search
farther, while the general reader will have gathered some few facts to
add to his store of interesting knowledge. We now pass on to the Metals.

[Illustration: Fig· 394—The manufacture of porcelain in China.]

[Illustration: Fig. 395.—Experiment showing affinity between arsenic
and chlorine.]



CHAPTER XXIX.

THE METALS.

  WHAT METALS ARE—CHARACTERISTICS AND GENERAL PROPERTIES OF
  METALS—CLASSIFICATION—SPECIFIC GRAVITY—DESCRIPTIONS.


We have learnt that the elements are divided into metalloids and
metals, but the line of demarcation is very faint. It is very difficult
to define what a metal is, though we can say what it is not. It is
indeed impossible to give any absolute definition of a metal, except as
“an element which does not unite with hydrogen, or with another metal
to form a chemical compound.” This definition has been lately given by
Mr. Spencer, and we may accept it as the nearest affirmative definition
of a metal, though obviously not quite accurate.

A metal is usually supposed to be solid, heavy, opaque, ductile,
malleable, and tenacious; to possess good conducting powers for heat
and electricity, and to exhibit a certain shiny appearance known as
“metallic lustre.” These are all the conditions, but they are by
no means necessary, for very few metals possess them all, and many
non-metallic elements possess several. The “alkali” metals are lighter
than water; mercury is a fluid. The opacity of a mass is only in
relation to its thickness, for Faraday beat out metals into plates so
thin that they became transparent. All metals are not malleable, nor
are they ductile. Tin and lead, for example, have very little ductility
or tenacity, while bismuth and antimony have none at all. Carbon is a
much better conductor of electricity than many metals in which such
power is extremely varied. Lustre, again, though possessed by metals,
is a characteristic of some non-metals. So we see that while we can
easily say what is not a metal, we can scarcely define an actual metal,
nor depend upon unvarying properties to guide us in our determination.

[Illustration: Fig. 396.—Laminater.]

The affinity of metals for oxygen is in an inverse ratio to their
specific gravity, as can be ascertained by experiment, when the
heaviest metal will be the least ready to oxidise. Metals differ in
other respects, and thus classification and division become easier. The
fusibility of metals is of a very wide range, rising from a temperature
below zero to the highest heat obtainable in the blow-pipe, and even
then in the case of osmium there is a difficulty. While there can be no
question that certain elements, iron, copper, gold, silver, etc., are
metals proper, there are many which border upon the line of demarcation
very closely, and as in the case of arsenic even occupy the debatable
land.

SPECIFIC GRAVITY is the relation which the weight of substance bears
to the weight of an equal volume of water, as already pointed out in
PHYSICS. The specific gravities of the metals vary very much, as will
be seen from the table following—water being, as usual, taken as 1:—

  Aluminium        2·56
  Antimony         6·7
  Arsenic          6·
  Bismuth          9·7
  Cadmium          8·6
  Calcium          1·5
  Chromium         6·8
  Cobalt           8·9
  Copper           8·9
  Gold            19·3
  Indium           7·3
  Iridium         21·1
  Iron             7·8
  Lead            11·3
  Lithium         ·593
  Magnesium       1·74
  Manganese        8·
  Mercury         13·5
  Molybdenum       8·6
  Nickel           8·8
  Osmium          21·4
  Palladium       11·8
  Platinum        21·5
  Potassium       ·865
  Rhodium         12·1
  Rubidium         1·5
  Ruthenium       11·4
  Silver          10·5
  Sodium          ·972
  Strontium        2·5
  Thallium        11·8
  Tin              7·2
  Titanium         5·3
  Tungsten        17·6
  Uranium         18·4
  Zinc             7·1
  Zircon           4·3

Some metals are therefore lighter and some heavier than water.

The table underneath gives the approximate fusing points of some of the
metals (Centigrade Scale)—

  (Ice melts at 0°.)

  Platinum[21]   about 1500°
  Gold             ”   1200°
  Silver           ”   1000°
  Cast iron       1000-1200°
  Wrought iron     ”   1500°
  Copper           ”   1100°
  Antimony         ”    432°
  Zinc             ”    400°
  Lead             ”    330°
  Bismuth          ”    265°
  Tin              ”    235°
  Sodium           ”     97°
  Potassium        ”     60°
  Mercury          ”     40°

There are some metals which, instead of fusing,—that is, passing from
the solid to the liquid state,—go away in vapour. These are volatile
metals. Mercury, potassium, and sodium, can be thus distilled. Some do
not expand with heat, but contract (like ice), antimony and bismuth,
for instance, while air pressure has a considerable effect upon the
fusing point. Some vaporise at once without liquefying; others, such as
iron, become soft before melting.

ALLOYS are combinations of metals which are used for many purposes, and
become harder in union. Amalgams are alloys in which mercury is one
constituent. Some of the most useful alloys are under-stated:—

  Name of Alloy.       Composition.

  Aluminium bronze     Copper and aluminium.
  Bell metal           Copper and tin.
  Bronze                  ”      ”
  Gun metal               ”      ”
  Brass                Copper and zinc.
  Dutch metal             ”      ”
  Mosaic gold             ”      ”
  Ormulu                  ”      ”
  Tombac                  ”      ”
  German silver        Copper, nickel, and zinc.
  Britannia metal      Antimony and tin.
  Solder                   ”       ”
  Pewter               Tin and lead.
  Type metal           Lead and antimony (also copper at times).
  Shot                 Lead and arsenic.
  Gold currency        Gold and copper.
  Silver currency      Silver and copper.
  Stereotype metal     Lead, antimony, and bismuth.
          Metals combine with chlorine, and produce chlorides,
          Metals    ”     ”   sulphur    ”     ”    sulphides,
          Metals    ”     ”   oxygen     ”     ”    oxides, and so on.

The metals may be classed as follows in divisions:—

  Metals of the alkalies        as POTASSIUM, SODIUM, LITHIUM, AMMONIUM.
  Metals of the alkaline earths as BARIUM, CALCIUM, MAGNESIUM, STRONTIUM.
  Metals of the earths          as {ALUMINIUM, Cerium, Didymium, Erbium,
                                   {Glucinium, Lanthanum, Terbium,
                                   {Thorium, Yttrium, Zirconium.
  Metals proper—
      Common Metals             as {IRON, MANGANESE, COBALT, NICKEL,
                                   {COPPER, Bismuth, LEAD, TIN, ZINC,
                                   {CHROMIUM, Antimony.
      Noble Metals              as {MERCURY, SILVER, GOLD, PLATINUM,
                                   {Palladium, Rhodium, Ruthenium,
                                   {Osmium, Iridium.

We cannot attempt an elaborate description of all the metals, but we
will endeavour to give a few particulars concerning the important ones,
leaving many parts for Mineralogy to supplement and enlarge upon. We
shall therefore mention only the most useful of the metals in this
place. We will commence with POTASSIUM.


METALS OF THE ALKALIES.

POTASSIUM has a bright, almost silvery, appearance, and is so greatly
attracted by oxygen that it cannot be kept anywhere if that element be
present—not even in water, for combustion will immediately ensue on
water; and in air it is rapidly tarnished. It burns with a beautiful
violet colour, and a very pretty experiment may easily be performed
by throwing a piece upon a basin of water. The fragment combines with
the oxygen of the water, the hydrogen is evolved, and burns, and the
potassium vapour gives the gas its purple or violet colour. The metal
can be procured by pulverizing carbonate of potassium and charcoal, and
heating them in an iron retort. The vapour condenses into globules in
the receiver, which is surrounded by ice in a wire basket. It must be
collected and kept in naphtha, or it would be oxidised. Potassium was
first obtained by Sir Humphrey Davy in 1807. Potash is the oxide of
potassium, and comes from the “ashes” of wood.

[Illustration: Fig. 397.—Preparation of potassium.]

The compounds of potassium are numerous, and exist in nature, and by
burning plants we can obtain potash (“pearlash”). Nitrate of potassium,
or nitre (saltpetre), (KNO_{3}), is a very important salt. It is found
in the East Indies. It is a constituent of gunpowder, which consists of
seventy-five parts of nitre, fifteen of charcoal, and ten of sulphur.
The hydrated oxide of potassium, or “caustic potash” (obtained from the
carbonate), is much used in soap manufactories. It is called “caustic”
from its property of cauterizing the tissues. Iodide, bromide, and
cyanide of potassium, are used in medicine and photography.

Soap is made by combining soda (for hard soap), or potash (for soft
soap), with oil or tallow. Yellow soap has turpentine, and occasionally
palm oil, added. Oils and fats combine with metallic oxides, and oxide
of lead with olive oil and resin forms the adhesive plaister with which
we are all familiar when the mixture is spread upon linen. Fats boiled
with potash or soda make soaps; the glycerine is sometimes set free
and purified as we have it. Sometimes it is retained for glycerine
soap. Fancy soap is only common soap coloured. White and brown Windsor
are the same soap—in the latter case browned to imitate age! Soap is
quite soluble in spirits, but in ordinary water it is not so greatly
soluble, and produces a lather, owing to the lime in the water being
present in more or less quantity, to make the water more or less “hard.”

[Illustration: Fig. 398.—Machine for cutting soap in bars.]

[Illustration: Fig. 399.—Soap-boiling house.]


SODIUM is not unlike potassium, not only in appearance, but in its
attributes; it can be obtained from the carbonate, as potassium is
obtained from its carbonate. _Soda_ is the oxide of sodium, but the
most common and useful compound of sodium is the chloride, or common
salt, which is found in mines in England, Poland, and elsewhere. Salt
may also be obtained by the evaporation of sea water. Rock salt is
got at Salzburg, and the German salt mines and works produce a large
quantity. The _Carbonate of Soda_ is manufactured from the chloride of
sodium, although it can be procured from the salsoda plants by burning.
The chloride of sodium is converted into sulphate, and then ignited
with carbonate of lime and charcoal. The soluble carbonate is extracted
in warm water, and sold in crystals as soda, or (anhydrous) “soda ash.”
The large quantity of hydrochloric acid produced in the first part
of the process is used in the process of making chloride of lime. A
few years back, soda was got from Hungary and various other countries
where it exists as a natural efflorescence on the shores of some lakes,
also by burning sea-weeds, especially the common bladder wrack (_Fucus
vesiculosus_), the ashes of which were melted into masses, and came
to market in various states of purity. The bi-carbonate of soda is
obtained by passing carbonic acid gas over the carbonate crystals. Soda
does not attract moisture from the air. It is used in washing, in glass
manufactories, in dyeing, soap-making, etc.

_Sulphate of Soda_ is “Glauber’s Salt”; it is also employed in
glass-making. Mixed with sulphuric acid and water, it forms a freezing
mixture. Glass, as we have seen, is made with silicic acid (sand),
soda, potassa, oxide of lead, and lime, and is an artificial silicate
of soda.


LITHIUM is the lightest of metals, and forms the link between alkaline
and the alkaline earth metals. The salts are found in many places in
solution. The chloride when decomposed by electricity yields the metal.


CÆSIUM and RUBIDIUM require no detailed notice from us. They were first
found in the solar spectrum, and resemble potassium.


AMMONIUM is only a conjectural metal. _Ammonia_, of which we have
already treated, is so like a metallic oxide that chemists have come
to the conclusion that its compounds contain a metallic body, which
they have named hypothetically AMMONIUM. It is usually classed amongst
the alkaline metals. The salts of ammonia are important, and have
already been mentioned. Muriate (chloride) of ammonia, or sal-ammoniac,
is analogous to chloride of sodium and chloride of potassium. It is
decomposed by heating it with slaked lime, and then gaseous ammonia is
given off.

[Illustration: Fig. 400.—Mottled soap-frames.]


THE METALS OF THE ALKALINE EARTHS.

BARIUM is the first of the four metals we have to notice in this
group, and will not detain us long, for it is little known in a free
condition. Its most important compound is heavy spar (_sulphate of
baryta_), which, when powdered, is employed as a white paint. The oxide
of barium, BaO, is termed baryta.

_Nitrate of Baryta_ is used for “green fire,” which is made as
follows:—Sulphur, twenty parts; chlorate of potassium, thirty-three
parts; and nitrate of baryta, eighty parts (by weight).

[Illustration: Fig. 401.—Soda furnace.]

       *       *       *       *       *

CALCIUM forms a considerable quantity of our earth’s crust. It is the
metal of lime, which is the oxide of calcium. In a metallic state it
possesses no great interest, but its combinations are very important to
us. _Lime_ is, of course, familiar to all. It is obtained by evolving
the carbonic acid from carbonate of lime (CaO).

The properties of this lime are its white appearance, and it develops
a considerable amount of heat when mixed with water, combining to make
hydrate of lime, or “slaked lime.” This soon crumbles into powder, and
as a mortar attracts the carbonic acid from the air, by which means it
assumes the carbonate and very solid form, which renders it valuable
for cement and mortar, which, when mixed with sand, hardens. Caustic
lime is used in whitewashing, etc.

_Carbonate of Lime_ (CaCO_{3}) occurs in nature in various forms, as
limestone, chalk, marble, etc. Calc-spar (arragonite) is colourless,
and occurs as crystals. Marble is white (sometimes coloured by metallic
oxides), hard, and granular. Chalk is soft and pulverizing. It occurs
in mountainous masses, and in the tiniest shells, for carbonate of lime
is the main component of the shells of the crustacea, of corals, and
of the shell of the egg; it enters likewise into the composition of
bones, and hence we must regard it as one of the necessary constituents
of the food of animals. It is an almost invariable constituent of
the waters we meet with in Nature, containing, as they always do, a
portion of carbonic acid, which has the power of dissolving carbonate
of lime. But when gently warmed, the volatile gas is expelled, and the
carbonate of lime deposited in the form of white incrustations upon the
bottom of the vessel, which are particularly observed on the bottoms of
tea-kettles, and if the water contains a large quantity of calcareous
matter, even our water-bottles and drinking-glasses become covered with
a thin film of carbonate of lime. These depositions may readily be
removed by pouring into the vessels a little dilute hydrochloric acid,
or some strong vinegar, which in a short time dissolves the carbonate
of lime.

_Sulphate of Lime_ (CaSO_{4}) is found in considerable masses, and
is commonly known under the name of _Gypsum_. It occurs either
crystallized or granulated, and is of dazzling whiteness; in the latter
form it is termed _Alabaster_, which is so soft as to admit of being
cut with a chisel, and is admirably adapted for various kinds of works
of art. Gypsum contains water of crystallization, which is expelled at
a gentle heat. But when ignited, ground, and mixed into a paste with
water, it acquires the property of entering into chemical combination
with it, and forming the original hydrate, which in a short time
becomes perfectly solid. Thus it offers to the artist a highly valuable
material for preparing the well-known plaster of Paris figures, and
by its use the noblest statues of ancient and modern art have now
been placed within the reach of all. Gypsum, moreover, has received a
valuable application as manure. In water it is slightly soluble, and
imparts to it a disagreeable and somewhat bitterish, earthy taste. It
is called “selenite” when transparent.

_Phosphate of Lime_ constitutes the principal mass of the bones of
animals, and is extensively employed in the preparation of phosphorus;
in the form of ground bones it is likewise used as a manure. It appears
to belong to those mineral constituents which are essential to the
nutrition of animals. It is found in corn and cereals, and used in
making bread; so we derive the phosphorus which is so useful to our
system.

_Chloride of Lime_ is a white powder smelling of chlorine, and is
produced by passing the gas over the hydrate of lime spread on trays
for the purpose. It is the well-known “bleaching powder.” It is also
used as a disinfectant. The _Fluoride of Calcium_ is Derbyshire spar,
or “Blue John.” Fluor spar is generally of a purple hue. We may add
that hard water can be softened by adding a little powdered lime to it.

       *       *       *       *       *

MAGNESIUM sometimes finds a place with the other metals, for it bears a
resemblance to zinc. Magnesium may be prepared by heating its chloride
with sodium. Salt is formed, and the metal is procured. It burns very
brightly, and forms an oxide of magnesia (MgO). Magnesium appears in
the formation of mountains occasionally. It is ductile and malleable,
and may be easily melted.

_Carbonate of Magnesia_, combining with carbonate of lime, form the
Dolomite Hills. When pure, the carbonate is a light powder, and when
the carbonic acid is taken from it by burning it is called Calcined
Magnesia.

The _Sulphate of Magnesia_ occurs in sea-water, and in saline springs
such as Epsom. It is called “Epsom Salts.” Magnesium wire burns
brightly, and may be used as an illuminating agent for final scenes in
private theatricals. _Magnesite_ will be mentioned among Minerals.

       *       *       *       *       *

STRONTIUM is a rare metal, and is particularly useful in the
composition of “red-fire.” There are the carbonate and sulphate of
strontium; the latter is known as _Celestine_. The red fire above
referred to can be made as follows, in a _dry mixture_. Ten parts
nitrate of strontia, 1½ parts chlorate of potassium, 3½ parts of
sulphur, 1 part sulphide of antimony, and ½ part charcoal. Mix well
without moisture, enclose in touch paper, and burn. A gorgeous crimson
fire will result.


METALS OF THE EARTHS.

ALUMINIUM (Aluminum) is like gold in appearance when in alloy with
copper, and can be procured from its chloride by decomposition with
electricity. It occurs largely in nature in composition with clays
and slates. Its oxide, _alumina_ (Al_{2}O_{3}), composes a number of
minerals, and accordingly forms a great mass of the earth. Alumina is
present in various forms (_see_ Minerals) in the earth, all of which
will be mentioned under Crystallography and Mineralogy. The other nine
metals in this class do not call for special notice.


HEAVY METALS

IRON, which is the most valuable of all our metals, may fitly head
our list. So many useful articles are made of it, that without
consideration any one can name twenty. The arts of peace and the
glories of war are all produced with the assistance of iron, and its
occurrence with coal has rendered us the greatest service, and placed
us at the head of nations. It occurs native in meteoric stones.

Iron is obtained from certain ores in England and Sweden, and these
contain oxygen and iron (_see_ Mineralogy). We have thus to drive away
the former to obtain the latter. This is done by putting the ores in
small pieces into a blast furnace (fig. 402) mixed with limestone
and coal. The process of severing the metal from its ores is termed
smelting, the air supplied to the furnace being warmed, and termed the
“hot blast.” The “cold blast” is sometimes used. The ores when dug from
the mine are generally stamped into powder, then “roasted,”—that is,
made hot, and kept so for some time to drive off water, sulphur, or
arsenic, which would prevent the “fluxes” acting properly. The fluxes
are substances which will mix with, melt, and separate the matters to
be got rid of, the chief being charcoal, coke, and limestone. The ore
is then mixed with the flux, and the whole raised to a great heat; as
the metal is separated it melts, runs to the bottom of the “smelting
furnace,” and is drawn off into moulds made of sand; it is thus cast
into short thick bars called “pigs,” so we hear of pig-iron, and
pig-lead. Iron is smelted from “ironstone,” which is mixed with coke
and limestone. The heat required to smelt iron is so very great, that
a steam-engine is now generally employed to blow the furnace. (Before
the invention of the steam-engine, water-mills were used for the same
purpose.) The smelting is conducted in what is called a blast furnace.
When the metal has all been “reduced,” or melted, and run down to the
bottom of the furnace, a hole is made, out of which it runs into the
moulds; this is called “tapping the furnace.”

[Illustration: Fig. 402.—Blast furnace.]

Smelting is often confounded with melting, as the names are somewhat
alike, but the processes are entirely different; in melting, the metal
is simply liquefied, in smelting, the metal has to be produced from
ores which often have no appearance of containing any, as in the case
of ironstone, which looks like brown clay.

The cone of the furnace, A, is lined with fire-bricks, _i_ _i_, which
is encased by a lining, _l_ _l_; outside are more fire-bricks, and
then masonry, _m_ _n_; C is the throat of the furnace; D the chimney.
The lower part, B, is called the _boshes_. As soon as the ore in
the furnace has become ignited the carbon and oxygen unite and form
carbonic acid, which escapes, and the metal fuses at last and runs
away. The coal and ore are continually added year after year. The
glassy scum called “slag “ protects the molten iron from oxidation.

The metal drawn from the blast furnace is “pig iron,” or “cast” iron,
and contains carbon. This kind of iron is used for casting operations,
and runs into sand-moulds. It contracts very little when cooling. It is
hard and brittle.

[Illustration: Fig. 403.—General foundry, Woolwich Arsenal.]

[Illustration: Fig. 404.—Wire rollers.]

