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THE BOY'S PLAYBOOK OF SCIENCE:

INCLUDING THE

Various Manipulations and Arrangements

OF

CHEMICAL AND PHILOSOPHICAL APPARATUS REQUIRED FOR THE SUCCESSFUL
PERFORMANCE OF SCIENTIFIC EXPERIMENTS. IN ILLUSTRATION OF THE
ELEMENTARY BRANCHES OF CHEMISTRY AND NATURAL PHILOSOPHY.

BY JOHN HENRY PEPPER, F.C.S., A. INST. C.E.; LATE PROFESSOR OF CHEMISTRY
AT THE ROYAL POLYTECHNIC, ETC. ETC. AUTHOR OF "THE PLAYBOOK OF METALS."

_NEW EDITION._

Illustrated with 470 Engravings, CHIEFLY EXECUTED FROM THE AUTHOR'S
SKETCHES, BY H. G. HINE.

  LONDON:
  GEORGE ROUTLEDGE AND SONS,
  THE BROADWAY, LUDGATE.
  NEW YORK: 416, BROOME STREET.
  1869.

  LONDON.
  SAVILL, EDWARDS AND CO., PRINTERS, CHANDOS STREET.
  COVENT GARDEN.


[Illustration: Wheatstone's telephonic concert at the Polytechnic, in
which the sounds and vibrations pass _inaudible_ through an intermediate
hall, and are reproduced in the lecture-room unchanged in their
qualities and intensities.

_Frontispiece._]


TO

PROFESSOR LYON PLAYFAIR, C.B., F.R.S.

PROFESSOR OF CHEMISTRY IN THE UNIVERSITY OF EDINBURGH.


DEAR SIR,

    I dedicate these pages to your Children, whom I often had the
    pleasure of seeing at the Polytechnic during my direction of that
    Institution. I do so as a mark of respect and appreciation of your
    talent and zeal, and of your public-spirited advocacy of the Claims
    of Science in this great and commercial country.

    Without making you responsible in any way for the shortcomings of
    this humble work on Elementary Science, allow me to subscribe
    myself,

    Dear Sir,

    Yours most respectfully,

    JOHN HENRY PEPPER.




  CONTENTS.


                                                         PAGE

  INTRODUCTION                                             1


  CHAPTER I.

  THE PROPERTIES OF MATTER--IMPENETRABILITY                3


  CHAPTER II.

  CENTRIFUGAL FORCE                                       17


  CHAPTER III.

  THE SCIENCE OF ASTRONOMY                                19


  CHAPTER IV.

  CENTRE OF GRAVITY                                       32


  CHAPTER V.

  SPECIFIC GRAVITY                                        48


  CHAPTER VI.

  ATTRACTION OF COHESION                                  59


  CHAPTER VII.

  ADHESIVE ATTRACTION                                     67


  CHAPTER VIII.

  CAPILLARY ATTRACTION                                    69


  CHAPTER IX.

  CRYSTALLIZATION                                         73


  CHAPTER X.

  CHEMISTRY                                               81


  CHAPTER XI.

  CHLORINE, IODINE, BROMINE, FLUORINE                    129


  CHAPTER XII.

  CARBON, BORON, SILICON, SELENIUM, SULPHUR, PHOSPHORUS  151


  CHAPTER XIII.

  FRICTIONAL ELECTRICITY                                 173


  CHAPTER XIV.

  VOLTAIC ELECTRICITY                                    193


  CHAPTER XV.

  MAGNETISM AND ELECTRO-MAGNETISM                        206


  CHAPTER XVI.

  ELECTRO-MAGNETIC MACHINES                              211


  CHAPTER XVII.

  THE ELECTRIC TELEGRAPH                                 218


  CHAPTER XVIII.

  RUHMKORFF'S, HEARDER'S, AND BENTLEY'S COIL APPARARATUS 230


  CHAPTER XIX.

  MAGNETO-ELECTRICITY                                    241


  CHAPTER XX.

  DIA-MAGNETISM                                          247


  CHAPTER XXI.

  LIGHT, OPTICS, AND OPTICAL INSTRUMENTS                 255


  CHAPTER XXII.

  THE REFRACTION OF LIGHT                                298


  CHAPTER XXIII.

  REFRACTING OPTICAL INSTRUMENTS                         303


  CHAPTER XXIV.

  THE ABSORPTION OF LIGHT                                327


  CHAPTER XXV.

  THE INFLECTION OR DIFFRACTION OF LIGHT                 328


  CHAPTER XXVI.

  THE POLARIZATION OF LIGHT                              335


  CHAPTER XXVII.

  HEAT                                                   352


  CHAPTER XXVIII.

  THE STEAM-ENGINE                                       406


  CHAPTER XXIX.

  THE STEAM-ENGINE--_continued_                          418




[Page 1]

INTRODUCTION.


Although "The South Kensington Museum" now takes the lead, and surpasses
all former scientific institutions by its vastly superior collection of
models and works of art, there will be doubtless many thousand young
people who may remember (it is hoped) with some pleasure the numerous
popular lectures, illustrated with an abundance of interesting and
brilliant experiments, which have been delivered within the walls of the
Royal Polytechnic Institution during the last twenty years.

On many occasions the author has received from his young friends
letters, containing all sorts of inquiries respecting the mode of
performing experiments, and it has frequently occurred that even some
years after a lecture had been discontinued, the youth, now become the
young man, and anxious to impart knowledge to some "home circle" or
country scientific institution, would write a special letter referring
to a particular experiment, and wish to know how it was performed.

The following illustrated pages must be regarded as a series of
philosophical experiments detailed in such a manner that any young
person may perform them with the greatest facility. The author has
endeavoured to arrange the manipulations in a methodical, simple, and
popular form, and will indeed be rewarded if these experiments should
arouse dormant talent in any of the rising generation, and lead them on
gradually from the easy reading of the present "Boy's Book," to the
study of the complete and perfect philosophical works of Leopold Gmelin,
Faraday, Brande, Graham, Turner, and Fownes.

Every boy should ride "a hobby-horse" of some kind; and whilst play, and
plenty of it, must be his daily right in holiday time, he ought not to
forget that the cultivation of some branch of the useful Arts and
Sciences will afford him a delightful and profitable recreation when
[Page 2]satiated with mere _play_, or imprisoned by bad weather, or
gloomy with the unamused tediousness of a long winter's evening.

The author recollects with pleasure the half-holidays he used to devote
to Chemistry, with some other King's College lads, and in spite of
terrible pecuniary losses in retorts, bottles, and jars, the most
delightful amusement was enjoyed by all who attended and assisted at
these juvenile philosophical meetings.

It has been well remarked by a clever author, that bees are
_geometricians_. The cells are so constructed as, with the least
quantity of material, to have the largest sized spaces and the least
possible interstices. The mole is a _meteorologist_. The bird called the
nine-killer is an _arithmetician_, also the crow, the wild turkey, and
some other birds. The torpedo, the ray, and the electric eel are
_electricians_. The nautilus is a _navigator_. He raises and lowers his
sails, casts and weighs anchor, and performs nautical feats. Whole
tribes of birds are _musicians_. The beaver is an _architect_,
_builder_, and _wood-cutter_. He cuts down trees and erects houses and
dams. The marmot is a _civil engineer_. He does not only build houses,
but constructs aqueducts, and drains to keep them dry. The ant maintains
a regular _standing army_. Wasps are _paper manufacturers_. Caterpillars
are _silk-spinners_. The squirrel is a _ferryman_. With a chip or a
piece of bark for a boat, and his tail for a sail, he crosses a stream.
Dogs, wolves, jackals, and many others, are _hunters_. The black bear
and heron are _fishermen_. The ants are _day-labourers_. The monkey is a
_rope dancer_. Shall it, then, be said that any boy possessing the
Godlike attributes of Mind and Thought with Freewill can only eat,
drink, sleep, and play, and is therefore lower in the scale of
usefulness than these poor birds, beasts, fishes, and insects? No! no!
Let "Young England" enjoy his manly sports and pastimes, but let him not
forget the mental race he has to run with the educated of his own and of
other nations; let him nourish the desire for the acquisition of
"scientific knowledge," not as a mere school lesson, but as a treasure,
a useful ally which may some day help him in a greater or lesser degree
to fight "The Battle of Life."


[Page 3]

THE BOY'S PLAYBOOK OF SCIENCE.




CHAPTER I.

THE PROPERTIES OF MATTER--IMPENETRABILITY.


In the present state of our knowledge it seems to be universally agreed,
that we cannot properly commence even popular discussions on astronomy,
mechanics, and chemistry, or on the imponderables, heat, light,
electricity, and magnetism, without a definition of the general term
"matter;" which is an expression applied by philosophers to every
species of substance capable of occupying space, and, therefore, to
everything which can be seen and felt.

The sun, the moon, the earth, and other planets, rocks, earths, metals,
glass, wool, oils, water, alcohol, air, steam, and hosts of things, both
great and small, all solids, liquids and gases, are included under the
comprehensive term _matter_. Such a numerous and varied collection of
bodies must necessarily have certain qualities, peculiarities, or
properties; and hence we come in the first place to consider "The
general powers or properties of matter." Thus, if we place a block of
wood or stone in any position, we cannot take another substance and put
it in the space filled by the wood or stone, until the latter be
removed. Now this is one of the first and most simple of the properties
of matter, and is called _impenetrability_, being the property possessed
by all solid, liquid, and gaseous bodies, of filling a space to the
exclusion of others until they be removed, and it admits of many amusing
illustrations, both as regards the proof and modification of the
property.

Thus, a block of wood fills a certain space: how is it (if impenetrable)
that we can drive a nail into it? A few experiments will enable us to
answer this question.

Into a glass (as depicted at fig. 1) filled with spirits of wine, a
quantity of cotton wool many times the bulk of the alcohol may (if the
experiment is carefully performed) be pushed without causing a drop to
overflow the sides of the vessel.

Here we seem to have a direct contradiction of the simple and [Page 4]
indisputable truth, that "two things cannot occupy the same space at
once." But let us proceed with our experiments:--

We have now a flask full of water, and taking some very finely-powdered
sugar, it is easy to introduce a notable quantity of that substance
without increasing the bulk of the water; the only precaution necessary,
is not to allow the sugar to fall into the flask in a mass, but to drop
it in grain by grain, and very slowly, allowing time for the air-bubbles
(which will cling to the particles of sugar) to pass off, and for the
sugar to dissolve. Matter, in the experiments adduced, appears to be
penetrable, and the property of impenetrability seems only to be a
creation of fancy: reason, however, enables us to say that the latter is
not the case.

[Illustration: Fig. 1.]

[Illustration: Fig. 2.]

A nail may certainly be hammered into wood, but the particles are
_thrust aside_ to allow it to enter. Cotton wool may be placed in
spirits of wine because it is simply greatly extended and bulky matter,
which, if compressed, might only occupy the space of the kernel of a
nut, and if this were dropped into a half-pint measure full of alcohol,
the increase of bulk would not cause the spirit to [Page 5] overflow.
The cotton-wool experiment is therefore no contradiction of
_impenetrability_. The experiment with the sugar is the most troublesome
opponent to our term, and obliges us to amend and qualify the original
definition, and say, that the ultimate or smallest particles or atoms of
bodies only are impenetrable; and we may believe they are not in close
contact with each other, because certain bulks of sugar and water occupy
more space separately than when mixed.

[Illustration: Fig. 3.]

If we compare the flask of water to a flask full of marbles, and the
sugar to some rape-seed, it will be evident that we may almost pour
another flask full of the latter amongst the marbles, because they are
not in close contact with each other, but have spaces between them; and
after pouring in the rape-seed, we might still find room for some fine
sand.

The particles of one body may thus enter into the spaces left between
those of another without increasing its volume; and hence, as has been
before stated, "The atoms only of bodies are truly impenetrable."

This spreading, as it were, of matter through matter assumes a very
important function when we come to examine the constitution of the air
we breathe, which is chiefly a mechanical mixture of gases: seventy-nine
parts by volume or measure of nitrogen gas, twenty-one parts of oxygen
gas, and four parts of carbonic acid vapour in every ten thousand parts
of air having the following relations as to weight:--

                   Specific gravity.

  Nitrogen               972
  Oxygen                1105
  Carbonic acid         1524

[Illustration: Fig. 4.]

[Illustration: Fig. 5.

A. The porous cell. B. The jar of hydrogen. C. The brass cap and glass
tube D, the end of which dips into the tumbler containing the solution
of indigo E. F F. The wire and stand supporting the porous cell and tube
in tumbler.]

It might be expected that these gases would arrange themselves in our
atmosphere in the above order, and if that were the case, we should have
the carbonic-acid _gas_ (a most poisonous one) at the bottom, and
touching the earth, then the oxygen, and, last of all, the nitrogen;
[Page 6] a state of things in which _organized_ life could not exist.
The gases do not, however, separate: indeed, they seem to act as it were
like _vacuums_ to one another, and "the diffusion of gases" has become a
recognised fact, governed by fixed laws. This fact is curiously
illustrated, as shown in our cut, by filling a bottle with carbonic
acid, and another with hydrogen; and having previously fitted corks to
the bottles, perforated so as to admit a tube, place the bottle
containing the carbonic acid on the table, then take the other full of
hydrogen, keeping the mouth downwards, and fit in the cork and tube:
place this finally into the cork of the carbonic-acid bottle, which may
be a little larger than the other, in order to make the arrangement
stand firmer; and after leaving them for an hour or so, the carbonic
acid, which is twenty-two times heavier than the hydrogen, will ascend
to the latter, whilst the hydrogen will descend to the carbonic acid.
The presence of the carbonic acid in the hydrogen bottle is easily
proved by pouring in a wine-glassful of clear lime-water, which speedily
becomes milky, owing to the production of carbonate of lime; whilst the
proof of the hydrogen being present in the carbonic acid is established
by absorbing the latter with a little cream of lime--_i.e._, slacked
lime mixed to the consistence of cream with some water--and setting fire
to the hydrogen that remains, which burns quietly with a yellowish flame
if unmixed with air; but if air be admitted to the bottle, the mixture
of air and hydrogen inflames rapidly, and with some noise. One of the
most elegant modes of showing the diffusion of gases is by taking a
large round dry porous cell, such as would be employed in a voltaic
battery, and having cemented a brass cap with a glass tube attached to
its open extremity, it may then be supported by a small tripod of iron
[Page 7] wire, and the end of the glass tube placed in a tumbler
containing a small quantity of water coloured blue with sulphate of
indigo. If a tolerably large jar containing hydrogen is now placed over
the porous cell, bubbles of gas make their escape at the end of the
tube, because the hydrogen diffuses itself more rapidly into the porous
cell than the air which it already contains passes out. When the jar is
removed, the reverse occurs, hydrogen diffuses out of the porous cell,
and the blue liquid rises in the tube.

This diffusive force prevents the accumulation of the various noxious
gases on the earth, and spreads them rapidly through the great bulk of
the atmosphere surrounding the globe.

Although air and other gases are invisible, they possess the property of
impenetrability, as may be easily proved by various experiments. Having
opened a pair of common bellows, stop up the nozzle securely, and it is
then impossible to shut them; or, fill a bladder with air by blowing
into it, and tie a string fast round the neck; you then find that you
cannot, without breaking the bladder, press the sides together.

[Illustration: Fig. 6. represents the water overflowing, as the glass,
with the orifice closed, is pressed down, proving the impenetrability of
air.]

[Illustration: Fig. 7. The orange has entered the glass vessel, and the
air having passed from the orifice, no water overflows.]

It is customary to say that a vessel is empty when we have poured out
the water which it contained. Having provided two glass vessels full of
water, place each of them in an empty white pan, to receive the
overflow, then lay an orange upon the surface of the water of one of
them, and being provided with a cylindrical glass, open at one end, with
a hole in the centre of the closed end, place your finger firmly over
the orifice, and endeavour, by inverting the glass over the orange, and
pressing upon the surface of the water, to make it enter the interior of
the glass cylinder; the resistance of the air will now cause the water
to overflow into the white pan, whilst the orange will not enter. The
[Page 8] orange may now be transferred to the other vessel of water, and
on removing the finger from the orifice of the cylindrical glass, and
inverting it as before over the orange, the air will rush out and the
orange and water will enter, whilst there will be no overflow as in the
preceding experiment. The comparison of the two is very striking, and at
once teaches the fact desired.

[Illustration: Fig. 8. Gas-jar with stop-cock closed, and potassium in
ladle; air prevents the entrance of the water.]

[Illustration: Fig. 9. Gas-jar; stop-cock open; the air passes, the
water enters, and the potassium is inflamed.]

Whilst the vessels of water are still in use, another pretty experiment
may be made with the metal potassium. First throw a small piece of the
metal on the surface of the water, to show that it takes fire on contact
with that fluid; then, having provided a gas-jar, fitted with a cap and
stop-cock, and a little spoon screwed into the bottom of the stop-cock
inside the gas-jar, place another piece of potassium in the little
spoon, and, after closing the stop-cock, push the jar into one of the
vessels of water: as before, the impenetrability of the air prevents the
water flowing up to the potassium; but, on opening the stop-cock, the
air escapes, the water rushes up, and directly it touches the potassium,
combustion ensues.

Having sufficiently indicated the nature and meaning of impenetrability,
we may proceed to discuss experimentally three other marked and special
qualities of matter--viz., _inertia_, _gravity_, and _weight_.

[Page 9]

INERTIA, OR PASSIVENESS.

_Inertia_ is a power which (according to Sir Isaac Newton) is implanted
in all matter of resisting any change from a state of rest. It is
sometimes called _vis inertiæ_, and is that property possessed by all
matter, of remaining at rest till set in motion, and _vice versâ_; and
it expresses, in brief terms, resistance to motion or rest.

[Illustration: Fig. 10. Tin tray, with glass bottom, full of water;
candle placed underneath.]

A pendulum clock wound up and ready to go, does not commence its
movements, until the inertia of the pendulum is overcome, and motion
imparted to it. On the other hand, when seated in a carriage, should any
obstruction cause the horse to stop suddenly, it is only perhaps by a
violent effort, if at all, that we can resist the onward movement of our
bodies. To illustrate inertia, construct a metal tray, about three feet
long, two feet wide, and two inches deep, with a glass bottom, and
arrange it on a framework supported by legs, like a table, and having
filled it with water, let the room be darkened, and then place under the
tank a lighted candle, at a sufficient distance from the glass to
prevent the heat cracking it. If a piece of calico or paper, stretched
on a framework, be now held over the water at an angle of about thirty
degrees, all that occurs on the surface of the water will be rendered
visible on such screen. Attention may now be directed to the quiescence,
or the inertia of the water, while the opposite condition of movement
and formation of the waves may be beautifully shown by touching the
surface of the water with the finger; the miniature waves being depicted
on the screen, and continuing their motion till set at rest by striking
against the sides of the tin tray.

[Illustration: Fig. 11. Same tray, with calico screen; showing the waves
as they are produced by touching the surface of the water with the
finger.]

[Page 10]

Should the above experiment be thought too troublesome or expensive to
prepare, inertia may be demonstrated by filling a tea-cup or other
convenient vessel with water, and after moving rapidly with it in any
direction, if we stop suddenly, the rigidity of all parts of the cup we
hold brings them simultaneously to a state of rest; but the mobility of
the liquid particles allows of their continuing in motion in their
original direction, and the liquid is spilled. Thus, carelessness in
handing and spilling a cup of tea (though not to be recommended) serves
to illustrate an important principle. The inertia of bodies in motion is
further and lamentably illustrated by the accidents caused from the
sudden stoppage of a railway train whilst in rapid motion, when heads
and knees come in contact with frightful results.--It is more especially
demonstrated by the earth, the moon, and the other planets continuing
their motion for ever in the absence of any friction or resistance to
oppose their onward progress. It is the friction arising from the
roughness of the ground, the resistance of the air, and the force of the
earth's attraction, which puts a stop to bodies set in motion about the
surface of the earth.

[Page 11]

GRAVITATION.

_Inertia_ represents a passive force, _gravitation_, an active condition
of matter; and this latter may truly be termed a force of attraction,
because it acts between masses at sensible or insensible distances: it
is illustrated by a stone, unsupported, falling to the ground; by the
stone pressing with force on the earth, and requiring power to raise it
from the ground: indeed, it is commonly reported that it was by an
accident--"an apple falling from a tree"--that the great Newton was led
to reflect on the universal law of gravitation, and to pronounce upon it
in the following memorable words:--

"_Every particle of matter in the universe attracts every other particle
of matter with a force or power directly proportional to the quantity of
matter in each, and decreasing as the squares of the distances which
separate the particles increase._"

These words may appear very obscure to our juvenile readers; but when
dissected and examined properly, they clearly define the property of
gravitation. For instance, "every particle attracts every other with a
force proportional to the quantity of matter in each." This statement
was verified some years back by Maskelyne, who, having sought out and
discovered a steep, precipitous rock in the Schichallion mountains, in
Scotland, suspended from it a metal weight by a cord, and going to a
convenient distance with a telescope, and observing the weight, he found
that it did not hang perpendicularly, like an ordinary plumb line, but
was attracted, or impelled, to the sides of the rock by some kind of
attraction, which, of course, could be no other than that indicated by
Newton as the attraction of gravitation.

[Illustration: Fig. 12. The Schichallion Rocks. The dotted line and
weight A represent the ordinary position of a plumb line, whilst the
line of the weight B indicates (of course, with some exaggeration) the
attractive power of the mass of the rock drawing it from the
perpendicular.]

This truly wonderful power of attraction pervades all masses; and being,
as before stated, proportional to the quantity of matter, if a man could
be transported to the surface of the sun, he would become about thirty
times heavier: he would be attracted, or impelled, to the sun with
thirty times more gravitating force than on the surface of the earth,
and would weigh about two tons. Of course, nursing a baby on the sun's
surface would be a very serious affair with our ordinary strength;
whilst on some of the smaller planets, such as Ceres and Pallas, we
should probably gravitate with a force of a few pounds only, and with
the same muscular power now possessed, we should quite emulate the
exploits of those domestic little creatures sometimes called "the
industrious fleas," and our jumping would be something marvellous.

[Page 12]

There is no very good lecture-table experiment that will
illustrate gravitation, although attention may be directed to the fact
of a piece of potassium thrown on the surface of water in a plate
generally rushing to the sides, and, as if attracted, attaching itself
with great force to the substance of the pottery or porcelain; or, if a
model ship, or lump of wood, be allowed to float at rest in a large tank
of water, and a number of light chips of wood or bits of straw be thrown
in, they generally collect and remain around the larger floating mass.

[Illustration: Fig. 13.

A. The centre ball, representing the earth's centre of gravity.

W W W W. Four wires fixed into centre ball, and passing through and
secured in the hoop, projecting about one foot from the circumference.

B B B B. Two balls--a model ship and toy--working on the wires like
beads, with vulcanized India-rubber straps attached to them and the
circumference of the hoop.]

A very good idea, however, may be afforded of the universal action of
gravity maintaining all things in their natural position on the earth
by [Page 13] taking a hoop and arranging in and upon it balls, or a
model ship, or other toy, and wires, as depicted in our diagram.

With this simple apparatus we may illustrate the upward, downward, and
sideway movement of bodies from the earth, and the counteraction by the
force of gravitation of any tendency of matter to fall away from the
globe, which is represented in the model by the india-rubber springs
pulling the balls and toys back again to the circumference of the hoop.

The attraction of gravitation decreases (quoting the remainder of
Newton's definition) as the squares of the distances which separate the
particles increase--_i.e._, it obeys the principle called "inverse
proportion"--viz., the greater the distance, the less gravitating power;
the less the distance, the greater the power of gravitation. Gravitation
is like the distribution of light and other radiant forces, and may be
thus illustrated.

[Illustration: Fig. 14. Place a lighted candle, marked A, at a certain
distance from No. 1, a board one foot square; at double the distance the
latter will shadow another board, No. 2, four feet square; at three
times, No. 3, nine feet square; at four, No. 4, sixteen feet; and so
on.]

To make the comparison between the propagation of light and the
attraction of gravitation, we have only to imagine the candle, _a_, to
represent the point where the force of gravity exists in the highest
degree of intensity; suppose it to be the sun--the great centre of this
power in our planetary system. A body, as at No. 1, at any given
distance will be attracted (like iron-filings to a magnet) with a
certain force; at twice the distance, the square of two being four, and
by inverse proportion, the attraction will be four times less; at thrice
the distance, nine times less; at the fourth distance, sixteen times
less; and so on. With the assistance of this law, we may calculate,
roughly, the depth of a well, or a precipice, or a column, by
ascertaining the time occupied in the fall of a stone or other heavy
substance. A falling body descends about 16 feet in one second, 64 feet
in two seconds, 144 feet in three seconds, 256 feet in four seconds, 400
feet in five seconds, 576 feet in six seconds; the spaces passed over
being as the squares of the times.

Suppose a stone takes three seconds in falling to the surface of the
water in a well, then 3 × 3 = 9 × 16 = 144 feet would be a rough
estimate of the depth. The calculation will exceed the truth in
consequence of the stone being retarded in its passage by the resistance
of the air.

[Page 14]

All bodies gravitate equally to the earth: for instance, if an open box,
say one foot in length, two inches broad, and two inches deep, be
provided with a nicely-fitted bottom, attached by a hinge, a number of
substances, such as wood, cork, marble, iron, lead, copper, may be
arranged in a row; and directly the hand is withdrawn, the moveable flap
flies open, and if the manipulation with the disengagement of the
trap-door is good, the whole of the substances are seen to proceed to
the earth in a straight line, as shown in our drawing.

[Illustration: Fig. 15.]

[Illustration: Fig. 16.]

If a heavy substance, like gold, be greatly extended by hammering and
beating into thin leaves, and then dropped from the hand, the resistance
of the air becomes very apparent; and a gold coin and a piece of
gold-leaf would not reach the earth at the same time if allowed to fall
from any given height. This fact is easily displayed by the assistance
of a long glass cylindrical vessel placed on the air-pump, with suitable
apparatus arranged with little stages to carry the different substances;
upon two of them may be placed a feather and a gold coin, and on the
third, another gold coin and a piece of gold-leaf.

In arranging the experiment, great care ought to be taken that the
little stages are all nicely cleaned, and free from any oil, grease, or
other matter which might cause the feathers or the gold-leaf to cling to
the stages when they are disengaged, by moving the brass stop round that
works in the collar of leathers. Sometimes these leathers are oiled,
[Page 15] and in that case, when the vacuum is made, the oil, by the
pressure, is squeezed out, and, passing down, may reach the stages and
spoil the experiment, by causing the feathers and gold-leaf to stick to
the brass, producing great disappointment, as the illustration, usually
called the "_guinea and feather glass experiment_" takes some time to
prepare. The air-pump being in good order, the long glass is first
greased on the lower welt or edge, and then placed firmly on the
air-pump plate. The top edge, or welt, may now be greased, and the gold
coins, feathers, and gold-leaf arranged in the drop-apparatus; this is
carefully placed on the top of the glass, and firmly squeezed down. The
author has always found a tallow candle, rolled in a sheet of paper (so
as to leave about half the candle exposed), the best grease to smear the
glass with for air-pump experiments; if the weather is cold, the candle
may be placed for a few minutes before an ordinary fire to soften the
tallow. Pomatum answers perfectly well when the surfaces of glass and
brass are all nicely ground; but as air-pumps and glasses by use get
scratched and rubbed, the tallow seems to fill up better all ordinary
channels by which air may enter to spoil a vacuum.

[Illustration: Fig. 17.]

[Illustration: Fig. 18.]

The apparatus being now arranged, the air is pumped out; and here,
again, care must be taken not to shake the gold off the stages. When a
proper vacuum has been obtained, which will be shown by the pump-gauge,
the stop is withdrawn from one of the stages, and the gold and feather
are seen to fall simultaneously to the air-pump plate. Another stage,
with the gold-leaf and coin, may now be detached; both showing
distinctly, that when the resistance of the air is withdrawn, all
bodies, whether called _light_ or _heavy_, gravitate equally to the
earth. Then, the screw at the bottom of the pump barrels [Page 16]
being opened, attention may be directed to the whizzing noise the air
makes on entering the vacuum, and when the air is once more restored to
the long glass vessel, the last stage may be allowed to fall; and now,
the gold coin reaches the pump-plate first, and the feather, lingering
behind, loses (as it were) the race, and touches the plate after the
gold coin; thus demonstrating clearly the resistance of the air to
falling bodies.

Another, and perhaps less troublesome, mode of showing the same fact, is
to use a long glass tube closed at each end with brass caps cemented on.
One cap should have the largest possible aperture closed by a brass
screw, and the other may fit a small hand-pump.

If a piece of gold and a small feather are placed in the tube, it may be
shown that the former reaches the bottom of the tube first, whilst it is
full of air, and when the air is withdrawn by means of the pump, and the
tube again inverted, both the gold and the feather fall in the same
time.

[Illustration: Fig. 19. A B. Glass tube containing a piece of gold and a
feather, which are placed in at the large aperture A. C. Small
hand-pump.]

For this reason, all attempts to measure heights or depths by observing
the time occupied by a falling body in reaching the earth must be
incorrect, and can only be rough approximations. An experiment tried at
St. Paul's Cathedral, with a stone, which was allowed to fall from the
cupola, indicated the time occupied in the descent to be four and a half
seconds: now, if we square this time, and multiply by 16, a height of
324 feet is denoted; whereas the actual height is only 272 feet, and the
difference of 52 feet shows how the stone was retarded in its passage
through the air; for, had there been no obstacle, it would have reached
the ground in 4-3/20ths seconds.

[Illustration: Fig. 20.]

The force of gravitation is further demonstrated by the action of the
sun and moon raising the waters of the ocean, and producing the tides;
and also by the earth and moon, and other planets and satellites, being
prevented from flying from their natural paths or orbits around the sun.
It is also very clearly proved that there must be some kind of
attractive force resident in the earth, or else all moveable things, the
water, the air, the living and dead matters, would fly away from the
surface of the earth in obedience to what is called "centrifugal force."
Our earth is twenty-four hours in performing one rotation on its axis,
which is an imaginary line drawn from pole to pole, and represented by
the _wire_ round which we cause a sphere to rotate. All objects,
therefore, on the earth are moving with the planet at an enormous
velocity; and this movement is called the earth's diurnal, or daily
rotation. Now, [Page 17] it will be remembered, that mud or other fluid
matter flies off, and is not retained by the circumference of a wheel in
motion: when a mop is trundled, or a dog or sheep, after exposure to
rain, shake themselves, the water is thrown off by what is called
centrifugal force (_centrum_, a centre, _fugio_, to fly from).




CHAPTER II.

CENTRIFUGAL FORCE.


That power which drives a revolving body from a centre, and it may be
illustrated by turning a closed parasol, or umbrella, rapidly round on
its centre, the stick being the axis--the ribs fly out, and if there is
much friction in the parts, the illustration is more certain by
attaching a bullet to the end of each rib, as shown in our drawing.

[Illustration: Fig. 21.]

[Illustration: Fig. 22.]

The same fact may be illustrated by a square mahogany rod, say one inch
square and three feet long, with two flaps eighteen inches in length,
hanging by hinges, and parallel to the sides of the centre rod, which
immediately fly out on the rotation of the long centre piece.

The toy called the centrifugal railway is also a very pretty
illustration of the same fact. A glass of water, or a coin, may be
placed in the little carriage, and although it must be twice hanging
perpendicular in a line with the earth, the carriage does not tumble
away from its appointed track, and the centrifugal force binds it firmly
to the interior of the circle round which it revolves.

[Page 18]

[Illustration: Fig. 23.]

Another striking and very simple illustration is to suspend a
hemispherical cup by three cords, and having twisted them, by turning
round the cup, it may be filled with water, and directly the hand is
withdrawn, the torsion of the cord causes the cup to rotate, and the
water describes a circle on the floor, flying off at a tangent from the
cup, as may be noticed in the accompanying cut.

[Illustration: Fig. 24.]

A hoop when trundled would tumble on its side if the force of
gravitation was not overcome by the centrifugal force which imparts to
it a motion in the direction of a tangent (_tango_, to touch) to a
circle. The same principle applies to the spinning-top--this toy cannot
be made to stand upon its point until set in rapid motion.

Returning again to the subject of gravitation, we may now consider it in
relation to other and more magnificent examples which we discover by
studying the science of astronomy.

[Page 19]




CHAPTER III.

THE SCIENCE OF ASTRONOMY.


In a work of this kind, professedly devoted to a very brief and popular
view of the different scientific subjects, much cannot be said on any
special branch of science; it will be better, therefore, to take up one
subject in astronomy, and by discussing it in a simple manner, our young
friends may be stimulated to learn more of those glorious truths which
are to be found in the published works of many eminent astronomers, and
especially in that of Mr. Hind, called "The Illustrated London
Astronomy." One of the most interesting subjects is the phenomenon of
the eclipse of the sun; and as 1858 is likely to be long remembered for
its "annular eclipse," we shall devote some pages and illustrations to
this subject.

Eclipses of the sun are of three kinds--partial, annular, and total.
Many persons have probably seen large partial eclipses of the sun, and
may possibly suppose that a total eclipse is merely an intensified form
of a partial one; but astronomers assert that no degree of partial
eclipse, even when the very smallest portion of the sun remains visible,
gives the slightest idea of a total one, either in the solemnity and
overpowering influence of the spectacle, or the curious appearances
which accompany it.

The late Mr. Baily said of an eclipse (usually called that of Thales),
which caused the suspension of a battle between the Lydians and Medes,
that only a total eclipse could have produced the effect ascribed to it.
Even educated astronomers, when viewing with the naked eye the sun
nearly obscured by the moon in an annular eclipse, could not tell that
_any part_ of the sun was hidden, and this was remarkably verified in
the annular eclipse of the 15th March of this year.

During the continuance of a total eclipse of the sun, we are permitted a
hasty glance at some of those secrets of Nature which are not revealed
at any other time--glories that hold in tremulous amazement even veteran
explorers of the heavens and its starry worlds.

The general meaning of an eclipse may be shown very nicely by lighting a
common oil, or oxy-hydrogen lantern in a darkened room, and throwing the
rays which proceed from it on a three-feet globe. The lantern may be
called the sun, and, of course, it is understood that correct
comparative sizes are not attempted in this arrangement; if it were so,
the globe representing the earth would have to be a mere speck, for if
we make the model of the sun in proportion to a three-feet globe, no
ordinary lecture hall would contain it. This being premised, attention
is directed to the lantern, which, like the sun, is self-luminous, and
is giving out its own rays; these fall upon the globe we have designated
the earth, and illuminate one-half, whilst the other is shrouded in
darkness, reminding us of the opacity of the earth, and teaching, in a
familiar [Page 20] manner, the causes of day and night. Another globe,
say six inches in diameter, and supported by a string, may be compared
to the moon, and, like the earth, is now luminous, and shines only by
borrowed light: the moon is simply a reflector of light; like a sheet of
white cardboard, or a metallic mirror. When, therefore, the small globe
is passed between the lantern and the large globe, a shadow is cast on
the large globe: it is also seen that only the half of the small globe
turned towards the lantern is illuminated, while the other half,
opposite the large globe, is in shadow or darkness. And here we
understand why the moon appears to be black while passing before the
sun; so also by moving the small globe about in various curves, it is
shown why eclipses are only visible at certain parts of the earth's
surface; and as it would take (roughly speaking) fifty globes as large
as the moon to make one equal in size to our earth, the shadow it casts
must necessarily be small, and cannot obscure the whole hemisphere of
the earth turned towards it. An eclipse of the sun is, therefore, caused
by the opaque mass of moon passing between the sun and the earth. Whilst
an eclipse of the moon is caused by the earth moving directly between
the sun and the moon: the large shadow cast by the earth renders a total
eclipse of the moon visible to a greater number of spectators on that
half of the earth turned towards the moon. All these facts can be
clearly demonstrated with the arrangement already described, of which we
give the following pictorial illustration:--

[Illustration: Fig. 25.]

In using this apparatus, it should be explained that if the moon were as
large as the sun, the shadow would be cylindrical like the figure 1, and
of an unlimited length. If she were of greater magnitude, it would
precisely resemble the shadow cast in the experiment already adduced
with the lantern and shown at No. 2. But being so very much smaller than
the sun, the moon projects a shadow which converges to a point as shown
in the third diagram.

[Page 21]

[Illustration: Fig. 26.]

[Illustration: Fig. 27.]

[Illustration: Fig. 28.]

In order to comprehend the difference between an annular and a total
eclipse of the sun, it is necessary to mention the apparent sizes of the
sun and moon: thus, the former is a very large body--viz., eight hundred
and eighty-seven thousand miles in diameter; but then, the sun is a very
long way off from the earth, and is ninety millions of miles distant
from us; therefore, he does not appear to be very large: indeed, the sun
seems to be about the same size as the moon; for, although the sun's
diameter is (roughly speaking) four hundred times greater than that of
the moon, he is four hundred times further away from us, and,
consequently, the sun and moon _appear_ to be the same size, and when
they come in a straight line with the eye, the nearer and smaller body,
the moon, covers the larger and more distant mass, the sun; and hence,
we have either an annular, or a total eclipse, showing how a small body
may come between the eye and a larger body, and either partially or
completely obscure it.

[Page 22]

With respect to an annular eclipse, it must be remembered, that the
paths of all bodies revolving round others are elliptical; _i.e._, they
take place in the form of an ellipse, which is a figure easily
demonstrated; and is, in fact, one of the conic sections.

If a slice be taken off a cone, parallel with the base, we have a circle
thus--

[Illustration: Fig. 29.]

If it be cut obliquely, or slanting, we see at once the figure spoken
of, and have the ellipse as shown in this picture.

[Illustration: Fig. 30.]

Now, the ellipse has two points within it, called "the foci," and these
are easily indicated by drawing an ellipse on a diagram-board, in which
two nails have been placed in a straight line, and about twelve inches
apart. Having tied a string so as to make a loop, or endless cord, a
circle may first be drawn by putting the cord round one of the nails,
and holding a piece of chalk in the loop of the string, it may be
extended to its full distance, and a circle described; here a figure is
produced round one point, and to show the difference between a circle
and an ellipse, the endless cord is now placed on the two nails, and the
chalk being carried round inside the string, no longer produces the
circle, but that familiar form called the oval. As a gardener would say,
an oval has been struck; and the two points round which it has been
described, [Page 23] are called the _foci_. This explanation enables us
to understand the next diagram, showing the motion of the earth round
the sun; the latter being placed in one of the foci of a very moderate
ellipse, and the various points of the earth's orbit designated by the
little round globes marked A, B, C, D, where it is evident that the
earth is nearer to the sun at B than at D. In this diagram the ellipse
is exaggerated, as it ought, in fact, to be very nearly a circle.

[Illustration: Fig. 31.]

[Illustration: Fig. 32.]

We are about three millions of miles nearer to the sun in the winter
than we are in the summer; but from the more oblique or slanting
direction of the rays of the sun during the winter season, we do not
derive any increased heat from the greater proximity. The sun,
therefore, apparently varies in size; but this seeming difference is so
trifling that it is of no importance in the discussion: and here we may
ask, why does [Page 24] the earth move round the sun? Because it is
impelled by _two forces_, one of which has already been fully explained,
and is called the _centrifugal_ power, and the other, although termed
the _centripetal_ force, is only another name for the "attraction of
gravitation."

[Illustration: Fig. 33.]

[Illustration: Fig. 34.]

To show their mutual relations, let us suppose that, at the creation of
the universe, the earth, marked A, was hurled from the hand of its
Maker; according to the law of inertia, it would continue in a straight
line, A C, for ever through space, provided it met with no resistance or
obstruction. Let us now suppose the earth to have arrived at the point
B, and to come within the sphere of the attraction of the sun S; [Page
25] here we have at once contending forces acting at right angles to
each other; either the earth must continue in its original direction, A
C, or fall gradually to the sun. But, mark the beauty and harmony of the
arrangement: like a billiard-ball, struck with equal force at two points
at right angles to each other, it takes the mean between the two, or
what is termed the diagonal of the parallelogram (as shown in our
drawing of a billiard-table), and passes in the direction of the curved
line, B D; having reached D, it is again ready to fly off at a tangent;
the centrifugal force would carry it to E, but again the gravitating
force controls the centripetal, and the earth pursues its elliptical
path, or orbit, till the Almighty Author who bade it move shall please
to reverse the command.

[Illustration: Fig. 35.]

The mutual relations of the centripetal and centrifugal forces may be
illustrated by suspending a tin cylindrical vessel by two strings, and
having filled it with water, the vessel may be swung round without
spilling a single drop; of course, the movement must be commenced
carefully, by making it oscillate like a pendulum.

[Illustration: Fig. 36.]

The cord which binds it to the finger may be compared to the centripetal
force, whilst the centrifugal power is illustrated by the water pressing
against the sides and remaining in the vessel. Upon the like principles
the moon revolves about the earth, but her orbit is more elliptical than
that of the earth around the sun; and it is evident from our diagram
that the moon is much further from the earth at A than at B. As a
natural consequence, the moon appears sometimes a little larger and
sometimes smaller than the sun; the apparent mean diameter of the latter
being thirty-two minutes, whilst the moon's apparent diameter varies
from twenty-nine and a half to thirty-three and a half minutes. Now, if
the moon passes exactly between us and the sun when she is apparently
largest, then a total eclipse takes place; whereas, if she glides
between the sun and ourselves when smallest--_i.e._, when furthest off
from the earth--then she is not sufficiently [Page 26] large to cover
the sun entirely, but a ring of sunlight remains visible around her, and
what is called an annular eclipse of the sun occurs. This fact may be
shown in an effective manner by placing the oxy-hydrogen lantern before
a sheet, or other white surface, and throwing a bright circle of light
upon it, which may be called the sun; then, if a round disc of wood be
passed between the lantern and the sheet, at a certain distance from the
nozzle of the lantern, all the light is cut off, the circle of light is
no longer apparent, and we have a resemblance to a total eclipse.

[Illustration: Fig. 37.]

By taking the round disc of wood further from the lantern, and repeating
the experiment, it will be found that the whole circle of light is not
obscured, but a ring of light appears around the dark centre,
corresponding with the phenomenon called the annular (ring-shaped)
eclipse.

If a bullet be placed very near to one eye whilst the other remains
closed, a large target may be wholly shut out from vision; but if the
bullet be adjusted at a greater distance from the eye, then the centre
only will be obscured, and the outer edge or ring of the target remains
visible.

When the advancing edge, or first _limb_, as it is termed, of the moon
approaches very near to the second limb of the sun, the two are joined
together for a time by alternations of black and white points, called
Baily's beads.

This phenomenon is supposed to be caused partly by the uneven and
mountainous edge of the moon, and partly by that inevitable fault of
telescopes, and of the nervous system of the eye, which tends to enlarge
the images of luminous objects, producing what is called irradiation. It
is exceedingly interesting to know that, although the clouds obscured
the annular eclipse of 1858, in many parts of England, we are yet [Page
27] left the recorded observations of one fortunate astronomer, Mr. John
Yeats, who states that--

"All the phenomena of an annular eclipse were clearly and beautifully
visible on the Fotheringay-Castle-mound, which is a locality easily
identified. Baily's beads were perfectly plain on the completion of the
_annulus_, which occurrence took place, according to my observation, at
about seventy seconds after 1 o'clock; it lasted about eighty seconds.
The 'beads,' like drops of water, appeared on the upper and under sides
of the moon, occupying fully three-fourths of her circumference.

"Prior to this, the upper edge of the moon seemed dark and rough, and
there were no other changes of colour. At 12.43, the cusps, for a few
moments, bore a very black aspect.

"There was nothing like intense darkness during the eclipse, and less
gloom than during a thunderstorm. Bystanders prognosticated rain; but it
was the shadow of a rapidly-declining day. At 12 o'clock, a lady living
on the farm suddenly exclaimed, 'The cows are coming home to be milked!'
and they came, all but one; that followed, however, within the hour.
Cocks crowed, birds flew low or fluttered about uneasily, but every
object far and near was well defined to the eye.

"A singular broadway of light stretched north and south for upwards of a
quarter of an hour; from about 12.54 to 1.10 P.M."

[Illustration: Fig. 38.]

[Illustration: Fig. 39.]

If the annular eclipse of the sun be a matter for wonderment, the total
eclipse of the same is much more surprising; no other expression than
that of _awfully grand_, can give an idea of the effects of totality,
and of the suddenness with which it obscures the light of heaven. The
darkness, it is said, comes dropping down like a mantle, and as the
moment of full obscuration approaches, people's countenances become
livid, the horizon is indistinct and sometimes invisible, and there is a
general appearance of horror on all sides. These are not simply the
inventions [Page 28] of active human imaginations, for they produce
equal, if not greater effects, upon the brute creation. M. Arago quotes
an instance of a half-starved dog, who was voraciously devouring some
food, but dropped it the instant the darkness came on. A swarm of ants,
busily engaged, stopped when the darkness commenced, and remained
motionless till the light reappeared. A herd of oxen collected
themselves into a circle and stood still, with their horns outward, as
if to resist a common enemy; certain plants, such as the convolvulus and
silk-tree acacia, closed their leaves. The latter statement was
corroborated during the annular eclipse of the 15th of March, 1858, by
Mr. E. S. Lane, who states, that crocuses at the Observatory, Beeston,
had their blossoms expanded before the eclipse; they commenced closing,
and were quite shut at about one minute previous to the greatest
darkness; and the flowers opened partially about twenty minutes
afterwards. A "_total eclipse_" of the sun has always impressed the
human mind with terror and wonder in every age: it was always supposed
to be the forerunner of evil; and not only is the mind powerfully
impressed, as darkness gradually shuts out the face of the sun, but at
the moment of totality, a magnificent corona, or glory of light, is
visible, and prominences, or flames, as they are often termed, make
their appearance at different points round the circle of the dark mass.
This glory does not flash suddenly on the eye; but commencing at the
first limb of the sun, passes quickly from one limb to the other. Our
illustration shows "the corona" and the "rose-coloured prominences,"
whose nature we shall next endeavour to explain. Professor Airy
describes the change from the last narrow crescent of light to the
entire dark moon, surrounded by a ring of faint light, as most curious,
striking, and magical in effect. The progress of the formation of the
corona was seen distinctly. [Page 29] It commenced on the side of the moon
opposite to that at which the sun disappeared, and in the general decay
and disease which seemed to oppress all nature, the moon and the corona
appeared almost like a local sore in that part of the sky, and in some
places were seen double. Its texture appeared as if fibrous, or composed
of entangled threads; in other places brushes, or feathers of light
proceeded from it, and one estimate calculated the light at about
one-seventh part of a full moon light. The question, whether the corona
is concentric with the sun and moon, was specially mooted by M. Arago,
and Professor Baden Powell has produced such excellent imitations of the
"corona" by making opaque bodies occult, or conceal, very bright points,
that it cannot be considered as material or real, although it ought to
be remembered that the best theory of the zodiacal light represents it
to be a nebulous mass, increasing in density towards the sun, and yet no
portion of this nebulous mass was seen during the totality. But by far
the most remarkable of all the appearances connected with a "total
eclipse" are the rose-coloured prominences, mountains, or flames,
projecting from the circumference of the moon to the inner ring of the
corona; and, although they had been observed by Vaserius (a Swedish
astronomer) in 1733, they took the modern astronomers entirely by
surprise in 1842, and they were not prepared with instruments to
ascertain the nature of these strange and almost portentous forms. In
1851, however, great preparations were made to throw further light on
the subject. Professor Airy went to make his observations, and he says,
"That the suddenness of the darkness in 1851 appeared much more striking
than in 1842, and the forms of the rose-coloured mountains were most
curious. One reminded him of a boomerang (that curious weapon thrown so
skilfully by the aborigines of Australia); this same figure has been
spoken of by others as resembling a Turkish scimitar, strongly coloured
with rose-red at the borders, but paler in the centre. Another form was
a pale-white semicircle based on the moon's limbs; a third figure was a
red detached cloud, or balloon, of nearly circular form, separated from
the moon by nearly its own breadth; a fourth appeared like a small
triangle, or conical red mountain, perhaps a little white in the
interior;" and the Professor proceeds to say, "I employed myself in an
attempt to draw roughly the figures, and it was impossible, after
witnessing the increase in height of some, and the disappearance of
another, and the arrival of new forms, not to feel convinced that the
phenomena belonged to the sun, and _not_ to the moon."

Still the question remains unanswered, what are these "rose-coloured
prominences?" If they belong to the sun, and are mountains in that
luminary, they must be some thirty or forty thousand miles in height.

M. Faye has formally propounded the theory, that they are caused by
refraction, or a kind of mirage, or the distortion of objects caused by
heated air. This phenomenon is not peculiar to any country, though most
frequently observed near the margin of lakes and rivers, and on hot
sandy plains. M. Monge, who accompanied Buonaparte in his [Page 30]
expedition to Egypt, witnessed a remarkable example between Alexandria
and Cairo, where, in all directions, green islands appeared surrounded
by extensive lakes of pure, transparent water. M. Monge states that
"Nothing could be conceived more lovely or picturesque than the
landscape. In the tranquil surface of the lake, the trees and houses
with which the islands are covered were strongly reflected with vivid
and varied hues, and the party hastened forward to enjoy the refreshment
apparently proffered them; but when they arrived, the lake, on whose
bosom the images had floated--the trees, amongst whose foliage they
arose, and the people who stood on the shore, as if inviting their
approach, had all vanished, and nothing remained but the uniform and
irksome desert of sand and sky, with a few naked and ragged Arabs."

If M. Monge and his party had not been undeceived, by actually going to
the spot, they would, one and all, have been firmly convinced that these
visionary trees, lakes, and buildings had a real existence. This kind of
mirage is known in Persia and Arabia by the name of "serab" or
miraculous water, and in the western districts of India by that of
"scheram." This illusion is the effect of unusual refraction, and M.
Faye attempts to account for the rose-coloured mountains by something of
a similar nature.

It is right, however, to mention, that learned astronomers do not
consider this theory of any value.

Lieutenant Patterson, one of the observers of the eclipse of 1851, says,
that "It is very remarkable that the flames or prominences correspond
exactly (at least as far as he could judge) with the spots on the sun's
surface." Taking this statement with that of M. Faye, it may be assumed,
as a new idea, and nothing more, that these prominences are, after all,
mere aerial pictures of these openings in the sun's atmosphere, or what
are called "sun spots." In the "Edinburgh Philosophical Journal," it is
said, that although it has lately been shown in the Edinburgh
Observatory that it is possible to produce, by certain optical
experiments, red flames on the sun's limb of precisely the rose-coloured
tint described, yet, on weighing the whole of the evidence, there does
seem a great preponderance in favour of the eclipse flames being real
appendages of the sun, and in that case they must be masses of such vast
size as to play no unimportant part in the economy of that stupendous
orb.

During the last eclipse great disappointment was felt that the darkness
was so insignificant, although, when we consider the enormous
light-giving power of the sun, and know that it was not wholly obscured,
we could hardly have expected any other result. There can be no doubt
that a decided change in the amount of light is only to be observed
during a total eclipse of the sun, one of which occurred on the 7th of
September, 1858; but, unfortunately, it was only visible in South
America; we must therefore content ourselves with the descriptions of
those astronomers who can be fully relied on. From the graphic account
given by Professor Piazzi Smyth, the astronomer-royal for Scotland,
[Page 31] of a total eclipse as seen by him on the western coast of
Norway, we may form some notion of the imposing appearance of the
surrounding country when obscured during the occurrence of this rare
astronomical phenomenon.

The Professor remarks, "To understand the scene more fully, the reader
must fancy himself on a small, rocky island on a mountainous coast, the
weather calm, and the sky at the beginning of the eclipse seven-tenths
covered with thin and bright cirro-strati clouds. As the eclipse
approaches, the clouds gradually darken, the rays of the sun are no
longer able to penetrate them through and through, and drench them with
living light as before, but they become darker than the sky against
which they are seen. The air becomes sensibly colder, the clouds still
darker, and the whole atmosphere murkier.

"From moment to moment as the totality approaches, the cold and darkness
advance apace; and there is something peculiarly and terribly convincing
in the two different senses, so entirely coinciding in their indications
of an unprecedented fact being in course of accomplishment. Suddenly,
and apparently without any warning (so immensely greater were its
effects than those of anything else which had occurred), the totality
supervenes, and darkness _comes down_. Then came into view lurid lights
and forms, as on the extinction of candles. This was the most striking
point of the whole phenomenon, and made the Norse peasants about us flee
with precipitation, and hide themselves for their lives.

"Darkness reigned everywhere in heaven and earth, except where, along
the north-eastern horizon, a narrow strip of unclouded sky presented a
low burning tone of colour, and where some distant snow-covered
mountains, beyond the range of the moon's shadow, reflected the faint
mono-chromatic light of the partially eclipsed sun, and exhibited all
the detail of their structure, all the light, and shade, and markings of
their precipitous sides with an apparently supernatural distinctness.
After a little time, the eyes seemed to get accustomed to the darkness,
and the looming forms of objects close by could be discerned, all of
them exhibiting a dull-green hue; seeming to have exhaled their natural
colour, and to have taken this particular one, merely by force of the
red colour in the north.

"Life and animation seemed, indeed, to have now departed from everything
around, and we could hardly but fear, against our reason, that if such a
state of things was to last much longer, some dreadful calamity must
happen to us all; while the lurid horizon, northward, appeared so like
the gleams of departing light in some of the grandest paintings by Danby
and Martin, that we could not but believe, in spite of the alleged
extravagances of these artists, that Nature had opened up to the
constant contemplation of their mind's-eye some of those magnificent
revelations of power and glory which others can only get a glimpse of on
occasions such as these."

It can be easily imagined, that under such peculiar and awful
circumstances, the careful observation of these effects must be somewhat
difficult, [Page 32] and the only wonder is that the astronomical
observations are conducted with any certainty at all.

In the eclipse of 1842, it was not only the vivacious Frenchman who was
carried away in the impulse of the moment, and had afterwards to plead
that "_he was no more than a man_" as an excuse for his unfulfilled part
in the observations, but the same was the case with the grave Englishman
and the more stolid German. In 1851, much the same failure in the
observations occurred; and on some person asking a worthy American, who
had come with his instruments from the other side of the world expressly
to observe the eclipse, what he had succeeded in doing? he merely
answered, with much quiet impressiveness, "_That if it was to be
observed over again, he hoped he would be able to do something, but
that, as it was, he had done nothing: it had been too much for him._"
This is not quite so bad as the fashionable lady who had been invited to
look at an eclipse of the sun through a grand telescope, but arriving
too late, inquired whether "it could not be shown _over again_."

With this brief glance at the science of astronomy, we once more return
to the term "gravity," which will introduce to us some new and
interesting facts, under the head of what is called "centre of gravity."




CHAPTER IV.

CENTRE OF GRAVITY.


_That point about which all the parts of a body do, in any situation,
exactly balance each other._

The discovery of this fact is due to Archimedes, and it is a point in
every solid body (whatever the form may be) in which the _forces_ of
_gravity_ may be considered as _united_. In our globe, which is a
sphere, or rather an oblate spheroid, the centre of gravity will be the
centre. Thus, if a plummet be suspended on the surface of the earth, it
points directly to the centre of gravity, and, consequently, two
plummet-lines suspended side by side cannot, strictly speaking, be
parallel to each other.

If it were possible to bore or dig a gallery through the whole substance
of the earth from pole to pole, and then to allow a stone or the fabled
Mahomet's coffin to fall through it, the momentum--_i.e._, the force of
the moving body, would carry it beyond the centre of gravity. This
force, however, being exhausted, there would be a retrograde movement,
and after many oscillations it would gradually come to rest, and then,
unsupported by anything material, it would be suspended by the force of
gravitation, and now enter into and take part in the general attracting
force; and being equally attracted on every side, the stone or coffin
must be totally without weight. _Momentum_ is prettily illustrated by a
series of inclined planes [Page 33] cut in mahogany, with a grooved
channel at the top, in imitation of the famous Russian ice mountains:
and if a marble is allowed to run down the [Page 34] first incline, the
momentum will carry it up the second, from which it will again descend
and pass up and down the third and last miniature mountain.

[Illustration: Fig. 40. F. The centre. A B C D E. Plummet-lines, all
pointing to the centre, and therefore diverging from each other.]

[Illustration: Fig. 41. P P P. Inclined planes, gradually decreasing in
height, cut out of inch mahogany, with a groove at the top to carry an
ordinary marble. B B B. Different positions of the marble, which starts
from B A.]

In a sphere of uniform density, the centre of gravity is easily
discovered, but not so in an irregular _mass_; and here, perhaps, an
explanation of terms may not be altogether unacceptable.

_Mass_, is a term applied to solids, such as a mass of lead or stone.

_Bulk_, to liquids, such as a bulk of water or oil.

_Volume_, to gases, such as a volume of air or oxygen.

[Illustration: Fig. 42. A B D, The three points of suspension. C, The
point of intersection, and, therefore, the centre of gravity. P, The
line of plummet.]

To find the centre of gravity of any mass, as, for example, an ordinary
school-slate, we must first of all suspend it from any part of the
frame; then allow a plumb-line to drop from the point of suspension, and
mark its direction on the slate. Again, suspend the slate at various
other points, always marking the line of direction of the plummet, and
at the point where the lines intersect each other, there will be the
centre of gravity.

[Illustration: Fig. 43.]

If the slate be now placed (as shown in Fig. 43) on a blunt wooden point
at the spot where the lines cross each other, it will be found to
balance exactly, and this place is called the _centre of gravity_, being
the point with which all other particles of the body would move with
parallel and equable motion during its fall. The equilibrium of bodies
is therefore much affected by the position of the centre of gravity.
Thus, if we cut out an elliptical figure from a board one inch in
thickness, and rest it on a flat surface by one of its edges (as at No.
1, fig. 44), this point of contact is called the point of support, and
the centre of gravity is immediately above it.

In this case, the body is in a state of secure equilibrium, for any
motion on either side will cause the centre of gravity to ascend in
these directions, and an oscillation will ensue. But if we place it upon
the smaller end, as shown at No. 2 (fig. 44), the position will be one
of [Page 35] equilibrium, but not stable or secure; although the centre of
gravity is directly above the point of support, the slightest touch will
displace the oval and cause its overthrow. The famous story of Columbus
and the egg suggests a capital illustration of this fact; and there are
two modes in which the egg may be poised on either of the ends.

[Illustration: Fig. 44. The point of support. C, The centre of gravity.]

The one usually attributed to the great discoverer, is that of scraping
or slightly breaking away a little of the shell, so as to flatten one of
the ends, thus--

[Illustration: Fig. 45. A Represents the egg in its natural state, and,
therefore, in unstable equilibrium; B, another egg, with the surface, S,
flattened, by which the centre of gravity is lowered, and if not
disturbed beyond the extent of the point of support the equilibrium is
stable.]

The most philosophical mode of making the egg stand on its end and
without disturbing the exterior shell is to alter the position of the
yolk, which has a greater density than the white, and is situated about
the centre. If the egg is now shaken so as to break the membrane
enclosing the yolk, and thus allow it to sink to the bottom of the
smaller end, the centre of gravity is lowered; there is a greater
proportion of weight [Page 36] concentrated in the small end, and the
egg stands erect, as depicted at fig. 46.

It is this variable position of the centre of gravity in ivory balls
(one part of which may be more dense than another) that so frequently
annoys even the best billiard-players; and on this account a ball will
deviate from the line in which it is impelled, not from any fault of the
player, but in consequence of the ivory ball being of unequal density,
and, therefore, not having the centre corresponding with the centre of
gravity. A good billiard-player should, therefore, always try the ball
before he engages to play for any large sum.

[Illustration: Fig. 46.--No. 1. Section of egg. C. Centre of gravity. Y.
The yolk. W. The white.

No. 2. C. Centre of gravity, much lowered. Y. The yolk at the bottom of
the egg.]

The toy called the "tombola" reminds us of the egg-experiment, as there
is usually a lump of lead inserted in the lower part of the hemisphere,
and when the toy is pushed down it rapidly assumes the upright position
because the centre of gravity is not in the lowest place to which it can
descend; the latter position being only attained when the figure is
upright.

[Illustration: Fig. 47.--No. 1. C. Centre of gravity in the lowest
place, figure upright.

No. 2. C. Centre of gravity raised as the figure is inclined on either
side, but falling again into the lowest place as the figure gradually
comes to rest.]

There is a popular paradox in mechanics--viz., "a body having a tendency
to fall by its own weight, may be prevented from falling by adding to it
a weight on the same side on which it tends to fall," and the paradox is
demonstrated by another well-known child's toy as depicted in the next
cut.

[Page 37]

[Illustration: Fig. 48. The line of direction falling beyond the base;
the bent wire and lead weight throwing the centre of gravity under the
table and near the leaden weight; the hind legs become the point of
support, and the toy is perfectly balanced.]

[Illustration: Fig. 49.--No. 1. Sword balanced on handle: the arc from C
to D is very small, and if the centre, C, falls out of the line of
direction it is not easily restored to the upright position.

No 2. Sword balanced on the point: the arc from C to D much larger, and
therefore the sword is more easily balanced.]

After what has been explained regarding the improvement of the stability
of the egg by lowering the situation of the centre of gravity, it may at
first appear singular that a stick loaded with a weight at its upper
extremity can be balanced perpendicularly with greater ease and
precision than when the weight is lower down and nearer the hand; and
that a sword can be balanced best when the hilt is uppermost; [Page 38]
but this is easily explained when it is understood that with the handle
downwards a much smaller arc is described as it falls than when
reversed, so that in the former case the balancer has not time to
re-adjust the centre, whilst in the latter position the arc described is
so large that before the sword falls the centre of gravity may be
restored within the line of direction of the base.

[Illustration: Fig. 50.--No. 1. The two pieces of mahogany, carved to
represent a man and a boy, one being 10 and the other 5 inches long,
attached to board by hinges at H H.]

[Illustration: Fig. 51.--No. 2. The board pushed forward, striking
against a nail, when the short piece falls first, and the long one
second.]

For the same reason, a child tripping against a stone will fall quickly;
whereas, a man can recover himself; this fact can be very nicely shown
by fixing two square pieces of mahogany of different lengths, by hinges
on a flat base or board, then if the board be pushed rapidly forward and
struck against a lead weight or a nail put in the [Page 39] table, the
short piece is seen to fall first and the long one afterwards; the
difference of time occupied in the fall of each piece of wood (which may
be carved to represent the human figure) being clearly denoted by the
sounds produced as they strike the board.

Boat-accidents frequently arise in consequence of ignorance on the
subject of the centre of gravity, and when persons are alarmed whilst
sitting in a boat, they generally rise suddenly, raise the centre of
gravity, which falling, by the oscillation of the frail bark, outside
the line of direction of the base, cannot be restored, and the boat is
upset; if the boat were fixed by the keel, raising the centre of gravity
would be of little consequence, but as the boat is perfectly free to
move and roll to one side or the other, the elevation of the centre of
gravity is fatal, and it operates just as the removal of the lead would
do, if changed from the base to the head of the "tombola" toy.

A very striking experiment, exhibiting the danger of rising in a boat,
may be shown by the following model, as depicted at Nos. 1 and 2, figs.
52 and 53.

[Illustration: Fig. 52.--No. 1. Sections of a toy-boat floating in
water. B B B. Three brass wires placed at regular distances and screwed
into the bottom of the boat, with cuts or slits at the top so that when
the leaden bullets, L L L, which are perforated and slide upon them like
beads, are raised to the top, they are retained by the brass cuts
springing out; when the bullets are at the bottom of the lines they
represent persons sitting in a boat, as shown in the lower cuts, and the
centre of gravity will be within the vessel.]

We thus perceive that the stability of a body placed on a base depends
upon the position of the line of direction and the height of the centre
of gravity.

Security results when the line of direction falls within the base.
Instability when just at the edge. Incapability of standing when falling
without the base.

[Page 40]

[Illustration: Fig. 53.--No. 2. The leaden bullets raised to the top now
show the result of persons suddenly rising, when the boat immediately
turns over, and either sinks or floats on the surface with the keel
upwards.]

[Illustration: Fig. 54. F. Board cut and painted to represent the
leaning-tower of Pisa. G. The centre of gravity and plummet line
suspended from it. H. The hinge which attaches it to the base board. I.
The string, sufficiently long to unwind and allow the plummet to hang
outside the base, so that, when cut, the model falls in the direction of
the arrow.]

The leaning-tower of Pisa is one hundred and eighty-two feet in height,
and is swayed thirteen and a half feet from the perpendicular, but yet
remains perfectly firm and secure, as the line of direction falls
considerably within the base. If it was of a greater altitude it could
no longer stand, because the centre of gravity would be so elevated that
the line of direction would fall outside the base. This fact may be
illustrated by taking a board several feet in length, and having cut
[Page 41] it out to represent the architecture of the leaning-tower of
Pisa, it may then be painted in distemper, and fixed at the right angle
with a hinge to another board representing the ground, whilst a
plumb-line may be dropped from the centre of gravity; and it may be
shown that as long as the plummet falls within the base, the tower is
safe; but directly the model tower is brought a little further forward
by a wedge so that the plummet hangs outside, then, on removing the
support, which may be a piece of string to be cut at the right moment,
the model falls, and the fact is at once comprehended.

The leaning-towers of Bologna are likewise celebrated for their great
inclination; so also (in England) is the hanging-tower, or, more
correctly, the massive wall which has formed part of a tower at
Bridgenorth, Salop; it deviates from the perpendicular, but the centre
of gravity and the line of direction fall within the base, and it
remains secure; indeed, so little fears are entertained of its tumbling
down, that a stable has been erected beneath it.

[Illustration: Fig. 55.--No. 1. Two billiard-cues arranged for the
experiment and fixed to a board: the ball is rolling _up_.

No. 2. Sections showing that the centre of gravity, C, is higher at A
than at B, which represents the thick end of the cues; it therefore, in
effect, rolls down hill.]

One of the most curious paradoxes is displayed in the ascent of a
billiard-ball from the thin to the thick ends of two billiard-cues
placed at an angle, as in our drawing above; here the centre of gravity
is raised at starting, and the ball moves in consequence of its actually
_falling_ from the high to the low level.

Much of the stability of a body depends on the height through which the
centre of gravity must be elevated before the body can be overthrown.
The greater this height, the greater will be the immovability of the
mass. One of the grandest examples of this fact is shown in the ancient
Pyramids; and whilst gigantic palaces, with vast columns, [Page 42] and
all the solid grandeur belonging to Egyptian architecture, have
succumbed to time and lie more or less prostrate upon the earth, the
Pyramids, in their simple form and solidity, remain almost as they were
built, and it will be noticed, in the accompanying sketch, how
difficult, if not impossible, it would be to attempt to overthrow bodily
one of these great monuments of ancient times.

[Illustration: Fig. 56. C. Centre of gravity, which must be raised to D
before it can be overthrown.]

[Illustration: Fig. 57. No. 1. The centre of gravity is near the ground,
and falls within the wheels. No. 2. The centre of gravity is much
elevated, and the line of direction is outside the wheels.]

The principles already explained are directly applicable to the
construction or secure loading of vehicles; and in proportion as the
centre of gravity is elevated above the point of support (that is, the
wheels), so is the insecurity of the carriage increased, and the
contrary takes place if the centre of gravity is lowered. Again, if a
waggon be loaded [Page 43] with a very heavy substance which does not
occupy much space, such as iron, lead, or copper, or bricks, it will be
in much less danger of an overthrow than if it carries an equal weight
of a lighter body, such as pockets of hops, or bags of wool or bales of
rags.

In the one instance, the centre of gravity is near the ground, and falls
well within the base, as at No. 1, fig. 57. In the other, the centre of
gravity is considerably elevated above the ground, and having met with
an obstruction which has raised one side higher than the other, the line
of direction has fallen outside the wheels, and the waggon is
overturning as at No. 2.

The various postures of the human body may be regarded as so many
experiments upon the position of the centre of gravity which we are
every moment unconsciously performing.

To maintain an erect position, a man must so place his body as to cause
the line of direction of his weight to fall within the base formed by
his feet.

[Illustration: Fig. 58.]

The more the toes are turned outwards, the more contracted will be the
base, and the body will be more liable to fall backwards or forwards;
and the closer the feet are drawn together, the more likely is the body
to fall on either side. The acrobats, and so-called "India-Rubber
Brothers", dancing dogs, &c., unconsciously acquire the habit of
accurately balancing themselves in all kinds of strange positions; but
as these accomplishments are not to be recommended to young people, some
other marvels (such as balancing a pail of water on a stick laid upon a
table) may be adduced, as illustrated in fig. 59.

Let A B represent an ordinary table, upon which place a broomstick, C D,
so that one-half shall lay upon the table and the other extend from
[Page 44] it; place over the stick the handle of an empty pail (which
may possibly require to be elongated for the experiment) so that the
handle touches or falls into a notch at H; and in order to bring the
pail well under the table, another stick is placed in the notch E, and
is arranged in the line G F E, one end resting at G and the other at E.
Having made these preparations, the pail may now be filled with water;
and although it appears to be a most marvellous result, to see the pail
apparently balanced on the end of a stick which may easily tilt up, the
principles already explained will enable the observer to understand that
the centre of gravity of the pail falls within the line of direction
shown by the dotted line; and it amounts in effect to nothing more than
carrying a pail on the centre of a stick, one end of which is supported
at E, and the other through the medium of the table, A B.

[Illustration: Fig. 59.]

This illustration may be modified by using a heavy weight, rope, and
stick, as shown in our sketch below.

[Illustration: Fig. 60.]

Before we dismiss this subject it is advisable to explain a term
referring to a very useful truth, called the centre of percussion; a
knowledge of which, gained instinctively or otherwise, enables the
workman to wield his tools with increased power, and gives greater force
to the cut of the swordsman, so that, with some physical strength, he
may perform the feat of cutting a sheep in half, cleaving a bar of lead,
or [Page 45] neatly dividing, _à la Saladin_, in ancient Saracen
fashion, a silk handkerchief floating in the air. There is a feat,
however, which does not require any very great strength, but is
sufficiently startling to excite much surprise and some inquiry--viz.,
the one of cutting in half a broomstick supported at the ends on
tumblers of water without spilling the water or cracking or otherwise
damaging the glass supports.

[Illustration: Fig. 61.]

These and other feats are partly explained by reference to time: the
force is so quickly applied and expended on the centre of the stick that
it is not communicated to the supports; just as a bullet from a pistol
may be sent through a pane of glass without shattering the whole square,
but making a clean hole through it, or a candle may be sent through a
plank, or a cannon-ball pass through a half opened door without causing
it to move on its hinges. But the success of the several feats depends
in a great measure on the attention that is paid to the delivery of the
blows at the _centre of percussion_ of the weapon; this is a point in a
moving body where the percussion is the greatest, and about which the
impetus or force of all parts is balanced on every side. It may be
better understood by reference to our drawing below. Applying this
principle to a model sword made of wood, cut in half in the centre of
the blade, and then united with an elbow-joint, the handle being fixed
to a board by a wire passed through it and the two upright pieces of
wood, the fact is at once apparent, and is well shown in Nos. 1, 2, 3,
fig. 62.

[Page 46]

[Illustration: Fig. 62. No. 1, is the wooden sword, with an elbow-joint
at C. No. 2. Sword attached to board at K, and being allowed to fall
from any angle shown by dotted-line, it strikes the block, W, outside
the centre of percussion, P, and as there is unequal motion in the parts
of the sword it bends down (or, as it were, breaks) at the elbow-joint,
C.

No. 3 displays the same model; but here the blow has fallen on the
block, W, precisely at the centre of percussion of the sword, P, and the
elbow-joint remains perfectly firm.]

When a blow is not delivered with a stick or sword at the centre of
percussion, a peculiar jar, or what is familiarly spoken of as a
_stinging_ sensation, is apparent in the hand; and the cause of this
disagreeable result is further elucidated by fig. 63, in which the post,
A, corresponds with the handle of the sword.

[Page 47]

[Illustration: Fig. 63. A. The post to which a rope is attached. B and C
are two horses running round in a circle, and it is plain that B will
not move so quick as C, and that the latter will have the greatest
moving force; consequently, if the rope was suddenly checked by striking
against an object at the centre of gravity, the horse C would proceed
faster than B, and would impart to B a backward motion, and thus make a
great strain on the rope at A. But if the obstacle were placed so as to
be struck at a certain point nearer C, viz., at or about the little
star, the tendency of each horse to move on would balance and neutralize
the other, so that there would be no strain at A. The little star
indicates the _centre of percussion_.]

All military men, and especially those young gentlemen who are intended
for the army, should bear in mind this important truth during their
sword-practice; and with one of Mr. Wilkinson's swords, made only of the
very best steel, they may conquer in a chance combat which might
otherwise have proved fatal to them. To Mr. Wilkinson, of Pall Mall, the
eminent sword-cutler, is due the great merit of improving the quality of
the steel employed in the manufacture of officers' swords; and with one
of his weapons, the author has repeatedly thrust through an iron plate
about one-eighth of an inch in thickness without injuring the point, and
has also bent one nearly double without fracturing it, the perfect
elasticity of the steel bringing the sword straight again. These, and
other severe tests applied to Wilkinson's swords, show that there is no
reason why an officer should not possess a weapon that will bear
comparison with, nay, surpass, the far-famed _Toledo_ weapon, instead of
submitting to mere army-tailor swords, which are often little better
than hoops of beer barrels; and, in dire combat with Hindoo or Mussulman
fanatics' Tulwah, may show too late the folly of the owner.

[Illustration: Fig. 64.]

[Page 48]




CHAPTER V.

SPECIFIC GRAVITY.


It is recorded of the great Dr. Wollaston, that when Sir Humphry Davy
placed in his hand, what was then considered to be _the_ scientific
wonder of the day--viz., a small bit of the metal potassium, he
exclaimed at once, "How heavy it is," and was greatly surprised, when
Sir Humphry threw the metal on water, to see it not only take fire, but
actually _float_ upon the surface; here, then, was a philosopher
possessing the deepest learning, unable, by the sense of touch and by
ordinary handling, to state correctly whether the new substance (and
that a metal), was heavy or light; hence it is apparent that the
property of specific gravity is one of importance, and being derived
from the Latin, means _species_, a particular sort or kind; and
_gravis_, heavy or weight--i.e., the particular weight of every
substance compared with a fixed standard of water.

[Illustration: Fig. 65. A. A large cylindrical vessel containing water,
in which the egg sinks till it reaches the bottom of the glass. B. A
similar glass vessel containing half brine and half water, in which the
egg floats in the centre--viz., just at the point where the brine and
water touch.]

We are so constantly in the habit of referring to a standard of
perfection in music and the arts of painting and sculpture, that the
youngest will comprehend the office of water when told that it is the
philosopher's unit or starting-point for the estimation of the relative
weights of solids and liquids. A good idea of the scope and meaning of
the term specific gravity, is acquired by a few simple experiments,
thus: if a cylindrical [Page 49] glass, say eighteen inches long, and
two and a half wide, is filled with water, and another of the same size
is also filled, one half with water and the other half with a saturated
solution of common salt, or what is commonly termed brine, a most
amusing comparison of the relative weights of equal bulks of water and
brine, can be made with the help of two eggs; when one of the eggs is
placed in the glass containing water, it immediately sinks to the
bottom, showing that it has a greater specific gravity than water; but
when the other egg is placed in the second glass containing the brine,
it sinks through the water till it reaches the strong solution of salt,
where it is suspended, and presents a most curious and pretty
appearance; seeming to float like a balloon in air, and apparently
suspended upon nothing, it provokes the inquiry, "whether magnetism has
anything to do with it?" The answer, of course, is in the negative, it
merely floats in the centre, in obedience to the common principle, that
all bodies float in others which are heavier than themselves; the brine
has, therefore, a greater weight than an equal bulk of water, and is
also heavier than the egg. A pleasing sequel to this experiment may be
shown by demonstrating how the brine is placed in the vessel without
mixing with the water above it; this is done by using a glass tube and
funnel, and after pouring away half the water contained in the vessel
(Fig. 65.), the egg can be floated from the bottom to the centre of the
glass, by pouring the brine down the funnel and tube. The saturated
solution of salt remains in the lower part of the vessel and displaces
the water, which floats upon its surface like oil on water, carrying the
egg with it.

[Illustration: Fig. 66. A vessel half full of water, and as the brine is
poured down the tube the egg gradually rises.]

The water of the Dead Sea is said to contain about twenty-six per cent.
of saline matter, which chiefly consists of common salt. It is perfectly
clear and bright, and in consequence of the great density, a person may
easily float on its surface, like the egg on the brine, so that if a
ship could be heavily laden whilst floating on the water of the Dead
Sea, it would most likely sink if transported to the Thames. This
illustration of specific gravity is also shown by a model ship, which
being first floated on the brine, will afterwards sink if conveyed to
another vessel containing water. One of the tin model ships sold as a
magnetic [Page 50] toy answers nicely for this experiment, but it must be
weighted or adjusted so that it just floats in the brine, A; then it
will sink, when placed, in another vessel containing only water.

[Illustration: Fig. 67.

A. Vessel containing brine, upon which the little model floats.

B. Vessel containing water, in which the ship sinks.]

Another amusing illustration of the same kind is displayed with goldfish,
which swim easily in water, floating on brine, but cannot dive to
the bottom of the vessel, owing to the density of the saturated solution
of salt. If the fish are taken out immediately after the experiment, and
placed in fresh water, they will not be hurt by contact with the strong
salt water.

These examples of the relative weights of equal bulks, enable the
youthful mind to grasp the more difficult problem of ascertaining the
specific gravity of any solid or liquid substance; and here the strict
meaning of terms should not be passed by. _Specific_ weight must not be
confounded with _Absolute_ weight; the latter means the entire amount of
ponderable matter in any body: thus, twenty-four cubic feet of sand
weigh about one ton, whilst specific weight means the _relation_ that
subsists between the _absolute weight_ and the _volume_ or _space_ which
that _weight_ occupies. Thus a cubic foot of water weighs sixty-two and
a half pounds, or 1000 ounces avoirdupois, but changed to gold, the
cubic foot weighs more than half a ton, and would be equal to about
19,300 ounces--hence the relation between the cubic foot of water and
that of [Page 51] gold is nearly as 1 to 19.3; the latter is therefore
called the specific gravity of gold.

Such a mode of taking the specific gravity of different
substances--viz., by the weight of equal bulks, whether cubic feet or
inches, could not be employed in consequence of the difficulty of
procuring exact cubic inches or feet of the various substances which by
their peculiar properties of brittleness or hardness would present
insuperable obstacles to any attempt to fashion or shape them into exact
volumes. It is therefore necessary to adopt the method first devised by
Archimedes, 600 B.C., when he discovered the admixture of another metal
with the gold of King Hiero's crown.

This amusing story, ending in the discovery of a philosophical truth,
may be thus described:--King Hiero gave out from the royal treasury a
certain quantity of gold, which he required to be fashioned into a
crown; when, however, the emblem of power was produced by the goldsmith,
it was not found deficient in weight, but had that appearance which
indicated to the monarch that a surreptitious addition of some other
metal must have been made.