[Illustration: Fig. 405.—Cutting edges.]

_Bar Iron_ is the almost pure metal. It is remarkably tenacious, and
may be drawn into very fine wire or rolled. But it is not hard enough
for tools. It is difficult to fuse, and must be welded by hammering at
a red heat. Wire-drawing is performed by taking the metal as a bar,
and passing it between rollers (fig. 404), which flattens it, and
then between a new set, which form cutting edges on the rolled plate
(fig. 405), the projections of one set fitting into the hollows of the
other closely as in the illustration. The strips of metal come out at
the aperture seen at A in the next illustration. These rods are drawn
through a series of diminishing holes in a steel plate, occasionally
being heated to keep it soft and ductile. When the wire has got to a
certain fineness it is attached to a cylinder and drawn away, at the
same time being wound round the cylinder over a small fire. Some
metals can be drawn much finer than others. Gold wire can be obtained
of a “thickness” (or thinness) of only the 5,000th part of an inch,
550 feet weighing one grain! But platinum has exceeded this marvellous
thinness, and wire the 30,000th part of an inch has been produced.
Ductility and malleability are not always found in the same metal in
proportion. The sizes of wires are gauged by the instrument shown in
the margin. The farther the wire will go into the groove the smaller
its “size.”

[Illustration: Fig. 406.—Rollers.]

[Illustration: Fig. 407.—Wire size.]

Steel contains a certain amount of carbon, generally about 1 to 2
per cent. Cast steel is prepared from cast iron. Steel from bar-iron
has carbon added, and is termed bar-steel. The process is called
“cementation,” and is carried on by packing the bars of iron in
brick-work boxes, with a mixture of salt and soot, or with charcoal,
which is termed “cement.” Steel is really a carbide of iron, and Mr.
Bessemer founded his process of making steel by blowing out the excess
of carbon from the iron, so that the proper amount—1·5 per cent.—should
remain.

[Illustration: Fig. 408.—Coarse wire-drawing.]

A brief summary of the Bessemer process may be interesting. If a bar of
steel as soft as iron be made red-hot and plunged into cold water, it
will become very hard. If it be then gently heated it will become less
hard, and is then fitted for surgical instruments. The various shades
of steel are carefully watched,—the change of colour being due to
the varying thickness of the oxide; for we know that when light falls
upon very thin films of a substance,—soap-bubbles, for instance,—the
light reflected from the under and upper surfaces interfere, and cause
colour, which varies with the thickness of the film. These colours in
steel correspond to different temperatures, and the “temper” of the
steel depends upon the temperature it has reached. The following table
extracted from Haydn’s “Dictionary of Science” gives the “temper,” the
colour, and the uses of the various kinds of steel.

[Illustration: Fig. 409.—Fine wire-drawing.]

  Temperature |      Colour.      |           Uses of Steel.
  Cent.  Fahr.|                   |
  ------------+-------------------+-------------------------------------
  220° = 430° | Faint yellow      | Lancets.
  232° = 450° | Pale straw        | Best razors and surgical instruments.
  243° = 470° | Yellow            | Ordinary razors, pen-knives, etc.
  254° = 490° | Brown             | Small shears, scissors, cold chisels,
                                     etc.
  265° = 510° | Brown and purple  | Axes, pocket-knives, plane-irons, etc.
                 spots
  277° = 530° | Purple            | Table-knives, etc.
  288° = 550° | Light blue        | Swords, watch-springs, etc.
  293° = 560° | Full blue         | Fine saws, daggers, etc.
  316° = 600° | Dark blue         | Hand and pit saws.

The _Bessemer_ process transfers the metal into a vessel in which
there are tubes, through which air is forced, which produces a much
greater heat than a bellows does. Thus in the process the carbon of the
iron acts as fuel to maintain the fusion, and at the same time by the
bubbling of the carbonic acid mixes the molten iron thoroughly.

During the bubbling up of the whole mass of iron, and the extreme
elevation of temperature caused by the union of the carbon of the
impure iron with the oxygen of the air, the oxide of iron is formed,
and as fast as it forms fuses into a sort of glass; this unites with
the earthy matters of the “impure” iron, and floats on the upper part
as a flux, thus ridding the “cast iron” of all its impurities, with no
other fuel than that contained in the metal itself, and in the air
used. When the flame issuing from the “converter” contracts and changes
its colour, then the time is known to have arrived when the iron is “
de-carbonized.” The amount of carbon necessary is artificially added,
ebullition takes place, a flame of carbonic oxide comes out, and the
metal is then run into ingots.

The compounds of iron which are soluble in water have a peculiar
taste called _chalybeate_ (like ink). Many mineral springs are so
flavoured, and taste, as the immortal Samuel Weller put it, “like warm
flat-irons.” Iron is frequently used as a medicine to renew the blood
globules.

_Protoxide of Iron_ is known only in combination.

_Sesqui-Oxide of Iron_ is “red ironstone.” Powdered it is called
English _rouge_, a pigment not altogether foreign to our use. In a pure
state it is a remedy for arsenical poisoning, and is really the “rust”
upon iron.

[Illustration: Fig. 410.—Bessemer’s process.]

_Bisulphide of Iron_ is iron pyrites, and is crystalline.

_Chloride of Iron_ is dissolved from iron with hydrochloric acid. It is
used in medicine.

_Cyanide of Iron_ makes, with cyanide of potassium, the well-known
prussiate of potash (ferro-cyanide of potassium), which, when heated,
precipitates Prussian blue (cyanogen and iron).

The _Sulphate of the Protoxide_ is known as copperas, or _green
vitriol_, and is applied to the preparation of Prussian blue.

       *       *       *       *       *

MANGANESE is found extensively, but not in any large quantities, in one
place; iron ore contains it. It is very hard to fuse, and is easily
oxidised. The binoxide is used to obtain oxygen, and when treated with
potassium and diluted, it becomes the permanganate of potassium, and is
used as “Condy’s fluid.” It readily oxides organic matters, and is thus
a disinfectant. It crystallizes in long, deep, red needles, which are
dissolved in water. It is a standard laboratory test. There are other
compounds, but in these pages we need not detail them.

       *       *       *       *       *

COBALT and NICKEL occur together. They are hard, brittle, and fusible.
The salts of cobalt produce beautiful colours, and the chloride yields
an “invisible” or sympathetic ink. The oxide of cobalt forms a blue
pigment for staining glass which is called “smalt.” Nickel is chiefly
used in the preparation of German silver and electro-plating. The
salts of nickel are green. Nickel is difficult to melt, and always is
one of the constituents of meteoric iron, which falls from the sky in
aërolites. It is magnetic like cobalt, and is extracted from the ore
called kupfer-nickel. A small United-States coin is termed a “nickel.”

       *       *       *       *       *

COPPER is the next metal we have to notice. It has been known for
centuries. It is encountered native in many places. The Cornish copper
ore is the copper pyrites. The fumes of the smelting works are very
injurious, containing, as they do, arsenic and sulphur. The ground near
the mines is usually bare of vegetation in consequence of the “smoke.”
Sheet copper is worked into many domestic utensils, and the alloy with
zinc, termed _Brass_, is both useful and ornamental. _Red brass_ is
beaten into thin leaves, and is by some supposed to be “gold leaf”; it
is used in decorative work. Bronze is also an alloy of copper, as are
gun-metal, bell-metal, etc.

[Illustration: Fig. 411.—Native copper.]

Next to silver, copper is the best conductor of electricity we have.
It is very hard and tough, yet elastic, and possesses malleability
and ductility in a high degree. It forms two oxides, and there are
several sulphides; the principal of the latter are found native, and
worked as ores. The sulphate of copper is termed _blue vitriol_, and
is used in calico-printing, and from it all the (copper) pigments are
derived. It is also used in solution by agriculturists to protect wheat
from insects. When copper or its alloys are exposed to air and water,
a carbonate of copper forms, which is termed _verdigris_. All copper
salts are poisonous; white of eggs is an excellent remedy in such cases
of poisoning.

       *       *       *       *       *

LEAD is obtained from galena, a sulphide of lead. It is a soft
and easily-worked metal. When freshly cut it has quite a bright
appearance, which is quickly tarnished. Silver is often present in
lead ore, and is extracted by Pattison’s process, which consists in the
adaptation of the knowledge that lead containing silver becomes solid,
after melting, at a _lower_ temperature than lead does when pure. Pure
lead therefore solidifies sooner.

One great use of lead is for our domestic water-pipes, which remind us
in winter of their presence so disagreeably. Shot is made from lead,
and bullets are cast from the same metal. Shot-making is very simple,
and before the days of breech-loading guns and cartridges, no doubt
many readers have cast bullets in the kitchen and run them into the
mould over a basin of water or a box of sand. For sporting purposes
lead is mixed with arsenic, and when it is melted it is poured through
a sort of sieve (as in the cut) at the top of a high tower. (_See_
figs. 413 and 414). The latter illustration gives the section of the
shot tower; A is the furnace, B is the tank for melting the lead, and
the metal is permitted by the workman at C to run through the sieve
in fine streams. As the lead falls it congeals into drops, which are
received in water below to cool them. They are, of course, not all
round, and must be sorted. This operation is performed by placing them
on a board tilted up, and under which are two boxes. The round shot
rush over the first holes and drop into the second box, but the uneven
ones are caught lagging, and drop into box No. 1. They are accordingly
sent to the furnace again.

[Illustration: Fig. 412.—Shot tower.]

The next process is to sort the good shot for size. This is done
by sieves—one having holes a _little larger_ than the size of shot
required. This sieve passes through it all of the right size and
smaller, and keeps the bigger ones. Those that have passed this
examination are then put into another sieve, which has holes in it a
_little smaller_ than the size of shot wanted. This sieve retains the
right shot, and lets the smaller sizes pass, and so on. The shot are
sized and numbered, glazed by rolling them in a barrel with graphite,
and then they are ready for use. Bullets are made by machinery by the
thousand, and made up into cartridges with great speed.

[Illustration: Fig. 413.—Sieve for making shot.]

The compounds of lead are also poisonous, and produce “colic,” to
which painters are subject. Red lead, or minium, is a compound of the
protoxide and the binoxide, and may be found native. The former oxide
is _litharge_; _white_ lead, or the _carbonate of lead_, is a paint,
and is easily obtained by passing a stream of carbonic acid into a
solution of acetate of lead. It is used as a basis of many paints.

[Illustration: Fig. 414.—Section of shot tower.]

[Illustration: Fig. 415.—Preparing lead for bullets.]


TIN is another well-known metal. It is mentioned by Moses. It possesses
a silver-like lustre, and is not liable to be oxidised. The only really
important ore is called Tinstone, from which the oxygen is separated,
and the metal remains. Cornwall has extensive tin mines. Tin is
malleable and ductile, and can be beaten into _foil_ or “silver leaf,”
or drawn into wire. It prevents oxidation of iron if the latter be
covered with it, and for tinning copper vessels for culinary purposes.
The Romans found tin in Cornwall, and the term “Stanneries” was applied
to the courts of justice among the tin miners in Edward the First’s
time. We have already mentioned the alloys of tin. The oxides of tin,
“Stannous” and “Stannic,” are useful to dyers. The latter is the
tinstone (SnO_{2}). Sulphide of tin is called “Mosaic gold,” and is
much used for decorative purposes.


ZINC is procured from _calamine_, or carbonate of zinc, and _blende_,
or sulphide of zinc. It has for some years been used for many purposes
for which lead was once employed, as it is cheap and light. Zinc is a
hard metal of a greyish colour, not easily bent, and rather brittle;
but when made nearly red-hot, it can be rolled out into sheets or
beaten into form by the hammer. Zinc is about six-and-three-quarter
times heavier than water. Like many other metals, it is volatile
(when heated to a certain extent it passes off into vapour), and the
probable reason that it was not known or used of old is that it was
lost in the attempt to smelt its ores. Zinc is now obtained by a sort
of distillation; the ores are mixed with the flux in a large earthen
crucible or pot.

We have already noticed the alloy of zinc with copper (brass), and the
use of zinc to galvanize iron by covering the latter with a coating
of zinc in a bath is somewhat analogous to electro-plating. The metal
is largely used as the _positive_ element in galvanic batteries, and
for the production of hydrogen in the laboratory. Zinc forms one oxide
(ZnO), used for zinc-white. The sulphate of zinc is white vitriol, and
the chloride of zinc is an “antiseptic.” Certain preparations of the
metal are used in medicine as “ointments” or “washes,” and are of use
in inflammation of the eyelids.


CHROMIUM. This “metallic element” is almost unknown in the metallic
state. But although little known, the beautiful colours of its
compounds make it a very interesting study. The very name leads one to
expect something different to the other metals—_chroma_, colour. The
metal is procured from what is known as chrome-ironstone, a combination
of protoxide of iron and sesqui-oxide of chromium (FeOCr_{2}O_{3}). By
ignition with potassium we get chromic acid and chromate of potassium,
a yellow salt which is used to make the other compounds of chromium.
The metal is by no means easy to fuse.

Sesqui-Oxide of Chromium is a fine green powder employed in painting
porcelain.

Chromate of Lead is termed “chrome yellow,” and in its varieties is
employed as a paint.

Chromate of Mercury is a beautiful vermilion. There are numerous other
combinations which need not be mentioned here.

[Illustration: Fig. 416.—Type-casting.]


ANTIMONY was discovered by Basil Valentine. The Latin term is Stibium,
hence its symbol, Sb. It is very crystalline, and of a peculiar
bluish-white tint. It will take fire at a certain high temperature,
and can be used for the manufacture of “Bengal Lights,” with nitre and
sulphur in the proportions of antimony “one,” the others two and three
respectively.

The compounds of antimony are used in medicine, and are dangerous when
taken without advice. They act as emetics if taken in large quantities.
Our “tartar emetic” is well known.

Antimony, in alloy with lead and a little tin, form the _type metal_
to which we are indebted for our printing. Type-casting is done by
hand, and requires much dexterity. A ladle is dipped into the molten
metal, and the mould jerked in to fill it properly, and then the
type is removed and the mould shut ready for another type; and a
skilful workman can perform these operations five hundred times in an
hour,—rather more than eight times a minute,—producing a type each
time; this has afterwards to be finished off by others. The metal of
which type is made consists of lead and antimony; the antimony hardens
it and makes it take a sharper impression. The letters are first cut in
steel, and from these “dies” the moulds are made in brass, by stamping,
and in these the types are cast.

Stereotype consists of plates of metal taken, by casting, from a
forme of type set up for the purpose: an impression was formerly
carried on by plaster-of-Paris moulds, but lately what is termed the
_papier-maché_ process is adopted. The paper used is now made in
England, and the prepared sheet is placed upon the type and beaten
upon it. Paste is then filled in where there are blanks, and another
and thicker sheet of the prepared paper is placed over all, dried,
and pressed. When this is properly done the paper is hardened, and
preserves an impression of the type set up. The paper mould is then
put into an iron box, and molten metal run in. In a very short time a
“stereotype” plate is prepared from the paper, which can be used again
if necessary. The metal plate is put on the machine.

There are several compounds of antimony, which, though valuable to
chemists, would not be very interesting to the majority of readers. We
will therefore at once pass to the Noble Metals.


THE NOBLE METALS.

There are nine metals which rank under the above denomination:—Mercury,
Silver, Gold, Platinum, Palladium, Rhodium, Ruthenium, Osmium, Iridium.
We will confine ourselves chiefly to the first four on the list.


MERCURY, or QUICKSILVER, is the first of the metals which remain
unaltered by exposure to atmospheric air, and thus are supposed to earn
their title of nobility. Mercury is familiar to us in our barometers,
etc., and is fluid in ordinary temperatures, though one of the heaviest
metals we possess. It is principally obtained from native cinnabar, or
sulphide of mercury (vermilion), and the process of extraction is very
easy. Mercury was known to the ancients, and is sometimes found native.
In the mines the evil effects of the contact with mercury are apparent.

This metal forms two oxides,—the black (mercurous) oxide, or suboxide
(Hg_{2}O), and the red (mercuric) oxide, or red precipitate. The
chlorides are two,—the subchloride, or calomel, and the perchloride,
or corrosive sublimate. The sulphides correspond with the oxides; the
mercuric sulphide has been mentioned. Its crimson colour is apparent in
nature, but the Chinese prepare it in a particularly beautiful form.
Many amalgams are made with mercury, which is useful in various ways
that will at once occur to the reader.


SILVER is the whitest and most beautiful of metals, and its use for
our plate and ornaments is general. It is malleable and ductile, and
the best conductor of electricity and heat that we have. It is not
unfrequently met with in its native state, but more generally it is
found in combination with gold and mercury, or in lead, copper, and
antimony ores. The mines of Peru and Mexico, with other Western States
of America, are celebrated—Nevada, Colorado, and Utah in particular.
The story of the silver mine would be as interesting as any narrative
ever printed. The slavery and the death-roll would equal in horror
and in its length the terrible records of war or pestilence. We have
no opportunity here to follow it, or its kindred metals with which
it unites, on the sentimental side; but were the story of silver
production written in full, it would be most instructive.

[Illustration: Fig. 417.—Native silver.]

Silver is found with lead (galena), which is then smelted. The lead
is volatilized, and the silver remains. It is also extracted by the
following process, wherein the silver and golden ore is crushed and
washed, and quicksilver, salt, and sulphate of copper added, while heat
is applied to the mass. From tank to tank the slime flows, and deposits
the metals, which are put into retorts and heated. The mercury flies
off; the silver and gold remain in bars.

In some countries, as in Saxony and South America, recourse is had
to another process, that of amalgamation, which depends on the easy
solubility of silver and other metals in mercury. The ore, after being
reduced to a fine powder, is mixed with common salt, and roasted at a
low red heat, whereby any sulphide of silver the ore may contain is
converted into chloride. The mixture is then placed, with some water
and iron filings, in a barrel which revolves round its axis, and the
whole agitated for some time, during which process the chloride of
silver becomes reduced to the metallic state. A portion of mercury is
then introduced, and the agitation continued. The mercury combines
with the silver, and the amalgam is then separated by washing. It
is afterwards pressed in woollen bags to free it from the greater
part of the mercury, and then heated, when the last trace of mercury
volatilizes and leaves the silver behind.

_Nitrate of Silver_ is obtained when metallic silver is dissolved in
nitric acid. It is known popularly as _lunar caustic_, and forms the
base of “marking inks.” _Chloride of silver_ is altered by light, but
the iodide of silver is even more rapidly acted on, and is employed
in photography. _Fulminating silver_ is oxide of silver digested in
ammonia. It is very dangerous in inexperienced hands. It is also
prepared by dissolving silver in nitric acid, and adding alcohol. It
cools in crystals. Fulminating mercury is prepared in the same way.


GOLD is the most valuable of all metals,—the “king of metals,” as it
was termed by the ancients. It is always found “native,” frequently
with silver and copper. Quartz is the rock wherein it occurs. From the
disintegration of these rocks the gold sands of rivers are formed, and
separated from the sands by “washing.” In Australia and California
“nuggets” are picked up of considerable size.

It is a rather soft metal, and, being likewise costly, is never used
in an absolutely pure state. Coins and jewellery are all alloyed with
copper and silver to give them the requisite hardness and durability.
Gold is extremely ductile, and very malleable. One grain of gold may
be drawn into a wire five hundred feet in length, and the metal may be
beaten into almost transparent leaves 1/200000 of an inch in thickness!

[Illustration: Fig. 418.—Native gold.]

Aqua-regia, a mixture of hydrochloric and nitric acids, is used to
dissolve gold, which is solved only by selenic acid, though the free
chlorine will dissolve it. Faraday made many experiments as to the
relation of gold to light. (_See_ “Phil. Trans.,” 1857, p. 145.) The
various uses of gold are so well known that we need not occupy time
and space in recording them. Gilding can be accomplished by immersing
the articles in a hot solution of chloride of gold and bicarbonate of
potash mixed; but the electro process is that now in use, by which the
gold precipitates on the article to be plated.

We have already described the process of electro-plating in the case
of silvered articles, and we need only mention that electro-gilding is
performed very much in the same way. But gilding is also performed in
other ways; one of which, the so-called water gilding, is managed as
follows. Gilding with the gold-leaf is merely a mechanical operation,
but water-gilding is effected by chemistry.

Water-gilding is a process (in which, however, no water is used)
for covering the surface of metal with a thin coating of gold; the
best metal for water-gilding is either brass, or a mixture of brass
and copper. A mixture of gold and mercury, in the proportion of one
part of gold to eight of mercury, is made hot over a fire till they
have united; it is then put into a bag of chamois-leather, and the
superfluous mercury pressed out. What remains is called an “amalgam”;
it is soft, and of a greasy nature, so that it can be smeared over any
surface with the fingers. The articles to be gilt are made perfectly
clean on the surface, and a liquid, made by dissolving mercury in
nitric acid (aqua-fortis), is passed over them with a brush made of
fine brass wire, called a “scratch-brush.” The mercury immediately
adheres to the surface of the metal, making it look like silver; when
this is done, a little of the amalgam is rubbed on, and the article
evenly covered with it. It is then heated in a charcoal fire till all
the mercury evaporates, and the brass is left with a coating of gold,
which is very dull, but may be burnished with a steel burnisher and
made bright if necessary. In former times articles were inlaid with
thin plates of gold, which were placed in hollows made with a graver,
and melted in, a little borax being applied between.

When a solution of “chloride of gold” is mixed with ether, the ether
takes the gold away from the solution, and may be poured off the top
charged with it. This solution, if applied to polished steel by means
of a camel-hair pencil, rapidly evaporates, leaving a film of gold
adhering to the steel, which, when burnished with any hard substance,
has a very elegant appearance. In this way any ornamental design in
gold may be produced, but it is not very durable. The gilt ornaments,
scrolls, and mottoes on sword-blades, are sometimes done in this way.


PLATINUM is the heaviest of all metals, gold being next. Platinum
is practically infusible, and quite indifferent to reagents. It is
therefore very useful in certain manufactories, and in the laboratory.
It can be dissolved by _aqua-regia_. The stills for sulphuric acid are
made of platinum, and the metal is used for Russian coinage, but must
be very difficult to work on account of its infusible property.

In the finely-divided state it forms a gray and very porous mass, which
is known as _spongy platinum_, and possesses the remarkable property
of condensing gases within its pores. Hence, when a jet of hydrogen
is directed upon a piece of spongy platinum, the heat caused by its
condensation suffices to inflame the gas. This singular power has been
applied to the construction of a very beautiful apparatus, known as
Döbereiner’s lamp, which consists of a glass jar, _a_, covered by a
brass lid, _e_, which is furnished with a suitable stop-cock, _c_,
and in connection with a small bell jar, _f_, in which is suspended,
by means of a wire, a cylinder of metallic zinc, _z_. When required
for use, the outer jar is two-thirds filled with a mixture of one
part sulphuric acid and four parts water, and the stop-cock opened to
allow the escape of atmospheric air, the spongy platinum contained
in the small brass cylinder, _d_, being covered by a piece of paper.
The stop-cock is then closed, and the bell jar, _f_, allowed to fill
with hydrogen, and after it has been filled and emptied several times,
the paper is removed from the platinum and the cock is again opened,
when the gas, which escapes first, makes the metal red-hot and finally
inflames. This property of platinum is also used in the “Davy” lamp.

[Illustration: Fig. 419.—Döbereiner’s lamp.]

The remaining metals do not call for detailed notice.

In conclusion, we may refer to the following statement, which in
general terms gives the properties of the metals, their oxides and
sulphides for ordinary readers.


GENERAL CLASSIFICATION OF THE METALS.

(Transcriber’s Note: The table has been divided into two parts to fit
within width constraints)

The metals admit of being really distinguished by the following table,
in which they are presented in several groups, according to their
peculiar properties, and each distinguished by a particular name:—


  ---------------------------------------
  |                                     |
  |               METALS.               |
  |                                     |
  --------------------------------------+
  |     (A.) _Light Metals._            |
  |                                     |
  |Specific gravity from 0·8 to 1;      |
  |  never occur in the uncombined      |
  |  state.                             |
  --------------------------------------+
  |     (a.) _Alkaline Metals._         |
  |                                     |
  | 1. Potassium.                       |
  | 2. Sodium.                          |
  |      (Ammonium.)                    |
  |                                     |
  |                                     |
  |                                     |
  --------------------------------------+
  |(b.) _Metals of the Alkaline Earths._|
  |                                     |
  | 3. Calcium.                         |
  | 4. Barium.                          |
  | 5. Strontium.                       |
  --------------------------------------+
  |(c.) _Metals of the Earths proper._  |
  |                                     |
  | 6. Magnesium.                       |
  | 7. Aluminium.                       |
  --------------------------------------+
  |     (B.) _Heavy Metals._            |
  |                                     |
  |Specific gravity from 5 to 21; are   |
  |  found chiefly in combination       |
  |  with oxygen, and frequently        |
  |  with sulphur and arsenic; some     |
  |  are native.                        |
  --------------------------------------+
  |     (a.) _Common Metals._           |
  |                                     |
  |    Become oxidized in the air.      |
  |                                     |
  | 8. Iron.            14. Lead.       |
  | 9. Manganese.       15. Tin.        |
  |10. Cobalt.          16. Zinc.       |
  |11. Nickel.          17. Chromium.   |
  |12. Copper.          18. Antimony.   |
  |13. Bismuth.                         |
  --------------------------------------+
  |     (b.) _Noble Metals._            |
  |                                     |
  |     Unchangeable in the air.        |
  |                                     |
  |19. Mercury.         21. Gold.       |
  |20. Silver.          22. Platinum.   |
  ---------------------------------------


  --------------------------------------------------------------------------
  |                           Properties of the                            |
  |------------------------------------------------------------------------|
  |             Oxides.              |             Sulphides.              |
  +----------------------------------+-------------------------------------|
 A|Powerful bases; possessing a      |Powerful bases, which oxidize in     |
  |  strong affinity for water, and  |  the air, and form sulphates;       |
  |  form with it hydrates. They     |  when treated with acids evolve     |
  |  yield their oxygen to carbon    |  hydrosulphuric acid.               |
  |  only at a white heat.           |                                     |
  +----------------------------------+-------------------------------------|
 a|Highly caustic; powerful bases,   |Caustic; strong bases; very soluble  |
  |  separate all other oxides from  |  in water, and dissolve a           |
  |  their combinations with acids;  |  large quantity of sulphur, which   |
  |  are very soluble in water, and  |  is separated on addition of an     |
  |  do not lose their water of      |  acid as a white powder, termed     |
  |  hydration at the highest        |  _milk of sulphur_; they were       |
  |  temperatures; attract carbonic  |  formerly termed _liver of sulphur_.|
  |  acid rapidly from the air.      |                                     |
  +----------------------------------+-------------------------------------|
 b|Caustic; strong bases; slightly   |Caustic; strong bases; dissolve      |
  |  soluble in water; lose their    |  sulphur, and are partly soluble    |
  |  water of hydration at a moderate|  in water, and partly insoluble.    |
  |  heat, and powerfully absorb     |                                     |
  |  carbonic acid.                  |                                     |
  +----------------------------------+-------------------------------------|
 c|                  { Weak bases,   |Insoluble in water.                  |
  |Feebly caustic.   {   insoluble in|                                     |
  |Not caustic.      {   water.      |                                     |
  |                                  |                                     |
  +----------------------------------+-------------------------------------|
 B|Feebler bases than the foregoing, |Neutral compounds; insoluble in      |
  |  some are acids; insoluble in    |  water; antimony and several        |
  |  water, and lose their water of  |  of the rarer metals produce        |
  |  hydration at a moderate heat.   |  compounds with sulphur, which      |
  |                                  |  deport themselves as acids.        |
  |                                  |                                     |
  |                                  |                                     |
  +----------------------------------+-------------------------------------|
 a|With few exceptions, are soluble  |Those occurring in nature are        |
  |  in acids, and, when ignited with|  somewhat brass-like in appearance, |
  |  carbon at a red heat, yield     |  and are termed _pyrites_           |
  |  their oxygen; are, for the most |  and _blendes_. Those which are     |
  |  part, fusible and non-volatile. |  artificially prepared have         |
  |                                  |  peculiarcolours; by heat they      |
  |                                  |  are converted into sulphates.      |
  |                                  |                                     |
  |                                  |                                     |
  |                                  |                                     |
  +----------------------------------+-------------------------------------|
 b|Have more the properties of acids |With the exception of sulphide       |
  |  than of bases; are decomposed   |  of mercury, they leave the pure    |
  |  by ignition into oxygen and     |  metal when ignited.                |
  |  metal.                          |                                     |
  |                                  |                                     |
  |                                  |                                     |
  --------------------------------------------------------------------------


FOOTNOTES:

[21] Requires oxy-hydrogen blow-pipe.



CHAPTER XXX.

ORGANIC CHEMISTRY.

 RADICALS—ACIDS—BASES—NEUTRALS.


In the introduction to these brief chapters upon Chemistry, we said
that the science was divided into two sections, the first section
consisting of the simple combinations, and the other of compound
combinations. The latter being met with chiefly in animal and vegetable
matter, as distinguished from dead or inert matter, was termed
_Organic_. This distinction will be seen below.

  COMBINATIONS OF SIMPLE GROUPS.

  INORGANIC.

   I. Elements and their Combinations.
        (1) Non-Metallic.
        (2) Metallic.

  II. Peculiar Decompositions of the above.
        (1) By Electricity.
        (2) By Light.

  COMBINATIONS OF COMPOUND GROUPS.

  ORGANIC.

   I. Compound Radicals and their Combinations.

  II. Peculiar Decompositions of the above.
        (1) Spontaneous.
        (2) Dry Distillation.

We have already placed before our readers the elements and their
simple combinations, and have incidentally mentioned the decomposition
by electricity and by light. In the section upon Electricity the
positive and negative poles are explained. Oxygen appears always at
the positive pole, potassium at the negative. The other simple bodies
vary. Chlorine, in combination with oxygen, is evolved at the negative
pole, but when with hydrogen at the positive pole. In the series below
each element behaves electro-_negatively_ _to those following it_, and
electro-_positively_ to those _above it_; and the farther they are
apart the stronger their opposite affinities are.

ELECTRICAL RELATION OF THE ELEMENTS.

  Oxygen.
  Sulphur.
  Nitrogen.
  Chlorine.
  Bromine.
  Iodine.
  Fluorine.
  Phosphorus.
  Arsenic.
  Carbon.
  Chromium.
  Boron.
  Antimony.
  Silicium.
  Gold.
  Platinum.
  Mercury.
  Silver.
  Copper.
  Bismuth.
  Lead.
  Cobalt.
  Nickel.
  Iron.
  Zinc.
  Hydrogen.
  Manganese.
  Aluminium.
  Magnesium.
  Calcium.
  Strontium.
  Barium.
  Sodium.
  Potassium.

The importance of these facts to science is unmistakable, and, indeed,
many attempts have been made to explain, from the electrical condition
of the elements, the nature of chemical affinity, and of chemical
phenomena in general.

Electrotyping is another instance of decomposition by means of
electricity, and respecting decomposition by light we know how powerful
the action of the sun’s rays are upon plants, and for the evolution
of oxygen. The daguerreotype and photographic processes are also
instances which we have commented upon. So we can pass directly to the
consideration of the compound groups.

In nearly every complex organic compound we have a relatively simple
one of great stability, which is termed the _radical_, which forms,
with other bodies, a compound radical.[22] In these complex groups we
find certain elements generally,—viz., carbon, hydrogen, nitrogen,
sulphur, and phosphorus. Some compounds may consist of two of these,
but the majority contain three (hydrogen, oxygen, and carbon). Many
have four (carbon, oxygen, hydrogen, and nitrogen), and some more
than four, including phosphorus and sulphur. Others, again, may
contain chlorine and its relatives, arsenic, etc., in addition. Now
we will all admit that in any case in which carbon is present in
composition with other simple bodies forming an organic body, and if
that body be ignited in the air, it burns and leaves (generally) a
black mass. This is a sure test of the presence of carbon, and forms
an organic compound. Similarly in decomposition nitrogen and sulphur
in combination inform us they are present by the odour they give off.
We need not go farther into this question of radicals and compound
radicals than to state that a compound radical plays the part of
an element in combination. We find in alcohol and ether a certain
combination termed _Ethyl_. This “compound radical” occurs in same
proportions in ether, chloride of ethyl, iodide of ethyl, etc., as
C_{2}H_{5}; so it really acts as a simple body or element, though
it is a compound of carbon and hydrogen. A simple radical is easily
understood; it is an element, like potassium, for instance. We may now
pass to the organic combinations classified into ACIDS, BASES, and
INDIFFERENT, or NEUTRAL, BODIES.


I. ACIDS.

There are several well-known organic acids, which we find in fruits
and in plants. They are volatile and non-volatile; acids are sometimes
known as “Salts of Hydrogen.” We have a number of acids whose names are
familiar to us,—viz., acetic, tartaric, citric, malic, oxalic, tannic,
formic, lactic, etc.

_Acetic acid_ (HC_{2}H_{4}O_{2}) is a very important one, and is easily
found when vegetable juices, which ferment, are exposed to the air, or
when wood and other vegetable matter is subjected to the process of
“dry distillation.” Vinegar contains acetic acid, which is distilled
from wood, as we shall see presently. Vinegar is made abroad by merely
permitting wine to get sour; hence the term _Vin-aigre_. In England
vinegar is made from “wort,” of malt which is fermented for a few days,
and then put into casks, the bung-holes of which are left open for
several weeks, until the contents have become quite sour. The liquid is
then cleared by isinglass. The vinegar of commerce contains about 6 per
cent. of pure acetic acid, and some spirit, some colouring matter, and,
of course, water. Wood vinegar (pyroligneous acid) is used for pickles.
The ordinary vinegar when distilled is called white vinegar, and it may
also be obtained from fruits, such as gooseberries or raspberries.

[Illustration: Fig. 420.—Vinegar ground.]

[Illustration: Fig. 421.—Boiler or copper.]

Acetic acid, or “wood vinegar,” is prepared as follows:—There are
some large iron cylinders set in brickwork over furnaces, and these
cylinders have each a tube leading to a main pipe in which the liquid
is received for condensation. The cylinders, which contain about seven
or eight hundredweight, are filled with logs of wood, either oak,
beech, birch, or ash, the door is closely fastened, and the joints
smeared with clay; the fires are now lighted and kept up all day,
till the cylinders are red-hot; at night they are allowed to cool. In
the morning, the charcoal, into which the wood is now converted, is
withdrawn, and a fresh charge supplied; it is then found that about
thirty or forty gallons of liquid has condensed in the main tube from
each cylinder, the remainder being charcoal and gases which pass off;
the liquid is acid, brown, and very offensive, and contains acetic
acid, tar, and several other ingredients, among which may be named
creosote; it is from this source all the creosote, for the cure of
toothache, is obtained. To purify this liquid it is first distilled,
and this separates much of the tar; it is then mixed with lime,
evaporated to dryness, and heated to expel the remaining tar and other
impurities; it is next mixed with sulphate of soda and water, and the
whole stirred together; the soda, now in union with the acetic acid,
is washed out from the lime and strained quite clear; it is afterwards
evaporated till it crystallises, and vitriol (sulphuric acid) then
added; finally the acetic acid is distilled over, and the sulphuric
acid left in union with the soda, forming sulphate of soda, to be used
in a similar process for the next batch of acid. The acetic acid is now
quite colourless, transparent, and very sour, possessing a fragrant
smell. This is not pure acetic acid, but contains a considerable
quantity of water. The acetic acid of commerce, mixed with seven times
its bulk of water, forms an acid of about the strength of malt vinegar,
perfectly wholesome, and agreeable as a condiment.

[Illustration: Fig. 422.—Vinegar-cooling process.]

[Illustration: Fig. 423.—Tan-yard and pits.]

Pure acetic acid may be made by mixing dry acetate of potash with oil
of vitriol in a retort, and distilling the acetic acid into a very cold
receiver; this, when flavoured with various volatile oils, forms the
aromatic vinegar sold by druggists. It is a very strong acid, and if
applied to the skin will quickly blister it.

Acetate of lead, or sugar of lead, is obtained by dissolving oxide of
lead in vinegar. A solution of this salt makes the goulard water so
familiar to all. Acetate of lead is highly poisonous.

Acetate of copper is _verdigris_, and poisonous. Other acetates are
used in medicine.

We may pass quickly over some other acids. They are as follows:—


TARTARIC ACID (C_{4}H_{6}O_{6}) is contained in grape juice, and
crystallizes in tabular form. The purified powdered salt is cream of
Tartar.


CITRIC ACID (C_{6}H_{8}O_{7}) is found native in citrons and lemons, as
well as in currants and other fruits. It is an excellent anti-scorbutic.


MALIC ACID (C_{4}H_{6}O_{5}) is found chiefly in apples, as its name
denotes (malum, an apple). It is prepared from mountain-ash berries.


OXALIC ACID (C_{2}H_{2}O_{4}). If we heat sugar with nitric acid we
shall procure this acid. It is found in sorrel plants.


TANNIC ACID (C_{27}H_{22}O_{17}). It is assumed that all vegetables
with an astringent taste contain this acid. _Tannin_ is known for its
astringent qualities. The name given to this acid is derived from the
fact that it possesses a property of forming an insoluble compound with
water, known as _leather_. Tanning is the term employed. _Tannin_ is
found in many vegetable substances, but oak bark is usually employed,
being the cheapest. The “pelts,” hides, or skins, have first to be
freed from all fat or hair by scraping, and afterwards soaking them
in lime and water. Then they are placed in the tan-pit between layers
of the bark, water is pumped in, and the hides remain for weeks,
occasionally being moved from pit to pit, or relaid, so as to give
all an equal proportion of pressure, etc. The longer the leather is
tanned—it may be a year—the better it wears.

Skins for gloves and binding are tanned with “sumach,” or alum and
salt. Sometimes the leather is split by machinery for fine working.
_Parchment_ is prepared from the skins of asses, sheep, goats, and
calves, which are cleaned, and rubbed smooth with pumice stone.

Tannic acid, with oxide of iron, produces _Ink_, for the gall-nut
contains a quantity of the acid. All the black inks in use generally
are composed of green vitriol (sulphate of iron) in union with some
astringent vegetable matter; the best is the gall-nut, although, for
cheapness, logwood and oak bark have each been used. An excellent black
ink may be made by putting into a gallon stone bottle twelve ounces of
bruised galls, six ounces of green vitriol, and six of common gum, and
filling up the bottle with rain water; this should be kept three or
four weeks before using, shaking the bottle from time to time.

Blue ink has lately been much used; it is made by dissolving
newly-formed Prussian blue in a solution of oxalic acid. To make it,
dissolve some yellow prussiate of potash in water in one vessel, and
some sulphate of iron in another, adding a few drops of nitric acid to
the sulphate of iron; now mix the two liquids, and a magnificent blue
colour will appear, in the form of a light sediment; this is to be put
upon a paper filter, and well washed by pouring over it warm water, and
allowing it to run through; a warm solution of oxalic acid should now
be mixed with it, and the Prussian blue will dissolve into a bright
blue ink.

[Illustration: Fig. 424.—Unhairing the hide.]

Red ink is made by boiling chips or raspings of Brazil wood in vinegar,
and adding a little alum and gum; it keeps well, and is of a good
colour. A red ink of more beautiful appearance, but not so durable,
may be made by dissolving a few grains of carmine in two or three
teaspoonfuls of spirit of hartshorn.

Marking ink is made by dissolving nitrate of silver in water, and then
adding some solution of ammonia, a little gum water, and some Indian
ink to colour it. Printers’ ink is made by grinding drying oil with
lamp-black.

The powdered gall-nut is an excellent test for iron in water. It will
turn violet if any iron be present.


FORMIC ACID (CH_{2}O_{2}) is the caustic means of defence employed
by ants, hence the term _formic_. It can be artificially prepared by
distilling a mixture of sugar, binoxide of manganese, and sulphuric
acid. On the skin it will raise blisters.

[Illustration: Fig. 425.—Drying rooms for hides.]


LACTIC ACID (C_{3}H_{6}O_{3}) is present in vegetable and animal
substances. Sour whey contains it, and the presence of the acid in
the whey accounts for its power of removing from table-linen stains.
When what is called “lactic fermentation” occurs, milk is said to be
“turned.”


II.

BASES.

The definition of a base is not easy. We have described bases as
substances which, combining with acids, form salts, but the definition
of a base is as unsatisfactory as that of acid or salt. All vegetable
bases contain nitrogen, are usually very bitter, possess no smell or
colour, and are insoluble in water. They are usually strong poisons,
but very useful in medicine.