It may be assumed that King Hiero consulted his friend and philosopher
Archimedes, and he might have said, "Tell me, Archimedes, without
pulling my crown to pieces, if it has been adulterated with any other
metal?" The philosopher asked for time to solve the problem, and going
to take his accustomed bath, discovered then specially what he had never
particularly remarked before--that, as he entered the vessel of water,
the liquid rose on each side of him--that he, in fact, displaced a
certain quantity of liquid. Thus, supposing the bath to have been full
of water, directly Archimedes stepped in, it would overflow. Let it be
assumed that the water displaced was collected, and weighed 90 pounds,
whilst the philosopher had weighed, say 200 pounds. Now, the train of
reasoning in his mind might be of this kind:--"My body displaces 90
pounds of water; if I had an exact cast of it in lead, the same _bulk_
and _weight_ of liquid would overflow; but the weight of my body was,
say 200 pounds, the cast in lead 1000 pounds; these two sums divided by
90 would give very different results, and they would be the specific
gravities, because the rule is thus stated:--'Divide the gross weight by
the loss of weight in water, the water displaced, and the quotient gives
the specific gravity.'" The rule is soon tested with the help of an
ordinary pair of scales, and the experiment made more interesting by
taking a model crown of some metal, which may be nicely gilt and
burnished by Messrs. Elkington, the celebrated electro-platers of
Birmingham. For convenience, the pan of one scale is suspended by
shorter chains than the other, and should have a hook inserted in the
middle; upon this is placed the crown, supported by very thin copper
wire. For the sake of argument, let it be supposed that the crown weighs
17½ ounces avoirdupois, which are duly placed in the other scale-pan,
and without touching these weights, the crown is now placed in a vessel
of water. It might be supposed that directly the crown enters the water,
it would gain weight, in consequence of being wetted, [Page 52] but the
contrary is the case, and by thrusting the crown into the water, it may
be seen to rise with great buoyancy so long as the 17½ ounces are
retained in the other scale-pan; and it will be found necessary to place
at least two ounces in the scale-pan to which the crown is attached
before the latter sinks in the water; and thus it is distinctly shown
that the crown weighs only about 15½ ounces in the water, and has
therefore _lost_ instead of _gaining_ weight whilst immersed in the
liquid. The rule may now be worked out:

                              Ounces.

  Weight of crown in air      17½
  Ditto in water              15½
                              ---
  Less in water                2

                 17½ / 2 = 8¾

The quotient 8¾ demonstrates that the crown is manufactured of
copper, because it would have been about 19¼ if made of pure gold.

[Illustration: Fig. 68. A. Ordinary pair of scales. B. Scale-pan,
containing 17½ ounces, being the weight of the crown in air. C. Pan,
with hook and crown attached, which is sunk in the water contained in
the vessel D; this pan contains the two ounces, which must be placed
there to make the crown sink and exactly balance B.]

[Page 53]

_Table of the Specific Gravities of the Metals in common use._

  Platinum            20.98
  Gold                19.26 to 19.3 and 19.64
  Mercury             13.57
  Lead                11.35
  Silver              10.47 to 10.5
  Bismuth              9.82
  Copper               8.89
  Iron                 7.79
  Tin                  7.29
  Zinc                 6.5 to 7.4

The simple rule already explained may be applied to all metals of any
size or weight, and when the mass is of an irregular shape, having
various cavities on the surface, there may be some difficulty in taking
the specific gravity, in consequence of the adhesion of _air-bubbles_;
but this may be obviated either by brushing them away with a feather,
or, what is frequently much better, by dipping the metal or mineral
first into alcohol, and then into water, before placing it in the vessel
of water, by which the actual specific gravity is to be taken.

The mode of taking the specific gravity of liquids is very simple, and
is usually performed in the laboratory by means of a thin globular
bottle which holds exactly 1000 grains of pure distilled water at 60°
Fahrenheit. A little counterpoise of lead is made of the exact weight of
the dry globular bottle, and the liquid under examination is poured into
the bottle and up to the graduated mark in the neck; the bottle is then
placed in one scale-pan, the counterpoise and the 1000-grain weight in
the other; if the liquid (such as oil of vitriol) is heavier than water,
then more weight will be required--viz., 845 grains--and these figures
added to the 1000 would indicate at once that the specific gravity of
oil of vitriol was 1.845 as compared with water, which is 1.000. When
the liquid, such as alcohol, is lighter than water, the 1000-grain
weight will be found too much, and grain weights must be added to the
same scale-pan in which the bottle is standing, until the two are
exactly balanced. If ordinary alcohol is being examined, it will be
found necessary to place 180 grains with the bottle, and these figures
deducted from the 1000 grains in the other scale-pan, leave 820, which,
marked with a dot before the first figure (_sic_ .820), indicates the
specific gravity of alcohol to be less than that of water.

The difference in the gravities of various liquids is displayed in a
very pleasing manner by an experiment devised by Professor Griffiths, to
whom chemical lecturers are especially indebted for some of the most
ingenious and beautiful illustrations which have ever been devised. The
experiment consists in the arrangement of five distinct liquids of
various densities and colours, the one resting on the other, and
distinguished not only by the optical line of demarcation, but by little
balls of wax, which are adjusted by leaden shot inside, so as to sink
through [Page 54] the upper strata of liquids, and rest only upon the
one that it is intended to indicate.

The manipulation for this experiment is somewhat troublesome, and is
commenced by procuring some pure bright quicksilver, upon which an iron
bullet (black-leaded, or painted of any colour) is placed, or one of
those pretty glass balls which are sold in such quantities at the
Crystal Palace.

Secondly. Put as much white vitriol (sulphate of zinc) into a half pint
of boiling water as it will dissolve, and, when cold, pour off the clear
liquid, make up a ball of coloured wax (say red), and adjust it by
placing little shot inside, until it sinks in a solution of sulphate of
copper and floats on that of the white vitriol.

Thirdly. Make a solution of sulphate of copper in precisely the same
manner, and adjust another wax ball to sink in water, and float on this
solution.

Fourthly. Some clear distilled water must be provided.

Fifthly. A little cochineal is to be dissolved in some common spirits of
wine (alcohol), and a ball of cork painted white provided.

Finally. A long cylindrical glass, at least eighteen inches high, and
two and a half or three inches diameter, must be made to receive these
five liquids, which are arranged in their proper order of specific
gravity by means of a long tube and funnel.

The four balls--viz., the iron, the two wax, and the cork balls, are
allowed to slide down the long glass, which is inclined at an angle; and
then, by means of the tube and funnel, pour in the tincture of
cochineal, and all the balls will remain at the bottom of the glass. The
water is poured down next, and now the cork ball floats up on the water,
and marks the boundary line of the alcohol and water. Then the solution
of blue vitriol, when a wax ball floats upon it. Thirdly, the solution
of white vitriol, upon which the second wax ball takes its place; and
lastly, the quicksilver is poured down the tube, and upon this heavy
metallic fluid the iron or glass ball floats like a cork on water.

[Illustration: Fig. 69. Long cylindrical glass, 18 × 3 inches,
containing the five liquids.]

The tube may now be carefully removed, pausing at each liquid, so that
no mixture take place between them; and the result is the arrangement of
five liquids, giving the appearance of a cylindrical glass painted with
[Page 55] bands of crimson, blue, and silver; and the liquids will not
mingle with each other for many days.

A more permanent arrangement can be devised by using liquids which have
no affinity, or will not mix with each other--such as mercury, water,
and turpentine.

The specific weight or weights of an equal measure of air and other
gases is determined on the same principle as liquids, although a
different apparatus is required. A light capped glass globe, with
stop-cock, from 50 to 100 cubic inches capacity, is weighed full of air,
then exhausted by an air-pump, and weighed empty, the loss being taken
as the weight of its volume of air; these figures are carefully noted,
because _air_ instead of _water_ is the standard of comparison for all
gases. When the specific gravity of any other gas is to be taken, the
glass globe is again exhausted, and screwed on to a gas jar provided
with a proper stop-cock, in which the gas is contained; and when perfect
accuracy is required, the gas must be dried by passing it over some
asbestos moistened with oil of vitriol, and contained in a glass tube,
and the gas jar should stand in a mercurial trough. (Fig. 70.) The
stop-cocks are gradually turned, and the gas admitted to the exhausted
globe from the gas jar; when full, the cocks are turned off, the globe
unscrewed, and again weighed, and by the common rule of proportion, as
the weight of the air first found is to the weight of the gas, so is
unity (1.000, the density of air) to a number which expresses the
density of the gas required. If oxygen had been the gas tried, the
number would be 1.111, being the specific gravity of that gaseous
element. If chlorine, 2.470. Carbonic acid, 1.500. Hydrogen being much
less than air, the number would only be 69, or decimally 0.069.

[Illustration: Fig. 70. A. Glass globe to contain the gas. B. Gas jar
standing in the mercurial trough, D. C. Tube containing asbestos
moistened with oil of vitriol.]

A very good approximation to the correct specific gravity (particularly
where a number of trials have to be made with the same gas, such as
[Page 56] ordinary coal gas) is obtained by suspending a light paper
box, with holes at one end, on one arm of a balance, and a counterpoise
on the other. The box can be made carefully, and should have a capacity
equal to a half or quarter cubic foot; it is suspended with the holes
downward, and is filled by blowing in the coal gas until it issues from
the apertures, and can be recognised by the smell. The rule in this case
would be equally simple: as the known weight of the half or quarter
cubic foot of common air is to the weight of the coal gas, so is 1.000
to the number required. (Fig. 71.)

[Illustration: Fig. 71. A. The balance. B. The paper box, of a known
capacity. C. Gas-pipe blowing in coal-gas, the arrows showing entrance
of gas and exit of the air.]

As an illustration of the different specific weights of the gases, a
small balloon, containing a mixture of hydrogen and air, may be so
adjusted that it will just sink in a tall glass shade inverted and
supported on a pad made of a piece of oilcloth shaped round and bound
with list. On passing in quickly a large quantity of carbonic acid, the
little balloon will float on its surface; and if another balloon,
containing only hydrogen, is held in the top part of the open shade, and
a sheet of glass carefully slid over the open end, the density of the
gases (although they are perfectly invisible) is perfectly indicated;
and, as a climax to the experiment, a third balloon can be filled with
laughing gas, and may be placed in the glass shade, taking care that the
one full of pure hydrogen does not escape; the last balloon will sink to
the bottom of the [Page 57] jar, because laughing gas is almost as
heavy as carbonic acid, and the weight of the balloon will determine its
descent. (Fig. 72.)

[Illustration: Fig. 72. Inverted large glass shade, containing half
carbonic acid and half common air.]

[Illustration: Fig. 73. A. Inverted glass shade, containing the
material, B, for generating carbonic acid gas. C. The soap-bubble. D D.
The glass tube for blowing the bubbles. E. Small lantern, to throw a
bright beam of light from the oxy-hydrogen jet upon the thin
soap-bubble, which then displays the most beautiful iridescent colours.]

A soap-bubble will rest most perfectly on a surface of carbonic acid
gas, and the aerial and elastic cushion supports the bubble till it
bursts. The experiment is best performed by taking a glass shade twelve
inches broad and deep in proportion, and resting it on a pad; half a
pound of sesquicarbonate of soda is then placed in the vessel, and upon
this is poured a mixture of half a pint of oil of vitriol and half a
pint of water, the latter being previously mixed and allowed to cool
before use. An enormous quantity of carbonic acid gas is suddenly
generated, and rising to the edge, overflows at the top of the glass
shade. A well-formed soap-bubble, detached neatly from the end of a
glass-tube, oscillates gently on the surface of the heavy gas, and
presents a most curious and pleasing appearance. The soapy water is
prepared by cutting a few pieces of yellow soap, and placing them in a
two-ounce bottle containing distilled water. (Fig. 73.) The specific
gravity of the gases, may therefore be either greater, or less than
atmospheric air, [Page 58] which has been already mentioned as the
standard of comparison, and examined by this test the vapours of some of
the compounds of carbon and hydrogen are found to possess a remarkably
high gravity; in proof of which, the vapour of ether may be adduced as
an example, although it does not consist only of the two elements
mentioned, but contains a certain quantity of oxygen. In a cylindrical
tin vessel, two feet high and one foot in diameter, place an ordinary
hot-water plate, of course full of boiling water; upon this warm surface
pour about half an ounce of the best ether; and, after waiting a few
minutes until the whole is converted into vapour, take a syphon made of
half-inch pewter tube, and warm it by pouring through it a little hot
water, taking care to allow the water to drain away from it before use.
After placing the syphon in the tin vessel, a light may be applied to
the extremity of the long leg outside the tin vessel, to show that no
ether is passing over until the air is sucked out as with the
water-syphon; and after this has been done, several warm glass vessels
may be filled with this heavy vapour of ether, which burns on the
application of flame. Finally, the remainder of the vapour may be burnt
at the end of the syphon tube, demonstrating in the most satisfactory
manner that the vapour is flowing through the syphon just as spirit is
removed by the distillers from the casks into cellars of the
public-houses. (Fig. 74.)

[Illustration: Fig. 74. A. Tin vessel containing the hot-water plate, B,
upon which the ether is poured. C. The syphon. D. Glass to receive the
vapour. E. Combustion of the ether vapour in another vessel.]

[Page 59]

Before dismissing the important subject of specific gravity (or, as it
is termed by the French _savants_, "density"), it may be as well to
state that astronomers have been enabled, by taking the density of the
earth and by astronomical observations, to calculate the gravity of the
planets belonging to our solar system; and it is interesting to observe
that the density of the planet Venus is the only one approaching the
gravity of the earth:--

  The Earth  1.000
  The Sun     .254
  The Moon    .742
  Mercury    2.583
  Venus      1.037
  Mars        .650
  Jupiter     .258
  Saturn      .104
  Herschel    .220




CHAPTER VI.

ATTRACTION OF COHESION.


In previous chapters one kind of attraction--viz., that of gravitation,
has been discussed and illustrated in a popular manner, and pursuing the
examination of the invisible, active, and real forces of nature, the
attraction of cohesion will next engage our attention. There is a
peculiar satisfaction in pursuing such investigations, because every
step is attended by a reasonable proof; there is no ghostly mystery in
philosophic studies; the mind is not suddenly startled at one moment
with that which seems more than natural; it is not carried away in an
ecstasy of wonder and awe, as in the so-called _spirit-rapping_
experiments, to be again rudely brought back to the material by the
disclosure of trickeries of the most ludicrous kind, such as those
lately exposed by M. Jobert de Lamballe, at the Academy of Sciences at
Paris. This gentleman has unmasked the effrontery of the spirit-rappers
by merely stripping the stocking from the heel of a young girl of
fourteen. M. Velpeau declares that the rapping is produced by the
muscles of the heel and knee acting in concert, and quotes the case of a
lady once celebrated as a medium, who has the power of producing the
most curious and interesting music with the tendons of the thigh. This
music is said to be loud enough to be heard from one end of a long room
to the other, and has often played a conspicuous part in the revelations
made by the medium. M. Jules Clocquet also explained the method by which
the famous girl pendulum had so long abused the credulity of the Paris
public. This girl, whose self-styled faculty is that of striking the
hour at any time of the day or night, was attended at the Hospital St.
Louis by M. Clocquet, who states that the vibrations in [Page 60] this
case were produced by a rotatory motion in the lumbar regions of the
vertebral column. The sound of these (_à la_ rattlesnake) was so
powerful, that they might be distinctly heard at a distance of
twenty-five feet.

In studying the powers of nature, which the most sceptical mind allows
must exist, there is an abundant field for experiment without attempting
the exploits of Macbeth's witches, or the fanciful powers of Manfred;
and, returning to the theme of our present chapter, it may be asked, how
is cohesion defined? and the answer may be given, by directing attention
to the three physical conditions of water, which assumes the form of
ice, water, or steam.

In the Polar regions, and also in the Alpine and other mountains where
glaciers exist, there the traveller speaks of ice twenty, thirty, forty,
nay, three hundred feet in thickness. Here the withdrawal of a certain
quantity of heat from the water evidently allows a new force to come
into full play. We may call it what we like; but cohesion, from the
Latin _cum_, together, and _hæreo_, I stick or cleave, appears to be the
best and most rational term for this power which tends to make the atoms
or particles of the same kind of matter move towards each other, and to
prevent them being separated or moved asunder. That it is not merely
hypothetical is shown by the following experiments.

If two pieces of lead are cast, and the ends nicely scraped, taking care
not to touch the surfaces with the fingers, they may by simple pressure
be made to cohere, and in that state of attraction may be lifted from
the table by the ring which is usually inserted for convenience in the
upper piece of lead; they may be hung for some time from a proper
support, and the lower bit of lead will not break away from the upper
one; they may even be suspended, as demonstrated by Morveau, in the
vacuum of an air-pump, to show that the cohesion is not mistaken for the
pressure of the atmosphere, and no separation occurs. And when the union
is broken by physical force, it is surprising to notice the limited
number of points, like pin points, where the cohesion has occurred;
whilst the weight of the lump of lead upheld against the force of
gravitation reminds one forcibly of the attraction of a mass of soft
iron by a powerful magnet, and leads the philosophic inquirer to
speculate on the principle of cohesion being only some masked form of
magnetic or electrical attraction. (Fig. 75.)

[Illustration: Fig. 75. A A. Two pieces of lead, scraped clean at the
surfaces B B. C. Stand, supporting the two pieces of lead attached to
each other by cohesion.]

[Page 61]

A fine example of the same force is shown in the use of a pair of flat
iron surfaces, planed by the celebrated Whitworth, of Manchester. These
surfaces are so true, that when placed upon each other, the upper one
will freely rotate when pushed round, in consequence of the thin film of
air remaining between the surfaces, which acts like a cushion, and
prevents the metallic cohesion. When, however, the upper plate is slid
over the lower one gradually, so as to exclude the air, then the two may
be lifted together, because cohesion has taken place. (Fig. 76.)

[Illustration: Fig. 76. A. Whitworth's planes, with film of air between
them.

B. Film of air excluded when cohesion occurs.]

A glass vessel is a good example of cohesion. The materials of which it
is composed have been soft and liquid when melted in the fire, and on
the removal of the excess of heat it has become hard and solid, in
consequence of the attractive force of cohesion binding the particles
together; in the absence of such a power, of course, the material would
fall into the condition of dust, and a mere shapeless heap of silicates
of potash and lead would indicate the place where the moulded and
coherent glass would otherwise stand.

A lump of lead, six inches long by four broad, and half an inch thick,
may be supported by dexterously taking off a thick shaving with a proper
plane, and after pressing an inch or more of the strip on the planed
surface of the large lump of lead, the cohesion is so powerful that the
latter may be lifted from the table by the strip of metal.

The bullets projected from Perkins' steam-gun, at the rate of three
hundred per minute, are thrown with such violence, that, when received
on a thick plate of lead backed up with sheet iron, a cold welding takes
place between the two surfaces of metal in the most perfect manner, just
as two soft pieces of the metal potassium may be squeezed and welded
together. The surfaces of an apple torn asunder will not readily cohere,
but if cut with a sharp knife, cohesion easily occurs; so with a wound
produced by a jagged surface, it is difficult to make the parts
[Page 62] heal, whereas some of the most desperate sabre-cuts have been
healed, the cohesion of the surfaces of cut flesh being very rapid;
hence, if the top of a finger is cut off, it may be replaced, and will
grow, in consequence of the natural cohesion of the parts.

The art of plating copper with silver, which is afterwards gilt, and
then drawn out into flattened wire for the manufacture of gold lace and
epaulets, usually termed bullion, is another example of the wonderful
cohesion of the particles of gold, of which a single grain may be
extended over the finest plate wire measuring 345 feet in length.

The process of making wax candles is a good illustration of the
attraction of cohesion; they are not generally cast in moulds, as most
persons suppose, but are made by the successive applications of melted
wax around the central plaited wick. Other examples of cohesion are
shown by icicles, and also stalagmites; which latter are produced by the
gradual dropping of water containing chalk (carbonate of lime) held in
solution by the excess of carbonic acid gas; the solvent gradually
evaporates, and leaves a series of calcareous films, and these cohere in
succession, producing the most fantastic forms, as shown in various
remarkable caverns, and especially in the cave of Arta, in the island of
Majorca.

In metallic substances the cohesion of the particles assumes an
important bearing in the question of relative toughness and power of
resisting a strain; hence the term cohesion is modified into that of the
property of "tenacity."

The tenacity of the different metals is determined by ascertaining the
weight required to break wires of the same length and gauge. Iron
appears to possess the property of tenacity in the greatest, and lead in
the least degree. (Fig. 77.)

[Illustration: Fig. 77. B. Pan supported by leaden wire broken by a
weight which the iron wire at A easily supports.]

[Page 63]

The tenacity of iron is taken advantage of in the most scientific manner
by the great engineers who have constructed the Britannia Tube, and that
eighth wonder of the world, the _Leviathan_, or _Great Eastern_
steam-ship. In both of these sublime embodiments of the genius and
industrial skill of Great Britain the advantage of the cellular
principle is fully recognised. The magnitude of this colossal ship is
better realized when it is remembered that the _Great Eastern_ is six
times the size of the _Duke of Wellington_ line-of-battle ship, that her
length is more than three times that of the height of the Monument,
while in breadth it is equal to the width of Pall Mall, and that a
promenade round the deck will afford a walk of more than a quarter of a
mile. Up to the water-mark the hull is constructed with an inner and
outer shell, two feet ten inches apart, each of three-quarter-inch
plate; and between them, at intervals of six feet, run horizontal webs
of iron plates, which convert the whole into a series of continuous
cells or iron boxes. (Fig. 78.)

[Illustration: Fig. 78. Transverse section of _Great Eastern_, showing
the cellular construction from keel to water-line, A A.]

This double ship is useful in various ways; in the first place, the
danger arising from collision is diminished, as it is supposed that the
outer web only would be broken through or damaged; so that the water
would not then rush into the steam-ship, but merely fill the space
between the shells. In the second place, if there should be any
difficulty in procuring ballast, the space can be filled with 2500 tons
of water, or again pumped out, according to the requirements of the
vessel. The strength of a continued cellular construction can be easily
imagined, and may be well illustrated by a thin sheet of common tin
plate. If the ends be rested on blocks of wood, so as to lap over the
wood about one inch, they are easily displaced, and the mimic bridge
broken down from its [Page 64] supports by the addition to the centre
of a few ounce weights; whilst the same tin plate rolled up in the
figure of a tube, and again rested on the same blocks, will now support
many pounds weight without bending or breaking down. (Fig. 79.)

[Illustration: Fig. 79. A. Flat tin plate, breaking down with a few
ounce weights.

B. Same tin plate rolled up supports a very heavy weight.]

The deck of the ship is double or cellular, after the plan of Stephenson
in the Britannia Tubular Bridge, and is 692 feet in length. The tonnage
register is 18,200 tons, and 22,500 tons builder's measure; the hull of
the _Great Eastern_ is considered to be of such enormous tenacity, that,
if it were supported by massive blocks of stone six feet square, placed
at each end, at stem and stern, it would not deflect, curve, or bend
down in the middle more than _six inches_ even with all her machinery,
coals, cargo, and living freight.

In adducing remarkable instances of the adhesive power and tenacity of
inorganic matter, it may not be altogether out of place to allude to the
strength and force of living matter, or muscular power. It is stated
that Dr. George B. Winship, of Roxbury in America, a young physician,
twenty-five years old, and weighing 143 pounds, is the strongest man
alive; in fact, quite the Samson of the nineteenth century. He can raise
a barrel of flour from the floor to his shoulders; can raise himself
with either _little_ finger till his chin is half a foot above it; can
raise 200 pounds with either little finger; can put up a church bell of
141 pounds; can lift with his hands 926 pounds dead weight without the
aid of straps or belts of any kind. As compared with Topham, the Cornish
strong man, who could raise 800 pounds, or the Belgic one, his power is
greater; and as the use of straps and belts increases the power of
lifting by about four times, it is stated that Winship could lift at
least 2500 pounds weight.

[Illustration: Fig. 80. A. Ordinary glass water hammer. B. Copper tube
ditto, showing exhausting syringe at D, the height of the water at B,
and the end to be placed in the fire at C.]

With these illustrations of cohesion we may return again to the abstract
consideration of this power with reference to water, in which we have
noticed that the antagonist to this kind of attraction is the force or
power termed caloric or heat. The latter influence removes the frozen
bands of winter and converts the ice to the next condition, water. In
this state cohesion is almost concealed, although there is just a
slight [Page 65] excess to hold even the particles of water in a state
of unity, and this fact is beautifully illustrated by the formation of
the brilliant diamond drops of dew on the surfaces of various leaves, as
also in the force and power exercised by great volumes of water, which
exert their mighty strength in the shape of breaker-waves, dashing
against rocks and lighthouses, and making them tremble to their very
base by the violence of the shock; here there must be some unity of
particles, or the collective strength could not be exerted, it would be
like throwing a handful of sand against a window--a certain amount of
noise is produced, but the glass is not fractured; whilst the same sand
united by any glutinous material, would break its way through, and soon
fracture the brittle glass. It is so usual to see the particles of water
easily separated, that it becomes difficult to recognise the presence of
cohesion; but this bond of union is well illustrated in the experiment
of the water hammer. The little instrument is generally made of a glass
tube with a bulb at one end; in this bulb the water which it contains is
boiled, and as the steam issues from the other extremity, drawn out to a
capillary tube, the opening is closed by fusion with the heat of a
blowpipe flame. As the water cools the steam condenses, and a vacuum, so
far as air is concerned, is produced; if now the tube is suddenly
inverted, the whole of the water falls _en masse_, collectively, and
striking against the bottom of the tube, produces a metallic ring, just
as if a piece of wood or metal were contained within the tube. If the
end to which the water falls is not well cushioned by the palm of the
hand, the water hammers itself through and breaks away that part of the
glass tube. Hence it is better to construct the water hammer of copper
tube, about three-quarters of an inch in diameter and three feet long;
at one end a female screw-piece is inserted, into which a stop-cock is
fitted; when the tube is filled to the height of about six inches with
water, and shaken, the air divides the descending volume of water, and
the ordinary splashing sound is heard; there is no unity or cohesion of
the parts; if, however, the end of the copper tube is thrust into a fire
and the water boiled so that steam issues from the cock, which is then
closed, and the tube removed and cooled, a smart blow is given, and
distinctly heard when the copper tube is rapidly inverted or shaken so
as to cause the water to rise [Page 66] and fall. The experiment may be
rendered still more instructive by turning the cock and admitting the
air, which rushes in with a whizzing sound, and on shaking the tube the
metallic ring is no longer heard, but it may be again restored by
attaching a small air syringe or hand pump, and removing the air by
exhaustion. (Fig. 80.)

In the fluid condition water still possesses a surplus of cohesion over
the antagonistic force of heat; when, however, the latter is applied in
excess, then the quasi-struggle terminates; the heat overpowers the
cohesive attraction, and converts the water into the most willing slave
which has ever lent itself to the caprices of man--viz., into
steam--glorious, useful steam: and now the other end of the chain is
reached, where heat triumphs; whilst in solids, such as ice, cohesion is
the conqueror, and the intermediate link is displayed in the fluid state
of water. If any fact could give an idea of the gigantic size of the
_Great Eastern_, it is the force of the steam which will be employed to
move it at the rate of about eighteen miles per hour with a power
estimated at the nominal rate of 2600 horses, but absolutely of at least
12,000 horses. This steam power, coupled with the fact that she has been
enormously strengthened in her sharp, powerful bows, by laying down
three complete iron decks forward, extending from the bows backward for
120 feet, will demonstrate that in case of war the _Great Eastern_ may
prove to be a powerful auxiliary to the Government. These decks will be
occupied by the crew of 300 or 400 men, and with this large increase of
strength forward, the _Great Eastern_, steaming full power, could
overtake and cut in two the largest wooden line-of-battle ship that ever
floated. Should war unhappily spread to peaceful England, and the
enormous power of this vessel be realized, her name would not
inappropriately be changed from the _Great Eastern_ to the _Great
Terror_ of the ocean. The _Times_ very properly inquires, "What fleet
could stand in the way of such a mass, weighing some 30,000 tons, and
driven through the water by 12,000 horse-power, at the rate of
twenty-two or twenty-three miles per hour. To produce the steam, 250
tons of coal per diem will be required, and great will be the honourable
pride of the projectors when they see her fairly afloat, and gliding
through the ocean to the Far West."

A good and striking experiment, displaying the change from the liquid to
the vapour state, is shown by tying a piece of sheet caoutchouc over a
tin vessel containing an ounce or two of water. When this boils, the
india-rubber is distended, and breaks with a loud noise; or in another
illustration, by pouring some ether through a funnel carefully into a
flask placed in a ring stand. If flame is applied to the orifice, no
vapour issues that will ignite, provided the neck of the flask has not
been wetted with the ether. When, however, the heat of a spirit-lamp is
applied, the ether soon boils, and now on the application of a lighted
taper, a flame some feet in length is produced, which is regulated by
the spirit-lamp below, and when this is removed, the length of the flame
diminishes immediately, and is totally extinguished if the bottom of the
flask is plunged into cold water; the withdrawal of the heat restores
the power of cohesion. Another illustration of the vast power of steam
[Page 67] will be shortly displayed in the Steam Ram; and, "Supposing,"
says the _Times_, "the new steam ram to prove a successful design, the
finest specimens of modern men-of-war will be reduced by comparison to
the helplessness of cock boats. Conceive a monstrous fabric floating in
mid-channel, fire proof and ball proof, capable of hurling broadsides of
100 shot to a distance of six miles; or of clapping on steam at pleasure
and running down everything on the surface of the sea with a momentum
utterly irresistible.

"This terrible engine of destruction is expected to be itself
indestructible. We are told that she may be riddled with shot (supposing
any shot could pierce her sides), that she may have her stem and her
stern cut to pieces, and be reduced apparently to a shapeless wreck,
without losing her buoyancy or power. Supposing that she relies upon the
shock of her impact instead of fighting her guns, it is calculated that
she would sink a line-of-battle ship in three minutes, so that a
squadron as large as our whole fleet now in commission would be
destroyed in about one hour and a quarter."




CHAPTER VII.

ADHESIVE ATTRACTION.


The term cohesion must not be confounded with that of adhesion, which
refers to the clinging to or attraction of bodies of a dissimilar kind.
The late Professor Daniell defines cohesion to be an attraction of
homogeneous ([Greek: _homos_], like, and [Greek: _genos_], kind) or
similar particles; adhesion to be an attraction subsisting between
particles of a heterogeneous, [Greek: _eteros_], different, and [Greek:
_genos_], kind.

There are numerous illustrations of adhesion, such as mending china, and
the use of glue, or paste, in uniting different surfaces, or mortar, in
building with bricks; it is also well shown at the lecture table by
means of a pair of scales, one scale-pan of which being well cleaned
with alkali at the bottom, may then be rested on the surface of water
contained in a plate; the adhesion between the water and the metal is so
perfect, that many grain weights may be placed in the other pan before
the adhesion is broken; and after breakage, if the pan be again placed
on the water, and a few grains removed from the other, so as to adjust
the two pans, and make them nearly equal, a drop of oil of turpentine
being added, instantly spreads itself over the water, and breaking the
adhesion between the latter and the metal, the scale-pan is immediately
and again broken away, as the adhesion between the turpentine and the
metal is not so great as that of water and metal. The adhesion of air
and water is well displayed in an apparatus recommended for ventilating
mines, in which a constant descending stream of water carries with it a
quantity of air, which being disengaged, is then forced out of a proper
orifice. The same kind of adhesion between air and water is displayed in
the ancient [Page 68] Spanish Catalan forge, where the blast is
supplied to the iron furnace on a similar principle, only, a natural
cascade is taken advantage of instead of an artificial fall of water
through a pipe.

The adhesion of air and water becomes of some value when a river flows
through a large and crowded city, because the water in its passage to
and fro, must necessarily drag with it, a continuous column of air, and
assist in maintaining that constant agitation of the air which is
desirable as a preventive to any accumulation of noxious air charged
with foetid odours, arising from mud banks or from other causes. The
fact of adhesion, existing between water and air, is readily shown, by
resting one end of a long glass tube, of at least one inch diameter, on
a block of wood one foot high. If water is allowed to flow down the
tube, so as to leave a sufficient space of air above it, the adhesion
between the two ancient elements becomes apparent, directly a little
smoke is produced, near the top end of the glass tube resting on the
block of wood. The smoke, which has a greater tendency to rise than to
fall, is dragged down the glass tube, and accompanies the water as it
flows from the higher to the lower level. The same truth is also
illustrated in horizontal troughs or tubes through which water is caused
to flow.

The adhesion between air and glass is so great, that it is absolutely
necessary to boil the mercury in the tubes of the best barometers; and
if this is not carefully attended to, the adhering air between the glass
and mercury gradually ascends to, and destroys, the Torricellian vacuum
at the top of the barometer tube. Even after the mercury is boiled, the
air will creep up in course of years; and in order to prevent its
passage between the glass and quicksilver, it has been recommended, that
a platinum ring should be welded on to the end of the glass tube,
because mercury has the power of wetting or enfilming the metal
platinum, and the two being in close contact, would, as it were, shut
the only door by which the air could enter the barometer tube.

[Illustration: Fig. 81. Model of the apparatus for drawing down air. A,
cistern of water, supplied by ball-cock, and kept at one level, so that
the water just runs down the sides of the tube, and draws down the air
in the centre, B C. The vessel to which the air and water are conveyed
by a gutta-percha tube, T. There is another ball-cock to permit the
waste water to run away when it reaches a certain level; the end of the
pipe always dips some inches into this water, whilst the air escapes
from the jet, D.]

[Page 69]




CHAPTER VIII.

CAPILLARY ATTRACTION.


This kind of attraction is termed capillary, in consequence of tubes, of
a calibre, or bore, as fine as hair, attracting and retaining fluids.

If water is poured into a glass, the surface is not level, but cupped at
the edges, where the solid glass exerts its adhesive attraction for the
liquid, and draws it from the level. If the glass be reduced to a very
narrow tube, having a hair-like bore, the attraction is so great that
the water is retained in the tube, contrary to the force of gravitation.
Two pieces of flat glass placed close together, and then opened like a
book, draw up water between them, on the same principle. A mass of salt
put on a plate containing a little water coloured with indigo displays
this kind of attraction most perfectly, and the water is quickly drawn
up, as shown by the blue colour on the salt. A little solution of the
ammonio-sulphate of copper imparts a finer and more distinct blue colour
to the salt. A piece of dry Honduras mahogany one inch square, placed in
a saucer containing a little turpentine, is soon found to be wet with
the oil at the top, which may then be set on fire.

Almost every kind of wood possesses capillary tubes, and will float, on
account of these minute vessels being filled with air; if, however, the
air is withdrawn, then the wood sinks, and by boiling a ball made of
beech wood in water, and then placing it under the vacuum of an air pump
in other cold water, it becomes so saturated with water that it will no
longer float. A remarkable instance of the same kind is mentioned by
Scoresby, in which a boat was pulled down by a whale to a great depth in
the ocean, and after coming to the surface it was found that the wood
would neither swim nor burn, the capillary pores being entirely filled
with salt water.

A piece of ebony sinks in water on account of its density, closeness,
and freedom from air. A gauge made of a piece of oak, with a hole bored
in it of one inch diameter, accurately receives a dry plug of willow
wood which will not enter the orifice after it is wetted. Millstones are
split by inserting wedges of dry hard wood, which are afterwards wetted
and swelled, and burst the stone asunder. One of the most curious
instances of capillary attraction is shown in the currying of leather, a
process which is intended to impart a softness and suppleness to the
skin, in order that it may be rendered fit for the manufacture of boots,
harness, machine bands, &c. The object of the currier is to fill the
pores of the leather with oil, and as this cannot be done by merely
smearing the surface, he prepares the way for the oil by wetting the
leather thoroughly with water, and whilst the skin is damp, oil is
rubbed on, and it is then exposed to the air; the water evaporates at
ordinary temperatures, but oil does not; the consequence is that the
[Page 70] pores of the leather give up the water, which disappears in
evaporation, and the oil by capillary attraction is then drawn into the
body of the leather, the oil in fact takes the place vacated by the
water, and renders the material very supple, and to a considerable
extent waterproof. In paper making, the pores of this material, unless
filled up or sized, cause the ink to blot or spread by capillary
attraction. The porosity of soils is one of the great desideratums of
the skilful agriculturist, and drainage is intended to remove the excess
of water which would fill the pores of the earth, to the exclusion of
the more valuable dews and rains conveying nutritious matter derived
from manures and the atmosphere.

A cane is an assemblage of small tubes, and if a piece of about six
inches in length (cut off, of course, from the joints) be placed in a
bottle of turpentine, the oil is drawn up and may be burnt at the top;
it is on this principle that indestructible wicks of asbestos, and wire
gauze rolled round a centre core, are used in spirit lamps. Oil, wax,
and tallow, all rise by capillary attraction in the wicks to the flame,
where they are boiled, converted into gas, and burnt.

[Illustration: Fig. 82. Geber's filter. A. The solution of acetate of
lead. B. The dilute sulphuric acid. C. The clear liquid, separated from
the sulphate of lead in B.]

[Illustration: Fig. 83. Prawn syphon.]

The capillary attraction of skeins of cotton for water was known and
appreciated by the old alchemists; and Geber, one of the most ancient of
these pioneers of science, and who lived about the seventh century,
describes a filter by which the liquid is separated from the solid. This
experiment is well displayed by putting a solution of acetate of lead
into a glass, which is placed on the highest block of a series of three,
arranged as steps. Into this glass is placed the short end of a skein
[Page 71] of lamp cotton, previously wetted with distilled water; the
long end dips into another glass below, containing dilute sulphuric
acid, and as the solution of lead passes into it, a solid white
precipitate of sulphate of lead is formed; then another skein of wetted
cotton is placed in this glass, the long end of which passes into the
last glass, so that the clear liquid is separated and the solid left
behind. (Fig. 82.)

In this filter the lamp cotton acts as a syphon through the capillary
pores which it forms. On the same principle, a prawn may be washed in
the most elegant manner (as first shown by the late Duke of Sussex), by
placing the tail, after pulling off the fan part, in a tumbler of water,
and allowing the head to hang over, when the water is drawn up by
capillary attraction, and continues to run through the head. (Fig. 83.)
The threads of which linen, cotton, and woollen cloths are made are
small cords, and the shrinkage of such textile fabrics, is well known
and usually inquired about, when a purchase is made; here again
capillary attraction is exerted, and the fabric contracts in the two
directions of the warp and woof threads; thus, twenty-seven yards of
common Irish linen will permanently shrink to about twenty-six yards in
cold water. In these cases the water is attracted into the fibres of the
textile material, and causing them to swell, must necessarily shorten
their length, just as a dry rope strained between two walls for the
purpose of supporting clothes, has been known to draw the hooks after
being suddenly wetted and shortened by a shower of rain.

In order to tighten a bandage, it is only necessary to wind the dry
linen round the limbs as close as possible, and then wet it with water,
when the necessary shrinkage takes place.

If a piece of dry cotton cloth is tied over one end of a lamp glass, the
other may be thrust into, or removed from the basin of water very
easily, but when the cotton is wetted, the fibres contract and prevent
air from entering, so that the glass retains water just as if it were an
ordinary gas jar closed with a glass stopper.

[Illustration: Fig. 84. A. Basin of water. B. Cylinder of wire gauze
closed at both ends with gauze. When full of water it may be lifted from
the basin by the handle, C.]

A Spanish proverb, expressing contempt, says, "go to the well with a
sieve," but even this seeming impossibility is surmounted by using a
cylinder of wire gauze, which may be filled with water, and by means of
the capillary attraction between the meshes of the copper-wire gauze and
the water, the whole is retained, and may be carefully lifted from a
basin of water; the experiment only succeeds when the air is completely
driven out of the interstices of the gauze, and the little cylinder
completely filled with water; this may be done [Page 72] by repeatedly
sinking and drawing out the cylinder, or still more effectually, by
first wetting it with alcohol and then dipping the cylinder in water.

A balloon, made of cotton cloth, cannot be inflated by means of a pair
of bellows, but if the balloon is wetted with water, then it may be
swelled out with air just as if it had been made of some air-tight
material; hence the principle of varnishing silk or filling the pores
with boiled oil, when it is required in the manufacture of balloons.

Biscuit ware, porous tubes for voltaic batteries, alcarrazas, or water
coolers, are all examples of the same principle.

Whilst speaking most favourably of the benevolent labours of many
gentlemen (beginning with Mr. Gurney) who have erected "Drinking
Fountains" in London's dusty atmosphere and crowded streets, it must not
be forgotten that pious Mohammedans have, in bygone times, already set
us the example in this respect; and in the palmy days of many of the
Moorish cities, the thirsty citizen could always be refreshed by a
draught of cool water from the porous bottles provided and endowed by
charitable Mussulmans, and placed in the public streets.

[Illustration: Fig. 85. Moorish niche and porous earthenware bottle,
containing water.]

[Page 73]




CHAPTER IX.

CRYSTALLIZATION.


[Illustration: Fig. 86. Crystals of snow.]

It has been already stated that the force of cohesion binds the similar
particles of substances together, whether they be _amorphous_ or
shapeless, _crystalline_ or of a regular symmetrical and mathematical
figure. The term crystal was originally applied by the ancients to
silica in the form of what is usually termed rock crystal, or Brazilian
pebble; and they supposed it to be water which had been solidified by a
remarkable intensity of cold, and could not be thawed by any ordinary or
summer heat. Indeed, this idea of the ancients has been embodied (to a
certain extent) in the shape of artificial ice made by crystallizing
large quantities of sulphate of soda, which was made as flat as
possible, and upon [Page 74] which skaters were invited to describe the
figure of eight, at the usual admittance fee, representing twelve pence.
A crystal is now defined to be an inorganic body, which, by the
operation of affinity, has assumed the form of a regular solid
terminated by a certain number of planes or smooth surfaces.

Thousands of minerals are discovered in the crystallized state--such as
cubes of iron pyrites (sulphuret of iron) and of fluor spar (fluoride of
calcium), whilst numerous saline bodies called salts are sold only in
the form of crystals. Of these salts we have excellent examples in Epsom
salts (sulphate of magnesia), nitre (nitrate of potash), alum (sulphate
of alumina), and potash; the term salt being applied specially to all
substances composed of an acid and a base, as also to other combinations
of elements which may or may not take a crystalline form. Thus, nitre is
composed of nitric acid and potash; the first, even when much diluted,
rapidly changes paper, dipped in tincture of litmus and stained blue, to
a red colour, whilst potash shows its alkaline nature, by changing
paper, stained yellow with tincture of turmeric, to a reddish-brown. The
latter paper is restored to its original yellow by dipping it into the
dilute nitric acid, whilst the litmus paper regains its delicate blue
colour by being passed into the alkaline solution. An acid and an alkali
combine and form a neutral salt, such as nitre, which has no action
whatever on litmus or turmeric; whilst the element iodine, which is not
an acid, unites with the metallic element potassium, and therefore not
an alkali, and forms a salt that crystallizes in cubes called iodide of
potassium. Again, cane sugar, which is composed of charcoal, oxygen, and
hydrogen, crystallizes in hard transparent four-sided and irregular
six-sided prisms, but is not called a salt. Silica or sand is found
crystallized most perfectly in nature in six-sided pyramids, but is not
a salt; it is an acid termed silicic-acid. Sand has no acid taste,
because it is insoluble in water, but when melted in a crucible with an
alkali, such as potash, it forms a salt called silicate of potash.
Magnesia, from being insoluble, or nearly so, in water, is all but
tasteless, and has barely any alkaline reaction, yet it is a very strong
alkaline base; 20.7 parts of it neutralize as much sulphuric acid as 47
of potash. A salt is not always a crystallizable substance, and _vice
versâ_. The progress of our chemical knowledge has therefore demanded a
wider extension and application of the term _salt_, and it is not now
confined merely to a combination of an acid and an alkali, but is
conferred even on compounds consisting only of sulphur and a metal,
which are termed _sulphur salts_.

So also in combinations of chlorine, iodine, bromine, and fluorine, with
metallic bodies, neither of which are acid or alkaline, the term _haloid
salts_ has been applied by Berzelius, from the Greek ([Greek: _als_], sea
salt, and [Greek: _eidos_] form), because they are analogous in
constitution to sea salt; and the mention of sea salt again reminds us
of the wide signification of the term salt, originally confined to this
substance, but now extended into four great orders, as defined by
Turner:--

ORDER I. _The oxy-salts._--This order includes no salt the acid or base
of which is not an oxidised body (ex., nitrate of potash).

[Page 75]

ORDER II. _The hydro-salts._--This order includes no salt the acid or
base of which does not contain hydrogen (ex., chloride of ammonium).

ORDER III. _The sulphur salts._--This order includes no salt the
electro-positive or negative ingredient of which is not a sulphuret
(ex., hydrosulphuret of potassium).

ORDER IV. _The haloid salts._--This order includes no salt the
electro-positive or negative ingredient of which is not haloidal. (Exs.,
iodide of potassium and sea salt). To fix the idea of salt still better
in the youthful mind, it should be remembered that alabaster, of which
works of art are constructed, or marble, or lime-stone, or chalk, are
all salts, because they consist of an acid and a base.

In order to cause a substance to crystallize it is first necessary to
endow the particles with freedom of motion. There are many methods of
doing this chemically or by the application of heat, but we cannot by
any mechanical process of concentration, compression, or division,
persuade a substance to crystallize, unless perhaps we except that
remarkable change in wrought or fibrous iron into crystalline or brittle
iron, by constant vibration, as in the axles of a carriage, or by
attaching a piece of fibrous iron to a tilt hammer.

If we powder some alum crystals they will not again assume their
crystalline form; if brought in contact there is no freedom of motion.
It is like placing some globules of mercury on a plate. They have no
power to create motion; their inertia keeps them separated by certain
distances, and they do not coalesce; but incline the plate, give them
motion, and bring them in contact, they soon unite and form one globule.
The particles of alum are not in close contact, and they have no freedom
of motion unless they are dissolved in water, when they become
invisible; the water by its chemical power destroys the mechanical
aggregation of the solid alum far beyond any operation of levigation.
The solid alum has become liquid, like water; the particles are now free
to move without let or hindrance from friction. A solution, (from the
Latin _solvo_, to loosen) is obtained. The alum must indeed be reduced
to minute particles, as they are alike invisible to the eye whether
assisted by the microscope or not. No repose will cause the alum to
separate; the solvent power of the water opposes gravitation; every part
of the solution is equally impregnated with alum, and the particles are
diffused at equal distances through the water; the heavy alum is
actually drawn up against gravity by the water.

How, then, is the alum to be brought back again to the solid state? The
answer is simple enough. By evaporating away the excess of water, either
by the application of heat or by long exposure to the atmosphere in a
very shallow vessel, the minute atoms of the alum are brought closer
together, and crystallization takes place. The assumption of the solid
state is indicated by the formation of a thin film (called a _pellicle_)
of crystals, and is further and still more satisfactorily proved by
taking out a drop of the solution and placing it on a bit of glass,
which rapidly becomes filled with crystals if the evaporation has been
carried sufficiently far (Fig. 87).

[Page 76]

After evaporating away sufficient water, the dish is placed on one side
and allowed to cool, when crystals of the utmost regularity of form are
produced, and, denoted by a geometrical term, are called octohedral or
eight-sided crystals, when in the utmost state of perfection (Fig. 88).

[Illustration: Fig. 87. R R. Ring-stand. S S. Spirit-lamps. A. Flask
containing boiling solution of alum.--_Solution._ B. Funnel, with a bit
of lamp-cotton stuffed in the bottom.--Filtration. C. Evaporating
dish.--Evaporation. D. Drop on glass.--Crystallization.]

[Illustration: Fig. 88.]

The science of crystallography is too elaborate to be discussed at
length in a work of this kind; the various terms connected with crystals
will therefore only be explained, and experiments given in illustration
of the formation of various crystals.

When the apices--_i.e._, the tips or points of crystals--are cut off,
they are said to be truncated; and the same change occurs on the edges
of numerous crystals.

If some of the salt called chloride of calcium in the dry and amorphous
state is exposed to the air, it soon absorbs water, or what is termed
_deliquesces_: the same thing occurs with the crystals of carbonate of
potash, and if four ounces are weighed out in an evaporating dish, and
then exposed for about half an hour to the air, a very perceptible
increase in weight is observed by the assistance of the scales and grain
weights. _Deliquescence_ is a term from the Latin _deliqueo_, to melt,
and is in fact a gradual melting, caused by the absorption of water from
the atmosphere. The reverse of this is illustrated with various
crystals, such as Glauber's salt (sulphate of soda), or common washing
soda (carbonate of soda); if a fine clear crystal is taken out of the
solution, called the mother liquor, in which it has been crystallized,
wiped dry, and placed under a glass shade, this salt may remain for a
long period [Page 77] without change, but if it receive one scratch
from a pin, the door is opened apparently for the escape of the water
which it contains, chemically united with the salt, and called water of
crystallization; the white crystal gradually swells out, the little
_quasi_ sore from the pin-scratch spreads over the whole, which becomes
opaque, and crumbling down falls into a shapeless mass of white dust;
this change is called _efflorescence_, from _effloresco_, to blow as a
flower--caused by the abstraction from them of chemically-combined water
by the atmosphere. With reference to the preservation of crystals,
Professor Griffiths recommends them to be oiled and wiped, and placed
under a glass shade, if of a deliquescent nature; or if efflorescent,
they are perfectly preserved by placing them under a glass shade with a
little water in a cup to keep the air charged with moisture and prevent
any drying up of the crystal.

Deliquescent crystals may be preserved by placing them, when dry, in
naphtha, or any liquor in which they are perfectly insoluble. Some
salts, like Glauber's salts, contain so much water of crystallization
that when subjected to heat they melt and dissolve in it, and this
liquefaction of the solid crystal is called "watery fusion." Other
salts, such as bay salt, chlorate of potash, &c., when heated, fly to
pieces, with a sharp crackling noise, which is due sometimes, to the
unequal expansion of the crystalline surface, or the sudden conversion
of the water (retained in the crystal by capillary attraction) into
steam; thus nitre behaves in this manner, and frequently retains water
in capillary fissures, although it is an anhydrous salt, or salt
perfectly free from combined water. The crackling sound is called
_decrepitation_, and is well illustrated by throwing a handful of bay
salt on a clear fire; but this property is destroyed by powdering the
crystals.

Many substances when melted and slowly cooled concrete into the most
perfect crystals; in these cases heat alone, the antagonist to cohesion,
is the solvent power. Thus, if bismuth be melted in a crucible, and when
cooling, and just as the pellicle (from _pellis_, a skin or crust) is
forming on the surface, if two small holes are instantly made by a rod
of iron and the liquid metal poured out from the inside (one of the
holes being the entrance for the air, the other the exit for the metal);
on carefully breaking the crucible, the bismuth is found to be
crystallized in the most lovely cubes. Sulphur, again, may be
crystallized in prismatic crystals by pursuing a similar plan; and the
great blocks of spermaceti exhibited by wax chandlers in their windows,
are crystallized in the interior and prepared on the same principle.

There are other modes of conferring the crystalline state upon
substances--viz., by elevating them into a state of vapour by the
process called sublimation (from _sublimis_, high or exalted), the
lifting up and condensation of the vapour in the upper part of a vessel;
a process perfectly distinct from that of _distillation_, which means to
separate drop by drop. Both of these processes are very ancient, and
were invented by the Arabian alchemists long antecedent to the seventh
century. Examples of sublimation are shown by heating iodine, and
especially [Page 78] benzoic acid; with the latter, a very elegant
imitation of snow is produced, by receiving the vapour, on some sprigs
of holly or other evergreen, or imitation paper snowdrops and crocuses,
placed in a tasteful manner under a glass vessel. The benzoic acid
should first be sublimed over the sprigs or artificial flowers in a gas
jar, which may be removed when the whole is cold, and a clear glass
shade substituted for it. (Fig. 89.)

All electro deposits on metals are more or less crystalline; and copper
or silver may be deposited in a crystalline form by placing a scraped
stick of phosphorus in a solution of sulphate of copper or of nitrate of
silver. The phosphorus takes away the oxygen from the metal, or
deoxidizes the solution, and the copper or silver reappears in the
metallic form. The surface of the phosphorus must not be scraped in the
air, but under water, when the operation is perfectly safe.

A singular and almost instantaneous crystallization can be produced by
saturating boiling water with Glauber's salt, of which one ounce and a
half of water will usually dissolve about two ounces; having done this,
pour the solution, whilst boiling hot, into clean oil flasks, or vials
of any kind, previously warmed in the oven, and immediately cork them,
or tie strips of wetted bladder, over the orifices of the flasks or
vials, or pour into the neck a small quantity of olive oil, or close the
neck with a cork through which a thermometer tube has been passed. When
cold, no crystallization occurs until atmospheric air is admitted; and
it was formerly believed that the pressure of the air effected this
object, until some one thought of the oil, and now the theory is
modified, and crystallization is supposed to occur in consequence of the
water dissolving some air which causes the deposit of a minute crystal,
and this being the turning point, the whole becomes solid. However the
fact may be explained, it is certain that when the liquid refuses to
crystallize on the admission of air, the solidification occurs directly
a minute crystal of sulphate of soda, or Glauber's salt, is dropped into
the vessel.

When the crystallization is accomplished, the whole mass is usually so
completely solidified, that on inverting the vessel, not a drop of
liquid falls out.

[Illustration: Fig. 89. A. Gas-jar, with stopper open at first, to be
shut when the lamp is withdrawn. B. Wooden stand, with hole to carry the
cup C, containing the benzoic acid, heated below by the spirit-lamp, S.
F. Flowers or sprigs arranged on pieces of rock or mineral.]

[Page 79]

It may be observed that the same mass of salt will answer any number of
times the same purpose. All that is necessary to be done, is to place
the vial or flask, in a saucepan of warm water, and gradually raise it
to the boiling point till the salt is completely liquefied, when the
vessel must be corked and secured from the air as before. When the
solidification is produced much heat is generated, which is rendered
apparent by means of a thermometer, or by the insertion of a copper wire
into the pasty mass of crystal in the flask, and then touching an
extremely thin shaving or cutting of phosphorus, dried and placed on
cotton wool. Solidification in all cases produces heat. Liquefaction
produces cold.

In Masters's freezing apparatus certain measured quantities of
crystallized sal-ammoniac, nitre, and nitrate of ammonia, are placed in
a metallic cylinder, surrounded with a small quantity of spring water
contained in an outer vessel. Directly the crystals are liquefied by the
addition of water, intense cold is produced, which freezes the water and
forms an exact cast of the inner cylinder in ice, and this may
afterwards be removed, by pouring away the liquefied salts, and filling
the inner cylinder, with water of the same temperature as the air, which
rapidly thaws the surrounding ice, and allows it to slip off into any
convenient vessel ready to receive it. (Fig. 90.)

[Illustration: Fig. 90. A. The inner cylinder which contains the
freezing mixture. B B. The outer one containing spring water. C C. The
ice slipping away from the inner cylinder.]

For an ingenious method of obtaining large and perfect crystals of
almost any size, experimentalists are indebted to Le Blanc. His method
consists in first procuring small and perfect crystals--say, octohedra
of alum--and then placing them in a broad flat-bottomed pan, he pours
over the crystals a quantity of saturated solution of alum, obtained by
evaporating a solution of alum until a drop taken out crystallizes on
cooling. The positions of the crystals are altered at least once a day
with a glass rod, so that all the faces may be alternately exposed to
the action of the solution, for the side on which the crystal rests, or
is in contact with the vessel, never receives any increment. The
crystals will thus gradually grow or increase in size, and when they
have done so for some time, the best and most symmetrical, may be
removed and placed separately, in vessels containing some of the same
saturated [Page 80] solution of alum, and being constantly turned they
may be obtained of almost any size desired.

Unless the crystals are removed to fresh solutions, a reaction takes
place, in consequence of the exhaustion of the alum from the water, and
the crystal is attacked and dissolved. This action is first perceptible
on the edges and angles of the crystal; they become blunted and
gradually lose their shape altogether. By this method crystals may be
made to grow in length or breadth--the former when they are placed upon
their sides, the latter if they be made to stand upon their bases.

On Le Blanc's principle, beautiful crystal baskets are made with alum,
sulphate of copper, and bichromate of potash. The baskets are usually
made of covered copper wire, and when the salts crystallize on them as a
nucleus or centre, they are constantly removed to fresh solutions, so
that the whole is completely covered, and red, white, and blue sparkling
crystal baskets formed. They will retain their brilliancy for any time,
by placing them under a glass shade, with a cup containing a little
water.

The sketch below affords an excellent illustration of some of Nature's
remarkable concretions in the peculiar columnar structure of basalt.

[Illustration: Fig. 91. The Giant's Causeway.]

[Page 81]




CHAPTER X.

CHEMISTRY.


[Illustration: Fig. 92. Alchemists at work.]

There is hardly any kind of knowledge which has been so slowly acquired
as that of chemistry, and perhaps no other science has offered such
fascinating rewards to the labour of its votaries as the _philosopher's
stone_, which was to produce an unfailing supply of gold; or _the elixir
of life_, that was to give the discoverer of the gold-making art the
time, the prolonged life, in which he might spend and enjoy it.

Hundreds of years ago Egypt was the great depository of all learning,
art, and science, and it was to this ancient country that the most
celebrated sages of antiquity travelled.

Hermes, or Mercurius Trismegistus, the favourite minister of the
Egyptian king Osiris, has been celebrated as the inventor of the art of
alchemy, and the first treatise upon it has been attributed to Zosymus,
of Chemnis or Panopolis. The Moors who conquered Spain were remarkable
[Page 82] for their learning, and the taste and elegance with which they
designed and carried out a new style of architecture, with its lovely
Arabesque ornamentation. They were likewise great followers of the art
of alchemy, when they ceased to be conquerors, and became more
reconciled to the arts of peace. Strange that such a people, thirsting
as they did in after years for all kinds of knowledge, should have
destroyed, in the persons of their ancestors, the most numerous
collection of books that the world had ever seen: the magnificent
library of Alexandria, collected by the Ptolemies with great diligence
and at an enormous expense, was burned by the orders of Caliph Omar;
whilst it is stated that the alchemical works had been previously
destroyed by Diocletian in the fourth century, lest the Egyptians should
acquire by such means sufficient wealth to withstand the Roman power,
for gold was then, as it is now, "the sinews of war."

Eastern historians relate the trouble and expense incurred by the
succeeding Caliphs, who, resigning the Saracenic barbarism of their
ancestors, were glad to collect from all parts the books which were to
furnish forth a princely library at Bagdad. How the learned scholar
sighs when he reads of seven hundred thousand books being consigned to
the ignominious office of heating forty thousand baths in the capital of
Egypt, and of the magnificent Alexandrian Library, a mental fuel for the
lamp of learning in all ages, consumed in bath furnaces, and affording
six months' fuel for that purpose. The Arabians, however, made amends
for these barbarous deeds in succeeding centuries, and when all Europe
was laid waste under the iron rule of the Goths, they became the
protectors of philosophy and the promoters of its pursuits; and thus we
come to the seventh century, in which Geber, an Arabian prince lived,
and is stated to be the earliest of the true alchemists whose name has
reached posterity.

Without attempting to fill up the alchemical history of the intervening
centuries, we leap forward six hundred years, and now find ourselves in
imagination in England, with the learned friar, Roger Bacon, a native of
Somersetshire, who lived about the middle of the thirteenth century; and
although the continual study of alchemy had not yet produced the
"stone," it bore fruit in other discoveries, and Roger Bacon is said,
with great appearance of truth, to have discovered gunpowder, for he
says in one of his works:--"From saltpetre and _other_ ingredients we
are able to form a fire which will burn to any distance;" and again
alluding to its effects, "a small portion of matter, about the size of
the thumb, _properly disposed_, will make a tremendous sound and
coruscation, by which cities and armies might be destroyed." The
exaggerated style seems to have been a favourite one with all
philosophers, from the time of Roger Bacon to that of Muschenbroek of
the University of Leyden, who accidentally discovered the Leyden jar in
the year 1746, and receiving the first shock, from a vial containing a
little water, into which a cork and nail had been fitted, states that
"he felt himself struck in his arms, shoulders, and breast, so that he
lost his breath, and was _two days_ before he recovered from the effects
of the blow and the terror;" [Page 83] adding, that "he would not take
a _second_ shock for the kingdom of France." Disregarding the numerous
alchemical events occurring from the time of Roger Bacon, we again
advance four hundred years--viz., to the year 1662, when, on the 15th of
July, King Charles II. granted a royal charter to the Philosophical
Society of Oxford, who had removed to London, under the name of the
Royal Society of London for Promoting Natural Knowledge, and in the year
1665 was published the first number of the _Philosophical Transactions_;
this work contains the successive discoveries of Mayow, Hales, Black,
Leslie, Cavendish, Lavoisier, Priestley, Davy, Faraday; and since the
year 1762 has been regularly published at the rate of one volume per
annum. With this preface proceed we now to discuss some of the varied
phenomena of chemical attraction, or what is more correctly termed


CHEMICAL AFFINITY.

The above title refers to an endless series of changes brought about by
chemical combinations, all of which can be reduced to certain fixed
laws, and admit of a simple classification and arrangement. A mechanical
aggregation, however well arranged, can be always distinguished from a
chemical one. Thus, a grain of gunpowder consists of _nitre_, which can
be washed away with boiling water, of _sulphur_, which can be sublimed
and made to pass away as vapour, of _charcoal_, which remains behind
after the previous processes are complete; this mixture has been
perfected by a careful proportion of the respective ingredients, it has
been wetted, and ground, and pressed, granulated, and finally dried; all
these mechanical processes have been so well carried out that each
grain, if analysed, would be similar to the other; and yet it is, after
all, only a mechanical aggregation, because the sulphur, the charcoal,
and the nitre are unchanged. A grain of gunpowder moistened, crushed,
and examined by a high microscopic power, would indicate the yellow
particles of sulphur, the black parts of charcoal, whilst the water
filtered from the grain of powder and dried, would show the nitre by the
form of the crystal. On the other hand, if some nitre is fused at a dull
red heat in a little crucible, and two or three grains of sulphur are
added, they are rapidly oxidized, and combine with the potash, forming
sulphate of potash; and after this change a few grains of charcoal may
be added in a similar manner, when they burn brightly, and are oxidized
and converted into carbonic acid, which also unites in like manner with
the potash, forming carbonate of potash; so that when the fused nitre is
cooled and a few particles examined by the microscope, the charcoal and
sulphur are no longer distinguishable, they have undergone a chemical
combination with portions of the nitre, and have produced two new salts,
perfectly different in taste, gravity, and appearance from the original
substances employed to produce them. Hence chemical combination is
defined to be "_that property which is possessed by one or more
substances, of uniting together and producing a third or other body
perfectly different [Page 84] in its nature from either of the two or more
generating the new compound_".

To return to our first experiment with the gunpowder: take sulphur,
place some in an iron ladle, heat it over a gas flame till it catches
fire, then ascend a ladder, and pour it gently, from the greatest height
you can reach, into a pail of warm water: if this experiment is
performed in a darkened room a magnificent and continuous stream of fire
is obtained, of a blue colour, without a single break in its whole
length, provided the ladle is gradually inclined and emptied. The
substance that drops into the warm water is no longer yellow and hard,
but is red, soft, and plastic; it is still sulphur, though it has taken
a new form, because that element is dimorphous ([Greek: _dis_] twice,
and [Greek: _morphê_] a form), and, Proteus-like, can assume two forms.
Take another ladle, and melt some nitre in it at a dull red heat, then
add a small quantity of sulphur, which will burn as before; and now,
after waiting a few minutes, repeat the same experiment by pouring the
liquid from the steps through the air into water; observe it no longer
flames, and the substance received into the water is not red and soft
and plastic, but is white, or nearly so, and rapidly dissolves away in
the water. The sulphur has united with the oxygen of the nitre and
formed sulphuric acid, which combines with the potash and forms sulphate
of potash; here, then, oxygen, sulphur, and potassium, have united and
formed a salt in which the separate properties of the three bodies have
completely disappeared; to prove this, it is only necessary to dissolve
the sulphate of potash in water, and after filtering the solution, or
allowing it to settle, till it becomes quite clear and bright, some
solution of baryta may now be added, when a white precipitate is thrown
down, consisting of sulphate of baryta, which is insoluble in nitric or
other strong acids. The behaviour of a solution of sulphate of potash
with a nitrate of baryta may now be contrasted with that of the elements
it contains; on the addition of sulphur to a solution of nitrate of
baryta no change whatever takes place, because the sulphur is perfectly
insoluble. If a stream of oxygen gas is passed from a bladder and jet
through the same test, no effect is produced; the nitrate of baryta has
already acquired its full proportion of oxygen, and no further addition
has any power to change its nature; finally, if a bit of the metal
potassium is placed in the solution of nitrate of baryta it does not
sink, being lighter than water, and it takes fire; but this is not in
any way connected with the presence of the test, as the same thing will
happen if another bit of the metal is placed in water--it is the oxygen
of the latter which unites rapidly with the potassium, and causes it to
become so hot that the hydrogen, escaping around the little red-hot
globules, takes fire; moreover, the fact of the combustion of the
potassium under such circumstances is another striking proof of the
opposite qualities of the three elements--sulphur, oxygen, and
potassium--as compared with the three chemically combined and forming
sulphate of potash. The same kind of experiment may be repeated with
charcoal; if some powdered charcoal is made red-hot, and then puffed
into the air with a blowing machine, numbers of sparks are produced, and
the [Page 85] charcoal burns away and forms carbonic acid gas, a little
ash being left behind; but if some more nitre be heated in a ladle, and
charcoal added, a brilliant deflagration (_deflagro_, to burn) occurs,
and the charcoal, instead of passing away in the air as carbonic acid,
is now retained in the same shape, but firmly and chemically united with
the potash of the nitre, forming carbonate of potash, or pearl-ash,
which is not black and insoluble in water and acids like charcoal, but
is white, and not only soluble in water, but is most rapidly attacked by
acids with effervescence, and the carbon escapes in the form of carbonic
acid gas. Thus we have traced out the distinction between mechanical
aggregation and chemical affinity, taking for an example the difference
between gunpowder as a whole (in which the ingredients are so nicely
balanced that it is almost a chemical combination), and its
constituents, sulphur, charcoal, and nitre, when they are chemically
combined; or, in briefer language, we have noticed the difference
between the mechanical mixture, and some of the chemical combinations,
of three important elements. Our very slight and partial examination of
three simple bodies does not, however, afford us any deep insight into
the principles of chemistry; we have, as it were, only mastered the
signification of a few words in a language; we might know that _chien_
was the French for dog, or _cheval_ horse, or _homme_ man; but that
knowledge would not be the acquisition of the French language, because
we must first know the alphabet, and then the combination of these
letters into words; we must also acquire a knowledge of the proper
arrangement of these words into sentences, or grammar, both syntax and
prosody, before we can claim to be a French scholar: so it is with
chemistry--any number of isolated experiments with various chemical
substances would be comparatively useless, and therefore the "alphabet
of chemistry," or "table of simple elements," must first be acquired.
These bodies are understood to be solids, fluids, and gases, which have
hitherto defied the most elaborate means employed to reduce them into
more than one kind of matter. Even pure light is separable into seven
parts--viz., red, orange, yellow, green, blue, indigo, and violet; but
the elements we shall now enumerate are not of a compound, but, so far
as we know, of an absolutely simple or single nature; they represent the
boundaries, not the finality, of the knowledge that may be acquired
respecting them.

The elements are sixty-four in number, of which about forty are
tolerably plentiful, and therefore common; whilst the remainder,
twenty-four, are rare, and for that reason of a lesser utility: whenever
Nature employs an element on a grand scale it may certainly be called
common, but it generally works for the common good of all, and fulfils
the most important offices.

[Page 86]

  CLASSIFICATION OF THE ALPHABET OF CHEMISTRY.

       13 _Non-Metallic Bodies._

       Name.  Symbol.    Combining
                        proportion
                         or atomic
                          weight.

   1. Oxygen      O   =        8
   2. Hydrogen    H   =        1
   3. Nitrogen    N   =       14
   4. Chlorine    Cl  =       35.5
   5. Iodine      I   =      127.1
   6. Bromine     Br  =       80.
   7. Fluorine    F   =       18.9
   8. Carbon      C   =        6
   9. Boron       B   =       10.9
  10. Sulphur     S   =       16
  11. Phosphorus  P   =       32
  12. Silicon     Si  =       21.3
  13. _Selenium_  Se  =       39.5

              51 _Metals._

   1. Aluminum     Al  =       13.7
   2. Antimony     Sb  =      129
   3. Arsenic      As  =       75
   4. Barium       Ba  =       68.5
   5. Bismuth      Bi  =      213
   6. Cadmium      Cd  =       56
   7. Calcium      Ca  =       20
   8. _Cerium_     Ce  =       47
   9. Chromium     Cr  =       26.7
  10. Cobalt       Co  =       29.5
  11. Copper       Cu  =       31.7
  12. _Donarium_
  13. _Didymium_   D
  14. _Erbium_     E
  15. Gold         Au  =      197
  16. _Glucinum_   Gl
  17. Iron         Fe  =       28
  18. _Ilmenium_   Il
  19. _Iridium_    Ir  =       99
  20. Lead         Pb  =      103.7
  21. _Lanthanium_ La
  22. _Lithium_    Li  =        6.5
  23. Magnesium    Mg  =       12.2
  24. Manganese    Mn  =       27.6
  25. Mercury      Hg  =      100
  26. _Molybdenum_ Mo  =       46
  27. Nickel        Ni  =     29.6
  28. _Norium_
  29. _Niobium_     Nb
  30. _Osmium_      Os  =     99.6
  31. Platinum      Pt  =     98.7
  32. Potassium     K   =     39.2
  33. Palladium     Pd  =     53.3
  34. _Pelopium_    Pe
  35. Rhodium       R   =     52.2
  36. _Rhuthenium_  Ru  =     52.2
  37. Silver        Ag  =    108.1
  38. Sodium        Na  =     23
  39. Strontium     Sr  =     43.8
  40. Tin           Sn  =     59
  41. _Tantalum_    Ta  =    184
  42. _Tellurium_   Te  =     64.2
  43. _Terbium_     Tb
  44. _Thorium_     Th  =     59.6
  45. _Titanium_    Ti  =     25
  46. Tungsten      W[A]=     95
  47. Uranium       U   =     60
  48. _Vanadium_    V   =     68.6
  49. _Yttrium_     Y
  50. Zinc          Zn  =     32.6
  51. _Zirconium_   Zr  =     22.4

  (N.B. The elements printed in italics are at present unimportant.)

[Footnote A: From the mineral Wolfram, and now exceedingly valuable, as
when alloyed with iron it is harder than, and will bore through steel.]

A few words will suffice to explain the meaning of the terms which head
the names, letters, and numbers of the Table of Elements. [Page 87] The
names of the elements have very interesting derivations, which it is
not the object of this work to go into; the symbols are abbreviations,
ciphers of the simplest kind, to save time and trouble in the frequent
repetition of long words, just as the signs + plus, and - minus, are
used in algebraic formulæ. For instance--the constant recurrence of
water in chemical combinations must be named, and would involve the most
tedious repetition; water consists of oxygen and hydrogen, and by taking
the first letter of each word we have an instructive symbol, which not
only gives us an abbreviated term for water, but also imparts at once a
knowledge of its composition by the use of the letters, HO.

Again, to take a more complex example, such as would occur in the study
of organic chemistry--a sentence such as _the hydrated oxide of
acetule_, is written at once by C_{4}H_{4}O_{2}, the figures referring
to the number of equivalents of each element--viz., 4 equivalents of C,
the symbol for carbon, 4 of H (hydrogen), and 2 of O (oxygen).

The long word paranaphthaline, a substance contained in coal tar, is
disposed of at once with the symbols and figures C_{30}H_{12}.

The figures in the third column are, however, the most interesting to
the precise and mathematically exact chemist. They represent the united
labours of the most painstaking and learned chemists, and are the exact
quantities in which the various elements unite. To quote one example: if
8 parts by weight of oxygen--viz., the combining proportions of that
element--are united with 1 part by weight of hydrogen, also its
combining number, the result will be 9 parts by weight of water; but if
8 parts of oxygen and 2 parts of hydrogen were used, one only of the
latter could unite with the former, and the result would be the
formation again of 9 parts of water, with an overplus of 1 equivalent of
hydrogen.

It is useless to multiply examples, and it is sufficient to know that
with this table of numbers the figures of analysis are obtained.
Supposing a substance contained 27 parts of water, and the oxygen in
this had to be determined, the rule of proportion would give it at once,
9: 27:: 8: 24. 9 parts of water are to 27 parts as 8 of oxygen (the
quantity contained in 9 parts of water) are to the answer
required--viz., 24 of oxygen. The names, symbols, and combining
proportions being understood, we may now proceed with the performance of
many interesting


CHEMICAL EXPERIMENTS.

As the permanent gases head the list, they will first engage our
attention, beginning with the element oxygen--Symbol O, combining
proportion 8. There is nothing can give a better idea of the enormous
quantity of oxygen present in the animal, vegetable, and mineral
kingdoms, than the statement that it represents _one-third_ of the
weight of the whole crust of the globe. Silica, or flint, contains about
half its weight of oxygen; lime contains forty per cent.; alumina about
thirty-three per cent. In these substances the element oxygen remains
inactive and powerless, chained by the strong fetters of chemical
affinity to [Page 88] the silicium of the flint, the calcium of the
lime, and the aluminum of the alumina. If these substances are heated by
themselves they will not yield up the large quantity of oxygen they
contain.

Nature, however, is prodigal in her creation, and hence we have but to
pursue our search diligently to find a substance or mineral containing
an abundance of oxygen, and part of which it will relinquish by what
used to be called by the "old alchemists" the _torture_ of heat. Such a
mineral is the black oxide of manganese, or more correctly the binoxide
of manganese, which consists of one combining proportion of the metal
manganese--viz., 27.6, and two of oxygen--viz., 8 × 2 = 16. If three
proportions of the binoxide of manganese are heated to redness in an
iron retort, they yield one proportion (equal to 8) of oxygen, and all
that has just been explained by so many words is comprehended in the
symbols and figures below:--

  3 MnO_{2} = Mn_{3}O_{4} + _O_.

Thus the 3 MnO_{2} represent the three proportions of the binoxide of
manganese before heat is applied, whilst the sign =, the sign of
equation (equal to), is intended to show that the elements or compounds
placed _before_ it produce those which _follow_ it; hence the sequel
Mn_{3}O_{4} + _O_ shows that another compound of the metal and oxygen is
produced, whilst the + _O_ indicates the liberated oxygen gas. The iron
retort employed to hold the mineral should be made of cast iron in
preference to wrought iron, as the latter is very soon worn out by
contact with oxygen at a red heat. A gun-barrel will answer the purpose
for an experiment on the small scale, to which must be adapted a cock
and piece of pewter tubing. Such a make-shift arrangement may do very
well when nothing better offers; but as a question of expense, it is
probably cheaper in the end to order of Messrs. Simpson and Maule, or of
Messrs. Griffin, or of Messrs. Bolton, a cast-iron bottle, or cast-iron
retort, as it is termed, of a size sufficient to prepare two gallons of
oxygen from the binoxide of manganese, which, with four feet of iron
conducting-pipe, and connected to the bottle with a screw, does not
[Page 89] cost more than six shillings--an enormous dip, perhaps, in
the juvenile pocket, and therefore we shall indicate presently a still
cheaper apparatus for the same purpose. (Fig. 93.)

[Illustration: Fig. 93. A. The iron bottle, containing the black oxide
of manganese, with pipe passing to the pneumatic trough, B B, in which
is fixed a shelf, C, perforated with a hole, under which the end of the
pipe is adjusted, and the gas passes into the gas-jar, D.]

[Illustration: Fig. 94. A A. Pneumatic trough, with gas jar raised to
shelf; bubbles of air are rushing in at B, as the level of the water is
below the shelf--viz., at C C. D D. Same trough and gas jar with water
kept over the shelf by the introduction of the stone pitcher E, full of
water.]

The oxygen is conveyed to a square tin box provided with a shelf at one
end, perforated with several holes at least one inch in diameter, called
the pneumatic trough; any wooden trough, butter or wash-tub, foot-pan or
bath, provided with a shelf, may be raised by the same title to the
dignity of a piece of chemical apparatus. The gas jar must be filled
with water by withdrawing the stopper and pressing it down into the
trough, and when the neck is below the level of the water, the stopper
is again inserted, and the jar with the water therein contained lifted
steadily on to the shelf, the entry of atmospheric air being prevented
by keeping the lower part of the gas jar, called the welt, under the
water. Sometimes the pneumatic trough contains so small a quantity of
water that on raising the gas jar to the shelf the liquid does not cover
the bottom, and the air rushes up in large bubbles. Under these
circumstances it is better to provide a gallon stone jug full of water,
so that when the jar is being raised to the shelf it may be thrust into
the trough (on the same principle as the crow and the pitcher in the
fable), and thus by its bulk (as the stones in the pitcher) raise the
water to the proper level. When the gas jar is about half filled with
gas the jug may be withdrawn. This arrangement saves the trouble of
constantly adding and baling out water from the pneumatic trough. (Fig.
94.)

There are other solid oxygenized bodies in which the affinities are less
powerful, and hence a lower degree of heat suffices to liberate the
oxygen gas, and one of the most useful in this respect is the salt
termed chlorate of potash. If the substance is heated by itself, the
temperature required to expel the oxygen is almost as high as that
demanded for the black oxide of manganese; but, strange to say, if the
two substances are reduced to powder, and mixed in equal quantities by
weight, then a very moderate increase of heat is sufficient to cause the
chlorate of [Page 90] potash to give up its oxygen, whilst the oxide of
manganese undergoes no change whatever. It seems to fulfil only a
mechanical office--possibly that of separating each particle of chlorate
of potash from the other, so that the heat attacks the substance in
detail, just as a solid square of infantry might repel almost any
attack, whilst the same body dispersed over a large space might be of
little use; so with the chlorate of potash, which undergoes rapid
decomposition when mixed with and divided amongst the particles of the
oxide of manganese; less so with the red oxide of iron, and still less
with sand or brick-dust. (Fig. 95.)

[Illustration: Fig. 95. Preparation of oxygen from chlorate of potash
and oxide of manganese.

  KO.ClO_{5} = { O_{6}
               { KCl.

]

This curious fact is explained usually by reference to what is called
catalytic action, or _decomposition by contact_ ([Greek: _kata_],
downwards, and [Greek: _luô_], I unloosen), _being a power possessed by a
body of resolving another into a new compound without undergoing any
change itself_. To make this term still clearer, we may notice another
example in linen rags, which may be exposed for any length of time to
the action of water without fear of conversion into sugar; if, however,
oil of vitriol is first added to the linen rags, and they are
subsequently digested at a proper temperature with water, then the rags
are converted into sugar (the author has seen a specimen made of an "old
shirt"); but, curious to relate, the oil of vitriol is unchanged in the
process, and if the process be commenced with a pound of acid, the same
quantity is discoverable at the end of the chemical decomposition of the
linen rags, and their conversion into sugar.

If a mixture of equal parts of oxide of manganese and chlorate of potash
is placed in a clean Florence flask, with a cork, and pewter, or glass
tube attached, great quantities of oxygen are quickly liberated, on the
application of the heat of a spirit lamp. Such a retort would cost about
fourpence, and if the flask is broken in the operation it can be easily
replaced by another, value one penny, as the same cork and tube will
generally suit a number of these cheap glass vessels. Corks may always
[Page 91] be softened by using either a proper cork squeezer, or by
placing them under a piece of board or a flat surface, and rolling and
pressing the cork till quite elastic.

Whilst fitting the latter into the neck of a flask, it is perhaps safer
to hold the thin and fragile vessel in a cloth, so that if the flask
breaks the chemical experiment may not be arrested for many days by the
severe cutting and wounding of the fingers. After the cork is fitted, it
is to be removed from the flask and bored with a cork borer. This useful
tool is sold in complete sets to suit all sizes of glass tubes, and the
pewter or glass being inserted, the flask and tube will be ready for
use, provided the tube is bent to the proper curve. This is easy enough
to perform with the pewter, but not quite so easy with the glass tube,
which must be held over the flame of a spirit lamp till soft, and then
bent very gradually to the proper curve. If a short length of the glass
tube is heated, it bends too sharply, and the convexity of the glass is
flattened, whilst the internal diameter of the tube is lessened, so that
at least three inches in length should be warmed, and the heat must not
be continued in one place only, but should be maintained in the
direction of the bend, the whole manipulation being conducted without
any hurry. (Fig. 96.)

[Illustration: Fig. 96. A. The cork squeezer. B. The cork borers. C. The
operation of bending the glass tube over the flame of the spirit-lamp.
D. The neck of the flask, with cork and tube bent and fitted complete
for use.]