The most important are the following bases:—


QUININE is contained in the cinchona (yellow) bark. One hundred parts
of the bark have been calculated to yield three of quinine.


MORPHINE is the poisonous base of opium, which is the juice of the
poppy, and is prepared chiefly in India and China.


NICOTINE is the active principle of tobacco, and varies in quantity
in different tobaccos. Havannah tobacco possesses the least. It is a
powerful poison, very oily, volatile, and inflammable.


CONIA is prepared from the hemlock. It is fluid and volatile. It is
also a deadly poison, and paralyses the spine directly.

[Illustration: Fig. 426.—Hemlock.]


STRYCHNINE is found in poisonous trees, particularly in the nux-vomica
seeds of Coromandel. It produces lock-jaw and paralysis. There is no
antidote for strychnine; emetics are the only remedy.

The above are chiefly remarkable for their uses in medicine, and in
consequence of their highly poisonous character are best left alone by
unpractised hands.

A German chemist, named Serturner, was the first to extract the active
principle from Opium. The question of opium importation has lately been
attracting much attention, and the opinions concerning its use are
divided. Probably in moderation, and when used by ordinary people (not
demoralized creatures), it does little harm.

[Illustration: Fig. 427.—The Poppy.]

Opium is the juice of the “common” poppy, and derives its name from
the Greek _opos_, juice. The plant is cultivated in India, Persia,
and Turkey. After the poppy has flowered the natives go round, and
with a sharp instrument wound, or puncture, every poppy head. This is
done very early in the morning, and under the influence of the sun
during the day the juice oozes out. Next morning the drops are scraped
off. The juice is then placed in pots, dried, and sent for export.
The “construction” of opium is very complicated, for it contains a
number of ingredients, the most important being morphia, narcotine,
meconic acid, and codeia. It is to the first named constituent that the
somnolent effect of opium is due.


III.

INDIFFERENT SUBSTANCES.

There are a great number of so-called “indifferent” substances to which
we cannot be indifferent. Such bodies as these have neither acid nor
basic properties, and stand no comparison with salts. They are of great
importance, forming, as they do, the principal nutriment of animals.
Some contain nitrogen, some do not; they may therefore be divided
into nitrogenous and non-nitrogenous substances; the former for solid
portions of the body, the latter for warmth.

We will take the latter first, and speak of some of them—such as
starch, gum, sugar, etc.


STARCH is found in the roots of grain, in the potato, dahlia,
artichoke, etc., and by crushing the parts of the plant, and washing
them, the starch can be collected as a sediment. In cold water and
in spirits of wine starch is insoluble. The various kinds of starch
usually take their names from the plants whence they come. Arrowroot is
obtained from the West Indian plant _Maranta Arundinacea_. Cassava and
tapioca are from the manioc; sago, from the sago palm; wheat starch,
and potato starch are other examples.

[Illustration: Fig. 428.—Plantation of sugar-canes.]

If starch be baked in an oven at a temperature of about 300° it
becomes, to a great extent, soluble in cold water, forming what is
called “British gum”; this is largely used for calico printing and
other purposes; if boiled in water under great pressure, so that the
temperature can be raised to the same degree, it is also changed into
an adhesive sort of gum, “mucilage”; this is the substance made use
of by the government officials to spread over the backs of postage
and receipt stamps to make them adhere. The starch of grain, during
germination, or growth, contains _diastase_, which converts the starch
into gum and sugar; the same effect can be produced by heating starch
with diluted sulphuric acid.


GUM found in plants is chiefly procured from the Mimosa trees, from
which it flows in drops, and is called _Gum Arabic_. There are other
so-called “gums,” but this is the one generally referred to.


SUGAR exists in fruits, roots, and in the stalks of plants, in the
juice of the cane, maple, and beet-root particularly. The canes are
crushed, the juice is clarified with lime to prevent fermentation, and
the liquid is evaporated. It is then granulated and cleared from the
molasses. Sugar, when heated, becomes dark, and is called “caramel.” It
is used for colouring brandy, and gives much difficulty to the sugar
refiners.

[Illustration: Fig. 429.—Refining vacuum pan.]

Sugar refining is conducted as follows. The raw (brown) sugar is
mixed into a paste with water, and allowed to drain. The sugar thus
becomes white. It is then dissolved in water, with animal charcoal and
bullocks’ blood. The liquid is boiled, and put into a dark cistern with
holes at the bottom, and cotton fibres being fastened in the holes,
are hung into another dark cistern, into which the liquid runs pure
and white. It is then pumped into a copper vessel,—vacuum pan,—and
condensed to the proper consistence. Subsequently it is poured into
conical moulds, and pure syrup poured upon the crystal shapes. The
caramel is then removed through a hole at the end. The moulds or
loaves are then dried, and if not even or elegant they are turned in
a lathe. Finally they are packed up as “loaf sugar.” Sugar undergoes
no decomposition, and is the cause of non-decomposition in other
substances. For this reason it is employed in “preserving” fruit, etc.
Sugar is obtained from beet by crushing and rasping the roots, as the
cane is treated.

[Illustration: Fig. 430.—Sugar moulds.]

[Illustration: Fig. 431.—Turning the loaves.]


SPIRIT OF WINE, OR ALCOHOL, is not a natural product. It is found by
the decomposition of grape-sugar by fermentation. There is a series of
alcohols which exhibit a regular gradation, founded, so to speak, upon
one, two, or three molecules of water. They are called respectively
alcohols, glycols, and glycerins. Thus we have—

  _Alcohols._

  Methylic alcohol.
  Ethylic     ”
  Prophylic   ”
  Amylic      ”

  _Glycols._

  Enthelein  glycol.
  Prophylene   ”
  Butylene     ”
  Amylene      ”

  _Glycerins._

  (Ordinary Glycerine is the
  only one known.)

The cetyl and melissylic alcohols are contained in spermaceti and
bees-wax respectively. The usual alcohol is the _Vinic_, a transparent,
colourless liquid, which is the spirituous principle of wine, spirits,
and beer, and when sugar is fermented the alcohol and carbonic acid
remain.

Spirits of wine has a very powerful affinity for water, and thus
the use of stimulants in great quantity is to be deprecated, for
alcohol absorbs the water from the mucous membranes of the stomach
and the mouth, making them dry and hard. The state of “intoxication,”
unfortunately so familiar, is the effect produced by alcohol upon the
nerves. We append a list of the beverages which are most in use, and
the percentage of alcohol in each according to Professor Hart:—

  Port       15   per cent.
  Madeira    14·5    ”
  Sherry     14      ”
  Claret      8      ”
  Ale         6      ”
  Porter      5      ”

Spirit of wine is contained in many mixtures, and for the purpose of
ascertaining how much alcohol may be in wine, or any other liquid, a
hydrometer is used (fig. 432). This instrument consists of a glass tube
with a bulb at the end. It is put into water, and the place the water
“cuts” is marked by a line on the stem, and called zero 0°. Spirit of
wine has less specific gravity than water, so in absolute alcohol the
instrument will sink lower than in water, and will descend to a point
which is marked 100. In any mixture of alcohol and water, of course the
hydrometer will rise or sink between the extreme points accordingly
as the mixture may contain less alcohol or more. So a scale can be
furnished. The instrument, as described, was invented by MM. Gay-Lussac
and Tralles, and called the “percentage” hydrometer. There are many
other instruments marked in a more or less arbitrary manner. We append
a comparative table of a few hydrometers. (_See_ page 420.)

[Illustration: Fig. 432.—Hydrometer.]


ETHER, or _sulphuric_ ether, is a mixture of spirits of wine with
sulphuric acid, and distilled. It loses water, and the product
is ether, which is volatile, and transparent, with a peculiarly
penetrating odour. It will not mix with water, and if inhaled will
produce a similar effect to chloroform.

                     COMPARATIVE TABLE OF HYDROMETERS.

  ---------+------------+------------+-----------+-----------+-----------
  Specific | Percentage | Percentage |  Degree,  |  Degree,  |  Degree,
  Gravity. |   Volume   |   Weight,  | according | according | according
           | (Tralles). |  at 60° F. |to Cartier.|  to Beck. | to Baumé.
  ---------+------------+------------+-----------+-----------+-----------
    1·000  |      0     |       0    |     10    |     0     |     10
    0·991  |      5     |     4·0    |    ...    |   ...     |    ...
    0·985  |     10     |     8·0    |     12    |   ...     |    ...
    0·980  |     15     |    12·1    |    ...    |     3     |     13
    0·975  |     20     |    16·2    |    ...    |   ...     |    ...
    0·970  |     25     |    20·4    |     14    |     5     |    ...
    0·964  |     30     |    24·6    |     15    |     6     |     15
    0·958  |     35     |    28·9    |    ...    |   ...     |     16
    0·951  |     40     |    33·4    |    ...    |     9     |     17
    0·942  |     45     |    37·9    |     18    |   ...     |    ...
    0·933  |     50     |    42·5    |    ...    |    12     |     20
    0·923  |     55     |    47·2    |     21    |    14     |    ...
    0·912  |     60     |    52·2    |    ...    |    16     |     24
    0·901  |     65     |    57·2    |     24    |    19     |    ...
    0·889  |     70     |    62·5    |     27    |   ...     |     28
    0·876  |     75     |    67·9    |    ...    |    24     |    ...
    0·863  |     80     |    73·5    |     30    |    27     |     32
    0·848  |     85     |    79·5    |     35    |    30     |     35
    0·833  |     90     |    85·7    |    ...    |    34     |     38
    0·815  |     95     |    92·4    |     40    |    38     |     42
    0·793  |    100     |   100·0    |     44    |    44     |     48
  ---------+------------+------------+-----------+-----------+-----------

Chloroform is transparent, and will sink in water. Diluted alcohol
with hypo-chloride of lime, will produce it. When inhaled, chloroform
produces a pleasing insensibility to pain, and is useful in surgery.

A certain compound of alcohol with mercury dissolved in nitric acid
will cause decomposition, and white crystals will eventuate. These
compound crystals are termed _fulminating mercury_.

       *       *       *       *       *

We must now pass rapidly over the few remaining subjects we have to
notice, such as fats and soaps, wax, oils, etc.

Fats are of the greatest use to man, particularly in cold climates, for
upon them depends the heat of the body. Fatty acid, if liquid, is known
as oleic acid; if solid, stearic acid. Soaps are compounds of fatty
acids. Many “fats” are consumed as food, others as fuel or for lighting
purposes, in the shape of oils. Such oils are not primarily useful
for burning. Petroleum and other mineral oils are found in enormous
quantities in America.

There are what we term fixed oils, and essential or volatile oils. A
list is annexed as given by “Hadyn’s Dictionary of Science”:—

FIXED OILS.

  _Drying._
  Linseed oil.
  Poppy oil.
  Sunflower oil.
  Walnut oil.
  Tobacco-seed oil.
  Cress-seed oil.

  _Non-Drying._

  Almond oil.
  Castor oil.
  Colza oil.
  Oil of mustard.
  Rape-seed oil.
  Olive oil, etc.

ESSENTIAL OILS.

  Oil of anise.
  Oil of bergamot.
  Oil of carraway.
  Oil of cassia.
  Oil of cedar.
  Oil of cloves.
  Oil of lavender.
  Oil of lemon.
  Oil of mint.
  Oil of myrrh.
  Oil of nutmeg.
  Oil of peppermint.
  Oil of rose.
  Oil of turpentine.

Vegetable oils are obtained by crushing seeds; animal oils come from
the whale and seal tribe. Paraffin oil comes from coal. Linseed is a
very drying oil, and on it depends the drying power of paint. We know
olive oil will not dry on exposure to the air. Oiled silk is made
with linseed oil. When oil is drying in the air considerable heat is
evolved, and if oiled substances be left near others likely to catch
fire, spontaneous combustion may ensue. Oil of turpentine is found in
the pine and fir trees, and many of the oils above mentioned are used
by perfumers, etc., the rose oil, or attar of roses, being an Eastern
compound.

[Illustration: Fig. 433.—Crushing mill.]

Allied to the volatile oils are the RESINS, which are non-conductors of
electricity. They are vegetable products. They are soluble in alcohol,
in the volatile oils, or in ether, and these solutions are called
_varnishes_; the solvent evaporates and leaves the coating. Turpentine,
copal, mastic, shellac, caoutchouc, and gutta-percha are all resinous
bodies. Amber is a mineral resin, which was by the ancients supposed to
be the “tears of birds” dropped upon the seashore. Moore refers to this
in his poetic “Farewell to Araby’s Daughter”—

    “Around thee shall glisten the loveliest amber
    That ever the sorrowing sea-bird has wept.”

Amber is not soluble either in water or alcohol; it is, however,
soluble in sulphuric acid. It takes a good polish, and when rubbed is
very electrical. It is composed of water, an acid, some oil, and an
inflammable gas, which goes off when the amber is distilled.

The well-known camphor is got from a tree called the “Laurus Camphora”;
it is a white, waxy substance, and can be obtained by oxidizing certain
volatile oils. It is generally produced from the Laurus Camphora in a
“still.” The behaviour of a piece of camphor in pure water is curious,
but its motions can be at once arrested by touching the water or
dropping oil on the surface. This phenomenon is due to the surface
tension of the liquid, which diminishes when it is in contact with the
vapour of the substance.


NITROGENOUS SUBSTANCES.

There are certain albuminous compounds which we must mention here.
These are albumen, fibrine, and caseine. Albumen is the white of egg;
fibrine is, when solid, our flesh and muscular fibre, while caseine is
the substance of cheese. These are very important compounds, and the
albuminous bodies are of the very highest importance as food, for the
solid portion of blood, brain, and flesh consist, in a great measure,
of them. Albumen, fibrine, and caseine contain carbon, hydrogen,
nitrogen, and oxygen, with sulphur and phosphorus.


ALBUMEN. The most familiar and the almost pure form of albumen is in
the white of eggs, which is albuminate of sodium. It also exists in
the _serum_ of the blood, and therefore it is largely found in the
animal kingdom. It can also be extracted from seed or other vegetable
substances, but it is essentially the same. Albumen is very useful as
an antidote to metallic poisons. It forms about 7 per cent. of human
blood. It is soluble up to about 140° Fah.; it then solidifies, and is
precipitated in a white mass. Albumen is used in the purification of
sugar, etc.


FIBRINE is found in a liquid condition in blood. The vegetable fibrine
(gluten) is prepared by kneading wheat flour in a bag till the washings
are no longer whitened. Like albumen it is found both in a solid and
liquid state.

_Caseine_ is seen in the skin which forms upon milk when heated, and
forms about 3 per cent. of milk, where it exists in a soluble state,
owing to the presence of alkali; but caseine, like albumen, is only
soluble in alkaline solutions. As we have said, it is the principal
constituent of cheeses. Caseine is precipitated by the _lactic_ acid
of milk, which is produced by keeping the milk too warm. Caseine, or
curds, as they are called, are thus precipitated. The milk is said to
be “sour,” or turned.


MILK, the food of the young of all mammalia, is composed chiefly of
water, a peculiar kind of sugar, butter, and caseine. It is this sugar
in milk which causes the lactic acid mentioned above. The actual
constituents of milk are as follows:—

  Water                       873·00
  Butter                       30·00
  Sugar                        43·90
  Caseine                      48·20
  Calcium (phosphate)           2·31
  Magnesia                      0·42
  Iron                          0·07
  Potassium (chloride)          1·44
  Sodium                        0·24
  Soda (with caseine)           0·42
                             -------
                             1000·00

The sugar of milk is non-fermenting, and can be procured from whey by
evaporation.


DECOMPOSITION.

We have seen that animals and plants are composed of many different
substances, and so it will be at once understood that these substances
can be separated from each other, and then the decomposition of the
body will be completed. When the sap sinks or dries up in plants they
are dead. When our heart ceases to beat and our blood to flow we die,
and then, gradually but surely, decay sets in. There is no fuel left
to keep the body warm; cold results, and the action of oxygen of the
air and light or water decays the body, according to the great and
unalterable laws of Nature. “Dust thou art, and unto dust shalt thou
return,” is an awful truth. The constituents of our bodies must be
resolved again, and the unfailing law of _chemical attraction_ is
carried out, whereby the beautiful organism, deprived of the animating
principle, seeks to render itself into less complicated groups and
their primary elements.

This resolution of the organic bodies is decomposition, or “spontaneous
decomposition,” and is called decay, fermentation, or putrefaction,
according to circumstances. The Egyptians, by first drying the bodies
of the dead (and then embalming them), removed one great source of
decay—viz., water, and afterwards, by the addition of spices, managed
to arrest putrefaction.

Fermentation is familiar in its results, which may be distilled for
spirituous liquors, or merely remain fermented, as beer and wine. Fusel
oil is prepared from potatoes, rum from cane sugar, arrack from rice.
The power of fermentation exists in nature everywhere, and putrefaction
is considered to be owing to the presence of minute germs in the
atmosphere, upon which Professors Tyndall and Huxley have discoursed
eloquently.

Plants are subjected to a process of decomposition, which has been
termed “slow carbonization,” under certain circumstances which exclude
the air. The gases are given off, and the carbon remains and increases.
Thus we have a kind of moss becoming peat, brown coal, and coal. The
immense period during which some beds of coal must have lain in the
ground can only be approximately ascertained, but the remains found in
the coal-measures have guided geologists in their calculations.

Having already mentioned some products of distillation, we may now
close this portion of the subject and pass on to a brief consideration
of minerals and crystals. We have left many things unnoticed, which in
the limited space at our disposal we could not conveniently include in
our sketch of chemistry and chemical phenomena.


FOOTNOTES:

[22] Cyanogen, ethyl, and cacodyl, are compound radicals.



CHAPTER XXXI.

MINERALOGY AND CRYSTALLOGRAPHY.

 THE MINERALS—CHARACTERISTICS—CRYSTALS AND THEIR FORMS—DESCRIPTIONS OF
 MINERALS.


MINERALS are constituent parts of the earth. All parts of minerals are
alike. There are simple minerals and mixed. The former are the true
minerals, and are generally considered under the heading MINERALOGY.
The others constitute a branch of GEOLOGY, as they form aggregate
masses, and as such compose a large portion of the earth. We must learn
to distinguish minerals and crystals as inorganic forms of nature. In
the animal and vegetable kingdoms we have forms which are possessed of
organs of sight, smell, taste, and certain structures indispensable
to their existence and development. But in minerals we have no such
attributes. They are INORGANIC, and have a similar structure; a
fragment will tell us the story as well as a block of the same mineral.
These inorganic substances are possessed of certain attributes or
characteristics. We find they have FORM. They have chemical properties,
and they behave differently when exposed to light and electricity. They
are generally solid. All the elements are found in the mineral kingdom,
and a mineral may be an element itself, or a chemical combination of
elements. These compounds are classed according as the combination is
more or less simple. An alliance of two elements is termed a _binary_
compound, of three a _ternary_ compound, forming a base and an acid.

We have learnt from our chemistry paper that there are between sixty
and seventy elementary bodies in nature. When we speak of “elements,”
we do not mean to apply the popular and erroneous definition of the
word. Earth, air, fire, and water are not elements; they are compounds,
as we have seen. The list of elements has been given; we will now
give the names of the more important minerals. We have no space for a
detailed description, but in the British Museum the cases contain some
hundreds, and the student will find them classified and described with
the greatest care, and according to the arrangement of Berzelius.


PRINCIPAL MINERALS AS ARRANGED BY PROFESSOR ANSTED.

I.

  Diamond.
  Graphite.
  Anthracite.
  Coal.
  Lignite.
  Bitumen.
  Amber.
  Sulphur.
  Quartz.
  Amethyst.
  Agate.
  Chalcedony.
  Flint.
  Jasper.
  Opal.

II.

  Sal-ammoniac.
  Nitre.
  Rock-salt.
  Borax.

III.

  Witherite.
  Spar.
  Strontianite.
  Celestine.
  Calc-spar.
  Marble.
  Dolomite.
  Fluor-spar.
  Gypsum.
  Apatite.
  Magnesite.
  Corundum.
  Sapphire.
  Emery.
  Turquoise.
  Alum-stone.

IV.

  Cyanite.
  Christolite.
  Clay.
  Fullers-earth.
  Garnet.
  Iolite.
  Jade.
  Emerald.
  Beryl.
  Felspar.
  Obsidian.
  Pumice.
  Talc.
  Serpentine.
  Zircon.
  Hornblende.
  Asbestos.
  Augite.
  Diallage.
  Topaz.
  Tourmaline.
  Lapis-lazuli.
  Chrysoberyl.