Having filled a gas jar with oxygen, it may be removed from the
pneumatic trough by sliding it into a plate under the surface of the
water, and to prevent the stopper being thrust out accidentally from the
jar by the upward pressure of the gas, whilst a little compressed,
during the act of passing it into the plate, it is advisable to hold the
stopper of the jar firmly but gently, so that it cannot slip out of its
place. A number of jars of oxygen may be prepared and arranged in
plates, all of which of course must contain a little water, and enough
to cover the welt of the jar.

[Page 92]

EXPERIMENTS WITH OXYGEN GAS.

This gas was originally discovered by Priestley, in August, 1774, and
was first obtained by heating red precipitate--_i.e._, the red oxide of
mercury.

  HgO = Hg + O.

We leave these symbols and figures to be deciphered by the youthful
philosopher with the aid of the table of elements, &c., and return to
the experiments.

There are certain thin wax tapers like waxed cord, called bougies, which
can be bent to any shape, and are very convenient for experiments with
the gases. If one of these tapers is bent as in Fig. 97, then lighted
and allowed to burn for some minutes, a long snuff is gradually formed,
which remains in a state of ignition when the flame of the taper is
blown out. On plunging this into a jar of oxygen, it instantly re-lights
with a sort of report, and burns with greatly-increased brilliancy, as
described by Dr. Priestley in his first experiment with this gas, and so
elegantly repeated by Professor Brande in his refined dissertation on
the progress of chemical science.

[Illustration: Fig. 97.]

"The 1st of August, 1774, is a _red-letter day_ in the annals of
chemical philosophy, for it was then that Dr. Priestley discovered
dephlogisticated air. Some, sporting in the sunshine of rhetoric, have
called this the birthday of pneumatic chemistry; but it was even a more
marked and memorable period; it was then (to pursue the metaphor) that
this branch of science, having eked out a sickly and infirm infancy in
the ill-managed nursery of the early chemists, began to display symptoms
of an improving constitution, and to exhibit the most hopeful and
unexpected marks of future importance. The first experiment, which led
to a very satisfactory result, was concluded as follows:--A glass jar
was filled with quicksilver, and inserted in a basin of the same; some
red precipitate of quicksilver was then introduced, and floated upon the
quicksilver in the jar; heat was applied to it in this situation with a
burning-lens, and to use Priestley's own words, _I presently found that
air was expelled from it very readily. Having got about three or four
times as much as the bulk of my materials, I admitted water into it, and
found that it was not imbibed by it. But what surprised me more than I
can well express was, that a candle burned in this air with a remarkably
vigorous flame, very much like that enlarged flame with which a candle
burns in nitrous air exposed to iron or lime of sulphur_ (_i.e._,
laughing gas); _but as I had got nothing like this remarkable appearance
from any kind of air besides this peculiar [Page 93] modification of
nitrous air, and I knew no nitrous acid was used in the preparation of
mercurius calcinatus, I was utterly at a loss how to account for it._"
(Fig. 98.)

[Illustration: Fig. 98. A. Glass vessel full of mercury, containing the
red precipitate at the top, and standing in the dish B, also containing
mercury. C. The burning-glass concentrating the sun's rays on the red
precipitate, being Priestley's original experiment.]


_Second Experiment._

The term oxygen is derived from the Greek ([Greek: _ozus_], acid, and
[Greek: _gennaô_], I give rise to), and was originally given to this
element by Lavoisier, who also claimed its discovery; and if this honour
is denied him, surely he has deserved equal scientific glory in his
masterly experiments, through which he discovered that the mixture of
forty-two parts by measure of azote, with eight parts by measure of
oxygen, produced a compound precisely resembling our atmosphere. The
name given to oxygen was founded on a series of experiments, one of
which will now be mentioned.

[Illustration: Fig. 99. A. The deflagrating spoon, B. The cork. C. The
zinc, or brass, or tin plate. D D. The gas jar.]

Place some sulphur in a little copper ladle attached to a wire, and
called a deflagrating spoon, passed through a round piece of zinc or
brass plate and cork, so that the latter acts as an adjusting
arrangement to fix the wire at any point required. The combustion of the
sulphur, previously feeble, now assumes a remarkable intensity, and a
peculiar coloured light is generated, whilst the sulphur unites with the
oxygen, and forms sulphurous acid gas. It produces, in fact, the same
gas which is formed by burning an ordinary sulphur match. This compound
is valuable as a disinfectant, and is a very important bleaching agent,
being most extensively employed in the whitening of straw employed in
the manufacture of straw bonnets. It is an acid gas, as Lavoisier found,
and this property may be detected by pouring a little tincture of litmus
into the bottom of the plate in which the gas jar stands. The blue
colour of the litmus is rapidly changed to red, and it might be thought
that no further argument could possibly be required to prove that oxygen
was _the_ acidifying agent, the more particularly as the result is the
same in the next illustration.

[Page 94]

_Third Experiment._

Cut a small piece from an ordinary stick of phosphorus under water, take
care to dry it properly with a cloth, and after placing it in a
deflagrating spoon, remove the stopper from the gas-jar, as there is no
fear of the oxygen rushing away, because it is somewhat heavier than
atmospheric air; and then, after placing the spoon with the phosphorus
in the neck of the jar, apply a heated wire and pass the spoon at once
into the middle of the oxygen; in a few seconds a most brilliant light
is obtained, and the jar is filled with a white smoke; as this subsides,
being phosphoric acid, and perfectly soluble in water, the same litmus
test may be applied, when it is in like manner changed to red. The acid
obtained is one of the most important constituents of bone.


_Fourth Experiment._

A bit of bark-charcoal bound round with wire is set on fire either by
holding it in the flame of a spirit-lamp, or by attaching a small piece
of waxed cotton to the lower part, and igniting this; the charcoal may
then be inserted into a bottle of oxygen, when the most brilliant
scintillations occur. After the combustion has ceased and the whole is
cool, a little tincture of litmus may also be poured in and shaken
about, when it likewise turns red, proving for the third time the
generation of an acid body, called carbonic acid--an acid, like the
others already mentioned, of great value, and one which Nature employs
on a stupendous scale as a means of providing plants, &c., with solid
charcoal. Carbonic acid, a virulent poison to animal life, is, when
properly diluted, and as contained in atmospheric air, one of the chief
alimentary bodies required by growing and healthy plants.

In three experiments acid bodies have been obtained; can we speculate on
the result of the next?


_Fifth Experiment._

Into a deflagrating spoon place a bit of potassium, set this on fire by
holding it in the spoon in the flame of a spirit-lamp, and then rapidly
plunge the burning metal into a bottle of oxygen. A brilliant ignition
occurs in the deflagrating spoon for a few seconds, and there is little
or no smoke in the jar. The product this time is a solid, called potash,
and if this be dissolved in water and filtered, it is found to be clear
and bright, and now on the addition of a little tincture of litmus to
one half of the solution, it is wholly unaffected, and remains blue; but
if with the other half a small quantity of tincture of turmeric is
mixed, it immediately changes from a bright yellow solution to a
reddish-brown, because turmeric is one of the tests for an alkali; and
thus is ascertained by the help of this and other tests that the result
of the combustion is not an _acid_, but an _alkali_. The experiment is
made still more satisfactory by burning another bit of potassium in
oxygen and dissolving the product in water, and if any portion of [Page
95] the reddened liquid derived from the sulphurous, phosphoric, and
carbonic acids taken from the previous experiments, be added to separate
portions of the alkaline solution, they are all restored to their
original blue colour, because an acid is neutralized by an alkali; and
the experiment is made quite conclusive by the restoration of the
reddened turmeric to a bright yellow on the addition of a solution of
either of the three acids already named. Moreover, an acid need not
contain a fraction of oxygen, as there is a numerous class of
_hy_dracids, in which the acidifying principle is hydrogen instead of
oxygen, such as the hydrochloric, hydriodic, hydro-bromic, and
hydrofluoric acids.


_Sixth Experiment._

A piece of watch-spring is softened at one end, by holding it in the
flame of a spirit-lamp, and allowing it to cool. A bit of waxed cotton
is then bound round the softened end, and after being set on fire, is
plunged into a gas jar containing oxygen; the cotton first burns away,
and then the heat communicates to the steel, which gradually takes fire,
and being once well ignited, continues to burn with amazing rapidity,
forming drops of liquid dross, which fall to the bottom of the
plate--and also a reddish smoke, which condenses on the sides of the
jar; neither the dross which has dropped into the plate, nor the reddish
matter condensed on the jar, will affect either tincture of litmus or
turmeric; they are neither acid nor alkaline, but _neutral_ compounds of
iron, called the sesquioxide of iron (Fe_{2}O_{3}), and the magnetic
oxide (Fe_{3}O_{4}=FeO.Fe_{2}O_{3}).


_Seventh Experiment._

[Illustration: Fig. 100. A. Bladder containing oxygen, provided with a
stop-cock and jet leading to, B, B. Finger glass containing boiling
water. C. The cup of melted phosphorus under the water. The gas escapes
from the bladder when pressed.]

Some oxygen gas contained in a bladder provided with a proper jet may be
squeezed out, and upon, some liquid phosphorus contained in a cup at the
bottom of a finger glass full of boiling water, when a most brilliant
combustion occurs, proving that so long as the principle is complied
with--viz., that of furnishing oxygen to a combustible substance--it
will burn under water, provided it is insoluble, and possesses the
remarkable affinity for oxygen which belongs to phosphorus. The
experiment should be performed with boiling water, to keep the
phosphorus in the liquid state; and it is quite as well to hold a [Page
96] square foot of wire gauze over the finger glass whilst the
experiment is being performed. (Fig. 100.)


_Eighth Experiment._

Oxygen is available from many substances when they are mixed with
combustible substances, and hence the brilliant effects produced by
burning a mixture of nitre, meal powder, sulphur, and iron or steel
filings; the metal burns with great brilliancy, and is projected from
the case in most beautiful sparks, which are long and needle-shaped with
steel, and in the form of miniature rosettes with iron filings; it is
the oxygen from the nitre that causes the combustion of the metal, the
other ingredients only accelerate the heat and rate of ignition of the
brilliant iron, which is usually termed a gerb.


_Ninth Experiment._

A mixture of nitrate of potash, powdered charcoal, sulphur, and nitrate
of strontium, driven into a strong paper case about two inches long, and
well closed at the end with varnish, being quite waterproof, may be set
on fire, and will continue to burn under water until the whole is
consumed; the only precaution necessary being to burn the composition
from the case with the mouth downward, and if the experiment is tried in
a deep glass jar it has a very pleasing effect. (Fig. 101.)

[Illustration: Fig. 101. A. Case of red fire burning downwards, and
attached by a copper wire to a bit of leaden pipe B, to sink it. C C.
Jar containing water.]

The red-fire composition is made by mixing nitrate of strontia 40 parts
by weight, flowers of sulphur 13 parts, chlorate of potash 5 parts,
sulphuret of antimony 4 parts. These ingredients must first be well
powdered separately, and then mixed carefully on a sheet of paper with a
paper-knife. They are liable to explode if rubbed _together_ in a
mortar, on account of the presence of sulphur and chlorate of potash,
and the composition, if kept for any time, is liable to take fire
spontaneously.


_Tenth Experiment._

Some zinc is melted in an iron ladle, and made quite red hot; if a
little dry nitre is thrown upon the surface, and gently stirred into the
metal, it takes fire with the production of an intense white light,
whilst large quantities of white flakes ascend, and again descend when
cold, being the oxide of zinc, and called by the alchemists the
"Philosopher's Wool" (ZnO). In this experiment the oxygen from the nitre
effects the oxidation of the metal zinc.

[Page 97]

_Eleventh Experiment._

A mixture of four pounds of nitre with two of sulphur and one and a half
of lamp black produces a most beautiful and curious fire, continually
projected into the air as sparks having the shape of the rowel of a
spur, and one that may be burnt with perfect safety in a room, as the
sparks consume away so rapidly, in consequence of the finely divided
condition of the charcoal, that they may be received on a handkerchief
or the hand without burning them. The difficulty consists in effecting
the complete mixture of the charcoal. The other two ingredients must
first be thoroughly powdered separately, and again triturated when
mixed, and finally the charcoal must be rubbed in carefully, till the
whole is of a uniform tint of grey and very nearly black, and as the
mixture proceeds portions must be rammed into a paper case, and set on
fire; if the stars or pinks come out in clusters, and spread well
without other and duller sparks, it is a sign that the whole is well
mixed; but if the sparks are accompanied with dross, and are projected
out sluggishly, and take some time to burn, the mixture and rubbing in
the mortar must be continued; and even that must not be carried too far,
or the sparks will be too small. N.B.--If the lamp-black was heated red
hot in a close vessel, it would probably answer better when cold and
powdered.


_Twelfth Experiment._

Into a tall gas jar with a wide neck project some red-hot lamp-black
through a tin funnel, when a most brilliant flame-like fire is obtained,
showing that finely divided charcoal with pure oxygen would be
sufficient to afford light; but as the atmosphere consists of oxygen
diluted with nitrogen, compounds of charcoal with hydrogen, are the
proper bodies to burn, to produce artificial light.


_Thirteenth Experiment. The Bude Light._

This pretty light is obtained by passing a steady current of oxygen gas
(escaping at a very low pressure) through and up the centre pipe of an
argand oil lamp, which must be supplied with a highly carbonized oil and
a very thick wick, as the oxygen has a tendency to burn away the cotton
unless the oil is well supplied, and allowed to overflow the wick, as it
does in the lamps of the lighthouses. The best whale oil is usually
employed, though it would be worth while to test the value of Price's
"Belmontine Oil" for the same purpose. (Fig. 102.)

[Illustration: Fig. 102. A. Reservoir of oil. B. The flexible pipe
conveying oxygen to centre of the argand lamp.]

[Page 98]

_Fourteenth Experiment. A Red Light._

Clear out the oil thoroughly from the Bude light apparatus; or, what is
better, have two lamps, one for oil, and the other for spirit; fill the
apparatus with a solution of nitrate of strontia and chloride of calcium
in spirits of wine, and let it burn from the cotton in the same way as
the oil, and supply it with oxygen gas.


_Fifteenth Experiment. A Green Light._

Dissolve boracic acid and nitrate of baryta in spirits of wine, and
supply the Bude lamp with this solution.


_Sixteenth Experiment. A Yellow Light._

Dissolve common salt in spirits of wine, and burn it as already
described in the Bude light apparatus.


_Seventeenth Experiment. The Oxy-calcium Light._

[Illustration: Fig. 103.--No. 1. A. Oxygen jet. B. The ball of lime,
suspended by a wire. C. Spirit lamp.

No. 2. D. Oxygen jet. E. Gas (jet connected with the gas-pipe in the
rear by flexible pipe) projected on to ball of lime, F.]

This very convenient light is obtained in a simple manner, either by
using a jet of oxygen as a blowpipe to project the flame of a spirit
lamp on to a ball of lime; or common coal-gas is employed instead of the
spirit lamp, being likewise urged against a ball of lime. By this plan
one bag containing oxygen suffices for the production of a brilliant
light, not equal, however, to the oxy-hydrogen light, which will be
explained in the article on hydrogen. (Fig. 103.)


_Eighteenth Experiment._

To show the weight of oxygen gas, and that it is heavier than air, the
stoppers from two bottles containing it may be removed, one bottle may
be left open for some time and then tested by a lighted taper, when it
[Page 99] will still indicate the presence of the gas, whilst the other
may be suddenly inverted over a little cup in which some ether, mixed
with a few drops of turpentine, may be burning--the flame burns with
much greater brilliancy at the moment when the oxygen comes in contact
with it.


_Nineteenth Experiment._

The theory of the effect of oxygen upon the system when inhaled would be
an increase in the work of the respiratory organs; and it is stated that
after inhaling a gallon or so of this gas, the pulse is raised forty or
fifty beats per minute: the gas is easily inhaled from a large
india-rubber bag through an amber mouthpiece; it must of course be quite
pure, and if made from the mixture of chlorate of potash and oxide of
manganese, should be purified by being passed through lime and water, or
cream of lime.


_Twentieth Experiment._

There are certain colouring matters that are weakened or destroyed by
the action of light and other causes, which deprive them of oxygen gas
or deoxidize them. A weak tincture of litmus, if long kept, often
becomes colourless, but if this colourless fluid is shaken in a bottle
with oxygen gas it is gradually restored; and if either litmus,
turmeric, indigo, orchil, or madder, paper, or certain ribbons dyed with
the same colouring matters, have become faded, they may be partially
restored by damping and placing them in a bottle of oxygen gas. The
effect of the oxygen is to reverse the _de_oxidizing process, _and_ to
impart oxygen to the colouring matters. By a peculiar process indigo may
be obtained quite white, and again restored to its usual blue colour,
either by exposure to the air or by passing a stream of oxygen through
it.


_Twenty-first Experiment._

Messrs. Matheson, of Torrington-street, Russell-square, prepare in the
form of wire some of the rarest metals, such as magnesium, lithium, &c.
A wire of the metal magnesium burns magnificently in oxygen gas, and
forms the alkaline earth magnesia. The metal lithium, to which such a
very low combining proportion belongs--viz., 6.5, can also be procured
in the state of wire, and burns in oxygen gas with an intense white
light into the alkaline lithia, which dissolved in alcohol with a little
acetic acid, and burnt, affords a red flame, making a curious contrast
between the effects of colour produced by the metallic and oxidized
state of lithium.


THE ALLOTROPIC CONDITION OF OXYGEN GAS.

The term allotropy (from [Greek: _allotropos_], of a different nature)
was first used by the renowned chemist Berzelius. Dimorphism, or
diversity in crystalline form, is therefore a special case of allotropy,
and is most amusingly illustrated with the iodide of mercury (HgI),
which is made either by rubbing together equal combining proportions
[Page 100]of mercury and iodine (both of which are to be found in the
Table of Elements, page 86), or by carefully precipitating a solution of
corrosive sublimate (chloride of mercury (HgCl)) with one of iodide of
potassium, just enough and no more of the latter being added to
precipitate the metal, or else the iodide of mercury is redissolved by
the excess of the precipitant. It is first of a dirty yellow, and then
gradually changes when stirred to a scarlet; if this be collected on a
filter, and washed and drained, it is a beautiful scarlet, and when some
of this substance is rubbed across a sheet of paper, a bright scarlet is
apparent, which may be rapidly changed to a lemon-yellow by heating the
paper over the flame of a spirit lamp; and the iodide of mercury is
again brought back to a scarlet colour by rubbing down the yellow
crystals with the fingers. This experiment may be repeated over and over
again with the like results. If some of the scarlet iodide of mercury is
sublimed from one bit of glass to another, it forms crystals, derived
from the right rhombic prism; when these are scratched with a pin they
change again to the scarlet state, the latter when crystallized being in
the form of the square-based octohedron.

Other cases of dimorphism may be mentioned--viz., with sulphur,
carbonate of lime, and lead, and many others, whilst allotropy is
curiously illustrated in the various conditions of charcoal, which, in
the more numerous examples, is black and opaque, and in another instance
transparent like water. Lamp-black is soft, but the diamond is the
hardest natural substance. The allotropic state of sulphur has been
already alluded to; phosphorus, again, exists in three modifications:
1st, Common phosphorus, which shines in the dark and emits a white
smoke. 2nd, White phosphorus. 3rd, Red or amorphous phosphorus, which
does not shine or emit white smoke when exposed to the air, and is so
altered in its properties that it may be safely carried in the pocket.

Enough evidence has therefore been offered to show that the allotropic
property is not confined to one element or compound, but is discoverable
in many bodies, and in no one more so than in the allotropic state of
the element oxygen called


OZONE.

The Greek language has again been selected by the discoverer, Schönbein,
of Basle, for the title or name of this curious modification of oxygen,
and it is so termed from [Greek: _oxein_], to smell. The name at once
suggests a marked difference between ozone and oxygen, because the
latter is perfectly free from odour, whilst the former has that peculiar
smell which is called electric, and is distinguishable whenever an
electrical machine is at work, or if a Leyden jar is charged by the
powerful Rhumkoff, or Hearder coil; it is also apparent when water is
decomposed by a current of electricity and resolved into its elements,
oxygen and hydrogen. When highly concentrated it smells like chlorine;
and the author recollects seeing the first experiments by Schönbein, in
England, at Mr. Cooper's laboratory in the Blackfriars-road. Ozone is
prepared by taking a clean empty bottle, and pouring therein a very
[Page 101] little distilled water, into which a piece of clean scraped
phosphorus is introduced, so as to expose about one-half of its diameter
to the air in the bottle, whilst the other is in contact with the water.
(Fig. 104.)

For the sake of precaution, the bottle may stand in a basin or soup
plate, so that if the phosphorus should take fire, it may be instantly
extinguished by pouring cold water into the bottle, and should this
crack and break, the phosphorus is received into the plate.

[Illustration: Fig. 104. A. A quart bottle, with the stopper loosely
placed therein. B. The stick of clean phosphorus. C. The water level
just to half the thickness of the phosphorus. D D. A soup-plate.]

When the ozone is formed the phosphorus can be withdrawn, and the
phosphorous-acid smoke washed out by shaking the bottle; it is
distinguishable by its smell, and also by its action on test paper,
prepared by painting with starch containing iodide of potassium on some
Bath post paper; when this is placed in the bottle containing ozone, it
changes the test blue, or rather a purplish blue.

[Illustration: Fig. 105. V. A small voltaic battery standing on the
stool with glass legs, S S, and capable of heating a thin length of
platinum wire about two inches long, and bent to form a point between
the conducting wires, W W.--N.B. The voltaic current can be cut off at
pleasure, so as to cool the wire when necessary. A is the prime
conductor of an ordinary cylinder electrical machine. B is the wire
conveying the frictional electricity to the conducting wires of the
voltaic battery, where the point P being the sharpest point in the
arrangement, delivers the electrified and ozonized air.]

Ozone is a most energetic body, and a powerful bleaching agent; if a
point is attached to the prime conductor of an electrical machine, and
the electrified air is received into a bottle, it will be found to
smell, and has the power of bleaching a _very_ dilute solution of
indigo. Ozone [Page 102] is not a mere creation of fancy, as it can not
only be produced by certain methods, but may be destroyed by a red heat.
If a point is prepared with a loop of platinum wire, and this latter,
after being connected with a voltaic battery, made red hot, and the
whole placed on an insulating stool, and connected with the prime
conductor of an electrical machine, it is found that the electrified air
no longer smells, the ozone is destroyed; on the other hand, if the
voltaic battery is disconnected, and the electrified air again allowed
to pass from the cold platinum wire, the smell is again apparent, the
air will bleach, and if caused to impinge at once upon the iodide of
starch test, changes it in the manner already described. (Fig. 105.)

Ozone is insoluble in water, and oxidizes silver and lead leaf, finely
powdered arsenic and antimony; it is a poison when inhaled in a
concentrated state, whilst diluted, and generated by natural processes,
it is a beneficent and beautiful provision against those numerous smells
originating from the decay of animal and vegetable matter, which might
produce disease or death: ozone is therefore a powerful disinfectant.
The test for ozone is made by boiling together ten parts by weight of
starch, one of iodide of potassium, and two hundred of water; it may
either be painted on Bath post paper, and used at once, or blotting
paper may be saturated with the test and dried, and when required for
use it must be damped, either before or after testing for ozone, as it
remains colourless when _dry_, but becomes blue after being moistened
with water.

Paper prepared with sulphate of manganese is an excellent test for
ozone, and changes brown rapidly by the oxidation of the proto-salt of
manganese, and its conversion into the binoxide of the metal.

Ozone is also prepared by pouring a little sulphuric ether into a quart
bottle, and then, after heating a glass rod in the flame of the spirit
lamp, it may be plunged into the bottle, and after remaining there a few
minutes ozone may be detected by the ordinary tests.


NITROGEN, OR AZOTE.

[Greek: _Nitron_], nitre; [Greek: _gennaô_], I form; [Greek: _a_],
privative; [Greek: _zôê_], life. Symbol, N; combining proportion, 14.
Also termed by Priestley, _phlogisticated_ air.

In the year 1772, Dr. Rutherford, Professor of Botany in the University
of Edinburgh, published a thesis in Latin on fixed air, in which he
says:--"_By the respiration of animals healthy air is not merely
rendered mephitic_ (i.e., charged with carbonic acid gas), _but also
suffers another change. For after the mephitic portion is absorbed by a
caustic alkaline lixivium, the remaining portion is not rendered
salubrious; and although it occasions no precipitate in lime-water, it
nevertheless extinguishes flame and destroys life._" Such is the
doctor's account of the discovery of nitrogen, which may be separated
from the oxygen in the air in a very simple manner. The atmosphere is
the great storehouse of nitrogen, and four-fifths of its prodigious
volume consist of this element.

[Page 103]

         _Composition of Atmospheric Air._

                                  Bulk.      Weight.

  Oxygen                            20        22.3
  Nitrogen                          80        77.7
                                   ---        ----
                                   100        100.

The usual mode of procuring nitrogen gas is to abstract or remove the
oxygen from a given portion of atmospheric air, and the only point to be
attended to, is to select some substance which will continue to burn as
long as there is any oxygen left. Thus, if a lighted taper is placed in
a bottle of air, it will only burn for a certain period, and is
gradually and at last extinguished; not that the whole of the oxygen is
removed or changed, because after the taper has gone out, some burning
sulphur may be placed in the vessel, and will continue to burn for a
limited period; and even after these two combustibles have, as it were,
taken their fill of the oxygen, there is yet a little left, which is
snapped up by burning phosphorus, whose voracious appetite for oxygen is
only appeased by taking the whole. It is for this reason that phosphorus
is employed for the purpose of removing the oxygen, and also because the
product (phosphoric acid) is perfectly soluble in water, and thus the
oxygen is first combined, and then washed out of a given volume of air,
leaving the nitrogen behind.


_First Experiment._

To prepare nitrogen gas, it is only necessary to place a little dry
phosphorus in a Berlin porcelain cup on a wine glass, and to stand them
in a soup plate containing water. The phosphorus is set on fire with a
hot wire, and a gas jar or cylindrical jar is then carefully placed over
it, so that the welt of the jar stands in the water in the soup plate.
At first, expansion takes place in consequence of the heat, but this
effect is soon reversed, as the oxygen is converted into a solid by
union with the phosphorus, forming a white smoke, which gradually
disappears. (Fig. 106.)

[Illustration: Fig. 106. A. Cylindrical glass vessel, open at one end,
and inverted over B, the wine-glass, supporting C, the cup containing
the burning phosphorus, and the whole standing in a soup-plate, D D,
containing water.]

Supposing two grains of phosphorus had been placed in a platinum tube,
and just enough atmospheric air passed over it to convert the whole into
phosphoric acid, the weight of the phosphorus would be increased to 4½
grains by the addition of 2½ grains of oxygen; [Page 104] now one cubic
inch of oxygen weighs 0.3419, or about 1/3rd of a grain, hence 7.3 cubic
inches of oxygen disappear, which weigh as nearly as possible 2½ grains,
so that as 36.5 cubic inches of air contain 7.3 cubic inches of oxygen,
that quantity of air must have passed over the 2 grains of phosphorus to
convert it into 4½ grains of phosphoric acid.

For very delicate purposes, nitrogen is best prepared by passing air
over finely-divided metallic copper heated to redness; this metal
absorbs the whole of the oxygen and leaves the nitrogen. The
finely-divided copper is procured by passing hydrogen gas over pure
black oxide of copper.


_Second Experiment._

[Illustration: Fig. 107. A. Glass jar, with collar of leather, through
which the stamper, C, works. B B. The tube containing the finely-divided
lead, part of which falls out, and is ignited, and retained by the
little tray just below, being part of the iron stand, D D, with crutches
supporting the ends of the glass tube, and the whole stands in the dish
of water, E E.]

A very instructive experiment is performed by heating a good mass of
tartrate of lead in a glass tube which is hermetically sealed, and being
placed on an iron support, is then covered by a capped air jar with a
sliding rod and stamper, the whole being arranged in a plate containing
water. When the stamper is pushed down upon the glass the latter is
broken (Fig. 107), and the air gradually penetrates to the finely
divided lead, when ignition occurs, and the oxygen is absorbed, as
demonstrated by the rise of the water in the jar. On the same principle,
if a bottle is filled about one-third full with a liquid amalgam of lead
and mercury, and then stopped and shaken for two hours or more, the
finely divided lead absorbs the oxygen and leaves pure nitrogen. Or if a
mixture of equal weights of sulphur and iron filings, is made into a
paste with water in a thin iron cup, and then warmed and placed under a
gas jar full of air standing on the [Page 105] shelf of the pneumatic
trough, or in a dish full of water, the water gradually rises in the jar
in about forty-eight hours, in consequence of the absorption of the
oxygen gas.


_Third Experiment._

Nitrogen is devoid of colour, taste, smell, of alkaline or acid
qualities; and, as we shall have occasion to notice presently, it forms
an _acid_ when chemically united with oxygen, and an alkali in union
with hydrogen. A lighted taper plunged into this gas is immediately
extinguished, while its specific gravity, which is lighter than that of
oxygen or air, is demonstrated by the rule of proportion.

  Weight of 100 cubic             Weight of 100 cubic          Specific
  inches of air at 60°   Unity.   inches of nitrogen at       gravity of
  Fahr., bar. 29.92 in.           60° Fahr., bar. 29.92 in.    nitrogen.

       30.829         :    1     ::      29.952             :    971

And its levity may be shown very prettily by a simple experiment. Select
two gas jars of the same size, and after filling one with oxygen gas and
the other with nitrogen gas, slide glass plates over the bottoms of the
jars, and proceed to invert the one containing oxygen, placing the neck
in a stand formed of a box open at the top; then place the jar
containing nitrogen over the mouth of the first, withdrawing the glass
plates carefully; and if the table is steady the top gas jar will stand
nicely on the lower one. Then (having previously lighted a taper so as
to have a long snuff) remove the stopper from the nitrogen jar and
insert the lighted taper, which is immediately extinguished, and as
quickly relighted by pushing it down to the lower jar containing the
oxygen. This experiment may be repeated several times, and is a good
illustration of the relative specific gravities of the two gases, and of
the importance of the law of universal diffusion already explained at p.
6, by which these gases _mix_, not _combine_ together, and the
atmosphere remains in one uniform state of composition in spite of the
changes going on at the surface of the earth. Omitting the aqueous
vapour, or steam, ever present in variable quantities in the atmosphere,
ten thousand volumes of dry air contain, according to Graham:--

[IIlustration: Fig. 108. A. Gas jar containing nitrogen, N, standing on
B, another jar full of oxygen, O. The taper, C, is extinguished at N,
and relighted at O. D D. Stand supporting the jars.]

[Page 106]

  Nitrogen                          7912
  Oxygen                            2080
  Carbonic acid                        4
  Carburetted hydrogen (CH_{2})        4
  Ammonia                        a trace
                                 -------
                                  10,000


_Fourth Experiment._

It was the elegant, the accomplished, but ill-fated Lavoisier who
discovered, by experimenting with quicksilver and air, the compound
nature of the atmosphere; and it was the same chemist who gave the name
of azote to nitrogen; it should, however, be borne in mind that it does
not necessarily follow because a gas extinguishes flame that it is a
_poison_. Nitrogen extinguishes flame, but we inhale enormous quantities
of air without any ill effects from the nitrogen or azote that it
contains; on the other hand, many gases that extinguish flame are
_specific poisons_, such as carbonic acid, carbonic oxide, cyanogen, &c.

Lavoisier's experiment may be repeated by passing into a measured jar,
graduated into five equal volumes, four measures of nitrogen and one
measure of oxygen; a glass plate should then be slid over the mouth of
the vessel, and it may be turned up and down gently for some little time
to mix the two gases, and when the mixture is tested with a lighted
taper, it is found neither to increase nor diminish the illuminating
power and the taper burns as it would do in atmospheric air. (Fig. 109.)

[Illustration: Fig. 109. A. Gas jar divided into five equal parts. B B.
Section of pneumatic trough, to show the decantation of gas from one
vessel to another. The gas is being passed from C to A, through the
water.]

[Page 107]

HYDROGEN.

Hydrogen ([Greek: _udôr_], water; [Greek: _gennaô_], I give rise to), so
termed by Lavoisier--called by other chemists inflammable air, and
phlogiston. Symbol, H; combining properties, 1. The lightest known form
of matter.

Every 100 parts by weight of water contain 11 parts of hydrogen gas; and
as the quantity of water on the surface of the earth represents at least
two-thirds of the whole area, the source of this gas, like that of
oxygen or nitrogen, is inexhaustible. Van Helmont, Mayow, and Hales had
shown that certain inflammable and peculiar gases could be obtained, but
it was reserved for the rigidly philosophic mind of Cavendish to
determine the nature of the elements contained in, and giving a
speciality to, the inflammable gases of the older chemists. By acting
with dilute acids upon iron, zinc, and tin, Cavendish liberated an
inflammable elastic gas; and he discovered nearly all the properties we
shall notice in the succeeding experiments, and especially demonstrated
the composition of water in his paper read before the Royal Society in
the year 1784.


_First Experiment._

Hydrogen is prepared in a very simple manner, by placing some zinc
cuttings in a bottle, to which is attached a cork and pewter or bent
glass tube, and pouring upon the metal some dilute sulphuric or
hydrochloric acid. Effervescence and ebullition take place, and the gas
escapes in large quantities, water being decomposed; the oxygen passes
to the zinc, and forms oxide of zinc, and this uniting with the
sulphuric acid forms sulphate of zinc, which may be obtained after the
escape of the hydrogen by evaporation and crystallization. (Fig. 110.)

  Zn + HO.SO_{3} = ZnO.SO_{3} + H;
               or,
      Zn + HCl = ZnCl + H.

In nearly all the processes employed for the generation of hydrogen gas,
a metal is usually employed, and this fact has suggested the notion that
hydrogen may possibly be a metal, although it is the lightest known form
of matter; and it will be observed in all the succeeding experiments
that a metallic substance will be employed to take away the oxygen and
displace the hydrogen.

[Illustration: Fig. 110. A. Bottle containing zinc cuttings and water
and fitted with a cap and two tubes, the one marked B, containing a
funnel, conveys the sulphuric acid to the zinc and water, whilst the gas
escapes through the pipe C.]

[Page 108]

Whenever hydrogen is prepared it should be allowed to escape from the
generating vessel for a few minutes before any flame is applied, in
order that the atmospheric air may be expelled. The most serious
accidents have occurred from carelessness in this respect, as a mixture
of hydrogen and air is explosive, and the more dangerous when it takes
fire in any closed glass bottle.


_Second Experiment._

If a piece of potassium is confined in a little coarse wire gauze cage,
attached to a rod, and thrust under a small jar full of water, placed on
the shelf of the pneumatic trough, hydrogen gas is produced with great
rapidity, and is received into the gas jar. The bit of potassium being
surrounded with water, is kept cool, whilst the hydrogen escaping under
the water is not of course burnt away, as it is whenever the metal is
thrown on the _surface_ of water.


_Third Experiment._

[Illustration: Fig. 111. A. Flask containing water, and producing steam,
which passes to the iron tube, B B, containing the iron borings heated
red hot in the charcoal stove C. The hydrogen passes to the jar D,
standing on the shelf of the pneumatic trough.]

Across a small iron table-furnace is placed about eighteen inches of
1-inch gas-pipe containing iron borings, the whole being red-hot; and
attached to one end is a pipe conveying steam from a boiler, or flask,
or retort, whilst another pipe is fitted to the opposite end, and passes
to the pneumatic trough. Directly the steam passes over the red hot iron
borings it is deprived of oxygen, which remains with the iron, forming
the rust or oxide of iron, whilst the hydrogen, called in this case
_water gas_, escapes with great rapidity. When steam is passed over
red-hot charcoal, hydrogen is also produced with carbonic oxide gas, and
this in fact is the ordinary process of making _water gas_, which being
purified is afterwards saturated with some volatile hydrocarbon and
burnt. At first sight, such a mode of making gas would be thought
extremely profitable, and in spite of the numerous failures the
_discovery_ (so called) of _water gas_ is reproduced as a sort of
_chronic wonder_; but experience and practice have clearly demonstrated
that _water gas_ is a fallacy, and as long as we can get coal it is not
worth while going through the round-about processes of first burning
coal to produce steam; secondly, [Page 109] of burning coal to heat
charcoal, over which the steam is passed to be converted into gas, which
has then to be purified and saturated with a cheap hydrocarbon obtained
from coal or mineral naphtha; whilst ordinary coal gas is obtained at
once by heating coal in iron retorts. (Fig. 111.)

Thus, by the metals zinc, tin, potassium, red-hot iron (and we might add
several others), the oxygen of water is removed and hydrogen gas
liberated.


_Fourth Experiment._

If bottles of hydrogen gas are prepared by all the processes described,
they will present the same properties when tested under similar
circumstances. A lighted taper applied to the mouths of the bottles of
hydrogen, which should be inverted, causes the gas to take fire with a
slight noise, in consequence of the mixture of air and hydrogen that
invariably takes place when the stopper is removed; on thrusting the
lighted taper into the bulk of the gas it is extinguished, showing that
hydrogen possesses the opposite quality to oxygen--viz., that it takes
fire, but does not support combustion. By keeping the bottles containing
the hydrogen upright, when the stopper is removed the gas escapes with
great rapidity, and atmospheric air takes its place, so much so that by
the time a lighted taper is applied, instead of the gas burning quietly,
it frequently astonishes the operator with a loud pop. This sudden
attack on the nerves may be prevented by always experimenting with
inverted bottles. (Fig. 112.)

[Illustration: Fig. 112. A. Bottle opened upright, and hydrogen
exploding. B. Bottle opened inverted, and hydrogen burning quietly at
the mouth.]


_Fifth Experiment._

Hydrogen is 14.4 lighter than air, and for that reason may be passed
into bottles and jars without the assistance of the pneumatic trough.
One of the most amusing proofs of its levity is that of filling paper
bags or balloons with this gas; and we read, in the accounts of the
fêtes at [Page 110] Paris, of the use of balloons ingeniously
constructed to represent animals, so that a regular aerial hunt was
exhibited, with this drawback only, that nearly all the animals
preferred ascending with their legs upwards, a circumstance which
provoked intense mirth amongst the volatile Frenchmen. The lightness of
hydrogen may be shown in two ways--first, by filling a little
gold-beater's skin balloon with _pure_ hydrogen (prepared by passing the
gas made from zinc and dilute pure sulphuric acid through a strong
solution of potash, and afterwards through one of nitrate of silver),
and allowing the balloon to ascend; and then afterwards, having of
course secured the balloon by a thin twine or strong thread, it may be
pulled down and the gas inhaled, when a most curious effect is produced
on the voice, which is suddenly changed from a manly bass to a ludicrous
nasal squeaking sound. The only precautions necessary are to make the
gas quite pure, and to avoid flame whilst inhaling the gas. It is
related by Chaptal that the intrepid (quære, foolish) but unfortunate
aeronaut, Mons. Pilate de Rosio, having on one occasion inhaled hydrogen
gas, was rash enough to approach a lighted candle, when an explosion
took place in his mouth, which he says "_was so violent that he fancied
all his teeth were driven out._" Of course, if it were possible to
change by some extraordinary power the condition of the atmosphere in a
concert-room or theatre, all the bass voices would become extremely
nasal and highly comic, whilst the sopranos would emulate railway
whistles and screech fearfully; and supposing the specific gravity of
the air was continually and materially changing, our voices would never
be the same, but alter day by day, according to the state of the air, so
that the "familiar voice" would be an impossibility.

A bell rung in a gas jar containing air emits a very different sound
from that which is produced in one full of hydrogen--a simple experiment
is easily performed by passing a jar containing hydrogen over a
self-acting bell, such as is used for telegraphic purposes. (Fig. 113.)

[Illustration: Fig. 113. A. Stand and bell. B B. Tin cylinder full of
hydrogen, which may be raised or depressed at pleasure, by lifting it
with the knob at the top, when the curious changes in the sound of the
bell are audible.]


_Sixth Experiment._

Some of the small pipes from an organ may be made to emit the most
curious sounds by passing heavy and light gases through them; in these
experiments bags containing the gases should be employed, which may
drive air, oxygen, carbonic acid, or hydrogen, through the organ pipes
at precisely the same pressure.

[Page 111]

_Seventh Experiment._

One of those toys called "The Squeaking Toy" affords another and
ridiculous example of the effect of hydrogen on sound, when it is used
in a jar containing this gas. (Fig. 114.)

[Illustration: Fig. 114. The squeaking toy, used in a jar of hydrogen.]


_Eighth Experiment._

An accordion played in a large receptacle containing hydrogen gas
demonstrates still more clearly what would be the effect of an orchestra
shut up in a room containing a mixture of a considerable portion of
hydrogen with air, as the former, like nitrogen, is not a poison, and
only kills in the absence of oxygen gas.


_Ninth Experiment._

Some very amusing experiments with balloons have been devised by Mr.
Darby, the eminent firework manufacturer, by which they are made to
carry signals of three kinds, and thus the motive or ascending power may
be utilized to a certain extent.

Mr. Darby's attention was first directed to the manufacture of a good,
serviceable, and cheap balloon, which he made of paper, cut with
mathematical precision; the gores or divisions being made equal, and
when pasted together, strengthened by the insertion of a string at the
juncture; so that the skeleton of the balloon was made of string, the
whole terminating in the neck, which was further stiffened with calico,
and completed when required by a good coating of boiled oil. These
balloons are about nine feet high and five feet in diameter in the
widest part, exactly like a pear, and tapering to the neck in the most
graceful and elegant manner. They retain the hydrogen gas remarkably
well for many hours, and do not leak, in consequence of the paper of
which they are made being well selected and all holes stopped, and also
from the circumstance of the pressure being so well distributed over the
interior by the almost mathematical precision with which they are cut,
and the careful preparation of the paper with proper varnish. One of
their greatest recommendations is cheapness; for whilst a gold-beater's
skin balloon of the same size would cost about 5_l._, these can be
furnished at 5_s._ each in large quantities.

A balloon required to carry one or more persons must be constructed of
the best materials, and cannot be too carefully made; it is therefore a
somewhat costly affair, and as much as 200_l._, 500_l._, and even
1000_l._ have been expended in the construction of these aerial
chariots.

The chief points requiring attention are:--first, the quality of the
silk; secondly, the precision and scrupulous nicety required in cutting
[Page 112] out and joining the gores; thirdly, the application of a good
varnish to fill up the pores of the silk, which must be insoluble in
water, and sufficiently elastic not to crack.

The usual material is Indian silk (termed Corah silk), at from 2_s._ to
2_s._ 6_d._ per yard.

The _gores_ or parts with which the balloon is constructed require, as
before stated, great attention; it being a common saying amongst
aeronauts, "_that a cobweb will hold the gas if properly shaped_" the
object being to diffuse the pressure equally over the whole bag or
balloon.

The varnish with which the silk is rendered air-tight can be made
according to the private recipe of Mr. Graham, an aeronaut, who states
that he uses for this purpose two gallons of linseed oil (boiled), two
ditto (raw), and four ounces of beeswax; the whole being simmered
together for one hour, answers remarkably well, and the varnish is tough
and not liable to crack.

For repairing holes in a balloon, Mr. Graham recommends a cement
composed of two pounds of black resin and one pound of tallow, melted
together, and applied on pieces of varnished silk to the apertures.

The actual cost of a balloon will be understood from information also
derived from Mr. Graham. His celebrated "Victoria Balloon," which has
passed through so many hairbreadth escapes, was sixty-five feet high,
and thirty-eight feet in diameter in the broadest part; and the
following articles were used in its construction:--

                                                      £   _s._ _d._
  1400 yards of Corah silk, at 2_s._ 6_d._ per yard  175   0    0
  The netting weighed 70 lbs.                         20   0    0
  Extra ropes weighed 20 lbs. at 2_s._ per lb.         2   0    0
  The car weighed 25 lbs.                              7   0    0
  Varnish, wages, &c.                                 16   0    0
                                                    -------------
                                                    £220   0    0

Thirty-eight thousand cubic feet of coal gas were required to fill this
balloon, charged by one company 20_l._, by others from 9_l._ to 10_l._;
and eight men were required to hold the inflated baggy monster.

Such a balloon as described above is a mere soap bubble when compared
with the "New Aerial Ship" now building in the vicinity of New York; the
details are so practical and interesting, that we quote nearly the whole
account of this mammoth or Great Eastern amongst balloons, as given in
the _New York Times_.

    "An experiment in scientific ballooning, greater than has yet been
    undertaken, is about to be tried in this city. The project of
    crossing the Atlantic Ocean with an air-ship, long talked of, but
    never accomplished, has taken a shape so definite that the apparatus
    is already prepared and the aeronaut ready to undertake his task.

    "The work has been conducted quietly, in the immediate vicinity of
    New York, since the opening of spring. The new air-ship, which has
    [Page 113] been christened the City of New York, is so nearly
    completed, that but few essentials of detail are wanting to enable
    the projectors to bring it visibly before the public.

    "The aeronaut in charge is Mr. T. S. C. Lowe, a New Hampshire man,
    who has made thirty-six balloon ascensions.

    "The dimensions of the City of New York so far exceed those of any
    balloon previously constructed, that the bare fact of its existence
    is notable. Briefly, for so large a subject, the following are the
    dimensions:--Greatest diameter, 130 feet; transverse diameter, 104
    feet; height, from valve to boat, 350 feet; weight, with outfit,
    3½ tons; lifting power (aggregate), 22½ tons; capacity of gas
    envelope, 725,000 cubic feet.

    "The City of New York, therefore, is nearly five times larger than
    the largest balloon ever before built. Its form is that of the usual
    perpendicular gas-receiver, with basket and lifeboat attached.

    "Six thousand yards of twilled cloth have been used in the
    construction of the envelope. Reduced to feet, the actual
    measurement of this material is 54,000 feet--or nearly 11 miles.
    Seventeen of Wheeler and Wilson's sewing machines have been employed
    to connect the pieces, and the upper extremity of the envelope,
    intended to receive the gas-valve, is of triple thickness,
    strengthened with heavy brown linen, and sewed in triple seams. The
    pressure being greatest at this point, extraordinary power of
    resistance is requisite. It is asserted that 100 women, sewing
    constantly for two years, could not have accomplished this work,
    which measures by miles. The material is stout and the stitching
    stouter.

    "The varnish applied to this envelope is a composition the secret of
    which rests with Mr. Lowe. Three or four coatings are applied, in
    order to prevent leakage of the gas.

    "The netting which surrounds the envelope is a stout cord,
    manufactured from flax expressly for the purpose. Its aggregate
    strength is equal to a resistance of 160 tons, each cord being
    capable of sustaining a weight of 400 lbs. or 500 lbs.

    "The basket which is to be suspended immediately below the balloon
    is made of rattan, is 20 feet in circumference and 4 feet deep. Its
    form is circular, and it is surrounded by canvas. This car will
    carry the aeronauts. It is warmed by a lime-stove, an invention of
    Mr. O. A. Gager, by whom it was presented to Mr. Lowe. A lime-stove
    is a new feature in air voyages. It is claimed that it will furnish
    heat without fire, and is intended for a warming apparatus only. The
    stove is 1½ feet high, and 2 feet square. Mr. Lowe states that he
    is so well convinced of the utility of this contrivance, that he
    conceives it to be possible to ascend to a region where water will
    freeze, and yet keep himself from freezing. This is to be tested.

    "Dropping below the basket is a metallic lifeboat, in which is
    placed an Ericsson engine. Captain Ericsson's invention is therefore
    to be tried in mid-air. Its particular purpose is the control of a
    propeller, rigged upon the principle of the screw, by which it is
    proposed to obtain [Page114] a regulating power. The application of
    the mechanical power is ingeniously devised. The propeller is fixed
    in the bow of the lifeboat, projecting at an angle of about
    forty-five degrees. From a wheel at the extremity twenty fans
    radiate. Each of these fans is 5 feet in length, widening gradually
    from the point of contact with the screw to the extremity, where the
    width of each is 1½ feet. Mr. Lowe claims that by the application of
    these mechanical contrivances his air-ship can be readily raised or
    lowered, to seek different currents of air; that they will give him
    ample steerage way, and that they will prevent the rotatory motion
    of the machine. In applying the principle of the fan, he does not
    claim any new discovery, but simply a practical development of the
    theory advanced by other aeronauts, and partially reduced to
    practice by Charles Green, the celebrated English aeronaut.

    "Mr. Lowe contends that the application of machinery to aerial
    navigation has been long enough a mere theory. He proposes to reduce
    the theory to practice, and see what will come of it. It is
    estimated that the raising and lowering power of the machinery will
    be equal to a weight of 300 lbs., the fans being so adjusted as to
    admit of very rapid motion upward or downward. As the loss of three
    or four pounds only is sufficient to enable a balloon to rise
    rapidly, and as the escape of a very small portion of the gas
    suffices to reduce its altitude, Mr. Lowe regards this systematic
    regulator as quite sufficient to enable him to control his movements
    and to keep at any altitude he desires. It is his intention to
    ascend to a height of three or four miles at the start, but this
    altitude will not be permanently sustained. He prefers, he says, to
    keep within a respectable distance of mundane things, where 'he can
    see folks.' It is to be hoped his machinery will perform all that he
    anticipates from it. It is a novel affair throughout, and a variety
    of new applications remain to be tested. Mr. Lowe, expressing the
    utmost confidence in all the appointments of his apparatus, assured
    us that he would certainly go, and, as certainly, would go into the
    ocean, or deliver a copy of Monday's _Times_ in London on the
    following Wednesday. He proposes to effect a landing in England or
    France, and will take a course north of east. A due easterly course
    would land him in Spain, but to that course he objects. He hopes to
    make the trip from this city to London in forty-eight hours,
    certainly in sixty-four hours. He scouts the idea of danger, goes
    about his preparations deliberately, and promises himself a good
    time. As the upper currents, setting due east, will not permit his
    return by the same route, he proposes to pack up the City of New
    York, and take the first steamer for home.

    "The air-ship will carry weight. Its cubical contents of 725,000
    feet of gas suffice to lift a weight of 22½ tons. With outfit
    complete its own weight will be 3½ tons. With this weight 19 tons of
    lifting power remain, and there is accordingly room for as many
    passengers as will care to take the venture. We understand, however,
    that the company is limited to eight or ten. Mr. Lowe provides sand
    for ballast, regards his chances of salvation as exceedingly
    favourable, [Page 115] places implicit faith in the strength of his
    netting, the power of his machinery, and the buoyancy of his
    lifeboat, and altogether considers himself secure from the hazard of
    disaster. If he accomplish his voyage in safety, he will have done
    more than any air navigator has yet ventured to undertake. If he
    fail, the enterprise sinks the snug sum of 20,000 dollars. Wealthy
    men who are his backers, sharing his own enthusiasm, declare failure
    impossible, and invite a patient public to wait and see."

A night ascent witnessed at any of the public gardens is certainly a
stirring scene, particularly if the wind is rather high. On approaching
the balloon, swayed to and fro by the breeze, it seems almost capable of
crushing the bold individual who would venture beneath it; seen as a
large dark mass in the yet dimly-lighted square, it appears to be
incapable of control; when the inflation is completed, the aeronaut, all
importance, seats himself in the car, and blue lights, with other
fireworks, display the victim who is to make a "last ascent," or perhaps
_descent_. Finally the word is given, the ropes are cast off, and the
bulky chariot rises majestically to the sound of the National Anthem.
The crowd see no more, but the next day's _Times_ reports the end of the
aerial journey.

Balloons can never be of any permanent value as means of locomotion
until they can be steered; and this is a problem, the solution of which
is something like _perpetual motion_. In the first place, a balloon of
any size exposes an enormous surface to the pressure and force of the
winds; and when we consider that they move at the rate of from three to
eighty miles per hour, it will be understood that the fabric of the
balloon itself must give way in any attempt to tear, work, or pull it
against such a force. Secondly and lastly, the power has not yet been
created which will do all this without the inconvenience of being so
_heavy_ that the steering engine fixes the balloon steadily to the earth
by its obstinate gravity. When engines of power are constructed without
the aeronaut's obstacle of weight--when balloons are made of thin copper
or sheet-iron, then we may possibly hear of the voyage of the good ship
_Aerial_, bound for any place, and quite independent of dock, port, and
the host of dues (_quere_), which the sea-going ships have to disburse.
It is, however, gratifying to the zeal and perseverance of those who
dream of aerial navigation, to know that a balloon is not quite useless;
and here we may return to the consideration of Mr. Darby's signals,
which are of various kinds, and intended to appeal to the senses by
night as well as by day; and first, by _audible sounds_. Such means have
long been recognised, from the ancient float and bell of the "Inchcape
Rock," to the painful minute-gun at sea, or the shrill railway whistle
and detonating signals employed to prevent the horrors of a collision
between two trains. The signal sounds are produced by the explosion of
shells capable of yielding a report equal to that of a six-pounder
cannon, and they are constructed in a very simple manner. A ball,
composed of wood or copper, and made up by screwing together the two
hemispheres, is attached to a shaft or tail of cane or lance-wood,
properly feathered like an arrow; at the side opposite to that of the
arrow--viz., at its antipodes, is placed a slight protuberance
[Page 116]containing a minute bulb of glass filled with oil of vitriol, and
surrounded with a mixture of chlorate of potash and sugar, the whole
being protected with gutta-percha, and communicating by a touch-hole
with the interior, which is of course filled with gunpowder. These
shells are attached to a circular framework by a strong whipcord, which
passes to a central fuse, and are detached one after the other as the
slow fuse (made hollow on the principle of the argand lamp) burns
steadily away. Directly a shell falls to the ground, the little bulb
containing the oil of vitriol breaks, and the acid coming in contact
with the chlorate of potash and sugar, causes the mixture to take fire,
when the gunpowder explodes. During the siege of Sebastopol many similar
mines were prepared by the Russians in the earth, so that when an
unfortunate soldier trod upon the spot, the concealed mine blew up and
seriously injured him; such petty warfare is as bad as shooting
sentries, and a cruel application of science, that unnecessarily
increases the miseries of war without producing those grand results for
which the truly great captains, Wellington and Napoleon, only warred.
(Fig. 115.)

[Illustration: Fig. 115. A. Ring attached to balloon, carrying an
hexagonal framework with six shells. B. Hollow fuse, which burns slowly
up to the strings, and detaches each shell in succession. C. Section of
shell. The shaded portion represents the gunpowder.]

[Illustration: Fig. 116. The bill distributor, consisting of three
hollow fuses, with bills attached in packets.]

The bill distributor consists of a long piece of wood, to which are
[Page 117] attached a number of hollow fuses, with packets of bills,
protected from being burned or singed by a thin tin plate; 10,000 or
20,000 bills can thus be delivered, and the wind assists in scattering
them, whilst the balloon travels over a distance of many miles. It must
be recollected that in each case the shells and the bills are detached
by the string burning away as the fire creeps up from the fuse. (Fig.
116.)

Another most ingenious arrangement, also prepared by Mr. Darby, is
termed by the inventor, the "Land and Water Signal," and may be thus
described:--A short hollow ball of gutta-percha, or other convenient
material, five or six inches in diameter, and filled with printed bills,
or the information, whatever it may be, that is required to be sent, is
attached to a cap to which a red flag, having the words "_Open the
shell_" and four cross sticks, canes, or whalebones with bits of cork at
equal distances, are fitted. The whole is connected by a string to the
fuse as before described. These signals are adapted for land and water:
in either case they fall upright, and in consequence of the sticks
projecting out they float well in the water, and can be seen by a
telescope at a distance of three miles. (Fig. 117.) Many of these
signals were sent away by Mr. Darby from Vauxhall; one was picked up at
Harwich, another at Brighton, a third at Croydon; in the latter case it
was found by a cottager, who, fearing gunpowder and combustibles, did
not examine the shell, but having mentioned the circumstance to a
gentleman living near him, they agreed to cut it open; and intelligence
of their arrival, in this and the other cases, was politely forwarded to
Mr. Darby at Vauxhall Gardens.

[Illustration: Fig. 117. The land and water signal, which remains
upright on land, or floats on the surface of water. A. The water-tight
gutta-percha shell, containing the message or information. B B B. Sticks
of cane to keep the flag in an upright position; at the ends are
attached cork bungs.]

Balloons, like a great many other clever inventions, have been despised
by military men as new-fangled expedients, toys, which may do very well
to please the gaping public, but are and must be useless in the field.
Over and over again it has been suggested that a balloon corps for
observation should be attached to the British army, but the scheme
has [Page 118] been rejected, although the expense of a few yards of silk
and the generation of hydrogen gas would be a mere bagatelle as compared
with the transport and use of a single 32-pounder cannon. The antiquated
notions of octogenarian generals have, however, received a great shock
in the fact that the Emperor Napoleon III. was enabled, by the
assistance of a captive balloon, to watch the movements and dispositions
of the Austrian troops; and with the aid of the information so obtained,
he made his preparations, and was rewarded by the victory of Solferino;
and as soon as the battle was over Napoleon III. occupied at Cavriana
the very room and ate the dinner prepared for his adversary, the Emperor
Francis Joseph.

Over and over again the most excellent histories have
been written of aerostation, but they all tend to one truth, and that
is, the great danger and risk of such excursions; and to enable our
readers to form their own judgment, a chronological list of some of the
most celebrated aeronauts, &c., is appended.

  1675. Bernair attempted to fly--_killed_.
  1678. Besnier attempted to fly.
  1772. L'Abbé Desforges announced an aerial chariot.
  1783. Montgolfier constructed the first air balloon.
    "   Roberts _frères_, first gas balloon, destroyed by the peasantry
          of Geneva, who imagined it to be an evil spirit or the moon.
  1784. Madame Thiblé, the first lady who was ever up in the clouds;
          she ascended 13,500 feet.
    "   Duke de Chartres, afterwards _Egalité_ Orleans, travelled 135
          miles in five hours in a balloon.
    "   Testu de Brissy, equestrian ascent.
    "   D'Achille, Desgranges, and Chalfour--Montgolfier balloon.
    "   Bacqueville attempted a flight with wings.
    "   Lunardi--gas balloon.
    "   Rambaud--Montgolfier balloon, which was burnt.
    "   Andreani--Montgolfier balloon.
  1785. General Money--gas balloon, fell into the water, and not rescued
          for six hours.
    "   Thompson, in crossing the Irish Channel, was run into with the
          bowsprit of a ship whilst going at the rate of twenty miles
          per hour.
    "   Brioschi--gas balloon ascended too high and burst the balloon;
          the hurt he received ultimately caused his _death_.
    "   A Venetian nobleman and his wife--gas balloon--_killed_.
    "   Pilatre de Rozier and M. Romain--gas balloon took fire--both
          _killed_.
  1806. Mosment--gas balloon--_killed_.
    "   Olivari--Montgolfier balloon--_killed_.
  1808. Degher attempted a flight with wings.
  1812. Bittorf--Montgolfier balloon--_killed_.
  1819. Blanchard, Madame--gas balloon--_killed_. [Page 119]
  1819. Gay Lussac--gas balloon, ascended 23,040 feet above the level of
          the sea. Barometer 12.95 inches; thermometer 14.9 Fah.
    "   Gay Lussac and Biot--gas balloon for the benefit of science.
          Both philosophers returned safely to the earth.
  1824. Sadler--gas balloon--_killed_.
    "   Sheldon--gas balloon.
    "   Harris--gas balloon--_killed_.
  1836. Cocking--parachute from gas balloon--_killed_.
  1847. Godard--Montgolfier balloon fell into and extricated from the
          Seine.
  1850. Poitevin, a successful French aeronaut.
    "   Gale, Lieut.--gas balloon--_killed_.
    "   Bixio and Barral--gas balloon.
    "   Graham, Mr. and Mrs.--gas balloon.--Serious accident ascending
          near the Great Exhibition in Hyde Park.
    "   Green, the most successful living aeronaut of the present time.

Of the 41 persons enumerated, 14 were killed, and nearly all the
aeronauts met with accidents which might have proved fatal.

[Illustration: Fig. 118. Flying machine (_theoretical_).]

[Page 120]

_Tenth Experiment._

Soap bubbles blown with hydrogen gas ascend with great rapidity, and
break against the ceiling; if interrupted in their course with a lighted
taper they burn with a slight yellow colour and dull report.


_Eleventh Experiment._

By constructing a pewter mould in two halves, of the shape of a
tolerably large flask, a balloon of collodion may be made by pouring the
collodion _inside_ the pewter vessel, and taking care that every part is
properly covered; the pewter mould may be warmed by the external
application of hot water, so as to drive off the ether of the collodion,
and when quite dry the mould is opened and the balloon taken out. Such
balloons may be made and inflated with hydrogen by attaching to them a
strip of paper, dipped in a solution of wax and phosphorus, and
sulphuret of carbon; as the latter evaporates, the phosphorus takes fire
and spreads to the balloon; which burns with a slight report. The pewter
mould must be very perfectly made, and should be bright inside; and if
the balloons are filled with oxygen and hydrogen, allowing a sufficient
excess of the latter to give an ascending power, they explode with a
loud noise directly the fire reaches the mixed gases.


_Twelfth Experiment._

In a soup-plate place some strong soap and water; then blow out a number
of bubbles with a mixture of oxygen and hydrogen; a loud report occurs
on the application of flame, and if the room is small the window should
be placed open, as the concussion of the air is likely to break the
glass.


_Thirteenth Experiment._

Any noise repeated at least thirty-two times in a second produces a
musical sound, and by producing a number of small explosions of hydrogen
gas inside glass tubes of various sizes, the most peculiar sounds are
obtained. The hydrogen flame should be extremely small, and the glass
tubes held over it may be of all lengths and diameters; a trial only
will determine whether they are fit for the purpose or not.


_Fourteenth Experiment._

Flowers, figures, or other designs, may be drawn upon silk with a
solution of nitrate of silver, and the whole being moistened with water,
is exposed to the action of hydrogen gas, which removes the oxygen from
the silver, and reduces it to the metallic state.

In like manner designs drawn with a solution of chloride of gold are
produced in the metallic state by exposure to the action of hydrogen
gas. Chloride of tin, usually termed muriate of tin, may also be reduced
in a similar manner, care being taken in these experiments that [Page
121] the fabric upon which the letters, figures, or designs are painted
with the metallic solution be kept quite damp whilst exposed to the
hydrogen gas.


_Fifteenth Experiment._

A mixture of two volumes of hydrogen with one volume of oxygen explodes
with great violence, and produces two volumes of steam, which condense
against the sides of the strong glass vessel, in which the experiment
may be made, in the form of water. As the apparatus called the Cavendish
bottle, by which this experiment only may be safely performed, is
somewhat expensive, and requires the use of an air-pump, gas jars with
stop-cocks, and an electrical machine and Leyden jar, other and more
simple means may be adopted to show the combination of oxygen and
hydrogen, and formation of water.

If a little alcohol is placed in a cup and set on fire, whilst an empty
cold gas jar is held over the flame, an abundant deposition of moisture
takes place from the combustion of the hydrogen of the spirits of wine.
Alcohol contains six combining properties of hydrogen, with four of
charcoal and two of oxygen. If a lighted candle, or an oil, camphine,
Belmontine, or gas flame, is placed under a proper condenser, large
quantities of water are obtained by the combustion of these substances.
(Fig. 119.)

[Illustration: Fig. 119. A. A burning candle, or oil or gas lamp. Copper
head and long pipe fitting into B C, the receiver from which the
condensed water drops into D. E E. Two corks fitted, between which is
folded some wet rag.]

[Page 122]

_Sixteenth Experiment._

During the combustion of a mixture of two volumes of hydrogen with one
of oxygen, an enormous amount of heat is produced, which is usefully
applied in the arrangement of the oxy-hydrogen blowpipe. The flame of
the mixed gases produces little or no light, but when directed on
various metals contained in a small hole made in a fire brick, a most
intense light is obtained from the combustion of the metals, which is
variously coloured, according to the nature of the substances employed.
With cast-iron the most vivid scintillations are obtained, particularly
if after having fused and boiled the cast-iron with the jet of the two
gases, one of them, viz., the hydrogen, is turned off, and the oxygen
only directed upon the fused ball of iron, then the carbon of the iron
burns with great rapidity, the little globule is enveloped in a shower
of sparks, and the whole affords an excellent notion of the principle of
Bessemer's patent method of converting cast-iron at once into pure
malleable iron, or by stopping short of the full combustion of carbon,
into cast-steel.

The apparatus for conducting these experiments is of various kinds, and
different jets have been from time to time recommended on account of
their alleged safety. It may be asserted that all arrangements proposed
for burning any quantity of the _mixed_ gases are extremely dangerous:
if an explosion takes place it is almost as destructive as gunpowder,
and should no particular damage be done to the room, there is still the
risk of the sudden vibration of the air producing permanent deafness. If
it is desired to burn the _mixed_ gases, perhaps the safest apparatus is
that of Gurney; in this arrangement the mixed gases bubble up through a
little reservoir of water, and thus the gas-holder--viz., a bladder, is
cut off from the jet when the combustion takes place. (Fig. 120.) This
jet is much recommended by Mr. Woodward, the highly respected President
of the Islington Literary and Scientific Institution, and may be fitted
up to show the phenomena of polarized light, the microscope, and other
interesting optical phenomena.

[Illustration: Fig. 120. Gurney's jet. A. Pipe with stop-cock leading
from the gas-holder. B. The little reservoir of water through which the
mixed gases bubble. C. The jet where the gases burn. D. Cork, which is
blown out if the flame recedes in the pipe, C.]

Mr. Woodward states, that a series of experiments, continued during many
years, has proved, that while the bladder containing the mixed gases is
under pressure, the flame cannot _be made_ to pass the safety chambers,
and consequently an explosion is impossible; and even if through extreme
carelessness or design, as by the removal of pressure or the contact of
a spark with the bladder, an explosion occurs, it can produce no other
than the momentary effect of the alarm occasioned by the report; [Page
123] whereas, when the gases are used in separate bags under a pressure
of two or three half hundredweights, if the pressure on one of the bags
be accidentally removed or suspended, the gas from the other will be
forced into it, and if not discovered in time, will occasion an
explosion of a very dangerous character; or if through carelessness one
of the partially emptied bags should be filled up with the wrong gas,
effects of an equally perilous nature would ensue.

[Illustration: Fig. 121. A. The bladder of mixed gases, pressed by the
board, B B, attached by wire supports to another board, C C, which
carries the weights, D D. E E. Pipe to which the bladder, A, is screwed,
and when A is emptied, it is re-filled from the other bladder, R. F F F.
Pipe conveying mixed gases to the lantern, G G, where they are burnt
from a Gurney's jet, H.]

In the oxy-hydrogen blowpipe usually employed, the gases are kept quite
separate, either in gasometers or gas bags, and are conveyed by distinct
pipes to a jet of very simple construction, devised by the late
Professor Daniell, where they mix in very small volumes, and are burnt
at once at the mouth of the jet. (Fig. 122.)

[Illustration: Fig. 122. Daniell's jet. O O. The stop-cock and pipe
conveying oxygen, and fitting inside the larger tube H H, to which is
attached a stop-cock, H, connected with the hydrogen receiver. A. The
orifice near which the gases mix, and where they are burnt.]

The gases are stored either in copper gasometers or in air-tight bags of
Macintosh cloth, capable of containing from four to six cubic feet of
gas, and provided with pressure boards. The boards are loaded with two
or three fifty-six pound weights to force out the gas with sufficient
[Page 4] pressure, and of course must be equally weighted; if any change
of weight is made, the stop-cocks should be turned off and the light put
out, as the most disastrous results have occurred from carelessness in
this respect. (Fig. 123.)

[Illustration: Fig. 123. Gas bag and pressure boards.]

The oxy-hydrogen jet is further varied in construction by receiving the
gases from separate reservoirs, and allowing them to mix in the upper
part of the jet, which is provided with a safety tube filled with
circular pieces of wire gauze. (Fig. 124.) With this arrangement a most
intense light is produced, called the Drummond or lime light, and coal
gas is now usually substituted for hydrogen.

[Illustration: Fig. 124. A A. Board to which B B is fixed. O. Oxygen
pipe. H. Hydrogen pipe. C C. Space filled with wire gauze. D. Lime
cylinder.]

[Page 125]

_Seventeenth Experiment._

There are many circumstances that will cause the union of oxygen and
hydrogen, which, if confined by themselves in a glass vessel, may be
preserved for any length of time without change; but if some powdered
glass, or any other finely-divided substance with sharp points, is
introduced into the mixed gases at a temperature not exceeding 660°
Fahrenheit, then the gases silently unite and form water.

This curious mode of effecting their combination is shown in a still
more interesting manner by perfectly clear platinum foil, which if
introduced into the mixed gases gradually begins to glow, and becoming
red-hot causes the gases to explode. Or still better, by the method
first devised by Dobereiner, in 1824, by which finely prepared spongy
platinum--_i.e._, platinum in a porous state, and exposing a large
metallic surface--is almost instantaneously heated red-hot by contact
with the mixed gases. When this fact became known, it was further
applied to the construction of an instantaneous light, in which hydrogen
was made to play upon a little ball of spongy platinum, and immediately
kindled. These Dobereiner lamps were possessed by a few of the curious,
and would no doubt be extensively used if the discovery of phosphorus
had not supplied a cheaper and more convenient fire-giving agent. When
the spongy platinum is mixed with some fine pipeclay, and made into
little pills, they may (after being slightly warmed) be introduced into
a mixture of the two gases, and will silently effect their union. The
theory of the combination is somewhat obscure, and perhaps the simplest
one is that which supposes the platinum sponge to act as a conductor of
electric influences between the two sets of gaseous particles; although,
again, it is difficult to reconcile this theory with the fact that
powdered glass at 660°, a bad conductor of electricity, should effect
the same object. The result appears to be due to some effects of surface
by which the gases seem to be condensed and brought into a condition
that enables them to abandon their gaseous state and assume that of
water.

When Sir H. Davy invented the safety-lamp, he was aware that, in certain
explosive conditions of the air in coal mines, the flame of the lamp was
extinguished, and in order that the miner should not be left in the
dreary darkness and intricacies of the galleries without some means of
seeing the way out, he devised an ingenious arrangement with thin
platinum wire, which was coiled round the flame of the lamp, and fixed
properly, so that it could not be moved from its proper place by any
accidental shaking. When the flame of the safety-lamp, having the
platinum wire attached, was accidentally extinguished by the explosive
atmosphere in which it was burning, the platinum commenced glowing with
an intense heat, and continued to emit light as long as it remained in
the dangerous part of the mine. Sir H. Davy warned those who might use
the platinum to take care that no portion of the thin wire passed
_outside_ the wire gauze, for the obvious reason that, if ignited
outside the wire gauze protector, it would inflame the fire-damp.

[Illustration: Fig. 125. P P. Two platinum plates connected with wires
to the cups. The wires are passed through holes in the finger-glass, B
B, and are fixed perfectly steady by pouring in cement composed of resin
and tallow to the line L L. Two glass tubes filled with water acidulated
with sulphuric acid, and placed over the platinum plates in
finger-glass, which also contains dilute sulphuric acid to improve the
conducting power of the water. The wires of the battery are placed in
the cups, and the arrows show the direction of the current of
electricity.]

[Page 126]

_Eighteenth Experiment._

Water is decomposed by passing a current of voltaic electricity through
it by means of two platinum plates, which may be connected with a
ten-cell Grove's battery. The gases are collected in separate tubes, and
the experiment offers one of the most instructive illustrations of the
composition of water. (Fig. 125.)

There is a current of electricity passing from and between two platinum
plates decomposing water, offering the converse of the Dobereiner
experiment, and highly suggestive of the probability of the theory
already advanced in explanation of the singular combination of oxygen
and hydrogen in the presence of clean platinum foil, and more especially
when we consider the operation of Grove's gas battery, in which a
current of electricity is produced by pieces of platinum foil covered
with finely-divided platinum, called platinum black; each piece is
contained in a separate glass tube filled alternately with oxygen and
hydrogen, and by connecting a great number of these tubes a current of
electricity is obtained, whilst the oxygen and hydrogen are slowly
absorbed and disappear, having combined and formed water, although
placed in separate glass tubes. (Fig. 126.)

The analysis of water is shown very perfectly on the screen by fitting
up some very small tubes and platinum wires in the same manner as shown
in fig. 125. The vessel in which the tubes and wires are contained with
the dilute sulphuric acid must be small, and arranged so as to pass
nicely into the space usually filled by the picture in an ordinary magic
lantern, or, still better, in one lighted by the oxy-hydrogen or lime
light. If the dilute acid is coloured with a little solution of indigo,
the gradual displacement of the fluid by the production of the two gases
is very perfectly developed on the screen when the small voltaic battery
is attached to the apparatus; and of course a large number of persons
may watch the experiment at the same time.

With respect to the application of the light produced from a jet of the
[Page 127] mixed gases thrown upon a ball of lime, it may be stated that
for many years the dissolving view lanterns and other optical effects
have been produced with the assistance of this light; and more lately
Major Fitzmaurice has condensed the mixed gases in the old-fashioned oil
gas receivers, and projected them on a ball of lime; and it was this
light thrown from many similar arrangements that illuminated the British
men-of-war when Napoleon III. left her Majesty's yacht at night in the
docks at Cherbourg.

[Illustration: Fig. 126. Grove's gas battery consists of tubes
containing oxygen and hydrogen alternately, and having a thin piece of
platinum foil, P, inserted by the blowpipe in each glass tube. The foil
hangs down the full length of the interior of the glass. Each pair of
tubes is contained in a little glass tumbler containing some dilute
sulphuric acid, and the hydrogen tube, H, of one pair, is connected with
the oxygen tube, O, of the next. W W. The terminal wires of the series.]

Mr. Sykes Ward, of Leeds, has also proposed a most simple and excellent
application of the oxy-hydrogen light for illumination _under_ the
[Page 128] surface of water, and for the convenience of divers, who are
frequently obliged to cease their operations in consequence of the want
of light. Mr. Ward's submarine lamp consists of a series of very strong
copper tubes, which are filled with the mixed gases by means of a
force-pump; and in order to prevent the lamp being extinguished, it
burns under _double_ glass shades, which are desirable in order to
prevent the glass immediately next to the light cracking by contact with
the cold water.

[Illustration: Fig. 127. Cherbourg.]

The author tried this lamp at Ryde, and although the coast-guards
objected to the production of a brilliant light at night, which they
stated might be mistaken for a signal and would cause some confusion
amongst the war vessels in the immediate neighbourhood, enough
experiments were made, to show that the Ward lamp would burn for a
considerable time under water, and could be kept charged with the gas by
means of a process that was easily workable in the boat. The gases were
taken out mixed in gas bags, and pumped into the reservoir when
required. With a much larger reservoir greater results could be
obtained; and if nautilus diving bells are to be used in modern warfare,
they will require a powerful light to show them their prey, so that they
may attach the explosives which are to blow great holes in the
men-of-war.

[Illustration: Fig. 128. A A. Tube reservoir to hold the mixed gases. B.
The jet and lime ball. D. The first glass shade, held down by a cap and
screw. C. The second glass shade. E E. The handle by which it is lowered
into the water.]

[Illustration: Fig. 129. Submarine lamp.]

[Page 129]




CHAPTER XI.

CHLORINE, IODINE, BROMINE, FLUORINE.

_The four Halogens, or Producers of Substances like Sea Salt._


Chlorine ([Greek: _chlôros_], green). Symbol, Cl. Combining proportion,
35.5. Specific gravity, 2.44. Scheele termed it dephlogisticated
muriatic acid; Lavoisier, oxymuriatic acid; Davy, chlorine.

The consideration of the nature of this important element introduces to
our notice one of the most original chemists of the eighteenth
century--viz., the illustrious Scheele, who was born at Stralsund, in
1742, and in spite of every obstacle, fighting his "battle of life" with
sickness and sorrow, he succeeded in making some of the most valuable
discoveries in science, and amongst them that of chlorine gas. It was in
the examination of a mineral solid--viz., of manganese--that Scheele
made the acquaintance of a new gaseous element; and in a highly original
dissertation on manganese, in 1774, he describes the mode of procuring
what he termed _dephlogisticated muriatic acid_--a name which is
certainly to be regretted, from its absurd length, but a title which was
strictly in accordance with the then established theory of phlogiston;
and if the latter is considered synonymous with hydrogen, quite in
accordance with our present views of the nature of this element. Scheele
discovered the leading characteristics of chlorine, and especially its
power of bleaching, which is alone sufficient to place this gas in a
high commercial position, when it is considered that all our linen used
formerly to be sent to Holland, where they had acquired great dexterity
in the ancient mode of bleaching--viz., by exposure of the fabric to
atmospheric air or the action of the damps or dews, assisted greatly by
the agency of light. Some idea may be formed of the present value of
chlorine, when it is stated that the linen goods were retained by the
Dutch bleachers for nine months; and if the spring and summer happened
to be favourable, the operation was well conducted; on the other hand,
if cold and wet, the goods might be more or less injured by continual
exposure to unfavourable atmospheric changes. At the present time, as
much bleaching can be done in nine weeks as might formerly have been
conducted in the same number of months; and the whole of the process of
chlorine bleaching is carried on independent of external atmospheric
caprices, whilst the money paid for the process no longer passes to
Holland, but remains in the hands of our own diligent bleachers and
manufacturers.


_First Experiment._

[Illustration: Fig. 130. A. Flask containing the fuming hydrochloric
acid, which is gently boiled by the heat of the spirit lamp. B. Tube
passing to the Wolfe's bottle, containing pumice-stone or asbestos
moistened with sulphuric acid. C. Second tube passing into a dry empty
bottle, which receives the hydrochloric acid gas.]

As Scheele first indicated, chlorine is obtained by the action of the
black oxide of manganese, on "the Spirit of Salt," or hydrochloric acid;
and the most elementary and instructive experiment showing its
preparation can be made in the following manner:--

[Page 130]

Place in a clear Florence oil-flask, to which a cork and bent tube have
been first fitted, some strong fuming hydrochloric acid. Arrange the
flask on a ring-stand, and then pass the bent tube either to a Wolfe's
bottle containing some pumice-stone moistened with oil of vitriol, or to
a glass tube containing either pumice or asbestos wetted with the same
acid. Another glass tube, bent at right angles, passes away from the
Wolfe's bottle into a receiving bottle. (Fig. 130). On the application
of heat, the hydrochloric gas is driven off from its solution in water,
and any aqueous vapour carried up is retained by the asbestos or pumice
stone wetted with oil of vitriol; the application of the latter is
called _drying the gas_--i.e., depriving it of all moisture; sometimes
the salt called chloride of calcium is used for the same purpose, and it
must be understood by the juvenile chemist that gases are not dried like
towels, by exposure to heat, or _by putting them in bladders before the
fire_, as we once heard was actually recommended, but by causing the gas
charged with invisible steam to pass over some substance having a great
affinity for water. The dry hydrochloric gas falls into the bottle, and
displaces the air, being about one-fourth heavier than the latter, and
gradually overflowing from the mouth of the vessel, produces a white
smoke, which is found to be acid by litmus paper, but has no power to
bleach, and is not green; it is, in fact, a combination of one combining
proportion of chlorine with one of hydrogen, and to detach the latter,
and set the chlorine free, it is necessary to convey the hydrochloric
gas to some body which has an affinity for hydrogen. Such a substance is
provided in the use of the black oxide of manganese, which is placed
either in a small flask or in a tube provided with two bulbs, and when
heated with the lamp it separates the hydrogen from the hydrochloric
gas, and forms water, which partly condenses in the second bulb. And now
the gas that escapes is no longer acid and fuming with a white smoke on
contact with the air; but is green, has a strong odour, bleaches, and is
so powerful in its action on all living tissues, that it must be
carefully avoided and not inhaled; if a small quantity is accidentally
inhaled, it produces a violent fit of coughing, which lasts a [Page
131] considerable time, and is only abated by inhaling the diluted
vapour of ammonia, or ether, or alcohol, and swallowing milk and other
softening drinks. (Fig. 131.)