V.

  Wolfram.
  Molybdenite.
  Chromite.
  Pitch-blende.
  Uranite.
  Pyrolusite.
  Wad.
  Manganese-spar.
  Arsenic.
  Realgar.
  Orpiment.
  Antimony (grey).
  Bismuth.
  Blende.
  Calamine.
  Spartalite.
  Tinstone.
  Galena.
  Pyromorphite.
  Iron-pyrites.
  Mispickel.
  Magnetic iron ore.
  Micaceous iron.
  Hematite.
  Spathic iron.
  Cobalt.
  Copper.
  Oxides of copper.
  Copper pyrites.
  Azurite.
  Malachite.
  Mercury.
  Cinnabar.
  Silver.
  Gold.
  Platinum.
  Palladium.

The above is the arrangement best suited for beginners.


Professor Nichol prefers the following arrangement:—

ORDER I.—THE OXIDISED STONES.

  Quartz.
  Felspar.
  Scapolite.
  Haloid stones.
  Leucite.
  Zeolite.
  Mica.
  Serpentine.
  Hornblende.
  Clays.
  Garnet.
  Cyanite.
  Gems.
  Metallic stones.

ORDER II.—SALINE STONES.

  Calc-spar.
  Fluor-spar.
  Heavy-spar.
  Gypsum.
  Rock-salt.

ORDER III.—SALINE ORES.

  Sparry iron ores.
  Iron salts.
  Copper salts.
  Lead salts.

ORDER IV.—OXIDIZED ORES.

  Iron ores.
  Tinstone.
  Manganese ores.
  Red copper ores.
  White antimony ores.

ORDER V.—THE NATIVE METALS.

ORDER VI.—SULPHURETTED METALS.

  Iron pyrites.
  Galena.
  Grey antimony ore.
  Grey copper ore.
  Blende.
  Ruby-blende.

INFLAMMABLES.

  Sulphur.
  Diamond.
  Coal.
  Mineral resins.
  Combustible salts.

These are only a portion of the minerals, but it would be scarcely
interesting to give the list at greater length. In the foregoing we
recognize the metals and various combustible and non-combustible
substances familiar to us, existing, as people say sometimes, in
“lumps.” But if any one will take the trouble to examine a “lump,” he
will find the shape is definite and even. These regular forms of the
minerals are called CRYSTALS, from the Greek word krustallos, _ice_.
The term was originally applied to quartz, for in olden times it
was thought that quartz was really congealed water. We can define a
crystal as “an inorganic solid bounded by plane surfaces arranged round
imaginary lines known as _axes_.” It must not be imagined that crystals
are small bodies; they may be of any size. There are crystals of many
hundredweight; and although the usual crystal is comparatively small,
it may be any size.

Crystallization has occurred by cooling, or by other natural means;
and we can form crystals by evaporation from certain salts deposited
in water. So we may conclude also that the evaporation of water in the
early periods deposited many forms of crystals. We have crystals in the
air, such as snowflakes, which are vapours crystallized. Carbon, when
crystallized, is the diamond. Boron is very like it. Oxygen cannot be
crystallized. Alumina makes sapphires and ruby with silica. Alumina
and earth give us spars, tourmaline, and garnets. Limestone also has
beautiful forms, as in Iceland spar. Crystals, therefore, are certain
forms of nature, corresponding in the inorganic kingdom to the animals
and plants of the organic.

Let us look a little more at these. Here we have a group of crystals
of different forms. Earths are metals combined with oxygen, and the
principal earths are alumina, lime, and silica. To these three we are
chiefly indebted for the ground we live on, and from which we dig so
many useful metals and other minerals. Earths are coloured by the
substances mixed with them. We can thus find copper, silver, gold,
lead, etc., by noting the appearance of the soil. True earths are
white. Strontia and baryta are also earths, and the latter is used
in firework manufactories. Our chief assistants are ALUMINA, which
furnishes us with bricks and slate; LIME, which gives us marble or
stones for building in a carbonate form. Quicklime, by which is meant
lime freed from the carbonic acid, is well known; and plaster of Paris
is only lime and sulphuric acid in combination. The SILICATES, such as
sand and flint, are in daily demand. Agate, cornelian, Scotch pebbles,
rock-crystal, etc., belong to the same family. Even our gems are
crystallized earths, and, as already stated, diamonds are merely carbon.

Stone, as we know, is quarried; that is, it is dug out of the earth.
But perhaps many readers do not know why a stone-mine is called a
“quarry.” Most kinds of stone (granite and marble are the exceptions)
are found in layers, or strata, rendering them easy of removal. The
blocks of stone are cut with reference to these layers in a more or
less square manner, and “squared up” before they are carried away. Thus
the term “quarry,” from an old French word, _quarré_, or _carré_, as
now written, signifying a square. In granite quarries the stone being
very hard is bored, and loosened by means of gunpowder or dynamite
blasting. Slate, on the contrary, is easily divided into slabs. We will
now resume the subject of Crystals.

[Illustration:
                {1.—Emerald.      3.—Garnet.      5.—Diamond.
  Fig. 434. {
                {2.—Agate.        4.—Ruby.        6.—Rock crystal.]

We have said that crystals vary in size, and this variety may be traced,
in the cases of crystallization from fluids, to the slowness or the
rapidity of the cooling process. If the work be done slowly, then the
crystals obtain a size commensurate with the time of cooling, as they
are deposited one upon the other. The form of minerals is the first
important point, and to ascertain their forms and structure we must
study CRYSTALLOGRAPHY. We shall find faces, or _planes_,—the lines of
contact of any two planes,—called _edges_, and the _angles_ formed
where these planes meet. We may add that crystals have, at least, four
planes, making six edges and four angles. Nearly all crystals have
more than this, for the forms are, if not infinite, very numerous, and
are divided into six (by some writers into seven) different systems or
fundamental forms from which the varieties are derived. The axis of a
crystal is an imaginary line drawn from an angle to the opposite one.

The first form, the _monometric, or cubic system_, with three equal
axes at right angles, is represented by fig. 436. This crystal is
limited by eight equilateral triangles. It has twelve edges and six
angles. If we describe a line from any one angle to an opposite one,
that line is called an _axis_, and in the case before us there are
three such axes, which intersect each other at right angles.[23] Such
crystals are regular octohedra. There are irregular forms also, whose
axes do not come at right angles, or they may be of unequal length.
The substances which we find crystallized in this form or system are
the diamond, nearly all metals, chloride of sodium (salt), fluor-spar,
alum, etc.

[Illustration: Fig. 435.—Stone quarry.]

When we say in this form we do not mean that all the minerals are
shaped like the illustration (fig. 436). We shall at once see that the
system admits of other shapes. For instance, a regular crystal may have
been cut or rubbed (and the experiment can be made with a raw turnip).
Suppose we cut off the angles in fig. 436; we then shall have a totally
different appearance, and yet the crystal is the same, and by cutting
that down we can obtain a cube (fig. 437). Take off its angles again we
obtain a regular octohedron once more, as shown in the diagram opposite.

[Illustration: Fig. 436.—Regular octahedron—first system.]

We will exhibit the gradations. Suppose we cut fig. 437; we will obtain
(fig. 438) the cube. The next is merely the cube with angles and edges
cut off; and if we proceed regularly we shall arrive at fig. 442, the
rhombic dodecahedron, or twelve-sided figure, whose equal planes are
rhombs.

We can, by taking away alternate angles or edges situated opposite,
arrive at other secondary crystals. From the original octohedron we
can thus obtain figs. 443 and 444. These are known as _tetrahedron_.
The _pentagonal dodecahedron_ is another secondary form (fig. 445).

[Illustration: Fig. 437.—Octohedron angles removed.]

[Illustration: Fig. 438.—The cube.]

[Illustration: Fig. 439.—Cube with angles removed.]

The cube, or hexahedron, the octohedron, and the rhombohedron are
all simple forms, being each bounded by equal and similar faces, or
surfaces. We can thus understand how certain primary or original
natural forms of crystals can be changed in appearance by connection.
Of the various substances crystallizing in this system we find salt,
iron pyrites, gold, silver, copper, and platinum, and the sulphide
of lead called _galena_, in the cube or hexahedron form. The diamond
and fluor-spar, alum, etc., appear in the first form (I), fig. 436
(octohedron). The cube, we see, has six equal _faces_, eight equal
_angles_, and twelve equal _edges_. Galena, as will be observed from
the illustration herewith, shows this peculiarity in a very marked
manner (fig. 446).

[Illustration:

 Fig. 440.—Another intermediate form of octohedron between figs. 436
 and 438.]

[Illustration: Fig. 441.—Cube deprived of edges and angles.]

[Illustration: Fig. 442.—Rhombic dodecahedron (garnet crystal).]

[Illustration: Figs. 443 and 444.—Secondary forms of first system.]

[Illustration: Fig. 445.—Pentagonal dodecahedron.]

The _second_ crystalline form is the HEXAGONAL, and in this system
three of the four axes are equal and in the same plane, inclined at an
angle of 60°, with a principal axis at right angles to the others. In
crystals of this system are found quartz and calc-spar.

The _third_ system is termed the QUADRATIC or the _diametric_. This
form has three axes, all at right angles, two being equal and the other
longer or shorter than the former two. In this system crystallize
sulphate of nickel, zircon, oxide of tin, etc.

[Illustration: Fig. 446.—Galena, or sulphide of lead.]

[Illustration: Fig. 447.—Oxide of tin.]

The _fourth_, or RHOMBIC system, or the _trimetric_. Here we have three
rectangular axes, all unequal and intersecting at right angles. The
sulphate and nitrate of potassium crystallize in this system.

[Illustration: Fig. 448.—Rock crystal—second system.]

[Illustration: Figs. 449 and 450.—Quadratic, or third system.]

[Illustration: Fig. 451.—Prism of quadratic system.]

The _fifth_ is the _oblique_, or MONOCLINIC system, which displays
three unequal axes, two of which are at right angles; the third, or
principal axis, is at right angles to one and oblique to the other of
the preceding. Ferrous sulphate, tartaric acid, and gypsum crystallize
in this system.

[Illustration: Fig. 452.—Rhombic, or fifth system of crystals.]

[Illustration: Fig. 453.—Crystals of the fifth system.]

The _sixth_, or TRICLINIC system, or the _doubly oblique_. In this
system we have three axes differing in length, and all forms which can
be arranged about these unequal and oblique axes. Sulphate of copper
will be found in this group. The system has been called anorthic, or
triclinic, because the axes are unequal and inclined, as in the oblique
prism based upon an obliqued angled parallelogram. Axinite crystal, as
annexed, will show one form in this system.

[Illustration: Fig. 454.—Sixth system.]

As may be gathered from the foregoing, it is not easy to determine
a crystalline form with certainty,—a great part of the crystal may
be invisible. A crystalline mass is a mineral, which consists of an
arrangement of crystals heaped together. If it does not possess these
the mineral is _amorphous_, or shapeless. We will now endeavour to
describe some of the physical characteristics of minerals.

[Illustration: Fig. 455.—Wollaston’s Goniometer, an instrument for
measuring the angles of crystals.]

The GONIOMETER (_see_ fig. 455) is the instrument used for measuring
the angles of crystals. Wollaston’s reflecting instrument is most
generally used. It consists of a divided circle, graduated to degrees,
and subdivided with the vernier. The manner of working is easy, though
apparently complicated. The vernier is brought to zero, when an object
is reflected in one face of the crystal. The crystal is turned till the
same object is viewed from another face. The angle of reflection is
then measured, and can be read off on the circle.

We have already referred to the physical characteristics of the
minerals, and one of these attributes is _cohesion_. When we find a
substance is difficult to break, we say it is “hard.” This means that
the cohesion of the different particles is very great. Minerals vary in
hardness; some are extremely difficult to act upon by force, and a file
appears useless. At the other side we find some which can be pricked
or scratched with a pin; and these degrees of hardness being put as
extremes, we can in a manner relatively estimate the hardness of all
other minerals. We can test this by scratching one against another;
whichever scratches the other is the harder of the two, and thus by
taking up and discarding alternately, we can at length arrive at a
comparative estimate of the hardness of all. Such a scale was arrived
at by Mohs, and arranged in the following order. The softest mineral
comes first:—

   1. Talc.
   2. Gypsum (rock-salt).
   3. Calcareous spar.
   4. Fluor-spar.
   5. Apatite-spar.
   6. Felspar.
   7. Quartz.
   8. Topaz.
   9. Corundum.
  10. Diamond.

Talc, we see, is the softest, and diamond the hardest. Thus “diamond
cut diamond” has passed into a proverb expressive of the difficulties
one “sharp” person has to circumvent or “cut out” another. Diamond
is used by glass-cutters. When geologists wish to express the degree
of hardness of any substance, they mention it with reference to the
foregoing list; and if the substance be harder than fluor-spar, but
not so hard as felspar, they determine its hardness five, or perhaps
between five and six, or between four and five, according as it is
harder or less hard than apatite. Thus hardness, or power of cohesion,
resistance to exterior force and pressure, is a prime characteristic of
the mineral kingdom. The file is the best test.

We now come to another phase of the physical character of our
minerals—_cleavage_. This is the term employed to express the
facility of cutting in a certain direction which in the mineral is
its direction of cleavage. Take mica, for instance. There is no
difficulty in separating mica into thin layers; we can do so with our
fingers. The layers, or flakes, or laminæ are so arranged that they
exhibit less cohesion in one direction than when tried in other ways.
We cut with the grain, as it were in the direction of the fibre when
wood is concerned. Here we have another popular saying expressive of
this,—“against the grain,”—which signifies an act performed unwillingly
and unpleasantly. Cleavability, therefore, means cutting with the
grain, as it were, and various minerals are possessed of different
degrees of cleavage. It sometimes happens that electric excitement is
observed when cleavage takes place. One place will become positive, and
the other negative. Mica, arragonite, and calcareous spar will exhibit
this action after cleavage or pressure. When a crystal of tourmaline
is heated, it will develop positive electricity at one end of its
principal axis, and negative at the other. Even if it be broken, the
extremities of the fragments will exhibit similar phenomena, and so
far like a magnet, which, as we have seen, possesses this attribute
of “polarity.” But a curious fact in connection with this is that, if
the heating cease the polarity ceases for a second or two, and yet as
cooling goes on the polarity is restored, with the difference that the
positive end has become negative, and the end previously negative has
come over to the opposite pole. Electricity, therefore, must be hidden
away in every portion of our globe, and will some day be proved to be
the mainspring of all life.

_Fracture_ in minerals is also to be noticed. Those substances which we
cannot laminate we are obliged to break, and we may require to break a
mineral in a direction different from or opposed to its direction of
cleavage. Under such circumstances we must break it, disintegrate it,
and observe the fracture. Sometimes we shall find the surfaces very
even, or uneven, or what is termed _conchoidal_. This is observable in
the breaking of flint. There are various ways in which minerals display
fracture, and the particular manner and appearance denotes the class to
which the mineral belongs.

We may pass over the question of the specific gravity of minerals, as
we have in a former part explained this. It is important, however, to
ascertain the specific gravity. As a general rule, minerals containing
heavy metals are of high specific gravity.

But the relation of minerals (crystals) with regard to light is of
great interest and importance. When we were writing of polarization,
we mentioned the faculty a crystal has for double refraction, by which
it divides a ray of light into two prolonged rays taking different
directions, the plane of vibration of one being at right angles to
that of the other. This property is not possessed by all crystals.
Some act as ordinary transparent media. Some crystals transmit only
one polarized ray, and tourmaline is called a polarizer; and if light
be passed through it to another polarizer, it will be transmitted
if the latter be similarly held; but if the second be held at right
angles to it the ray will be stopped. We can easily understand this
if we suppose a grating through which a strip of tin is passed; but
the strip will be stopped by bars at right angles to it. The coloured
rings in crystals can be observed when a slice of a double refracting
crystal is examined. The rings are seen surrounding a black cross in
some instances, and a white cross in another. The effect when examined
in the polariscope is very beautiful. Selenite is probably the best
crystal for exhibiting colours.

Minerals sometimes reflect, sometimes refract light; they are said
to possess lustre and phosphorescence. All these properties may
be considered as belonging to the crystals which are transparent,
semi-transparent, translucent, or opaque, according to the degrees
in which they permit light to pass through them. All minerals are
electric or non-electric, and the variety can be ascertained by
rubbing and placing the mineral near the electrometer. But all do
not exhibit magnetic properties. Taste and smell are strongly marked
in some minerals—salts, for instance, and sulphur; some are soapy to
the touch, some appear cold to the fingers. Chemistry is very useful
to us in determining the nature of the mineral, and the amount of it
enclosed in the substance under examination. These delicate operations
are termed qualitative and quantitative analysis. The application of
heat is increased by means of the blowpipe, which is in effect a small
bellows. We can thus, and particularly by means of the oxy-hydrogen
blowpipe, obtain a very intense heat with little trouble. When the
fragments of a mineral are held in the flame by platinum “tweezers,” or
tongs, then the _fusibility_ of the substance, and the colour of the
blow-pipe flame will be of great assistance in determining the nature
of the mineral. It is also curious to observe the different forms into
which the various substances expand or contract under the influence
of the blowpipe. We may have a rugged slag, an enamel, or a glass,
or a bead, or “drop” of metal. The varied substances produce various
colours—yellow, green, orange, or red, according to circumstances.
Strontia is a vivid red, copper is green, lime orange, and so on.

[Illustration: Fig. 456.—The blowpipe.]

It is very little use to attempt a study of mineralogy without some
acquaintance with chemistry. In dealing with minerals, and in studying
geology, we must try to keep our knowledge of chemical science in our
minds, and thus fortified we can more easily understand the steps
leading to the classification of minerals. It is impossible to teach
mineralogy or geology from books. Nature must be studied, the specimens
must be seen, the earth must be examined. The advance in mineralogy may
be—probably will be—slow, but crystals will teach something; and when
we can pass a _viva voce_ examination in chemistry and crystallography,
expressing, by the symbols, the various substances under discussion, we
shall have made a considerable advance in the science. We shall have
an idea of the component parts of various substances, and be able to
class the various minerals according to their chemical constitution.
Beginning with the metalloids, we shall pass to the metals and various
compounds, salts, resinous substances, etc., such as amber.

It is impossible in the space at our command to describe all the
minerals, and yet it is necessary to enumerate the most important.
We may, therefore, take them in the following order. It should be
added that most of the simple minerals occur in comparatively small
quantities, but sometimes we find them in aggregate masses (rocks). We
append a table.


SYNOPTICAL TABLE OF THE MINERALS.

  First Class.—Metalloids.
    Sulphur.
    Boron.
    Carbon.
    Silicium (Silicon).

  Second Class.—Light Metals.
    Potassium.
    Sodium.
    Ammonium.
    Calcium.
    Barium.
    Strontium.
    Magnesium.
    Aluminum.

  Heavy Metals.
    Iron.
    Manganese.
    Cobalt.
    Copper.
    Bismuth.
    Lead.
    Tin.
    Zinc.
    Chromium.
    Antimony.
    Arsenic.
    Mercury.
    Silver.
    Gold.
    Platinum.

  Third Class.
    Salts.
    Earthy resins.


SULPHUR is found in Sicily and Italy and other parts of Europe, in
a native state, but as such has to be purified. The crystals take
the form as shown in the margin. Cleavage imperfect; it is brittle.
Sulphuric acid is a very important combination, and a very dangerous
one in inexperienced hands. Sulphur combines with a number of elements,
which combinations are “Sulphides.” (_See_ Chemistry section.)

[Illustration: Fig. 457.—Crystals of sulphur.]


SELENIUM is a metalloid resembling sulphur, but less common. It is
inodorous.


BORON is usually found near volcanic springs, and in combination
with oxygen. It is soluble. Taste, acid bitter, and white in colour;
friable. It is known as SASSOLINE, or boracic acid. (_See_ Biborate of
Soda for one of the borates.)


CARBON is one of the most important of our minerals. In the form of
coal we have it in daily use, and in the form of diamond it is our most
valuable gem. In the latter form it is the hardest of all minerals, a
powerful refractor of light, lustrous, and transparent. It is found in
the East Indies and Brazil; more lately Cape diamonds have been brought
to Europe, but they do not equal the Eastern gem. Almost fabulous
prices have been given for diamonds, which, after all, are only carbon
in a pure state. Another form of carbon is _graphite_ (plumbago, or
blacklead). It is much used for pencils and in households. It is found
in Cumberland, and in many other localities in Europe and Canada.

Carbon appears in one or other of the above forms in regular
octahedrons or their allied shapes. _Anthracite_, another form of
carbon, is used as fuel for strong furnaces. It leaves little “ash,”
and is smokeless when burned. _Coal_, in all its forms, is evidently
derived from wood. Thousands of years ago vegetable matter must
have been embedded in the ground and subjected to carbonization.
There are different kinds of coal, all of which come under one or
other of the following heads: cubical coal, slate coal, cannel coal,
glance-lignite,—the last being, as its name implies, an imperfect
development of wood; it is a brown coal. We are not here concerned with
coal as a fuel. Charcoal is also a form of carbon prepared from wood
and finds a counterpart in coke, which is prepared from coal. Carbon,
as we have already seen, plays an important part in electric lighting
and in the Voltaic Battery. Peat, or as it is called in Ireland,
“turf,” is one of the most recent of the carboniferous formations.
It is much used as fuel. It is cut from moors (“bogs,” as they are
sometimes called), and the various deposits can be traced. Bog-oak
is no doubt the first step towards peat, as peat is the step towards
coal. The brown turf is newer than the black, and both kinds may be
seen stacked in small square “bricks” along the Irish canals and in the
yards of retailers of fuel.

[Illustration: Fig. 458.—Crystals of carbon.]


SILICON. Silica occurs generally in combination with alumina, and never
in a free state. In combination with oxygen it is called silicic acid.
Silica, when crystallized, is usually called _quartz_.


QUARTZ has several varieties. We need only enumerate them, they will
all be immediately recognized. We give illustrations of the crystals of
quartz (fig. 459):—

 1. Rock crystal appears in beautiful six-sided prisms.

 2. Amethyst is coloured by protoxide of manganese, supposed by the
 ancients to be a charm against drunkenness.

 3. Common quartz, or quartz rock, forms granite in combination, and is
 also known as “cat’s-eye,” “rose” quartz, etc.

 4. Chalcedony, sometimes termed _cornelian_: used for seals, etc.

 5. Flint: much used in potteries. “Flint and steel” have been
 superseded by phosphorus.

 6. Hornstone: something like flint, resembling horn.

 7. Jasper: of various colours; opaque and dull in appearance.

 8. Silicious slate: a combination; used as a whetstone.

 9. Agate: a mixture of quartz, amethyst, jasper, and cornelian; very
 ornamental.

 10. Opal: a peculiar variety, containing water. It is not found in the
 form of crystal, but in vitreous masses. Its changeableness of hue is
 proverbial. The “noble” opal is much prized.

 11. Smoky quartz, or cairngorm.

 12. Onyx and Sardonyx.

[Illustration: Fig. 459.—Quartz crystals in various forms.]

We now arrive at some minerals which contain metals.


POTASSIUM. This metal is so frequently combined in minerals with
alumina that we may refer to it with the latter in sequel. There are
two natural potassa salts—nitre, and sulphate of potassa. Nitre is
known as saltpetre, and is of great use in medicine. It is the chief
ingredient in the composition of gunpowder.


SODIUM. We have a number of minerals in this group—viz., _nitrate of
soda_ (nitratine), which occurs in large quantities in Peru; _rock
salt_, chloride of sodium, known as salt. It crystallizes in the cubic
system. Colour usually white, but it occurs in secondary rocks in
company with gypsum, etc. It is sometimes of a reddish colour, or even
green and yellow. Biborate of soda is _borax_, and is found in and on
the borders of a Thibetian lake. There are several other combinations
with soda: the sulphates of soda—viz., thenardite and glauberite,
anhydrous and hydrated respectively, carbonate of soda, and so on.


AMMONIA combinations occur in lava fissures, and are not often met with
in consequence of their volatile nature.


[Illustration: Fig. 460.—Spar crystal.]

CALCIUM. This forms an important group of the minerals, which are
very white in colour, and not very hard in substance. Calcium is the
metallic basis of lime. Fluoride of calcium, known as _fluor-spar_,
most frequently crystallizes in cubes in the first system. _Anhydrite_
is the anhydrous sulphate of calcium. The hydrated sulphate is called
gypsum. One variety of the hydrated sulphate is selenite, another is
known as alabaster. Apatite, or asparagus stone, and pharmacolite are
in this group.

  [Illustration: Rhombohedron (_r_).
  Primary rhombohedron (_r_).
  Six-sided prism (_g_) regular.
  Primitive rhombohedron (_r_), with acute form (_r½_).
  Obtuse rhomtrahedron (_r½_), ending in prism (_g_).
  Equal six-sided prism (_a_), ending in regular (_r_).
  Obtuse rhombohedron.

  Fig. 461.—CRYSTALS OF CARBONATE OF LIME.]

_Carbonate of lime_, not content with one system of crystals, makes its
appearance in two. It is therefore divided into two minerals—namely,
_calcareous spar_ and _arragonite_. In the former it possesses various
forms, as will be observed in the accompanying diagrams. It is a very
important mineral, as will be readily acknowledged; it enters largely
into the composition of all shells and bones. The minute shells,
deposited by millions at the bottom of the sea, have combined to raise
our chalk cliffs. Carbonate of lime is a constituent of water, as the
deposits at the bottoms of kettles, upon the sides and bottoms of
water-bottles, and the stalactites all testify. A little good vinegar
will quickly dissolve this deposit. Calc-spar is crystallized, and the
Iceland spar is celebrated. Marble, which is another form of carbonate
of lime, is white, hard, and granular. It is sometimes varied, but the
pure white is the most valuable. Chalk, we know well, is soft, and is
useful for writing. We have also _aphrite_, schiefer spar,—compact
limestone in various forms,—and finally, _arragonite_, called from the
place of its nativity, Arragon,—a colourless and somewhat transparent
vitreous crystal.


BARYTES. The sulphate of baryta is known as _heavy spar_; the crystals
are of tabular forms, but numerous modifications exist. One of the
forms is represented in the margin.

[Illustration: Fig. 462.—Tabular form of heavy spar.]


STRONTIUM is the metallic basis of strontia. Sulphate of strontium
is _celestine_, the mineral which colours the blow-pipe flame a fine
crimson. There are certain varieties. Strontia salts are chemical
preparations. A beautiful pyrotechnic “red fire” is produced by mixing
nitrate of strontia with sulphur, antimony, charcoal, and chlorate of
potassia.[24] There is a carbonate of strontia in the same crystalline
system.


MAGNESIUM. With this metal we have a large group of minerals.
_Magnesite_ is carbonate of magnesia, and occurs as talc-spar. The
magnesium limestone crystallizes as _bitter spar_. This dolomite
is like marble or common limestone, according to colour. Talc is a
combination of magnesia with silicic acid. The hydrated carbonate is
termed “white magnesia.” The sulphate of magnesia is found in Siberia,
and we have _boracite_, and native magnesia called _periclase_. The
sulphate is generally present in mineral waters, such as the Seidlitz
and Epsom Springs. Large masses have been found in the extensive
caverns of Kentucky and Tennessee, etc.

_Meerschaum_ is a hydrated silicate of magnesia. It is found in
Anatolia and Negropont, also in France and Australasia. Serpentine
is another similar composition. It is found in Cornwall, where it is
carved into various ornaments. It is sometimes called snakestone.
There are many other hydrated silicates of magnesia—viz., gymnite,
picrosmine, pycrophyll, etc.

[Illustration: Fig. 463.—Crystal of augite.]

There is another family allied to magnesia, called AUGITES. These
minerals are black or dark-green, and are contained in lava and basalt:
AUGITE and HORNBLENDE are the chief representatives of this family. The
former crystallizes in the fourth system (_see_ fig. 463), and there
are several varieties—diallage, bronzite, diopside, etc. HORNBLENDE
belongs to the same system, and is a large factor in the composition
of gneiss, syenite, and porphyry. _Tremolite_ is a hornblende, and
_asbestos_ (_amianthos_), and _mountain-cork_ are also varieties. The
attribute of asbestos for sustaining heat is well known, and may be
usefully employed for fire-proof purposes. The well-known _jade-stone_
of China and _calamite_ are other varieties.


[Illustration: Fig. 464.—Alum crystals.]

ALUMINUM, or ALUMINIUM, gives us a large class of minerals. It is the
metallic basis of alumina, which, combined with silica, is the chief
component of our clay. Silicic acid and this base combine to form many
minerals, and contains nearly all the precious stones. _Corundums_
consist of pure alumina, and crystallize in the hexagonal system. The
following stones are varieties of this mineral:—_Sapphire_, a beautiful
blue; _ruby_, a red oriental; _topaz_, yellow oriental; _amethyst_,
violet; all being sapphires more or less. The finest crystals are found
in the East Indies in the sands of rivers and diluvial soils. The
_common corundum_ is very hard, and is used for polishing. _Emery_ is
well known, and is found in mica-slate. It is of a bluish-grey colour,
and is also a polisher.


ALUM forms another family, of which we may first mention aluminite, a
“basic sulphate” of alumina and found in small quantities. _Alum-stone_
is found in Italy. _Alum_ occurs in large crystallized masses.
(_See_ illustration, fig. 464.) There are different minerals with a
composition very similar to alum, in which the potassa base of alum is
supplied by others. Thus we have the potassa alum, soda alum, manganese
alum, ammonia alum—all being very nearly of the same constituents, and
having similar crystals in the regular system, and are thus termed
isomorphous, or similarly-formed. The potassa, or potash alum, is the
commonest form, and is found abundantly in England, on the Continent of
Europe, and the United States. Soda alum is called _salfatarite_, and
magnesia alum _pickeringite_; manganese alum is _apjohnite_; phosphate
of alumina is _wavellite_.

There are compounds of alumina and magnesia called SPINELS. They are
hard minerals, and the same isomorphous changes take place with them as
are observable with the bases of alum. There are therefore varieties
such as the _spinel ruby_ found in the East Indies, very red in colour;
the _balas ruby_ not so red, and the orange-red, termed _rubicelle_.
Ceylon is remarkable for some fine specimens of spinels. _Chromite_ is
like the spinel, but is known as chrome iron.


ZEOLITES are principally compositions of silica and alumina. They
contain water, and are white, vitreous, and transparent. There are
several varieties of them—natrolite, stilbite, etc. We will now pass on
to the _Clays_, which are a very important family of the aluminum group.

There are a number of hard minerals which, when disintegrated, form
certain earthy masses. These we term clay, or clays, which possess
various colours and receive certain names, according to the proportion
of metallic oxides they contain. All clays have an affinity for water,
and retain it to a very great extent. The earth has also a peculiar
smell. Clay is used in various ways; pottery, for instance, we read
in the Bible as having been an employment from very ancient times.
One attribute of clays, the retention of water, is of the greatest
use to the world in providing moisture for plants and seeds. We may
mention other characteristics of clay. It absorbs oil very quickly, and
therefore is useful for removing grease-spots. It cannot be burned,
so we have fire-bricks and fire-clay in our stoves and furnaces.
There are various clays—pipe-clay, for instance, which is white;
potters’ clay is coarser. There is porcelain clay as well as porcelain
earth, of which more below. _Yellow ochre_ and _sienna_ are clays
used by artists. _Bole_ is a reddish clay; and _tripoli_ is employed
for polishing. There are, besides, _andalusite_, or chiastolite and
disthene, crystalline forms of clay.

       *       *       *       *       *

Porcelain has been known to the Chinese for centuries. In 1701 it was
discovered in Germany by Böttcher, a chemist, who while endeavouring
to make gold by Royal command, found the porcelain, and was thereby
enriched. Porcelain earth is frequently found; is known in many places
as kaolin, and usually comes from the decomposition of felspar. But in
Cornwall we find it as decomposed granite, and the filtering process
can be viewed from the railway, while both gneiss and granulite have
been known to yield kaolin. It is also found in China, Saxony, and
France. It is free from iron, and when ground and mixed with silicic
acid, it is handed to the potter or moulder. After the vessels have
been dried in the air they are put into the kiln, and then become white
and hard. After that they are glazed in a mixture of porcelain earth
and gypsum, or ground flints and oxide of lead, made fluid with water
in the glazing of earthenware. The vessel is then put into the furnace
again, or “fired,” as the process is called, and then comes out white,
hard, and partly transparent.

[Illustration: Fig. 465.—Porcelain furnace.]

Earthenware utensils are made of a coarser material,—clay and powdered
flints,—from which all the gross matter has been eliminated. Flint
is not difficult to break, if made hot and thrown into cold water. A
stamper is then used to break the flints. They are first ground in a
mill and purified like the clay, then they are mixed and beaten, while
moist, into “putty,” and turned, or forced, into moulds. The handles
are fixed on afterwards. The ware is baked for two days and glazed. The
various colours are obtained by mixing different clays and oxides—iron
or manganese. Biscuit porcelain is made by pouring a creamy mixture of
porcelain earth into plaster-of-Paris moulds, and when a thin case has
formed within, the liquid is poured out again. It is then dried in the
mould and shrinks. The mould is taken to pieces, and the thin biscuit
porcelain is left.

[Illustration: Fig. 466.—Stampers.]

[Illustration: Fig. 467.—Flint mill.]


FELSPARS are very like the zeolites, except that the former contain no
water. Felspar crystallizes in a number of different forms. We annex
illustrations of specimens. This spar is found in rocks, granite,
gneiss, etc. One variety is the _moonstone_, of a peculiar lustre.
Felsite is amorphous felspar. _Albite_ contains soda instead of potash.
_Labradorite_ is nearly a pure lime felspar, and is remarkable for
its colours, like a pigeon’s breast. _Spodumene_ is like albite, and
leucite, soda-lite, etc., belong to this family.

[Illustration: Fig. 468.—Felspar.]

[Illustration: Fig. 469.—Felspar crystal.]


LAPIS-LAZULI is a felspar distinguished by its blue tint. It was used
for ultramarine colouring at one time, which colour can also be made
chemically. Lapis-lazuli is found in Siberia and China. It is a mixture
of mineral species. _Hauyne_ is something like it. _Obsidian_ is a sort
of black glass, and occurs in various colours in vitreous masses. It is
derived from the fusion of rocks, and is employed in the manufacture of
boxes, etc. _Pumice stone_ bears a resemblance in composition to the
foregoing, but is porous, and so called spongy. It contains both potash
and soda in some quantities. _Pearlstone_ and _pitchstone_ also attach
themselves to this family group.


The GARNETS possess many curious forms of crystals, which are coloured
and used as gems. _Tourmaline_ is a very particularly useful crystal,
and is used in the investigations concerning the polarization of
light. It is found of nearly all colours. The garnet and staurolite
crystals are shown (figs. 470, 471).

The former is silicate of alumina with the silicate of some other
oxide, which is not always the same. This change, of course, gives us a
series, as in the case of _alum_ above mentioned.

[Illustration: Fig. 470.—Garnet crystal.]

The red varieties, called _almandine_, or precious garnets, are
distinguished from the duller, “common” species by their clear colour.
Bohemia is the most productive soil for the garnets.


MICA includes, as we have already noticed, a group of minerals which
have a peculiarly _laminated_ structure. These layers are by no means
all alike, but they present a smoothness to the fingers which is
highly characteristic. The chief constituents are alumina and silica,
occasionally with magnesia. Mica slate is very common, and is often
used instead of glass in window-frames. _Muscovite_, _lepidolite_, and
_phlogopite_ are all micas of the “potash,” “lithia,” and “magnesia”
varieties.

In the list of minerals associated with the lighter metals, we need
only now mention the _Gems_, so well known. These stones are very hard
in many instances, infusible, and exhibiting beautiful colours. Amongst
them are diamonds, sapphires, and rubies, of which we have spoken; the
topaz, noticed under corundum. The chrysoberyl (of a pale green, or
occasionally reddish hue), of which the alexandrite of Siberia is a
variety, is a compound of glucina with alumina; the beryl, a silicate
of the same, and the emerald of beautiful green. Zircon is another gem,
and “hyacinth” is its most valued form. The latter is found in basaltic
rocks. The emerald crystallizes in the hexagonal system.

[Illustration: Fig. 471.—Staurolite crystal.]

We may now consider the minerals formed by the heavier metals, such as
Iron, Copper, Nickel, etc.


IRON. This well-known metal fills a very important place in our mineral
arrangements, for the substances formed with iron ores occur in great
variety of structure, and occasionally in very large masses. They are
highly magnetic, and very hard. Were we here treating of iron as a
metal, we could give some information respecting its extraction and
manifold uses. All we need mention here is the fact that iron occurs
in nature in various ores which are essentially composed of iron and
oxygen. The iron is extracted in the blast furnace, in which the
process is continued for years. The “slag,” or glassy scum, protects
the molten iron from the air; its presence is necessary in all blast
furnaces. The most important of the iron group of minerals are MAGNETIC
IRON (magnetite), or loadstone. This mineral occurs in Sweden and
North America, and is found in primary rocks, and in Scandinavia forms
mountains. It crystallizes in the regular (octahedron) system, and
often in the form in illustration in the margin. It is highly magnetic,
as its name implies.

[Illustration: Fig. 472.—Magnetic iron.]

Native iron very rarely occurs, and then only in thin layers. The most
extraordinary specimens are those termed meteoric iron, which fall from
the atmosphere in great masses; and the meteoric stones, which contain
ninety per cent of iron, together with other constituents in small
quantities—viz., nickel, cobalt, copper, manganese, carbon, sulphur,
arsenic, etc.

_Red hematite_ crystallizes in the hexagonal system. It possesses much
the same (chemical) constitution as corundum (_q. v._). It is brightly
metallic, and shows a red streak. It occurs in various forms, as
iron _glance_ or specular iron, which is found in Sweden and Russia;
micaceous iron, bloodstone, clay, ironstone, etc.

_Brown hematite_ has not been found in crystals, but brown ironstone
(fibrous) is crystalline. The earthy brown, containing clay, gives us
_yellow ochre_ and _umber_. _Pea-iron ore_ and “_morass_” or “bog” ore
also belong to this class. _Limonite_ is the name given to these more
recent formations, of which yellow ochre is a pure specimen.

[Illustration: Fig. 473.—Native oxide of iron.]

The combinations of iron with sulphur (pyrites) are also important.
_Iron pyrites_ and _magnetic iron pyrites_ are two which may be
mentioned. The latter first.

Magnetic iron pyrites (or _pyrrhotin_) crystallizes in six-sided
prisms, and is attracted by the magnet. The composition of this mineral
has not been exactly ascertained, and no chemical formula has been
found for it.


IRON PYRITES (bisulphide of iron) is known as _cubic pyrites_, _yellow
pyrites_, and _mundic_. It is generally found in the regular system
of crystals, either as a cube or as a pentagonal dodecahedron. (_See_
first system of crystals, _ante_.) Its colour is yellowish. It is
known also as _green vitriol_ when oxidised, and forms beautiful green
crystals (copperas). This salt is used in the preparation of Prussian
blue and violet dyes. With gallic acid it makes ink.

There are many other “ferruginous” minerals, such as _vivianite_,
_green ironstone_, white iron pyrites, arsenical pyrites, or mispickel,
etc.

A carbonate of iron, called _chalybite_, or spathic ironstone, is
very abundant in nature, and forms obtuse rhombohedrons. It is very
useful for the production of steel, as it forms the clay iron ore found
in coal districts in combination. In a fibrous form it is known as
_sphærosiderite_. It is a most useful mineral.

Chrome iron (chromite) is useful for the preparation of chromium
compounds. It crystallizes in the cubic system. It is magnetic,
especially when treated. Chromic acid forms scarlet “needle” crystals,
and by its assistance chromate of lead, or _chrome yellow_, is
prepared. (Chromate of lead is found in a native state as crocoisite).
_See_ Chromium.


MANGANESE is contained in several minerals. It usually occurs as an
oxide. It colours minerals variously. In a pure state manganese is
white and brittle. The chief minerals are—

_Pyrolusite_ (the binoxide of manganese of commerce) occurs in
crystals. It is black. It is used in the preparation of chlorine and
oxygen. The other minerals are known as _manganite_, which is also
found associated with pyrolusite, as are _hausmannite_ and _braunite_,
the other oxides.