[Illustration: Fig. 131. A. The flask containing the fuming hydrochloric
acid, heated by spirit lamp. B. Tube passing to Wolfe's bottle,
containing the pumice-stone or asbestos wetted with oil of vitriol. C.
Second tube, which passes into a wide-mouthed small flask containing
black oxide of manganese, partly in powder and partly in lump; and the
third tube conveys the chlorine to any convenient vessel. The double
bulb tube, E E, may be substituted for the flask, the oxide of manganese
being contained in the bulb M.--N.B. Any tube may be joined on to
another by a bit of india-rubber tubing, which is tied by string.]

[Illustration: Tube A is joined to tube B by the caoutchouc pipe C, tied
with packthread.]


_Second Experiment._

The mode of preparing chlorine, as already given, though very
instructive, is troublesome to perform; a more simple process may
therefore be described:--

Pour some strong hydrochloric acid upon powdered black oxide of
manganese contained in a Florence oil-flask, taking care that the whole
of the black powder is wetted with the acid so that none of it clings to
the bottom of the flask in the dry state to cause the glass to crack on
the application of heat. A cork and bent glass tube is now attached, and
conveyed to the pneumatic trough; on the application of heat to the
mixture in the flask the chlorine is evolved, and may be collected in
stoppered bottles, the first portion that escapes, although it contains
atmospheric air, should be carefully collected in order to prevent any
[Page 132] accident from inhaling the gas, and it will do very well to
illustrate the bleaching power of the gas, and therefore need not be
wasted. The above process may be described in symbols, all of which are
easily deciphered by reference to the table of elements, page 86.

  MnO_{2} + 2 HCl = MnCl + 2HO + Cl.


_Third Experiment._

Another and still more expeditious mode of preparing a little chlorine,
is by placing a small beaker glass, containing half an ounce of
chlorinated lime, usually termed chloride of lime or bleaching powder,
carefully at the bottom of a deep and large beaker glass, and then, by
means of a tube and funnel, conveying to the chloride of lime some
dilute oil of vitriol, composed of half acid and half water;
effervescence immediately occurs from the escape of chlorine gas, and as
it is produced it falls over the sides of the small beaker glass into
the large one, when it may be distinguished by its green colour. If a
little gas be dipped out with a very small beaker glass arranged as a
bucket, and poured into a cylindrical glass containing some dilute
solution of indigo, and shaken therewith, the colour disappears almost
instantaneously; and if a piece of Dutch metal is thrown into the beaker
glass it will take fire if enough chlorine has been generated, or some
very finely-powdered antimony will demonstrate the same result. Thus,
with a few beaker glasses, some chloride of lime, sulphuric acid, a
solution of indigo, and a little Dutch metal, the chief properties of
chlorine may be displayed. (Fig. 132.)

[Illustration: Fig. 132. A A. The large beaker glass. B. The small one,
containing the chloride of lime. C. The tube and funnel down which the
dilute sulphuric acid is poured. D D. Sheet of paper over top of large
glass, with hole in centre to admit the tube. E. The little beaker used
as a bucket.]


_Fourth Experiment._

Into a little platinum spoon place a small pellet of the metal sodium,
and after heating it in the flame of a spirit lamp, introduce the metal
[Page 133] into a bottle of chlorine, when a most intense and brilliant
combustion occurs, throwing out a vivid yellow light, and the heat is
frequently so great that the bottle is cracked. After the combustion,
and when the bottle is cool, it is usually lined with a white powder,
which will be found to taste exactly the same as salt, and, in fact, is
that substance, produced by the combination of chlorine, a virulent
poison, with the metal sodium, which takes fire on contact with a small
quantity of water; and hence the use of salt for the preparation of
chlorine gas when it is required on the large scale.

                                        Parts.
  Common salt                              4
  Black oxide of manganese                 1
  Sulphuric acid                           2
  Water                                    2


_Fifth Experiment._

Some Dutch metal, or powdered antimony, or a bit of phosphorus,
immediately takes fire when introduced into a bottle containing chlorine
gas, forming a series of compounds termed chlorides, and demonstrating
by the evolution of heat and light, the energetic character of chlorine,
and that oxygen is not the only supporter of combustion; chlorine gas
has even, in some cases, greater chemical power, because some time
elapses before phosphorus will ignite in oxygen gas, whilst it takes
fire directly when placed in a bottle of chlorine.


_Sixth Experiment._

The weight and bleaching power of chlorine are well shown by placing a
solution of indigo in a tall cylindrical glass, leaving a space at the
top of about five inches in depth. By inverting a bottle of chlorine
over the mouth of the cylindrical glass, it pours out like water, being
about two and a half times heavier than atmospheric air, and then, after
placing a ground glass plate over the top of the glass, the chlorine is
recognised by its colour, whilst the bleaching power is demonstrated
immediately the gas is shaken with the indigo solution.


_Seventh Experiment._

As a good contrast to the last experiment, another cylindrical jar of
the same size may be provided, containing a solution of iodide of
potassium with some starch, obtained by boiling a teaspoonful of
arrowroot with some water; any chlorine left in the bottle (sixth
experiment) may be inverted into the top of this glass and shaken, when
it turns a beautiful purple blue in consequence of the liberation of
iodine by the chlorine, whose greater affinity for the base produces
this result. The colour is caused by the union of the iodine and the
starch, which form together a beautiful purple compound, and thus the
apparent anomaly of destroying and producing colour with the same agent
is explained.

[Page 134]


_Eighth Experiment._

Dry chlorine does not bleach, and this fact is easily proved by taking a
perfectly dry bottle, and putting into it two or three ounces of fused
chloride of calcium broken in small lumps, then if a bottle full of
chlorine is inverted over the one containing the chloride of calcium,
taking the precaution to arrange a few folds of blotting paper with a
hole in the centre on the top of the latter to catch any water that may
run out of the chlorine bottle at the moment it is inverted, the gas
will be dried by contact with the chloride of calcium, and if a piece of
paper, with the word chlorine written on it with indigo, and previously
made hot and dry, is placed in the chlorine, no change occurs, but
directly the paper is removed, dipped in water, and placed in a bottle
of damp chlorine, the colour immediately disappears. (Fig. 133.)

[Illustration: Fig. 133. A A. Dry bottle, containing chloride of
calcium. B. Bottle of chlorine. The arrow indicates the gas. C C. The
blotting-paper, to catch any water from the bottle, B. D. The bottle
closed, and containing the paper.]

This experiment shows that chlorine is only the means to the end, and
that it decomposes water, setting free oxygen, which is supposed to
exert a high bleaching power in its _nascent_ state, a condition which
many gases are imagined to assume just before they take the gaseous
state, a sort of intermediate link between the solid or fluid and the
gaseous condition of matter. The nascent state may possibly be that of
ozone, to which we have already alluded as a powerful bleaching agent.


_Ninth Experiment._

A piece of paper dipped in oil of turpentine emits a dense black smoke,
and frequently a flash of fire is perceptible, directly it is plunged
into a bottle containing chlorine gas; here the gas combines only with
the hydrogen of the turpentine, and the carbon is deposited as soot.


_Tenth Experiment._

If a lighted taper is plunged into a bottle of chlorine it continues to
burn, emitting an enormous quantity of smoke, for the reason already
explained, and demonstrating the perfection of the atmosphere in which
[Page 135] we live and breathe, and showing that had oxygen gas
possessed the same properties as chlorine, the combustion of compounds
of hydrogen and carbon would have been impossible, in consequence of the
enormous quantity of soot which would have been produced, so that some
other element that would freely enter into combination with it must have
been provided to produce both artificial light and heat. Chlorine is a
gas which cannot be inhaled, and ozone presents the same features, as a
mouse confined for a short time with an excess of ozone soon died; but
ozone is the extraordinary condition of oxygen; the element in the
ordinary state is harmless, and is the one which enters so largely into
the composition of the air we breathe.


_Eleventh Experiment._

When one volume of olefiant gas (prepared by boiling one measure of
alcohol and three of sulphuric acid) is mixed with two volumes of
chlorine, and the two gases agitated together in a long glass vessel for
a few seconds, with a glass plate over the top, which should have a welt
ground perfectly flat, they unite on the application of flame, with the
production of a great cloud of black smoke, arising from the deposited
carbon, whilst a sort of roaring noise is heard during the time that the
flame passes from the top to the foot of the glass. (Fig. 134.)

[Illustration: Fig. 134. Remarkable deposition of carbon during the
combustion of one volume of olefiant gas with two of chlorine.]


_Twelfth Experiment._

Formerly Bandannah handkerchiefs were in the highest estimation, and no
gentleman's toilet was thought complete without one. The pattern was of
the simplest kind, consisting only of white spots on a red or other
coloured ground. These spots were produced in a very ingenious manner by
Messrs. Monteith, of Glasgow, by pressing together many layers of silk
with leaden plates perforated with holes; a solution of chlorine was
then poured upon the upper plate, and pressure being applied it
penetrated the whole mass in the direction of the holes, bleaching out
the colour in its passage. This important commercial result may be
imitated on the small scale by placing a piece of calico dyed with
Turkey red between two thick pieces of board, each of which is [Page
136] perforated with a hole two inches in diameter, and corresponding
accurately when one is placed upon the other. The pieces of board may be
squeezed together in any convenient way, either by weights, strong
vulcanized india-rubber bands or screws, and when a strong solution of
chlorine gas or of chloride of lime is poured into the hole and
percolates through the cloth, the colour is removed, and the part is
bleached almost instantaneously by first wetting the calico with a
little weak acid, and then pouring on the solution of chloride of lime.
On removing and washing the folded red calico it is found to be bleached
in all the places exposed to the solution, and is now covered with white
spots. (Fig. 135.)

[Illustration: Fig. 135. A. Circular hole in the upper piece of wood, a
similar one being perforated in the lower one. B B. The strong
india-rubber bands. The bleaching solution is poured into A.]


IODINE.

Iodine ([Greek: _Iôdês_], violet coloured). Symbol, I; combining
proportion, 127.1; specific gravity, 4.948. Specific gravity of iodine
vapour, 8.716.

In the previous chapter, devoted to the element chlorine, little or
nothing has been said of that inexhaustible storehouse of chlorine,
iodine, and bromine--viz., the boundless ocean. Some one has remarked
that, as it is possible the air may contain a little of everything
capable of assuming the gaseous form, so the ocean may hold in a state
of solution a modicum of every soluble substance, in proof of which we
have lately read of some very important experiments resulting in the
separation of the metal silver from sea water, not certainly in any
profitable quantity, but quite enough to prove its presence in the
ocean.

No elaborate research is necessary to ascertain the presence of
chlorine, when it is remembered that Schafhäutl has calculated, that all
the oceans on the globe contain three millions fifty-one thousand three
hundred and forty-two cubic geographical miles of salt, or about five
times more than the mass of the Alps.

Now, salt contains about 60 per cent. of chlorine gas, and therefore the
bleachers can never stand still for want of it; but iodine is not so
plentiful, and was discovered by M. Courtois, of Paris, in _kelp_, a
substance from which he prepared carbonate of soda, or washing soda; but
as this is now more cheaply prepared from common salt, the kelp is at
present required only for the iodine salts it contains, as also for the
chloride of potassium. Kelp is obtained by burning dried sea-weeds in a
[Page 137] shallow pit; the ashes accumulate and melt together, and this
fused mass broken into lumps forms kelp. The ocean bed no doubt has its
fertile and barren plains and mountains, and amongst the so-called
"oceanic meadows" are to be mentioned the two immense groups and bands
of sea-weed called the Sargasso Sea, which occupy altogether a space
exceeding six or seven times the area of Germany.

The iodine is contained in the largest proportion in the deep sea
plants, such as the long elastic stems of the fucus palmatus, &c. The
kelp is lixiviated with water, and after separating all the
crystallizable salts, there remains behind a dense oily-looking fluid,
called "iodine ley," to which sulphuric acid is added, and after
standing a day or two the acid "ley" is placed in a large leaden retort,
and heated gently with black oxide of manganese. The chlorine being
produced very slowly, liberates the iodine, as already demonstrated in
experiment seven, p. 133, and it is collected in glass receivers.

Iodine, when quite pure and well crystallized, has a most beautiful
metallic lustre, and presents a bluish-black colour, affording an odour
which reminds one at once of the "sea smell."


_First Experiment._

A few grains of iodine placed in a flask may be sublimed at a very
gentle heat, and afford a magnificent violet vapour, which can be poured
out of the flask into a warm bottle. If the bottle is cold the iodine
condenses in minute and brilliant crystals. (Fig. 136.)

[Illustration: Fig. 136. A. Flask containing iodine heated by spirit
lamp. B. Cold flask above to receive the vapour. C C. Sheet of cardboard
to cut off the heat from the spirit lamp.]


_Second Experiment._

Upon a thin slice of phosphorus place a few small particles of iodine;
the heat produced by the combination of the two elements soon causes the
phosphorus to take fire.


_Third Experiment._

Heat a brick, and then throw upon it a few grains of iodine; by holding
a sheet of white paper behind, the splendid violet colour of the vapour
is seen to great advantage. It was by the discovery of iodine in the
ashes of sponge--which had long been used as a remedy for goitre, a
remarkable glandular swelling--that this element began to be used for
medical purposes, and the important salt called iodide of potassium is
now used in large quantities, not only in medicine, but likewise for
that most fascinating art, which has made its way steadily, and is now
practised so extensively, under the name of _photography_.

[Page 138]


THE ART OF PHOTOGRAPHY.

It was the great George Stephenson who asked the late Dean Buckland the
posing question, "Can you tell me what is the power that is driving that
train?" alluding to a train which happened to be passing at the moment.
The learned dean answered, "I suppose it is one of your big engines."
"But what drives the engine?" "Oh, very likely a canny Newcastle
driver." "What do you say to the light of the sun?" "How can that be?"
asked Buckland. "It is nothing else," said Stephenson. "It is light
bottled up in the earth for tens of thousands of years; light, absorbed
by plants and vegetables, being necessary for the condensation of carbon
during the process of their growth, if it be not carbon in another form;
and now, after being buried in the earth for long ages in fields of
coal, that latent light is again brought forth and liberated, made to
work--as in that locomotive--for great human purposes."

Such was the opinion of the most original and practical man that ever
reasoned on philosophy; and could he have lived to realize the thorough
adaptation and business use of light in the art of photography, he would
have said, man is only imitating nature, and in producing photographs he
must employ the same agent which in ages past assisted to produce the
coal.

In another part of this elementary work we shall have to consider the
nature of light; here, however, the chemical part only of the process of
photography will be discussed.

Many years ago (in the year 1777) Jenny Lind's most learned countryman,
Scheele, discovered that a substance termed chloride of silver, obtained
by precipitating a solution of chloride of silver with one of salt,
blackened much sooner in the violet rays than in any other part of the
spectrum. He says, "Fix a glass prism at the window, and let the
refracted sunbeams fall on the floor; in this coloured light put a paper
strewed with luna cornua (horn silver or chloride of silver), and you
will observe that this horn silver grows sooner black in the violet ray
than in any of the other rays."

In 1779, Priestley directed especial attention to the action of light on
plants; and the famous Saussure, following up these and other
experiments, determined that the carbonic acid of plants was more
generally decomposed into carbon and oxygen in the blue rays of the
spectrum; these facts probably suggested the bold theory of Stephenson
already alluded to. Passing by the intermediate steps of photography, we
come to the second year of the present century, and find in the Journal
of the Royal Institution a paper by Wedgwood, entitled "An Account of a
Method of Copying Paintings upon Glass, and of making Profiles, by the
Agency of Light upon Nitrate of Silver; with observations, by H. Davy."
Such a paper would lead the reader to suppose that very little remained
to be effected, and that mere details would quickly establish the art;
but in this case the experimentalists were doomed to [Page 139]
disappointment, as, after producing their photographs, they could not
make them permanent; they had not yet discovered the means of _fixing_
the pictures. Nearly fourteen years elapsed, when the subject was again
taken up by Niépcè, of Chalons, with little success, so far as the
fixing was concerned; and twenty-seven years had passed away since the
experiments of Wedgwood and Davy, when, in 1829, Niépcè and Daguerre
executed a deed of co-partnership for mutually investigating the matter.
These names would suggest a rapid progress; but, strange to relate, ten
years again rolled away, the father Niépcè had in the meantime died, and
a new contract was made between the son and M. Daguerre, when, in
January, 1839, the famous discovery was made known to the world, and in
July of the same year the French Government granted a pension for life
of six thousand francs to Daguerre, and four thousand to the son of
Niépcè, who had so worthily continued the experiments commenced by his
father. The triumph of the industrious French experimentalists was not,
however, to be unique; across the Channel another patient and laborious
philosopher had completed on paper precisely the same kind of results as
those obtained by Daguerre on silver plates. Mr. Fox Talbot, in England,
had immortalized himself by a discovery which was at once called the
Talbotype, and for which a patent was secured in 1841. Having thus
hastily sketched a brief history of the art, we may now proceed to the
details of the process.


_First Experiment._

A photogenic drawing, so called, but now termed _a positive copy_, is
prepared by placing some carefully selected paper, which is free from
spots or inequalities (good paper is now made by several English
manufacturers, although some kinds of French paper, such as Cansan's,
are in high repute), in a square white hard porcelain dish containing a
solution of common salt in distilled water, 109 grains of salt to the
pint. The paper is steeped in this solution for ten minutes, and then
taken out and pressed in a _clean_ wooden press, or it should be dabbed
dry on a _clean_ flat surface with a _clean_ piece of white calico,
which may be kept specially for this duty and not used for anything
else, and it is well that all would-be photographers should understand
that neatness and cleanliness are perfectly indispensable in conducting
these processes. If a design were required for the armorial bearings of
the art of photography, it might certainly be most fanciful, but the
motto must be _cleanliness_ and _neatness_, and in preparing paper it
should not be unnecessarily handled, but lifted by the corners only. The
object of dabbing the paper is to prevent the salt accumulating in large
quantities in one part of the paper and the reverse in another, and to
distribute the salt equally through the whole. The paper being now
dried, is called salted paper, and is rendered sensitive when required
by laying it down on a solution of ammonio-nitrate of silver, prepared
by adding ammonia to a solution containing sixty grains of nitrate of
silver to the ounce of distilled water, until the whole of the oxide of
silver is re-dissolved, except [Page 140] a very small portion. A few
drops of nitric acid are also recommended to be added, and after
allowing the solution to stand, it may be poured off quite clear, and is
ready for use either in the bath, or if economy must be rigidly adhered
to, the salted paper may be laid flat on a board, and held in its place
with four pins at the corners, and then just enough to wet the surface
of the paper may be run along the side of a glass spreader, and the
liquid gently drawn over the surface of the salted paper, which is
allowed to dry on a flat surface for a few minutes, and afterwards hung
up by one corner to a piece of tape stretched across the room, until
quite dry, and then placed in a blotting-book fitting into a case which
completely excludes the light. Copying-paper should be made at night, as
the day is then free for all photographic operations requiring an
abundance of light. It will not keep long, and should be used the next
day.

[Illustration: Fig. 137. A. The glass spreader with cork handle. B. The
silver solution clinging to rod and paper by capillary attraction. C C C
C. Four pins holding down the paper on a board.--N.B. The spreader is
made of glass rod three-eighths thick.]

A piece of lace, a skeleton leaf, a sharp engraving on thin paper, and
above all things, a negative photograph on glass or paper, is easily
copied by placing the prepared paper with the prepared side (carefully
protected from the light) upwards on any flat surface, such as plate
glass; upon this is arranged the bit of lace or the negative photograph
with the face or picture downwards, another bit of plate glass is then
placed over it, and weights arranged at the corners; after exposure to
the sun's rays for thirty minutes, more or less (according to the
dullness or bright aspect of the day), the picture is brought into a
dark room and examined by the light of a candle or by the light from a
window covered with yellow calico, and after placing a paper weight on
one corner of the lace, or [Page 141] negative picture, or copying
paper, it may be carefully lifted in one part, and if the copy is
sufficiently dark, is ready for fixing, but if it is faint the lifted
corner is carefully replaced, the upper glass is laid on, and the
picture again exposed to the light. Should the position of the lace or
negative be changed during the examination, re-exposure is useless, and
would only produce a double and confused picture, as it would be
impossible to lay the lace or the negative exactly in the same place
again on the copying paper.

The manipulations just described are much facilitated by using a
copying-frame or press, which consists of a square wooden frame with a
thick plate-glass window; upon this are placed the negative picture and
the copying paper, and the two are brought in close contact by means of
a board at the back pressed by a hand-screw. (Fig. 138.) After the
photogenic drawing or positive copy is taken, it is fixed by being
placed in a solution of hyposulphite of soda, consisting of one fluid
ounce of saturated solution to eight of water. The saturated solution of
hyposulphite of soda is conveniently kept in a large bottle for use, and
in order to improve the colour a very little chloride of gold is added
to the fixing solution, the picture must now be thoroughly washed,
dried, and pressed.

[Illustration: Fig. 138. The back of the copying-frame, showing the
hand-screw and pressure-board. The plate glass inside is set in the base
of the frame, and is of course the part exposed to the light.]


_Second Experiment._

Another mode of preparing the copying paper, called albumen paper, is to
take the whites of four eggs, and four ounces of distilled water
containing one hundred and sixty grains of chloride of ammonium; these
are beaten up with a fork or a bundle of feathers, and as the froth is
produced it is skimmed off by a silver spoon into another basin, or a
beaker glass, and being allowed to settle for twelve hours it is
strained through fine muslin, and is ready for use. The best paper is
floated on the surface of this liquid for three minutes, taken out, and
dried at once on a hot plate.

In floating paper one corner is first laid down, and care taken not to
enclose any air bubbles, which would prevent the fluid wetting the
paper, whilst the remainder of the paper is slowly laid upon the surface
of the fluid.

The albumen paper is excited by laying it for five minutes on a solution
of nitrate of silver, seventy-two grains to the ounce of water, [Page
142] and when dry it will keep for three days. This copying paper is
used in the same manner as the last, and fresh eggs only must be used in
its preparation, because stale ones soon cause the copy to change and
blacken all over from the liberation of sulphur, which unites with the
silver. The colour of the copy is sometimes improved by a solution of
hot potash, and by dipping the well-washed picture, after the use of the
hyposulphite of soda, in a very dilute solution of hydrosulphuret of
ammonia.


_Third Experiment._

In the Daguerreotype process, a silver plate, after being thoroughly
cleaned and polished, is exposed to the vapour of iodine, and is thus
rendered so sensitive that it may be at once exposed in the camera. In
the Talbotype process, the same principle is apparent, and paper is
prepared by first covering its surface with iodide of silver, which is
afterwards rendered sensitive to the action of light by means of an
excess of nitrate of silver, as follows:--

One side of a sheet of selected Cansan's paper is first covered (by
means of a spreader) with a solution of nitrate of silver (thirty grains
to the ounce of water), hung up in a dark room and dried; it is then
immersed in a solution of iodide of potassium of five hundred grains to
a pint of distilled water, for five or ten minutes, and immediately
changes to a yellow colour in consequence of the precipitation of the
yellow iodide of silver; it is then well washed with plenty of water,
and being dried, may be kept for any length of time, and is called
"iodized paper." Light has no action whatever upon it. To render the
paper sensitive, three solutions are prepared in separate bottles, and
marked 1, 2, 3.

No. 1, contains a solution of nitrate of silver, fifty grains to the
ounce of water.

No. 2, glacial acetic acid.

No. 3, a saturated solution of gallic acid.

With respect to No. 3, Mr. William Crookes has shown, that when a
saturated solution of gallic acid is required in large quantities, that
it is better to dissolve at once two ounces of gallic acid in six ounces
of alcohol (60° over proof); to hasten solution, the flask may be
conveniently heated by immersion in hot water; when cold it should be
filtered, mixed with half a drachm of glacial acetic acid, and preserved
in a stoppered bottle for use; so prepared it will keep unaltered for a
considerable length of time. The gallic acid is not precipitated from
this solution by the addition of water; consequently, if in any case
desirable, the development of a picture may be effected with a much
stronger bath than the one usually employed. To obtain a solution of
about the same strength as a saturated aqueous solution, such as No. 3,
half a drachm of the alcoholic solution is mixed with two ounces of
water; but for my particular purpose, says Mr. Crookes, referring to the
wax-paper process, "I prefer a weaker bath, which is prepared by mixing
half a drachm with ten ounces of water." In either case it [Page 143]
will be found necessary to add solution of nitrate of silver in small
quantities, as the developing picture seems to require it.

Returning again to the solutions marked 1, 2, 3, the numbers will assist
the memory in mixing the proportions of each. If the paper is required
to be used _at once_, a drachm of each may be mixed together and spread
over the iodized paper (of course, in a dark room), which is then
transferred to a clean blotting-book of white bibulous paper, and being
placed in the paper-holder may be taken to the camera and exposed at
once. If the paper is not required to be used immediately, the solutions
are mixed in the proportions of the numbers--viz., one of No. 1, two of
No. 2, three of No. 3; and in making the mixture, it is advisable to
keep a measure specially for No. 3, the gallic acid, or else the
measure, if used for the three solutions, will have to be washed out
every time, which is very troublesome, particularly where water is not
plentiful.

If the excited paper is required to be kept some hours before use, No. 3
must be added in still larger proportion, as much as ten or even twenty
measures of No. 3 to two of No. 2, and one of No. 1, being used, and
even this large dilution is frequently insufficient to prevent the paper
spoiling in hot weather; therefore if the temperature is high, too much
reliance must not be placed on this paper, as it is peculiarly
disappointing, after walking some miles to romantic and beautiful
scenery, to find, when developing the pictures in the evening, that the
paper used was all spoilt before exposure; and it will be seen presently
that when the excited paper is to be carried about for use, it is better
to adopt the wax-paper process.

After the excited iodized paper is exposed in the camera--and the time
of exposure cannot be taught, as that speciality is only acquired by
experience, and may vary from five to thirty minutes, or even more--the
invisible picture is developed and rendered visible, not by exposure to
the vapour of mercury, as in Daguerre's process with silver plates, but
by a mixture of one of No. 1 with four of No. 3. The development is
carefully watched by looking through the negative placed before a
lighted candle, and the time of development may vary from ten to thirty
minutes, and all the time the picture must be kept wet with the
solution, so that it is better perhaps to make a bath of the solution
and lay the picture on its surface than to pour the liquid over the
picture. After the development is matured, the picture is now washed in
clean water, and fixed temporarily, if required, by immersion in a bath
containing 200 grains of bromide of potassium in one pint of water, or
permanently by the hyposulphite of soda, made by mixing one part of a
saturated solution with five or ten of water, or one ounce of the salt
to six or twelve of water; but, as before mentioned, it is better to
keep a Winchester quart full of a saturated solution of hyposulphite of
soda, and then it is always ready for use instead of employing the
weights and scales, and continually weighing out portions of the salt.
The picture after fixing is thoroughly washed with water, and being
[Page 144] dried is now placed between the folds of a wax book--_i.e._,
some leaves of blotting-paper are kept saturated with white wax, and
when a picture is placed between them, and a hot iron passed over the
outside sheet, the wax enters the pores of the paper, and after removing
any excess of wax by passing the picture through a book of bibulous
paper, over which the hot flat iron is passed, the negative picture at
last is ready for use, and any number of positive copies may be taken
from it, as already described in the first experiment, page 139.

This mode of manipulation is called the Talbotype, and before dismissing
the subject another process of iodizing the paper may be explained.

To a solution of nitrate of silver of twenty, thirty, or fifty grains to
the ounce of water, a sufficient number of the crystals of iodide of
potassium is added, first to produce the yellow iodide of silver, and
then to dissolve it, so that the yellow precipitate appears with a small
quantity, and disappears with an excess of the iodide. If this solution
is spread over sheets of paper, and these latter then placed in a bath
of water, the iodide of silver is precipitated on the surface, and after
plenty of washing to remove the excess of iodide of potassium, the paper
may be dried, and will keep for any length of time without change. This
paper may be excited, exposed, developed, fixed, and waxed, as already
explained.


_Fourth Experiment. The Wax-paper Process._

This mode of taking negative photographs begins where the talbotype
ends--viz., by _first_ waxing the paper perfectly and evenly, as already
explained, Cansan's negative paper being preferred. The wax paper is now
well soaked in a bath, made by dissolving one hundred grains of iodide
of potassium, six grains of cyanide of potassium, four grains of
fluoride of potassium, ten grains of bromide of potassium, ten grains of
chloride of sodium, in one pint of fresh whey, with the addition of a
little alcohol and a few grains of iodine. When soaked in this solution
for about one hour, the paper is taken out and hung up to dry.

N.B. With respect to iodizing the wax paper, it is almost better to
obtain it ready prepared, and then every sheet may be relied on. Mr.
Melhuish, of Blackheath and Holborn, supplies it in any quantity, and
his paper never fails; the operator has then only to perform the
sensitizing and developing processes. To render the iodized paper
sensitive it is immersed for about six minutes in a bath containing a
solution of nitrate of silver (thirty-five grains to the ounce of water,
with forty drops of glacial acetic acid); the paper is now removed, and
washed in two trays of common clear rain-water or distilled water, and
is then dried off between folds of blotting-paper.

This process may be performed on the previous evening by the light of a
candle, or by day in a room lit by one window covered with four
thicknesses of yellow calico, and after the paper is dry it will keep
for three [Page 145] weeks or a month, and may be exposed in a camera
with a three-inch lens of eighteen-inch focus, with the inch diaphragm,
on a bright day from five to fifteen minutes; in bad weather the
exposure must be longer. The picture may be carried home and rendered
visible or developed by immersion in a bath containing a saturated
solution of gallic acid, and as the developing continues, a few drops of
the sensitizing solution of nitrate of silver and glacial acetic acid
may be added. Finally, the picture is fixed by immersion for a quarter
of an hour in a solution of hyposulphite of soda (four ounces of the
crystal to one pint of water, or one part of the saturated solution to
eight of water), and being well washed, is then dried, hung before the
fire to melt the wax, and is now ready to print from.


_Fifth Experiment. Albumen on Glass Process._

Albumen is the scientific name for the white of egg, of which four
ounces by measure are mixed with one ounce and a half of distilled
water, and after being whisked to a froth, are removed by a spoon into
another basin or a beaker glass, and allowed to stand for several hours
and then filtered. Mr. Crookes has recommended a very ingenious, simple,
and useful filter. (Fig. 139.) He says: "This simple and inexpensive
piece of apparatus, which any instrument maker or glass-blower can
supply at a few hours' notice, will be found invaluable in almost every
photographic process on glass. The sponge has this great advantage over
all other kinds of filters, that thick gelatinous liquids--_e.g._,
honey, albumen, gelatine, meta-gelatine, or the various preservative
syrups--flow through it with the utmost readiness; whilst at the same
time dust, air bubbles, or froth, and dried particles floating in the
liquid, are effectually kept back, and if fitted with stoppers,
collodion might be filtered in it; or if the ends were fitted together
with a bit of flexible pipe, the stoppers might be dispensed with
altogether.

[Illustration: Fig. 139. A B. Glass tube, bent as in picture. C. Piece
of damp sponge squeezed into the head of the tube. Any liquid poured in
at B will flow through the sponge until it has attained the same level
in A.]

Having poured the albumen on a perfectly clean glass plate, taking care
to have sufficient to run freely over the surface of the glass, the
excess is then gently drained off and the plate turned so as to have the
coated side downwards; it is then fixed in a sling made by taking a
stout bit of string about three feet long, which is doubled and knotted
at the fold, leaving the two ends free; two small triangles or stirrups
of silver wire looped at one corner are now tied on to the ends of the
string, and these form a support for the opposite edges of the glass
plate to rest on; the two strings are knotted together at a [Page 146]
convenient distance from the stirrups to prevent the glass slipping out,
and the plate is now rotated rapidly over a heated metallic surface,
such as an iron box containing some burning charcoal or the _warming
pan_, care being taken to avoid dust as much as possible, and to use
only the whites of new-laid eggs. (Fig. 140.) The glass plate, covered
with dry albumen, is now iodized to a straw colour by exposure over a
box containing iodine, as in the Daguerreotype process, and is
sensitized by immersion for three or four minutes in a bath containing a
solution of nitrate of silver (twenty-five grains to an ounce of water);
the plate is afterwards washed in distilled water and left to dry
spontaneously, of course in a darkened room. The plates may then be
placed ready for use in a very ingenious tin box devised by Mr. Crookes,
which keeps them perfectly light-tight even in the sun, and at the same
time is less bulky than the ordinary wooden ones. It is made of tin
plate, the cover sliding tight over the top, and more than half way down
the sides; light is further excluded by means of an outer jacket of tin,
which is soldered to the box a little below the centre. The cover thus
slides between the case and the jacket, and renders injury to the plates
by the entrance of light an impossibility. (Fig. 141.)

[Illustration: Fig. 140. A. Loop for finger. B. The knot which prevents
the stirrups of silver wire, C C, slipping off the corners of the glass
plate. D D. The opposite corners of the glass plate on which the
stirrups are placed.]

[Illustration: Fig. 141. A A. Tin box, with partitions to hold glass
plates, B B. The outer jacket, between which and the box, A, the lid or
cover, C, slides.]

The sensitive albumenized glass plate is exposed in the camera from
fifteen to thirty minutes, and developed (much in the same way as the
paper pictures) with one ounce of a saturated [Page 147] solution of
gallic acid containing ten or fifteen drops of the sensitizing solution.
The plate is usually placed on a levelling stand, and the solution
poured on the glass plate; the development is slow, and may be quickened
sometimes by the application of heat.

The picture is fixed by immersion for a short time in a bath containing
one part of a saturated solution of hyposulphite of soda in eight of
water. The pictures produced by this process are exquisitely defined,
provided always the camera is well focussed, and to assist this
operation a magnifying glass may be employed. After removal from the
hyposulphite of soda the plate is well washed with water, and being
allowed to dry spontaneously, is now ready to print from.


_Sixth Experiment. The Collodion on Glass Process._

The glass plates for this, as well as the albumen on glass process,
should be cleaned by rubbing them over first with a mixture of Tripoli
powder and ammonia, which is washed off under a tap, and the glass being
drained is rubbed dry and polished with a clean calico duster kept
exclusively for this purpose.

The iodized collodion is now poured on, and the excess returned to the
bottle. Collodion can be made very easily, but if prepared without due
precautions, it cannot be used afterwards, and reminds one of the old
story of the enthusiastic son, who, when asking his father's permission
to espouse the beloved, enumerated amongst her other accomplishments,
the fact that she _could_ make a pudding, and was answered by the bluff
question, "But can you eat it afterwards?" So it is with collodion: a
great deal of messing and loss of time is saved by purchasing it of the
various makers, amongst whom may be specially noticed Mr. Richard
Thomas, of 10, Pall Mall, who has devoted the whole of his attention to
the preparation of this important photographic chemical, and with a
success which his numerous patrons can well testify. The collodion is
sold either mixed with the iodizing solution, or the two can be obtained
separately, with directions on the bottles as to the quantities to be
mixed together.

The plate covered with the iodized collodion is quickly transferred to a
bath containing a solution prepared in the following manner:--Dissolve
four ounces of nitrate of silver in eight ounces of water, and to this
add twenty grains of iodide of potassium in one ounce of water; shake
them together, and then pour the whole into fifty-six ounces of
distilled water, and in half an hour add one ounce of alcohol and half
an ounce of ether; agitate the whole and filter the next morning. The
collodion plate is kept in this solution for a certain period, only
learnt by experience, and should be occasionally lifted out to see if a
uniform transparency is obtained; say that the immersion may be
continued for five minutes, it is now ready for the camera, and may be
exposed from about one to two minutes, or more if the light is
deficient; the time of exposure is also a matter of _practice_, mere
directions can be of no use in this stage of the process.

The picture is developed on a levelled stand, with a solution of three
[Page 148] grains of pyrogallic acid in three ounces of water, to which
sixty drops of glacial acetic acid have been added. When fully developed
the plate is washed with water and fixed with a solution of hyposulphite
of soda, consisting of one part of the saturated solution to eight of
water, again thoroughly but gently washed, so as not to endanger the
separation of the film from the glass; it is allowed to dry
spontaneously, and being coated with amber varnish (a solution of amber
in chloroform) is now ready to print from. (Fig. 123.) It is, perhaps,
hardly necessary to add, that the sensitizing and developing processes
must be performed in a dark room.

[Illustration: Fig. 142. A. Glass or gutta-percha bath to hold the
sensitizing solution. B. Glass, with piece cemented on the end to hold
the prepared glass plate, C, whilst dipped in the bath, A. The plate C
has a cross in one corner to show prepared side.]

[Illustration: Fig. 143. First effect of peripatetic photography on the
rural population.]

[Page 149]

BROMINE.

Bromine ([Greek: _brômos_], a bad odour). Symbol, Br. Combining
proportion, 80. Specific gravity, 2.966.

In a previous portion of this work, the connexion between chlorine,
iodine, and bromine has been pointed out; and as we have to notice the
colour of the element bromine, the chromatic union of the triad may be
alluded to. These elements present very nearly all the colours of the
spectrum:

  Bromine             red to orange.
  Chlorine            yellow to green.
  Iodine              blue, indigo, violet.

These three elements also furnish examples of the three conditions of
matter; iodine being a solid, bromine a fluid, chlorine a gas; the
relation of their combining proportions is also curious: as might be
expected, the fluid bromine takes an intermediate position, and
(according to the axiom that half the sum of the extremes is equal to
the mean) by dividing the combining proportions of iodine and chlorine,
and adding them together, we have, as nearly as possible, the combining
proportion of bromine:

  Chlorine             35 ÷ 2 = 17.75
  Iodine              126 ÷ 2 = 63
                                -----
                                80.75

The combining proportion of bromine is 80, but 80.75 is so near, that it
may reasonably be conjectured future experiments will reduce the number
of the three elements, and may prove that they are only modifications of
a single one. This is the only kind of alchemy which is tolerated in the
nineteenth century, and any philosopher who will reduce the number of
elements, and prove that some of them are only modifications of others,
will achieve a renown that must transcend the _éclat_ of all previous
discoverers.

Bromine was discovered by Balard, in 1826, and, like chlorine and
iodine, is a constituent of sea water. The chief source of bromine is a
mineral spring at Kreutznach, in Germany. The process by which it is
obtained offers a good example of chemical affinity; the water of the
mineral spring is evaporated, all crystallizable salts removed, and a
current of chlorine gas passed through the remaining solution, which
changes to a yellow colour, in consequence of the liberation of the
bromine by the combinations of chlorine with the bases previously united
with the former; the liquid is then shaken with ether, which dissolves
out the bromine. In the next place, the etherial solution is agitated
with strong solution of potassa, and is thus obliged to part with the
bromine which is converted into bromate of potassa; this is ultimately
changed by fusion to bromide of potassium; and by distillation with
black oxide of manganese and sulphuric acid, the bromine is finally
obtained. Six [Page 150] processes are therefore necessary before the
small quantity of bromine contained in the mineral spring-water, is
separated.


_First Experiment._

Bromine is a very heavy fluid, which should be preserved by keeping it
in a bottle covered with water; when required, a few drops may be
removed by means of a small tube, and dropped into a warm bottle, which
is quickly filled with the orange-red vapour. If some phosphorus is
placed in a deflagrating spoon, and exposed to the action of bromine
vapour, it takes fire spontaneously.


_Second Experiment._

Powdered antimony sprinkled into the vapour of bromine immediately takes
fire.


_Third Experiment._

A burning taper immersed in a bottle containing the vapour of bromine is
gradually extinguished.


_Fourth Experiment._

Liquid bromine exposed to a freezing mixture of ice and salt, or reduced
to a temperature of about eight degrees below zero, solidifies into a
yellowish-brown, brittle, crystalline mass.


_Fifth Experiment._

A solution of indigo shaken with a small quantity of the vapour of
bromine is quickly bleached. Many substances, when brought in contact
with liquid bromine, combine with explosive violence, and therefore
experiments with liquid bromine are not recommended, as all the most
instructive and conclusive results can be obtained by the use of the
vapour of bromine, which is easily procured by allowing a few drops to
fall into a warm, dry bottle.

Bromine, as already mentioned, is used in the art of photography.


FLUORINE.

Symbol, F. Combining proportion, 19.

This singular element seems almost to embody the ancient idea of the
alchemists, being a sort of _alkahest_, or universal solvent; or in
plainer language, its affinities for other bodies are so powerful, that
it attacks every substance (not even excepting gold), at the moment of
its liberation, and combines therewith, so that its isolation has not
yet been effected. Chemists who assert that they have been able to
obtain fluorine in the elementary condition, pronounce it to be a gas
which possesses the colour of chlorine; but the experiments, as hitherto
conducted, render that statement extremely doubtful.

[Page 151]

The only interesting fact connected with fluorine, is the remarkable
property of attacking glass and other silicious bodies, belonging to its
combination with hydrogen gas, called hydrofluoric acid. This acid is
easily obtained and used by placing some powdered fluorspar in a leaden
tray six inches square and two inches deep. If sulphuric acid is now
mixed with the powdered spar, so as to form a thin paste, and heat
applied, the vapour of the hydrofluoric acid quickly rises, and can be
employed to etch a glass plate upon which a drawing may have been
previously traced by scratching away the wax, with which it is first
coated. By heating the glass plate before a fire, a sufficient quantity
of wax is soon melted on to it by merely rubbing the wax against the
glass plate; any excess should be avoided, if a well-executed drawing is
required to be etched on its surface. (Fig. 144.)

[Illustration: Fig. 144. A A A. The glass plate, with the waxed side
downwards, placed on the leaden tray containing the fluorspar and
sulphuric acid. B. Spirit lamp.]

The wax plate must not remain too long over the leaden tray, as the heat
is apt to melt the wax, when the acid not only attacks those parts from
which the wax has been removed by the etching needle, but also the
surface of the glass generally, and thus the clearness of the design is
spoilt. After exposure--and it is as well to prepare two or three glass
plates for the experiment--the wax is quickly removed by rubbing and
washing with oil of turpentine, and the design (beautifully etched into
the glass) is then apparent.




CHAPTER XII.

CARBON, BORON, SILICON, SELENIUM, SULPHUR, PHOSPHORUS.


This group of non-metallic elements has been frequently styled
"Metalloids," meaning substances allied to, but not possessing, all the
properties belonging to a metallic substance; and therefore perhaps the
expression, non-metallic solids, is the best that can be adopted. They
may be subdivided into two classes of three each, which have properties
more or less allied to each other--viz.,

  Carbon, Boron, Silicon; and
  Selenium, Sulphur, Phosphorus.

[Page 152]

CARBON.

Symbol, C; Combining Proportion, 6.

This element has almost the property of ubiquity, and is to be found not
only in all animal and vegetable substances, in common air, sea, and
fresh water, but also in various stones and minerals, and especially in
chalk and limestone.

There is, perhaps, no element which offers a greater variety of amusing
experiments and elementary facts than carbon, whether it be considered
either in its simple or combined state.

A piece of carbon, in the shape of the Koh-i-Noor, was one of the chief
attractions at the first Exhibition in Hyde Park. The diamond is the
hardest and most beautiful form of charcoal; how it was made in the
great laboratory of nature, or how its particles came together, seems to
be a mystery which up to the present time has not yet been solved, at
all events no artificial process has yet produced the diamond.

Sir D. Brewster, speaking of the Koh-i-Noor, remarks that on placing it
under a microscope, he observed several minute cavities surrounded with
sectors of polarized light, which could only have been produced by the
expansive action of a _compressed gas or fluid_, that had existed in the
cavities when the diamond was in the _soft_ state.

Now it is known that bamboo, which is of a highly silicious nature, has
the property of depositing in its joints a peculiar form of silica,
called tabasheer. Silicon is one of the triad with carbon--_i.e._, it is
allied to carbon on account of certain analogies; may it not then be
supposed that, in times gone by, ages past, when the atmosphere was
known to be highly charged with carbonic acid gas, there might possibly
have existed some peculiar tree which had not only the power of
decomposing carbonic acid (possessed by all plants at the present
period), but was enabled, like the bamboo, to deposit, not silica, which
is the oxide of silicium, but carbon, the purest form of charcoal--viz.,
the diamond? Speculation in these matters is ever more rife than stern
proof, and it may be stated, that all attempts to manufacture this
precious gem (like those of the alchemists with gold and silver) have
most signally failed.


_First Experiment._

Box and various woods, dried bones, and different organic matters,
placed in a nearly close iron or other vessel, and heated red hot, so
that all volatile matter may escape, leave behind a solid black
substance called charcoal. If that kind obtained from bones, and termed
bone black or ivory black, is roughly powdered, and placed in a flask
with some solution of indigo or some vinegar, or syrup obtained by
dissolving common moist sugar in water, and boiled for a short period,
the colour is removed, and on filtering the liquid it is found to be as
clear and colourless as water, provided sufficient ivory black has been
employed.

[Page 153]


_Second Experiment._

Charcoal is a disinfectant, and is used for respirators; it has even
been recommended medically, and charcoal lozenges can be bought at
various chemists' shops. If a few drops of a strong solution of
hydrosulphuret of ammonia (which has the agreeable odour belonging to
putrid eggs) is mixed with half a pint of water, it will of course smell
strongly, and likewise precipitate Goulard water, or a solution of
acetate of lead black; but on shaking the water with a few ounces of
charcoal, it no longer smells of sulphuretted hydrogen, and if filtered
and poured into a solution of lead does not turn it black. This chemical
action of charcoal, independent of its seeming mechanical attraction for
colouring matter, would appear to show that the pores of charcoal
contain oxygen, which in that peculiar condensed state destroys
colouring matter, and oxidizes other bodies.


_Third Experiment._

A very satisfactory experiment, proving that the diamond and plumbago or
black lead are identical with charcoal, although differing in outward
form and purity, can be made at a little cost, by purchasing a fragment
of refuse diamond, called "_boart_," of Mr. Tennant of the Strand. A
small piece costs about five shillings. The fragment should be carefully
supported by winding some _thin_ platinum wire round it, as, if the wire
is too thick, it cools down the heat of the bit of diamond and prevents
it kindling in the oxygen gas. A difficulty may arise in preparing the
fragment, in consequence of the wire continually slipping off. The
"boart" should therefore be grasped by the thumb and first finger, and
the wire wound round; then it must be carefully turned and again wound
across with the platinum wire, as in the sketch below. (Fig. 145.)

[Illustration: Fig. 145. A. The platinum wire. B. The fragment of
"boart" or refuse diamond.]

A piece of black lead (so called) may now be taken from a lead pencil
and also supported by platinum wire; likewise a bit of common bark
charcoal or hard coke. Three bottles of oxygen should now be prepared
from chlorate of potash and oxide of manganese, an extra bottle being
provided for the diamond in case there should be any failure in its
ignition. The bark charcoal can be first ignited by holding a corner in
the spirit lamp for a few seconds; when plunged into oxygen it
immediately kindles and burns with rapidity, and if the cork is well
fitted, the product of combustion--viz., carbonic acid gas--is retained
for future examination. The small piece of black lead is next heated red
hot in the flame of the spirit lamp, and being attached by its platinum
support to a stiff copper wire thrust through a cork, which fits the
bottle of oxygen, is placed whilst red hot in the gas, and continues to
glow until consumed. The fragment of diamond is by no means, however,
so [Page 154] easily ignited, the flame of the spirit lamp must be
urged upon it with the blowpipe; when quite red hot, an assistant may
remove the stopper from the bottle of oxygen, and the person heating the
diamond should plunge it instantly into the gas; if this is dexterously
managed, the fragment of _boart_ glows like a little star, and the
combustion frequently continues till the piece diminishes so much that
it falls out of its platinum support.

Sometimes the diamond cools down without igniting, the same process must
therefore be repeated, and a few extra bottles of oxygen will prevent
disappointment, as every failure destroys the purity of the gas by
admixture with atmospheric air when the stopper is removed. (Fig. 146.)

[Illustration: Fig. 146. A. Bottle containing bark charcoal. B. Ditto
the plumbago or black lead. C. Ditto the diamond.]

The combustion having ceased in the three bottles, the corks are
removed, and the glass stoppers again fitted for the purpose of testing
the _products_, which offer no apparent indication of any change, as
oxygen and carbonic acid gas are both invisible. In each bottle a new
combination has been produced; the charcoal, the black lead, the diamond
have united with the oxygen, in the proportion of six parts of carbon to
sixteen parts of oxygen, to form twenty-two parts of carbonic acid gas,
which may be easily detected by pouring into each bottle a small
quantity of a solution of slacked lime in water, called lime water. This
test is easily made by shaking up common slacked lime with rain or
distilled water for about an hour, and then passing it through a calico
or paper filter. The test, though perfectly clear when poured in,
becomes immediately clouded with a white precipitate, usually termed a
_milkiness_, no doubt in allusion to the London milk, which is supposed
to contain a notable proportion of chalk and water, for in this case the
precipitate is chalk, the carbonic acid from the diamond and the
charcoal having united with the lime held in solution by the water and
formed carbonate of lime, or chalk, a substance similar in composition
to marble, limestone, Iceland or double refracting spar, these three
being nearly similar in composition, and differing only, like carbon and
the diamond, in external appearance.

[Page 155]

The milkiness, however, must not be held as conclusive of the presence
of carbonic acid gas until a little vinegar or other acid, such as
hydrochloric or nitric, has been finally added; if it now disappears
with effervescence (like the admixture of tartaric acid, water, and
carbonate of soda), the little bubbles of carbonic acid gas again
escaping slowly upwards, leaving the liquid in the three bottles quite
clear, then the experimentalist may sum up his labours with these
effects, which prove in the most decisive manner that common charcoal,
black lead, and the diamond, are formed of one and the same
element--viz., carbon.


_Fourth Experiment._

Having effected the synthesis (or combining together) of the diamond and
oxygen, it is no longer possible to recover it in its brilliant and
beautiful form. If the product of combustion is retained in a flask made
of thin, hard glass, and two or three pellets of the metal potassium are
placed in directly after the diamond has ceased to burn, and the flame
of a spirit lamp applied till the potassium ignites, then the metal, by
its great affinity for oxygen, takes away and separates it again from
that which was formerly the diamond; but instead of the jewel being
deposited, there is nothing but _black_, shapeless, and minute particles
of carbon obtained, if the potash produced is dissolved in water, and
the charcoal separated by a filter.


_Fifth Experiment._

Chalk is made by uniting carbonic acid gas with lime; it may therefore
be employed as a source of the gas, by placing a few lumps of chalk, or
marble, or limestone, in a bottle such as was used in the generation of
hydrogen gas; on the addition of some water and hydrochloric acid,
effervescence takes place from the escape of carbonic acid gas, and the
cork and pewter pipe being adapted, it may be conveyed by its own
gravity into glasses, jugs, or any other vessels, and a pneumatic trough
will not be required. Carbonic acid gas has a specific gravity of 1.529,
and is therefore rather more than half as heavy again as atmospheric
air.


_Sixth Experiment._

In order to satisfy the mind of the operator that the gas obtained from
chalk is similar to the _product of combustion from the diamond_, some
lime-water may be placed in a glass, and the gas from the bottle allowed
to bubble through it; instantly the same milkiness is apparent, which
again vanishes on the addition of acid. And this experiment is rendered
still more striking if a lighted taper be placed in the glass just after
the addition of the acid, when it will be immediately extinguished.


_Seventh Experiment._

If a lady's muff-box, supported by threads or chains, is hung on one end
of a scale-beam, and counterbalanced by a scale pan and a few shot, it
is [Page 156] immediately depressed on pouring into the muff-box a
quantity of carbonic acid gas, which may have been previously collected
in a large tin vessel. After showing the weight of the gas, the box is
detached from the scale-beam, and the contents poured upon a series of
lighted candles, which are all extinguished in succession. (Fig. 147.)

[Illustration: Fig. 147. A. Carbonic acid gas poured out of the tin box
into B, the muff-box. B B. Detached muff-box, and candles extinguished
by the carbonic acid gas poured from it.]


_Eighth Experiment._

The property of carbonic acid gas of extinguishing flame, as compared
with the contrary property of oxygen, is nicely shown by first passing
into a large and tall gas jar one half of its volume of oxygen gas; a
large cork perforated with holes may be introduced, so as to float upon
the surface of the water in the gas jar, and is usefully employed to
break the violence with which the carbonic acid enters the gas jar, as
it is passed in to fill up the remaining half volume of the gas jar,
which now contains oxygen at the top, and carbonic acid gas at the
bottom. On testing the contents of the jar with a lighted taper, it
burns fiercely in the oxygen, but is immediately extinguished in the
[Page 157] carbonic acid gas, being alternately lighted and put out as
it is raised or depressed in the gas jar.


_Ninth Experiment._

A little treacle, water, and a minute portion of size, may be placed
with some yeast in a quart bottle, to which a cork and pewter or glass
pipe is attached; directly the fermentation begins, quantities of
carbonic acid gas may be collected, and tested either with lime-water or
the lighted taper.


_Tenth Experiment._

Some clear lime-water placed in a convenient glass is quickly rendered
milky on passing through it the air from the lungs by means of a glass
tube; thus proving that respiration and (as shown by the ninth
experiment) fermentation, as well as the combustion of charcoal, produce
carbonic acid gas.


_Eleventh Experiment._

[Illustration: Fig. 148. A A. The box model of the Grotto del Cane. B B.
Cardboard fixed in front of box, and painted to imitate rocks. C.
Carbonic acid gas bottle, with bent tube passing through hole in the
side of the box. A taper introduced at D burns in the upper, and is
extinguished in the lower, part of the model.]

Carbonic acid gas is not only generated by the above processes, but is
liberated naturally in enormous quantities from volcanoes, and from
certain soils: hence the peculiar nature of the air in the Grotto del
Cane. Dogs thrust into this cave drop down immediately, and are
immediately revived by the tender mercies of the guides, who throw them
into the adjoining lake. This natural phenomenon is well imitated by
taking a box, open at the top, and nailing on to it a frame of
cardboard, [Page 158] which may be painted to represent rocks, taking
care that a portion (about three inches deep) at the lower part is well
pasted to the box at the edges, so that the gas may be retained; a hole
is perforated at the top side to admit a lighted taper, and another at
the side for the pipe from the carbonic acid bottle; when the bottom is
filled with gas, a taper is applied, which is found to burn in the upper
part, but is immediately extinguished when it reaches the lower
division, where the three inches of pasteboard prevent it falling out:
thus showing in a simple manner why a guide may enter the cave with
impunity, whilst the dog is rendered insensible because immersed in the
gas. (Fig. 148.)


_Twelfth Experiment._

Many fatal accidents have occurred in consequence of the air in deep
pits, graves, &c., becoming unfit for respiration by the accumulation of
carbonic acid gas, which may arise either from cavities in the soil,
where animal matter has undergone decomposition, or it may happen from
the depth and narrowness of the hole or well preventing a proper draught
or current of air, so that it becomes foul by the breathing of the man
who is digging the pit. Air which contains one or two per cent. of
carbonic acid will support the respiration of man, or maintain the flame
of a candle; but it produces the most serious results if inhaled for any
length of time; a lighted candle let down into a well (suspected to
contain foul air) before the descent of the person who is to work in it,
may burn, but does not indicate the presence of the small percentage of
the poison, carbonic acid. Frequently no trouble is taken to test the
air with a lighted candle; a man is lowered by his companions, who see
him suddenly become insensible, another is then lowered quickly to
rescue him, and he shares the same fate; and indeed cases have occurred
where even a third and a fourth have blindly and ignorantly rushed to
their death in the humane attempt to rescue their fellow creatures. What
is to be done in these cases? Are the living to remain idle whilst the
unfortunate man is suffocating rapidly at the bottom of the pit? No;
provided they do not venture themselves into the pit, they may try every
known expedient to alter the condition of the foul air, so as to enable
them to descend to the rescue. One should be despatched to any
neighbouring house or cottage for a pan of burning coals; if any slacked
lime is to be had, it may be rapidly mixed with water, and poured down
the side of the pit; a bundle of shavings set on fire and let down,
keeping it to one side, so as to establish a current; or even the empty
buckets constantly let down empty and pulled up full of the noxious air,
may appear a somewhat absurd step to take, but under the circumstances
any plan that will change the air sufficiently to enable another person
to descend must be adopted; in proof of which the following experiments
may be adduced:

Fill a deep glass jar with carbonic acid, and ascertain its presence
with a lighted taper; if a beaker glass to which a string is attached is
let down into the vessel and drawn up, and then inverted over a lighted
[Page 159] taper, the utility of this simple plan is at once rendered
apparent; the beaker glass represents the empty bucket, and can be let
down and pulled up full of carbonic acid until a sensible change in the
condition of the atmosphere is produced. The best plan, however, is to
set the air in motion by heat obtained from burning matter, or even a
kettle of boiling water, lowered by a cord, and this fact is well shown
by putting a small flask full of boiling water, and corked, at the
bottom of the deep glass jar containing the carbonic acid gas, which
rises like other gases when sufficiently heated, and passing away, mixes
with the surrounding air. (Fig. 149.)

[Illustration: Fig. 149. A. Deep jar containing carbonic acid gas, which
is being removed by the little glass bucket. B. Jar containing corked
flask of boiling water on a pad; the heated gas rises and the cold air
descends to take its place.]


_Thirteenth Experiment._

Carbonic acid gas dissolved in water under considerable pressure, forms
that most agreeable drink called soda-water; the gas is not only useful
in this respect, but has been applied most successfully by Mr. Gurney to
extinguish a fire on a gigantic scale, which had been burning for years
in the waste of a coal mine in Scotland. The same gas, generated
suddenly by the combustion of a mixture of nitre, coke dust, and clay,
or plaster of Paris, in vessels of a peculiar construction, has formed
the subject of a patent by Phillips, since merged into the Fire
Annihilator Company. The instrument is peculiarly adapted for shipping,
and might, if properly used, be the means of saving many ships and
valuable lives. (Fig. 150.)

Its practical value is established by the test of actual use: in the
streets, by the Leeds Fire Brigade, and by firemen of the Fire
Annihilator Company, temporarily stationed at Liverpool and Manchester.

The Fire Annihilator has been formally recognised by the Government
Emigration Commissioners, who introduced into the Passengers' Act, 1852,
in §24, the alternative, "_Or other apparatus for extinguishing fire_,"
with distinct reference to this invention, and subsequently by formal
order authorized their officers to pass ships carrying Fire
Annihilators.

[Page 160]

[Illustration: Fig. 150. A. A carriage with six fire annihilators, No. 5
size, fitted with moveable pipes. The body of the carriage forms a tank
for forty gallons of water; the tank is filled at a bunghole in the
platform; a patent tap is fitted to the rear of the carriage; a spigot
is placed near the end upright of the rail; a hand-pump is placed in the
box at rear of carriage; a leather bucket with foot-holds and three
canvas buckets are hung on the carriage; a hammer for removing and
driving on the cover of the fire annihilator, and a nut wrench for the
No. 10 truck, are placed in the box. B. A fire annihilator, No. 10 size,
with moveable pipe, on a spring truck, is attached to the carriage.

The battery is fitted with shafts for one horse. A pole is also provided
to fix across the shafts, so that the battery may be drawn by hand.]

[Illustration: Fig. 151. A. Tank containing acid, communicating by a
pipe with B, half filled with chalk and water. C C C C. Pipes conveying
carbonic acid from the generator B, to the ceiling, where it is
discharged from numerous holes on the fire beneath.]

Monsieur Adolphe Girard has proposed that all houses should be provided
with an apparatus for the generation of carbonic acid gas, placed [Page
161] outside the building, which is to be conveyed along the ceiling by
means of pipes perforated with numerous holes, and to be put in
operation directly a fire breaks out. This plan, however ingenious,
could hardly supply the carbonic acid gas with sufficient rapidity, and
it is to be feared would utterly fail in practice. (Fig. 151.)


BORON.

Symbol, B; combining proportion, 10.9.

Discovered by Homberg, in 1702, in borax, which is a biborate of soda
(NaO,2BO_{3}), and is used very extensively in the manufacture of glass;
also for glazing stoneware and soldering metals; it is also a valuable
flux in various crucible operations, whilst in testing minerals with the
blowpipe it is invaluable. Borax is made either from tincal, a substance
that occurs naturally in some parts of India, China, and Persia, or by
the addition of carbonate of soda to boracic acid, a substance obtained
from the volcanic districts of Tuscany, whence it is imported to this
country, and used in the manufacture of borax.

The element boron may be obtained by placing some pure boracic acid and
some small bits of potassium in a tube together, and applying the flame
of a spirit-lamp, a glow of heat takes place, and when the tube is cold
the potash may be washed away, and the boron remains as a dark brownish
powder somewhat resembling carbon. M. St. Claire Deville and Wöhler have
lately made some important discoveries with respect to this element, and
disproved the statement that it is uncrystallizable. Their researches
prove it to be producible under three forms and of various colours, such
as honey-yellow and garnet-red, the crystals in some cases being like
diamonds of the purest water--_i.e._, limpid and transparent. A new
combination of aluminium and boron is stated to possess the most
remarkable properties. It is harder than the diamond, and in the state
of powder will cut and drill rubies, and even the diamond itself, with
more facility than diamond powder. Deville and Wöhler incline to the
belief that the diamond is dimorphous, and capable (in conditions yet to
be described) of assuming the same forms as boron. At a high
temperature, boron, like titanium, absorbs _nitrogen_ only from the
atmosphere, and rejects the oxygen. (Query, may not some of those
remarkably hard black diamonds prove to be boron?)


SILICON.

Symbol, Si; combining proportion, 21.3.

The great Berzelius was the first to obtain this element in 1823.
Silicon in the pure state is a dark brown powder; if ignited at a very
high temperature it assumes a chocolate colour, which is supposed to be
the allotropic condition, because it no longer burns when heated
moderately in oxygen or air, and is not attacked by hydrofluoric acid.
[Page 162] The most interesting combination of silicon is the teroxide
called silicic acid, silica (SiO_{3}). Silicon is next to oxygen so far
as regards its plentifulness, and is found in the state of silica in
nearly every mineral, but especially in rock crystal, quartz, flint,
sand, jasper, agate, and tripoli. It is largely used in the manufacture
of glass, and a most useful "soluble glass" is obtained by melting
together in a crucible fifteen parts of sand, ten parts of carbonate of
potash, and one part of charcoal.

Cold water merely washes away the excess of alkali, and after this is
done the powdered soluble glass may be boiled with water in the
proportion of one of the former with five of the latter, when it
gradually dissolves; the solution may be evaporated to a thick pasty
fluid, which looks like jelly when cool, and on exposure to the air in
thin films changes to a transparent, colourless, brittle, but not hard
glass. Wood, cotton, and linen fabrics are rendered less combustible
when coated with this glass, which excludes the oxygen of the air, and
it has lately been employed to fill up the porous and capillary openings
in stone exposed to the atmosphere, and is stated to be very efficacious
as a preservative of the stone in some cases.


SULPHUR.

Symbol, S; combining proportion, 16.

Sulphur, like charcoal, is of common occurrence in nature, and is
chiefly supplied from the volcanic districts of Tuscany and Sicily:
there is an abundance of this element in the United Kingdom, but then it
is locked up in combination with iron, copper, and lead, under the name
of iron pyrites, copper pyrites, galena; and whilst Sicily and Tuscany
supply thousands of tons weight in the uncombined state, it is not, of
course, worth while to go through expensive operations at home for the
separation of sulphur from the ores. During the dispute between Sicily
and England, several patents were secured for new and economical
processes by which sulphur was obtained from various minerals; and had
this country been excluded from a supply of native sulphur, no doubt
some of these patents would now be in active operation.

It is almost possible to estimate the commercial prosperity of a country
by the sulphur it consumes, not, happily, by their warlike operations,
but in the manufacture of oil of vitriol or sulphuric acid, which is the
starting point of a great number of useful arts and manufactures.


_First Experiment._

Some very curious results may be obtained by heating sulphur at certain
temperatures; in the ordinary state it is a pale yellow solid, and when
subjected to a temperature of 226° Fahr. it melts to a brownish-yellow,
transparent, thin fluid; according to all preconceived notions of the
properties of substances which liquify by an increase of heat, it might
be imagined that every additional degree of heat would only [Page 163]
render the melted sulphur still more liquid, but strange to say, when it
reaches a temperature of about 320° Fahr. it changes red, and thick like
treacle; and as the heat rises it becomes so tenacious, that the ladle
in which it is contained may be inverted, and the sulphur will hardly
flow out: at about 482° Fahr. it again becomes liquid, but not so fluid
as at the lower temperature. If allowed to cool from 482° Fahr., the
above results are simply inverted; the sulphur becomes thick, again
liquid, and finally crystallizes in long, thin, rhombic prisms, which
are seen most perfectly by first allowing a crust of sulphur to form on
the liquid portion, and then having made two holes in this crust, the
sulphur is poured out, when the remainder is found in the interior of
the crucible crystallized in the form already mentioned. Sulphur takes
fire in the air when exposed to a heat of about 560° Fahr., and burns
with a pale blue flame; and, as already stated, it may be poured from a
considerable height on a still dark night, and produces a continuous
column of blue fire, just like an unbroken current of electricity. If
the melted and burning sulphur is received into a vessel containing
boiling water, it is no longer yellow, but assumes a curious
_allotropic_ state, in which it is a reddish-brown, transparent,
shapeless mass, that may be easily kneaded and used for the purpose of
taking casts of seals, which become yellow in a few days, and are found
then to be hard and crystallized.


_Second Experiment._

Sulphur vapour, in one sense, may be regarded as a supporter of
combustion: if a clean Florence oil-flask is filled with copper
turnings, and a little roughly-powdered sulphur sprinkled in, and heat
applied, the copper glows with an intense heat, and burning in the
vapour of the sulphur, produces a sulphuret of copper; from this
compound the sulphur may be again obtained by boiling the powdered
sulphuret with weak nitric acid, which oxidizes and dissolves the
copper, leaving the greater part of the sulphur behind, which may be
collected, melted, and burnt, and will be found to display all the
properties belonging to that element. This experiment is a very good
example of simple analysis; and if the copper is weighed and likewise
the combined sulphur, a good notion may be formed of the principles of
combining proportions.


_Third Experiment._

A little sulphur burnt under a gas jar, or in any convenient box (a
hat-box, for instance), produces sulphurous acid (SO_{2}), which will
bleach a wetted red rose or dahlia, and many other flowers. This gas is
employed most extensively in bleaching straw, and sundry woollen goods,
such as blankets and flannel, and likewise silk, and is perhaps one of
the best disinfectants that can be employed; when fever has been raging
in the dwellings of the poor, as in cottages, &c., all metallic
substances should be removed, the doors and windows closed, the bedding,
&c., well exposed, and then a quantity of sulphur should be burnt in an
old [Page 164] frying-pan placed on a brick, taking care to avoid the
chance of setting the place on fire; after a few hours the doors and
windows may be opened, and the disinfectant will be found to have done
its work cheaply and surely.


_Fourth Experiment._

The presence of sulphur in various organic substances, such as hair, the
white of egg, and fibrine, is easily detected by heating them in a
solution of potash, and adding acetate of lead as long as the
precipitate formed is redissolved; finally the solution must be heated
to the boiling point, when it instantly becomes black by the separation
of sulphuret of lead.


_Fifth Experiment._

Sulphuric acid, HO,SO_{3}, or oil of vitriol, is made in such enormous
quantities that it is never worth while to attempt its preparation on a
small scale. In consequence of its great affinity for water, many
energetic changes are produced by its action. Oil of vitriol poured on
some loaf sugar placed in a breakfast-cup with the addition of a
dessert-spoonful of boiling water, rapidly boils and deposits an
enormous quantity of black charcoal. If a word be written on a piece of
white calico with dilute sulphuric acid, and then rapidly and thoroughly
washed out, no visible change occurs; but if the calico is exposed to
heat, so that the excess of water is driven off, the remaining and now
concentrated oil of vitriol attacks the calico, and the word is
indelibly printed in black by the decomposition of the fabric of cotton.
A very remarkable process has lately been introduced by Mr. Warren de la
Rue, by which paper is converted into a sort of tough parchment-like
material, called ametastine, by the action of oil of vitriol and water
of a certain fixed strength; and any departure from the exact
proportions destroys the toughness of the paper. After the paper has
been acted upon by the acid, it becomes extremely tenacious, and will
support a considerable weight without breaking. Mr. Smee has used this
ametastine in the construction of an hygrometer, and states that it may
save many a traveller from catching a severe rheumatism in a damp bed.


_Sixth Experiment._

When the vapour of sulphur is passed over red-hot charcoal and the
product carefully condensed, a peculiar liquid is obtained, called
bisulphide of carbon (CS_{2}), which possesses a peculiar odour, is
extremely transparent and brilliant-looking, and enjoys a high
refractive power. This liquid is used as a solvent for phosphorus and
other substances, and is extremely volatile and combustible, and burns
silently with a pale blue flame. The combustion of its vapour, mixed
with certain gases, offers a good example of the fact that slow burning
may be a peaceful experiment, whilst very rapid combustion often
resolves itself into an explosion. Thus, if a few drops of bisulphide of
carbon are dropped into a narrow-mouthed dry quart bottle containing
common air, and flame applied, the combustion takes place with rapidity,
a rushing or [Page 165] roaring sound being audible, in consequence of
the diffused vapour being supplied with more oxygen, and burning more
rapidly than it would do if simply consumed from a stick or glass rod
wetted with the fluid. A still greater rapidity of combustion is ensured
by dropping some bisulphide of carbon into a long stout cylindrical jar,
fifteen inches long and three inches in diameter, containing nitric
oxide gas (NO_{2}); when flame is applied the mixture burns with a
bright flash and some noise, and if burnt in a narrow mouthed bottle
would most likely blow it to atoms.

The greatest rapidity of combustion, and of course the loudest noise, is
obtained by shaking some bisulphide of carbon in a similar stout and
strong cylindrical jar filled with oxygen gas, but in this case the jar
must be protected with a double cylinder of stout wire gauze; it does
not always break, but if it is blown to fragments each particle becomes
a lancet-shaped piece of glass, which is capable of producing the most
dangerous wounds. (Fig. 152.)

[Illustration: Fig. 152. A. Air and bisulphide of carbon. B. Nitric
oxide and ditto. C. Oxygen and ditto. D D. Stout cylinder of double wire
gauze, open top and bottom.]


SELENIUM.

Selenium ([Greek: _selênê_], the Moon[B]); symbol, Se; combining
proportion, 39.5.

This new metallic element is allied to sulphur, and is a species of
chemical curiosity, being found in minute quantities in various
minerals; it may be melted and cast into any form. Medallions of the
discoverer (Berzelius) of selenium, in little cases, are imported from
Germany, for the cabinets of the curious.

[Footnote B: Called selenium on account of its strong analogy to the
metal tellurium (_tellus_, the earth).]

[Page 166]


PHOSPHORUS.

Phosphorus ([Greek: _phôs_], light; [Greek: _pherein_], to bear; symbol, P;
combining proportion, 32.)

Monsieur Salverte, in his work on the Occult Sciences of the Ancients,
quotes a remarkable story respecting the probable discovery of the
nature of phosphorus in 1761:--"A Prince San Severo, at Naples,
cultivated chemistry with some success; he had, for example, the secret
of penetrating marble with colour, so that each slab sawed from the
block presented a repetition of the figure imprinted on its external
surface. In 1761, he exposed some human skulls to the action of
different reagents, and then to the heat of a glass furnace, but paying
so little attention to his manner of proceeding, that he acknowledged he
did not expect to arrive a second time at the same result. From the
product he obtained a vapour, or rather a gas was evolved, which
kindling at the approach of a light, burned for several months without
the matter appearing to die or diminish in weight. San Severo thought he
had found, the impossible secret of the inextinguishable lamp, but he
would not divulge his process, for fear that the vault in which were
interred the princes of his family should lose the unique privilege with
which he expected to enrich it, of being illuminated with a _perpetual
lamp_." Had he acted like a philosopher of the present day, San Severo
would have attached his name to the important discovery of the existence
of _phosphorus_ in the _bones_, and made public the process by which it
might be obtained.

This element, formerly sold at four or five shillings the ounce, has now
fallen so much in price, from the greater demand and larger production,
that it may be bought for a few shillings the pound, and is imported in
tin cases in large quantities from Germany. It was discovered about two
hundred years ago by Brandt, a merchant of Hamburg, and may be prepared
on a small scale by distilling at a red heat phosphoric acid previously
fused with one-fourth of its weight of powdered charcoal.


_First Experiment._

Phosphorus, when pure, is without taste or colour, but generally of a
very pale buff-colour, and semi-transparent; it is extremely
combustible, and is usually preserved under the surface of water; when
perfectly dry, a thin slice will take fire at 60° Fah., and burns with
great brilliancy. Considering the heat produced during the combustion of
phosphorus, it might be thought that it would infallibly set fire to any
ordinary combustible, such as paper or wood, but this is not the case
when phosphorus is employed by itself, as may be proved by the following
experiment.

Cut five very small pieces of phosphorus, and place them like the five
of diamonds on a sheet of cartridge-paper laid upon the table, set the
bits of phosphorus on fire, when they will be rapidly burnt away [Page
167] leaving only five black spots, but not firing the paper, as would
be the case if some red-hot coals or charcoal were placed in the same
position. The cause is very simple. Phosphorus in burning produces
phosphoric acid, which is an anti-combustible, and coats the surface of
the paper round the spot where the combustion occurs, and acting as a
kind of glaze or glass, excludes the oxygen of the air, and prevents the
fire spreading.

If some powdered sulphur is sprinkled round the spot where the bit of
phosphorus is to be burnt, the case is very different; the heat melts
and sets fire to the sulphur, which being uncoated with the phosphoric
acid, communicates to the paper; and it is on this principle that
lucifer-matches can be used as instantaneous lights. The tip of the wood
of which they are composed is first dipped in sulphur, and then the
phosphorus composition made of gum, chlorate of potash, vermilion, and
phosphorus, is placed over it; and if the latter were used alone without
the sulphur, not one match in a hundred would take fire properly.


_Second Experiment._

Common phosphorus is perfectly and rapidly dissolved by bisulphide of
carbon. The solution must be carefully preserved, as it is a liquid
combustible, which takes fire spontaneously after the bisulphide of
carbon evaporates; so that wherever it is dropped, a flame, arising from
the spontaneous combustion of the finely-divided phosphorus, is sure to
be produced. This liquid was recommended many years ago to the
Government for the purpose of setting sails of ships or other
combustible matter on fire. The solution of phosphorus alone did not
answer the purpose, as already explained in the first experiment; but
when wax was dissolved with the phosphorus, it then became a most
dangerous fluid, which it was recommended should be used in shells, and
discharged from a mortar or howitzer in the ordinary manner. Dr. Lyon
Playfair was the first to make this proposed application of the
solution, and it has since, we believe, been recommended by Captain
Norton in his liquid-fire shells.


_Third Experiment._

One of the most curious facts in connexion with phosphorus, is its
assumption of the allotropic state in what is termed _amorphous_
(shapeless) or red phosphorus. This substance, when handled for the
first time, might be mistaken for a lump of badly-made Venetian red.
There is no risk of its taking fire like the common phosphorus, and it
does not (according to Schrötter, of Berlin, who discovered this
peculiar condition) exhale those fumes which are so prejudicial to the
lucifer-match makers. When the vapour of common phosphorus is
continually inhaled, it is said to cause a peculiar and disgusting
disease, which terminates in the destruction of the jaw-bone; whilst the
bones in other parts of the body become brittle, and arm-bones thus
affected are fractured with the slightest blow.

The difference between common and red phosphorus is well shown first,
[Page 168] by placing a few small pieces of both kinds in separate
bottles or vials containing bisulphide of carbon; the common phosphorus,
as already explained, quickly dissolves in the liquid, and if poured on
a sheet of paper, and hung up, is soon on fire; whilst the red variety
is wholly unaffected, and if the bisulphide of carbon is poured off on
to paper, it merely evaporates, and no combustion occurs.

The similarity in composition, though not in outward form, is further
shown by filling two jars with oxygen gas, and having provided two
deflagrating spoons, some common phosphorus is placed in one, and red
phosphorus in the other; a wire, gently heated by dipping it into some
boiling water, is now applied to the former, which immediately takes
fire, and may be plunged into the jar of oxygen gas, when it burns with
the usual brilliancy. The red phosphorus, however, must be brought to a
much higher temperature (500° Fah.) before it will even shine in the
dark, and then with a still further increase of heat it takes fire, and
on being placed in the other jar of oxygen burns up much more slowly
than the yellow phosphorus, but at last exhibits that brilliant flash of
light which is so characteristic of the combustion of phosphorus in
oxygen.

The amorphous or red phosphorus is employed in the manufacture of
_safety chemical matches_, and M. A. Meunons has secured a patent in
England for an improvement in lucifer matches, with a view to obviate
the risks of accidental ignition. To attain this end the matches are
first cut by a machine from cubes of wood, the cut being stopped at a
short distance from the end of each cube, so as to leave the lower
extremities adherent. The upper or free extremity of each packet of
splints thus formed being coated with wax or sulphur, is dipped in one
of the following preparations:--Chlorate of potash, two parts;
pulverized charcoal, one part; umber, one part; or, chlorate of potash,
sulphur, and umber, in equal parts, thoroughly mixed with glue. The
opposite extremity or "cut" of each packet is then painted over with
amorphous phosphorus blended with size, so that on separating the
matches the phosphorus is only found on the top of each. The matches
thus prepared are ignited by breaking off a small piece of the
phosphorised end and rubbing it on the opposite extremity covered with
the inflammable preparation.

Loud exploding and dangerous lucifers were formerly made by dipping
bundles of matches, previously coated with sulphur at the tips, into a
thick solution of gum, at a temperature of 104° Fahr., coloured with
smalt or red lead, in which was dissolved a certain proportion of
chlorate of potash, and also containing finely divided particles of
phosphorus obtained by the constant stirring and rubbing of the
materials in a mortar. When dry the matches exploded if rubbed against a
gritty surface, and there was always a risk of a fragment flying off and
entering the eye. To obviate this danger, _silent_ or _noiseless lucifer
matches_ were invented, and the composition used (according to Böttger)
is as follows:--Gum arabic, 16 parts by weight; phosphorus, 9 parts;
nitre, 14 parts; powdered black oxide of manganese, 16 parts. The above
ingredients are worked up in a mortar with water, at 104° Fahr., and the
matches previously tipped with sulphur are dipped therein and afterwards
dried.

[Page 169]


_Fourth Experiment._

The combustion of phosphorus under water is easily demonstrated by
placing some ordinary stick phosphorus in a metallic cup, and then
plunging it rapidly under the surface of boiling water. If a jet of
oxygen gas is now directed upon the liquid phosphorus, it burns with
great brilliancy. When the oxygen escapes too rapidly from the jet, it
causes some small particles to be thrown out of the water, so that it is
advisable to defend the face with a sheet of wire gauze held a few
inches above the glass whilst the experiment is being conducted. (Fig.
153.)

[Illustration: Fig. 153. A A. Finger-glass of boiling water containing a
metallic cup with melted phosphorus. C. Jet of oxygen gas. D D. Sheet of
wire gauze.]


_Fifth Experiment._

Phosphorus burns and emits beautiful flashes of light in the presence of
the gas called peroxide of chlorine (ClO_{4}), which must be very
carefully generated under the surface of water by first placing some cut
phosphorus and chlorate of potash at the bottom of a long and stout
cylindrical glass nearly full of water; sulphuric acid is then conveyed
to the chlorate of potash by means of a syphon, the end of which must be
drawn out to a small opening, or else the oil of vitriol will descend
too rapidly, and the glass will be cracked by the heat. Immediately the
peroxide of chlorine comes in contact with the phosphorus it explodes,
and passes again to its original elements, oxygen and chlorine. These
bubbles envelope minute particles of phosphorus, which rapidly ascend,
like water-spiders, to the surface, and burn as they pass upwards,
producing a continual series of sparks of fire, which have an extremely
pretty effect. (Fig. 154.) The syphon is of course first filled with
water, and as that is displaced, the oil of vitriol takes its place.

[Illustration: Fig. 154. A A. Tall glass nearly full of water; at the
bottom are the chlorate of potash and phosphorus. B. Wolfe's bottle and
syphon, conveying the oil of vitriol to bottom of A A.]

[Page 170]


_Sixth Experiment._

If a little phosphorus is placed in a small copper boiler, and the steam
allowed to escape from a jet, it is observed to be luminous, in
consequence of a minute portion of phosphorus being carried up
mechanically with the steam. The same fact is shown very prettily by
boiling water in a flask containing some phosphorus.


_Seventh Experiment._

Phosphorus explodes violently when rubbed with a little chlorate of
potash, and in order to perform this experiment safely, it should be
made in a strong iron mortar, the pestle of which must be surrounded
with a large circle of cardboard and wire gauze; so that when it is
brought down upon the phosphorus and chlorate of potash, any particles
that may fly out are detained by the shield. Without this precaution the
experiment is one of the most dangerous that can be made. (Fig. 155.)

[Illustration: Fig. 155. A. The iron mortar containing the phosphorus
and chlorate of potash. B. The pestle, with the shield, C C, composed of
a circle of wire gauze, covered with one of cardboard.]


_Eighth Experiment._

Phosphuretted hydrogen owes its property of spontaneous combustion to
the presence of the vapour of a liquid, phosphide of hydrogen (PH_{2}),
which may be prepared by placing some phosphide of calcium into a flask
with water heated to a temperature of 140° Fah., and conveying the gas
into a U-shaped tube surrounded with a mixture of ice and salt. The
liquid obtained is colourless, and must be preserved from contact with
air, as it takes fire spontaneously directly it is exposed to the
atmosphere. (Fig. 156.)

[Illustration: Fig. 156. A. The flask containing the phosphide of
calcium and water, and placed in a water-bath heated to 140° Fah. B.
Bent tube conveying the gas to C C, the U-shaped tube, to which it is
attached by india-rubber tubing, C C. The U-shaped tube, surrounded with
a freezing mixture. D D. Bent tube, passing into a cup of water to
prevent contact with air.]

[Page 171]


_Ninth Experiment._

Phosphide of calcium is quickly prepared by placing some small pieces of
lime in a crucible and making them red-hot; if lumps of dry phosphorus
are thrown into the crucible, and the cover placed on quickly, and
immediately after the phosphorus, the latter unites with the calcium,
and forms a brown substance which produces gaseous phosphide of hydrogen
(PH_{3}) when placed in water, and the gas takes fire spontaneously when
it comes in contact with the air.


_Tenth Experiment._

Phosphorus placed in a retort with a tolerably strong solution of
potash, and a small quantity of ether, affords a large quantity of
phosphide of hydrogen (commonly called phosphuretted hydrogen) when
boiled. The neck of the retort must dip into a basin of water, and the
object of the ether is to prevent the combustion of the first bubbles of
gas _inside_ the retort, which by their explosion would probably break
the glass. If the neck of the retort is kept under water in which potash
is dissolved, the gas may be generated for many days at pleasure,
although it is not a desirable experiment to renew too often, on account
of the disagreeable odour produced. (Fig. 157.)

[Illustration: Fig. 157. A retort containing the phosphorus, water,
potash, and ether. B. Neck dipping into a basin of water. C. The gas
burning, and producing beautiful rings of smoke.]


_Eleventh Experiment._

When a jar of oxygen is held over the neck of the retort generating the
phosphuretted hydrogen, a bright flash of light and explosion are
observed; and if the experiment is performed in a darkened room, it is
just like a sudden flash of lightning. A bottle of chlorine held over
the neck [Page 172] of the retort, and dipping of course in the water
of the basin, produces a green flame every time the bubble of gas passes
into it. That curious appearance of light, sometimes seen in marshy
districts, called will-o'-the-wisp, is supposed to be due to the escape,
from decomposing matter, of bubbles of hydrogen, nitrogen, &c., through
which the spontaneously inflammable phosphide of hydrogen is diffused.

At a place called Dead Man's Island, near Sheerness, magnificent effects
of this kind are sometimes apparent when the mud banks are accidentally
stirred at night by a boat-hook. A credible observer says, he once saw
there a flash of yellowish-green light, accompanied with noise, about
thirty feet in height. The apparent height might be due to the duration
of the impression of the flash on the eye, as the light from the burning
phosphuretted hydrogen ascended rapidly upwards. The source of this gas
appears to be due to the fact, that during the time some Russian ships
were watched by the Brest fleet, a number of the sailors died of
cholera, and were buried in the banks; the decomposition of the bone
containing phosphorus would account for the appearance of light already
described.

With the discussion of some of the most interesting properties of the
thirteen non-metallic elements we take leave of the subject of
chemistry, reserving the consideration of the metals for another popular
juvenile work, of which they will form the subject.

In answer to the oft-repeated question, "Where can I get the _things_
for the experiments?" it may be stated that every kind of glass vessel
and the chemicals mentioned in this chapter, can be procured either of
Messrs. Simpson, Maule, and Co., Kennington, or of Griffin and Co.,
Bunhill-row, or Bolton and Co., High Holborn.

[Illustration: Fig. 158. Will-o'-the-wisp.]


[Page 173]




CHAPTER XIII.

FRICTIONAL ELECTRICITY.


[Illustration: Fig. 159. Franklin and his kite.]

Of all the agents with which man is acquainted, not one can afford a
greater source of wonderment to the ignorant, of meditation to the
learned, than the effects of that marvellous force pervading all matter
called electricity. We look at matter endowed with life, and matter
wanting this divine gift, with some degree of interest, depending on our
various tastes and occupations; we know at a glance a bird, a beast, or
a fish; we observe with pleasure and admiration the wonderful changes of
nature, and know that a few seeds thrown into the broken clods and
well-tilled earth may become either the waving, golden corn-field or in
time may grow from the tender little shrub to the stately forest-tree;
we know all these things because they belong to the visible world, and
are continually passing before our eyes: but in looking at the visible,
we must not forget and ignore the invisible. It may with truth be [Page
174] stated that the greatest powers of nature are all concealed, and if
any truth would lead us from Nature to Nature's God, it is the fact that
no visible, solid, tangible agent can work with so much force and power
as invisible electricity. Many centuries passed away since the
commencement of the Christian era, before the human mind was prepared to
appreciate this great power of nature; other forces had claimed
attention, and the difference in the presence or absence of two of the
imponderable agents, heat and light, as derived from the sun, in the
effects of the change of the seasons, and other common facts, had led
philosophers to speculate early upon their nature; but electricity, from
its peculiar properties, long escaped observation, and it was not until
the beginning of the eighteenth century (about 1730) that any material
facts had been discovered in this science, when Mr. Stephen Grey, a
pensioner of the Charterhouse, discovered what he termed _electrics_ and
_non-electrics_, and also the use of insulating materials, such as silk,
resin, glass, hair, &c.; and it is obvious that, until the latter fact
was discovered, the science would remain in abeyance, because there
would be no mode of preserving the electrical excitement in the absence
of non-conductors of this force.

The year 1750 was remarkable for Volta's discoveries and Dr. Franklin's
identification of the electricity of the machine with the stupendous
effects of the thunderstorm. Sir Humphry Davy, in 1800, with his
commanding genius, threw fresh light upon the already numerous
electrical effects discovered. In 1821, Faraday commenced his studies in
this branch of philosophy; which he has since so diligently followed up,
that he has been for some years, and is still the first electrician of
the age. From the commencement of the present century, discoveries have
succeeded each other in regular order and with the most amazing results;
and now electricity is regularly employed as a money-getting agent in
the process of the electrotype and electro-silvering and gilding; also
in the electric telegraph; and in a few years we may possibly see it
commonly employed as a source of artificial light.

The nature of electricity, says Turner, like that of heat, is at present
involved in obscurity. Both these principles, if really material, are so
light, subtle, and diffuse, that it has hitherto been found impossible
to recognise in them the ordinary characteristics of matter; and
therefore electric phenomena may be referred, not to the agency of a
specific substance, but to some property or state of common matter, just
as sound and light are produced by a vibrating medium. But the effects
of electricity are so similar to those of a mechanical agent, it appears
so distinctly to emanate from substances which contain it in excess, and
rends asunder all obstacles in its course so exactly like a body in
rapid motion, that the impression of its existence as a distinct
material substance _sui generis_ forces itself irresistibly on the mind.
All nations, accordingly, have spontaneously concurred in regarding
electricity as a material principle; and scientific men give a
preference to the same view, because it offers an easy explanation of
phenomena, and suggests a natural language intelligible to all.

[Page 175]

There are five well-ascertained sources of electricity, and three which
are considered to be uncertain. The five sources are friction, chemical
action, heat, magnetism, peculiar animal organisms. The three uncertain
sources are contact, evaporation, and the solar rays.


_First Experiment._

A stick of sealing-wax or a bit of glass tube, perfectly dry, rubbed
against a warm piece of flannel, has elicited upon its surface a new
power, which will attract bits of paper, straw, or other light
materials; and after these substances are endowed with the same force, a
repellent action takes place, and they fly off. One of the most
convenient arrangements for making experiments with the attractive and
repellent powers of electricity is to fix with shell-lac varnish round
discs of gilt paper, of the size of a half-crown, at each end of a long
straw that is supported about the centre with a silk thread, which may
hang from the ceiling or any other convenient support. (Fig. 160.)

[Illustration: Fig. 160. A. The glass pillar support. B. Straw with
discs, hanging by a silk thread.]

The varnish is easily prepared by placing four or eight ounces of
shell-lac in a bottle, and pouring enough pyroxylic spirit (commonly
termed wood naphtha) upon the lac to cover it. After a short time, and
by agitation, solution takes place. In a variety of ways friction is
proved to be a source of electricity, and forms a distinct branch of the
science, under the name of _frictional_ electricity.


_Second Experiment._

The nature of chemical action has been already explained, and is alluded
to here as a source of electricity of which the proof is very simple. A
piece of copper and a similar-sized plate of zinc have attached to them
copper wires; these plates are placed opposite to, but do not touch each
other, in a vessel containing water acidulated with a small quantity of
sulphuric acid. When the wires are brought in contact, a current of
electricity circulates through the arrangement, but has no power to
attract bits of paper, straw, &c. In order to ascertain whether the
current of electricity passes or not, a piece of covered copper wire is
bent several times round a magnetic needle, so that it has freedom of
motion inside the core or hollow formed by twisting the copper wire.
This arrangement, properly constructed, is called the galvanometer
[Page 176] needle, and is invaluable as a means of ascertaining the
passage of electricity derived from chemical action. (Fig. 161.)

[Illustration: Fig. 161. A. The galvanometer needle. B. Vessel
containing weak acid and the zinc and copper plates. The arrows show the
path of the electric current.]

When the wires leading from the metal plates are connected with the
extremities of the coil in the galvanometer, the needle is deflected or
pushed aside to the right hand or to the left, according to the
direction of the current.


_Third Experiment._

The third source of electricity is heat, and the effect of this agent is
well shown by twisting together a piece of platinum and silver wire, so
as to form one length. If the silver end is attached to any screw of the
galvanometer, and the platinum end to the second screw, no movement of
the magnetic needle takes place until the heat of a spirit-lamp is
applied for a moment to the point of juncture between the silver and
platinum wires, when the magnetic needle is immediately deflected.

[Illustration: Fig. 162. A. The galvanometer needle, with wires
attached. S, S. Silver wire joined to P, P, the platinum wire. The heat
of the spirit-lamp is applied at the point of juncture, +.]


_Fourth Experiment._

The fourth source of electricity--viz., magnetism--requires a somewhat
more complicated arrangement; and a most delicate galvanometer needle
must be provided, to which is attached the extremities of a long spiral
coil of copper wire covered with cotton or silk. Every time a bar magnet
is introduced inside the coil, so that the conducting wire cuts the
magnetic curves, a deflection of the galvanometer needle takes place,
[Page 177] and the same effect is produced on the withdrawal of the
magnet, the needle being deflected in the opposite direction.

The magnetic spark can be obtained by employing a magnet of sufficient
power; and the arrangement for this purpose is very simple. A cylinder
of soft iron is provided, and round its centre are wound a few feet of
covered thin copper wire, one end of which is terminated with a copper
disc well amalgamated, and the other end, after being properly cleaned
and coated with mercury, is brought into contact with the disc. Directly
this cylinder is laid across the poles of the magnet, and as quickly
removed, the point and disc, from the elasticity of the former, separate
for the moment, the contact is broken between the point and disc, and a
brilliant but tiny spark is apparent.

[Illustration: Fig. 163. A B. Horse-shoe magnet. C. Cylinder of soft
iron. D. Coil of copper wire and contact breaker.]


_Fifth Experiment._

The fifth mode of procuring electricity would require the assistance of
an electrical eel, a fine specimen of which (forty inches in length) was
exhibited at the Adelaide Gallery some years ago. Various experiments
were made with this animal, and the author had the pleasure of
witnessing all the ordinary phenomena of frictional electricity,
illustrated by Dr. Faraday, with the assistance of the animal
electricity derived from this curious creature. Recent experiments have,
however, proved that the electric current is induced through the agency
of the nervous [Page 178] system. This important fact has been
communicated by M. Dubois-Raymond, whose experiment is thus recorded:--A
cylinder of wood is firmly fixed against the edge of a table; two
vessels filled with salt and water are placed on the table, in such a
position that a person grasping the cylinder may, at the same time,
insert the fore-finger of each hand in the water. Each vessel contains a
metallic plate, and communicates, by two wires, with an extremely
sensitive galvanometer. In the instrument employed by M. Dubois-Raymond,
the wire is about 3¼ miles in length. The apparatus being thus arranged,
the experimenter grasps the cylinder of wood firmly with both hands, at
the same time dipping the fore-finger of each hand in the saline water.
The needle of the galvanometer remains undisturbed; the electric
currents passing by the nerves of each arm, and being of the same force,
neutralize each other. Now, if the experimenter grasp with energy the
cylinder of wood with the right hand, the left hand remaining relaxed
and free, immediately the needle will move from west to south, and
describe an angle of 30°, 40°, and even 50°; on relaxing the grasp, the
needle will return to its original position. The experiment may be
reversed by employing the left arm, and leaving the right arm free: the
needle will, in this case, be deflected from west to north. The
reversing of the action of the needle proves the influence of the
nervous force. The conditions, it may be added, essential to the success
of the experiment are: 1st, Great muscular and nervous energy; 2nd, The
contraction of only one arm at a time; 3rd, Dryness and cleanliness of
skin; and 4th, Freedom from any kind of wound on the immersed part.


_Sixth Experiment._

In making electrical experiments of the simplest kind, it soon becomes
apparent that certain substances, such as glass, sealing-wax, &c.,
retain the condition of electrical excitement; whilst other bodies, and
especially the metals, seem wholly incapable of electrical excitation:
hence the classification of bodies into conductors and non-conductors of
electricity. This arrangement is not strictly correct, because no
substance can be regarded as absolutely a conductor, or _vice versâ_. It
is better to consider these terms as meaning the two extremes of a long
chain of intermediate links, which pass by insensible gradations the one
into the other. In the manufacture of electrical apparatus, glass is of
course largely employed, and this substance, with brass and wood,
constitute the usual materials. One of the most instructive pieces of
apparatus is the electroscope, which can be made with a gas jar, a cork,
a piece of glass tube, brass wire and ball, or a flat disc of brass,
with some Dutch metal, or still better, gold leaf. The latter is first
cut into strips by retaining the leaf between a sheet of well-glazed
paper and cutting through the paper and the copper or gold leaf,
otherwise it would be impossible to cut the metal, on account of its
excessive thinness, except with a gilder's knife and cushion. The cork
is next fitted to the gas jar, and perforated with a hole to admit the
glass tube, which must be thoroughly dry, and [Page 179] is best coated
both inside and out with the shell-lac varnish described at page 175.
Some dry silk is wound round the brass wire, so that it remains fixed
and upright in the glass tube, the end outside the jar having a ball, or
still better, a flat disc of brass attached, and the other extremity
being split so as to act like a pair of forceps, to retain a piece of
card to which the gold leaves are attached. By removing the cork, tube,
and brass wire bodily from the neck of the gas jar, and then in a
perfectly still atmosphere carefully bringing the card, slightly wetted
with gum at the extremity, on two of the cut gold leaves, they may be
stuck on, and the whole is again arranged inside the dry gas jar, and
forms the important instrument called the electroscope. (Fig. 164.) With
the help of this arrangement, a number of highly instructive experiments
are performed.

[Illustration: Fig. 164. A. The brass wire, with flat disc outside, and
forceps holding gold leaf B inside the jar. C C. The glass tube.]


_Seventh Experiment._

First, the difference between conductors and non-conductors is admirably
shown by rubbing a bit of sealing-wax against a piece of woollen cloth
or flannel; on bringing the wax to the brass disc of the electroscope
the gold leaves no longer hang quietly side by side, but stand out and
repel each other, in obedience to the law "_that bodies similarly
electrified repel each other_." If the brass cap is touched whilst the
leaves are in this electrical state, they fall again to their original
position, showing that sealing-wax, after being excited, retains its
electrical condition, as also the gold leaves, because they are
supported on glass, or what is termed _insulated_--_i.e._, cut off from
conducting communication with surrounding objects. When, however, the
sealing-wax is passed through a damp hand, or the brass disc of the
electroscope touched, the electricity is conveyed away to the earth,
because the human body is a conductor of electricity.


_Eighth Experiment._

When a brass wire is rubbed and brought to the electroscope, the leaves
do not move, in consequence of the electricity passing away to the earth
through the body as fast as it is generated: it is just like pouring
water into a leaky cistern; but if the brass wire is tied to a long
stick of sealing-wax, and this latter held in the hand whilst the wire
is rubbed with a bit of flannel, then the gold leaves of the
electroscope are affected, on account of the insulation of the metal, as
every substance which can be rubbed (even fluids, as water) produces
electricity.

[Page 180]


_Ninth Experiment._

An insulating stool is merely a piece of strong square board, supported
on glass legs, which should be well varnished. If the assistant stands
on this stool and touches the disc of the electroscope, no movement of
the leaves takes place until his coat is briskly struck with a piece of
dry silk or skin, when the usual repulsion occurs.

[Illustration: Fig. 165. Assistant standing on the insulating stool and
touching the disc of the electroscope whilst being struck with a dry
handkerchief.]


_Tenth Experiment._

If a little powdered chalk is placed inside a pair of bellows, and then
forcibly ejected on to the disc of the electroscope, the friction of the
particles of chalk against the inside of the nozzle of the bellows and
against the disc of the instrument soon liberates sufficient electricity
to cause the gold leaves to stand out and repel each other.


_Eleventh Experiment._

Whilst the leaves of the electroscope are repelled from each other by
the application of a bit of rubbed sealing-wax, they may be again caused
to approach each other on bringing a dry glass tube previously rubbed
with a silk-handkerchief; because the electricity obtained from
sealing-wax is different from that procured from glass: the former is
called _resinous_ or _negative_ electricity, the latter _positive_ or
_vitreous_ electricity. Either, separately, is _repulsive_ of its own
particles, but _attractive_ of the other. [Page 181] No electrical
excitation can occur without the separation of these two curious states
of electricity, and electrical quiescence takes place when the two
electricities are brought together; hence the fall of the gold leaves
repelled by rubbed wax when the excited glass is brought towards the
disc of the electroscope. This experiment may be reversed by repelling
the leaves first with the excited glass, and then bringing the rubbed
wax, when the same effect takes place.


_Twelfth Experiment._

To show the important elementary truth, that in all cases of electrical
excitation the two kinds of electricity are generated, take a dry roll
of flannel, and holding it as lightly as possible, rub it against a bit
of wax. If the flannel is brought to the electroscope, the leaves repel
each other, and they immediately fall when the wax is now approached,
because the flannel is in the positive or vitreous state of electricity,
whilst the sealing-wax is in the negative or resinous condition.


_Thirteenth Experiment._

Any kind of friction generates electricity. A little roll of brimstone
placed in a dry mortar and powdered, and then thrown on to the
electroscope, quickly causes the repulsion of the leaves.


_Fourteenth Experiment._

A sheet of dry brown paper laid on a flat surface, and vigorously rubbed
with a piece of india-rubber, produces so much electricity that sparks
and flashes of light are apparent in a dark room when it is lifted from
the table; and it affects the leaves of the electroscope very
powerfully, so much so that care must be taken to apply it very
carefully to the disc, or the violence of the repulsion may cause the
fracture of the gold leaves, and then a great deal of time is wasted
before they can be put on again.


_Fifteenth Experiment._

A dry wig or bunch of horse-hair when combed becomes electrical, and
likewise affects the leaves of the electroscope.


_Sixteenth Experiment._

Two dry silk ribbons, the one white and the other black, passed rapidly
together through the fingers, exhibit sparks and flashes of light when
drawn asunder, and also cause the gold leaves to repel each other.


_Seventeenth Experiment._

Much instructive amusement is afforded by testing the gold leaves when
separated from each other during either of the former experiments,
[Page 182] with an excited piece of sealing-wax. If the electricity
produced is negative, they repel each other further when the excited wax
is approached; if positive, they fall when the excited wax is brought
near them.


_Eighteenth Experiment._

When fresh, dry, ground coffee is received on to the disc of the
electroscope, as it falls from the mill, powerful electrical excitation
is displayed, and this is sometimes so apparent, that the particles
cling around the lower part of the mill or to the sides of the cup or
basin held to catch it.


_Nineteenth Experiment._

After playing a tune on a violin, hold the bow (well rosined) to the
electroscope, when the usual divergence of the leaves will be apparent.


_Twentieth Experiment._

Cut some chips from a piece of wood with a knife attached to a glass
handle, and as they fall on to the electroscope the leaves are repelled.


_Twenty-first Experiment._

Warm a piece of bombazine by the fire and then draw out some of the
threads (which are of two kinds--viz., silk and wool), and place them on
the electroscope, when divergence of the leaves immediately takes place.


_Twenty-second Experiment._

Put upon the same leg a worsted stocking and over that a silk one, if
the latter is now quickly rubbed all over with a dry hand and near the
fire, and then suddenly slipped off, the sides repel each other, and the
silk stocking retains very much the same shape as if the leg still
remained in it, and of course collapses as the electricity passes away.


_Twenty-third Experiment._

Electrical machines consist only in the better arrangement of larger
pieces of glass and a more convenient mechanical contrivance for rubbing
them, and are of two kinds--viz., the cylinder and plate machines; it is
usual to give directions for the manufacture of an electrical machine
from a common bottle, and doubtless such rude instruments have been
made, but as Messrs. Elliott Brothers, of 30, Strand, now supply
excellent small machines at a very low cost, it is hardly worth while to
incur even a small expense for an instrument that must at the best be a
very imperfect one and frequently out of order. (Fig. 166.)

[Page 183]

[Illustration: Fig. 166. A cylinder electrical machine.]

[Illustration: Fig. 167. The ordinary plate electrical machine.]

Plate machines are somewhat more expensive than cylinder ones, but at
the same time are more quickly prepared for experiments, and Mr.
Hearder, of Plymouth, states, that the secret in obtaining the greatest
amount of electricity from a cylinder machine, is to keep the inside of
the glass absolutely clean, dry, and free from dust. Sometimes the glass
of which electrical machines are made is wholly unfit for electrical
[Page 184] purposes, in consequence of the decomposition of the surface
from imperfect manufacture and the liberation of the alkali. (Figs. 167,
168.)

[Illustration: Fig. 168. Woodward's double plate electrical machine,
giving a much larger quantity of electricity than Fig. 167.]


_Twenty-fourth Experiment._

Cylinder and plate machines are furnished with proper rubbers, and
before using the instrument it is usual to remove them, and after
carefully cleaning the glass with a dry silk handkerchief before a fire,
the rubbers are scraped with a paper-knife to remove the old amalgam,
and fresh applied by first melting the end of a tallow candle slightly,
and after passing this over the rubber, the finely powdered amalgam is
now dusted on to it. Electrical amalgam is prepared by fusing one part
of zinc with one of tin, and then agitating the liquid mass with two
parts of hot mercury placed in a wooden box; when cold it should be
carefully powdered and kept in a well-stoppered bottle for use. When the
amalgam has been applied, the rubbers are again screwed in their places,
and the machine when turned (if the atmosphere is tolerably dry) will
emit an abundance of bright sparks.


_Twenty-fifth Experiment._

Attraction and repulsion are shown on a larger scale, with the
assistance of electrical machines, by placing a fishing rod (the last
joint of [Page 185] which is made of glass) in an erect position, and
attaching to the extremity a long tassel of paper from which a thin wire
passes to the prime conductor of the electrical machine; on turning the
instrument, the strips of paper all stand out and repel each other.
(Fig. 169.)

[Illustration: Fig. 169. A A. The glass joint of the fishing-rod, from
which the last joint, carrying the paper tassel, B, projects. C. The
electrical machine.]

[Illustration: Fig. 170. A. Prime conductor. B. Upper brass-plate. C.
Lower ditto. The figures are seen between B and C.]


_Twenty-sixth Experiment._

Suspend from the prime conductor by a chain a circular brass plate and
under this place another supported by a brass adjusting stand. If pith
figures of men and women are placed on the lower plate, they rise
directly the machine is turned, although sometimes, in consequence of
irregularity in the adjustment of the centre of gravity, they perversely
dance on their heads instead of the usual position; out of half a dozen
figures, one only perhaps will be found to dance well, by alternately
jumping to the upper plate and falling to the lower one to discharge the
excess of electricity; and indeed the experiment will be found to
succeed better with one or two only on the plate instead of a number, as
they cling together and impede each other's movements. (Fig. 170.)

[Page 186]


_Twenty-seventh Experiment._

An assistant provided with a wig of well-combed hair presents a most
ridiculous appearance when standing on the insulating stool and
connected by a wire with the prime conductor of the electrical machine,
every hair, when not matted together, standing out in the most absurd
manner, when the machine is put in motion.


_Twenty-eighth Experiment._

Whilst standing on the stool, sparks may be obtained from his body, and
if some tow is tied over a brass ball, and moistened with a little
ether, and presented to the tip of his finger, a spark flies off which
quickly sets fire to the inflammable liquid.

[Illustration: Fig. 171. A A A. A ring of brass wire supported on a
glass pillar inside which the spiral tube, B, revolves, and produces
beautiful and ever-changing circles of light, when connected with the
conductor, C, of the electrical machine.]


_Twenty-ninth Experiment._

If small discs of tinfoil, cut out with a proper stamp, are pasted in
continuous lines over plate glass, or spirally round glass tubes, a
very [Page 187] pretty effect is produced when they receive the sparks
from the electrical machine, and the passage of the electricity from one
disc to the other produces a vivid spiral or other line of light. When
the tube is mounted in a proper apparatus, so as to revolve whilst the
sparks pass down the spiral tube, the effect of the continuous electric
sparks is much heightened. (Fig. 171.)


_Thirtieth Experiment._

A great variety of experiments, depending on the proper arrangement of
discs of tinfoil on various tubes of coloured glass are manufactured,
and some in the form of windmills, the sails being made luminous by the
passage of the electricity. The names of illustrious electricians,
beautiful crescents, stars, and even profile portraits, have been
produced in continuous streams of electric sparks.


_Thirty-first Experiment._

When an electrified body is brought towards another which is not
electrical, the latter is thrown into the opposite state of electricity
as long as the excited body remains in its neighbourhood; and this
condition of electrical disturbance, set up without any contact or
supply of electricity, is called _induction_, and involves a vast number
of interesting facts, which are thoroughly discussed in Dr. Noad's
excellent work on electricity, but can only be briefly alluded to here.

[Illustration: Fig. 172. The lengths of brass wire supported on glass
rod pillars indented by blowpipe, so as to retain the brass wires with
the pith balls hanging from each series, the letters P and N mean
Positive and Negative, and the signs for these terms are placed above.
The letters P and N are painted on the blocks which support the glass
rods.]

If a number of lengths of brass wire, supplied with balls at the
extremities, are supported on glass legs and arranged in a line, with a
little pith ball attached to a thread hanging from each end of the
length of brass wire, the effect of induction is shown very nicely; and
when an excited glass rod is brought towards one end of the series, the
rising of the pith balls to each other betrays the change which has
occurred in the electrical state of the brass wires by the mere
neighbourhood of the excited glass tube. The glass tube is electrified
positively, and attracts the negative electricity from the brass wire
towards the end nearest to it; [Page 188] the other extremity of the
brass wire is found to be in the positive state, and this re-acting on
the next, and so on throughout the lengths, completes the electrical
disturbance in the whole series. (Fig. 172.)


_Thirty-second Experiment._

[Illustration: Fig. 173. A A. Large circular tin or brass disc with
turned-up edge half an inch deep, and containing the resinous mixture B,
which is rubbed with the warm flannel. C C. The upper plate supported by
the glass handle D, a pith ball attached to a wire shows the electrical
excitation, and the spark is supposed to be passing to the hand E.]

If an insulated brass rod (such as has been described in the last
experiment) is touched by the finger whilst under induction, it remains
permanently electrified on the removal of the disturbing electrified
body; and it is on this principle that the useful electrical machine
called the Electrophorus is constructed. This _constant_ electrical
machine--for it will remain in action during weeks and months if kept
sufficiently dry--was invented by Volta in the year 1774, and has been
brought to great perfection by Mr. Lewis M. Stuart, of the City of
London School; so that with a little additional apparatus the whole of
the fundamental principles of electricity can be demonstrated. It
consists of a flat brass or tin circular dish about two feet in diameter
and half an inch deep, which is filled with a composition of equal parts
of black rosin, shell-lac, and Venice turpentine; the rosin and the
Venice turpentine being first melted together, and the shell-lac added
afterwards, care of course being taken that the materials do not boil
over and catch fire, in which case the pot must be removed from the
heat, and a piece of wet baize or other woollen material thrown over it.
Another tin or brass circular plate of twelve inches diameter, and
supported in the centre with a varnished glass handle nine inches long,
is also provided, and the resinous plate being first excited by several
smart blows with a warm roll of flannel, the plate held by the glass
handle is now laid upon the centre of the resinous one, and if removed
directly afterwards, does not afford the electric spark; but if, whilst
standing upon the excited resinous plate, it is touched, and then
removed by the glass handle, a powerful electric spark is obtained; and
this may be repeated over and over again with the like results, provided
the plate with the glass handle is touched with the finger just before
lifting it from the resinous plate. (Fig. 173.) [Page 189] The
electricity excited on the resinous plate is not lost, and by induction
sets up the opposite condition in the plate with the glass handle. The
resinous plate, being excited with negative electricity, disturbs the
electrical quiescence of the upper plate, and positive electricity is
found on the surface touching the resinous plate, and negative
electricity on the upper one, so that when it is removed without being
touched, the two electricities come together again, and no spark is
obtained; but if, as already described, the upper plate is touched
whilst under induction, then positive electricity appears to pass from
the finger to the negative electricity on the upper side of the plate,
when the two temporarily neutralize each other, and then, when the plate
is removed, the excess of electricity derived from the earth through the
finger becomes apparent. Induction requires no sensible thickness in the
conductors, and can be just as well produced on a leaf of gold as on the
thickest plate of metal; and it should be remembered that non-conductors
do not retain their state of electrical excitation when the disturbing
cause is removed, whereas conductors possess this power, and this fact
brings us to the consideration of the Leyden jar.


_Thirty-third Experiment._

If one side of a dry glass plate is held before and touches a brass ball
proceeding from the prime conductor of an electrical machine whilst in
action, the other side is soon found to be electrical; this does not
arise from the conduction of the electricity through the particles of
the glass, but is produced by induction, the side nearest the ball being
in the positive state, and the other side negative: as glass is a
non-conductor of electricity, the effect is much increased by coating
each side with tinfoil, leaving a margin of about two inches of
uncovered glass round the covered portion, then, if one side of such a
plate is held to the prime conductor of the electrical machine, and the
other connected with the ground, a powerful charge is accumulated; and
if the opposite sides are brought in contact with a bent brass wire, a
loud snapping noise is heard, and the two electricities resident on
either side of the glass come together with the production of a
brilliant spark, or if the hands are substituted for the bent brass
wire, that most disagreeable result is obtained--viz., an _electric
shock_; hence these glass plates are sometimes fitted up as pictures,
and when charged and handed to the unsuspecting recipient, he or she
receives the electric discharge to the great discomfort of their nervous
system.

Mica is sometimes substituted for glass, and the late Mr. Crosse, the
celebrated electrician, constructed a powerful combination of coated
plates of this mineral. It consisted of seventeen plates of thin mica,
each five inches by four, coated on both sides with tinfoil within half
an inch of the edge. They were arranged in a box with a glass plate
between each mica plate, all the upper sides were connected by strips of
tinfoil to one side of the box, and all the under surfaces in the same
manner with the opposite extremity of the box. They were charged like an
ordinary Leyden battery.

[Page 190]


_Thirty-fourth Experiment._

If the glass plate coated with tinfoil is charged, and then placed
upright on a stand, it may be slowly discharged by placing a bent wire
on the edge with the extremities covered with pith balls. The wire
balances itself, and continues to oscillate with noise until the
electricities of the two surfaces neutralize each other. (Fig. 174.)

[Illustration: Fig. 174. A A. Glass plate or stand coated with tinfoil
on each side, B. C. Wire with pith balls oscillating during the
discharge of the glass plate.]


_Thirty-fifth Experiment._

It is easy to imagine the glass plate of the last experiment rolled up
into the more convenient form of the Leyden jar, which consists of a
glass vessel lined both inside and out with tinfoil, leaving some two or
three inches of the glass round the mouth uncovered and varnished with
shell-lac; a piece of dry wood is fitted into the mouth of the jar,
through which a brass wire and chain are passed, and the end outside is
fitted with a ball. The Leyden jar is charged by holding the ball to the
prime conductor of the electrical machine until a sort of whizzing noise
is heard, caused by the excess of electricity passing round the
uncovered part of the jar and not through it, as the smallest crack in
the glass of the Leyden jar would render it useless. Electricity is
sometimes called a fluid, and the fact of collecting it like water in a
jar, helps us to understand this analogy. The noise, the bright spark,
or the shock are obtained by grasping the outside with one hand and
touching the ball with a brass wire held in the other. (Fig. 175.)

[Illustration: Fig. 175. The Leyden jar and brass wire discharger.]


_Thirty-sixth Experiment._

The jar is silently discharged if the balls are removed from the
discharger and points used instead; so, also, the whole of the
electricity produced by an electrical machine in full action may be
readily drawn off by a pointed conductor, such as a needle, placed at
the end of a brass wire. Electricity passes much more rapidly through
points than rounded surfaces, hence the reason why all parts of
electrical apparatus are free from sharp points and rough asperities.

[Page 191]

_Thirty-seventh Experiment._

Extremely thin wires may be burnt by passing the charge of a large
Leyden jar through them. The show jars, called specie jars, usually
decorated and placed in the windows of chemists' shops, make excellent
Leyden jars, when not too thick; and with two of the largest, all the
interesting effects produced by accumulated electricity may be
displayed. To pass the discharge through wires, nothing more is required
than to strain them across a dry mahogany board, between two brass wires
and balls, and if a sheet of white paper is placed under them, most
curious markings are produced by the fine particles of the deflagrated
metal blown into the surface of the paper. An arrangement of two or more
Leyden jars is usually called a Leyden Battery, just as a single cannon
is spoken of as a gun, whilst two or more constitute a battery. (Fig.
176.)

[Illustration: Fig. 176. A. Mahogany board with a sheet of white paper
and three pairs of brass wires and balls fixed in the wire, three on
each side. The thin wires are stretched between the balls, and the lower
one is in course of deflagration. B B. Charged large Leyden battery of
two jars; the arrows indicate the path of the electricity.]


_Thirty-eighth Experiment._

Little models of houses, masts of ships, trees, and towers are sold by
the instrument makers, and by placing a long balanced wire on the top of
the pointed wire of a large Leyden jar, having one end furnished with
wool to represent a cloud, a most excellent imitation of the effects of
a charged thunder-cloud is produced. The mechanical effect of a flash of
lightning has been analysed, and it has been stated, in one instance,
that the power developed through fifty feet was equal to a 12,220
horse-power engine, or about the power of the engines of the _Great
Eastern_, and that the explosive power was equal to a pressure of three
hundred millions of tons. (Fig. 177.)

[Page 192]

[Illustration: Fig. 177. A. Charged Leyden jar with balanced wire and
wool at B, representing a thunder-cloud. C. The obelisk overturned with
the discharge. D. Another model of the gable end of a house; the square
pieces of wood fly out when the continuity of the conductor is broken.]

It was the learned but humble minded Dr. Franklin who established the
identity between the mimic effects of the electrical machines (such as
have been described), and the awe-inspiring thunder and lightning of
nature. A copper rod, half an inch thick, pointed and gilt at the
extremity, and carried to the highest point of a building, will protect
a circle with a radius of twice its length. The bottom of the rod must
be passed into the earth till it touches a damp stratum.

[Illustration: Fig. 178. A storm.]


[Page 193]




CHAPTER XIV.

VOLTAIC ELECTRICITY.


In describing the various means by which electricity may be obtained, it
was stated that "Chemical Action" was a most important source of this
remarkable agent; at the same time it must be understood that it is not
every kind of chemical action which is adapted for the purpose; there
are certain principles to be rigidly adhered to--first, in the
generation of the force; and secondly, in carrying it by wires so as to
be applicable either for telegraphic purposes, or for the highly
valuable processes of electrotyping and electro-silvering, plating, and
gilding.

A lighted candle, or an intense combustion of coal, coke, or charcoal,
no doubt involves the production of electricity, but there are no means
at present known by which it may be collected and conducted; when that
problem is solved, the cheapest voltaic battery will have been
constructed, in which the element decomposed is charcoal, and not a
metal, such as iron or zinc. The first and most simple experiment that
can be adduced in proof of electrical excitation by chemical means, is
to take a bit of clean zinc and a clean half-crown, and placing one on
the tongue and the other below it, as long as they remain separate no
effect is observed, but directly they are made to touch each other,
whilst in that position, a peculiar thrill is rendered evident by the
nerves of the tongue, which in this case answers the same purpose as the
electroscope already described, and in a short time a peculiar metallic
taste is perceptible.

It has been stated over and over again that it was to a somewhat similar
circumstance we owe the discovery of voltaic electricity, and the story
of the skinned frogs agitated and convulsed by an accidental
communication with two different metals, or, as some say, with the
electricity from an ordinary machine, has been repeated in nearly every
work on the science. Professor Silliman, however, asserts that the
galvanic story is doubtful, and is a fabrication of Alibert, an Italian
writer of no repute, and that greater merit is due to Galvani than that
of being merely the accidental discoverer of this kind of electricity,
because he had been engaged for _eleven_ years in electro-physiological
experiments, using frogs' legs as electroscopes. It was whilst
experimenting on animal irritability, Galvani noticed the important fact
that when the nerve of a dead frog, recently killed, was touched with a
steel needle, and the muscle with a silver one, no convulsions of the
limb were produced until the two different metals were brought in
contact, and he explained the cause of these singular after-death
contortions by supposing that the nerves and muscles of all animals were
in opposite states of electricity, and that these nervous contractions
were caused by the annihilation, for the time, of this condition, by the
interposition of a good conductor between them.

This theory of Galvani had several opponents, one of whom, the [Page
194] celebrated Volta, succeeded in pointing out its fallacy; he
maintained that the electrical excitement was due entirely to the
metals, and that the muscular contractions were caused by the
electricity thus developed passing along the nerves and muscles of the
dead animal.

To Volta we are indebted for the first voltaic battery, and the
distinguished philosopher may truly be said to have laid the foundation
of this now _commercially_ valuable branch of science.


_First Experiment._

If a plate of clean bright zinc is placed in a vessel containing some
dilute sulphuric acid, energetic action occurs from the oxidation of the
metal, and its union as an oxide with the acid, and the escape of a
multitude of bubbles of hydrogen gas. After the action has proceeded
some time, the zinc may be removed, and if a little quicksilver is now
rubbed over the surface with a woollen rag tied on the end of a stick,
it unites with the metal, and the surface of the zinc assumes a
brilliant silvery appearance, and is said to be amalgamated. In that
condition it is no longer acted upon by dilute sulphuric acid, and for
the sake of economy this is the only form in which zinc should be
employed in the construction of voltaic batteries or single circles. If
a clean plate of copper, with a wire attached, is now placed in the
dilute acid opposite to and not touching the amalgamated zinc plate,
which may also be furnished with a conducting wire, no bubbles of
hydrogen escape until the wires from the two metals are brought in
contact, and then, singular to relate, the hydrogen escapes from the
copper plate, whilst the oxygen is rapidly absorbed by the zinc, and a
current of electricity will now be found to pass from the zinc through
the fluid to the copper, and back again through the wire to the starting
point, and if the wires are disconnected, the chemical action ceases,
and no more electricity is produced. (Fig. 179.)

[Illustration: Fig. 179. A single voltaic circle, consisting of a zinc
and copper plate (marked Z and C) in dilute acid. The arrows show the
direction of the current.]

The passage of the current of electricity is not discoverable by the
electroscope, because it is adapted only to indicate electricity of high
tension or intensity, such as that produced from the electrical machine,
which will pass rapidly through a certain thickness of air, and cause
pith balls to stand out and repel each other; such effects are not
producible by a single voltaic circle, or even an ordinary voltaic
battery, although one comprising some hundreds of alternations would
produce [Page 195] an effect on a delicate electrometer; hence voltaic
electricity is said to be of low intensity, and this property makes it
much more useful to mankind, because it has no desire to leave a
metallic path prepared for it, and does not seize the first opportunity,
like the electricity from the electrical machine, to run away to the
earth through the best and shortest conductor offered for it. If
electricity had only been producible by friction, we should never have
heard of electrotyping, and the other useful applications of electrical
force of low intensity.


_Second Experiment._

To ascertain the passage of a current of voltaic electricity, the
instrument called the galvanometer needle is provided, which consists of
a coil of copper wire surrounding a magnetic needle, so as to leave the
latter freedom of motion from right to left, or _vice versâ_. When this
coil is made part of the voltaic circle it becomes magnetic, and
reacting on the magnetized needle, deflects it to one side or the other,
according to the direction of the current. (Fig. 180.)

[Illustration: Fig. 180. A galvanometer needle, consisting of a coil of
covered copper wire, the ends of which terminate at the binding screws.
The magnetic needle is suspended on a point in the centre, and the coil
is surrounded with a graduated circle.]


_Third Experiment._

If a number of simple voltaic circles, such as the one described in the
first experiment, are connected together, they form a voltaic battery,
in which of course the quantity of electricity is greatly increased.
Batteries of all kinds, from the original Volta's pile, consisting of
round zinc and copper plates soldered together with interposed cloth
moistened with dilute sulphuric acid, or his _couronne des tasses_,
consisting of zinc and silver wires soldered together in pairs, and
placed in glass cups containing dilute acid, to the improved batteries
of Cruikshank, Wilkinson, Babington, Wollaston, and the still more
perfect arrangements of Daniell, Mullins, Shillibeer, and Grove, have
been from time to time recommended for their own peculiar features.

Amongst these several inventions, none will be found more useful than
the _constant_ battery of Daniell for electrotyping, silvering, gilding,
and other purposes, and Grove's battery for all the more brilliant
results, such as the deflagration of the metals or the production of the
electric light. The construction of the Daniell and Grove batteries will
therefore be described. The former consists of a cylindrical vessel made
of copper, in which is suspended or placed (as it is open at the top) a
membranous, brown-paper, canvas, or porous earthenware tube, containing
an amalgamated rod of zinc. To charge this arrangement, a strong
solution of sulphate of copper, with some sulphuric acid, is poured into
the copper vessel, which is provided usually with a sort of [Page 196]
colander at the top to hold crystals of sulphate of copper, and in the
porous tube containing the zinc rod is poured dilute sulphuric acid. A
number of these cylinders of copper, twenty inches high and three inches
and a half in diameter, arranged in wooden frames to the number of
twenty, afford a quantity of electricity sufficient to demonstrate all
the usual phenomena. (Fig. 181.)

[Illustration: Fig. 181. A A. Copper cylindrical vessel with colander to
hold the crystals of sulphate of copper. B. The amalgamated zinc rod
inside the porous cell C C. D. A series of single cells forming a
Daniell's battery.]

Professor Grove's battery consists of a flat glazed earthenware vessel
containing a flat porous cell. An amalgamated zinc plate is placed
outside the porous cell, and a platinum plate inside the latter. The
arrangement is put in action by pouring dilute sulphuric acid round the
zinc and strong nitric acid inside the porous cell. A set of Grove's
nitric acid battery, as manufactured by Messrs. Elliott, Brothers, of
30, Strand, with fifty pairs of sheet platinum, five inches by two
inches and a quarter, and double amalgamated zinc plates, flat porous
cells, and separate earthenware troughs for each pair, and stout
mahogany stand, arranged in ten series of five pairs, will evolve with a
proper voltameter one hundred cubic inches of the mixed gases per minute
from the decomposition of water, and will exhibit a most brilliant
electric light, when arranged as a single series of fifty pairs of
plates. Even thirty pairs exhibit the most splendid effects, whilst
forty may be regarded as the happy medium, giving all the results that
can be desired. (Fig. 182.)

The advantage of employing amalgamated zinc is very prominently
illustrated whilst using any powerful arrangements of either Daniell's
or Grove's batteries, as they will remain for hours quiescent, like a
giant asleep, until the terminal wires of the series are brought in
contact [Page 197] either through the intervention of some fluid under
decomposition or by means of charcoal points. The author had the
pleasure of witnessing at King's College some of the effects of an
enormous battery, prepared by the late Professor Daniell, and consisting
of seventy of his cells.

[Illustration: Fig. 182. A A. Amalgamated zinc plate in flat earthenware
trough. Attached to a binding screw is the platinum plate in porous
cell, C C. D. A series of single cells forming a Grove's battery.]

A continuous arch of flame was produced between two charcoal points,
when distant from each other three quarters of an inch, and the light
and heat were so intense that the professor's face became scorched and
inflamed, as if it had been exposed to a summer heat. The rays collected
by a lens quickly fired paper held in the focus.[C]

[Footnote C: By the light from the same battery photogenic drawings were
taken, and the heating power was so great as to fuse with the utmost
readiness a bar of platinum one-eighth of an inch square; and all the
more infusible metals, such as rhodium, iridium, titanium, &c., were
melted like wax when placed in small cavities in hard graphite and
exposed to the current of electricity.]


_Fourth Experiment._

It is by "chemical action" the electricity is produced, and as action
and reaction are always equal, but contrary, we are not surprised to
find that the electricity from the voltaic battery will in its turn
decompose chemically many compound bodies, of which water is one of the
most interesting examples. It was in the year 1800, and immediately
after Volta's announcement to Sir Joseph Banks of his discovery of the
pile, that Messrs. Nicholson and Carlisle constructed the first pile in
England, consisting of thirty-six half-crowns, with as many discs of
zinc and pasteboard soaked in salt water. These gentlemen, whilst
experimenting with the pile, observed that bubbles of gas escaped from
the platinum wires immersed in water and connected with the extremities
of the Volta's pile, and covering the wires with a glass tube full of
water, on the 2nd of May, 1800, they completed the splendid discovery of
the fact that the Volta's current had the power to decompose water and
other chemical compounds.

[Page 198]

In 1801, Davy had succeeded to a vacant post in the Royal Institution,
and on Oct. 6th, 1807, made his transcendent discovery of potassium with
the aid of the voltaic battery, and from that and other experiments
inferred that the whole crust of the globe was composed of the oxides of
metals. To exhibit the decomposition of water, two platinum plates with
proper connecting wires, passing to small metallic cups full of mercury,
are cemented inside a glass vessel, which is then filled with dilute
sulphuric acid. Just above the platinum plates and over them, stand two
glass tubes also containing the same fluid in contact with the battery.
Two measures of hydrogen are found in one tube, and one of oxygen in the
other. (Fig. 183.)

[Illustration: Fig. 183. A A. A finger glass with two holes drilled to
pass the wires through, which are imbedded in cement up to the platinum
plates. B B. Glass tubes, closed at one end and open at the other, which
are placed over the platinum plates to receive the liberated oxygen and
hydrogen. The scale at the side shows the respective volumes of two of H
to one of O.]

To measure the quantity power of the voltaic battery, an important
instrument invented by Faraday is used. It consists of separate platinum
plates cemented in a wooden stand, and over which a capped air-jar with
a bent pipe is also cemented. This apparatus contains dilute sulphuric
acid of the same strength as that used in the battery under examination,
and by taking the time, the quantity of the mixed oxygen and hydrogen
gases producible by a battery per minute is accurately determined, the
gases of course being collected in a graduated jar. (Fig. 184.)

[Illustration: Fig. 184. A. Gas jar with cap and bent tube passing to
the graduated tube C; the jar is cemented in the same stand which
carries the connecting cups, wires, and platinum plates, which are bent
round each other to improve the action of the voltameter.]

[Page 199]


_Fifth Experiment._

By grouping the simple circles forming a voltaic battery in various
numerical relations, the _quantity_ and _intensity_ effects are
modified.

Thus, if a series of thirty pairs of Grove's battery are all connected
together in consecutive order, the smallest _quantity_ and the largest
_intensity_ effect is produced.

If changed to two groups of fifteen each, the quantity is doubled--that
is to say, it will produce double the quantity of the mixed gases from
the voltameter with half the intensity.

If arranged in three groups of ten each, it is trebled with a
proportional loss of intensity, until the grouping reaches six series of
five each, when a maximum supply of the mixed gases is obtained from the
voltameter.

In arranging the groups, all the zinc ends of each series are connected,
and all the platinum ends are likewise joined by proper wires.


_Sixth Experiment._

A plate-glass trough, containing a few grains of iodide of potassium
dissolved in water with some starch, is quickly decomposed into its
elements by placing in two platinum plates and connecting them with the
wires of the voltaic battery. If the glass trough is divided in the
centre with a bit of cardboard, the purple colour of the iodine and
starch is shown very beautifully on one side, but not on the other, as
iodine is liberated at one pole and the alkali at the other. (Fig. 185.)

[Illustration: Fig. 185. A A. A glass trough containing the salt
dissolved in water, and divided temporarily with a bit of cardboard, B.
C C are the two platinum plates connected with the battery, and the
shaded side is supposed to represent the liberation of the iodine.]


_Seventh Experiment._

Some solution of common salt coloured with sulphate of indigo and placed
in the trough is decomposed into chlorine, which bleaches one side of
the indigo solution, and the alkali liberated on the other does not
affect it.


_Eighth Experiment._

Some nitrate of potash dissolved in water and coloured with litmus
placed in the glass trough, changes red on one side of the cardboard by
the liberation of acid, and is not affected on the other.

In these experiments the oxygen, iodine, chlorine, and nitric acid are
liberated at the electro-positive pole, and are hence termed
electro-negative bodies, whilst hydrogen and the alkalies are set free
at the electro-negative pole, and are therefore called electro-positive
bodies. [Page 200] Faraday has modified these terms, and calls the two
classes "_anions_" and "_cathions_," and the two poles "anodes" and
"cathodes."

Anode, from [Greek: _ana_], up, and [Greek: _hodos_], a way: the way which
the sun rises. Anions, from [Greek: _ana_], up, and [Greek: _eimi_], to go:
that which goes up; a substance which passes to the anode during the
passage of a current of electricity. Cathode, from [Greek: _kata_], down,
and [Greek: _hodos_], a way: the way which the sun sets. Cathion, from
[Greek: _kata_], down, and [Greek: _eimi_], to go: that which goes down; a
substance which passes to the cathode during the passage of electricity
from the anode to the cathode.


_Ninth Experiment._

In the process of the electrotype is presented a valuable application of
the chemical power of the voltaic circle or battery, and it may be
conducted either as a single cell operation or by distinct batteries. In
the former case the most simple arrangement will suffice; the only
articles necessary are--a large mug or tumbler; some brown paper and a
ruler; a bit of amalgamated zinc, four inches long and half an inch
wide; a short length of copper wire; some black lead, blue vitriol, and
oil of vitriol.

The mould from which the electrotype is to be taken can be made of
common sealing wax, plaster of Paris, white wax, gutta percha, or
fusible alloy. Supposing the first to be selected--viz., a common seal,
it is first thoroughly black-leaded,[D] then one end of the copper wire
is bent round the top of the amalgamated zinc, and the other is gently
warmed and melted into the side of the seal, leaving a small portion
uncovered by the wax, which is then well black-leaded. A few ounces of
blue vitriol are dissolved in boiling water, and when cold the solution
is poured into the tumbler, and the porous cell to contain the mixture
of eight parts water to one of sulphuric acid is made by rolling the
brown paper three or four times round the ruler and closing the end, and
fixing the side with a little sealing wax. The porous cell of brown
paper is now filled with the dilute acid, and placed in the tumbler
containing the solution of blue vitriol, the amalgamated zinc being
arranged in the paper cell, and the attached seal in the copper
solution; in about twelve hours a good deposit of copper is produced,
and a perfect cast in metal of the seal obtained. (Fig. 186.)

[Footnote D: The application of plumbago, or black lead, for electrotype
purposes, was first made by the late lamented Mr. Robert Murray.]

[Illustration: Fig. 186. A A. The tumbler containing the solution of
sulphate of copper. B B. The brown paper cell containing the dilute
sulphuric acid, inside which is the amalgamated zinc with wire attached
to the seal D.]

[Page 201]

Messrs. Elliott provide every kind of convenient vessel for the purpose,
and in the picture below it will be noticed that the single cell
apparatus, though not so economical as the simple tumbler arrangement
already described, is perhaps more convenient for electrotyping. (Fig.
187.)

[Illustration: Fig. 187. A. Single cell apparatus with proper vessel,
porous tube, and binding screws. B. A large trough divided by a
diaphragm of biscuit-ware or very thin porous wood.]


_Tenth Experiment._

[Illustration: Fig. 188. A. A single cell, Daniell's, attached to B, the
trough containing the mould and the plate of copper. Below is a Smee's
battery ready to be attached to a larger trough for the purpose of
electrotyping a great number of moulds at the same time.]

A single cell apparatus is only adapted to produce small electrotypes,
but when larger ones are required, a separate battery of three or four
[Page 202] Daniell's or Smee's cells is required; and it is usual to
place the mould to be copied in a separate wooden trough, attaching it
to the cathode wire, whilst a copper plate is connected with the anode,
so that as the solution of sulphate of copper undergoes decomposition by
the passage of the electricity, it is kept almost in a normal state, in
consequence of the oxygen of the water and the acid passing to the
copper plate, which they attack and dissolve as fast as the oxide of
copper and hydrogen are liberated at the cathode, where the latter
deoxidizes the oxide of copper, and by a secondary action deposits
metallic copper; the object being to dissolve fresh metal as the copper
is deposited on the mould. (Fig. 188.)


_Eleventh Experiment._

To silver electrotypes or other brass and copper articles, the first
attention must be paid to the cleanness of them; and when an electrotype
is just removed from the copper solution, and washed in clean water, it
is at once ready to receive the coating of silver; otherwise, if it has
been handled, or is slightly greasy, it should be first boiled in a
solution of common washing soda, and then the oxide removed by passing
it rapidly in and out of some "Dipping Acid," which is prepared by
mixing together equal parts of oil of vitriol and nitric acid; when
removed from the "Dipping Acid," it must be well washed in water, and
may remain under the surface of the water until the silvering solution
is ready. A silver solution may be prepared by dissolving a sixpence in
some nitric acid contained in a flask; it is then poured into a solution
of common salt, which precipitates the chloride of silver, and leaves
the copper in solution--the latter is poured off when the chloride has
subsided, and after being well washed in some boiling water, is
dissolved in a solution of cyanide of potassium. If a clean electrotype
is plunged into this solution, it is immediately covered with a very
thin coating of silver, which of course would soon wear off, and in
order to increase the thickness of the silver deposit, a single cell
arrangement may be constructed of a large gallipot containing a wide
porous cell and a circle of amalgamated zinc around it; the arrangement
is set in action by pouring a solution of salt (or, still better, sal
ammoniac) into and around the porous vessel, and the silvering solution
into the latter; a connecting wire passes from the zinc, and the article
being attached to it, is now plunged into the porous cell, when a
current of electricity slowly passes and deposits the silver on the
copper article. (Fig. 189.)

[Illustration: Fig. 189. The gallipot containing the solution of sal
ammoniac, with the circular amalgamated zinc with wire and binding screw
to which the medal is attached, and contained in the porous vessel
holding the silvering solution and medal.]

[Page 203]


_Twelfth Experiment._

Separate batteries and large troughs containing a solution of cyanide of
silver in cyanide of potassium are used on a grand scale in the
electro-plating establishment of Messrs. Elkington of Birmingham, where
the finest specimens of the art are to be obtained; a plate of silver
being attached to the anode to supply the loss of silver in these
troughs.


_Thirteenth Experiment._

The art of gilding by the agency of electricity is quite as simple as
the processes already described, although greater care is necessary to
avoid any loss of the precious metal. A small bit of gold is dissolved
in a mixture of three parts muriatic acid and one of nitric acid, which
forms the chloride of gold. This is then digested with an excess of
calcined magnesia, and the gold is precipitated as an oxide of the
metal; the latter is collected and washed, and then boiled in strong
nitric acid to remove the magnesia clinging to it, and being again
thoroughly washed with water, is dissolved in a solution of cyanide of
potassium, forming a solution of cyanide of gold and potassium, which
may be placed in the porous cell of the single cell arrangement already
described in the Eleventh Experiment.


_Fourteenth Experiment._

The safest and surest mode of making a gilding solution is to dissolve
some cyanide of potassium in water in a gallipot, and having placed a
porous vessel therein containing the same solution, put a plate of
copper into the porous cell, and some thin foil of pure gold into the
gallipot; connect the gold with the anode of a single cell of Daniell,
and the copper in the porous cell with the cathode, and in a few hours
sufficient gold will be dissolved for the purpose of gilding.

It is usually recommended to warm the gilding solution till it reaches a
temperature of about 150° Fahr., and a very moderate battery power is
employed in Electro Gilding. Indeed the same arrangement, shown in the
Eleventh Experiment, Fig. 189, will also answer for the gilding
solution. After being gilt, the articles may be rubbed with a little
tripoli, or burnished (with taste) by the handle of a key.


_Fifteenth Experiment._

Passing on to the more brilliant results obtainable from a powerful
voltaic battery (of at least thirty pairs of Grove), the beautiful
incandescence of platinum wire may first be noticed. If a wire of this
metal is stretched between the brass standards of two ring stands, the
length must be proportioned to the power of the battery; the adjustment
can be made very conveniently by twisting the platinum wire on one ring
stand, and then leaving the other end loose, the second ring stand may
be brought nearer and nearer to the first, until the desired intensity
of [Page 204] light from the incandescent wire is obtained. (Fig. 190.)
If the wire is contained in a glass tube the cooling effect of currents
of air is prevented, and a much greater length of wire can be made hot.

[Illustration: Fig. 190. A A. Two ring stands with the battery wires B B
(which should be a convenient length) attached. C. Platinum wire, fixed
end. D. The other end held in one hand and shortened as the stand is
moved by the other hand.]


_Sixteenth Experiment._

With the same arrangement, a chain composed of alternate links of silver
and platinum wire presents a very pretty effect, every alternate link of
platinum being incandescent, whilst the silver, from its excellent
conducting power, remains comparatively cool.


_Seventeenth Experiment._

Fireworks or gunpowder, arranged in proper cases, are fired at a great
distance from the voltaic battery by heating a thin iron or platinum
wire contained within them by the passage of the electricity; and
submarine and other explosions of gunpowder by the same agency have
become a common engineering operation. (Fig. 191.)

[Illustration: Fig. 191. A. A Gerb firework with two holes punctured,
through which the bit of iron wire passes, and is wound round the
battery wires tied to the outside of the case. C. A gut bladder
containing the thin wire and powder for a miniature submarine
explosion.]

During the operation of blasting the hard marl rocks in the River Severn
by Mr. Edwards, C.E., a number of holes were made side by side in the
bed of the river, and cartridges formed of strong duck or canvas,
tapered at the bottom, were filled with charges of powder from two to
four pounds, according to the depth of the marl; thus, two pounds for
four feet, three pounds for four feet six inches, and four pounds for
five feet. Into the bag were conveyed the wires of the voltaic battery,
or Bickford's fuse, and being then coated with pitch and tallow, and
finally greased all over and dusted with whitening, they rarely failed,
and were all fired simultaneously under water. The pitch and tallow
first, and afterwards the simple tallow, effectually excluded the water
from the gunpowder contained in the canvas bag.

[Page 205]


_Eighteenth Experiment._

The burning of various metals by the battery is displayed with great
effect by De la Rue's discharger, as also the incandescence of the
charcoal points producing the _electric light_. The illuminating power
derived from a forty-cell Grove's battery of the ordinary size is about
equal to the light of 500 candles.

[Illustration: Fig. 192. De la Rue discharger, containing a series of
six pairs of different substances, such as charcoal, iron, lead, zinc,
copper, antimony, in six pair of crayon holders, and turning on a
centre, so as to be changed at pleasure.]

Fizeau and Foucault have made a careful comparison of the light obtained
from 92 carbon couples as arranged in a Bunsen's battery, and of the
oxy-hydrogen, or Drummond Light, as compared with that of the sun, and
they state that "On a clear August day, with the sun two hours high, the
electric light (assuming the sun as unity) bore to it the ratio of one
to two and a half--_i.e._, the sun was two and a half times more
powerful, while the Drummond Light was only 1/146th that of the sun."
Bunsen found the light from 48 carbons equal to 572 candles. In Bunsen's
battery carbon is substituted for the platinum in Grove's arrangement;
and simultaneously with Bunsen, Cooper (in England) had applied charcoal
for the same purpose.

At night the giant ship (Polyphemus like) is to have an electric light
at the mast-head whilst steaming across the Atlantic.

[Illustration: Fig. 193. _Great Eastern_, with electric light.]

[Page 206]




CHAPTER XV.

MAGNETISM AND ELECTRO-MAGNETISM.


If a small helix, or coil of covered wire, is arranged with an
unmagnetized steel needle within it, so that the discharge of a large
Leyden jar may take place through the coil, the needle will be found
strongly magnetic after the discharge of the electricity. (Fig. 194.)
Many years before this was known, it had been noticed that when a ship
was struck by lightning, the compasses were generally reversed; and in a
special case, where a house was struck, the electricity entered a box of
knives, fusing some, tearing the handles off others, but leaving them
strongly magnetic. Electricians tried to repeat the effect by sending
the discharge of powerful Leyden batteries through bars of steel without
any important result; and it was not until Oersted, in the year 1819,
made his important discovery that the copper wire conveying the
electricity possessed peculiar magnetic power, that the principle began
to be understood, and then the electricians succeeded in imitating the
effects of lightning on steel, as already described in the beginning of
this chapter. (Fig. 194.)

[Illustration: Fig. 194. A A. A glass tube supported on two uprights of
wood, with coil of copper wire passing round it, terminating in the
balls B B. C. Needle to place inside glass tube.]

[Illustration: Fig. 195.]

When the electricity has passed away from the Leyden jar through the
coil of [Page 207] copper wire, it no longer possesses any power to
affect a piece of steel or iron, but if the wires of the voltaic battery
are now connected with the coil of copper wire, which should be covered
with cotton or silk, and many yards in length, then a bar of steel or
soft iron is not only rendered magnetic, but remains permanently so, as
long as the current of electricity continues to pass along the coil of
wire, so that if some nails or iron filings are brought to the bar of
iron, one end of which projects from the coil, they cling to it with
great force, and a great number of nails may be hung on in this manner,
but they immediately fall off when the contact is broken with the
battery. (Fig. 195.)

Electricity thus becomes a source of magnetism, and the discoverer,
Oersted, found that only needles or bars of steel or iron were thus
affected, and not those of brass, shell-lac, sulphur, and other
substances; he termed the conducting wire "a conjunctive wire," and
described the effect of the electric current or "_electric conflict_,"
as he called it, as resembling a Helix (from [Greek: _helissô_], to turn
round; a screw or spiral), and that it is not confined to the conducting
wire, but radiates an influence at some distance. This latter statement
is exactly in accordance with our present notions, and hence the coil
conveying the current is said to _induce_ magnetism in the iron or
steel, just as the phenomena of induction are produced with frictional
electricity. The effect of Oersted's discovery, says Silliman, was truly
_electric_; the scientific world was ripe for it, and the truth he thus
struck out was instantly seized upon by Arago, Ampère, Davy, Faraday,
and a crowd of philosophers in all countries. The activity with which
this new field of research has been cultivated, has never relaxed even
to this hour, while it has borne fruit in a multitude of theoretical and
practical truths, and above all, in the electro-magnetic telegraph,
truly called, and especially in connexion with the Atlantic telegraph
wire, "_the great international nerve of sensation_."

[Illustration: Fig. 196. A loadstone mounted in brass or silver, with
the iron cheeks B B attached. C. The bit of soft iron called the
armature.]

Magnetism is not only the result of a current of electricity through any
good conductor, but there are certain oxides of iron, called magnetic
iron ores, which have the property of attracting iron filings, and are
mostly found in primitive rocks, being abundant at Roslagen, in Sweden,
and called the loadstone, from its always pointing, when freely
suspended, to the Polar, North, or Load Star. If a tolerably large
specimen of this mineral is examined, there will be found usually two
points where the iron filings are attracted in larger quantities than in
other parts of the same specimen. These attractive points are called
poles, and the loadstone being properly mounted with soft iron bars,
termed cheeks, bound round it (in old-fashioned loadstones) with silver
plate and duly ornamented with [Page 208] engraving, has its magnetic power
greatly increased, and is then said to be endowed with magnetic
polarity; and to prevent the loss of power, a soft piece of iron, called
the armature, is placed across and attracted to the poles of the
loadstone. (Fig. 196.)


_Second Experiment._

If a needle of tempered steel (fitted with a little brass cup in the
centre to work upon a point) is rubbed with the loadstone in one
direction only, it is rendered permanently magnetic, and will now be
found to take a certain fixed position, pointing always in a direction
due north and south. The end which points towards the north is called
the north pole, and the other extremity the south pole, and it is usual
to mark the north pole with an indent or scratch to distinguish it at
all times.


_Third Experiment._

If another bar of steel is magnetized, and the north pole duly marked,
and then brought towards the same pole of the suspended magnet, instant
repulsion takes place; the magnet, of course, grasped in the hand is not
free to move, but the small magnet immediately shows the same fact
noticed with electricity, viz., "_that similar magnetisms repel_." Two
north poles repel each other, but when the bar of steel is reversed, the
opposite effect occurs, and the suspended magnet is attracted, showing
that _dissimilar magnetisms attract_, and a north will attract a south
pole. (Fig. 197.)

[Illustration: Fig. 197. A magnetic needle, the north pole N being
attracted to the south pole of the bar magnet S, and repelled from the
north end.]


_Fourth Experiment._

[Illustration: Fig. 198. The horse-shoe magnet, and another one
unmagnetized, placed end to end; the one shaded and lettered N and S is
the magnet. A A. The piece of soft iron moved in the direction of the
arrow.]

By contact, the magnetic power is transferred from the magnet to a piece
of unmagnetized steel, and it is stated that the highest magnetizing
effect is that produced by the simple method of Jacobi. A horse-shoe
magnet has its poles brought in contact with the intended poles of
another bar of steel, likewise bent in the form of a horse-shoe, and by
[Page 209] drawing the feeder over the unmagnetized horse-shoe in the
direction of the arrow in the cut, and when it reaches the curve,
bringing it back again to the same place, say at least twelve times, and
after turning the whole over without separating the poles, and repeating
the same operation on the other side likewise twelve times, the steel is
then powerfully magnetized; and it is said that a horse-shoe of one
pound weight may be thus charged so as to sustain twenty-six and a half
pounds, and that by the old method of magnetizing it would only have
sustained about twenty-two pounds. (Fig. 198.)


_Fifth Experiment._

If the horse-shoe magnet is placed on a sheet of paper, and some iron
filings are dusted between the poles, a very beautiful series of curves
are formed, called the magnetic curves, which indicate the constant
passage of the magnetic power from pole to pole.


_Sixth Experiment._

The magnetic force exerted by a horse-shoe-shaped piece of soft iron,
surrounded with many strands of covered copper wire in short lengths, is
extremely powerful (Fig. 199), and enormous weights have been supported
by an electro-magnet when connected with a voltaic battery. Supposing a
man were dressed in complete armour, he might be held by an
electro-magnet, without the power of disengaging himself, thus realizing
the fairy story of the bold knight who was caught by a rock of
loadstone, and, in full armour, detained by the unfriendly magician.

[Illustration: Fig. 199. A. Powerful electro-magnet supporting a great
weight. B. The battery.]

[Page 210]


_Seventh Experiment._

When a piece of soft iron is held sufficiently near one of the poles of
a powerful magnet, it becomes by _induction_ endowed with magnetic
poles, and will support another bit of soft iron, such as a nail,
brought in contact with it. When the magnet is removed, the inductive
action ceases, and the soft iron loses its magnetic power. This
experiment affords another example of the connexion between the
phenomena of electricity and magnetism. It is in consequence of the
inductive action of the magnetism of the earth that all masses of iron,
especially when they are perpendicular, are found to be endowed with
magnetic polarity; hence the reaction of the iron in ships upon the
compasses, which have to be corrected and adjusted before a voyage, or
else serious errors in steering the vessel would occur, and there is no
doubt that many shipwrecks are due to this cause. No other metals beside
iron, steel, nickel, cobalt, and possibly manganese, can receive or
retain magnetism after contact with a magnet.

The remarkable effect of magnetism upon all matter, so ably investigated
by Faraday and others, will be explained in another part of this
book--viz., in the article on Dia-Magnetism.

[Illustration: Fig. 200. Magician and his loadstone-rock.--Vide _Fairy
Tale_.]

[Page 211]




CHAPTER XVI.

ELECTRO-MAGNETIC MACHINES.


The experiments already described in illustration of some of the
phenomena of electro-magnetism are of such a simple nature that they may
be comprehended without difficulty; but it is not such an easy task to
appreciate the curious fact of an invisible power producing motion. It
has already been explained that a copper or other metallic wire
conveying a current of electricity becomes for the time endowed with a
magnetic power, and if held above, or below, or close to, a suspended
magnetized steel needle, affects it in a very marked degree, causing it
to move to the right or left, according to the _direction_ of the
electric current; and in order to form some notion of the condition of a
metallic wire whilst the electricity is passing through it, the annexed
diagrams may be referred to. (Figs. 201, 202.)

[Illustration: Fig. 201. Portion of a square copper conductor, in which
A B represents the direction of the electricity, and the small arrows, C
C C C, the magnetic current or whirl at right angles to the electrical
current, and exercising a tangential action.]

[Illustration: Fig. 202. A round conducting wire, in which the
electrical current is flowing in the direction of the large dart A B,
and the small arrows indicate the direction of the magnetic force.]

Dr. Roget says: "The magnetic force which emanates from the electrical
conducting wire is entirely different in its mode of operation from all
other forces in nature with which we are acquainted. It does not act in
a direction parallel to that of the current which is passing along the
wire, nor in any plane passing through that direction. It is evidently
exerted in a plane perpendicular to the wire, but still it has no
tendency to move the poles of the magnet in a right or radial line,
either directly towards, or directly from, the wire, as in every other
case of attractive or repulsive agency. The peculiarity of its action is
that it produces motion in a circular direction _all round_ the
wire--that is, in a direction at right angles to the radius, or in the
direction of the tangent to a circle described round the wire in a plane
perpendicular to it; hence the electro-magnetic force exerts a
tangential action, or that which Dr. Wollaston called a vertiginous or
whirling motion."

[Page 212]

Dr. Faraday concluded that there is no real attraction or repulsion
between the wire and either pole of a magnet, the action which imitates
these effects being of a compound nature; and he also inferred that the
wire ought to revolve round a magnetic pole of a bar magnet, and a
magnetic pole round a wire, if proper means could be devised for giving
effect to these tendencies, and for isolating the operations of a single
pole. For the first idea of electro-magnetic rotation the world is
indebted to Dr. Wollaston; but Dr. Faraday, with his usual ingenuity,
was the first who carried out the theory practically. The rotation of a
wire (conveying a current of voltaic electricity) round one of the poles
of a magnet is well displayed with the simple contrivance devised by
him. (Fig. 203.)

[Illustration: Fig. 203. N. A small bar magnet cemented into a
wine-glass, the north pole being at N. A is a moveable wire looped over
the hook, which is the positive (+) pole of the battery; the free
extremity rotates round the pole of the magnet when the current of
electricity passes. The dotted line represents the level of the mercury
which the glass contains. The electricity passes in at A, and out at the
wire B, as shown by the arrows. C is connected with the negative, and D
with the positive, pole of the battery.]

By a careful observation of the complex action of an electrified wire
upon a magnetic needle, Dr. Faraday was enabled to analyse the phenomena
with his usual penetration and exhaustive ability, and he found, as
Daniell relates,--

    "That if the electrified wire is placed in a perpendicular position,
    and made to approach towards one pole of the needle, the pole will
    not be simply attracted or repelled, but will make an effort to pass
    off on one side in a direction dependent upon the attractive or
    repulsive power of the pole; but if the wire be continually made to
    approach the centre of motion by either the one or the other side of
    the needle, the tendency to move in the former direction will first
    diminish, then become null, and ultimately the motion will be
    reversed, and the needle will principally endeavour to pass in the
    opposite direction. The opposite extremity of the needle will
    present similar phenomena in the opposite direction; hence Dr.
    Faraday drew the conclusion that the direction of the forces was
    _tangential_ to the circumference of the wire, that the pole of the
    needle is drawn by one force, not in the direction of a radius to
    its centre, but in that of a line touching its circumference, and
    that it is repelled by the other force in the opposite direction. In
    this manner the northern force acted all round the wire in one
    direction, and the southern in the opposite one. Each pole of the
    needle, in short, appeared to have a tendency to revolve round the
    wire in a direction opposite to the other, and, consequently, the
    wire round the poles. Each pole has the power of acting upon the
    wire by itself, and not as connected with the opposite pole, and the
    _apparent attractions_ and _repulsions_ are merely _exhibitions_ of
    the _revolving motions_ in different parts of their circles."

[Page 213]

The same fact illustrated at Fig. 203, is also demonstrated in a still
more striking manner by means of wire bent into a rectangular form, and
so arranged that whilst the current of electricity passes, it is free to
move in a circle; and when the poles of a magnet are brought towards the
electrified wire, it may be attracted or repelled at pleasure, and in
fact becomes a magnetic indicator, and places itself (if carefully
suspended) at right angles to the magnetic meridian. (Fig. 204.)

[Illustration: Fig. 204. A A A A. The rectangular wire covered with silk
and varnished, one end of which being pointed, rests on the little cup
B, connected with a covered wire passing down the centre of the brass
support to the binding screw C let into ivory. D. The other extremity of
the rectangular wire; this being covered and varnished, is not in
metallic contact with the end B, but is likewise pointed, and dips into
the mercury contained in the large cup E E. The upper and lower cups do
not touch, and are separated by ivory, marked by the shaded portion, and
the cup E E is in metallic communication with the brass pillar, and is
connected with the negative pole of the battery at F, whilst C is
connected with the positive pole of the battery, and the electricity
circulates round the wire in the direction of the arrows. When a bar
magnet, N, is brought towards the wire, the latter is immediately set in
motion, and by alternately presenting the opposite poles of the magnet,
the rectangular wire rotates freely round the cup B.]

These curious movements of a magnetized needle, and rotations of wires
and magnets, brought about by the agency of an active current of
electricity, have induced Sir David Brewster to advance his admirable
theory, which supposes the affection of the mariner's compass needle,
and all other suspended pieces of steel, to be due to the agency of
_electrical currents_ continually _circulating_ around the globe; and
Mr. Barlow contrived the following experiment in illustration of
Brewster's theory. A wooden globe, sixteen inches in diameter, was made
hollow, for the purpose of reducing its weight, and while still in the
lathe, grooves one-eighth of an inch deep and broad were cut to
represent an equator, and parallels of latitude at every four and a half
degrees each way from the equator to the poles. A groove of double depth
was also cut like a meridian from pole to pole, but only half round. The
grooves were cut to receive the copper wire covered with silk, and the
laying on was commenced by taking the middle of a length of ninety feet
of wire one-sixteenth of an inch in diameter, which was applied to the
equatorial groove so as to meet in the transverse meridian; it was then
made to pass round this parallel, returned again along the meridian to
the next parallel, and then passed round this again, and so on, till the
wire was thus led in continuation from pole [Page 214] to pole. The
length of wire still remaining at each pole was returned from each pole
along the meridian groove to the equator, and at this point, each wire
being fastened down with small staples, the wires from the remaining
five feet were bound together near their common extremity, when they
opened to form separate connexions for the poles of a voltaic battery.
When the battery was connected, and magnetic needles placed in different
positions, they behaved precisely as they would do on the surface of the
earth, the induction set up by the electrified wire being a perfect
imitation of that which exists on the globe.

The opposite effect to that already described--viz., the rotation of one
pole of a magnet round the electrified wire, was also arranged by
Faraday in the following manner. (Fig. 205.)

[Illustration: Fig. 205. N S. A little magnet floating in mercury
contained in the glass A A; the north pole is allowed to float above the
surface of the quicksilver, and the south pole is attached to the wire
passing through the bottom of the glass vessel. The electricity passes
in at B, and taking the course indicated by the arrows travels through
the glass of quicksilver to the other pole of the battery at C. Directly
contact is made with the battery, the little magnet rotates round the
electrified wire, W. The dotted line shows the level of the mercury in
glass.]

In the examination of the magnetic phenomena obtained from wires
transmitting a current of electricity, it should be borne in mind that
any conducting medium which forms part of a closed circuit--_i.e._, any
conductor, such as charcoal, saline fluids, acidulated water, which form
a link in the endless chain required for the path of the
electricity,--will cause a magnetic needle placed near it to deviate
from its natural position.

These positions of the electrified wire and the magnetic needle are of
course almost unlimited, and in order to assist the memory with respect
to the fixed laws that govern these relative movements, Monsieur Ampère
has suggested a most useful mechanical aid, and he says:--"Let the
observer regard himself as the conductor, and suppose a positive
electric current to pass from his head towards his feet, in a direction
parallel to a magnet; then its north pole in front of him will move to
his right side, and its south pole to his left.

"The plane in which the magnet moves is always parallel to the plane in
which the observer supposes himself to be placed. If the plane of his
[Page 215] chest is horizontal, the plane of the magnet's motion will be
horizontal, but if he lie on either side of the horizontally-suspended
magnet, his face being towards it, the plane of his chest will be
vertical, and the magnet will tend to move in a vertical plane."

This very lucid comparison will be seen to apply perfectly to the
direction of the rotations in Figs. 203 and 205.

The whole of this apparatus is made in the most elegant and finished
manner by Messrs. Elliott, of 30, Strand; and by a modification of the
latter arrangement (Fig. 206), the opposite rotations of the opposite
poles of the magnets round the electrified wire, are shown in the most
instructive manner. The apparatus (Fig. 206) was devised by the late Mr.
Francis Watkins, and consists of two flat bar magnets doubly bent in the
middle, and having agate cups fixed at the under part of the bend (by
which they are supported) upon upright pointed wires, the latter being
fixed upright on the wooden base of the apparatus, and the magnets turn
round them as upon an axis.

[Illustration: Fig. 206. A. Wire conveying the current of electricity. B
B. The magnets balanced on points rotating round the wires.]

Two circular boxwood cisterns, to contain quicksilver, are supported
upon the stage or shelf above the base. A bent pointed wire is directed
into the cup of each magnet, the ends of which dip into the mercury
contained in the boxwood circular troughs on the stage. By using a
battery to each magnet, and taking care that the currents of electricity
flow precisely alike, they will then rotate in opposite directions.