NICKEL and COBALT are generally found together, both being similar,
and the minerals are compounds of arsenic or sulphur, and occur under
similar circumstances. The principal are of NICKEL and of COBALT—

  Sulphide of nickel (ullmanite).
  Arsenical nickel (nickeline).
  Nickel glance (gersdorffite).
  Nickel pyrites (siegenite).
  Arsenical cobalt (smaltine).
  Cobalt glance (cobaltine).
  Cobalt bloom (erythrine).
  Cobalt pyrites (linnæite).

Nickel ores are used for extraction of the metal, which is used
as a substitute for silver. The name is derived from the German,
_kupfernickel_, or false copper. It was discovered in 1751.


COPPER, again, forms a number of minerals, and the chief is the _red
oxide_ of the metal, called _cuprite_. It crystallizes in the cubic
system. Its colour is red, and tinges a flame green. _Cuprite_ yields
excellent copper, and is found in Cornwall, and in many places on the
continent. The _black_ oxide is rarely found. It is known as melaconite.

_Malachite_ (carbonate of copper) is remarkable for its beautiful green
colour. In Australia it is worked for copper. It is chiefly ornamental.
Siberia yields the finest specimens, but the mineral is found in
Cornwall and Cumberland, as well as on the continent. Chessylite (from
Chessy, in France) is frequently found with malachite. It has been
called blue malachite, or the azure copper ore. It is used as a paint.

Besides the above, copper unites with sulphur to form minerals, such
as the needle ore (bismuthic sulphide of copper), antimonial sulphide,
bournonite; purple copper, and copper pyrites, which is very abundant,
and furnishes us with most of our copper. There is also the “grey”
copper ore, which contains various metals; even silver is obtained from
it at times.


BISMUTH gives us only a few minerals, of secondary importance. Native
bismuth resembles antimony, but is reddish in hue. Bismuth ochre,
bismuth blende, and bismuthine are the chief combinations.


LEAD is more important, and is obtained from _galena_, the sulphide
of lead, which is very abundant, and the principal lead ore. It can
be at once distinguished by its high specific gravity and metallic
lustre; the “cubic cleavage” also is very easy. It frequently is
found containing silver, and even gold, antimony, iron, etc. There
are several suphantimonites of lead, such as zinkenite, geocronite,
etc., and the salts, such as sulphate of lead and white lead ore, or
carbonate of lead (cerasite). The chromate of lead is found in the Ural
Mountains.


TIN is not found in a native state, but as _tinstone_, or binoxide
of tin, named _cassiterite_. It is found largely in Cornwall, and
the mines there have yielded great quantities for generations. Tin
pyrites, a union of sulphides of tin, iron, and copper, is also found
in Cornwall.


ZINC is produced from the ore called (zinc) blende, or sulphide of zinc
(black Jack). Its colour is very variable, sometimes red, but when pure
is greenish-yellow. It is also found black and brown. The red oxide of
zinc (or spartalite) is also worked for zinc. The carbonate, or zinc
spar, is common, and used to make brass, as is _calamine_, which is
possessed of a remarkable lustre, and is even luminous when rubbed. It
is a silicious oxide of zinc, and is found in the sedimentary rocks.
When heated, it displays strong electric properties.


CHROMIUM occurs in very few mineral combinations; chromate of lead,
chrome iron, and chrome ochre, or sesqui-oxide of chromium are the only
important ones.


ANTIMONY minerals are very hard; the tersulphide is the most common,
and from this the metallic antimony is produced. Red antimony, the
oxide, is a rarer ore.


ARSENIC resembles antimony, and occurs in combination with many
metals. White arsenic, or arsenious acid, is found in Bohemia, Alsace,
Transylvania, etc. Orpiment and realgar are sulphides of arsenic, and
are employed as colouring matters in paint and fireworks. Arsenic is
very poisonous.


MERCURY is occasionally found native, but more generally as _cinnabar_.
Chloride of mercury (or calomel) is found associated with the cinnabar,
or hepatic ore. Cinnabar is easily volatilized, and possesses high
specific gravity. The Californian mines are very rich. Spain also
produces a large quantity. It is opaque, and carmine in colour.


SILVER occurs native, or in ores. The latter are as follows:—The
sulphide, or the vitreous silver (argentite); antimonial silver;
and the combined sulphides, of antimony and silver. There are many
silver minerals, such as the chloride (horn silver, or kerargyrite),
bromide, and carbonate of silver, bismuthic silver, etc. The bromide
and iodide are bromargyrite and iodargyrite. Silver occurs most
frequently associated with gold; natural alloys of these two metals are
found, containing from 0·16 to 38·7 per cent. of silver, which causes
considerable variations both of colour and density. In addition to this
alloy, we may mention _sylvanite_ (graphic tellurium), which contains,
besides gold and silver, one of the rarer metals—viz., tellurium.

[Illustration: Fig. 474.—Gold crystals.]


GOLD is our most precious mineral, and is generally found native. It
exists in sand and in certain rocks. It crystallizes in various forms,
and in Mexico it is found in companionship with silver and copper
sulphides. Australia and California render the most valuable supplies
of the metal.


PLATINUM is also found native, and rarely is crystals. It is often
alloyed with other metals, chiefly with iron or gold; also with
diamonds. We have already considered it as a metal. Little remains
to be said about _salts_ and _resins_, for with the exception of
those we have referred to under Chemistry, they are of little value.
The bitumens, rock oil, etc., which exude from the earth, are very
useful, and as asphalt and petroleum play an important part in the
civilized world, but scarcely come under the strict rule of minerals
as we consider them, and with this reference we close our sketch of
Mineralogy.



FOOTNOTES:

[23] A crystal should be held so that one of its axes is vertical to
the spectator. This axis is termed the principal axis, and when there
is inequality the longest axis is the principal.

[24] _See_ “Strontium” in Chemistry.



CHAPTER XXXII.

NEW LOCOMOTIVE APPLIANCES.

 THE KITE—THE AEROPHANE—ICE YACHTS—SAILING TRUCKS—WATER VELOCIPEDES.


The kite, known from the earliest times, and constructed by a number
of people, is a very familiar object, which we shall not describe; for
we will now speak of some similar appliances of a more interesting and
uncommon description.

[Illustration: Fig. 475.—Mr. Penaud’s “High-flier.”]

M. Penaud has invented some appliances in which twisted india-rubber
is the principal agent. Fig. 475 represents a sort of kite, which
rises in the air if one twists and then looses the india-rubber round
the central bow. Fig. 476 represents another kind of invention; it is
an “aerophane,” with a screw at the back, so fixed that it receives
no shock from striking against any obstacle. After having twisted the
india-rubber, and loosened our hold of the apparatus in a horizontal
position, it will first descend for an instant, then, acquiring
increased speed, it rises seven or eight feet from the ground, and
describes a regular movement in the air for a distance of about fifty
yards; the motion lasts for several seconds.

Some models have also been constructed capable of traversing a distance
of over seventy yards, remaining for thirteen seconds in the air, as
lightly poised as a bird, and without any connection with the ground.
During the whole time the rudder restrains with perfect exactitude the
ascending and descending movements as they occur; and we can plainly
observe the various oscillations like those of sparrows, or more
especially woodpeckers. At last, when the movements are coming to an
end, the apparatus falls gently to the ground in a slanting line.

[Illustration: Fig 476.—M. Penaud’s “Aerophane.”]

M. Penaud has also succeeded in constructing a mechanical bird, that
we have seen set in motion, which will continue flying for several
seconds; we give an illustration of it in fig. 477.

Another scientist, M. Tatin, has also produced some remarkable results.
His efforts have been unceasingly directed towards the reproduction of
the flight of a bird by means of more or less complicated arrangements.
He has endeavoured to discover in the small appliances made with
indiarubber, and used by MM. Penaud and Hureau de Villeneuve, what
were the best shapes in which to reproduce the wings, in order to adapt
them to a large apparatus acting by compressed air. After several
attempts, he decided on the employment of long, narrow wings. Wenham
had previously proved that a wing may be equally effectual whether
it be narrow or wide, and M. Marcy has also declared that birds with
a quick, narrow wing-stroke have always very long wings. By means of
these long, narrow wings (fig. 478) M. Tatin has reduced the time
during which the wing reaches a suitable position for acting on the air
when it first descends. Granted the fact, so long established, that a
bird flies more easily if it rests its wing against a great volume of
air, it will be understood that the maximum speed of movement will also
be the most advantageous as regards the reduction of expended force.
The inventor, finding that he could not prevent his mechanical birds
from losing force in proportion as they attained considerable speed,
remedied this defect by _placing the centre of gravity in front_.
In consequence of this, the bird in full flight preserves the same
equilibrium as the bird hovering in the air, and its speed is, to a
certain extent, passive, the mass of air pressing of its own accord
against the wings, all expenditure of force therefore being utilized
for suspension. Thus has M. Tatin been enabled to increase the weight
of his appliances, without increasing the motive power, and yet obtains
a double course.

[Illustration: Fig. 477.—Mechanical bird.]

The movement made by the wing round a longitudinal axis, by means
of which it always exposes its lower surface in front on rising, is
obtained by the mechanism illustrated in fig. 478 _a_.


M. TATIN’S BIRD.

This apparatus, looked at sideways or from behind, is composed of a
light wooden frame, on which are two small supports crossed by an
axletree so as to form two cranks. This axle receives a circular
movement from an india-rubber spring. The crank on the foremost plane
causes the rising and falling of the wings, which move round a common
axis, and pass the dead points as the cranks of a locomotive do—so the
action is maintained.

[Illustration: Fig. 478.—M. Tatin’s bird.]

[Illustration: Fig. 478 _a_.—Detail of fig. 478.]

But the wing does not only move as a whole; every part of it,
particularly as it rises, shows a tendency to inclination, which
is most marked towards the extremity; the part near the body alone
preserves an invariable obliquity. M. Tatin was of opinion that it is
with the screw that it is necessary to direct the twisting movement;
and to obtain it with all its transitions, he has substituted for silk
wings, which fold up, some wings composed entirely of strong feathers,
arranged in such a manner that they slipped one over the other when
in motion. The arrangement was perfect, but still not suitable for
adaptation to the large bird. The inventor therefore again returned to
the use of the silk wings, which he appears to have definitely adopted.
By means of certain modifications which he has recently introduced
in his larger apparatus—viz., a change in the shape of the wings,
variation of the amplitude in the flapping, etc., M. Tatin has been
enabled to make great progress. The bird, acting by means of compressed
air, at first could only raise three-quarters of its own weight, but
finally lifted itself entirely. And we must take into consideration
that the apparatus has to struggle against the weight of the steering
apparatus, which nullifying the vertical and horizontal reactions of
the bird during flight, constantly fulfils the office of regulator.

[Illustration: Fig. 479.—Back view of apparatus.]

We will now pass to the consideration of two ingenious appliances of a
very clever inventor, M. Salleron.


SMALL ATMOSPHERIC BOAT.

The little boat shown in fig. 480, which is about the size of
an ordinary plaything, is a very ingenious, if not a practical,
application of the specific lightness of air acting as a propelling
force. In this instance steam plays but a secondary part, which
consists in carrying off the air that causes the moving of the boat.

[Illustration: Fig. 480.—Atmospheric boat.]

The apparatus, as represented in fig. 481, is of extreme simplicity,
as will be seen at a glance. A small cylindrical boiler, B, connected
with a capillary tube, is placed on two supports over a spirit-lamp,
in such a manner that the opening from which the steam issues is
directly opposite the mouth of the tube, T. This tube, after forming a
sudden inclination, terminates at the back of the boat in an inclined
drain, R. The steam driven through the tube, T, carries along with it
a certain quantity of air, which, forced under the water, propels the
boat along. The little vessel soon reaches considerable speed, leaving
a long track behind it. It will be seen that this is not a mechanical
apparatus, capable of absorbing force or diminishing the action of
steam by causing its condensation.

[Illustration: Fig. 481.—Section of “atmospheric” boat.]

Let us now calculate the force engendered by this apparatus. We know
that a litre of water at boiling point gives 1,700 times its volume.
The steam, as it quickly issues from the opening of the boiler,
carries along at least ten times its volume, or 17,000 litres of air,
which, driven under the water, assumes an ascending force equal to the
difference of the densities of water and air, or about the weight of
the displaced water. Therefore in a litre of water transformed into
1,700 litres of the steam, which carries off into the water 1,700 ×
10 = 17,000 litres of air, a force is developed represented by 34,000
kilograms. In fact, by reason of the inclined position of the drain
on which the pressure of air acts, and its restricted dimensions,
the quantity of force employed in the propulsion of the boat is but
a fraction of the total force produced. Moreover, the resistance of
traction increases with the size of the boat, and as the dimensions
of the inclined pipe cannot be indefinitely enlarged, the result is
that the propulsive action is soon insufficient, so that the invention
is not, in its present condition, applicable to navigation on a large
scale. Its superiority to the steam-engine cannot, therefore, be
demonstrated; and we are only now discussing the contrivance in order
to show that it is possible, with only moderately powerful generators
and extremely simple mechanical appliances, to obtain considerable
dynamic effects, susceptible of more serviceable application than is
commonly believed.


CIRCULATING FOUNTAIN.

The apparatus given in fig. 482 is the subject of a very charming
experiment, showing the influence of capillarity on the movements
of liquids. Two glass balls, B B´, are connected by two tubes; one
straight and of rather large diameter, the other extremely slender,
and winding in and out in a more or less complicated manner. The large
tube passes into ball B´, and forms a slender point, J, at the orifice
of the narrow tube. At the lower end of the ball is a bulb, which is
closed with a cork, and contains a coloured liquid. The apparatus is
fixed to a board with a ring at each end, by which it can be hung on
the wall. When commencing the experiment, it should be hung so that
the ball B´ is uppermost. The liquid then flows through into the ball
B, without presenting any particular phenomenon. The apparatus is then
turned, and the liquid descends again with great speed, shoots through
the opening, J, and rises into the twisted tube. The air displaced from
ball B´ also rises, however, and mingles with the liquid, and it can be
seen circulating through the winding tube in a number of air-bubbles,
mingled with drops of liquid, gradually transmitting the pressure of
the column contained in the upper ball and straight tube; so that by
means of a similar phenomenon to that of the fountain of Nero, the
liquid rises higher than the level of the reservoir, a part falling
into ball B, which causes the experiment to be a little prolonged. This
circulation of air-bubbles and coloured drops through the twisted tube
of the apparatus has a very pretty effect.

[Illustration: Fig. 482.—Circulating fountain.]


THE PNEUMATIC PENCIL.

This ingenious invention is productive of results similar to Edison’s
electric pen. It is the invention of an American gentleman, Mr. J.
W. Brickenridge, of Lafayette, Indiana. The illustration (fig. 483)
explains the mechanism of the pneumatic pencil. The whole apparatus is
figured on the left side of the picture, while the longitudinal section
of the pencil is shown on the right, the small cut at the top being
a vertical section of a portion of the motive power. Compressed air
furnishes the power of pressure, which is accomplished by means of a
perforating needle.

If the treadle is put in motion, a backward and forward movement is
imparted to a flexible diaphragm, as in the upper section in the
centre of the illustration. By this movement the air is permitted
to enter, and is compressed by the diaphragm into the flexible tube
with which the diaphragm is connected. The air is thus brought into
contact with another diaphragm at the end of the tube and presses
on it. The pencil is fixed to the latter. When it is desired to use
the pencil the apparatus is set in motion, and by a series of sharp,
quick perforations, any writing can be traced, as by the electric pen.
This indentation can be copied over and over again in a press, the
writing acting as the negative; and if ink be first run over it, as
in a stencil plate, by a proper “roller,” the latter will come out as
plainly as possible.

[Illustration: Fig. 483.—Pneumatic pencil.]


TUBE WELLS.

The principle upon which the tube well depends is very simple. It is
well known that in certain localities water lies a short distance
beneath the surface of the ground, and a very little trouble would
satisfy us upon the point, and render us quite independent of the water
companies’ supply. On the supposition that the water exists underneath
our garden at, say, twenty-five feet beneath the surface of the ground,
we have only to drive into the soil a tube for that distance, and by
the assistance of a common pump we shall obtain a pure supply of water.

We will now proceed to describe the manner in which these wells are
sunk. The first step is to fix a platform firmly upon the ground and
bore a hole, by which the tube is to enter the ground. This tube
should be very thick, with an aperture of two inches or rather
less, and three or four yards in length. The lower portion should be
pierced with holes, as in the illustration, and terminating in a point
of extremely fine-tempered steel. This tube can be driven into the
ground by mallets, or by the suspended hammer, worked as shown in the
illustration (fig. 484). This work will be easily accomplished, and
when the first length of tube has been driven in, another can be fixed
to it and hammered down in the same way.

[Illustration: Fig. 484.—Tube Well.]

When the tubes have been driven to the depth indicated it will be as
well to let down a sounding line, a simple cord sustaining a pebble. If
the stone be pulled up dry, another length of tube can be added, or the
tubes can be pulled up, and another trial made. If, on the contrary,
the pebble come up wet, the object is accomplished, and a small pump
can be fixed to the upper end of the tube, as in fig. 485. At first
the water will be found a little thick and muddy, in consequence of
the disturbance of the soil and the particles adhering to the end of
the first tube; but after an hour or so it will be found that the
water has become quite clear. It need scarcely be said that if the
water possesses sufficient ascensional force to rise to the level of
the ground a pump need not be employed. An Artesian Well will, in that
case, be the result.

The operation described on page 456 can usually be performed without
any difficulty. Sometimes, however, the tube may come in contact with a
large stone, and in that case the experiment must be tried elsewhere;
but, as a rule, the pointed tube, in consequence of its small size
and penetrative power, pushes any moderately-sized obstacle aside,
readily turns aside itself, or passes between pieces of stone to the
desired depth. Nine times out of ten the operation will be successful,
and the experiment will not occupy more than an hour, under ordinary
circumstances, and the tubing (and pump) may be obtained at a moderate
price, which can even be diminished by arrangement. Ordinary wells are
relatively very difficult to sink, and the soil thrown out from the pit
is in the way, while a parapet is necessary to protect the opening.
Besides, should water not be found after much work, the expense and
trouble of digging will be uselessly incurred. Thanks to the tube
system, we can search or probe for water anywhere with ease, and if we
do not find it in one spot we can easily move on to another without
incurring any serious trouble or expense.

[Illustration: Fig. 485.—Abyssinian Pump.]

We believe the idea of these “instantaneous wells” originated in the
United States during the War of Secession, when some soldiers of the
Northern army sunk rifle barrels into the ground, and obtained water
in a barren land. To Mr. Norton the development of the idea is due,
and in the Abyssinian Expedition the utility of the notion was fully
demonstrated. Since that time M. Donnet of Lyons has modified and
improved the tube-well, and arranged all the materials, including wider
tubing and the hammers upon a carriage, thus giving greater facilities
to the workmen and to those desirous of sinking such wells.

The general arrangement of M. Donnet, and the carriage with its
equipments utilized, is depicted in fig. 484; the actual sinking of the
well is carried out just as originally performed by Mr. Norton.


A NEW SWIMMING APPARATUS.

[Illustration: Fig. 486.—Swimming apparatus.]

We have to mention a novel means of swimming, which may prove useful
to those who distrust the natural buoyancy of water and their own
powers of keeping afloat or swimming. The simple apparatus, shown in
fig. 486, is the invention of an American named Richardson, a citizen
of Mobile, U.S.

[Illustration: Fig. 487.—Nautical Velocipede.]

The machine consists, essentially, of a shaft, upon which a float is
fixed, and at the end of the shaft is a small screw propeller. The
shaft is put in motion by a wheel arrangement worked by the hands, and
by a crank moved by the feet. The swimmer rests upon the float, with
his head well above water. The float sustains him, while the propeller
forces him through the water, without his feeling fatigued, at the
rate of about five miles an hour. A certain amount of practice is
necessary to obtain complete command of the machine, but when mastered
the swimmer can proceed, without much exertion, at a rapid rate. The
apparatus itself is not difficult to make, and persons who have tried
it speak highly of its convenience and of the facilities it may afford.
Captain Boyton’s swimming-dress is another useful invention, but the
means of mechanical propulsion are wanting, while in this new apparatus
the swimmer can drive himself through the sea with ease and expedition,
and even a non-swimmer may thereby save life without danger to himself,
or the person he wishes to rescue.

[Illustration: Fig. 488.—Trained seal drawing canoe.]