Directly after the ingenious experiments of Faraday became known, a
great number of electro-magnetic engine models were constructed, and
many thought that the time was fast approaching when steam would be
superseded by electricity; and really, to see the pretty
electro-magnetic models work with such amazing rapidity, it might be
supposed that if they were constructed on a larger scale, a great amount
of hard work could be obtained from them. This idea, however, has been
proved to be a fallacy, for reasons that will be presently explained.
The figure on p. 216 displays two of these engines, one of which
represents the rotation of electro-magnets within four _fixed steel
magnets_, and the other the rotation of steel magnets by the _fixed
electro-magnets_. The latter (No. 2) moves with such great velocity,
that unless the strength of the battery is carefully adjusted, the
connexions are soon destroyed. (Fig. 207.)

[Page 216]

[Illustration: Fig. 207.--No. 1 consists of vertical permanent steel
magnets and horizontal soft-iron electro-magnets which rotate.

No. 2 consists of two fixed soft-iron electro-magnets, and four bent
permanent steel magnets, which rotate, in both cases of course, only
when connected with the battery.]

Considering the prodigious power or _pull_ of a soft-iron
electro-magnet, and its capability of supporting considerable weight,
the most reasonable expectations of success might be entertained with
machines acting by the direct pull. It was, however, discovered that
they soon became inefficient, from the circumstance that the repeated
blows received by the iron so altered its character, that it eventually
assumed the quality of steel, and had a tendency to retain a certain
amount of permanent magnetism, and thus to interfere with the principle
of making and unmaking a magnet. It was this fact that induced Professor
Jacobi, of St. Petersburg, after a large expenditure of money, to
abandon arrangements of this kind, and to employ such as would at once
produce a rotatory motion. The engine thus arranged was tried upon a
tolerably large scale on the Neva, and by it a boat containing ten or
twelve people was propelled at the rate of three miles an hour.

Various engines have been constructed by Watkins, Botta, Jacobi,
Armstrong, Page, Hjorth; the engine made by the latter (Hjorth) excited
much attention in 1851-52, and consisted of an electro-magnetic piston
drawn within or repelled from an electro-magnetic cylinder; and by this
motion it was thought that a much greater length of stroke could be
secured than by the revolving wheels or discs, but the loss of power
(not only in this engine, but in others) through space is very great,
and the lifting power of any magnet is greatly reduced and [Page 217]
altered at the smallest possible distance from its poles. This loss of
power is therefore a great obstacle in the way of the useful application
of electro-magnetic force, and can be appreciated even with the little
models, all of which may be stopped with the slightest friction,
although they may be moving at the time with great velocity.

In the second place, supposing the reduced force exerted by the two
magnets, a few lines apart, was considered available for driving
machinery, the moment the magnets begin to move in front of one another
there is again a great loss of power, and as the speed increases, there
is curiously a corresponding diminution of available mechanical power, a
falling-off in the _duty_ of the engine as the rotations become more
rapid. In the third place, the cost of the voltaic battery, as compared
with the consumption of coal in the steam-engine, is very startling, and
extremely unfavourable to electro-magnetic engines.

Mr. J. P. Joule found that the economical duty of an electro-magnetic
engine at a given velocity and for a given resistance of the battery is
proportioned to the mean intensity of the several pairs of the battery.
With his apparatus, every pound of zinc consumed in a Grove's battery
produced a mechanical force (friction included) equal to raise a weight
of 331,400 pounds to the height of one foot, when the revolving magnets
were moving at the velocity of eight feet per second. Now, the _duty_ of
the best Cornish steam-engine is about one million five hundred thousand
pounds raised to the height of one foot by the combustion of each pound
of coal, or nearly five times the extreme _duty_ that could be obtained
from an electro-magnetic engine by the consumption of one pound of zinc.
This comparison is therefore so very unfavourable, that the idea of a
successful application of electricity as an _economic_ source of power,
is almost, if not entirely abandoned.

By instituting a comparison between the different means of producing
power, it has been shown that for every shilling expended there might be
raised by

                        Pounds.
  Manual power          600,000 one foot high in a day.
  Horse               3,600,000       "        "
  Steam              56,000,000       "        "
  Electro-magnetism     900,000       "        "

A powerful magnet has been compared to a steam-engine with an enormous
piston but with an exceedingly short stroke. Although motive power
cannot be produced from electricity and applied successfully to
commercial purposes, like the steam-engine, yet the achievements of the
electric telegraph as an application of a small motive power must not be
lost sight of, whilst the fall of the ball at Deal and other places, by
which the chronometers of the mercantile navy are regulated, as also the
means of regulating the time at the General Post Office and various
railway stations, are all useful applications of the power which fails
to compete in other ways with steam.

[Page 218]




CHAPTER XVII.

THE ELECTRIC TELEGRAPH.


The engineering and philosophical details of this important instrument
have grown to such formidable dimensions, that any attempt (short of
devoting the whole of these pages to the subject) to give a full account
of the history and application of the instrument, the failures and
successes of novel inventions, and the continued onward progress of this
mode of communication, must be regarded as simply impossible, and
therefore a very brief account of the _principle_ only will be attempted
in these pages.

For the complete history of the discovery and introduction of the
principle of the Electric Telegraph the reader is referred to the
Society of Arts Journal (Nos. 348-9, vol. viii.), where it is stated
that it is _half a century_, dating from August, 1859, since the first
galvanic telegraph was made. "It was the Russian Baron Schilling's
electro-magnetic telegraph which, without its being known to be his, was
brought to London, and caused the establishment of the first practically
useful telegraph lines, not only in Great Britain, but in the world."
Dr. Hamel says: "The small sprout nursed on the Neva, which had been
exhibited on the Rhine, and thence brought to the Thames, grew up here
to a mighty tree, the fruit-laden branches of which, along with those
from trees grown up since, extend more and more over the lands and seas
of the Eastern hemisphere, whilst kindred trees planted in the Western
hemisphere have covered that part of the world with their branches, some
of which will, ere long, be interwoven with those in our hemisphere."

The first telegraph line in England was constructed by Mr. Cooke from
Paddington along the Great Western Railroad to West Drayton in 1838-39;
and it must be remembered that it was in February, 1837, that Mr. Cooke
first consulted Professor Charles Wheatstone, having previously visited
Dr. Faraday and Dr. Roget, and on the 19th November, 1837, a partnership
contract was concluded between Messrs. Cooke and Wheatstone.

To the distinguished philosopher, Professor Wheatstone, the merit of the
ingenious construction of the vertical-needle telegraph is due; whilst
Mr. Cooke's name will always be associated with the practical
establishment of the first telegraph lines in England. The first line in
the United States, from Washington to Baltimore, was completed in 1844,
being arranged and worked by Professor Morse.

In British India, in April and May, 1839, the first long line of
telegraph, twenty-one miles in length, and embracing 7000 feet of river
surface, was constructed by Dr. (now Sir William) O'Shaughnessy.

[Page 219]

The construction of the electric telegraph may be considered under three
heads:

1st. The Battery, _the motive power_.

2nd. The Wires, _the carriers of the force_.

3rd. The Instruments to be worked--_the bell_ and the _needle
telegraph_.


THE BATTERY.

The construction and rationale of the batteries generally in use have
been explained in another part of this work; those used for telegraphic
purposes consist of one or more couples, of which zinc is one, the
second being copper, silver, platinum, or carbon. Each couple is termed
an element, and a series of such couples a _battery_.

The batteries employed chiefly on the English lines consist of a plate
of cast-zinc four inches square and 3/16ths of an inch thick, attached
by a copper strap one inch broad to a thin copper plate four inches
square. The zinc is well amalgamated with mercury. Twelve of these
couples are arranged in a trough of wood, porcelain, or gutta-percha,
divided by partitions into twelve water-tight cells, 1¼ inch wide.
The zinc and copper preserve the same order and direction throughout,
and when arranged, the trough is filled with the finest white sand, and
then moistened with water previously mixed with five per cent. by
measure of pure sulphuric acid. This mode of applying the acid is the
clever practical improvement of Mr. Cooke, and prevents any
inconvenience from the spilling of the acid, and at the same time
renders the battery quite portable. The voltaic arrangement thus
prepared is found to remain in action for several weeks, or even months,
with the occasional addition of small quantities of acid, and answers
well for working needle telegraphs in fine and dry weather. In fogs and
rains, at distances exceeding 200 miles at most, their action is not so
perfect, and a vast number of couples must be employed, 144 to 288 being
frequently in use. In France, Prussia, and America, sand batteries do
not appear to answer, and Daniell's arrangement is preferred. Sixty
couples suffice in France for some of the long lines--viz., from Paris
to Bordeaux, 284 miles; Paris to Brussels, 231¼ miles; and in fact,
the advantages of the Daniell's battery have become so apparent, that
they are now being used on English lines. In Prussia, Bunsen's carbon
battery is much used; in India, a modification of Grove's battery is
preferred, the zinc being acted upon by a solution of common salt in
water. Two of these elements were found sufficient to work a line of
forty miles totally uninsulated, and including the sub-aqueous crossing
of the Hooghly River, 6200 feet wide.

The continual energy of the battery, whatever may be its construction,
depends on the circulation of the electricity, the object being to pass
the force from the positive end of the series through the wires, back
again to the negative extremity of the voltaic series.

The wire (the carrier of the force) must be continuous throughout,
unless, of course, water or earth forms a part of the endless conducting
chain.

[Page 220]


THE CONDUCTING WIRES.

These roads for the electricity may be of any convenient metal, and the
one preferred and used is iron, which is well calculated from its great
tenacity (being the most tenacious metal known) and cheapness to convey
the electricity, although it is not such a good conductor as copper, and
offers about six times more resistance to the flow of the current than
the latter metal. The wire does not appear to be made of iron, because
it is galvanized or passed through melted zinc, which coats the surface
and defends it from destructive rust, at the same time does not destroy
its valuable property of tenacity or power of resisting a strain. About
one ton of wire is required for every five miles, and to support this
weight, stout posts of fir or larch are erected about fifty yards apart,
and from ten to twenty-five feet high. At every quarter mile, on many
lines, are straining-posts with ratchet wheel winders, for tightening
the wires. On some of the lines the wires are attached to the posts by
side brackets carrying the insulators invented by Mr. C. V. Walker,
which are composed of brown salt-glazed stoneware of the
hour-glass shape, as shown in the drawing. (Fig. 208.)

[Illustration: Fig. 208. Walker's insulator.]

There are some objections to the hour-glass insulators, and they have
been modified by Mr. Edwin Clark, [Page 221] who employs a very strong
stone-ware hook open at the side, so that the wire can be placed on the
hook without threading, and the hooks can be replaced in case of
breaking, without cutting the telegraph wire, which is securely fastened
to each insulator by turns of thinner wire. An inverted cap of zinc is
used to keep the insulator dry. (Fig. 209.)

[Illustration: Fig. 209. Clark's insulator.]

In India the conductor is rather a rod than a wire, and weighs about
half a ton per mile; it is erected in the most substantial manner, and
many miles of the rod are supported on granite columns, other portions
on posts of the iron-wood of Arracan, or of teak.

The number of wires required by the electric telegraph often puzzles the
railway traveller, and people ask why so many wires are used on some
lines and so few on others? The answer is very simple: they are for
convenience. Two wires only are required for the double needle
telegraph, and one for the single needle instrument. But as so many
instruments are required at the terminal stations, an increased number
of wires, like rails for locomotives, must be provided; thus, on the
Eastern Counties, seven wires are visible, and are thus employed. The
two upper wires pass direct from London to Norwich; the next pair
connect London, Broxbourne, Cambridge, Brandon, Chesterfield, Ely; the
third pair all the small stations between London and Brandon; and the
seventh wire is entirely devoted to the bell.

If the earth was not a conductor of electricity, and employed in the
telegraphic circuit, four wires would be required for the double needle
telegraph, and two for the single instrument. To understand this, let us
suppose a battery circuit extending from Paddington to the instrument at
Slough, and the wire returning from Slough to Paddington, it is evident
that one wire would take the electricity to Slough, and the other return
it to London, as in the diagram below. (Fig. 210.)

[Illustration: Fig. 210. A. The battery. B. The instrument. The arrows
show the passage of the electricity to the single needle telegraph
instrument by one wire, and the return current by the other.]

If the whole of the return wire is cut away except a few feet at each
end, which are connected by plates of copper with the damp earth, the
current not only passes as before, but actually has increased in
intensity, and will cause a much more energetic movement of the needle
in the telegraph instrument. (Fig. 211.) These plates are called "_Earth
Plates_;" and Steinheil, in 1837, was the first who proved that the
earth might perform the function of a wire.

[Page 222]

[Illustration: Fig. 211. A. The battery. B. The instrument. C. Earth
plate at Slough. D. Earth plate at London. The arrows show the direction
of the electric current.]

It must be obvious that a message may be received at any station without
a battery, but in order to be able to return an answer, every station
must have its own battery.

Ingeniously-constructed lightning-conductors are attached to the posts
which carry the wires, so that in case of a storm, the natural
electricity is conveyed to the earth, whilst the voltaic electricity
artificially produced pursues its own course without deviation.
Protectors are also required for the instruments at the stations, and
the plan devised by Mr. Highton is thus described by the inventor:--

"A portion of the wire circuit--say for six or eight inches--is
enveloped in blotting-paper or silk, and a mass of metallic filings, in
connexion with the earth, is made to surround it. This arrangement is
placed on each side of the telegraph instrument at a station. When a
flash of lightning happens to be intercepted by the wires of the
telegraph, the myriads of infinitesimally fine points of metal in the
filings surrounding the wire at the station, on having connexion with
the earth, at once draw off nearly the whole charge of lightning, and
carry it safely to the earth."


THE INSTRUMENTS TO BE WORKED--THE BELL AND THE TELEGRAPH.

The bell or alarum resembles in construction that of an ordinary clock,
and is in fact a piece of clockwork wound up and ready to ring a bell,
when the _detent_ or preventive is removed. The detent is connected with
a piece of soft iron placed before an electro-magnet, and directly the
current passes, the electro-magnet attracts the soft piece of iron
attached to a perpendicular lever which the bell-crank lever rests upon;
the detent is removed, and the bell rings, and again stops when the
current of electricity ceases to pass.

One of the most simple alarum clocks is a common American clock, wound
up daily. A small electro-magnet surrounded with thick wire is placed
below a moveable piece of tinned iron, so that when this is attracted,
the fly of the clock is released, and its bell tolls unceasingly while
[Page 223] the magnet is excited. This arrangement is employed by Sir W.
O'Shaughnessy in the Indian telegraph system. (Fig. 212.)

It will readily be comprehended from this description that the alarum is
sounded by ordinary mechanism, and that the duty of the current of the
electricity is simply comprised in the act of removing the lever and
liberating machinery, which may be large or small; and if it were
thought necessary, the bells of the great clock-tower of the Houses of
Parliament, which chime the quarters, or even "Big Ben" himself (when
his constitution is restored), could be rung by a person at York or
Edinburgh, supposing wires, batteries, and a powerful electro-magnet
with a detent mechanism for the bells, were properly arranged and
connected with the clockwork.

[Illustration: Fig. 212. A. The soft iron tinned, which is attracted to
the electro-magnet B, and liberates the detent.]

In certain cases, Mr. Charles V. Walker states that a single and
distinct wire is used for the bell only, with his special mechanism,
called the _ringing key_. If the bell was always on the same wire as the
needle-coil, the bell would not only call the attention of, but
seriously annoy the clerk (unless, of course, he happened to be a very
deaf person) by its ringing whilst he was reading the signals of the
needle. The nuisance is prevented by what is termed _joining over_ or
making the _short circuit_--in fact, by providing for the current a
shorter and much more capacious road to the needle coil than by going
through that of the bell-magnet, which is made with very fine wire; and
the control of the short circuit is put in the hands of the clerk.


COOKE AND WHEATSTONE'S DOUBLE NEEDLE TELEGRAPH.

The principle of this instrument, as already explained, is involved in
the elementary experiment of Oersted--viz., the deflection of a magnetic
needle from the inside of a coil of wire conveying a current of
electricity, and as it is difficult to give a good description and
drawing of the interior of the instrument that can really be understood,
it may be sufficient to state that the handles give the operator the
power of reversing the current of electricity, so that the needles are
deflected with the utmost certainty to [Page 224] one side or the
other, either separately or simultaneously. (Fig. 213.)

[Illustration: Fig. 213. The letters of the alphabet, figures, and a
variety of conventional signals, are indicated by the single and
combined movements of the needles on the dial. The left-hand needle
moving once to the left indicates the +, which is given at the end of a
word. Twice in the same way, A; thrice, B; first right, then left, C;
the reverse, D. Once direct to the right, E; twice, F; thrice, G. In the
same order with the other needle for H, I, K, L, M, N, O, P. The signals
below the centre of the dial are indicated by the parallel movements of
both needles simultaneously. Both needles moving once to the left
indicate R; twice, S; thrice, T. First right, then left with both, U;
the reverse, V. Both moving once to the right, W; twice, X; thrice, Y.
The figures are indicated in the same way as the letters nearest to
which they are respectively placed. To change from letters to figures
the operator gives H, followed by the +, which the recipient returns to
signify that he understands. If, after the above signs (H and +) were
given, C R H L were received, 1845 would be understood. A change from
figures to letters is notified by giving I, followed by the +, which the
recipient also returns. Each word is acknowledged. If the recipient
understand, he gives E; if not, the +, in which case the word is
repeated. Attention to a communication by this instrument is called by
the ringing of a bell (of any size), which is effected through the
agency of an electric current. The upper case contains the bell.]

Sir W. O'Shaughnessy, in his excellent work on the electric telegraph in
British India, gives a description of a telegraphic instrument of
remarkable simplicity, which is successfully employed in India, and is
[Page 225] highly spoken of by Mr. E. V. Walker and other gentlemen
practically acquainted with the working of telegraphs. It consists of a
coil of fine wire on a card or ivory frame, a magnetic needle with a
light index of paper pasted across it; two stops of thin sheet lead to
limit the vibrations of the index; a supporting board eight inches
square, and a square of glass in a frame of wood, or a common glass
tumbler placed over it as a shade, to prevent the index being moved by
currents of air. It is stated that the office boys, with the assistance
of a native Indian carpenter, make up these telegraphs at a price not
exceeding two shillings each.

In England of course they would be more expensive; but the simplicity
and perfection of the arrangement are so much to be commended that we
give the details for the benefit of those boys who might wish to
establish a telegraph on a small scale for amusement.


THE FRAME.

This is a piece of mahogany eight inches square and one inch thick, with
a hollow groove cut in its centre two inches and a half long, half an
inch wide, and a quarter of an inch deep; a ledge of the same wood one
inch wide and half an inch deep surrounds the frame, leaving the inner
surface seven inches square; this is stained black with ink to make the
motions of the index more conspicuous.


THE COIL.

This consists of fifty feet of the finest silk-covered copper wire wound
on a frame of card two inches long, half an inch broad, three-eighths
deep in the open part.

An edge or flange of card, three-eighths of an inch wide, is attached to
it at each side to keep the wire in its place. The frame may be of thin
wood or ivory, and the winding of the wire commences at the lower left
corner, and it is coiled from left to right, as the hands of a watch
would move in the same plane. (Fig. 214.)

[Illustration: Fig. 214. The coil.]

Two inches of each end of the coil wire are now stripped of their silk
covering by being rubbed with sand-paper. The coil is mounted in the
frame by inserting its lower edge or flange in the groove, so that the
lower part or floor of the inside of the coil is level with that of the
[Page 226] frame, as shown below, and it is now ready to receive the
magnetized needle. (Fig. 215.)

[Illustration: Fig. 215. The coil fitted into frame.]


THE NEEDLE.

This is one inch long, one-twelfth of an inch wide, of the thinnest
steel, and fitted with a little brass cap turned to a true cone to
receive the point on which it is balanced. These needles are of hard
tempered steel, and are magnetized by a single contact with the poles of
an electro-magnet or other ordinary powerful magnet.

The magnet is now to be balanced on a steel point one-eighth of an inch
high; these are nipped off with cutting pliers from common sewing
needles, and soldered into a slip of thin copper three inches long, half
an inch wide. (Fig. 216.)

[Illustration: Fig. 216. A. The needle. B. The point on the slip of
copper.]

As the north end of the needle will be found to dip, it is advisable to
counteract this by touching the south end with a little shell-lac
varnish, which dries rapidly, and soon restores the needle to a perfect
equilibrium.

The needle is completed for use by fixing to it an index of paper (cut
from glazed letter paper) two inches long, tapering from one-eighth of
an inch to a point, and fastened at right angles on to the needle with
lac varnish, so as to be truly balanced, and pointing the sharp end to
the east, when the needle placed on the point settles due north and
south, its north pole being opposite the observer's right hand, the
observer facing west. (Fig. 217.)

[Illustration: Fig. 217. The needle with the paper index.]

[Page 227]

The coil frame is placed north and south, and the needle is now
introduced by sliding the end of the slip of copper into the opening in
the frame.

To limit the vibrations of the paper index a _stop_ is placed at each
side. The stops are made of a strip of thin sheet-lead or copper, a
quarter of an inch broad, one inch and a half long, and turned up at a
right angle, so that one inch rests on the board and half an inch is
vertical. For ordinary practice these stops are placed each at half an
inch from the index.

The telegraph is placed in a box, which may have a piece of
looking-glass in the lid, so that the readings can be taken with the
needle in the vertical instead of the horizontal position, if required.
(Fig. 218.)

[Illustration: Fig. 218. Box containing the telegraph, with the
looking-glass in the lid. A small steel magnet is placed on or near the
frame, if required, the south pole of this magnet being opposite to the
north pole of the needle in the telegraph coil. The bar is four inches
long, half an inch broad, three-sixteenths of an inch thick, and it is
only used to counteract any local deviation which may arise in using the
instrument with miles of wire. It would not be required under ordinary
circumstances. The alphabet used is shown to the left.]

The ends of the fine wire of the telegraph coil are joined on to the
wires from the _reversing_ instrument, and this is connected with a
voltaic series of one or more elements, so that by the employment of the
reverser the needle is caused to move right or left at pleasure. The
[Page 228] white paper index on the black ground can be followed with
the greatest certainty, and Sir W. O'Shaughnessy states that with this
instrument a telegraph clerk may read at the rate of twenty words per
minute with a double needle wire, being equal to forty words per minute.


THE REVERSER

consists of a block of wood, two inches and a half square, in which four
hollows, half an inch deep, are cut, and these hollows are joined
diagonally by copper wires let into the substance of the wood, and most
carefully insulated from each other by melted cement, but exposing a
clean metallic surface in each cell, which is filled with mercury. (Fig.
219.)

[Illustration: Fig. 219. Block of wood with four holes; the positive
terminal is connected with the holes A and B, the negative with C and D;
the hollows are filled with mercury. T T are the wires from the
telegraph box, and it is obvious that by dipping them alternately into C
B and A D the current is reversed, and the needle deflected right or
left at pleasure.]

In practice a more elaborate reverser is employed, but to demonstrate
the principle the simple block above described is quite sufficient.

With the telegraph placed at the top of a house, or in a distant
cottage, and a single cell of Grove's battery, or at most two, for any
short distances, with the reverser, messages may be passed with great
rapidity from the bottom of the house to the top, or from a mansion to
the lodge, it being understood that a battery, reverser, and telegraph,
are required at both places where messages are received and _answered_;
but if no answers are required, the battery and reverser are placed at
one end of the wire in the house, and the telegraph at the other
extremity in the cottage, and earth plates may be arranged to return the
current, or another wire used for that purpose.

Whilst lauding to the utmost the invention of the electric telegraph, we
must remember "there is nothing new under the sun," and that after all
Nature claims the _principle_ of telegraphing, and with the silent
gesture, the speaking eye, interpreted and answered by others, she
proclaims herself to be the originator of communication by signs.
Whilst [Page 229] the language of flowers, and the mournful
requirements of the deaf and dumb in the use of the finger alphabet,
show how readily man has adopted the important principle, till he has
brought it to the highest state of perfection in the electric telegraph.

When the telegraph was first adopted on the Great Western Railway, the
most ridiculous ideas were formed of its capabilities, and many persons
firmly believed that the wires were used for the purpose of dragging
letters and different articles from station to station. "Wife," said a
man, looking at the telegraph wires, "I don't see, for my part, how they
send letters on them wires, without tearin' 'em all to bits." "Oh, you
stupid!" exclaimed his intellectual spouse; "why, they don't send the
paper: they just send the writin' in a _fluid_ state."

[Illustration: Fig. 220. One of the ideas of telegraphic
communication.]

[Page 230]




CHAPTER XVIII.

RUHMKORFF'S, HEARDER'S, AND BENTLEY'S COIL APPARATUS.


In the course of the popular articles on frictional and voltaic
electricity, it has already been mentioned that whilst the _intensity
effects_--such as the capability of the spark to pass through a certain
thickness of air, or the production of the peculiar physiological effect
of the shock--belong especially to the phenomena of frictional
electricity, they are not apparent with the _quantity effects_, such as
may be produced by an ordinary voltaic battery, unless the latter
consists of an immense number of elements, such as the famous water
battery of the late respected Mr. Crosse, which consisted of two
thousand five hundred pairs of copper and zinc cylinders, well insulated
on glass stands, and protected from dust and light. If, however, the
feeble intensity current of voltaic electricity, from four or five
elements, is permitted to pass into a coil of a peculiar construction,
fitted with a condenser, and manufactured either by Ruhmkorff of Paris,
or Mr. Hearder of Plymouth, then the most remarkable effects are
producible, which have created quite a new and distinct series of
phenomena, and further established in the most satisfactory manner the
connexion between the electricities derived from _friction_ and
_chemical action_.

The construction of these coils does not differ very materially, and
great merit is due to Messrs. Ruhmkorff, Hearder, and Bentley, who have
separately and independently worked out the construction of the most
formidable machines of this class. In a letter to the author Mr. Bentley
says:--

"I commence the formation of my coil by using as an axis an iron tube
ten inches long and half an inch diameter; around this is placed a
considerable number of insulated iron wires the same length as the tube,
and sufficiently numerous to form a bundle one inch and three quarters
diameter. This core is wrapped carefully in eight or nine layers of
waxed silk, the necessity of which will be obvious presently.

"My primary helix, which is formed of thirty yards of No. 14
cotton-covered copper wire, is wound carefully on this core, and
consists of two layers, each layer being carefully insulated one from
the other by waxed silk, for I find that if a wet string or fine
platinum wire be connected with the two ends of the primary wires of an
induction coil in action, there is scarcely an indication of an induced
current to be obtained from the secondary wire. That this is not owing
to any decrease of magnetic power is proved by testing the iron core
before and after the experiment, but is simply owing to the central
magnet or coil exerting the whole of its inductive powers upon the
nearest closed circuit; it therefore follows that if the two layers of
primary wire are connected by the cotton covering becoming moist, the
whole of the [Page 231] induced current will take this path instead of
traversing the secondary wire.

"Before describing my secondary wire I must again call attention to the
important fact that the magnetism of the iron exerts its inductive power
upon the nearest conducting medium; and I have constructed an instrument
to demonstrate this fact. It consists simply of an ordinary coil, giving
the third of an inch spark, but having the four inner layers of
secondary wire brought out separately. Now, I find that when I keep the
ends of this wire separate I obtain nearly the third of an inch spark,
but when I connect them metallically I can obtain no intensity spark
whatever from the seventeen coils which surround them.

"It follows from this that before winding the secondary wire the
striking distance of a single layer must be ascertained, and I find that
with my coil I can get a spark one-tenth of an inch long from one coil
of wire, and sufficiently intense to penetrate with facility six layers
of waxed silk.

"Waxed silk is therefore unsuited for the insulation of large coils, and
I find, after numerous experiments, that there is no substance so fitted
for the purpose as gutta-percha tissue, and I use five layers of this
substance to each layer of wire.

"The secondary helix then consists of three thousand yards of No. 35
silk-covered copper wire, and is insulated in the manner described
above; but as I do not use cheeks to my coil it assumes the form of a
cylinder having rounded ends.

"For the protection of this instrument I place it in a mahogany box of
the proper size, and it is supported and retained in its position by an
iron rod, which is thrust through the hollow axis of the core and the
two ends of the box, leaving half an inch of the iron projecting to work
the contact breaker, which is fixed to one end of the box, while the two
ends of the secondary wire are brought out of the other through gutta
percha tubes.

"The condenser is contained in a separate box, and is formed of one
hundred and twenty sheets of tinfoil between double that number of
sheets of varnished paper, the alternate sides of the foil being brought
out and connected to appropriate binding screws.

"This condenser forms a convenient stand for the coil, and can be used
for many interesting experiments."

The shock which the condenser gives to the system depends in a great
measure on the size of the coatings. The primary wire alone does not
produce any physiological results, or at least very feeble ones. Mr.
Hearder's coil is wound on a bobbin six inches in length, and four
inches and a half thick, and includes three thousand yards of covered
wire (No. 35). The iron core consists of a bundle of small wires capped
with solid ends, and the sparks obtained from it were five-eighths of an
inch in air when the primary coil was excited by four pairs of Grove's
series; and when connected with the Leyden jar, the most vigorous and
brilliant results were produced. The condenser is made of cartridge
paper, coated in the proper manner with tinfoil. The secondary [Page
232] coil is quite independent of the primary one, which is laid on in
different lengths, so that the coil can be adjusted to any battery
power, whether for quantity or intensity.

For the successful exhibition of the capabilities of the machine, it is
required to perform the experiments in a darkened room. (Fig. 221.)

[Illustration: Fig. 221. Ruhmkorff's apparatus. A B. The coil,
containing more than a mile of insulated wire. The stand it rests upon,
and with which it is in communication, contains the _condenser_.]

In using this apparatus, eight pairs of Grove's battery will be quite
sufficient to produce the effects, and the greatest care must be taken
to avoid the shock, which is most severe and painful, and might do a
great deal of harm to a weakly, sensitive, and nervous person. To avoid
any accidents of this kind, the convenient arrangement at one end shown
in Fig. 222 must be carefully attended to, and when manipulating with
any part of the apparatus, if the battery is attached, the contact
should first be broken by bringing the ivory (the non-conducting) part
of the cylinder A (Fig. 222) in communication with the conductors, B B,
where the wires from the battery are attached.

[Illustration: Fig. 222. One end of Ruhmkorff's coil. B B. Connexion to
receive the battery wires. A is the cylinder, one half of which is ivory
and the other metal. In this position no shock can be received, because
the electricity is cut off by the ivory from the coil.]


_First Experiment._

It is at the other extremity of the coil that the experiments are
performed; for instance, if an exhausted globe is connected with the
pillars B B (Fig. 223), and the connexion made with the battery, a
beautiful faint blue light is apparent on one of the knobs and wires,
and by reversing the current the light appears on the other knob and
wire. [Page 233] This effect is supposed to resemble some of those
magnificent streaks and undulations of coloured light called the Aurora
Borealis; and, if the globe is removed from the foot, and screwed on to
the air-pump plate, and a little alcohol, ether, naphtha, or turpentine
placed on wool or tow is held to the air-pump screw, where the air
usually rushes in, and the cock turned, so that the vacuum is destroyed,
a quantity of the vapour will necessarily fill the globe; and if this is
once more exhausted, it presents a different appearance, being full of
coloured light (varying according to the spirit employed) but stratified
and of a circular form. (Fig. 223.)

[Illustration: Fig. 223. End of coil where the experiments are
performed. B B. Connecting screws and wires passing to the exhausted
globe, C. The screws are supported on insulating glass pillars, P P.]


_Second Experiment._

The appearance of these bands of light is modified by the nature of the
glass tubes employed, and the subject has been carefully investigated by
Mr. Gassiott. At the last meeting of the British Association at
Aberdeen, Dr. Robinson made various experiments, arranged by Mr. Ladd,
for the purpose of showing the connexion between these miniature effects
of bands of light in tubes containing various gases, and the phenomena
of the Aurora Borealis. The title of the discourse, which was specially
delivered in the Music Hall by the learned Doctor, was "On Electrical
Discharges in Highly-rarefied Media," and it was illustrated by
experiments prepared by Mr. Gassiott and Mr. Ladd.

The kind of tubes employed may be understood from the next figure. They
are made in Germany, and by approaching a powerful magnet to [Page 234]
the outside of any of the glass tubes whilst the bands of light are
being produced, the most remarkable modifications of them are obtained.
Mr. Ladd has mounted one of these tubes in a rotatory arrangement
similar to that described at page 186. When connected with the coil and
battery, it furnishes one of the most lovely "electric fire-wheels" that
can possibly be described. (Fig. 224.) Mr. Grove placed a piece of
carefully-dried phosphorus in a little metallic cup, and covered it with
a jar having a cap and wire. On removing the air from the receiver, and
passing the current of electricity through it from the Ruhmkorff coil,
he obtained a light completely stratified, and blended transversely with
straight but vibrating dark bands.

[Illustration: Fig. 224. A, B, C, D, E, F. Various tubes of different
kinds of glass, and containing gases and vapours. Each tube has a
platinum wire inserted at both ends, with which the contact is made with
the coil. The tube A contains mercury, which has been boiled in it, and
the air expelled. By moving the conducting wire to G or H, the light
which otherwise passes through the whole of the tubes stops at these
points.]


_Third Experiment._

When two very thin iron wires are arranged in the upright pillars (Fig.
223), and held sufficiently close to each other, as in Fig. 225, light
passes from one to the other. The wire from which the light passes
remains _cold_, the other becomes so _hot_ that it melts into a little
globule of liquid iron, and if paper is held between the wires it
rapidly takes fire. (Fig. 225.)

[Illustration: Fig. 225. Melting of the iron wire.]

[Page 235]


_Fourth Experiment._

Remove the break. Attach two wires to X X (Fig. 226). Hold them so as at
pleasure to complete and interrupt the galvanic circle. Two other wires
are attached at P P, their ends being about three-quarters of an inch
asunder. When the current is closed or broken at A A, a spark passes
between B B. (Fig. 226.)

[Illustration: Fig. 226. The making and breaking of the circuit.]


_Fifth Experiment._

A Leyden jar may be charged and discharged with singular rapidity when
connected with the coil, and the snapping noise is so rapid, that it
produces a continuous sharp sound. (Fig. 227.) If a piece of paper is
held between the ball of the Leyden jar and the wire, it is instantly
perforated, but not set on fire.

[Illustration: Fig. 227. A B. Leyden jar coated with tinfoil, and
standing on any non-conductor, such as gutta percha or the resinous or
glass plate, C.]

[Page 236]


_Sixth Experiment._

When the Leyden jar is coated with spangles of tinfoil, a spark appears
at each break, and the whole jar is lit up with hundreds of brilliant
sparks each time it is charged and discharged, and as this occurs with
amazing rapidity, the light is almost continuous. (No. 1. Fig. 228.) The
larger the Leyden jar, the shorter the spark, and _vice versâ_. By the
employment of a nicely-made screw and inch-scale, the distance between
the discharging points connected with a Leyden jar can be accurately
determined; and Mr. Hearder states that supposing a Leyden jar has one
square foot of charging surface, it will give a spark of one inch in
length, but if a smaller jar is used, with only half a square foot of
charging surface, the spark would be about one inch and a quarter in
length. (Fig. 228.)

[Illustration: Fig. 228.--No. 1. Spangled Leyden jar. No. 2. Hoarder's
apparatus for measuring the length of spark for Leyden jar and coil. P
P. Glass pillars. No. 3. Two best forms of spangles to paste on a Leyden
jar.]


_Seventh Experiment._

The direction and rapidity of the current appear to influence greatly
the heating and fire-giving power of the coil, and the following
experiment, devised by Mr. Hearder, furnishes a curious illustration of
this fact.

When the current passes in the direction of the arrows (Fig. 229), the
[Page 237] platinum wire remains perfectly cool whilst the gunpowder is
fired; and the contrary takes place if the current is reversed--viz.,
the gunpowder does not blow up, but the platinum wire is heated. In the
second experiment, a Leyden jar is included in the circuit. (Fig. 229.)

[Illustration: Fig. 229. A. The coil. B. Hearder's discharger, with thin
platinum wire, P, hanging between the points. C. Another discharger, and
powder going off between the points from the little table. The pillars
of the dischargers are glass. The arrows show the direction of the
current of electricity.]


_Eighth Experiment._

Amongst so many beautiful experiments, it is somewhat difficult to say
which is the most pleasing, but for softness and exquisite colouring,
with the continuous vibrating motion of the flowing current of
electricity, nothing can surpass "the cascade experiment." [This
beautiful experiment is usually termed "Gassiott's Cascade," and is thus
described by that gentleman. Two-thirds of a beaker glass, four inches
deep by two inches, are coated with tinfoil, leaving one inch and a half
of the upper part uncoated. On the plate of an air-pump is placed a
glass plate, and over it the beaker, covering the whole with an
open-mouthed glass receiver, on which is placed a brass plate having a
thick wire passing through a collar of leather; the portion of the wire
within the receiver is covered with a glass tube; one end of the
secondary coil is attached to this wire, and the other to the plate of
the pump. As the vacuum improves the effect is very surprising; at first
a faint clear blue light appears to proceed from the lower part of the
beaker to the plate; this gradually becomes brighter, until by slow
degrees it rises, increasing in brilliancy until it arrives at that part
which is opposite, or on a line with the inner coating, the whole being
intensely illuminated; a discharge then commences, as if the electric
fluid were itself a material body running over.] This result is obtained
by coating the inside of a handsome glass goblet with tinfoil, and
placing it under a jar fitted with a collar of leather and ball, and
arranged in the usual manner on the air-pump. Directly a vacuum is
obtained, the ball is moved down to the inside of the goblet, and the
wires from the coil being attached, a continuous series of streams of
[Page 238] electric light seem to overflow the goblet all round the
edge, and it stands then the very embodiment of the brimming cup of
_fire_, and emblematical of the dangers of the wine-cup. (Fig. 230.)

[Illustration: Fig. 230. Gassiott's Cascade.]


_Ninth Experiment._

If a piece of wood five inches long and half an inch square is placed on
the table of the discharger, and one wire brought on to the top edge and
the other approached to within three inches of it, and touching the
wood, and the space between them moistened with the strongest nitric
acid, a curious effect is visible from the creeping along of the fire,
which gradually carbonizes and burns the wood. (Fig. 231.)

[Illustration: Fig. 231. Burning the piece of wood moistened with the
strongest nitric acid.]

[Page 239]


_Tenth Experiment._

A glass plate wetted with gum, and then sprinkled with various filings
of iron, zinc, lead, copper, &c., produces a very pretty effect of
deflagration as one of the conducting wires is moved over its surface,
the other of course being in contact with the plate. The gum quickly
dries by putting the plate in a moderately-heated oven.


_Eleventh Experiment._

When the continuous discharges from the Leyden jar are made to pass
through the centre of a large lump of crystal of alum, blue vitriol, or
ferroprussiate of potash, &c., the whole of the crystal is beautifully
lighted up during the passage of the electricity from one wire of the
discharger to the other. (Fig. 232.)

[Illustration: Fig. 232. A. The Leyden jar. B. Large lump of alum, with
a hole bored through it in a line with C D. The discharging wires are
brought within three-eighths of an inch of each other, and the whole
crystal is lighted up with the brilliant electric sparks.]


_Twelfth Experiment._

When a piece of paper slightly damped is placed between the wires of the
discharger, the spark is increased to a much greater length, on account
of the conducting power of the water contained in the pores of the
paper; and taking all things into consideration, the author considers he
has witnessed the grandest effects from the coil invented and
constructed by Mr. Hearder, the talented lecturer and electrician of the
West of England.


_Thirteenth Experiment._

Electro-magnetic coil machines have been employed for a very
considerable time in alleviating certain of "the ills which flesh is
heir to," [Page 240] by the administration of shocks. These may be so
regulated as to be hardly perceptible, or may be so powerful that the
pain becomes absolutely intolerable.

These coils are now made self-acting, and consist of two coils of
covered and insulated wire wound round a bundle of soft-iron wires, with
the necessary connecting screws for the voltaic battery. The contact
with the battery is made and broken with great rapidity by a simple form
of break, consisting of a tinned disc of iron held by a spring over the
axis of the bundle of iron wires; and the continual noise of the break,
which is alternately attracted down to the bundle and brought back by
the spring, when the coil is in contact with the battery, demonstrates
(without the pain of taking the shock) when the instrument is in full
working order.

The coil machine is not only useful in a medical point of view, but when
properly arranged offers a good reception to a run-away bellringer, and
is an excellent preventive against illicit attempts at cheap rides by
small boys.

[Illustration: Fig. 233. Boy, _evidently shocked_, behind doctor's
carriage provided with a small coil machine.]

[Page 241]




CHAPTER XIX.

MAGNETO-ELECTRICITY.


[Illustration: Fig. 234. Clarke's magneto-electrical machine.]

The correlation of the physical forces, heat, light, electricity,
magnetism, and motion, is one of the most interesting subjects for study
that can be suggested to the lover of science. The examination of the
precise meaning of the term correlation, so ably considered by Professor
Grove, indicates a necessary mutual or reciprocal dependence of one
force on the other. Thus, electricity will produce heat, and _vice
versâ_; motion, such as friction, produces electricity, and the latter,
by its attraction and repulsion, establishes itself as a source of
motion. Electricity produces light, also magnetism, and contrariwise
light is said to possess [Page 242] the power of magnetizing steel,
whilst magnetism again produces light and electricity. Such are the
intimate connexions that exist between these imponderable agents, and we
may trace cause and effect and its reversal amongst these forces, until
the mind is lost in the examination of the bewildering mazes, and is
content to return to the beaten track and work out experimentally the
practical truths. We have had occasion to notice in another part of this
playbook the fact that a current of electricity causes the evolution of
magnetism in its passage through various conducting media, and the truth
has been specially illustrated by the various experiments in the chapter
devoted to electro-magnetism. In commencing this portion of electrical
science, we have no new terms to coin for the title of the discourse, as
we merely reverse the other when we examine the nature and peculiarities
of


MAGNETO-ELECTRICITY.

The source of the power must necessarily be a bar or horse-shoe shaped
piece of steel permanently endowed with magnetism. If the former is
thrust into a cylinder of wood or pasteboard, around which coils of
covered copper wire have been carefully wound, so that the extremities
communicate with a galvanometer, an immediate deflection of the needle
occurs, which, however, quickly returns to its first position, but is
again deflected in the opposite direction on the withdrawal of the steel
magnet from the coil of copper wire. (Fig. 235.)

[Illustration: Fig. 235. A B. Coil of copper wire. C. Permanent bar
magnet placed inside the coil, when the galvanometer needle, D, is
deflected.]

The rapid entrance and exit of the steel magnet in the helix of copper
wire would be insufficient to produce any quantity of electricity, and
the ingenuity of man has been taxed to arrange a method by which a
magnet may be suddenly formed and destroyed inside a coil of insulated
copper wire. The difficulty, however, has been surmounted by several
ingenious contrivances, based on the principles first discovered by
Faraday; and the one especially to be noticed is the revolution of a
coil of copper wire enclosing a piece of soft iron, called the
_armature_, before the poles of a powerful magnet. The first machine was
invented [Page 243] by M. Hypolyte Pixii, of Paris, and in 1833, Mr.
Saxton improved upon this machine, and three years afterwards, Mr. E. M.
Clarke described a very ingenious modification of the electro-magnetic
machine, which is depicted at page 241 of this chapter. In this picture,
the letter A is the permanent fixed horse-shoe magnets, which are very
appropriately termed the _battery_ magnets, because they take the
position that would otherwise be occupied by a voltaic battery, and they
are indeed the prime source of the electrical power that is evoked. D is
the intensity _armature_ which screws into a brass mandril seated
between the poles of the magnets A, motion being communicated to it by
the multiplying wheel, E. This armature or _inductor_ has two coils of
fine insulated copper wire of 1500 yards in length, coiled on its
cylinders, the commencement of each coil being soldered to the bar D,
from which projects a brass stem, also soldered into D, carrying the
break-piece H, which is made fast in any position by a small
binding-screw in a hollow brass cylinder to which the other terminations
of the coils, F F, are soldered, these being insulated by a piece of
hard wood attached to the brass stem. O is an iron wire spring pressing
against one end of the hollow brass cylinder; P is a square brass
pillar; Q is a metal spring that rubs gently on the break-piece H; T is
a copper wire for connecting the brass pieces with the wood L between
them, and out of which P and O pass; R R are two handles of brass with
metallic wires, the end of one being inserted into either of the brass
pieces connected with P and O, and the other into the brass stem that
carries the break-piece H, delivers a most severe shock directly the
wheel is set in motion.

In Saxton's electro-magnetic machine, the permanent steel magnets are
placed horizontally instead of perpendicularly, and are composed of six
or more horse-shoe-shaped pieces of steel. The armatures, or inductors,
or electro-magnets (for they consist of pieces of soft round iron with
wire wound round them), are two in number, and are adapted to exhibit
either _quantity_ or _intensity_ effects. The quantity armature is
constructed of stout iron, and covered with thick insulating wire. The
intensity armature is made of slighter iron, and covered with from one
thousand to two thousand yards of fine copper wire coated with silk. The
_quantity_ armature is intended for the exhibition of results similar to
those which are procurable from a voltaic battery, such as the magnetic
spark, inducing magnetism in soft iron, heating platinum wire. The
intensity armature is employed for the chemical decomposition of water
and other bodies, and likewise for the administration of those terrible
blows to the nervous system which cause strong men of the mildest
deportment to become painfully excited, and to make those ejaculations
which are so peculiar to the genus John Bull.


EXPERIMENTS WITH THE MAGNETO-ELECTRIC MACHINE.


_First Experiment._

The decomposition of water by the passage of electricity from one
platinum plate to another, has already been illustrated at page 198.
The [Page 244] same fact may likewise be displayed by the following
arrangement of the machine. (Fig. 236.)

[Illustration: Fig. 236. A. Apparatus for decomposing water and
collecting the gases separately. B B. Wires proceeding from the machine
at M, N. Q, works on the single break, H.]


_Second Experiment._

The electric light obtained by the passage of the electricity from the
battery through the charcoal points, is also an effect that can be
produced by magneto-electric machines, the wires leading from the points
A B being insulated by glass handles, and placed in the holes M N. (Fig.
237.)

[Illustration: Fig. 237. The electric light obtained from the magneto
machine.]

[Page 245]


_Third Experiment._

The scintillation of iron wire is one of the most pleasing experiments
with this apparatus, and is performed by pressing gently one end of a
piece of thin iron wire (attached by means of a binding-screw to the
upright bar A) against the armature, D. (Fig. 238.)

[Illustration: Fig. 238. Deflagration of iron wire.]


_Fourth Experiment._

The combustion of ether or other inflammable spirit may also be
demonstrated with the aid of this powerful apparatus, and the
arrangement, in common with the others employed by Mr. Clarke, is shown
in Fig. 239.

[Illustration: Fig. 239. The break is removed, and the double blades, B,
fixed in its place. The brass cup, A, containing mercury is so adjusted
that the points will leave the surface of the mercury when the armature
is vertical. Ether or alcohol poured on the surface is quickly inflamed
by the electric spark.]

With the assistance of the magneto-electric machine, telegraphic
communication may be conducted without the assistance of a battery. It
has also been applied to the art of electro-plating by Mr. J. P.
Woolrich, of Birmingham; and whilst visiting that place, the author had
the opportunity of witnessing the arrangement employed.

It consists of a very powerful magneto-electric machine turned by a
steam-engine, and connected with the large troughs containing the
silvering solution. If it is required to deposit a thin coating of
silver on the article, a short period suffices for the action of the
machine, whilst a thick deposit of the precious metal is only obtained
by the constant operation of the magnets for several hours. At Mr.
Woolrich's factory, the goods which were being coated with silver were
all kept in motion, moving slowly backwards and forwards in the trough
by means of an eccentric connected [Page 246] with the same
steam-engine that worked the electro-magnetic machine. (Fig. 240.)

[Illustration: Fig. 240. Silvering and plating by the magneto machine,
turned by a steam-engine.]

The magneto-electric telegraph patented by Mr. Henley in 1848, offers
another example of the application of the electric current induced in
electro-magnetic coils, when they rotate in close proximity to the poles
of a powerful steel magnet. This telegraph is now in constant use by the
English and Irish Magnetic Telegraph Company, through a distance of more
than 2100 miles. The whole length of wires in use amounts to the
astonishing quantity of 13,900 miles, of which 6350 miles are hidden
underground, and 7500 conducted above.

This telegraph is considered to be one of the simplest and most
economical yet brought into practical working.

[Page 247]




CHAPTER XX.

DIA-MAGNETISM.


At the end of the chapter devoted to the subject of light, will be found
an experiment devised and carried out by Dr. Faraday, in which it is
shown that if a bar of a peculiar glass (called after the inventor,
_Faraday's heavy glass_, or silicated borate of lead) is subjected to
the inductive action of a very powerful electro-magnet, that it has the
power of changing the direction of a ray of polarized light transmitted
through it. This effect is not confined to the poles of an
electro-magnet, but is also perceptible (though in a diminished degree)
with ordinary magnets.

The result of this important experiment was communicated to the Royal
Society by Dr. Faraday on the 27th November, 1845, the enunciation of
the fact by this learned philosopher being, "that when '_the line of
magnetic force_' is made to pass through certain transparent bodies
parallel to a ray of polarized light traversing the same body, the ray
of polarized light experiences a rotation." Now, "_the line of magnetic
force_" means that continual flow of the magnetic current which passes
from pole to pole, and is indicated by iron filings sprinkled on paper
placed above the poles of a magnet, and usually termed _magnetic
curves_, or the curved lines of magnetic force. (Fig. 241.)

[Illustration: Fig. 241. The curved lines of magnetic force.]

The heavy glass already alluded to, upon which the magnet exerts a
certain influence, is called


THE DIA-MAGNETIC;

and by this term is meant a body through which the lines of magnetic
force are passing without affecting it like iron or steel. At page 212
is a picture representing (at Figs. 201 and 202) the direction of the
electricity and that of the magnetic current or whirl at right angles to
it. If, then, Fig. 202 be considered as a piece of glass, the arrow A B
[Page 248] will show "the line of magnetic force," the point B being the
north pole, and the shaft A the south pole of the magnet, and the arrows
traced round will represent direction. This simple drawing expresses the
whole of the law of the action of the magnet on the glass, and if kept
in view, will give every position and consequence of direction resulting
from it.

The phenomenon of the affection of the beam of polarized light is
immediately connected with the magnetic force, and this is supposed to
be proved by the _brightness_ of the polarized ray being developed
_gradually_, as the iron coiled with wire requires about two seconds to
acquire its greatest power after being connected with the battery.

In another experiment of Faraday's, where a beam of polarized light was
sent through a long glass tube containing water, and introduced as a
core _inside_ a powerful electro-magnetic coil, the image of a candle
viewed with a proper eye-piece, appeared or disappeared as the battery
connexion was made or broken with the coil; but this result is not
considered by many philosophers to be conclusive of the action of
magnetism on light, but rather as an alteration of the _refracting_
power of the medium through which the light passes. These experiments
were the precursors of the other effects of magnetism upon different
kinds of matter which Faraday discovered, and he commenced his
examination with a small bar of heavy glass suspended by a filament of
silk between the poles of an electro-magnet, and when the twisting or
effects of torsion had ceased, the battery was connected. Directly the
current passed, Faraday's keen eye detected a movement of the glass, and
on repeating the experiment, he discovered that the movement was not
accidental, but always took place in a certain fixed direction--viz., a
direction at right angles to a line drawn across and touching the two
poles of a horse-shoe-shaped magnet--_i.e._, supposing the feeder or bit
of soft iron usually placed in contact with the poles of the
horse-shoe-magnet to represent the "_axial line_," any line drawn across
it at right angles would be called the _equatorial line_, whilst the
general space included between the poles of the magnet is called "the
_magnetic field_." The movement of the heavy glass was therefore
_equatorial_, and it pointed east and west instead of north and south,
like iron and steel.

[Illustration: Fig. 242. A cube of copper suspended between the poles of
a powerful electro-magnet.]

By the use of the apparatus (Fig. 242) Faraday proved that every [Page
249] substance, whether solid, fluid, or gaseous, was subject to
magnetic influences, assuming either the axial or equatorial position.
The apparatus consists of a prolongation of the poles of a powerful
electro-magnet, between which _the_ cube of copper, weighing from a
quarter to half a pound, suspended by a thread, may be set spinning or
rotating. If the electro-magnet is connected with the battery, the cube
stops immediately, and whilst still in the same position or in the
_magnetic field_, with the magnet in full action, it is impossible to
set it spinning or twisting round again. (Fig. 242.)

A large number of other substances, solid, liquid, and gaseous, were
submitted to the action of the magnet, the liquids and gases being
hermetically sealed in glass tubes, and some of the results are detailed
in the following list:

_Bodies that point axially, or are paramagnetic, like a suspended
needle._

  Iron.
  Nickel.
  Cobalt.
  Manganese.
  Chromium.
  Cerium.
  Titanium.
  Palladium.
  Platinum.
  Osmium.
  Paper.
  Sealing-wax.
  Fluor spar.
  _Peroxide of lead._
  Plumbago.
  China ink.
  Berlin Porcelain.
  _Red-lead._
  Sulphate of zinc.
  Shell-lac.
  Silkworm-gut.
  Asbestos.
  Vermilion.
  Tourmaline.
  Charcoal.
  All salts of iron, when the latter is basic.
  Oxide of titanium.
  Oxide of chromium.
  Chromic acid.
  Salts of manganese.
  Salts of chromium.
  Oxygen, which stands alone as a paramagnetic gas.

_Bodies that point equatorially, or are diamagnetic, like Faraday's
heavy glass._

  Bismuth.
  Antimony.
  Zinc.
  Tin.
  Cadmium.
  Sodium.
  Mercury.
  _Lead._
  Silver.
  Copper.
  Gold.
  Arsenic.
  Uranium.
  Rhodium.
  Iridium.
  Tungsten.
  Rock crystal.
  The mineral acids.
  Alum.
  Glass.
  _Litharge._
  Common salt.
  Nitre.
  Phosphorus.
  Sulphur.  [Page 250]
  Resin.
  Spermaceti.
  Iceland spar.
  Tartaric acid.
  Citric acid.
  Water.
  Alcohol.
  Ether.
  Sugar.
  Starch.
  Gum-arabic.
  Wood.
  Ivory.
  Dried mutton.
  Fresh beef.
  Dried beef.
  Apple.
  Bread.
  Leather.
  Fresh blood.
  Dried blood.
  Caoutchouc.
  Jet.
  Turpentine.
  Olive oil.
  Hydrogen.
  Carbonic acid.
  Carbonic oxide.
  Nitrous oxide (moderately).
  Nitric oxide (very slightly).
  Olefiant gas.
  Coal gas.

Nitrogen is neither paramagnetic nor diamagnetic, and is equivalent to a
vacuum. Magnetically considered, it is like space itself, which may be
considered as zero.

The term _magnetic_ Faraday proposes should be a general one, like that
of _electricity_, and include _all_ the phenomena and effects produced
by the power, and he proposes that bodies magnetic in the sense of iron
should be called _paramagnetic_, so that the division would stand thus:

  Magnetic ... { Paramagnetic,
               { Diamagnetic;

and it is this division which has been observed in the preceding tables.

       *       *       *       *       *

All space above and within the limits of our atmosphere may be regarded
as traversed by lines of force, and amongst others are the lines of
magnetic force which affect bodies, as shown in the table of
paramagnetic and diamagnetic bodies, which have the same relation to
each other as positive and negative, or north and south, in electricity
and magnetism.

The lines of magnetic force are assumed to traverse void space without
change; but when they come in contact with matter of any kind they are
either concentrated upon it or scattered according to the nature of the
matter.

The power which urges bodies to the axial or equatorial lines is not a
central force, but a force differing in character in the axial or radial
directions. If a liquid paramagnetic body were introduced into the field
of force, it would dilate axially, and form a prolate spheroid like a
lemon, while a liquid diamagnetic body would dilate equatorially, and
form an oblate spheroid like an orange. Plücker has demonstrated that if
magnetic solutions are placed in watch glasses across the poles of the
[Page 251] electro-magnet, they are heaped up in a very curious manner.
The poles of the electro-magnet are pieces of soft iron, which may be
drawn away or approached at pleasure, and according as the poles are
nearer or further asunder, the magnetic liquids, such as solution of
iron, are heaped up in one or two directions, as shown at B and C in
Fig. 243.

[Illustration: Fig. 243. Glass dish holding magnetic solution of iron,
and placed in the magnetic field.]

"The diamagnetic power, doubtless," says Faraday, "has its appointed
office, and one which relates to the whole mass of the globe. For though
the amount of the power appears to be feeble, yet, when it is considered
that the crust of the earth is composed of substances of which by far
the greater portion belongs to the diamagnetic class, it must not be too
hastily assumed that their effect is entirely overruled by the action of
the magnetic matters, whilst the great mass of waters and the atmosphere
must exert their diamagnetic action uncontrolled."

Plücker has also announced--what at the time he believed to be true--the
highly interesting and important fact that the optic axis of Iceland or
calcareous spar is repelled by the magnet and placed equatorially--a
fact which Plücker thought true of many other crystals when the magnetic
axis is parallel to the longer crystallographic axis. A piece of
kyanite, which is a mineral composed of sand, clay, often lime, iron,
water, and is used in India, being cut and polished as a gem, and sold
frequently as an inferior kind of sapphire, will, it is said, even under
the influence of the earth's magnetism, arrange itself like a magnetic
needle.

Plücker believed that he had discovered an existing relation between the
forms of the ultimate particles of matter and the magnetic forces, and
he imagined that the results he obtained would lead gradually to the
determination of crystalline form by the magnet. The experiments of
Tyndal and Knoblauch lead, however, to a very opposite series of
conclusions, and by ingeniously powdering the crystals with water, and
making them into a paste, which was afterwards dried and suspended
[Page 252] as a model in "the magnetic field;" also by taking a slice of
apple about as thick as a penny-piece, with some bits of iron wire
through it, in a direction perpendicular to its flat surface, they were
found to set equatorially not by repulsion but by the attraction of the
iron wires; or instead of the iron by placing bismuth wires, the apple
now settled axially, not by attraction but by the repulsion of the
bismuth. Ipecacuanha lozenges, Carlisle biscuits also, suspended in the
magnetic field, exhibited a most striking directive action. The
materials in these two cases were _diamagnetic_; but owing to the
pressure exerted in their formation their largest horizontal dimensions
set from pole to pole, the line of compression being equatorial; and it
is a universal law "_that in diamagnetic bodies the line along which the
density of the mass has been induced by compression sets equatorial, and
in magnetic bodies axial_." Hence they assume, from these and many other
conclusive experiments, that crystallized bodies, such as Iceland spar,
take their position in the magnetic field without reference to the
existence of an "optic axis."

At the conclusion of a brilliant lecture at the Royal Institution by Dr.
Tyndal "On the influence of material aggregation upon the manifestations
of force," in which Plücker's experiments respecting the repulsion of
the optic axis were gracefully discussed and his theory refuted, the
learned doctor said: "This evening's discourse is in some measure
connected with this locality; and thinking thus, I am led to inquire
wherein the true value of a scientific discovery consists? Not in its
immediate results alone, but in the prospect which it opens to
intellectual activity--in the hopes which it excites--in the vigour
which it awakens. The discovery which led to the results brought before
us to-night was of this character. _That_ magnet[E] was the physical
birthplace of these results; and if they possess any value they are to
be regarded as the returning crumbs of that bread which in 1846 was cast
so liberally upon the waters. I rejoice, ladies and gentlemen, in the
opportunity here afforded me of offering my tribute to the _greatest
workman_ of the age, and of laying some of the blossoms of that prolific
tree which he planted at the feet of the great discoverer of
diamagnetism."[F]

[Footnote E: Alluding to a splendid magnet made by Logeman, which was
sent to the Exhibition in Hyde-park in 1851. It could sustain a weight
of 430 pounds, and was purchased by the Royal Institution for Dr.
Faraday.]

[Footnote F: Dr. Faraday.]

It was first observed by Father Bancalari, of Genoa, that when the flame
of a candle is placed between the poles of a magnet it is strongly
repelled. The flames of combustible gases from various sources are
differently affected, both by the nature of the combustible and by the
nearness of the poles. Faraday repeated Bancalari's experiments, and by
a certain arrangement of the poles of this magnet he obtained a powerful
effect in the _magnetic field_, and having the axial line of the
magnetic force horizontal, he found that when the flame of a wax taper
was held near the axial line (but on one side or the other), and about
one-third of the flame rising above the level of the upper surface of
the [Page 253] poles, as soon as the magnetic force was exerted the
flame receded from the axial line, moving equatorially until it took an
inclined position, as if a gentle wind was causing its deflection from
the upright position.

When the flame was placed so as to rise truly across the magnetic axis,
the effect of the magnetism was very curious, and is shown at A, Fig.
244.

On raising the flame a little more the effect of the magnetic force was
to intensify the results already mentioned, and the flame actually
became of a _fish-tailed shape_, as at C, Fig. 244; and when the flame
was raised until about two-thirds of it were above the level of the
axial line, and the poles approached very close, the flame no longer
rose between the poles, but spread out right and left on each side of
the axial line, producing a double flame with two long tongues, as at B,
Fig. 244.

[Illustration: Fig. 244. Effect of magnetism on candle-flame between the
poles of the magnet.]

It was these experiments that led to the important discovery of the
paramagnetic property of oxygen, and proved in a decided manner that
gaseous bodies when heated became more highly diamagnetic. Oxygen, which
(tried in the air) is powerfully magnetic, becomes diamagnetic when
heated. A coil of platinum wire heated by a voltaic current, and placed
beneath the poles of Faraday's apparatus, occasioned a strong upward
current of air; but directly the magnetic action commences the ascending
current divides, and a descending current flows down _between_ the
upward currents.

The discovery, says Silliman, of the highly paramagnetic character of
oxygen gas, and of the neutral character of nitrogen, the two
constituents of air, is justly esteemed a fact of great importance in
studying the phenomena of terrestrial magnetism. We thus see that
one-fifth of the air by volume consists of an element of eminent
magnetic capacity, after the manner of iron, and liable to great
physical changes of density, temperature, &c., and entirely independent
of the solid earth. In this medium hang the magnetic needles used as
tests, and as this magnetic medium is daily heated and cooled by the
sun's rays, its power of [Page 254] transmitting the lines of magnetic
force is then affected, influencing undoubtedly the diurnal changes of
the magnetic needle.

For a complete digest of Faraday's discoveries in diamagnetism the
reader is referred to the second edition of Dr. Noad's comprehensive and
learned work entitled "A Manual of Electricity."

Coming always from the highest walks of philosophy to lower and "_common
things_" one cannot help being reminded of the old-fashioned method of
_drawing up_ a sluggish fire, and the natural query is suggested whether
the poker is to be considered as a weak magnet, and does influence and
draw towards the fire a greater supply of magnetic oxygen gas? (Fig.
245.)

[Illustration: Fig. 245.]

[Illustration: The interior of the optical box at the
Polytechnic--looking towards the screen. The assistants are supposed to
be showing the dissolving views.]

[Page 255]




CHAPTER XXI.

LIGHT, OPTICS, AND OPTICAL INSTRUMENTS.


[Illustration: Fig. 246. "The moon shines bright:--In such a night as
this."--_The Merchant of Venice._]

  "To gild refined gold, to paint the lily,
  To throw a perfume on the violet,
  To smooth the ice, or add another hue
  Unto the rainbow, or with taper light
  To seek the beauteous eye of heaven to garnish,
  Is wasteful and ridiculous excess."


Perfection admits of no addition, and it is just this feeling that might
check the most eloquent speaker or brilliant writer who attempted to
offer in appropriate language, the praises due to that first great
creation of the Almighty, when the Spirit of God moved upon the face of
the waters and said, "Let there be light." If any poet might be
permitted to laud and glorify this transcendant gift, it should be the
inspired Milton; who having enjoyed the blessing of light, and witnessed
the varied and beautiful phenomena that accompany it, could, when
afflicted by blindness, speak rapturously of its creation, in those
sublime strains beginning with--

[Page 256]

  "'Let there be light,' said God, and forthwith light
  Ethereal, first of things, quintessence pure,
  Sprung from the deep: and from her native east
  To journey through the airy gloom began,
  Sphered in a radiant cloud, for yet the sun
  Was not; she in a cloudy tabernacle
  Sojourn'd the while. God saw the light was good,
  And light from darkness by the hemisphere
  Divided: light the day, and darkness night,
  He named."

There cannot be a more glorious theme for the poet, than the vast
utility of light, or a more sublime spectacle, than the varied and
beautiful phenomena that accompany it. Ever since the divine command
went forth, has the sun continued to shine, and to remain, "till time
shall be no more," the great source of light to the world, to be the
means of disclosing to the eye of man all the beautiful and varied hues
of the organic and inorganic world. By the help of light we enjoy the
prismatic colours of the rainbow, the lovely and ever changing and ever
varied tints of the forest trees, the flowers, the birds, and the
insects; the different forms of the clouds, the lovely blue sky, the
refreshing green fields; or even the graceful adornment of "the fair,"
their beautiful dresses of exquisite patterns and colours. Light works
insensibly, and at all seasons, in promoting marvellous chemical
changes, and is now fairly engaged and used for man's industrial
purposes, in the pleasing art of photography; just as heat, electricity,
and magnetism, (all imponderable and invisible agents,) are employed
usefully in other ways.

The sources from whence light is derived are six in number. The first is
the sun, overwhelming us with its size, and destroying life, sometimes,
with his intense heat and light, when the piercing rays are not
obstructed by the friendly clouds and vapours, which temper and mitigate
their intensity, and prevent the too frequent recurrence of that quick
and dire enemy to man, the _coup de soleil_.

The body of the sun is supposed to be a habitable globe like our own,
and the heat and light are possibly thrown out from one of the
atmospheric strata surrounding it. There are probably three of these
strata, the one believed to envelope the body of the sun, and to be
directly in contact with it, is called the _cloudy stratum_; next to,
and above this, is the luminous stratum, and this is supposed to be the
source of heat and light; the third and last envelope is of a
transparent gaseous nature. These ideas have originated from astronomers
who have carefully watched the sun and discovered the presence of
certain black spots called _Maculæ_, which vary in diameter from a few
hundreds of miles to 40 or 50,000 miles and upwards. There is also a
greyish shade surrounding the black spots called the _Penumbra_, and
likewise other spots of a more luminous character termed _Faculæ_;
indeed the whole disc of the sun has a mottled appearance, and is
stippled over with minute shady dots. The cause of this is explained by
supposing that these various spots represent openings or breaks in the
atmospheric strata, through which the black body of the sun is apparent
or other portions of the three strata, just as if a black ball was
covered with red, then with yellow, and finally with blue silk: on
cutting through the blue the yellow is apparent; by snipping out pieces
of the blue and yellow, the red becomes visible; and by slicing away a
portion of the three silk coverings the black ball at last comes into
view. On a similar principle it is [Page 257]supposed that the variety
of spots and eruptions on the sun's face or disc may be explained. The
evolution of light is not, however, confined to the sun, and it emanates
freely from terrestrial matter by mechanical action, either by friction,
or in some cases by mere percussion. Thus the axles of railway carriages
soon become red hot by friction if the oil holes are stopped up; indeed
hot axles are very frequent in railway travelling, and when this
happens, a strong smell of burning oil is apparent, and flames come out
of the axle box. The knife-grinder offers a familiar example of the
production of light by the attrition of iron or steel against his dry
grindstone.

The same result on a much grander scale is produced by the apparatus
invented by the late Jacob Perkins; the combustion of steel ensues under
the action, viz., the friction of a soft iron disc revolving with great
velocity against a file or other convenient piece of hardened steel.
(Fig. 247.)

[Illustration: Fig. 247. Instrument for the combustion of steel.]

The stand has a disc of soft iron fixed upon an axis, which revolves on
two anti-friction wheels of brass. The disc, by means of a belt worked
over a wheel immediately below it, is made to perform 5000 revolutions
per minute. If the hardest file is pressed against the edge of the
revolving disc, the velocity of the latter produces sufficient heat by
the great friction to melt that portion of the file which is brought in
contact with it, whilst some particles of the file are torn away with
violence, and being [Page 258] projected into the air, burn with that
beautiful effect so peculiar to steel. If the experiment is performed in
a darkened room, the periphery of the revolving disc will be observed to
have attained a luminous red heat. Thirty years ago every house was
provided with a "tinder-box" and matches to "strike a light." Since the
advent of prometheans and lucifers, the flint and steel, the tinder, and
the matches dipped in sulphur, have all disappeared, and now the box
might be deposited in any antiquarian museum under the portrait of Guy
Fawkes, and labelled, "an instrument for procuring a light, extensively
used in the early part of the nineteenth century." (Fig. 248.)

[Illustration: Fig. 248. C. The steel. B. The flint. E. The tinder. D.
The matches of the old-fashioned tinder-box, A.]

The rubbing of a piece of wood (hardened by fire, and cut to a point)
against another and softer kind, has been used from time immemorial by
savage nations to evoke heat and light; the wood is revolved in the
fashion of a drill with unerring dexterity by the hands of the savage,
and being surrounded with light chips, and gently aided by the breath,
the latent fire is by great and incessant labour at last procured. How
favourably the modern lucifers compare with these laborious efforts of
barbarous tribes! a child may now procure a light with a chemically
prepared metal, and great merit is due to that person who first devised
a method of mixing together phosphorus and chlorate of potash and so
[Page 259] adjusted these dangerous materials that they are as safe as
the "old tinder-box," and have now become one of our domestic
necessaries. Ignition, or the increase of heat in a solid body, is
another source of light, and is well illustrated in the production of
illuminating power from the combustion of tallow, oil, wax, camphine or
coal gas. The term _ignition_ is derived from the Latin (_ignis_, fire),
and is quite distinct, and has a totally different meaning from that of
_combustion_. If a glass jar is filled with carbonic acid gas, and a
little tray placed in it containing some gun cotton, it will be found
impossible to fire the latter with a lighted taper, _i.e._ by combustion
(_comburo_, to burn), because the gas extinguishes flame which is
dependent on a supply of oxygen; whereas if a copper or other metallic
wire is made red hot or ignited, the carbonic acid has no effect upon
the heat, and the red hot wire being passed through the gas, the gun
cotton is immediately fired.

Flame consists of three parts--viz., of an outer film, which comes
directly in contact with the air, and has little or no luminosity; also
of a second film, where carbon is deposited, and, first by _ignition_,
and finally by combustion, produces the light; and thirdly, of an
interior space containing unburnt gas, which is, as it were, waiting its
turn to reach the external air, and to be consumed in the ordinary
manner. (Fig. 249.)

Chemical action and electricity have been so frequently mentioned in
this work as a source of heat and light, that it will be unnecessary to
do more than to mention them here, whilst phosphorescence (the sixth
source of light) in dead and living matter, a spontaneous production of
light, is well known and exemplified in the "glow-worm," the "fire-fly,"
the luminosity of the water of the ocean, or the decomposing remains of
certain fish, and even of human bodies. Phosphorescence is still more
curiously exemplified by holding a sheet of white paper, a calcined
oyster-shell, or even the hand, in the sun's rays, and then retiring
quickly to a darkened room, when they appear to be luminous, and visible
even after the light has ceased to fall upon them.

[Illustration: Fig. 249. A candle flame. 1. Outer flame. 2. Inner flame,
which is badly supplied with oxygen, and where the carbon is deposited
and _ignited_. 3. The interior, containing unburnt gas.]

For the purpose of examining the temporary phosphorescence of various
bodies, M. Becquerel has invented a most ingenious instrument, called
the "phosphorescope." It [Page 260] consists of a cylinder of wood one
inch in diameter and seven inches long, placed in the angle of a black
box with the electric lamp inside, so that three-fourths of the cylinder
are visible outside, and the remaining fourth exposed to the interior
electric light.

By means of proper wheels the cylinder, covered with any substance (such
as Becquerel's phosphori), is made to revolve 300 times in a second, and
by using this or a lesser velocity, the various phosphori are first
exposed to a powerful light and then brought in view of the spectator
outside the box.

It is understood that light is produced by an emanation of rays from a
luminous body. If a stone is thrown from the hand, an arrow shot from a
bow, or a ball from a cannon, we perfectly understand how either of them
may be propelled a certain distance, and why they may travel through
space; but when we hear that light travels from the sun, which is
ninety-five millions of miles away from the earth, in about seven
minutes and a half, it is interesting to know what is the kind of force
that propels the light through that vast distance, and also what is
supposed to be the nature of the light itself.

There are two theories by which the nature of light, and its propagation
through space, are explained; they are named after the celebrated men
who proposed them, as also from the theoretical mechanism of their
respective modes of propulsion: thus we have the Newtonian or
_corpuscular_ theory of light, and the Huyghenian or _undulatory_
theory; the first named after Sir Isaac Newton, and the second after
Huyghens, another most learned mathematician. Many years before Newton
made his grand discovery of the composition of light in the year 1672,
mathematicians were in favour of the _undulatory_ theory, and it
numbered amongst its supporters not only Huyghens, but Descartes, Hook,
Malebranche, and other learned men. Mankind has always been glad to
follow renowned leaders, it is so much easier, and is in most cases
perhaps the better course, to resign individual opinion when more
learned men than ourselves not only adopt but insist upon the truth of
their theories; and this was the case with the corpuscular theory, which
had been written upon systematically and supported by Empedocles, a
philosopher of Agrigentum in Sicily, who lived some 444 years before the
Christian era, and is said to have been most learned and eloquent; he
maintained that light consisted of particles projected from luminous
bodies, and that vision was performed both by the effect of these
particles on the eye, and by means of a visual influence emitted by the
eye itself. In course of time, and at least 2000 years after this theory
was advanced, philosophers had gradually rejected the corpuscular
theory, until the great Newton, about the middle of the seventeenth
century, advanced as a champion to the rescue, and stamping the
hypothesis with his approval, at once led away the whole army of
philosophers in its favour, so that till about the beginning of the
nineteenth century the whole of the phenomena of light were explained
upon this hypothesis.

The corpuscular theory, reduced to the briefest definition, supposes
light to be really a material agent, and requires the student to
believe [Page 261] that this agent consists of particles so
inconceivably minute that they could not be weighed, and of course do
not gravitate; the corpuscles are supposed to be given out bodily (like
sparks of burning steel from a gerb firework) from the sun, the fixed
stars, and all luminous bodies; to travel with enormous velocity, and
therefore to possess the property of _inertia_; and to excite the
sensation of vision by striking bodily upon the expanded nerve, the
retina, the quasi-mind of the eye. Dr. Young remarks, "that according to
this projectile theory the force employed in the free emission of light
must be about a million million times as great as the force of gravity
at the earth's surface, and it must either act with equal intensity on
all the particles of light, or must impel some of them through a greater
space than others, if its action be more powerful, since the velocity is
the same in all cases--for example, if the projectile force is weaker
with respect to red light than with respect to violet light, it must
continue its action on the red rays to a greater distance than on the
violet rays. There is no instance in nature besides of a simple
projectile moving with a velocity uniform in all cases, whatever may be
its cause; and it is extremely difficult to imagine that such an immense
force of repulsion can reside in all substances capable of becoming
luminous, so that the light of decaying wood, or two pebbles rubbed
together, may be projected precisely with the same velocity as the light
emitted by iron burning in oxygen gas, or by the reservoir of liquid
fire on the surface of the sun." Now one of the most striking
circumstances respecting the propagation of light, is the _uniformity_
of its velocity in the same medium. These and other difficulties in the
application of the corpuscular theory aroused the attention of the late
Dr. Young, and in the year 1801 he again revived and supported the
neglected undulatory theory with such great ability that the attention
of many learned mathematicians was directed to the subject, and now it
may be said that the corpuscular theory is almost, if not entirely,
rejected, whilst the undulatory theory is once more, and deservedly,
used to explain the theory of light, and its propagation through space.
By this hypothesis it is assumed that the whole universe, including the
most minute pores of all matter, whether solid, fluid, or gaseous, are
filled with a highly elastic rare medium of a most attenuated nature,
called _ether_, possessing the property of _inertia_ but not of
gravitation. This _ether_ is not light, but light is produced in it by
the excitation on the part of luminous bodies of a vibratory motion,
similar to the undulation of water that produces waves, or the vibration
of air affording sound. Water set in motion produces waves. Air set in
motion produces waves of sound. Ether, _i.e._ the theoretical ether
pervading all matter, likewise set in motion, produces light. The nature
of a vibratory medium is indeed better understood by reference to that
which we know possesses the ordinary properties of matter--viz., the
air; and by tracing out the analogy between the propagation of sound and
light, the difficulties of the undulatory theory very quickly vanish. To
illustrate vibration it is only necessary to procure a finger glass, and
having supported a little ebony ball attached to a silk thread by a bent
brass [Page 262] wire directly over it, so that the ball may touch
either the outside or the inside of the glass, attention must be
directed to the quiescence of the ball when a violin bow is lightly
moved over the edge of the glass without producing sound, and to the
contrary effect obtained by so moving and pressing the bow that a sharp
sound is emitted, when immediately the little ball is thrown off from
the edge, the repulsive action being continued as long as the sound is
produced by the vibration of the glass. (Fig. 250.)

[Illustration: Fig. 250. A. The finger glass. B. The violin bow. C. The
ebony ball. The dotted ball shows how it is repelled during the
vibration of the glass.]

Here the vibrations are first set up in the glass, and being
communicated to the surrounding air, a sound is produced; if the same
experiment could be performed in a vacuum, the glass might be vibrated,
but not being surrounded with air, no sound would be produced. This fact
is proved by first ringing a bell with proper mechanism fixed under the
receiver placed on the air-pump plate; the sound of the bell is audible
until the pump is put in motion and the receiver gradually exhausted,
when the ringing noise becomes fainter and fainter, until it is
perfectly inaudible. This experiment is made more instructive by
gradually admitting the air again into the exhausted vessel, and at the
same time ringing the bell, when the sound becomes gradually louder,
until it attains its full power. The sun and other luminous bodies may
be compared to the finger glass, and are supposed to be endowed
naturally with a vibratory motion (a sort of perpetual ague), only
instead of the air being set in motion, the _ether_ is supposed to be
thrown into waves, which travel through space, and convey the impression
of light from the luminous object. Another familiar example of an
undulatory medium is shown by throwing a stone into a pool of water; the
former immediately forces down and displaces a certain number of the
particles of the latter, consequently the surrounding molecules of water
are heaped up above their level; by the force of gravitation they again
descend and throw up another wave, this in subsiding raises another,
until the force of the original and loftier [Page 263] wave dies away
at the edge of the pool into the faintest ripples. It must however be
understood that it is not the particles of water first set in motion
that travel and spread out in concentric circles; but the force is
propagated by the rising and falling of each separate particle of water
as it is disturbed by the momentum of the descending wave before it.
When standing at a pier-head, or on a rock against which the sea dashes,
it is usual to hear the observer cry out, if the weather is stormy and
the waves very high, "Oh! here comes a great wave!" as if the water
travelled bodily from the spot where it was first noticed, whereas it is
simply the force that travels, and is exerted finally on the water
nearest the rock. It is in fact a progressive action, just as the wind
sweeps over a wide field of corn, and bends down the ears one after the
other, giving them for the time the appearance of waves. The principle
of successive action is well shown by placing a number of billiard balls
in a row, and touching each other; if the first is struck the motion is
communicated through the rest, which remain immovable, whilst the last
only flies out of its place. The force travels through all the balls,
which simply act as carriers, their motion is limited, and the last only
changes its position. Progressive movement is also well displayed by
arranging six or eight magnetized needles on points in a row, with all
their north poles in one direction. (Fig. 252.)

[Illustration: Fig. 251. Boy throwing stones into water and producing
circular waves.]

[Illustration: Fig. 252. A B. Series of needles arranged as described.
C. The bar magnet, with the north pole N towards the needles. The dotted
lines show the direction gradually assumed by all the needles,
commencing at D.]

On approaching the north pole of a bar magnet to the same pole of one
[Page 264] end of the series of needles, it is very curious to see them
turn in the opposite direction progressively, one after the other, as
the repulsive power of the bar magnet gradually operates upon the
similar poles in the magnetic needles. The undulations of the waves of
water are also perfectly shown by using the apparatus consisting of the
trough with the glass bottom and screen above it, as described at page
10. The transmission of vibrations from one place to another is also
admirably displayed in Professor Wheatstone's Telephonic Concert (see
page picture), where the musical instruments, as at the Polytechnic,
were placed by the author in the basement, and the vibration only
conducted by wooden rods to the sounding-boards above, so that the music
was laid on like gas or water. These vibrations or undulations in air,
water, and the theoretical ether, have therefore been called waves of
water, waves of sound, and waves of light, just as if three clocks were
made of three different metals, the mechanism would remain the same,
though the material, or in this case the medium, be different in each.

Any increase in the number of vibrations of the air produces acute,
whilst a decrease attends the grave sounds, and when the waves succeed
each other not less than sixteen times in a second, the lowest sound is
produced. Light and colours are supposed to be due to a similar cause,
and in order to produce the red ray, no less than 477 millions of
millions of vibrations must occur in a second of time; the orange, 506;
yellow, 535; green, 577; blue, 622; indigo, 658; violet, 699; and white
light, which is made up of these colours, numbers 541 millions of
millions of undulations in a second.

Although light travels with such amazing rapidity, there is of course a
certain time occupied in its passage through space--there is no such
thing as instantaneity in nature. A certain period of time, however
small, must elapse in the performance of any act whatever, and it has
been proved by a careful observation of the time at which the eclipses
of the satellites of Jupiter are perceived, that light travels at the
rate of 192,500 miles per second, and by the aberration of the fixed
stars, 191,515, the mean of these two sets of observations would
probably afford the correct rate. Such a velocity is, however, somewhat
difficult to appreciate, and therefore, to assist our comprehension of
their great magnitude, Sir J. Herschel has given some very interesting
comparative calculations, and coming from such an authority we can
readily believe them to be correct.

"A cannon-ball moving uniformly at its greatest velocity would require
seventeen years to reach the sun. Light performs the same distance in
about seven minutes and a half.

"The swiftest bird, at its utmost speed, would require nearly three
weeks to make the tour of the earth, supposing it could proceed without
stopping to take food or rest. Light performs the same distance in less
time than is required for a single stroke of its wing."

Dismissing for the present the theory of undulations, it will be
necessary to examine the phenomena of light, regarding it as radiant
matter, without reference to either of the contending theories.

[Page 265]

Light issues from the sun, passes through millions of miles to the
earth, and as it falls upon different substances, a variety of effects
are apparent. There is a certain class of bodies which obstruct the
passage of the rays of light, and where light is not, a shadow is cast,
and the substance producing the shadow is said to be opaque. Wood,
stone, the metals, charcoal, are all examples of opacity; whilst glass,
talc, and horn allow a certain number of the rays to travel through
their particles, and are therefore called transparent. Nature, however,
never indulges in sudden extremes, and as no substance is so opaque as
not (when reduced in thickness) to allow a certain amount of light to
pass through its substance, so, on the other hand, however transparent a
body may be, a greater or lesser number of the rays are always stopped,
and hence opacity and transparency are regarded as two extremes of a
long chain; being connected together by numerous intermediate links,
they pass by insensible gradations the one into the other.

If a gold leaf, which is about the one two-hundredth part of an inch in
thickness, is fixed on a glass plate and held before a light, a green
colour is apparent, the gold appearing like a green, semi-transparent
substance. When plates of glass are laid one above the other, and the
flame of a candle observed through them, the light decreases enormously
as the number of glass-plates are increased. Even in the air a
considerable portion of light is intercepted. It has been estimated that
of the horizontal sunbeams passing through about two hundred miles of
air, one two-thousandth part only reaches us, and that no sensible light
can penetrate more than seven hundred feet deep into the sea;
consequently, the vast depths discovered in laying the Atlantic
telegraph must be in absolute darkness.

[Illustration: Fig. 253.]

Light is thrown out on all sides from a luminous body like the spokes of
a cart-wheel, and in the absence of any obstruction, the rays are
distributed equally on all sides, diverging like the radii drawn from
the centre of a circle. As a natural consequence arising from the
divergence of each ray from the other, the intensity of light decreases
as the distance from the luminous source increases, and _vice versâ_.
Perhaps the best mechanical notion of this law is afforded by an
ordinary fan; the point from which the sticks radiate, and where they
all meet, may be [Page 266] termed the light; the sticks are the rays
proceeding from it. (Fig. 253.)

The fan is held in one hand, and the first finger of the other can be
made to touch all the sticks if placed sufficiently near to A; and
supposing the sticks are called rays of light, the intensity must be
great at that point, because all the rays fall upon it; but if the
finger is removed towards the outer edge--viz., to B, it now only
touches some three or four sticks; and pursuing the analogy, a very few
rays fall upon that point--hence the light has decreased in intensity,
or to speak correctly, "Light decreases inversely as the squares of the
distance." This law has already been illustrated at page 13; and as an
experiment, the rays from the oxy-hydrogen lantern may be permitted to
pass out of a square hole (say two inches square), and should be thrown
on to a transparent screen divided into squares by dark lines, so that
the light at a certain distance illuminates one of them; then it will be
found that at twice the distance, four may be illuminated, at three
times nine, and so on. (Fig. 254.)

[Illustration: Fig. 254. Lantern at the three distances from the
transparent screen, which is divided into nine equal squares.]

Upon this law is based the use of photometers, or instruments for
measuring light, and supposing it was required to estimate roughly the
illuminating power of any lamp, as compared with the light of a wax
candle six to the pound, the experiment should be conducted in a dark
room, from which every other light but that from the lamp and candle
under examination must be excluded.

The lamp, with the chimney only, is now placed say twelve feet from the
wall, and a stick or rod is placed upright and about two inches from the
latter, so that a shadow is cast on the wall; if the candle is now
lighted and allowed to burn up properly, two shadows of the stick will
be apparent, the one from the lamp being black and distinct, and the
other from the candle extremely faint, until it is approached nearer
the [Page 267] wall--say to within three feet--when the two shadows may
be now equal in blackness. (Fig. 255.) After this is apparent to one or
more persons, the distances of the lamp and candle from the wall are
carefully measured, and being squared, and the greater divided by the
lesser number, the quotient gives the illuminating power. For example:

  The lamp was 12 feet from the wall       12 × 12 = 144.
  The candle was 3 feet     "               3 ×  3 =   9.
                            9) 144
                              ----
                                16

Therefore the illuminating power of the lamp is equal to 16 wax candles
six to the pound.

[Illustration: Fig. 255. A. The lamp. B. The candle. C. The rod throwing
the two shadows, marked D and E, on a white wall or a sheet of paper.]

There are other and more refined means of working out the same fact, but
for a rough approximation to the truth, the plan already described will
answer very fairly.

A most amusing effect can be produced on the principle that every light
casts its own shadow, called the "dance of death," or the "dance of the
witches;" either of these agreeable subjects are drawn, and the outlines
cut out of a sheet of cardboard. If a wet sheet is stretched or hung on
one side of a pair of folding doors partly open, and between which the
cardboard is tacked up, and the space left at the top and bottom closed
with a dark cloth, directly the room before the sheet is darkened and a
lighted candle held behind the figure cut out in the cardboard, one
shadow or image is thrown upon the sheet, and these shadows may be
increased according to the number of candles used, and if they are held
by two or three persons, and moved up and down, or sideways, the shadows
follow the direction of the candles, and present the appearance of a
dance. (Fig. 256.)

[Page 268]

[Illustration: Fig. 256. "Before the curtain."]

[Illustration: Fig. 257. "Behind the curtain."]

[Page 269]

Another very comic effect of shadow is that called "jumping up to the
ceiling," and when carried out on a large scale by the author on an
enormous sheet suspended in the centre transept of the Crystal Palace,
Sydenham, it had a most laughable effect, and caused the greatest
amusement to the children of all ages. (Fig. 258.)

[Illustration: Fig. 258. The laughable effect of the shadows at the
Crystal Palace.]

This very telling result is produced by placing an oxy-hydrogen light
some feet behind a large sheet, and of course if any one passes between
the two a shadow of the individual is cast upon the sheet, then by
walking towards the light the figure diminishes in size, and by jumping
over it the shadow appears to go up to the ceiling, and to come down
when the jump is made in the opposite direction over the light and
towards the sheet. The _rationale_ of this experiment is very simple,
and is [Page 270] another proof of the distribution of light from a
luminous source being in every direction. By jumping over the light the
radii projected from the candle over the sheet are crossed, and the
shadow rises or falls as the figure passes upwards or downward. (Fig.
259.)

[Illustration: Fig. 259. The rays of light marked A B C D E proceeding
from a lighted candle or oxy-hydrogen light. The arrow pointing to the
right shows how these rays are crossed in jumping up to the ceiling; and
the second arrow, pointing to the left, shows the reverse.]

A beam of light is defined to be a collection of rays, and it is a
convenient definition, because it prevents confusion to speak only of
one ray in attempting to explain how light is disposed of under peculiar
circumstances.

The smallest portion of light which it is supposed can be separated is
therefore called a ray, and it will pass through any medium of the same
density in a perfectly straight line; but if it passes out of that
medium into another of a different density, or into any other solid,
fluid, or gaseous matter, it may be disposed of in four different ways,
being either reflected, refracted, polarized, or absorbed.

The reflection of light is the first property that will be considered,
and it will be found that every substance in nature possesses in a
greater or lesser degree the power of throwing off the rays of light
which fall upon them. Thus if we go into a room perfectly darkened,
containing every kind of work produced by nature or art, such as
flowers, birds, boxes of insects, rich carpets, hangings, pictures,
statuary, jewellery, &c., they cannot excite any pleasure because they
are invisible, but directly a lighted lamp is brought into the chamber,
then the rays fall upon all the surrounding objects, and being reflected
from their surfaces enter the eye, and there produce the phenomena of
vision.

This connexion between luminous and non-luminous bodies becomes very
apparent when we consider that the sun would appear only as an intense
light in a dark background, if the earth was not surrounded with the
various strata of air, in which are placed clouds and vapours that
collectively reflect and scatter the light, so as to cause it to be
endurable to vision. It is when the sky is very clear during July or
August that the heat becomes so intense, directly clouds begin to form
and float about, the heat is then moderated.

Many years ago, Baron Alexander Funk, visiting some silver mines in
Sweden, observed, that in a clear day it was as dark as pitch
underground in the eye of the pit at sixty or seventy fathoms deep;
whereas, on a cloudy or rainy day he could even see to read at 106
fathoms deep. Inquiring of the miners, he was informed that this is
always the case, and [Page 271] reflecting upon it he imagined very
properly that it arose from this circumstance--that when the atmosphere
is full of clouds, light is reflected from them into the pit in all
directions, so that thereby a considerable proportion of the rays are
reflected perpendicularly upon the earth; whereas when the atmosphere is
clear there are no opaque bodies to reflect the light in this manner, at
least, in a sufficient quantity, and rays from the sun itself can never
fall perpendicularly in Sweden. The use of reflecting surfaces has now
become quite common in all crowded cities, and especially in London,
where even the rays of light are too few to be lost, and flat or
corrugated mirrors are placed at various angles, either to throw the
light from the outside on the white-washed ceiling within, and thus
obtain a better diffused light through the apartment, or it is reflected
bodily to some back room, or rather dark brick box, where perhaps for
half a century candles have been required at an early hour in the
afternoon. The brilliant cut in diamonds is such an arrangement of the
posterior facets, or cut faces of the jewel, that all light reaching
them shall be thrown back and reflected, and thus impart an
extraordinary brilliancy to the gem.

The intense glare of snow in the Alpine regions has long been noticed,
and the reflected light is so powerful, that philosophers were even
disposed to believe that snow possessed a natural or inherent
luminosity, and gave out its own light. Mr. Boyle, however, disproved
this notion by placing a quantity of snow in a room from which all
foreign light was excluded, and neither he nor his companion could
observe that any light was emitted, although, on the principle of
momentary phosphorescence, it is quite possible to conceive that if the
snow was suddenly brought into a darkened room after exposure to the
rays of the sun, that it would give out for a few seconds a perceptible
light. In trying such an experiment, one person should expose the snow
to the sun, and bring it into a perfectly darkened room to a second
person, whose eyes would be ready to receive the faintest impression of
light, and if any phosphorescence existed, it must be apparent.

The property of reflection is also illustrated on a grand scale in the
illumination of our satellite, the moon, and the various planetary
bodies which shine by light reflected from the sun, and have no inherent
self-luminosity. Aristotle was well aware that it is the reflection of
light from the atmosphere which prevents total darkness after the sun
sets, and in places where the sun's rays do not actually fall during the
daytime. He was also of opinion that rainbows, halos, and mock suns,
were all occasioned by the reflection of the sunbeams in different
circumstances, by which an imperfect image of the sun was produced, the
colour only being exhibited, but not the proper figure.

The image, Aristotle says, is not single, as in a mirror, for each drop
of rain is too small to reflect a visible image, but the conjunction of
all the images is visible. Aristotle ascribed all these effects to the
_reflection_ of light, and it will be noticed when we come to the
consideration of the refraction of light, that of course his views must
be seriously modified.

[Page 272]

The reflection of light is affected rather by the condition of the
surface than the whole body of a substance, as a piece of coal may be
covered with gold or silver leaf and caused to shine, whilst the
brightest mirror is dimmed by the thinnest film of moisture.

From whatever surface light is reflected, it always takes place in
obedience to two fixed laws.

First. _The incident and reflected rays always lie in the same plane._

Second. _The angle of incidence is equal to the angle of reflection._

With a single jointed two-foot rule, both of these laws are easily
illustrated. The rule may be held in the hand, and one end being marked
with a piece of white paper may be called the incident ray, _i.e._, the
ray that falls upon the surface; and the other is the reflected ray, the
one cast off or thrown back. A perpendicular is raised by holding a
stick upright at the joint. (Fig. 260.)

[Illustration: Fig. 260. A D. A two foot rule; the end A may be termed
the incident ray, and the end D the reflected ray. S. The stick held
perpendicularly. The angle A B C is equal to the angle D E F, and the
whole may be moved in any direction or plane, either horizontal or
perpendicular, G G. The reflecting surface.]

One of the most simple and pleasing delusions produced by the reflection
of light, is that afforded by cutting through the outline of a vase, or
statuette, or flower, drawn on cardboard, and if certain points are left
attached, so that the design may not fall out, all the effect of
solidity is given by bending back the edges of the cardboard, so that
the light [Page 273] from a candle placed behind it, may be reflected
from the back edge of one cardboard on to the design, which is bent
back. The light reflected from one surface on to the other, imparts a
peculiarly soft and marble-like appearance, and when the design is well
drawn and cut, and placed in a good position, the illusion is very
perfect, and it appears like a solid form instead of a mere design cut
out of cardboard. (Fig. 261.)

[Illustration: Fig. 261. Cardboard design in frame, cut and bent back.
The lighted candle is behind.]

The leaf at the side of the above picture is intended to give an idea of
the mode of cutting out the designs, and in this case the leaf would be
cut and bent back, and a small attachment slip of cardboard left to
prevent it falling out.

The cardboard design is always bent toward the light, which is placed
behind it. As a good illustration of the importance of reflected light
and its connexion with luminous bodies, a beam of light from the
oxy-hydrogen lantern may be allowed to pass above the surface of a
table, when it will be noticed that the latter is lighted up only when
the beam is reflected downward by a sheet of white paper.

By reference to the two laws of reflection already explained, it is easy
to trace out on paper, with the help of compasses and rule, the effect
of plane, concave, and convex surfaces on parallel, diverging, or
converging rays of light, and it may perhaps assist the memory if it is
remembered that a _plane_ surface means one that is flat on both sides,
such as a looking-glass: a _convex_ surface is represented by the
outside of a watch-glass; a _concave_ surface, by the inside of a
watch-glass; parallel [Page 274] rays are like the straight lines in a
copy-book; diverging and converging rays, are like the sticks of a fan
spread out as the sticks separate or diverge; the sticks of the fan come
together, or converge at the handle.

The reflection of rays from a plane surface may be better understood by
reference to the annexed diagram. (Fig. 262.)

[Illustration: Fig. 262. A I, A K. Two diverging rays incident on the
plane surface, D. A D is perpendicular, and is reflected back in the
same direction. A I is divergent, and is thrown off at I L. The incident
and reflected rays forming equal angles, as proved by the perpendicular,
H. Any image reflected in a plane mirror appears as far behind it as the
object is before it, and the dotted lines meeting at G show the apparent
position of the reflected image behind the glass, as seen at G. The same
fact is also shown in the second diagram, where the reflected picture, I
M, appears at the same distance behind the surface of the mirror as the
object, A B, is before it.]

By the proper arrangement of _plane_ mirrors, a number of amusing
delusions may be produced, one of which is sometimes to be met with in
the streets, and is called "the art of looking through a four-inch deal
board." The spectator is first requested to look into a tube, through
which he sees whatever may be passing the instrument at the time; the
operator then places a deal board across the middle of the tube, which
is cut away for that purpose, and to the astonishment of the juveniles
the view is not impaired, and the spectator still fancies he is looking
through a straight tube; this however is not the case, as the deception
is entirely carried out by reflection, and is explained in the next cut.
(Fig. 263.)

[Page 275]

[Illustration: Fig. 263. A A A A. The apertures through which the
spectator first looks. B. The piece of wood, four inches thick. C, D, E,
F, are four pieces of looking-glass, so placed that rays of light
entering at one end of the tube are reflected round to the other where
the eye of the observer is placed.]

During the siege of Sebastopol numbers of our best artillerymen were
continually picked off by the enemy's rifles, as well as by cannon shot,
and in order to put a stop to the foolhardiness and incautiousness of
the men, a very ingenious contrivance was invented by the Rev. Wm.
Taylor, the coadjutor of Mr. Denison in constructing the first "Big Ben"
bell. It was called the reflecting spy-glass, and by its simple
construction rendered the exposure of the sailors and soldiers, who
would look over the parapet or other parts of the works to observe the
effect of their shot, perfectly unnecessary; whilst another form was
constructed for the purpose of allowing the gunner to "lay" or aim his
gun in safety. The instruments were shown to Lord Panmure, who was so
convinced of the importance of the invention, that he immediately
commissioned the Rev. Wm. Taylor to have a number of these telescopes
constructed; and if the siege had not terminated just at the time the
invention was to have been used, no doubt a great saving of the valuable
lives of the skilled artillerymen would have been effected in the allied
armies. The principle of the reflecting spy-glass may be comprehended by
reference to the next cut. (Fig. 264.)

[Illustration: Fig. 264. A picture of enemy's battery is supposed to be
on the mirror, A, whence it is reflected to B, and from that to the
artilleryman at C.]

By placing two mirrors at an angle of 45°, the reflected image of a
person gazing into one is thrown into the other, and of course the
effect is somewhat startling when a death's head and cross bones, or
other [Page 276] cheerful subject, is introduced opposite one mirror,
whilst some person who is unacquainted with the delusion is looking into
the other. Two adjoining rooms might have their looking-glasses arranged
in that manner, provided there is a passage running behind them. (Fig.
265.)

[Illustration: Fig. 265. A. A mirror at an angle of 45 degrees. The
arrows show the direction of the reflected image. B. The second mirror,
also at an angle of 45 degrees; the face of the person looking in at A
is reflected at B. C is the partition between the rooms.]

One of the most startling effects that can be displayed to persons
ignorant of the common laws of the reflection of light, is called the
"magic mirror," and is described by Sir Walter Scott in his graphic
story of that name. The apparatus for the purpose must be well planned
and fixed in a proper room for that purpose, and if carefully conducted,
may surprise even the learned. A long and somewhat narrow room should be
hung with black cloth, and at one end may be placed a large mirror, so
arranged that it will turn on hinges like a door. The magician's circle
may be placed at the other end of the chamber in which the spectators
must be rigidly confined, and there is very little doubt that the
arrangement about to be described was formerly used by clever
astrologers who pretended to look into the future, and to hold
communication with the supernatural powers. The credulity of the persons
who consulted these "wise men," is not surprising when we consider the
ignorance of the public generally of common physical laws, and of the
wonders that may be worked without the assistance of the "evil one;"
moreover, the initiated took great care to conceal the machinery of
their mysteries, never imparting the illusive tricks even to their most
faithful dependents except under solemn oaths of secrecy, because they
derived in many cases considerable profit by their pretended
conjurations and juggling tricks, and therefore were interested in
keeping the outer world in ignorance. The wizards were always careful to
impress those who came to consult them with the awful nature of the
incantations they were about to perform, and with such a powerful
auxiliary as [Page 277] fear, and a well-darkened room, they diverted
the thoughts of the more curious, and prevented them watching the
proceedings too closely. Theatrical effects were not disdained, such as
suppressed and dismal groans, sham thunder, and the wizard usually
heightened his own inspiring personal appearance by wearing of course a
long beard and flowing robe trimmed with hieroglyphics, and with the
assistance of a ponderous volume full of cabalistic signs, a few skulls
and cross bones, an hour-glass, a pair of drawn swords, a black cat, a
charcoal fire, and sundry drugs to throw into it, a very tolerable
collection of imps, familiars, and demons, might be expected to attend
without the modern practice of spirit-rapping. As before stated, the
delusion must be carefully conducted, and a confederate is necessary in
order to use the phantasmagoria, or magic lantern. The slides of course
were painted to suit the fortune to be unfolded--an easy road to riches
for the gentlemen, a tale of love, ending in matrimony, for the ladies.

The spectators being placed in the magic circle, are directed to look
into the mirror; they may even be ordered singly to fetch a skull off
the mantel-shelf beside the mirror, and whilst doing so to look full
into the mirror, and then return to the circle. Absolute silence is
enjoined, and soft music is now heard; the darkened room is lit up for
the moment by a little yellow or green fire thrown on to the charcoal
fire, and now looking into the mirror, it no longer reflects surrounding
objects, but a picture, at first small and faint, and then gradually
becoming large and clearer, is apparent. The picture is made visible by
the confederate gently drawing the mirror from its position parallel
with the frame to an angle of 45 degrees, and then throwing on from the
side a picture from a magic-lantern. The picture is small and indistinct
whilst the confederate holds it near the mirror and out of focus, but as
he moves backwards and focuses the lenses, the picture gradually
increases in size, and the reflecting angles having been well planned
beforehand, only those in the circle will be able to see the picture,
and great fun may be elicited from the magic mirror by pretending to
tell the future fate of a very slim person, and introducing him by a
succession of pictures which gradually assume a John Bull rotundity of
figure, surrounded by dozens of children; whilst to young ladies who are
engaged, a provoking picture of an old maid may be introduced; indeed,
there is no end to the innocent fun that may be extracted from the magic
mirror, and the whole plan of the delusion may be better understood by
reference to the next picture. (Fig. 266.)

Monsieur Salverte very properly remarks that "man is credulous from his
cradle to his tomb; but the disposition springs from an honourable
principle, the consequences of which precipitate him into many errors
and misfortunes.... The novelty of objects, and the difficulty of
referring them to known objects, will not shock the credulity of
unsophisticated men. There are some additional sensations which he
receives without discussion, and their singularity is perhaps a charm
which causes him to receive them with greater pleasure. _Man almost
always_ loves and seeks the marvellous. Is this taste natural?

[Page 278]

[Illustration: The magic mirror.]

[Illustration: Fig. 266. Plan of room. A A. The frame of the
looking-glass. A B. Mirror put back to an angle of 45 degrees. C. The
confederate who manages the lantern and shuts the glass to the frame
after each fortune is told. D. The magic circle, to which the rays are
reflected.]

[Page 279]

Does it spring from the education which during many ages the human race
has received from its first instructors? A vast and novel question, but
with which I have nothing to do. It is sufficient to observe that as the
lover of the wonderful always prefers the most surprising to the most
natural account, this last has been too frequently neglected, and is
irrevocably lost. Occasionally, however (and we shall cite more than one
instance), simple truth has escaped from the power of oblivion.
Credulous man may be deceived once, or more frequently; but his
credulity is not a sufficient instrument to govern his whole existence.
The wonderful excites only a transient admiration. In 1798, the French
_savans_ remarked with surprise how little the spectacle of balloons
affected the indolent Egyptian.... But man is led by his passions, and
particularly by _hope_ and _fear_."

When parallel rays fall upon a convex mirror, they are scattered and
dispersed in all directions, and the image of an object reflected in a
convex mirror appears to be very small, being reduced in size because
the reflected picture I M is nearer the surface of the mirror than the
object A B. No. 1. (Fig. 267.)

[Illustration: Fig. 267. A B, D H. (No. 2) represent two parallel rays
incident on the convex surface B H, the one (A B) perpendicularly, the
other (D H) obliquely. C is the centre of convexity. H E is the
reflected ray of the oblique incident one, D H; whilst C H I is the
perpendicular.]

Convex mirrors are not employed in any optical deception on a large
scale, although some ingenious delusions are producible from cylindrical
and conical mirrors, and are thus described by Sir David Brewster:

"Among the ingenious and beautiful deceptions of the seventeenth
century, we must enumerate that of the re-formation of distorted
pictures by reflection from cylindrical and conical mirrors. In these
representations, the original image from which a perfect picture is
produced, [Page 280] is often so completely distorted, that the eye
cannot trace in it the resemblance to any regular figure, and the
greatest degree of wonder is of course excited, whether the original
image is concealed or exposed to view. These distorted pictures may be
drawn by strict geometrical rules, and I have shown a simple method of
executing them. Let M be an accurate cylinder made of tin-plate or of
thick pasteboard. Out of the further side of it cut a small aperture, _a
b c d_, and out of the nearer side cut a larger one, A B C D (white
letters), the size of the picture to be distorted; having perforated the
outline of the picture with small holes, place it in the opening A B C D
(white letters), so that its surface may be cylindrical; let a candle or
a bright luminous object--the smaller the better--be placed at S, as far
behind the picture A B C D (white letters) as the eye is afterwards to
be placed before it, and the light passing through the small holes will
represent on a horizontal plane a distorted image of the picture at A B
C D, which, when sketched in outline with a pencil, shaded, and
coloured, will be ready for use. If we now substitute a polished
cylindrical mirror of the same size in place of M, then the distorted
picture, when laid horizontally at A B C D, will be restored to its
original state when seen by reflection at A B C D (white letters) in the
polished mirror." The effect of a cylindrical mirror on a distorted
picture is shown at No. 2, being copied from an old one seen by Sir D.
Brewster.

[Illustration: Fig. 268.]

By looking at a reflection of the face in a dish-cover or the common
surface of a bright silver spoon or of a silver mug, the latter truly
becomes ugly as the image is seen reflected from its surface, and [Page
281] assumes the most absurd form as the mouth is opened or shut, and
the face advanced or removed from the silver vessel. (Fig. 269.)

[Illustration: Fig. 269. Distorted image produced by an irregular convex
surface.]

In the writings of the ancients there are to be found certain
indications of the results of illusions produced by simple optical
arrangements, and the sudden and momentary apparition (from the gloom of
perfect darkness) of splendid palaces, delightful gardens, &c., with
which-the concurrent voice of antiquity assures us-the eyes of the
beholders were frequently dazzled in the mysteries, such as the
evocation and actual appearance of departed spirits, the occasional
images of their _umbræ_, and of the gods themselves. From a passage in
"Pausanias," (Boeotic XXX.), when, speaking of Orpheus, he says there
was anciently at Aornos, a place where the dead were evoked, [Greek:
_nekuomanteion_], we learn that in those remote ages there were places set
apart for the evocation of the dead. Homer relates, in the eleventh book
of the "Odyssey," the admission of Ulysses alone into a place of this
kind, when his interview with his departed friend was interrupted by
some fearful voice, and the hero, apprehending the wrath of Proserpine,
withdrew; the priests who managed these deceptive exhibitions no doubt
adopted this method of getting rid of their visitor, who might become
too inquisitive, and discover the secret of the mysteries.

Of all the reflecting surfaces mentioned, none produce more interesting
deceptions than the concave mirror, and there is very little doubt that
silver mirrors of this form were known to the ancients, and employed in
[Page 282] some of their sacred mysteries. Mons. Salverte has
industriously collected in his valuable work the most interesting proofs
of their use, and quotes the following passage of "Damascius," in which
the results obtainable from a concave mirror are clearly apparent. (Fig.
270.)

[Illustration: Fig. 270. The picture of a human face, possibly reflected
from a concave mirror concealed below the floor of the temple; the
opening being hidden by a raised mass of stone, and the worshippers
confined to a certain part of the temple, and not allowed to approach
it.]

He says:--"In a manifestation which ought not to be revealed ... there
appeared on the wall of the temple a mass of light which at first seemed
very remote; it transformed itself in coming nearer into a face
evidently divine and supernatural, of a severe aspect, but mixed with
gentleness, and extremely beautiful. According to the institution of a
mysterious religion, the Alexandrians honoured it as Osiris and Adonis."

Parallel rays thrown upon a concave surface are brought to a focus or
converge, and when an object is seen by reflection from a concave
surface, the representation of it is various, both with regard to its
[Page 283] magnitude and situation, according as the distance of the
object from the reflecting surface is greater or less. (Fig. 271.) When
the object is placed between the _focus_ of parallel rays and the
centre, the image falls on the _opposite_ side of the centre, and is
larger than the object, and in an inverted position. The rays which
proceed from any remote terrestrial object are nearly parallel at the
concave mirror--not strictly so, but come diverging to it in separate
pencils, or, as it were, bundles of rays, from each point of the side of
the object next the mirror; therefore they will not be converged to a
point at the distance of half the radius of the mirror's concavity from
its reflecting surface, but in separate points at a little greater
distance from the concave mirror. The nearer the object is to the
mirror, the further these points will be from it, and an inverted image
of the object will be formed in them, which will seem to hang pendant in
the air, and will be seen by an eye placed beyond it (with regard to the
mirror), in all respects like the object, and as distinct as the object
itself. No. 2. (Fig. 271.)

[Illustration: Fig. 271. No. 1. A B, D H represent two parallel rays
incident on the concave surface B H, whose centre of concavity is C. B F
and H F are the reflected rays meeting each other in F, and A B being
perpendicular to the concave surface, is reflected in a straight line.
No. 2. A B. The object. I M. The image.]

[Illustration: Fig. 272. A B represents the object, S V the reflecting
surface, F its focus of parallel rays, and C its centre. Through A and
B, the extremities of the object, draw the lines C E and C N, which are
perpendicular to the surface, and let A R, A G, be a pencil of rays
flowing from A. These rays proceeding from a point beyond the focus of
parallel rays, will, after reflection, converge towards some point on
the opposite side of the centre, which will fall upon the perpendicular,
B C, produced, but at a greater distance from C than the radiant A from
which they diverged. For the same reason, rays flowing from B will
converge to a point in the perpendicular N C produced, which shall be
further from C than the radiant B, from whence it is evident that the
image I M is larger than the object A B, that it falls on the _contrary_
side of the centre, and that their positions are inverted with respect
to each other.]

[Page 284]

It appears, from a circumstance in the life of Socrates, that the
effects of burning-glasses were known to the ancients; and it is
probable that the Romans employed the concave speculum for the purpose
of lighting the "sacred fire." This is very likely to be true,
considering that the priests who conducted the heathen worship of Osiris
and Adonis were acquainted with the use of concave metallic specula, as
already described at page 282. The effects that can be produced with the
aid of concave mirrors are very impressive, because they are not merely
confined to the reflection of inanimate objects, but life and motion can
be well displayed by them; thus, if a man place himself directly before
a concave mirror, but further from it than its centre of concavity, he
will see an inverted image of himself in the air between him and the
mirror of a less size than himself; and if he hold out his hand towards
the mirror the hand of the image will come out towards his hand and
coincide with it, being of an equal bulk when his hand is in the centre
of concavity, and he will imagine he may shake hands with his image.
(Fig. 273.)

[Illustration: Fig. 273. A concave mirror, showing the appearance of the
inverted and reflected image in the air.]

By using a large concave mirror of about three feet in diameter, the
author was enabled to show all the results to a large audience that were
usually visible to one person only. Whilst experimenting with a concave
mirror, by holding out the hand in the manner described, a bystander
will see nothing of the image, because none of the reflected rays that
form it enter his eyes. This circumstance is well illustrated by placing
a concave mirror opposite the fire, and allowing the image of the flames
projected from it to fall upon a well-polished mahogany table. If the
door of the room opens towards the mirror, and a spectator unacquainted
with the properties of concave mirrors should enter the apartment, the
person would be greatly startled to see flames apparently playing over
the surface of the table, whilst another spectator might enter from
another door and see nothing but a long beam of light, rendered visible
by the floating particles of dust. To give proper effect to this
experiment the concave mirror should be large, and no other light must
illuminate the room except that from the fire.

On the same polished table the appearance of a planet with a revolving
[Page 285] satellite may be prettily shown by darkening the fire with a
screen, and placing a lighted candle before it, which will be reflected
by the concave mirror, and appear on the table as a brilliant star of
light, and the satellite may be represented by the flame of a small wax
taper moved around the large burning candle. The following is the
arrangement used by the author at the Polytechnic Institution for the
purpose of exhibiting the properties of the concave mirror. A lantern
enclosing a very brilliant light, such as the electric or lime light, is
required for the illumination of the objects which are to be projected
on to the screen. The lantern and electric lamp of Duboscq was
preferred, although, of course, any bright light enclosed in a box, with
a plain convex lens to project the beam of light when required, will
answer the purpose. (Fig. 274.)

[Illustration: Fig. 274. A B. Portable screen of light framework,
covered with black calico. C C C C. Square aperture just above the
shelf, D D, upon which the object--viz., a bottle half full of water--is
placed. E. Duboscq lantern to illuminate the object at D D.]

By removing the diaphragm required to project the picture of the
charcoal points on to the screen, a very intense beam of light is
obtained, which may be focussed or concentrated on any opaque object by
another double convex lens, conveniently mounted with a telescope stand,
so that it may be raised or lowered at pleasure. This lens is
independent of the lantern, and may be used or not at the pleasure of
the operator.

The object is now placed on a shelf fixed to the screen, with a square
aperture just above it. The object of the screen is to cut off all
extraneous rays of light reflected from the mirror, or to increase the
sharpness of the outline of the picture of the object. The screen and
object being arranged, and the light thrown on from the lantern, the
next step is to adjust the concave mirror, and by moving it towards the
[Page 286] object, or backwards, as the case requires, a good image,
solid and quasi-stereoscopic, is projected on to the screen. (Fig. 275.)

[Illustration: Fig. 275. A. The concave mirror. B. The lantern. C. The
portable screen, shelf, and object. D. The inverted image of the bottle
filling with water, with the neck downwards, and when thrown on the disc
at D producing a most curious illusion.]

The act of filling the bottle with water, or better still with mercury,
is one of the most singular effects that can be shown; and if all the
apparatus is enclosed in a box, so that the picture on the screen only
is apparent, the illusion of a bottle being filled in an inverted
position is quite magical, and invariably provokes the inquiry, how can
it be done? The study of numismatics, the science of coins and medals,
is generally considered to be limited to the taste of a very few
persons, and any description of a collection of coins at a lecture would
be voted a great bore, unless, of course, the members of the audience
happened to be antiquaries; great light, however, may be thrown on
history by a study of these interesting remains of bygone times, and a
lecture on this subject, illustrated with pictures of coins thrown on to
the disc by a concave mirror in the manner described, might be made very
pleasing and instructive.

Coins, or plaster casts of coins _gilt_, flowers, birds, white mice, the
human face and hands, may all, when fully illuminated, be reflected by
the concave mirror on to the disc. A Daguerreotype picture at a certain
angle appears, when reflected by the concave mirror, to be like any
ordinary collodion negative, and all the lights and shadows are
reversed, so that the face of the portrait appears black, whilst the
black coat is white. On placing the Daguerreotype in another position,
easily found by experiment, it is now reflected in the ordinary manner,
showing an enlarged and perfect portrait on the disc. In using the
Daguerreotype the glass in front of it must be removed. The pictures
from the concave mirror may be also projected on thick smoke procured
from [Page 287] smouldering damped brown paper, or from a mixture of
pitch and a little chlorate of potash laid on paper, and allowed to burn
slowly by wetting it with water.

An image reflected from smoke would be visible to a number of
spectators, just as the light from the furnace fires of the locomotive
is frequently visible at night, being reflected on the escaping column
of steam.

It was probably with the help of some kind of smoke and the concave
speculum that the deception practised on the worshippers at the temple
of Hercules at Tyre was carried out, as it is mentioned by Pliny that a
consecrated stone existed there "from which the gods easily rose." At
the temple of Esculapius at Tarsus, and that of Enguinum in Sicily, the
same kind of optical delusions were exhibited as a portion of the
religious ceremonies, from which no doubt the priests obtained a very
handsome revenue, much more than could be obtained in modern times by
the mere exhibition of such wonders at Adelaide Galleries, Polytechnics,
or Panopticons.

The smoke from brown paper is very useful in showing the various
directions of the rays of light when reflected from plane, convex, and
concave surfaces. The equal angles of the incident and reflected rays
may be perfectly shown by using the next arrangement of apparatus. (Fig.
276.)

[Illustration: Fig. 276. A. Rays of light slightly divergent issuing
from the lantern, and received on a little concave mirror, which brings
the rays almost parallel, and reflects them to E, a piece of
looking-glass, from which they are again reflected. C is the incident,
and D the reflected rays. F. Smoke from brown paper.]

A very dense white smoke is obtained by boiling in separate flasks (the
necks of which are brought close together) solutions of ammonia and
hydrochloric acid.

The opposite properties of convex and concave mirrors--the former
scattering and the latter collecting the rays of light which fall upon
them--are also effectively demonstrated by the help of the same
illuminating [Page 288] source and proper mirrors, the smoke tracing
out perfectly the direction of the rays of light. (Fig. 277.)

[Illustration: Fig. 277. The smoke shows the rays of light falling on a
convex mirror, and rendered still more divergent.]

The smoke developes the cone of rays reflected from a concave mirror in
the most beautiful manner, and by producing plenty of [Page 289] smoke,
and turning the mirror about--the position of the focus (_focus_, a
fire-place), is indicated by a brilliant spot of light, and the reason
the images of objects reflected by the concave mirror are reversed, may
be better understood by observing how the rays cross each other at that
point. (Fig. 278.)

[Illustration: Fig. 278. The smoke shows rays of light falling on the
concave mirror. In this experiment attention should be directed to the
bright point, E, the focus where the convergent rays meet.]

One of the most perfect applications of the reflection of light is shown
in the "Gregorian reflecting telescope," or in that magnificent
instrument constructed by Lord Rosse, at Parsonstown, in Ireland. (Fig.
279.)

[Illustration: Fig. 279. Lord Rosse's gigantic telescope.]

The description of nearly all elaborate optical instruments is somewhat
tedious, but we venture to give one diagram, with the explanation of the
Gregorian reflecting telescope. (Fig. 280.)

[Illustration: Fig. 280. The Gregorian reflecting telescope.]

At the bottom of the great tube T T T T, (Fig. 280), is placed the large
concave mirror D U V F, whose principal focus is at M; and in its middle
is a round hole P, opposite to which is placed the small mirror L,
concave towards the greater one, and so fixed to a strong wire M, that
it may be moved farther from the great mirror or nearer to it, by means
of a long screw on the outside of the tube, keeping its axis still in
the same line P M N with that of the great one. Now since in viewing a
very remote object we can scarcely see a point of it but what is at
least as broad as the great mirror, we may consider the rays of each
pencil, which flow from every point of the object, to be parallel to
each other, [Page 290] and to cover the whole reflecting surface D U V
F. But to avoid confusion in the figure, we shall only draw two rays of
a pencil flowing from each extremity of the object into the great tube,
and trace their progress through all their reflections and refractions
to the eye f, at the end of the small tube t t, which is joined to the
great one.

Let us then suppose the object A B to be at such a distance, that the
rays E flow from its lower extremity B, and the rays C from its upper
extremity A. Then the rays C falling parallel upon the great mirror at
D, will be thence reflected by converging in the direction D G; and by
crossing at i in the principal focus of the mirror, they will form the
upper extremity i of the inverted image i K, similar to the lower
extremity B of the object A B; and passing on the concave mirror L
(whose focus is at N) they will fall upon it at g and be thence
reflected, converging in the direction N, because g m is longer than g
n; and passing through the hole P in the large mirror, they would meet
somewhere about r, and form the lower extremity d of the erect image a
d, similar to the lower extremity B of the object A B. But by passing
through the plano-convex glass R in their way they form that extremity
of the image at b. In like manner the rays E which come from the top of
the object A B and fall parallel upon the great mirror at F, are thence
reflected converging to its focus, where they form the lower extremity K
of the inverted image i K, similar to the upper extremity A, of the
object A B; and passing on to the smaller mirror L and falling upon it
at h, they are thence reflected in the converging state h o; and going
on through the hole P of the great mirror, they would meet somewhere
about q, and form there the upper extremity a of the erect image a d,
similar to the upper extremity A of the object A B; but by passing
through the convex glass R, in their way, they meet and cross sooner, as
at a, where that point of the erect image is formed. The like being
understood of all those rays which flow from the intermediate points of
the object, between A and B, and enter the tube T T, all the
intermediate points of the image between a and b will be formed; and the
rays passing on from the image through the eye-glass S, and through a
small hole e in the end of the lesser tube t t, they enter the eye f
which sees the [Page 291] image a d (by means of the eye-glass), under
the large angle c e d, and magnified in length, under that angle, from c
to d.

To find the magnifying power of this telescope, multiply the focal
distance of the great mirror by the distance of the small mirror, from
the image next the eye, and multiply the focal distance of the small
mirror by the focal distance of the eye-glass; then divide the product
of the latter, and the quotient will express the magnifying power. (Fig.
280.)

We now come to that much disputed and often quoted experiment of
Archimedes, who is stated to have employed metallic concave specula or
some other reflecting surface by which he was enabled to set fire to the
Roman fleet anchored in the harbour of Syracuse, and at that time
besieging their city, in which the great and learned philosopher was
shut up with the other inhabitants. The story handed down to posterity
was not disputed till about the seventeenth century, when Descartes
boldly attacked the truth of it on philosophical grounds, and for the
time silenced those who supported the veracity of this ancient Joe
Miller. Nearly a hundred years after this time, the neglected Archimedes
fiction was again examined by the celebrated naturalist Buffon, and the
account of his experiments detailed by the author of "Adversaria," in
Chambers' Journal, is so logical and conclusive, that we give a portion
of it verbatim.

"For some years prior to 1747, the French naturalist Buffon had been
engaged in the prosecution of those researches upon heat which he
afterwards published in the first volume of the Supplement to his
'Natural History.' Without any previous knowledge, as it would seem, of
the mathematical treatise of Anthemius ([Greek: _peri paradoxôn
mêchanêmatôn_]), in which a similar invention of the sixth century is
described,[G] Buffon was led, in spite of the reasonings of Descartes,
to conclude that a speculum or series of specula might be constructed
sufficient to obtain results little, if at all, inferior to those
attributed to the invention of Archimedes.

[Footnote G: See Gibbon's "Decline and Fall," chap. xl., section v.,
note _g_.]

"This, after encountering many difficulties, which he had foreseen with
great acuteness, and obviated with equal ingenuity, he at length
succeeded in effecting. In the spring of 1747, he laid before the French
Academy a memoir which, in his collected works, extends over upwards of
eighty pages. In this paper, he describes himself as in possession of an
apparatus by means of which he could set fire to planks at the distance
of 200, and even 210 feet, and melt metals and metallic minerals at
distances varying from twenty-five to forty feet. This apparatus he
describes as composed of 168 plain glasses, silvered on the back, each
six inches broad by eight inches long. These, he says, were ranged in a
large wooden frame, at intervals not exceeding the third of an inch; so
that, by means of an adjustment behind, each should be moveable in all
directions independently of the rest--the spaces between the glasses
being further of use in allowing the operator to see from behind the
point on which it behoved the various disks to be converged.

[Page 292]

"These results ascertained, Buffon's next inquiry was how far they
corresponded with those ascribed to the mirrors of Archimedes--the most
particular account of which is given by the historians Zonaras and
Tzetzes, both of the twelfth century.[H] 'Archimedes,' says the first of
these writers, 'having received the rays of the sun on a mirror, by the
thickness and polish of which they were reflected and united, kindled a
flame in the air, and darted it with full violence on the ships which
were anchored within a certain distance, and which were accordingly
reduced to ashes.' The same Zonaras relates that Proclus, a celebrated
mathematician of the sixth century, at the siege of Constantinople, set
on fire the Thracian fleet by means of brass mirrors. Tzetzes is yet
more particular. He tells us, that when the Roman galleys were within a
bow-shot of the city-walls, Archimedes caused a kind of hexagonal
speculum, with other smaller ones of twenty-four facets each, to be
placed at a proper distance; that he moved these by means of hinges and
plates of metal; that the hexagon was bisected by 'the meridian of
summer and winter;' that it was placed opposite the sun; and that a
great fire was thus kindled, which consumed the Roman fleet.

[Footnote H: Quoted by Fabricius in his "Biblioth. Græc.," vol. ii., pp.
551, 552.]

"From these accounts, we may conclude that the mirrors of Archimedes and
Buffon were not very different either in their construction or effects.
No question, therefore, could remain of the latter having revived one of
the most beautiful inventions of former times, were there not one
circumstance which still renders the antiquity of it doubtful: the
writers contemporary with Archimedes, or nearest his time, make no
mention of these mirrors. Livy, who is so fond of the marvellous, and
Polybius, whose accuracy so great an invention could scarcely have
escaped, are altogether silent on the subject. Plutarch, who has
collected so many particulars relative to Archimedes, speaks no more of
it than the former two; and Galen, who lived in the second century, is
the first writer by whom we find it mentioned. It is, however, difficult
to conceive how the notion of such mirrors having ever existed could
have occurred, if they never had been actually employed. The idea is
greatly above the reach of those minds which are usually occupied in
inventing falsehoods; and if the mirrors of Archimedes are a fiction, it
must be granted that they are the fiction of a philosopher."

Supposing that Archimedes really did project the concentrated rays of
the sun on the Roman vessels, one cannot help pitying the ignorance of
the Admiral Marcellus. Had this officer been acquainted with the laws of
the reflection of light, he might have laughed to scorn the power of
Archimedes, and by receiving the unfriendly rays on one of the bright
brazen convex shields of his soldiers, Marcellus could have scattered
the concentrated rays, and prevented the burning of his vessels.

In these days of learning it therefore appears strange to find any one
advocating the possible use of specula or reflecting mirrors for the
purposes of offence or defence, but M. Peyrard a few years ago proposed
[Page 293] to produce great effects by mounting each mirror in a
distinct frame, carrying a telescope so that one person could direct the
rays to the object intended to be set on fire, and he gravely
calculated, presuming on the ignorance of the attacked, that with 590
glasses of about twenty inches in diameter, he could reduce a fleet to
ashes at the distance of a quarter of a league! and with glasses of
double that size at the distance of half a mile! What effect a shell or
shot would produce upon this ancient weapon is not stated; this we may
safely leave our readers to determine for themselves. The experiment of
Archimedes has long been a favourite one with the boys. (Fig. 281.)

[Illustration: Fig. 281. One of the "miseries of _reflection_."]

The total internal reflection of light by a column of water is an
experiment that admits of great variety so far as colour is concerned,
and is one of the most novel and beautiful experiments with light
presented to the public within the last few years. The author had the
pleasure of introducing it in the first place at the Polytechnic
Institution, where the optical novelty excited the greatest attention,
and received the approbation of her Most Gracious Majesty, and his Royal
Highness the Prince Consort, with the Royal Family, who were pleased to
pay a private evening visit to the Polytechnic, and amongst other things
minutely examined the "Illuminated Cascade," which had been erected by
Mons. Duboscq of Paris.

The illumination of the descending columns of water was obtained by
converging the rays from a powerful electric light upon the orifice
from [Page 294] which the water escaped, the Duboscq lantern already
explained being employed, and in front of it were placed three
cylinders, each having a circular window behind and opposite the lens,
and an aperture of about one inch in diameter on the opposite side for
the escape of water. The lantern used was of a peculiar shape, and had
three sides, the electric light being in the centre of them, and passing
through three separate plano-convex lenses to the three cylinders from
which the water escaped.

[Illustration: Fig. 282.--Fig. 1. A. The electric light. B C D. The
three sides and lenses of the lantern. E F G. The three cylinders of
water, each with a circular glass window and orifices at Z Z Z, from
which the water and rays of light pass out.--Fig. 2. H. Section of one
side of the Duboscq lantern. I I. Cylinder of water, which enters from
below. K K. The stream of illuminated water. L L. Bit of coloured glass
held between the lantern and the cistern of water.]

Attention may be directed to the fact that the light merely passes out
of the orifices as a diverging beam of light until the flow of water
commences, when the rays are immediately taken up and reflected from
[Page 295] point to point inside the arched column of water, and
illuminating the latter in the most lovely manner, it appears sometimes
like a stream of liquid metal from the iron furnace, or like liquid ruby
glass, or of an amethyst or topaz colour, according to the colours of
the plates of glass held between the mouths of the lantern and the
circular windows in the cylinders of water. The same experiment created
quite a _furore_ at the Crystal Palace when it was introduced in one of
the author's lectures delivered in that noble place of amusement. In
order that our readers may understand the arrangement of the apparatus,
we have given at page 294 a ground plan view of it, as also the
appearance of the cascade when exhibited at the Polytechnic to the Royal
party. (Fig. 284.)

[Illustration: Fig. 283. A B. The sides of the cascade. The dotted lines
show the reflection of only two rays of the beam of light passing down
inside the water.]

Another curious effect observed with the illuminated cascade, is the
descent of balls of light as the reflection is cut off for a moment by
passing the finger through the stream of water, showing that a certain
time is occupied in the reflection of light from one end of the cylinder
of water to the other; indeed the best idea of the _rationale_ of the
experiment is formed by substituting in imagination a silver tube highly
polished in the interior, for the descending jet of water. The
reflection of sound takes place precisely in the same manner, and the
vibrations of the air are reflected from plane, concave, and convex
surfaces. It is on this principle that waves of sound thrown off from
different surfaces (as of hard rocks), produce the effect of the _echo_.
The sounds arrive at the ear in succession, those reflected nearest the
ear being first, and the reflecting surfaces at the greatest distance
sending the waves of sound to the ear after the former. At Lurley Falls
on the Rhine, there is an echo which repeats seventeen times. Whispering
galleries, again, illustrate the reflection of sound from continuous
curved surfaces, just as the arched column of water reflects from its
interior curved surfaces the rays of light.

Speaking-tubes are well known in which the waves of sound are
successively reflected from the sides, exactly like the "Illuminated
Cascade" (Fig. 283). The speaking-trumpet is also another and familiar
example of the same principle. Probably when Albertus Magnus constructed
the brazen head, which had the power of talking, it was nothing more
than a metallic head with a few wheels and _visible_ mechanism inside,
but connected with a lower apartment by a hollow metal tube, where
Albertus Magnus descended, and astonished the ignorant with [Page 296]
the then unknown principle of the speaking tube. Light entering at one
end of a bright metallic tube is reflected from the sides of the tube
till it reaches the other, and precisely the same effect occurs in the
interior of the cascade of water. (Fig. 284).

[Illustration: Fig. 284. End of Polytechnic Hall, where the illuminated
cascade was displayed to her Majesty, H.R.H. the Prince Consort, and
Royal party. The cascades issued from behind some artificial rock-work.]


THE KALEIDOSCOPE.

If this article on light and optics had gone minutely into the
mathematical and purely scientific portion of the subject, we should
have had frequent occasion to mention the name of Sir David Brewster, a
distinguished philosopher, whose name is peculiarly identified with this
interesting branch of physics. It is always pleasing to find men of such
standing not only devoting themselves to arguments which college
wranglers would study with pleasure, but also descending to a lower
level, and inventing optical instruments that delight and amuse the
non-scientific and juvenile part of the community. The names of Sir
David Brewster and Professor Wheatstone have been connected during the
last few years with the invention of the stereoscope, an instrument
[Page 298] that will be noticed in another part of this book, but here
we shall describe one of the most original optical instruments ever
devised, and although it is now regarded as a mere toy, its merits are
very great. The title of the instrument is borrowed from the Greek
[Greek: _kalos_], beautiful, [Greek: _eidos_], a form or appearance,
[Greek: _skopeô_], to see; and the public certainly endorsed the name
when they purchased 200,000 of these instruments in London and Paris
during the space of three months. It is said that the sensation it
excited in London, throughout all ranks of the community, was
astonishing, and people were everywhere seen, even at the corners of the
streets, looking through the kaleidoscope. The essential parts of this
instrument are two mirrors of unsilvered black parallel glass, or plate
glass painted black on one side, which should be from six to ten inches
in length, and from one inch to an inch and a half in breadth at the
object end, while they are made narrower at the other end, to which the
eye is applied. The mirrors are united at their lower edges by a strip
of black calico fixed with common glue, and are left open at the upper
edges, and retained at the proper angle by a bit of cork properly
blackened. The angles are 36°, 30°, 25°-5/7, 22½°, 20°, 18°, which
divide the circumference into 10, 12, 14, 16, 18, 20 parts, thus 36 × 10
= 360, or 18 × 20 = 360, and the strictest attention must be paid to
this part of the adjustment, or the figures produced will not be
symmetrical. After the mirrors are adjusted to [Page 298] the proper
angle, the space between the two upper edges should be covered across
with black velvet and the mirrors placed in a tin or brass tube, so that
the broad ends shall barely project beyond the end, while the narrow end
is placed so that the angle formed by the junction of the mirrors shall
be a little below the middle of that end of the tube. A cover with a
circular aperture in the centre is then to be fitted to the narrow end
of the mirrors, which should in general be furnished with a convex lens
whose focal length is an inch or two greater than the length of the
mirrors. A case for holding the objects, and for communicating to them a
revolving motion, is fitted to the object end of the tube. The objects
best suited for producing pleasing effects are small fragments of
coloured glass, wires of glass, both spun and twisted, and of different
colours and shades of colours, and of various shapes, in curves, angles,
circles; also, beads, bugles, fine needles, small pieces of lace, and
fragments of fine sea-weed are very beautiful. M. Sturm, of Prague, has
lately fixed the images of the kaleidoscope, so that they are available
for the production of patterns in every branch of silk, cotton, and
mixed fabrics. Photographs could be taken of the most beautiful of these
accidental designs, which only occur once, and if not copied are lost.

[Illustration: Fig. 285. A B. The tube containing the two mirrors, shown
by dotted lines. A. is the small end where the eye is placed. B. The
object end. C D. Another view of the mirrors arranged to place in the
tube; the shaded portion represents the black velvet. E. Double convex
lens. F. Box to contain objects, and usually fitted with ground glass
outside.]




CHAPTER XXII.

THE REFRACTION OF LIGHT.


This term appears to be often confounded with that of reflection, and
signifies the bending or breaking back of a ray of light (_re_, back,
and _frango_, to break); and it will be remembered that when light falls
on the surface of a solid (either liquid or gaseous) body, it may be
reflected (_re_, back, and _flecto_, to bend), refracted, polarized, or
absorbed. In the previous chapter the property of the reflection of
light has been fully investigated, and in this one refraction only will
be considered. It is a property which has been, and will continue to be,
of the greatest practical utility in its application to the construction
of all magnifying glasses, whether belonging to the telescope,
microscope, magic lantern, or the dissolving views; or the minor
refracting instruments--such as spectacles, opera-glasses, &c.; and it
should be remembered that their magnifying power depends solely on the
property of refraction.

If substances such as glass had not been endowed with this property, it
would be difficult to understand how the great discoveries in the
science of astronomy could have been made, or what information we could
have gained respecting those interesting truths so constantly revealed
by the aid of the microscope. Numerous instances might be quoted of the
value of this latter instrument in the detection of adulteration, and
the examination of organic structures. When so many talented and
industrious scientific men are at work with this [Page 299] instrument,
it is perhaps invidious to point to one singly, though we must make an
exception in favour of Professor Ehrenberg, of Berlin, whose microscope
did such good service in procuring undeniable proof of the Simonides'
fraud; he has made use of it again to detect the thief that stole a
barrel of specie, which had been purloined on one of the railways. One
of a number of barrels, that should have contained coin, was found on
arrival at its destination to have been emptied of its precious
contents, and re-filled with sand. On Professor Ehrenberg being
consulted, he sent for samples of sand from all the stations along the
different lines of railway that the specie had passed, and by means of
his microscope identified the station from which the sand must have been
taken. The station once discovered, it was not difficult to hit upon the
culprit in the small number of _employés_ on duty there.

[Illustration: Fig. 286.]

The simplest case of refraction occurs in tracing the course of a ray of
light through the air, and into the medium water; in this case it passes
from a rare to a dense medium, and the fact itself is well illustrated
by the next diagram, in which the shaded portion represents water, and
the paper that it is drawn upon the air. The line A B is a perpendicular
ray of light, which passes straight from the air into and through the
water, without being changed in its direction. The line C D is another
ray, inclined from the perpendicular, and entering the water at an
angle, does not pass in the straight line indicated by the dotted line,
but is refracted or bent towards the perpendicular at D E.

This fact reduced to the brevity of scientific laws is thus
expressed:--When a ray of light falls perpendicularly on a refracting
surface, _it does not experience any refraction or change of direction_.
When light passes out of a rare into a dense medium, as from air into
water, _the angle of incidence is greater than the angle of refraction_.
And when light passes from a dense into a rare medium, as out of water
into air, _the reverse takes place_, and _the angle of incidence is
smaller than the angle of refraction_.

In order to illustrate these laws, a zinc-worker or tinman may construct
a little tank, with glass windows in the front and sides, the latter
being as deep as the half-circle described on the back metal plate of
the tank, which of course rises higher, in order to show the full
circle; this should be japanned white, and a perpendicular and
horizontal black line described upon it--the whole, with the exception
of the circle, being japanned black. If the Duboscq lantern is arranged
with the little mirror, as described in fig. 276, page 287, the ray of
light may be thrown perpendicularly, or at an angle, through the water,
[Page 300] and the actual breaking back of the ray of light is rendered
distinctly apparent. (Fig. 287.)

[Illustration: Fig. 287. A. Duboscq lantern. B. The mirror. B C. The
incident ray. C D. The refracted ray. E F. Tank, containing water up to
the horizontal line of the circle.]

The refraction of light is also well displayed by Duboscq's apparatus,
with the plano-convex lens, and a brass arrow as an object, with another
double convex lens to focus it. When a good sharp outline of the arrow
is obtained on the disc, a portion of the rays of light producing it may
then be truly broken out or refracted by laying across the brass arrow a
square bar of plate glass. (Fig. 288).

[Illustration: Fig. 288. A. Rays of light from the electric light. B.
The cap, with figure of arrow cut out. C. The bar of plate glass. D. The
double convex glass to focus E, the image on the disc, and portion
refracted at B.]

There are many simple ways in which the refraction of light is
displayed, such as the apparent breaking of an oar where it enters the
water, or the remarkable manner in which the bottom is lifted up when we
look, at any angle, through the clear water of a deep river or lake; the
latter circumstance has unhappily led to most serious accidents, in
consequence of children being induced by the apparent shallowness of
[Page 301] the water to get in and bathe. Fish, again, unless seen
perpendicularly from a boat, always appear nearer than their true
position, and the Indians, when they spear fish, always take care to
strike as near the perpendicular as possible; experienced shots know
they must aim a little lower and nearer than the apparent position of a
fish in order to hit it.

Having learnt that light is bent from its course, it might be supposed
that all objects looked at through plate glass should appear distorted;
but it must be remembered that the sides of the glass being nearly
parallel, an equal amount of refraction occurs in every direction--so
that, unless the window is glazed with uneven wavy glass, the object,
for all practical purposes, does not apparently change its position,
being neither moved to the right or the left, or upward or downward. In
order to bend the rays of light in the required direction, the glass
must be cut into certain figures called prisms, plane glasses, spheres,
and lenses, some of which are shown in the annexed cut. (Fig. 289.)

[Illustration: Fig. 289.]

[Illustration: Fig. 290. A B. A double convex lens. C is a ray of light,
which falls perpendicularly on A B, and therefore passes on straight to
F, the focus. D D. Rays falling at an angle on A B, refracted to focus,
F.]

It would be tedious to trace out, by a regular series of diagrams, the
passage of light through the variety of combinations of lenses; and as
the plane, convex, and concave surfaces have been examined with respect
to their effect on the reflection of light, they may be referred to
again with regard to their influence in refracting light. In the latter
it will be found that convex and concave lenses have just the opposite
properties of mirrors; thus, a convex lens receiving parallel rays will
cause them to converge to a focus. (Fig. 290.) The case of
_short-sighted_ persons arises from too great a convexity of the eye,
which makes a very near focus; and that of old people is a flattening of
the eye, by which the focus is thrown to a greater distance. The remedy
for the latter is a convex spectacle-glass, whilst a concave lens is
required for the former, to scatter the rays and prevent their coming to
a point too soon.

[Page 302]

The action of a concave refracting surface is again the opposite to a
concave reflecting surface--the former disperses the rays of light,
whilst, the latter collects them. A concave lens, as might be expected,
produces exactly the contrary effect on light to that of a concave
mirror. (Fig. 291.)

[Illustration: Fig. 291. A B. A double concave lens. C., is a ray of
light which falls perpendicularly on A B, and passes through without any
alteration of its course. D D. Rays falling at an angle on A B, are
refracted and diverged.]

These facts are well shown with the aid of the lantern and electric
light. The rays of light are refracted in a visible manner when received
on a concave or convex lens, provided a little smoke from paper is
employed, as in the mirror experiments. (Fig. 292.)

[Illustration: Fig. 292. A. The electric light. B. The lens.]

Bearing these elementary truths in mind, it will not be difficult to
follow out a complete set of illustrations explanatory of the
construction and use of various popular optical contrivances.

[Page 303]




CHAPTER XXIII.

REFRACTING OPTICAL INSTRUMENTS.


I. _The Magic Lantern._

No other optical instrument has ever caused so much wonderment and
delight, from its origin to the present time, as this simple
contrivance. For a long time its true value was overlooked, and only
ridiculous or comic slides painted, but its educational importance is
now being thoroughly appreciated, not only on account of the size of the
diagrams that may be represented on the disc, but also from the fact
that the attention of an audience is better secured in a room when the
only object visible is the diagram under explanation. The lenses it
contains are a "bull's eye" or plano-convex, nearest the light, and a
double convex glass, for the purpose of focussing the picture which is
inverted and placed between the two lenses. (Fig. 293.)

[Illustration: Fig. 293. The magic lantern.]

In many books full directions are given for painting the glass slides,
but this is an art that requires very great practice and experience. A
person may know how to draw and paint on paper or canvas, but it is
quite a different thing where glass is concerned, and unless the
juvenile artist has taken lessons from a regular painter on glass, his
or her efforts are likely to be very unsatisfactory. In many popular
works embracing the subject of optics, full directions are given on the
mode of painting the slides for the magic lantern, or dissolving views;
a new era, however, has dawned upon this mode of illustration, in the
preparation of photographs on glass of the most lovely description, and
now instead of exhibiting mere daubs of weak colouring, photographic
pictures of singular perfection can be procured of Messrs. Negretti and
Zambra, Holborn, who have turned their attention especially to this
branch, and supply slides of all sizes.


II. _The Dissolving Views._

This very pleasing modification of the ordinary magic lantern is
displayed with the assistance of two lanterns of the same size, provided
with lamps and lenses which are exactly alike. They are best arranged
[Page 304] on one board, side by side, and if kept parallel with each
other, the circles of light thrown from the two lanterns would not
coincide on the screen; it is therefore necessary to place one of them
at an angle which will vary according to the distance from the screen.
The task of making the two circles of light overlap each other precisely
on the disc, is called centering the lanterns, and is the first thing
that must be attended to before exhibiting the slides. The slides for
the dissolving views are all painted of the same size, and supposing a
scene such as a church with a bridal procession and the trees in full
foliage, to represent summer, is first thrown on to the disc, it may be
changed to winter by putting another picture of the same subject, but
painted to represent bare trees, and the church and ground covered with
snow, and a grave open, with a funeral procession. The two pictures must
not be projected on the screen at the same time, and here the dissolving
mechanism is required; it consists of two fans so arranged that they may
be raised or lowered by a rack-work and handle; one fan in descending
covers one of the nozzles of the lanterns, and the other leaves the
second lantern open, and free to project the picture; the dissolving is
managed by slowly moving the handle of the rack-work, so that one
quarter of the picture already on the disc is cut off, and one quarter
of the new one thrown on. As the movement proceeds, one half of the old
picture is shut out, and one half of the new slide takes its place, and
so on, till the whole of the original picture is cut off by the fan and
the new one comes into view, and it is in this way the effect of the
change from summer to winter is produced. (Fig. 294.)

[Illustration: Fig. 294. Nozzle of one lantern, with the fan, A, raised,
and in the position to throw a picture on the disc. B. The other fan
shutting off the second lantern.]

When two pictures such as those already described, dissolve one into the
other, of course the same building or other marked portion of the
subject, must strictly coincide in each picture on the disc, or else the
two pictures are apparent, and the illusion is destroyed. The pictures
must all be centered before the exhibition commences. By the arrangement
of Mons. Duboscq, one electric light serves to illuminate both lanterns
by making use of mirrors. The dissolving apparatus is likewise very
[Page 305] simple, and consists of two diamond-shaped openings in a
brass frame, which open and shut alternately by a slide worked with a
handle. The single light is not to be recommended, as it is somewhat
troublesome to manage properly. (Fig. 295.)

[Illustration: Fig. 295. A. The electric light. B B. The two sets of
lenses for the two pictures. C. The dissolving mechanism. D. The picture
on screen.]

When dissolving views are required on a grand scale, the lenses must be
exceedingly large, and the condenser (corresponding with the
"bull's-eye" of the simple magic lantern) should be at least nine or
eleven inches in diameter, and the front glasses must be of a superior
make. The lenses for a large lantern lit by the oxy-hydrogen light, are
arranged as in the next cut. (Fig. 296.)

[Illustration: Fig. 296. A. The lime light. B. The condensers. C. The
picture. D D. The front lenses for focussing, with rack-work.]

At the Polytechnic the author had no less than six lanterns working at
or about the same time, to produce effects, in the views illustrating
the voyages of Sinbad the Sailor; and in order to obtain the increased
[Page 306] results required for dioramic effects, such for instance as
the Siege of Delhi, showing the bursting of the shells, &c., the four
fixed lanterns (the fronts of which are shown in the next cut) were
always employed. The two upper lanterns are dissolved by discs of brass
worked by the hand, and the lower ones with the fans. (Fig. 297.)

[Illustration: Fig. 297. Fronts of the four lanterns, showing how the
dissolving mechanism is arranged.]

"Behind the scenes" always has a great attraction for young people; we
have, therefore, in the frontispiece, with the help of Mr. Hine (who
painted a great number of the photographs shown at the Polytechnic
during the author's management), given a section of the large theatre
taken whilst the effective scene of the Siege of Delhi was in progress.
The optical effects were assisted by various sounds in imitation of
war's alarms, for the production of which, more _volunteers_ than were
required would occasionally trespass behind the screen, and produce
those terrific sounds that some persons of a nervous temperament said
were really stunning. In a page picture, we have also given a correct
drawing of the interior of the optical box at the Polytechnic, with the
four fixed lanterns, and side cupboards to hold the glass pictures. The
four lanterns worked on a railway, with wheels and a circular
turn-table; they could be removed, and the microscope arranged in their
places.

[Illustration: Before and behind the screen at the Polytechnic during
the exhibition of the dioramic effects of the siege of Delhi. _p. 306_]

[Page 307]

III. _The Oxy-Hydrogen Microscope._

Many persons will recollect the first exhibition of this instrument in
Bond-street, by Mr. J. T. Cooper, and Mr. Cary, succeeded by the
Adelaide Gallery exhibition of scientific wonders and an oxy-hydrogen
microscope. The apparatus for this purpose consists of three condensing
lenses and an object glass. The objects, such as live aquatic insects,
are placed in glass troughs containing water; the other objects, ferns,
feathers, butterflies, algæ, &c. &c., being mounted on slides in the
ordinary way with Canada balsam. (Fig. 298.)

[Illustration: Fig. 298. A. The lime light. C C C. Condensers. D. The
object, such as a tank of water containing live insects. E. The object
glasses.]


IV. _The Physioscope._

This instrument, brought out at the Polytechnic during the time that Mr.
J. F. Goddard managed the optical department of the institution, always
excited the greatest mirth and astonishment amongst the numerous
visitors; and _habitués_ of the old place may remember the good-natured
inimitable maudlin simper with which poor Mr. Tait (who was one of the
living objects shown on the disc) used to drink off the glass of wine
and then wink at the audience. When we say Mr. Tait used to wink, of
course it is understood that he was personally invisible, and his
apparition or image only appeared on the disc. The countenance is
brilliantly illuminated by the oxy-hydrogen light, and being placed near
the lenses, the rays are reflected from the face into the physioscope,
and being properly focused, and the inversion of the image corrected,
the perfect representation of the human countenance is apparent on the
disc. The lenses and concave reflectors required are shown in the
section of the physioscope. Messrs. Carpenter and Westley, of
Regent-street, have brought the manufacture of magic lanterns to great
perfection; and Mr. Collins, of the Polytechnic, constructs every kind
of dissolving view apparatus, oxy-hydrogen microscopes, physioscopes,
&c. (Fig. 299.) With this instrument any opaque objects (provided they
reflect light properly) may be displayed to a large audience. Plaster
casts appear with singular beauty and softness, whilst flowers, stuffed
birds, and especially humming birds, are excellent objects for the
physioscope.

[Page 308]

[Illustration: Fig. 299. A. One or more lime lights, throwing rays
reflected by concave mirrors on to the face B, from whence they are
reflected to C C, the first condensers. D D. Object glasses. This
instrument is made by Mr. Collins, who has the tools for making the
reflectors with correct curves. The picture of the face on the disc is
covered with black spots if the reflectors are not perfect.]


V. _The Camera Obscura._

A "dark chamber" is the name of a most amusing, and now, in the improved
form, extremely valuable instrument for photographic purposes. It is
occasionally to be met with in public gardens, and there is a very good
one on the Hoe at Plymouth. The construction of the camera for observing
the surrounding country is very simple, and merely consists of a flat
mirror placed at an angle, by which the picture is reflected through a
double-convex lens on to a white table beneath. (Fig. 300.)

[Illustration: Fig. 300. A. The mirror. B. The convex lens. C. The white
table.]

[Page 309]

The term "focusing," or the art of moving the lenses so that a sharp
image may be obtained, has been frequently mentioned in this article,
and perhaps it may be as well to describe the mode of ascertaining the
focal distance of a lens by experiment.

Hold the lens opposite the window so that a bright picture of the
window-sash may be obtained on a sheet of paper pinned against the wall,
and the distance of the lens from the paper will be the focal length.

If the lens has a very long focal length, it may be determined as
follows:--Measure the distance between the lens and the object, and also
from the image; multiply these distances together, and divide the
product by their sums; the quotient will give the focal distance.


VI. _The Decomposition of Light--"its Analysis and Synthesis."_

It is in the Italian language that the bride, the emblem of purity, is
called Lucia (_Lux_, light); and surely if an illustration were required
of beauty and singleness, light would be named poetically as
appropriate; but physically it is not of a single nature, it is
composite, and made up of seven colours. The instrument required to
refract a ray of light sufficiently to break it into its elementary
colours is called the prism, and is a solid having two plane surfaces,
called its refracting surfaces, with a base equally inclined to them.
(Fig. 301.)

[Illustration: Fig. 301. The prism. The base, A B, is equally inclined
to the refracting surfaces, C A, C B.]

It was in 1672 that Sir Isaac Newton made his celebrated analysis of
light, by receiving a sunbeam (as it passed through a hole in a shutter)
on to the refracting surface of a prism, and throwing the image or
spectrum on to a screen, where he observed the seven colours, red,
orange, yellow, green, blue, indigo, and violet, and thus proved "_that
there are different species of light, and that each species is disposed
both to suffer a different degree of refrangibility in passing out of
one medium into another, and to excite in us the idea of a different
colour from the rest; and that bodies appear of that colour which arises
from the composition of those colours the several species they reflect
are disposed to excite_."

Sir Isaac Newton's name would have been immortalized by this discovery
alone, even if he had not possessed that transcendent ability which
raised him above all other mathematicians and physicists. It is at the
same time interesting to know that the ancient author Claudian (A.D.
420) inquires "whether colour really belongs to the substances
themselves, or whether by the reflection of light they cheat the
eye--_enquires sitve color proprius rerum, lucisne repulsa eludant
aciem_."

Sir Isaac Newton determined that the spectrum could be divided into 360
equal parts, of which red occupied 45, orange 27, yellow 48, green 60,
blue 60, indigo 40, violet 80. He also discovered that if the highly
refracted rays, the seven colours, or spectrum were received into [Page
310] a concave mirror or a double-convex lens, that they again united
and formed white light. In order to demonstrate the properties of the
prism in various positions, the next diagram may be adduced. (Fig. 302.)

[Illustration: Fig. 302. A. The ray of light passing through two prisms
B placed base to base. In this position the light passes through to the
second prism, C, without alteration. At C the decomposition of light
occurs, and the spectrum is shown at D D. The top prism at B used singly
would reflect the ray to E without decomposing it into the coloured
rays.]

The rainbow is the most beautiful natural optical phenomenon with which
we are acquainted; it is only seen in rainy weather when the sun
illuminates the falling rain, and the spectator has the sun at his back.
There are frequently two bows seen, the interior and exterior bow, or
the primary and secondary, and even within the primary rainbow, and in
contact with it, and outside the secondary one, there have been seen
other bows beyond the number stated.

The primary or inner rainbow consists of seven different coloured bows,
and is usually the brightest, being formed by the rays of light falling
on the upper parts of the drops of rain. The exterior bow is formed by
the rays of light falling on the lower parts of the drops of rain; and
in both cases the rays of light undergo refraction and reflection, hence
the opinion of Aristotle, that the rainbow is caused only by the
reflection of light, is not correct.

The first refraction occurs when the rays of light enter, and the second
when they emerge from the spheroids of water in the first bow; the
refracted rays undergo only one reflection, whereas in the second the
brilliancy of the colours is impaired by two reflections.

The spectrum from the electric light is one of the most gorgeous
exhibitions of colour that can be conceived; and the instruments
required for the purpose are illustrated in No. 1 (Fig. 303), whilst
the [Page 311] synthesis of the coloured rays and production of white
light is shown at No. 2 of the same figure. (Fig. 303.)

[Illustration: Fig. 303. No. 1. A. The electric light. B. The narrow
slit through which the light passes to the convex lens, C. D. The prism.
E. The spectrum. No. 2 is the same for A B C D; but F is the convex lens
collecting the scattered rays, and forming white light at G.]


VII. _Duration of the Impression of Light._

If a circular disc is painted with the prismatic colours taken in the
same proportion with respect to each other in which they are exhibited
in the spectrum made by the prism, and the wheel is turned swiftly, then
the individual colours disappear, and nearly white light is apparent.
The cause is due to the same principle that creates the appearance of a
complete circle of fire when a burning squib is moved quickly round
before it is thrown away to burst, and as it is evident that the burning
squib cannot be in every part of the circle at the same moment, there
must be some inherent faculty belonging to the human eye which enables
it to retain for a definite period the impression of images that may
fall upon it; and this principle has been so far pressed, as it were,
beyond its limits, that it is gravely asserted the image of a man's
murderer "might be discovered on the retina of the eye-ball if that
could be examined sufficiently quick after death." The fixture of the
picture is said to be due to a sort of natural photographic process; but
such fanciful statements often lead the mind into dream-land only, and
so we will return to the fact of the duration of the impression of light
on the eye as evidenced by several ingenious optical instruments, and
especially by the scientific inventions of Dr. Faraday, Dr. Paris, and
of Mr. Thomas Rose of Glasgow.

By careful experiment M. D'Arcy found that the light of a live coal
moving at the distance of 165 feet, maintained its impression on the
[Page 312] retina during the seventh part of a second. Hence the cause
of the recomposition of white light when the colours on the disc are
quickly rotated. Each colour at any point succeeds the other before the
impression of the last is gone from the eye, and provided the colours
move round within the seventh part of a second, they are all impressed
together on the eye, and meeting on the retina, produce the effect of
white light.


VIII. _The Phenakistiscope._

This amusing instrument consists of a turning wheel upon which figures
appear to jump, walk, or dance. The disc or wheel is of cardboard, upon
which are painted (towards the periphery) figures in eight, ten, or
twelve postures. Thus, if it is desired to represent clowns turning
round in a circle, twelve different positions of the figure in the act
of turning are painted on the disc, and above each of the figures on the
wheel a slit is cut about one inch long, and a quarter of an inch wide
in a direction corresponding with the radii of the circle. This simple
form of the instrument is used by placing the figured side towards a
looking-glass and then causing it to revolve at a certain speed, which
is ascertained by experiment; and as the spectator looks through the
slits into the looking-glass, the clowns appear to turn round. At the
Polytechnic Institution there are two of these wheels with
looking-glasses, and although the same designs have done duty for many
years, they still attract the public attention. (Fig. 304.)

[Illustration: Fig. 304. Design for the phenakistiscope. The spectator
is supposed to be looking towards a mirror through the slits. It is
supported by a handle through the centre, round which it is twirled by
the other hand.]

In the "Journal of the Royal Institution" Mr. Faraday has described some
very interesting experiments and optical illusions produced by the
revolution of wheels in different directions and velocities. The wheels
are made of cardboard, and by cutting out two cog wheels of an equal
size, and placing one above the other on a pin, the usual hazy tint when
the cogs are acting is apparent when they are whirled round; but if the
two cog wheels are made to move in opposite directions, there will be
the extraordinary appearance of a fixed spectral wheel. If the cogs are
cut in a slanting direction on both wheels, the spectral wheel will
exhibit slanting cogs; but if one wheel is turned so that the cogs shall
point in opposite directions, then the spectral wheel will have straight
cogs. A number of such wheels set in motion in a darkened room, and
illuminated suddenly with the light from the electric spark, appear to
stand perfectly still, although moving with a great velocity. An
expensive instrument has been constructed by Duboscq, for the [Page
313] purpose of showing the usual phenakistiscope effects on the screen
with the magic lantern; a very limited picture, however, is shown, and
there is still great room for the improvement of the apparatus. (Fig.
305.)

[Illustration: Fig. 305. Phenakistiscope made by Duboscq, of Paris. No.
1. Apparatus in elevation with the condensers. No. 2. Section of the
apparatus. A. The light. B. Condenser, or plano-convex lens. C. Round
glass disc with design painted on it. D. Wooden disc with four
double-convex lenses placed at equal distances from each other, so as to
coincide with C, whilst rotating. Both the latter and C rotate, and the
picture is focussed on the disc by the lenses F. No. 3. Glass plate,
with device painted thereon.]

[Page 314]

IX. _The Thaumatrope._

This very simple toy was invented by the late Dr. Paris, who gave it an
appropriate name, compounded of the Greek words, [Greek: _thauma_],
wonder, [Greek: _trepô_], to turn. The duration of the impressions of
light on the eye is very apparent whilst using this toy, which is
usually made of a circular piece of cardboard, having on one side a
painting of a man's head, and on the other a hat; or a picture of a
lighted candle on one face of the cardboard, and an extinguisher on the
other; or a gate, and a horseman leaping it. Each pair of designs
painted on opposite sides of the cardboard appear to be one when twisted
round by strings tied to the opposite edges of the cardboard circle. The
_rationale_ of this experiment being, that the picture of one
design--such as the head and face--is retained by the eye until the hat
appears, and being mutually impressed upon the nerve of vision at very
nearly the same instant of time, they appear as one picture.