The NAUTICAL VELOCIPEDE, which also deserves some notice at our
hands, is the invention of M. Croce-Spinelli, who tried it upon the
great lake of Vincennes and also on the Seine, when it was the object
of much curiosity; but when the Franco-German war broke out the
experiments were discontinued, and the inventor did not live to perfect
the apparatus. He fell a victim to his love for ballooning. But M.
Joberts, a practical machinist, has lately taken up the idea broached
by Croce-Spinelli, and has brought out a new water velocipede of very
ingenious construction, with satisfactory results. The machine is
described as follows. There are two hollow tin “floats” of cylindrical
form, and tapered at the ends. These floats are joined together by a
platform made of very light wood, on which the seat of the worker is
raised, and underneath is the machinery for propelling the velocipede.
The motive power is very simple, and corresponds to that employed to
propel the bicycle on land, by the feet of the rider, the wheel being
furnished with paddles in the water velocipede.

[Illustration: Fig. 489.—Double yachts.]

A rudder, which can easily be worked by cords, gives the velocipedist
complete control of the machine, the steering being performed by a
handle similar to that which the bicyclist uses to turn the machine
he rides. In fact, the “water” velocipede is an adaptation of the
“terrestrial” machine so familiar to all readers. This velocipede is
equally adapted for sea or lake progression, the waves of the former
being, under ordinary circumstances, no obstruction, for very little
motion is imparted to the sitter. For those desirous to bathe in deep
water the machine offers many facilities; and in the case of attack of
cramp or faintness, rescue would not be difficult, as the swimmer could
support himself upon the pointed cylinders of the water velocipede till
assistance arrived. On the other hand, it is very necessary to know how
to swim before attempting to work the machine.

[Illustration: Fig. 490.—Ice boats.]

Before describing the ice-yachts which are used in Canada when winter’s
cold grasp lies on water and land, we will mention a very curious
experiment in water locomotion made a year or two ago. The illustration
explains itself. It is not an imaginary sketch, it is the record of
fact.

This sagacious seal was exhibited in London, and was in the habit of
performing certain tricks, one item of his performance being to draw
the light canoe (as represented), and another accomplishment consisted
in “striking the light guitar,” to the astonishment of the spectators,
amongst whom was the writer. The instrument was placed between his
fins, or “flappers,” and the seal twanged it more or less melodiously.
He was very tame, and obedient to his master and trainer.

We all have heard of, even if we have not seen, the twin steamer
_Castalia_, which, pending the opening of the tunnel beneath the
Channel, was supposed to reduce sea-sickness to a minimum. The
_Castalia_ did not answer, however, but an American has planned
certain double yachts, of which we give an illustration. The
sailing-boats, as represented, have had much success upon the lake of
Cayuga, and are quite seaworthy,—in fact, it is impossible to overturn
them.

The weight of one of these yachts is about fifteen hundred pounds, and
the draught six inches. Having two keels they answer the helm very
readily. The boat, in the centre of the illustration, belongs to Mr.
Prentiss, and is called the _Pera Ladronia_. It is a very fast “ship.”

From navigation in water, we now come to navigation _on_ water. The
ice-boats are much used in Canada, and their simple but effective
construction will be readily perceived from the accompanying
illustration. The Americans state that these ice-yachts can run before
a good breeze as fast as an ordinary train. There are, or were, models
of some such (Finland) yachts in the South Kensington Museum with two
sails. The American yacht, as a rule, has only one sail, and the owners
say—but we will not vouch for the truth of the allegation—that they
frequently run far ahead of the wind that primarily propelled them!


SAILING ON LAND.

It is quite possible to sail upon land, although this statement may
appear contradictory in terms. “The force of the wind upon sails,” says
Bishop Wilkins in his work, “Mathematical Magic,” printed in London in
1648, “can be applied to vehicles on land as well as to ships at sea.
Such conveyances,” he adds, “have long been in use in China and in
Spain, as well as in flat countries, such as Holland, where they have
been employed with great success. In the last-named country they are
propelled with greater speed than are ships before a fair wind; so that
in a few hours a boat containing several persons actually travelled
nearly two hundred miles, with no trouble to any one on board except
the steersman, who had little difficulty in guiding the boat.”

[Illustration: Fig. 491.—Sailing carriage of the 17th century, from a
drawing of the period.]

The astonishment expressed by the good bishop was quite justified, for,
as a matter of fact, a carriage or boat on wheels, with sails, as shown
in the illustration, achieved a distance of nearly thirty-eight miles
in an hour. This pace was quite unknown at that time; such a rate of
travelling had never entered the minds of people then. “Men running in
front of the machine after a while appeared to be going backwards, so
quickly were they overtaken and passed.” “Objects at a distance were
approached in the twinkling of an eye, and were left far in the rear.”
So it is evident that, had locomotion by steam not been adopted, the
mode of sailing on land would have eventually become the most rapid
mode of transit, and it is rather remarkable that it was never adopted
as a mode of travel.

[Illustration: Fig. 492.—On the Kansas Pacific Railway.]

But Bishop Wilkins had not to reproach himself on this account, for he
adapted the principle of the windmill to carriages, “so that the sails
would turn and move his car, no matter in what direction the wind was
blowing.” He proposed to make these sails act upon the wheels of a
carriage, and trusted to “make it move in any direction, either with
the wind or against it!” This suggestion has been lately adopted in the
United States, and it is curious that after two hundred and fifty years
no better mode for utilizing wind-power on land has ever been found.
Perhaps the ice-boats already mentioned may be the forerunners of some
new system of “land transport,” for which enormous kites have been made
available.

It is somewhat remarkable that if the introduction of railroads quite
“took the wind out of the sails” of any other mode of locomotion
on _terra firma_, it is that very iron track which has led to the
reintroduction of sails as a mode of progression upon the rails. In
the United States at the present time there are many vehicles propelled
by sails across the immense prairies at a pace, with a strong wind,
which equals that of the trains. We are indebted to Mr. Wood, of
Hayes City, Kansas, for the photograph from which the picture of the
sailing-waggon, invented by Mr. Bascom, of the Kansas Pacific Railway,
is copied. This carriage travels usually at thirty miles an hour, and a
speed of forty miles an hour has been obtained when the wind has been
high and blowing directly “aft.” The distance of eighty-four miles has
been accomplished in four hours when the wind was “on the beam,” or a
little forward of it, and on some curves with an almost contrary breeze.

The newest machine has four wheels, each thirty inches in diameter;
it is six feet in length, and weighs six hundred pounds. The sails
are carried upon two masts, and they contain about eighty-one square
feet of canvas. The main, or principal mast, is eleven feet high, four
inches in diameter at the base, and two inches at the top. As in the
case of the ice-boats, it is claimed for the sailing carriage that
it frequently outstrips the wind that propels it along the track.
On the other hand, there is a difference between the best sailing
points of the two kinds of vehicle. The ice-boat goes quickest with
the wind “dead aft,” the carriage makes best time with the wind “on
the beam”—_i.e._, sideways. The greater friction and larger surface
exposed to the influence of a side-wind no doubt will account for the
difference between the speed of the railway sailing-carriage and the
ice-boat.

Mr. Bascom informs us that the carriage we have described is in
frequent use upon the Kansas Pacific Railway, where it is employed to
transport materials for the necessary repairs of the line, telegraph,
etc., etc. It is a very cheap contrivance, and a great economizer of
labour. We all have noticed the cumbrous method of “trolly-kicking” by
“navvies” along the line. A trolly fitted with a sail would, in many
cases, and on many English lines, save a great deal of trouble, time,
and exertion to the plate-layers.



CHAPTER XXXIII.

ASTRONOMY.

 INTRODUCTORY—HISTORY OF ASTRONOMY—NOMENCLATURE.


[Illustration: Fig. 493.—Celestial globe.]

Astronomy is the science which treats of the heavenly bodies and the
laws which govern them. The term is derived from two Greek words,
_astron_, a star, and _nomos_, a law. It may be included in the study
of Physics, for the motion of the planetary bodies and equilibrium,
gravity, etc., all have something to say to the arrangements and
positions of the stars. The space in which they are set is infinite,
and known as the “Firmament,” or “Heaven.” The number of the heavenly
bodies must therefore be infinite also. We can see a few stars,
comparatively speaking, and there must be numbers whose light has never
yet reached the earth. When we calmly reason upon the immeasurable
distances and the awful rapidity of motion, with the masses of matter
thus in movement, we are constrained to acknowledge that all our
boasted knowledge is as nothing in the wondrous dispensations of Him
“who telleth the number of the stars, and calleth them all by their
names.”

Astronomy, no more than any other of the physical sciences, cannot
stand by itself. We have seen how heat, light, electricity, etc., are
all, in a manner, inter-dependent. So astronomy is dependent upon
mathematics, particularly geometry and trigonometry, for the wondrous
problems to be solved. But in the following sketch we do not propose
to plunge the reader in the slough of calculations. We only desire to
put plainly before him the great phenomena of nature with regard to the
heavens, and the glorious orbs which so thickly stud the space above
us. We need not detail the laborious calculations by which philosophers
have arrived at certain discoveries. We may refer to the results and
explain general principles, thereby indicating the road by which the
student may arrive at the more difficult bypaths in the fields of
scientific discovery.

The history of astronomy is nearly as old as the world itself, or
rather as old as the human race. From the earliest ages we can picture
men gazing upon the “spangled heavens,” and the wandering tribes of the
desert were always very careful observers of the paths of the stars.
To the nomads of the East the planetary system served as compass and
clock, calendar and barometer.

We shall find, therefore, that many observations of the heavenly
bodies were made by the ancients, and have descended to more advanced
generations, and this leads us to remark that the science of astronomy
can be studied without any very special or costly apparatus. In other
branches of science numerous instruments are indispensable before
we can reveal to ourselves the desired results. In astronomy, a
telescope—even a good field glass, such as possessed by any household,
will reveal many interesting facts. We will, by means of more expensive
instruments, and by the aid of large telescopes particularly, enjoy
the sight of the moon and planets. But even with the naked eye a great
variety of phenomena may be observed. With a celestial globe in our
hands upon a fine starry night, we can easily find out the position of
the constellations, and trace their forms in the firmament.

It is to the Chaldeans, Indians, Chinese, and Egyptians, that our
knowledge of astronomy is primarily due. They did much to facilitate
the observation of the stars; they named the planets, grouped the
stars, and marked the sun’s track in the sky. _Astrology_ was
cultivated in very remote ages. The Jews practised it; and the
astrologers of subsequent periods played very important parts in
divining the future of individuals, and casting their _horoscopes_.
Many of these so-called predictions came true, “because,” as was
remarked by Pascal, “as misfortunes are common they” (the astrologers)
“are often right,” as they foretold misfortune oftener than good
fortune. Still the fact remains that occasionally a very startling
prediction was made, and proved true; such, for instance, as the laying
waste of Germany by Gustavus Adolphus, which was foretold by Tycho
Brahé after his consideration of a certain comet, and the date of the
king’s death was also correctly prophesied. Astrology, therefore, held
a very considerable influence over the human race during the Middle
Ages.

We can only give a very brief historical summary of the science. We
know that the destinies of individuals and nations were at a very early
period attributed to the influence of the stars. We read that “the
stars in their courses fought against Sisera,” and many expressions
surviving to the present time serve to remind us that the stars were at
one time paramount in men’s minds. Thus we have the phrases—“unlucky
star,” “born under a lucky star,” “mark my stars,” “moonstruck,”
etc. Even the common term “consider”—to take counsel of the stars—is
thus accounted for, and many men have a habit of looking up to the
ceiling of a room or to the sky when thinking deeply—considering
with the stars. “Contemplate” is another term signifying the same
thing; for _templum_, a temple, was formerly a space marked upon
the sky in imaginary lines, and traced on the ground in accordance
with the supposed diagram. Thus temple became a place for heavenly
“contemplation,” and by an easy transition to a place of worship. In
our old poets’ writings we have many allusions to the influences of the
stars.

                “Now glowed the firmament
    With living sapphires; Hesperus, that led
    The starry host, rode brightest, till the moon,
    Riding in clouded majesty, at length
    Apparent queen, unveiled her peerless light,
    And o’er the dark her silver mantle threw.”—MILTON.

Although from Thales, who lived B.C. 610, the real science of astronomy
may be allowed to date, there can be no doubt that the ancients were
acquainted with many phenomena. The Chaldeans were, doubtless, the
first to place on record the rising and setting of the celestial bodies
and eclipses, and used the water-clock (clepsydra). A list of eclipses
from 2234 B.C. is stated to have been found at Babylon by Alexander the
Great. The Chaldeans also divided the ecliptic into twelve equal parts,
and the day and night into twenty-four hours. The Chinese, again, have
recorded astronomical phenomena as far back as 2857 B.C.; and the
Egyptians also were well versed in the science, although no records
of much importance remain to us, unless the zodiac signs were their
invention.

Thales predicted the eclipse of the sun B.C. 610. Aristarchus and
Eratosthenes also made important observations. Hipparchus (160-125
B.C.) discovered the precession of the equinoxes, calculated eclipses,
determined the length of the year, etc., etc.

Ptolemy, of Alexandria, A.D. 130-150, was the founder of a theory
called the Ptolemaic System, which recognized the earth as the centre
of all—the sun, moon, stars, etc., all revolving in very complicated
courses around it, as figured in the diagram herewith. Even though his
theory turned out to be untenable, he paved the way for his successors
in other ways, and left a valuable collection of observations on
record. In this volume, called the “Almagest,” he reviewed the state of
the science, and gave a catalogue of stars, as well as a description of
the heavens. He discovered the lunar evection.

After his time astronomy, though it was not neglected, appeared to
droop, and it is at a comparatively late period that we again open
the records—viz., in 1543, the year in which Copernicus died. This
philosopher, who was born in 1473, promulgated the true theory of the
solar system. He placed the sun in the centre of the planets, and by
this he explained their motion around the sun, though they appeared to
be carried round the earth. The book in which he explained his theory,
“De Revolutionibus Orbium Celestium,” was not finished till a day or
two before he died.

[Illustration: Fig. 494.—Ptolemaic System.]

The justly celebrated Tycho Brahé was the most important of the
successors of Copernicus, but he opposed the Copernican theory, while
other able philosophers agreed with it. Brahé was a Dane; he died in
1601. He adopted the theory that the sun and moon revolved around the
earth, while the (other) planets moved around the sun. This theory
did not gain much credence, but he, again, though he could not defeat
Copernicus, and though he was wrong in his assumption, made many
important investigations. After him came Kepler, whose observations
upon the planet Mars cleared away many complications, and he laid down
three laws, which are as follows:—

1. Every planet describes an elliptic orbit about the sun, which
occupies one focus of each such ellipse.

2. If a line be drawn from the sun, continually, to any planet, this
line will sweep over equal areas in equal times.

3. The squares of the periodic times of the planets are proportional to
the cubes of their mean distances from the sun.

Kepler also remarked that gravity was a power existing between all
bodies, and reasoned upon the tides being caused by the attraction of
the moon for the waters.

[Illustration: Fig. 495.—Copernican System.]

It was about this time—viz., the beginning of the seventeenth
century—that the telescope was invented, and logarithms came into
use. The actual discoverer of this now almost perfected instrument is
uncertain. Borelli, who wrote in the seventeenth century, ascribes the
discovery to Zachariah Jansen and Hans Lippersheim, spectacle makers of
Middleburg. Baptista Porta, also a spectacle maker, has had the credit
of discovering the magnifying power of the lens, and, so far, the
originator of the telescope.

[Illustration: Fig. 496.—Ellipse.]

[Illustration: Fig. 497.—Radii Vectores.]

[Illustration: Fig. 498.—Ecliptic and Equator.]

But whoever invented it, the telescope did not penetrate into southern
Europe till 1608-9. Galileo then made inquiries concerning the new
instrument, and Kepler made some propositions for their construction.
But Harriot had used the instrument so far back as 1611 or 1612, and
had observed spots upon the sun’s disc. Galileo, in 1610, had also
made observations with the telescope, and discovered the satellites
of Jupiter. He thereby confirmed the Copernican theory;[25] and when
Newton promulgated his immortal discovery of gravitation, after
Picard’s researches, the relations of the sun and planets became
more evident. His researches were published in the _Principia_, and
then one-half the scientific world began to question the principle
of gravitation, which was supported by Newton and his adherents.
Subsequently the researches of Lagrange and Laplace, Adams and
Leverrier, Sir J. Herschel, etc., brought astronomy into prominence
more and more; and the innumerable stars have been indicated as new
planets have been discovered. The spectroscope, which gives us the
analyses of the sun and other heavenly bodies, has, in the able hands
of living astronomers, revealed to us elements existing in the vapours
and composition of the sun, etc. Stars are now known to be suns, some
bearing a great resemblance to our sun, others differing materially.
The nebulæ have been analysed, and found to be stars, or gas, burning
in space—hydrogen and nitrogen being the chief constituents of this
glowing matter. Instruments for astronomical observation have now been
brought to a pitch of perfection scarcely ever dreamed of, and month
by month discoveries are made and recorded, while calculations as to
certain combinations can be made with almost miraculous accuracy. The
transit of Venus, the approaches of comets, eclipses, and the movements
of stars, are now known accurately, and commented upon long before the
event can take place.

We will close this chapter by giving a brief explanation of the various
definitions most usually employed in astronomy.

1. The _Axis_ of the earth is an _imaginary_ line passing through the
centre (north and south); the _poles_ are the extremities of this line.

2. The _Equator_ is an imaginary circle passing round the globe,
dividing it into northern and southern hemispheres. The _equinoctial_
is the plane of the former circle extended to the heavens, and
when the sun appears in that line the days and nights are of equal
duration—twelve hours each.

3. The _Ecliptic_ is the sun’s path through the heavens—though, of
course, the sun does not actually move, and therefore the track, or
supposed circle, is really the earth’s motion observable from the sun.
When the moon is near this circle eclipses happen. The ecliptic cuts
the equinoctial at an angle of about 23°. One half is to the north and
the other to the south of the equinoctial.

[Illustration: Fig. 499.—The Zodiac.]

4. The _Zodiac_ is a girdle extending 8° on each side of the ecliptic,
in which space of 16° the planets move. The zodiac is divided into
twelve parts of 30° each, called the “Signs.” These names are as under
written:—

NORTHERN SIGNS.

_Spring._

  Aries, the Ram, March.
  Taurus, the Bull, April.
  Gemini, the Twins, May.

_Summer._

  Cancer, the Crab, June.
  Leo, the Lion, July.
  Virgo, the Virgin, August.

SOUTHERN SIGNS.

_Autumn._

  Libra, the Balance, September.
  Scorpio, the Scorpion, October.
  Sagittarius, the Archer, November.

_Winter._

  Capricornus, the Goat, December.
  Aquarius, the Waterbearer, January.
  Pisces, the Fishes, February.

5. _Colures_ are two circles dividing the ecliptic into four equal
parts, and making the seasons.

6. The _Horizon_ is the boundary line of our vision, and is called the
sensible (apparent) horizon. The true horizon is the circle—as on a
globe—dividing the heavens into two hemispheres. The sensible horizon
is enlarged according as the eye is elevated above the ground. A man
six feet high can see a distance of three miles when standing on a
plain. We can always find the distance visible when we know the height
at which we stand, or, inversely, we can tell the height of an object
if we know the distance. We have only to increase the height _one half
in feet_, and extract the square root for the distance in miles. On
giving the distance in miles reverse the operation.

[Illustration: Fig. 500.—Right ascension.]

For instance, for the man six feet high, as supposed, add three feet,
being half his height; that makes nine feet. The square root (or number
multiplied by itself to give nine) is three, which is the number of
miles the man can see on a plain. Or, again, suppose we can see a
tower on the level, and we know we are twelve miles away from it. The
square of twelve is one hundred and forty-four feet, one-third of that
is forty-eight feet, which represents the half of the original height
added to the whole tower in feet; so the whole tower is ninety-six feet
high. Reversing, as in the former case, we can prove this by taking the
tower at ninety-six feet high and trying to find the distance we can
see from its summit = 96 + 48 = 144; the square root of 144 = 12, the
distance required.

7. The _Nadir_ and the _Zenith_ are the poles of the horizon. The
zenith is exactly overhead, the nadir exactly under foot. Circles drawn
through these points are azimuth circles.

8. _Meridians_ are circles passing through the poles at right angles
to the equinoctial. Every place is supposed to have a meridian, but
only twenty-four are upon the globe, and they represent the sun’s, or
the planets’, “movements” every hour—15° being one hour, 360° being