X. _The Kalotrope._

This is an optical arrangement by Mr. Thomas Rose, of Glasgow, primarily
designed for showing the illusions of the phenakistiscope and kindred
devices to a numerous audience; but more remarkable for its
presentations of very beautiful spectra, composed of the multiplication,
combination, and involution of simple figures disposed around a disc.
The arrangement consists of a movement for giving considerable velocity
to two concentric wheels, working nearly in contact, and moving in
contrary directions. But the only part of the apparatus that requires
special explanation and illustration is the device disc and the disc of
apertures; the first of which is placed on the hinder wheel, and the
second on the front wheel. We give figures of the two discs, premising,
however, that each is capable of an almost infinite variety of
characters. No. 1 (Fig. 306) presents in its four quadrants the
perforations for four distinct discs of apertures; and No. 2 is a device
disc, consisting of twelve equidistant black balls. Under _a_ the balls
will be presented as twenty-four ovals; under _b_, as forty-eight
involved figures, beautifully variegated; under _c_, as an elaborate
lacework; and under _d_, as a rich variegation of form and colour. Every
fresh disc of devices and disc of apertures of course opens up a new
field of effect. Thus, if we take a disc bearing twelve repeats of a
ball in the interior of a ring, each repeat being so painted that its
position is advanced in the ring until it reaches in the twelfth ring
the point whence it started, and place this on the back disc of the
kalotrope, having previously removed the first one, no effect is
observed when the wheel is rotated beyond the spreading out of the
design and general appearance of hazy black circles. When, however, the
disc, with twelve slits or apertures, is now placed on the front wheel,
and the two rotated in opposite directions, then the whole figure starts
as it were into existence, and each ball apparently moves round the
interior of its circle. [Page 315] The apparatus was produced at the
Royal Polytechnic Institution by the author, and excited much interest.
(Fig. 306.)

[Illustration: Fig. 306. Nos. 1 and 2 are the discs. No 3. Kalotrope in
elevation. No. 4. Side view of kalotrope, showing the multiplying wheels
and the perforated and painted discs moving in opposite directions.]


XI. _The Photodrome._

This is a second optical arrangement by Mr. Rose for showing spectral
illusions; and it is superior to the last, inasmuch as it offers to the
public lecturer a most effective means of presenting these deceptions to
a large audience. It differs from the kalotrope in several important
points. It dispenses with the discs of apertures, and leaves the device
disc with its face fully exposed to the spectators. The effects are
produced by a powerful light, thrown through the tube of a lantern, and
broken by a wheel working across it. The apparatus, as it at present
stands in the inventor's possession, consists of two distinct parts; the
one a movement for the device discs, and the other for the light. A
wheel four feet in diameter is connected with a train of movement
capable of giving it five hundred or six hundred revolutions per minute.
On this wheel the device disc is placed, in full view of the spectators,
and set in motion. From an opposite gallery the light is thrown, and
[Page 316] broken by a wheel of such diameter and number of apertures as
will admit the velocity of the _photodrome_ (or light-runner) to be at
least _six_ times the velocity of the device disc; whilst the apertures
are of such width as to restrict the duration of the light-flash to
about one-two-thousandth of a second. The wheel working across the light
has a train of movement for raising the velocity to two thousand
revolutions per second. The management of the apparatus is very simple.
The device-wheel is brought to a steady, rapid rotation, and the
operator on the light then works his wheel with gradually increasing
velocity, until he overtakes the figures of the device, where, by mere
delicacy of touch, he is able to hold them stationary or give them
motion, at pleasure.

Theories of light and colour still agitate the scientific world,
although that man must be bold who will assert that his hypothesis is
fitted to explain every difficult point that arises as our experimental
knowledge increases. Mr. G. J. Smith, of the Perth Academy, has
propounded a very ingenious theory of light and colour, supported by
some clever experiments. But, as Solomon says, "there is nothing _new_
under the sun," and in an able paper Mr. Rose, of Glasgow, lays claim to
the anticipations of Mr. Smith's theory as follows:--

"My attention has been directed to a paper entitled 'The Theory of
Light,' by G. John Smith, Esq., M.A., of Perth Academy. I think it is
now nearly two years since I communicated an interesting fact to
Professor Faraday, and to a member of our local Philosophical
Institution, which may fairly claim to have anticipated Mr. Smith's
theory. The fact was this: that if a piece of intensely white card be
held in one hand, with the light of a powerful gas-jet falling upon it,
and if the other hand has command of the gas-tap, as the light is
gradually reduced, the card will assume the prismatic colours down to
intense blue, and as the light is restored the colours will present
themselves in inverse order. The experiment showed, very conclusively to
my mind, that light is homogeneous, and that what we name colour is only
the various affection of the optic nerve by a greater or lesser
radiation of light from a focal point in an imperfect reflector--say, in
the instance, a white card. I apprehend that Mr. Smith confuses his
theory when he speaks of alternations of light and shadow producing
colour. Shadow, or darkness, is mere negation of light. We do not see
mixtures of light and darkness, or blackness and whiteness, but light in
its several degrees of intensity. Mr. Smith's experiments present only
what my kalotrope has done, and what my later device, the photodrome
(now nearly three years old) is doing in a much more perfect manner. It
is one of the mysteries intelligible only to the initiated, that whilst
Mr. Smith's paper seems to have been received with great favour by the
British Association, my communication relative to the photodrome was
voted 'not _sufficiently practical_.'

       *       *       *       *       *

"Since I have come before the public with an experiment, which in any
view is an interesting one, permit me to reproduce it under several
distinct conditions, and to add a brief narrative of remarkable
presentations of colour that have come before me, and which, so far as I
am [Page 317] aware, are perfectly novel, or known only through the
more recent experiments of Mr. Smith. Professor Faraday very courteously
acknowledged my communication of the experiment with the card, but said
that it only partially succeeded with him, and added that probably this
was owing to some decay of sensitiveness in his eyes. More likely I
failed to state with sufficient clearness the conditions of the
experiment, since I have always found nine persons out of ten perfectly
agreed as to the effects produced when they have been at my side. The
transitions from white to yellow, orange, red, and thence to intense
blue, are, I may say, invariably admitted. Success depends on a very
slow and regular reduction and restoration of the light. I have given
one method of performing the experiment, and will add other two. Allow
the light to remain undisturbed, and begin by holding the card near to
it; then keep the hand steady and the eye intently fixed upon the card,
and retire gradually with your back to the light, and the colours will
change in the order of the prismatic spectrum from yellow to intense
blue. On returning backwards towards the light the colours will again
present themselves, but in inverse order. In this form of the experiment
we are certain that the light remains precisely the same throughout. The
third method is this: Place a circle of white card, about three inches
in diameter, in the centre of a black board, and let a spectator stand
within twelve inches of the board, with his eyes fixed upon the card.
Let an operator be provided with a light so covered that it shall not
fall on the eye of the spectator; then, as he retires with the light or
returns with it, the spectator will see the colours as before. This
arrangement evidently subjects the experiment to a severe test, since
the black board enhances the whiteness of the card, and tends to
preserve it.... Whilst pursuing my principal object, I frequently
noticed most remarkable presentations of colour; but, as the conditions
were for the most part unsuitable to the lecture-room, I gave them only
a passing regard. Allow me to instance a few of the experiments.

"The first refers to the kalotrope, which may be briefly described as an
arrangement of two concentric wheels, working nearly in contact and in
contrary directions. Discs of various devices are provided for the
hinder wheel, and a number of perforated black discs for the one in
front. When a disc charged with twelve _black_ radii is placed on the
hinder wheel, the six spokes of the front wheel, in passing rapidly
across it, convert the twelve black radii into twenty-four apparently
stationary _white_ radii upon a tinted ground. Here is a remarkable
presentation of the complementary, inasmuch as it is placed permanently
before the eye by persistence.

"The second experiment is performed with the photodrome, which consists
of an independent wheel to receive the device discs, and an apparatus
(altogether apart, and, if desired, out of sight) by which flashes of
light are thrown upon the disc in rapid and regular succession. Now, if
a disc charged with twelve dark blue balls, nearly in contact, be placed
upon the wheel, and a little natural light be allowed to fall [Page
318] upon it, so soon as it is thrown into rapid revolution, and flashes
of artificial light (insulated in a lantern) are duly measured out upon
it, we see twelve apparently stationary light-blue balls upon a zone of
bright orange. Here, again, there is nothing for which we are not
prepared; the complementary is suddenly presented, and it is maintained
permanently before the eye by persistence.

"A third experiment may prove interesting in its relation to Mr. Smith's
ingenious theory. Place the kalotrope opposite a bright northern noonday
sky, remove the front wheel, and affix to the hinder wheel one of the
perforated black discs used for the kalotropic effects. The
experimentalist stands at the back of the instrument, and can see the
sky only through the apertures in the black disc. Cause these apertures
to pass the eye at intervals varying from one-half to one-sixth of a
second, and very remarkable presentations of colour are seen. Under the
lower velocities the sky flashes, and assumes an unnatural brilliancy,
and the intervals of the fourth and fifth of a second give it sometimes
a crimson, at others a deep purple colour. Now, what are we to infer
from this experiment? Certainly _not_ that the pulsations have
absolutely produced variety of colour. At every pulsation the full
natural light falls upon the eye, and the intervals between the
pulsations give time for the reaction necessary to the suggestion of
complementary colour, and that under manifold modifications arising out
of the ever-changing condition of the eye during the experiment. If the
apertures pass the eye with a velocity exceeding one-sixth of a second,
the effect ceases. There is then perfect persistence, and the eye
apprehends nothing but the ordinary light of the sky, reduced in
intensity, with nothing to break its uniformity or give it a chromatic
character.

"A fourth experiment is kindred to the last. Place the kalotrope under
the same adjustment and management as before, in front of a brilliant
sunset, and the spectator will see, with more than a poet's vision,

  'The rich hues of all glorious things.'"


XII. _The Kaleidoscopic Colour-top._

This invention by John Graham, of Tunbridge, is designed to show that
when white or coloured light is transmitted to the eye through small
openings cut into patterns or devices, and when such openings are made
to pass before the eye in rapid successive jerks, both form and colour
are retained upon the nerve of the visual organ sufficiently long to
produce a compound pattern, all the parts of which appear
simultaneously, although presented in succession. The instrument forms,
therefore, a pleasing illustration of the law that the eye requires an
almost inappreciably short space of time to receive an impression, and
that such impression is not directly effaced, but remains for an
assignable though very limited period. The results are obtained by
rotating two discs on a wheel, the lower disc containing colours, and
the upper one the [Page 319] openings; this latter disc is made to
vibrate as well as to rotate, thus allowing the eye to receive the
coloured light reflected from below, which light assumes, at the same
time, the forms of the patterns through which it has been transmitted.
The instrument serves also to illustrate most of the important phenomena
of colour.


XIII. _Simple Microscopes and Telescopes._

The Stanhope lenses are now sold at such a cheap rate, and are so useful
as simple portable microscopes, that it is hardly worth while to detail
any plan by which a cheap single-lens magnifier may be obtained.
Eloquent vendors of cheap microscopes are to be found in the streets,
who make their instrument of a pill-box perforated with a pin-hole, in
which a globule of glass fixed with Canada balsam is placed; and the
spherical form of the drop affords the magnifying power: or a thin
platinum wire may be bent into a small circular loop, and into this may
be placed a splinter of flint-glass; if the flame of a spirit-lamp is
urged upon the loop of platinum wire and glass by the blowpipe until it
melts, a small double-convex lens may be obtained, which will answer
very well as a magnifying-glass. Practice makes perfect, and after two
or three trials, a good single lens may be obtained, which can be
mounted between two small pieces of lead, brass, or cardboard, properly
fixed together, with holes through them just large enough to retain the
edge of the tiny lens. A prism can be made of two small pieces of
window-glass stuck together with a lump of soft beeswax, and if a few
drops of water are placed in the angle, they are retained by capillary
attraction. The prism is used by holding it against a large pin-hole or
small slit in a bit of card, and directing them towards the sky, when
the beautiful colours of the spectrum will be apparent if the card and
prism are brought close to the eye.

The most simple form of the refracting telescope is made with a lens of
any focal length exceeding six inches, placed at one end of a tin or
cardboard tube, which must be six inches longer than the focal length of
the lens; the tube may be in two parts, sliding one within the other,
and when the eye is placed at the other end, an inverted image of the
object looked at, is apparent. By using two double-convex lenses, a more
perfect simple astronomical telescope is obtained. The object-glass,
_i.e._, the lens next the object looked at, must be placed at the end of
a tin or pasteboard tube larger than its focus, and the second lens,
called the eye-glass, because next the eye, is a smaller tube, termed
the eye-tube; and if the focal length of the object-glass is three feet,
the eye-glass must have a one-inch focus, and of course the eye-tube and
glass must slide freely in the tube containing the object-glass. An
object-glass of forty feet focus will admit of an eye-glass of only a
four-inch focus, and will, therefore, magnify one hundred and twenty
times. A tube of forty feet in length would of course be very
troublesome to manage, and therefore it is usual to adopt the plan
originally devised by Huygens, viz., that of placing the object-glass in
a short tube on the [Page 320] top of a high pole with a
ball-and-socket joint, whilst the eye-glass is brought into the same
line as the object-glass, and focused with a tube and rack-work properly
supported. In an ordinary terrestrial telescope there are four lenses,
in order that the objects seen by its assistance shall not be inverted;
and whenever objects are examined by a common telescope, they are found
to be fringed, or surrounded with prismatic colours. This disagreeable
effect is corrected by the use of _achromatic_ lenses, in which two
kinds of glass are united; and the light decomposed by one glass,
uniting with the colours produced by the other form white light, thus a
double convex lens of crown glass, C C, may be united with a
plano-convex lens of flint glass, F F, which must have a focus about
double the length of that of the crown-glass lens. The concave lens
corrects the colour or chromatic aberration of the other, and leaves
about one-half of the refracting power of the convex lens as the
effective magnifying power of the compound lens. The French opticians
cement the lenses very neatly together, and use them in ordinary spy and
opera glasses. (Fig. 307.)

[Illustration: Fig. 307. A compound achromatic lens, composed of C C,
the double-convex lens of crown-glass, and F F, the plano-concave lens
of flint-glass.]


XIV. _The Stereoscope._

This instrument has now attained a popularity quite equal to, if it does
not surpass, that formerly enjoyed by the kaleidoscope, and without
entering upon the much-vexed question of priority of discovery, it is
sufficient again to mention with the highest respect the names of Sir
David Brewster and Professor Wheatstone as identified with the discovery
and use of this most pleasing optical instrument.

The principle of the stereoscope (meaning, _solid I see_) is copied from
nature: _i.e._, when both eyes are employed in the examination of an
object, two separate pictures, embracing dissimilar forms, are impressed
upon the retinæ, and produce the effect of solidity; if the pictures
formed at the back of the eyes could be examined by another person with
a stereoscope, they would come together, and also produce the effect of
solidity.

Stereoscopic pictures are obtained by exposing sensitized paper in the
camera to the picture of an object taken in two positions, or two
cameras are employed to obtain the same result. If the latter mode is
adopted, the stereoscopic pictures must not be taken from positions too
widely separated from each other; or else, when the two pictures are
placed in the stereoscope, they will stand out with a relief that is
quite unnatural, and the object will appear like a very reduced solid
model, instead of having the natural appearance presented by pictures
which have been taken at positions too distant from each other.

Sir David Brewster says, "In order to obtain photographic pictures
mathematically exact, we must construct a binocular camera which will
[Page 321] take the pictures simultaneously, and of the same size; that
is, by a camera with two lenses of the same aperture and focal length,
placed at the same distance as the two eyes. As it is impossible to
grind and polish two lenses, whether single or achromatic, of exactly
the same focal lengths, even if we had the very same glass for each, I
propose to bisect the lenses, and construct the instrument with
semi-lenses, which will give us pictures of precisely the same size and
definition. These lenses should be placed with their diameters of
bisection parallel to one another, and at a distance of 2½ inches,
_which is the average distance of the eyes in man_; and when fixed in a
box of sufficient size, will form a binocular camera, which will give us
at the same instant, with the same lights and shadows, and of the same
size, such dissimilar pictures of statues, buildings, landscapes, and
living objects, as will reproduce them in relief in the stereoscope."
Thus with a single camera provided with semi-lenses, or two lenses of
the same focal length, stereoscopic pictures can be obtained.

To bring the images of the two pictures together, and produce the effect
of solidity; either of two instruments may be employed. The reflecting
stereoscope is the invention of Professor Wheatstone. The refracting or
lenticular stereoscope that of Sir David Brewster.

The former is constructed by placing two upright boards on a wooden
stand at a moderate distance from each other; the stereoscopic pictures
are attached to these boards, which may be made to move up or down, and
if the pictures are held in grooves, they may be pulled right or left at
pleasure, and thus four movements are secured--viz., upward, downward,
right, or left. Between the two stereoscopic pictures are placed two
looking-glasses, so adjusted that their backs form an angle of ninety
degrees with each other. (Fig. 308.)

[Illustration: Fig. 308. Wheatstone's reflecting stereoscope.]

The pictures are illuminated at night by a lamp or gas flame placed at
the back of the mirrors, which, when fixed together, have the same shape
as a prism; indeed, Professor Wheatstone substituted a prism for the
mirrors, and thus paved the way for the invention of the lenticular
stereoscope.

[Page 322]

The stereoscopic effect is obtained by bringing the eyes close to the
inclined mirrors, so that the two reflected images coincide at the
intersection of the optic axis; the coincidence of the images is further
secured by moving either picture a little to the right or left, and if
the upright boards move bodily in grooves to or from the centre mirror,
the greatest nicety of adjustment is procured.

During the last three years of the author's directorship of the
Polytechnic--viz., in 1856, 1857, 1858--nearly the whole of the pictures
shown by the dissolving-view apparatus were coloured photographs from
Mr. Hine's original pictures, painted two feet square in blue and white,
and reduced on the glass to about six inches square. The collodion film
being frequently thick and difficult to penetrate with light, was etched
and scratched away where required, and filled in with colour, and when
these pictures were looked at with _one_ eye only, they appeared to be
almost solid or stereoscopic on the disc.

The lenticular stereoscope consists of a box of a pyramidal shape, open
at the base, and provided with grooves in which are placed the
stereoscopic pictures; if the latter are taken on glass the base of the
box is held directly against the light, but if they are daguerreotypes
or paper pictures, then a side light is reflected upon them by means of
a lid covered in the inside with tinfoil, which is raised or lowered at
pleasure from the top part of the box. Two semi-lenses are now fitted
into the narrow part of the box, and are placed at such a distance from
each other that the centres of the semi-lenses correspond with the pupil
of the eyes, and this distance has already been stated to amount to
2½ inches. (Fig. 309.)

[Illustration: Fig. 309. Brewster's lenticular stereoscope.]

The principle of the lenticular stereoscope is perhaps better seen by
reference to the next diagram, in which the centres of the semi-lenses
(_i.e._, a lens cut in half) are placed at 2½ inches apart, with
their _thin_ edges towards each other, and marked, A B, Fig. 310. The
centres of the two stereoscopic pictures C D correspond with the centres
of the lenses, and the rays of light _diverging_ from C D fall upon the
semi-lenses, and being refracted nearly _parallel_ are, by the prismatic
form of the semi-lenses, deflected from their course, and leave the
surfaces of the lenses in the same direction as if they actually
emanated from E; and as all images of bodies appear to come in a
straight line from the point whence they are seen, the two pictures are
superimposed on each other, and together produce the appearance of
solidity, so that a stereoscopic result is obtained when the _spectral
images_ of the two stereoscopic pictures are made to overlap each other.
By taking one of the semi-lenses in each hand, and looking at the two
pictures, the over-lapping [Page 323] of the _spectral images_ becomes very
apparent, so that the combined _spectral images_, and not the _pictures_
themselves, are seen when we look into a stereoscope. (Fig. 310.)

[Illustration: Fig. 310.]

Sir David Brewster says, "In order that the two images may coalesce
without any effort or strain on the part of the eye, it is necessary
that the distance of the similar parts of the two drawings be equal to
twice the separation produced by the prism. For this purpose measure the
distance at which the semi-lenses give the most distinct view of the
stereoscopic pictures, and having ascertained by using one eye the
amount of the refraction produced at that distance, or the quantity by
which the image of one of the pictures is displaced, place the
stereoscopic pictures at a distance equal to twice that quantity--that
is, place the pictures so that the average distance of similar parts in
each is equal to twice that quantity. If this is not correctly done, the
eye of the observer will correct the error by making the images
coalesce, without being sensible that it is making any such effort. When
the dissimilar stereoscopic pictures are thus united, the solid will
appear standing as it were in relief between the two plane
representations."


XV. _The Stereomonoscope._

M. Claudet, whose name has long been celebrated in connexion with the
art of photography, has described an instrument by which a single
picture is made to simulate the appearance of solidity, and he states
that by means of this arrangement a number of persons may observe the
effect at the same time. The apparatus required is very simple,
consisting of a large double convex lens, and a screen of ground glass.
The [Page 324] object A, Fig. 311, is highly illuminated, and placed in
the focus of a double convex lens B, when an image of the object is
projected, and will be found suspended in the air in the conjugate focus
of the lens at C, and from this point the rays of light will diverge as
from a real object, which will be seen by separate spectators at D D and
E E; and if the screen of ground glass is placed at G G, the image will
appear with all the effect of length, breadth, and depth, which belong
to solid bodies. (Fig. 311.)

[Illustration: Fig. 311. The stereomonoscope.]

An image formed on ground glass in this manner can be seen only in the
direction of the incident rays, and the stereoscopic effect is not
apparent when the image is received on a calico or transparent screen,
on account of the rays being scattered in all directions.


XVI. _The Stereomoscope._

[Illustration: Fig. 312. The stereomoscope.]

This arrangement is an important modification of the other, and consists
of a screen of ground glass (A B, Fig. 312), and two convex [Page 325]
lenses (C D, and E F) arranged in such a manner that they will project
images of the stereoscopic pictures, G H, at the same point on the
screen, A B.

It might be thought that a confusion of images would result from
projecting two pictures on one point, P--viz., the focus of the two
lenses; but as each photograph can be seen only in the direction of its
own rays, it follows that if the eyes are so placed that each receives
the impression of one stereoscopic picture, the two images must
coalesce, and a stereoscopic effect will be the result, as is apparent
at K K and L L; so that several persons may look at the stereoscope at
one time. (Fig. 312.)


XVII. _The Pseudoscope._

[Illustration: Fig. 313. Horizontal section of the pseudoscope, showing
at A B two prisms placed against a block of wood about two inches long
and one inch and a half wide, and cut out in the centre to admit the
nose at D. The eyes are supposed to be looking at the globe, C, in the
direction of the arrows. E E. Brass plates blackened, which shut out the
side light, and assist in keeping the prisms in position.]

This curious optical instrument, as its name implies, produces a false
image by the refracting power of prisms, and is the invention of
Professor Wheatstone. When used with both eyes, the same as the
stereoscope, it inverts the relief of a solid body, and makes it appear
exactly as if it were an intaglio, or sunk beneath the line surrounding
it. For instance, a terrestrial globe when looked at through the
pseudoscope appears to be concave, like Wyld's Globe in
Leicester-square, instead of convex. A vase with raised ornaments upon
it looks as if it had been turned (to reverse the usual expression)
outside in, and [Page 326] the whole of its convexity is turned to
concavity; and of course a face seen under these circumstances looks
very curious. (Fig. 313.) The cause is perhaps somewhat difficult to
understand; but by taking other and more simple examples of the same
effect, the principle may be gradually comprehended.

Sir David Brewster, in his "Letters on Natural Magic," remarks that "one
of the most curious phenomena is that _false_ perception in vision by
which we conceive depressions to be elevations, and elevations
depressions--or by which intaglios are converted into cameos, and cameos
into intaglios. This curious fact seems to have been observed at one of
the early meetings of the Royal Society of London, when one of the
members, in looking at a guinea through a compound microscope of new
construction, was surprised to see the head upon the coin depressed,
while other members could only see it embossed, as it really was.... The
best method of observing this deception is to view the engraved seal of
a watch with the eye-piece of an achromatic telescope, or with a
compound microscope, or any combination of lenses which inverts the
objects that are viewed through it; a single convex lens will answer the
purpose, provided we hold the eye six or eight inches behind the image
of the seal formed in its conjugate focus."

After bringing forward various interesting experiments in further
explanation of the cause, Sir D. Brewster states it to be his belief
that the illusion is the result of an operation of our own minds,
whereby we judge of the forms of bodies by the knowledge we have
acquired of light and shadow. Hence, the illusion depends on the
accuracy and extent of our knowledge on this subject; and while some
persons are under its influence, others are entirely insensible to it.
This statement is borne out by experience, as the author, whilst
Resident Director of the Polytechnic, had four of Wheatstone's
pseudoscopes placed in the gallery, with proper objects behind them; and
he frequently noticed that some visitors would look through the
instrument and see no alteration of the convex objects, whilst others
would shout with delight, and call their friends to witness the strange
metamorphosis, who in their turn might disappoint the caller by being
perfectly insensible to its strange effects.

The pseudo-effects of vision are not confined to the results already
explained, but are to be observed especially whilst travelling in a
coach, when the eyes may be so fixed as to give the impression of
movement to the trees and houses, whilst the coach appears to stand
still. In railway carriages, after riding for some time and then coming
to a stand still, if another train is set slowly in motion by the one at
rest, it frequently happens that the latter appears to be moving instead
of the former.

[Page 327]




CHAPTER XXIV.

THE ABSORPTION OF LIGHT.


The analysis of light has been explained in a previous chapter, and it
has been shown how the spectrum is produced. Colour, however, may be
obtained by other means, and the property enjoyed by certain bodies, of
absorbing certain coloured rays in preference to others, offers another
mode of decomposing light.

The property of absorption is shown to us in every kind of degree by
innumerable natural and artificial substances; and by examining the
spectrum through a wedge of blue glass, Sir David Brewster was enabled
to separate the seven colours of the spectrum into the three primary
colours, red, yellow, and blue, which he proved existed at every point
of the spectrum, and by over-lapping each other in various proportions,
produce the compound colours of orange, green, indigo, and violet.

Connected with this property is the remarkable effect produced by
coloured light on ordinary colours, and the sickly hue cast upon the
ghost in a melodrama, or the fiery complexion imparted to the hair of
Der Freischutz, or the jaundiced appearance presented by every member of
a juvenile assembly when illuminated with a yellow light from the salt
and burning spirit of "snapdragon," are too well known to require a
lengthened description here.

If a number of colours are painted on cardboard, or groups of plants,
flowers, flags, and shawls, are illuminated by a mono-chromatic light,
and especially the light procured from a large _tow_ torch well supplied
with salt and spirit, the effect is certainly very remarkable; at the
same time it shows how completely substances owe their colour to the
light by which they are illuminated, and it also indicates why ladies
cannot choose colours by candle light, unless of course they propose to
wear the dress only at night, when it is quite prudent to see the
colours in a room lit with gas; and this fact is so well known that with
the chief drapers, such as at Messrs. Halling, Pearce, and Stone's,
Waterloo House, a darkened room lit with gas is provided during the
daytime to enable purchasers of coloured dresses to judge of the effect
of artificial light upon them. Whilst the flowers, &c., are lighted up
with the yellow light, a magical change is brought about by throwing on
suddenly the rays from the oxy-hydrogen light, when the colours are
again restored; or if the latter apparatus is not ready, the combustion
of phosphorus in a jar of oxygen will answer the same purpose. The light
obtained from the combustion of gas affords an excess of the yellow or
red rays of light, which causes the difference between candlelight and
daylight colours already alluded to.

[Page 328]




CHAPTER XXV.

THE INFLECTION OR DIFFRACTION OF LIGHT.


In this part of the subject it is absolutely necessary to return to the
theory of undulations with which the present subject was commenced. The
inflection of light offers a third method by which rays of light may be
decomposed and colour produced. The phenomena are extremely beautiful,
although the explanation of them is almost too intricate for a popular
work of this kind.

The cases where colour is produced by inflection are more numerous than
might at first be supposed; thus, if we look at a gaslight or the
setting sun through a wire gauze blind, protecting the eye with a little
tank of dilute ink, a most beautiful coloured cross is apparent. An
extremely thin film of a transparent matter, such as a little naphtha or
varnish dropped on the surface of warm water or soap bubbles, or a very
thin film of glass obtained by blowing out a bulb of red-hot glass till
it bursts, or an exquisitely thin plate of talc or mica, all present the
phenomena of colour, although they are individually transparent, and in
ordinary thicknesses quite colourless.

[Illustration: Fig. 314. The two lenses, with the plate or film of air
between them, and producing seven coloured rings when the lenses are
brought sufficiently close to each other by the screws.]

Sir Isaac Newton brought his powerful intellect to bear on these facts,
and as a preliminary step invented an instrument for measuring the exact
thickness of those transparent substances that afforded colour, and the
apparatus displaying Newton's rings is still a favourite optical
experiment. It consists of a plano-convex lens, A. (Fig. 314), a slice,
as it were, from a globe of glass twenty-eight feet in diameter, or the
radius of whose convex surface is fourteen feet. This plano-convex lens
is placed on another double convex lens, B., whose convex surfaces have
a radius of fifty feet each, consequently the lenses are very shallow,
and the space (C C) included between them being filled with air, can of
course be accurately measured. (Fig. 314.) It is usual to mount the
lenses in brass rings which are brought together with screws, when the
most beautiful coloured rings are apparent, and are produced by the
extreme thinness of the film or plate of air enclosed between the two
lenses; and [Page 329] the relative thicknesses of the plates of air at
which each coloured light is reflected are as follows:--

  Red         133       10 millionths of an inch.
  Orange      120            "           "
  Yellow      113½           "           "
  Green       105½           "           "
  Blue         98            "           "
  Indigo       92½           "           "
  Violet       83½           "           "

By dividing an inch into ten millions of parts, and by taking 133 of
such parts, the thickness of the film of air required to reflect the red
ray is obtained, and in like manner the other colours require the minute
thicknesses of air recorded in the table above. When the thickness of
the film of air is about 12/178,000dths of an inch, the colours cease to
become visible, owing to the union of all the separate colours forming
white light, but if the Newton rings are produced in mono-chromatic
light, then a greater number of rings are apparent, but of one colour
only, and alternating with black rings, _i.e._, a dark and a yellow
succeeding each other; this fact is of great importance as an
illustration of the undulatory theory, and demonstrates the important
truth, that _two rays of light may interfere with each other in such a
manner as to produce darkness_.

Sir David Brewster remarks that, "From his experiments on the colours of
thin and of thick plates, Newton inferred that they were produced by a
singular property of the particles of light, in virtue of which they
possess, at different joints of their paths, _fits_ or dispositions to
be reflected from or transmitted by transparent bodies. Sir Isaac does
not pretend to explain the origin of these _fits_, or the cause which
produces them, but terms them _fits of transmission_ and _fits of
reflexion_."

Sir Isaac Newton objected to the theory of undulations because
experiments seemed to show that light could not travel through bent
tubes, which it ought to do if propagated by undulations like sound; and
it was reserved for the late Dr. Young to prove that light could and
would turn a corner, in his highly philosophical experiments
illustrating the inflection or bending in of the rays of light.

Dr. Young placed before a hole in a shutter a piece of thick paper
perforated with a fine needle, and receiving through it the diverging
beams on a paper screen, found that when a slip of cardboard
one-thirtieth of an inch in breadth was held in such a beam of light,
that the shadow of the card was not merely a dark band, but divided into
light and dark parallel bands, and instead of the centre of the shadow
being the darkest part, it was actually white. Dr. Young ascertained
that if he intercepted the light passing _on one side_ of the slip of
card with any opaque body, and allowed the light to pass freely on the
other side of the slip of cardboard, that all the bands and the white
band in the centre disappeared, and hence he concluded that the bands or
fringes within the shadow were produced _by the interference [Page 330]
of the rays bent into the shadow by one side of the card, with the rays
bent into the shadow by the other side_. (Fig. 315).

[Illustration: Fig. 315.]

In order to show how two waves may interfere so as to exalt or destroy
each other, two sets of waves may be propagated on the surface of a
still tank or bath of water, from the two points A A (Fig. 315), the
black lines or circles representing the tops of the waves. It will be
seen that along the lines B B the waves interfere just half way between
each other, so that in all these directions there will be a smooth
surface, provided each set of waves is produced by precisely the same
degree of disturbing force, so as to be perfectly equal and alike in
every respect, and the first wave of one set exactly half a wave in
advance of the first wave of the other, while at the curve in the
direction of all the line C C, the waves coincide, and produce
elevations or undulations of double extent; in the intermediate spaces,
intermediate effects will, of course, be produced.

Professor Wheatstone has invented some very simple and beautiful
acoustic apparatus for the purpose of proving that the same laws of
interference exist also in sound, which, as already stated, consists in
the vibrations or undulation of the particles of air.

[Page 331]

The nature and effects of interference are also admirably illustrated by
the following models of Mr. Charles Woodward, President of the Islington
Scientific Institution, and to whom we have already alluded.

[Illustration: Fig. 316.--No. 1. A model of waves with moveable
rods.--No. 2. A model of fixed waves.--No. 3. Intensity of waves doubled
by the superposition and coincidence of two equal systems.--No. 4. Waves
neutralized by the superposition and interference of two equal systems,
the raised part of one wave accurately fitting into and making smooth
the hollow of the other, illustrating the fact that two waves of light
or sound may destroy each other.]

[Illustration: Fig. 317. Appearance of Newton's rings when produced in
yellow light, 1, 3, 5, 7, being the yellow rings, and 2, 4, 6, 8, the
dark rings. Light by the odd numbers; darkness by the even numbers. The
central spot, where the two surfaces are in contact, is dark.]

Returning again to the coloured rings, we find that Newton discovered
that at whatever thickness of the film of air the coloured ring first
appeared, there would be found at twice that thickness the dark ring, at
three times the coloured, at four times the dark, and so on, _the
coloured rings_ regularly occurring at the _odd numbers_, and the _dark
ones_ at the _even numbers_. This discovery is well illustrated by the
models (Fig. 316); and it maybe noticed at No. 3 that the highest and
the lowest parts of the waves [Page 332] interfere, but coincide and
produce a wave of double intensity; the little crosses of the upper
model are in a straight line with the numbers 1, 3, 5, 7, and are
supposed to represent the coloured rings, whilst in No. 4 the upper
series of waves is half an undulation in advance of the lower; and if
the eye is again directed from the little crosses downward, the figures
2, 4, 6, 8, even numbers, are apparent, and represent the dark rings,
when the waves of light destroy each other. The phenomena of thin
plates, such as colours from soap bubbles, and the films of varnish, are
well explained by the law of interference. The light reflected from the
second surface of the film of air (which must of course, however thin,
have two surfaces, viz., a upper and a lower one) interferes with the
light reflected from the first, and as they come from different points
of space, one set of waves is in advance of the other, No. 4, Fig. 316;
they reach the eye with different lengths of paths, and by their
_interference_ form alternately the luminous and dark fringes, bands, or
circles. Bridge's diffraction apparatus, manufactured only by Elliott
Brothers, offers itself specially as a most beautiful drawing-room
optical instrument. The purpose of this apparatus is to illustrate in
great variety, and in the most convenient and compact form, the
phenomena of the diffraction or interference of light. This is attained
by the assistance of photography. Transparent apertures in an opaque
collodion film are produced on glass, and a point of light is viewed
through the apertures. [Page 333] The forms of the apertures are
exceedingly various,--triangles, squares, circles, ellipses, parabolas,
hyperbolas, and combinations of them, besides many figures of fanciful
forms, are included in the set. When an image of the sun is viewed
through these apertures, figures of extraordinary beauty, both of form
and colour, are produced; and of each of these many variations may be
obtained by placing the eye-glass of the telescope at different
distances from the object glass. Many of the figures produced,
especially when the telescope is out of focus, might suggest very useful
hints to those concerned in designing patterns. Although the phenomena
are chiefly of interest to the student of science, in consequence of
their bearing on theories of light, yet their beauty and variety render
them amusing to all. A few words on the mode of using the apparatus may
be of service. (Fig. 318.)

[Illustration: Fig. 318. Elliott Brothers' diffraction apparatus.]

Choose a very bright day, for then only can the apparatus be used. Place
the mirror in the sun, and let the light be reflected on the back of the
blackened screen. The lens which is inserted into this screen will then
form an exceedingly bright image of the sun. Then at the distance of not
less than twelve feet, clamp the telescope to a table in such a position
as to view the image thus formed. Put the eccentric cap on the end of
the telescope, clean the glass objects carefully, and attach them to the
cap so that they may be turned each in order before the telescope. In
this manner, all those which consist of a series of figures may be
viewed. Then detach the eccentric cap, and replace it by the other. Into
it place any of the single objects. In viewing some of the figures,
brightness is advantageous--in others, delicacy; in the former case, let
the lens of long focus be inserted in the screen--in the latter case,
that of shorter focus. In every case, let the phenomena be observed not
only when the telescope is in focus, but also when the eye-glass is
pushed in to various distances.

Mr. Warren de la Rue has ingeniously taken advantage of the colours
produced by thin films of varnish, and actually _fixed_ the lovely
iridescent colour produced in that manner on highly polished paper,
which is termed "iridescent paper." A tank of warm water at 80° Fahr.,
about [Page 334] six inches deep, and two feet six inches square, is
provided, and a highly glazed sheet of white or black paper being first
wetted on a perforated metallic plate, is then sunk with the plate below
its surface, care being taken to avoid air bubbles. A peculiar varnish
is then allowed to trickle slowly down a sort of tongue of metal placed
in the middle of one of the sides of the tank, and directly the varnish
touches the surface of the water it begins to spread out in exquisitely
thin films, and by watching the operation close to a window and skimming
away all the imperfect films, a perfect one is at last obtained, and at
that moment the paper lying on the metal plate is raised from the bottom
of the tank, and the delicate film of varnish secured. When dry, the
iridescent colours are apparent, and the paper is employed for many
ornamental purposes. An extremely simple and pretty method of producing
Newton's rings has been invented by Reade, and is called "Reade's
iriscope." A plate of glass of any shape (perhaps circular is the best)
is painted on one side with some quickly drying black paint or varnish,
and after the other side has been cleaned, it is then rubbed over with a
piece of wet soap, and this is rubbed off with a clean soft duster. A
tube of about half an inch in diameter, and twelve inches long, is
provided, and is held about one inch above the centre of the soaped side
of the glass plate, and directly the breath is directed down the tube on
the glass, an immense number of minute particles of moisture are
deposited on the glass, and these by inflection decompose the light, and
all the colours of the rainbow are produced. (Fig. 319.)

[Illustration: Fig. 319. Reade's iriscope.]

The iridescent colours seen upon the surface of _mother-of-pearl_, which
Mr. Simonds' excellent commercial dictionary tells us is "the name for
the iridescent shell of the pearl oyster, and other molluscs," are
referrible to fine parallel lines formed by its texture, and are
reproducible, according to Brewster's experiments, by taking impressions
of them in soft wax. The gorgeous colours of certain shells and fish,
the feathers of birds, Barton's steel buttons, are not due to any
inherent _pigment_ or colouring matter that could be extracted from
them, but are owing either to the peculiar fibrous, or parallel-lined,
or laminated (plate-like) surfaces upon which the light falls, and being
reflected in paths of different lengths, interference occurs, and
coloured light is produced.

[Page 335]




CHAPTER XXVI.

THE POLARIZATION OF LIGHT.


This branch of the phenomena of light includes some of the most
remarkable and gorgeous chromatic effects; at the same time, regarded
philosophically, it is certainly a most difficult subject to place in a
purely elementary manner before the youthful minds of juvenile
philosophers, and unless the previous chapter on the diffraction of
light is carefully examined, the rationale of the illustrations of
polarized light will hardly be appreciated. We have first to ask, "What
is polarized light?" The answer requires us again to carry our thoughts
back to the consideration of the undulatory theory of light, already
illustrated and partly explained at pages 262, 330.

After perusing this portion of the subject, it might be considered that
waves of light were constituted of one motion only, and that an
undulation might be either perpendicular or horizontal, according to
circumstances. (Fig. 320.)

[Illustration: Fig. 320.--No. 1. A wire bent to represent a
perpendicular vibration, which if kept in the latter position, will only
pass through a perpendicular aperture.--No. 2. A wire bent to represent
a horizontal wave which will only pass through a horizontal aperture.]

This simple condition of the waves of light could not, however, be
reconciled theoretically with the actual facts, and it is necessary in
regarding a ray of light, to consider it as a combination of two
vibrating motions, one of which, for the sake of simplicity, may be
considered as perpendicular, and the other horizontal; and this idea of
the nature of [Page 336] an undulation of light originated with the
late Dr. Young, who while considering the results of Sir D. Brewster's
researches on the laws of double refraction, first proposed the theory
of transversal (cross-wise) vibration. Dr. Young illustrated his theory
with a stretched cord, which if agitated or violently shaken
perpendicularly, produces a wave that runs along the cord to the other
end, and may be often seen illustrated on the banks of a river overhung
with high bushes; the bargemen who drive the horses pulling the vessel
by a rope, would be continually stopped by the stunted thick bushes, but
directly they approach them, they give the horse a lash, and then
violently agitate the rope vertically, which is thrown into waves that
pass along the rope, and clear the bushes in the most perfect manner.
(Fig. 321.)

[Illustration: Fig. 321. Bargeman throwing his tow-rope into waves to
get it over the thick bushes.]

[Illustration: Fig. 322. A section of a wave of common light made up of
the transversal vibration, A B and C D.]

Now if a similar movement is made with the stretched rope from right to
left, another wave will be produced, which will run along the cord in an
horizontal position, and if the latter is compared with the
perpendicular undulation, it will be evident that each set of waves will
be in planes at right angles to and independent of each other. This is
supposed to be the mechanism of a wave of common light, so that if a
section is taken of such an undulation, it will be represented by a
circle A B C D (Fig. 322), with two diameters A B, and C D; or a better
mechanical notion of a wave of common [Page 337] light is acquired from
the inspection of another of Mr. Woodward's cardboard models. (Fig.
323.)

[Illustration: Fig. 323. Model of a wave of common light.]

The existence of an _alternating motion of some kind_ at minute
intervals along a ray is, says Professor Baden Powell, "as real as the
motion of translation by which light is propagated through space. _Both_
must essentially be _combined_ in any correct conception we form of
light. That this alternating motion must have reference to certain
directions _transverse_ to that of the ray is equally established as a
consequence of the phenomena; and these _two_ principles must form the
basis of any explanation which can be attempted." A beam of common light
is therefore to be regarded as a rapid succession of systems of waves in
which the vibrations take place in different planes.

If the two systems of waves are separated the one from the other, viz.,
the horizontal from the perpendicular, they each form separately a ray
of polarized light, and as Fresnel has remarked, _common light_ is
merely _polarized light_, having _two planes_ of polarization at _right
angles_ to each other. To follow up the mechanical notion of the nature
of polarized light, it is necessary to refer again to Woodward's card
wave model (Fig. 323), and by separating the two cards one from the
other it may be demonstrated how a wave of common light reduced to its
skeleton or primary form is reducible into two waves of polarized light,
or how the two cards placed together again in a transversal position
form a ray of common light. (Fig. 324.)

[Illustration: Fig. 324.--No. 1. Common light, made up of the two waves
of polarized light, Nos. 2 and 3. ]

The query with respect to the nature of polarized light being answered,
it is necessary, in the next place, to consider how the separation of
these transversal vibrations may be effected, and in fact to ask what
optical arrangements are necessary to procure a beam of polarized light?
Light may be polarized in four different ways--viz., by reflection,
single refraction, double refraction, and by the tourmaline--viz., by
absorption.

[Page 338]

_Polarization by Reflection, and by Single Refraction._

[Illustration: Fig. 325.--No. 1. A is the lime light. B. The condenser
lenses. C. The beam of _common_ light. Here the glass plates are
removed.--No. 2. A. Lime light. B. The condenser lenses. C C. The bundle
of plates of glass at an angle of 56° 45´. D is the ray of light
polarized by reflection from the glass plates, C C, and E is the beam of
polarized light by single refraction, having passed through the bundle
of plates of glass, C C.]

In the year 1810, the celebrated French philosopher, Mons. Malus, while
looking through a prism of Iceland spar, at the light of the setting
sun, reflected from the windows of the Luxemburg palace in Paris,
discovered that a beam of light reflected from a plate of glass at an
angle of 56 degrees, presented precisely the same properties as one of
the rays formed by a rhomb of Iceland spar, and that it was in fact
polarized. _One_ of the transversal waves of polarized light of the
common light, being reflected or thrown off from the surface of the
glass, whilst the other and second transversal vibration passed
_through_ the plate of glass, and was likewise polarized in another
plane, but by _single refraction_, so that the experiment illustrates
two of the modes of polarizing light-viz., by reflection, and by single
refraction. This important elementary truth is beautifully illustrated
by Mr. J. T. Goddard's new form of the oxy-hydrogen polariscope, by
which a beam of common light traverses a long square tin box without
change; but directly a bundle of plates of glass composed of ten plates
of thin flattened crown glass, or sixteen plates of thin parallel glass
plates used for microscopes, are slid into the box at an angle of 56°
45´, then the beam of common [Page 339] light is split into two beams
of polarized light, which pursue their respective paths, one passing by
single refraction through the glass, and the other being reflected, and
rendered apparent by opening an aperture over the glass plates, and then
again by using a little smoke from brown paper, the course of the rays
becomes more apparent.

[Illustration: Fig. 326. A A. Model in wood of a bundle of plates of
glass at an angle of 56° 45´. B. Beam of common light, with transversal
vibration. C. Light polarized by reflection. D. Light polarized by
refraction.]

The same truth is well illustrated by the cardboard model wave and a
wooden plane with horizontal and perpendicular slits, placed at an angle
of 56° 45´, as at Fig. 326.


POLARIZATION BY DOUBLE REFRACTION.

The name of _Double_-refracting or Iceland Spar is given to a very
clear, limpid, and perfectly transparent mineral, composed of carbonate
of lime, and found on the eastern coast of Iceland. Its crystallographic
features are well described by the Rev. Walter Mitchell in his learned
work on mineralogy and crystallography, and it is sufficient for the
object of this article to state that it crystallizes in rhombs, and
modifications of the rhomboidal system. It must not be confounded with
rock or mountain crystal, which, under the name of quartz, crystallizes
in six-sided prisms with six-sided pyramidal tops; quartz being composed
of silica, or silicic acid and calcareous spar of carbonate of lime.
Very large specimens of the latter mineral are rare and valuable, and
the _lion_ of specimens of calcareous, or double-refracting spar, is now
in the possession of Professor Tennant, the eminent mineralogist of the
Strand. It is nine inches high, seven and three-quarters inches broad,
and five and a half inches thick; its estimated value being 100_l._ This
beautiful specimen has been photographed, and its stereograph
illustrates in a very striking manner the double refracting properties
of the spar.

If a printed slip of paper is placed behind a rhomb of Iceland spar, two
images of the former are apparent, and the stereograph already alluded
to shows this fact very perfectly, at the same time illustrates the
value of the stereoscope. Out of the stereoscope the words "Stereoscopic
Magazine" appear doubled, but seem to lie in the same plane; but
directly the picture is placed in the instrument, then it is clearly
seen that one image is evidently in a very different plane from the
other. The double-refracting power of this mineral is illustrated by
holding a small rhomb of Iceland spar, placed in a proper brass tube
before the orifice as at Fig. 327, from which the rays of common light
are [Page 340] passing; if an opaque screen of brass perforated with a
small hole is introduced behind the rhomb, then, instead of one circle
of light being apparent on the screen, two are produced, and both the
rays issuing in this manner are polarized, one being termed the ordinary
and the other the extraordinary ray. (Fig. 327.)

[Illustration: Fig. 327. A. The condensers. B. The hole in the brass
screen or stop. C. The rhomb of Iceland spar. O. The ordinary, and E the
extraordinary, ray, both of which are polarized light.]

The polarizing property of the rhomb is perhaps better shown by the next
diagram, where A B represents the obtuse angles of the Iceland spar, and
a line drawn from A to B, would be the axis of the crystal. The
incidental ray of common light is shown at C, and the oppositely
polarized transmitted rays called the ordinary ray O, and extraordinary
ray E, emerge from the opposite face of the rhomboid. If a black line is
ruled on a sheet of paper as at K K, and examined by the eye at C, it
appears double as at K K and J J. (Fig. 328.)

[Illustration: Fig. 328. Rhomb of Iceland spar.]

The cardboard model is again useful in demonstrating the polarization of
light by double refraction, and if a model of a rhomb of Iceland [Page
341] spar is made of glass plates, one face of which has an aperture
like a cross, and the other a horizontal and perpendicular slit, as at
Nos. 1 and 2 (Fig. 329), the production of the ordinary and
extraordinary rays is demonstrated in a familiar manner, and is easily
comprehended.

[Illustration: Fig. 329.--No. 1. One face of the model rhomb to admit
the transversal vibration, represented by the cardboard model.--No. 2.
The opposite face of the rhomb, from which issue the polarized,
ordinary, and extraordinary rays.--No. 3. Side view of the model.]

In Newton's "Optics" we find the following description of Iceland
spar:--"This crystal is a pellucid fissile stone, clear as water or
crystal of the rock (quartz), and without colour.... Being rubbed on
cloth it attracts pieces of straw and other light things like amber or
glass, and with aquafortis it makes an ebullition.... If a piece of this
crystalline stone be laid upon a book, every letter of the book seen
through it will appear double by means of a double refraction."


POLARIZATION BY THE TOURMALINE.

This mineral was first discovered during the sixteenth century, in the
island of Ceylon, afterwards in Brazil, and since that period at various
localities in the four quarters of the globe. In the Grevillian
collection purchased many years ago by government for the British
Museum, there is a fine specimen of red tourmaline valued at 500_l._ The
green tourmaline is named Brazilian emerald, and the Berlin blue
tourmaline is called Brazilian sapphire; the mineral chiefly consists of
sand (silica) and alumina, with a small quantity of lime, or potash, or
soda, boracic acid, and sometimes oxide of iron or manganese. When light
is passed through a slice of this mineral it is immediately polarized,
one of the transversal vibrations being absorbed, stopped, or otherwise
disposed of, the other only emerging from the tourmaline, consequently
it is one of the most convenient polarizers, although the polarized
light partakes of the accidental colour of the mineral. Green, blue, and
yellow tourmalines are bad polarizers, but the brown and pink varieties
[Page 342] are very good, and it is a most curious fact that white
tourmaline does not polarize. (Fig. 330.)

[Illustration: Fig. 330. Crystal of tourmaline slit (parallel to the
axis) into four plates, which when ground and polished, may be used for
the polarization of light.]

The mineral crystallizes in long prisms, whose primitive form is the
obtuse rhomboid, having the axis parallel to the axis of the prism. The
term axis with reference to the earth, as shown at page 16, is an
imaginary _single line_ around which the mass rotates, but in a crystal
it means a _single direction_, because a crystal is made up of a number
of similar crystals, each of which must have its axis, thus the whitest
Carrara marble reduced to fine powder, moistened with water and placed
under a microscope, is found to consist chiefly of minute rhomboids,
similar to calcareous spar. The smallest crystal of this mineral is
divisible again and without limit into other rhombs, each of which
possesses an axis. (Fig. 331.)

[Illustration: Fig. 331 represents a crystal, the axis of which is the
direction A B. The dotted lines show the division of the large crystal
into four other and smaller ones, each of which has its axis, A C, C B,
D E, F G; and every line within the large crystal parallel to A B is an
axis, consequently the term is employed usually in the plural number
_axes_.]

If a plate of tourmaline is held before the eye whilst looking at the
sun (like the gay youth in Hogarth's picture who is being arrested
whilst absorbed with the wonders of a tourmaline, which was, in the
great painter's time, a popular curiosity,) it may be turned round in
all directions without the slightest difference in the appearance of the
light, which will be coloured by the accidental tint of the crystal, but
if a second slice of tourmaline is placed behind the other, there will
be found certain directions in which the light passes through both the
slices, whilst in other positions the light is completely cut off.

[Page 343]

When the axes of both plates coincide, the light polarized by one
tourmaline will pass through the other, but if the axes do not coincide,
and are at right angles to each other, then the polarized light is
entirely stopped, and the _rationale_ of this will be appreciated at
once if a tourmaline is regarded (mechanically) as if it were like a
grating with perpendicular bars through which the polarized light will
pass. Any number of such gratings with the bars parallel would not stop
the polarized light, but if the second grating is turned round ninety
degrees, the bars will be at right angles to those of the first grating,
and the perpendicular wave of polarized light cannot pass. (Fig. 332.)

[Illustration: Fig. 332. A. Model of the first slice of tourmaline into
which the transversal vibrations, B, are passing; the horizontal wave is
absorbed, and the perpendicular polarized one proceeds to the second
slice of tourmaline, C, where the bars (the axes) being at right angles
to those of A, it is stopped, and cannot pass through until the bars of
C are parallel with A.]


_Splendid Chromatic effects produced by Polarized Light._

Having discussed the various modes of obtaining polarized light, the
next step is to arrange an apparatus by which certain double refracting
crystals, and other bodies, shall divide a ray of polarized light, and
then by subsequent treatment with another polarizing surface, the
divided rays are caused to _interfere_ with each other, and afford the
phenomena of colour. Bodies that refract light singly, such as gases,
vapours or liquids, annealed glass, jelly, gums, resins, crystallized
bodies of the tessular system, such as the cube and octohedron, do not
afford any of the results which will be explained presently, except by
the influence of pressure, as in unannealed glass, or a bent cold glass
bar. By compression or dilatation, they are changed to double refractors
of light. The bodies that possess the property of double refraction
(though not to the visible extent of Iceland spar), are all other bodies
such as crystallized chemicals, salts, crystallized minerals, animal and
vegetable substances possessing a uniform structure, such as horn and
quill; all these substances divide the ray of polarized light into two
parts, and by placing a thin film of a crystal of selenite (which is one
of the best minerals that can be used for the purpose) in the path of
the beam of polarized light, coming either from the glass plates, as in
No. 2, (Fig. 325), page 338, or from a slice of tourmaline, and then
receiving it through the ordinary focusing lenses or object-glasses of
the oxy-hydrogen microscope, no colour is yet apparent in the image of
the selenite on the screen, until [Page 344] another tourmaline, or a
bundle of glass plates, is placed at an angle of 56° 45´, and at right
angles to the plane of reflection of the first set of plates; then the
most gorgeous colours suddenly appear over all parts of the film of
selenite as depicted on the screen, like other objects shown by the
oxy-hydrogen microscope. (Fig. 333.)

[Illustration: Fig. 333. Duboscq's polarizing apparatus, A. The light
and the condenser lens. B. The plates of glass at the proper angle, C.
The selenite object, D. The focusing lens. E. The second bundle of
plates of glass called the analyser, F. A stop for extraneous rays of
light, G. The image of the film of selenite most beautifully coloured.

Goddard's oxy-hydrogen polariscope is one of the most convenient,
because either the reflected or refracted polarized rays can be rendered
available; it consists of the apparatus shown at Fig. 325, and to this
is added a low microscope power, and stage to hold the selenite or other
objects, with another bundle of sixteen plates of the thin microscopic
glass or mica, called the analyser. A slice of tourmaline, or a Nicol's
prism may be employed, instead of the second bundle of reflecting
plates. When the ray of polarized light reflected from the first set of
glass plates enters the doubly refracting film of selenite, which is
about the fortieth or fiftieth part of an inch in thickness, it is split
into the ordinary and extraordinary rays, and is said to be
_dipolarized_, and forms two planes of polarized light, vibrating at
right angles to each other. When the latter are received on another
bundle of plates of glass called the analyser, at an angle of 56° 45´,
but at right angles to the first set of glass plates, they interfere,
because in the passage of the two rays from the selenite they have
traversed it in different directions, with different velocities; one of
these sets of waves will therefore, on emerging from the opposite face
of the selenite be retarded, and lie [Page 345] behind the other; but
being polarized in different planes, they cannot _interfere_ until their
planes of polarization are made to coincide, which is [Page 346]
effected by means of the second bundle of glass plates called the
analyser; and when this is brought into a position at right angles to
the first set of reflecting glass plates, half the ordinary wave
interferes with half the extraordinary wave; and being transmitted
through the analyser, produces, say red and orange, whilst the remaining
halves also interfere, and being reflected, afford the complementary
colours green and blue. (Fig. 334.) The term _complementary_ is intended
to define any two colours containing red, yellow, and blue, because the
three combined together produce white light; for example, the
complementary colour to red would be green, because the latter contains
yellow and blue; the complementary colour to orange would be blue,
because the former contains red and yellow. Any two colours, therefore,
which together contain red, yellow, and blue are said to be
_complementary_; and if this principle was better understood, ladies
would never commit such egregious blunders as they occasionally do in
the choice of colours for bonnets and dresses, and select a blue bonnet
to be worn with a green dress, or _vice versâ_. By rotating the
analyser, the reflected and refracted rays change colours, and if the
former is red and the latter green, by moving the analyser round 90°,
the reflected rays change to green and the refracted to red; at 180° the
colours again change places; at 270° the reflected ray will be again
green, and the refracted red; to be once more brought back at 360° to
the original position, viz., reflected rays red, refracted green. The
thickness of the films of selenite determines the particular colour
produced.

[Illustration: Fig. 334. The electric lamp and lantern of Duboscq,
showing the projection of the carbon poles on the disc. This experiment
is performed with the help of the plano-convex lens, A, and the rays
pass through a very narrow aperture at B.]

[Illustration: Fig. 335. A A. Card model of a beam of polarized light
coming from the first bundle of plates of glass, shown at Fig. 326, p.
339. B. Model of the film of selenite, which divides or dipolarizes the
ray A A into C and D, which, interfering by means of the second bundle
of plates of glass called the analyser Z, produce reflected chromatic
effects by interference at E, and refracted effects at F.]

If the selenite is of a uniform thickness, one colour only is obtained,
and by ingeniously connecting pieces of various thicknesses (in the same
forms as stained glass for cathedral windows), the most beautiful
designs were made by the late Mr. J. T. Cooper, jun., which have since
been manufactured in great quantity and variety by Mr. Darker, of
Paradise-street, Lambeth. The colours of these selenite objects are seen
by placing them in front of a piece of black glass, fixed at the
polarizing angle, and then examining the design with a slice of
tourmaline, or still better with a single-image Nicol prism, when the
most brilliant colours are obtained, and varied at every change of the
angle of the analyser.

Selenite, or sparry-gypsum, is the native crystallized sulphate of lime,
which contains water of crystallization (CaO, SO_{3}, 2H_{2}O). It
frequently occurs imbedded in London clay, and is called _quarry glass_
by the labourers who find it at Shotover Hill, near Oxford, and also in
the Isle of Sheppey.

At a very early period, before the discovery of glass, selenite was used
for windows; and we are told that in the time of Seneca, it was imported
into Rome from Spain, Cyprus, Cappadocia, and even from Africa. It
continued to be used for this purpose until the middle ages, for Albinus
informs us, that in his time, the windows of the dome of Merseburg were
of this mineral. The first greenhouses, those invented by Tiberius, were
covered with selenite. According to Pliny, beehives were encased in
selenite, in order that the bees might be seen at work.

[Page 347]

The late Dr. Pereira has placed the phenomena already described in the
form of a most instructive diagram, which we borrow from his elaborate
work on "Polarized Light." (Fig. 336.)

[Illustration: Fig. 336. A. A ray of common or unpolarized light,
incident on B. B. The polarizer (a plate of tourmaline). C. A ray of
plane polarized light, incident on D. D. The doubly-refracting film of
selenite. E. The extraordinary ray. O. The ordinary ray, produced by the
double refraction of the ray C. G. The analyser (or doubly-refracting or
Nicol's prism). E O. The ordinary ray. E E. The extraordinary ray,
produced by the double refraction of the extraordinary ray, E. O O. The
ordinary ray. O E. The extraordinary ray, produced by the double
refraction of the ordinary ray, O.]

[Illustration: Fig. 337. No. 1. Unannealed glass for the polariscope.
Nos. 2 and 3. Appearance of the black cross and coloured circles in a
square and circular piece of unannealed glass in the polariscope.]

The chromatic effects described are not confined to selenite objects
only, but are obtained from glass, provided the particles are in a state
of unequal tension, as in masses of unannealed glass of various forms.
(Fig. 337.) Consequently, polarized light becomes a most valuable means
for ascertaining the condition of particles otherwise invisible and
inappreciable. One of the most beautiful experiments can be made [Page
348] with a bar of plate-glass, which refracts light singly until
pressure is applied to the centre, in order to bend it into an arch or
curve, when the appearance presented in Fig. 338 is apparent.

[Illustration: Fig. 338. A B. Bar of glass under the pressure of the
screw C, and appearance of bands or fringes of coloured light, which
entirely disappear on the removal of the screw. An effect, of course,
only visible by polarized light.]

A quill placed in the polarizing apparatus is also discovered to be in a
state of unequal tension by the appearance of coloured fringes within
it, which change colour at every movement of the analyser.

Another series of beautiful appearances present themselves when a ray of
white polarized light is made to pass perpendicularly through a slice of
any crystallized substance with a single axis; if the analyser consist
of a slice of tourmaline, a number of concentric coloured rings are
rendered visible with a black cross in the centre, which is replaced
with a white one on moving the tourmaline through each quadrant of the
circle.

Crystals of Iceland spar present this phenomenon in great beauty; and if
the crystal (such as nitre) has two axes of double-refraction, a
double-system of coloured rings is apparent, with the most curious
changes and combinations of the black and white crosses with them. (Fig.
339.)

[Illustration: Fig. 339. Crystal of nitre with two axes, as seen in
polarized light.]

Mr. Goddard has recommended the optical arrangement (Fig. 340) for
showing the rings with great perfection, as also the number of rings
that increase in some crystals (the topaz, for example), with the
divergence of the rays of polarized light passing through them.

Mr. Woodward's table and oxy-hydrogen polariscope and microscope, made
by Smith and Beck, of Coleman-street, is well adapted, from its [Page
349] simplicity and perfection, to exhibit all the varied and beautiful
effects of polarized light; and we only regret that want of space
prevents us describing it in detail, although the reader may see the
body of the apparatus at page 123, where the modifications of the
oxy-hydrogen light are described and figured; and the polarizing
apparatus would be placed, of course, in front of the light issuing from
the lantern.

[Illustration: Fig. 340. A A A. Polarized light. B B. A lens of short
focus, transmitting a cone of light with an angle of divergence for its
rays, C C, of 45°. D D. The crystal of topaz, Iceland spar, or nitre. E
E. The slice of blue tourmaline for analysing.]

Finally, the question of utility (the _cui bono_) may be considered in
answer to the query, What is the use of polarized light?

The value to scientific men of a knowledge of the nature of this
modification of common light cannot be overrated. It has given the
philosopher a new kind of test, by which he discovers the structure of
things that would otherwise be perfectly unknown; it has given the
astronomer increased data for the exercise of his reasoning powers;
whilst to the microscopist the beauty of objects displayed by polarized
light has long been a theme of admiration and delight, and has served as
a guide for the identification of certain varieties of any given
substance, such as starch.

A tube provided with a polarizer of tourmaline, or a single-image Nicol
prism, is invaluable to the look-out at the mast-head in cases where
vessels are navigating either inland or sea water, where the presence of
hidden rocks is suspected, because the polarizer rejects all the glare
of light arising from unequal reflection at the surface of water, and
enables the observer to gaze into the depths of the sea and to examine
the rocks, which can only be perfectly visible by the refracted light
coming from their surfaces through the water.

Professor Wheatstone has invented an ingenious polarizing clock for
showing the hour of the day by the polarizing power of the atmosphere.
Birt, Powell, and Leeson have each invented instruments for examining
the circular polarization of fluids, by which a more intimate knowledge
of the relative values of saccharine solutions may be obtained, besides
unfolding other truths important to investigators in this branch of
science.

And last, but not least, it was with the assistance of polarized light
[Page 350] that Dr. Faraday established the relation that exists between
light and magnetism, and through the latter, with the force of
electricity; and the next figure indicates the necessary apparatus
required to repeat this highly important physical truth--viz., the
deviation of the plane of polarization of light by the influence of the
magnetic force from a powerful electro-magnet. (Fig. 341.)

[Illustration: Fig. 341. A. The light and condenser lens. B.
Single-image Nicol prism. C. Rock crystal of two rotations. D. A
double-convex lens. E E. Faraday's heavy glass. F F. The powerful
electro-magnet connected with battery. G. Double-refracting prisms. H.
Image, or screen where the deviation of the plane of polarization by the
magnetic force is shown.]

By another and equally beautiful experiment at the London Institution,
Professor Grove demonstrated the production of all the other kinds of
force from light, using the following arrangement for the purpose:

A prepared daguerréotype plate is enclosed in a box full of water having
a glass front with a shutter over it; between this glass and the plate
is a gridiron of silver wire; the plate is connected with one extremity
of a galvanometer coil, and the gridiron of wire with one extremity of a
Breguet's helix; the other extremities of the galvanometer and helix are
connected by a wire, and the needles brought to zero. As soon as a beam
of either daylight or the oxy-hydrogen light is, by raising the shutter,
permitted to impinge upon the plate, the needles are deflected. Thus,
light being the initiatory force, we get

  _Chemical action_ on the plate,
  _Electricity_ circulating through the wires,
  _Magnetism_ in the coil,
  _Heat_ in the helix,
  _Motion_ in the needle.

Such, then, are some of the glorious phenomena that we have endeavoured
to explain in this and the preceding chapters on light. Here we have
noticed specially how completely we owe their appreciation to the sense
of sight operating through the eye, the organ of vision. Well may those
who have lost this divine gift speak of their darkness as of a lost
world of beauty to be irradiated only by better [Page 351] and more
enduring light; and most feelingly does Sir J. Coleridge speak on this
point when he says:--

"Conceive to yourselves, for a moment, what is the ordinary
entertainment and conversation that passes around any one of your family
tables; how many things we talk of as matters of course, as to the
understanding and as to the bare conception of which sight is absolutely
necessary. Consider, again, what an affliction the loss of sight must
be, and that when we talk of the golden sun, the bright stars, the
beautiful flowers, the blush of spring, the glow of summer, and the
ripening fruit of autumn, we are talking of things of which we do not
convey to the minds of these poor creatures who are born blind, anything
like an adequate conception. There was once a great man, as we all know,
in this country, a poet--and nearly the greatest poet that England has
ever had to boast of--who was blind; and there is a passage in his works
which is so true and touching that it exactly describes that which I
have endeavoured, in feeble language, to paint. Milton says:--

                  'Thus with the year
  Seasons return; but not to me returns
  Day, or the sweet approach of even, or morn,
  Or sight of vernal bloom, or summer's rose,
  Or flocks, or herds, or human face divine;
  But cloud instead, and ever-during dark
  Surrounds me; from the cheerful ways of men
  Cut off, and for the book of knowledge fair
  Presented with a universal blank
  Of Nature's works, to me expunged and rased,
  And wisdom at one entrance quite shut out.
  So much the rather, thou, celestial light,
  Shine inward, and the mind through all her powers
  Irradiate; there plant eyes; all mist from thence
  Purge and disperse, that I may see and tell
  Of things invisible to mortal sight.'

The great poet, when intent upon his work, sought for celestial light to
accomplish it. And this brings me to that part of the labours of our
Blind Institutions upon which I dwell the most and which, after all, is
the greatest compensation we can afford to the inmates for the
affliction they suffer; and that is, the means we provide for them to
read the blessed Word of God, which they can read by day as well as by
night, for light in their case is not an essential."

[Illustration]

[Page 352]




CHAPTER XXVII.

HEAT.


[Illustration: Fig. 342. James Watt.]

Throughout the greater number of the preceding chapters it will be
evident that the active properties of matter may be summed up under one
general head, and may be considered as varieties of attraction--such as
the attraction of gravitation, cohesive attraction, adhesive attraction,
attraction of composition (or chemical attraction), electrical
attraction, magnetical attraction.

The absolute or autocratic system does not, however, prevail in the
works of nature; and she seems ever anxious, whilst imparting great and
peculiar powers to certain agents, to create other forces which may
control and balance them. Thus, for instance, the great force of
cohesive attraction is an ever-present power discernible, as has been
shown, in solids and liquids; but if this agent [Page 352] were allowed
to run riot in its full strength and intensity, it would tyrannically
hold in subjection all liquid matter, and every drop of water which is
at present kept in the liquid state, would succumb to its iron rule, and
retain the solid state of ice. Hence, therefore, the wise creation of an
antagonistic force--viz., heat; which is not provided in any niggardly
manner, but is liberally bestowed upon the globe from that
all-sufficient and enormous source, the sun. And it is by the softening
and liquifying influence of his rays that the greater proportion of the
water on the surface of the globe is maintained in the fluid condition,
and is enabled to resist the power of cohesion, that would otherwise
turn it all, as it were, to stone.

Cohesion, electricity, and magnetism fully embody the notion of powers
of attraction, or _a drawing together_; whilst heat stands almost alone
in nature as the type of repulsion, or _a driving back_.

Mechanically, repulsion is demonstrated by the rebound of a ball from
the ground; the parts which touch the earth are for the moment
compressed, and it is the subsequent repulsion between the particles in
those parts which causes them to expand again and throw off the ball.

The development of heat is produced from various causes, which may be
regarded as at least four in number. Thus, it was shown by Sir Humphrey
Davy, that even when two lumps of ice are rubbed together, sufficient
heat is obtained to melt the two surfaces which are in contact with each
other. Friction is therefore an important source of heat, and one of the
most interesting machines at the Paris Exposition consisted of an
apparatus by which many gallons of water were kept in the boiling state
by means of the heat obtained from the friction of two copper discs
against each other. The machine attracted a good deal of attention on
its own merits, and especially because it supplied boiling water for the
preparation of chocolate, which the public was duly informed was boiled
by the heat _rubbed out_ of the otherwise cold discs of copper. When
cannon made on the old system are bored with a drill, it is necessary
that the latter should be kept quite cool with a constant supply of
water, or else the hard steel might become red-hot, and would then lose
its _temper_, and be no longer capable of performing its duty.

Count Rumford endeavoured to ascertain how much heat was actually
generated by friction. When a blunt steel bore, three inches and a half
in diameter, was driven against the bottom of a brass cannon seven
inches and a half in diameter, with a pressure which was equal to the
weight of ten thousand pounds, and made to revolve thirty-two times in a
minute, in forty-one minutes 837 grains of dust were produced, and the
heat generated was sufficient to raise 113 pounds of the metal 70°
Fahrenheit--a quantity of heat which is capable of melting six pounds
and a half of ice, or of raising five pounds of water from the freezing
to the boiling point. When the experiment was repeated under water, two
gallons and a half of water, at 60° Fah., were made to boil in two hours
and a half.

Chemical affinity has been so often alluded to in these pages, that it
[Page 354] may be sufficient to mention only one good instance of its
almost magical power in evoking heat. When a bit of the metal sodium is
placed on the tip of a knife, and thrust into some warm quicksilver, or
if a pellet of sodium and a few globules of mercury are placed on a hot
plate just taken from the oven, and then gently squeezed together, a
vivid production of heat and light is apparent; and when the mixture of
the two metals is cold, it will be found that the quicksilver has lost
its fluidity, and a solid amalgam of sodium and mercury is obtained,
which gradually, by exposure to the air, returns to the liquid state,
the mercury being set free, whilst the sodium is oxidized, and forms
soda. Just as an ordinary alloy of copper and gold used by jewellers
would lose its colour and brilliancy by the oxidation of the copper; and
when the rusty, dirty film is removed by rubbing and polishing, the
surface is again brilliant, and remains so until another film of the
exposed copper is attacked: in like manner the sodium is attacked and
changed by the oxygen of the air, whilst the mercury being unaffected
retains its brilliancy, and at the same time regains its fluidity. The
evolution of heat in the above case indicates that a chemical union has
taken place between the two metals.

Examples of the production of heat by electricity and magnetism have
been abundantly shown in the chapters on these subjects; and one of the
best illustrations of this fact has been shown on the occasion of the
opening of the telegraphic communication between France and England by
means of the submarine cable, when cannon were fired alternately at both
ends of the conducting cable by means of electricity, and the event thus
inaugurated in both countries.

That heat is a product of living animal organization is shown, as it
were, visibly by the marvellous phenomena that proceed in our own
bodies. People do not very often trouble themselves to ask where the
heat comes from, or even to think that this invisible power must be
maintained in the body, and that slow combustion, or, as Liebig terms
it, _eremacausis_, must continually go on inside our frail mortal
tenements; and more than this, that we cannot afford to waste our heat.
If the body is deprived of heat faster than it can be generated, death
must inevitably occur; and a very melancholy instance of this remarkable
mode of death has lately occurred in Switzerland to a Russian gentleman.

Such another instance of a man being slowly frozen to death within sight
and sound of other beings, through whose veins the blood was flowing at
its accustomed temperature (about 90° Fahr.), it would be difficult to
find, and it stands forth, therefore, as a marked example and
illustration of the statement already made, that living animal organisms
are truly a source of heat, which is as essential to the well-being of
the body as meat, drink, and air.

Heat is of two kinds, and may be either apparent to our senses, and
therefore called _sensible_ heat; or it may be entirely concealed,
although present in solids, liquids, and gases, and is then termed
_insensible_ or _latent_ heat.

[Page 355]


_Sensible Heat._

The first effect of this force is a demonstration of its repulsive
agency, and the dilatation or expansion of the three forms of matter
whilst under the influence of heat, admits of very simple illustrations.
The expansion of a solid substance, as, for instance, a metal, on the
application of heat, is apparent by fitting a solid brass cylinder into
a proper metal gauge, which is accurately filed so as to admit the
former when perfectly cold. If the brass rod is then heated, either by
plunging it into boiling water or by the application of the flame of a
spirit lamp, its particles are separated from each other; they now
occupy a larger space, and expansion is the result, and this is clearly
proved by the application of the gauge, which is no longer capable of
receiving it. (Fig. 343.) When, however, the latter is cooled, the
opposite result occurs, the particles of brass return to their old
position, and _contraction_ takes place; hence it is stated that "Bodies
expand by heat and contract by cold;" and it is proper to state here
that the term "_cold_" is of a negative character, and simply means the
absence of heat.

[Illustration: Fig. 343. A B. Cylinder of brass. C D. Iron gauge,
admitting A B longitudinally, and also in the hole E when cold, but
excluding A B when the latter is heated and expanded.]

Solid bodies do not expand equally on the application of the same amount
of heat; thus, a bar of glass one inch square and one thousand inches
long would only expand one inch whilst heated from the freezing to the
boiling point of water. A bar of iron one inch square and eight hundred
inches long would expand one inch in length, through the same degrees of
heat; and a bar of lead one inch square and three hundred and fifty
inches long would also dilate one inch in length. Hence,

  Lead expands in volume      1/350th.
  Iron                        1/800th.
  Glass                       1/1000th.

The unequal expansion of the metals is well illustrated by an experiment
devised by Dr. Tyndal, the respected Professor of Natural Philosophy in
the Royal Institution of Great Britain, and is arranged as follows:--A
long bar of brass and another of iron are supported on the [Page 356]
edges of two pieces of wood placed at an angle, and resting against the
sides of a mahogany framework. The metallic bars only touch one end of
the frame, and are in metallic communication with a piece of brass
inserted there, and forming part of a conducting chain connected with a
voltaic battery; when heat is applied to both bars they expand
unequally; the brass bar dilates first, and filling up the minute space
left between the two ends of the frame, touches another brass plate and
instantly completes the voltaic circuit, when a coil of platinum wire
becomes ignited, showing the fact of expansion; and secondly, the
difference in the power of dilatation possessed by each is clearly shown
by removing the two angular supports of wood, when the iron falls away,
whilst the brass remains and still completes the voltaic circuit. (Fig.
344.)

[Illustration: Fig. 344. A A. The brass bar which has expanded by the
heat from the gas jet B, and making the contact between the brass plates
in connexion with the binding screws C C, the voltaic circuit is
completed, and a coil of platinum wire in the glass tube D, is
immediately ignited. The iron bar at E E has not expanded sufficiently,
which is shown afterwards by removing the angular wooden supports K K,
when the iron falls off, and the brass remains on the two ledges of the
mahogany framework L L L.]

The force exerted by the expansion of solids is enormous, and reminds us
again of the amazing power of all the imponderable agents; and it is
truly wonderful to notice how the entry of a certain amount of heat into
and between the particles of metals, or other solids, endues them with a
mechanical force which is almost irresistible, and is capable of
working much harm. Kussné made an experiment with an iron sphere,
which he heated from a temperature of 32° Fahr. to 212° Fahr., and
he found that the expansion of the ball exerted a force equal to 4000
atmospheres--_i.e._ 4000 × 15--on every square inch of surface, or a
pressure equal to thirty millions of pounds; the entry of only 180° of
heat into the iron sphere produced this remarkable result, just as
Faraday has calculated that a single drop of water contains a sufficient
quantity of electricity to produce a result equal to the most powerful
flash of lightning, provided the electricity of quantity in the drop of
water is converted into electricity of high tension or intensity.

The practical applications of this well-known property of solids with
respect to heat are very numerous; thus, the iron bullet-moulds are
always made a little larger than the requisite size, in order to allow
for the expansion of the hot liquid lead, and the contraction of the
cold metal. The tires of wheels and the hoops of casks are usually
placed on whilst hot, in order that the subsequent contraction may bind
the spokes [Pg 357] and fellies, or the staves, closely together. If an
allowance was not made for the expansion and contraction of the iron
rails on the permanent ways of railroads, the regularity of the level
would be constantly destroyed, and the position of the rails, chairs,
and sleepers would be most seriously deranged; indeed it is calculated
that the railway bars between London and Manchester are five hundred
feet longer in the summer than in the winter.

The walls of the Cathedral of Armagh, as also those of the Conservatoire
des Art et Métiers, were brought back to a nearly perpendicular
position, by the insertion (through the opposite walls) of great bars of
iron, which being alternately heated, expanded, and screwed up tight,
then cooled and contracted, gradually corrected the bulging out of the
walls or main supports of these buildings. The principle of these famous
practical experiments is neatly illustrated by means of an iron
framework with a bar of iron placed through both its uprights, and
screwed tight when hot; on cooling, contraction occurs, which is shown
by a simple index. (Fig. 345.)

[Illustration: Fig. 345. The iron frame, with C C, wrought-iron bar
heated by putting on the semicircular piece of iron E E, which is first
made red-hot, and as the heat is communicated to the wrought iron rod C
C, it is screwed up tight by the nut K. G G. The index attached to the
iron frame screwed up when hot; the arms come together at P, and
separate further to H H as the contraction takes place by cooling the
bar C D.]

It has often been remarked that there is no rule without an exception,
and this applies in a particular instance to the law that "bodies expand
by heat and contract by cold"--viz., in the case of Rose's fusible
metal, which consists of

  Two parts by weight of bismuth,
  One part        "      lead,
  One part        "      tin.

To make the alloy properly, the lead is first melted in an iron ladle,
and to this are added first the tin, and secondly the bismuth; the whole
is then well stirred with a wooden rod, and cast into the shape of a
bar.

[Page 358]

When placed in the pyrometer and heated, the bar expands progressively
till it reaches a temperature of 111° Fahr.; it then begins to
_contract_, and is rapidly _shortened_, until it arrives at 156° Fahr.,
when it attains a maximum density, and occupies no more space than it
would do at the freezing-point of water. The bar, after passing 156°,
again expands, and finally melts at about 201°, which is 11° below the
boiling-point of water. Fusible metal is sometimes made into teaspoons,
which soften and melt down when stirred in a cup of hot tea or basin of
soup, to the great surprise and bewilderment of the victim of the
practical joke.

Unequal expansion is familiarly demonstrated with a bit of toasted
bread, which curls up in consequence of the surface exposed to the fire
contracting more rapidly than the other; and the same fact is
illustrated with compound flat and thin bars of iron and brass, which
are fixed and rivetted together; when heated, the compound bar curves,
because the iron does not expand so rapidly as the brass, and of course
forms the interior of the curve, whilst the brass is on the exterior.

The experiment with the compound bar is made more conclusive and
interesting by arranging it with a voltaic battery and platinum lamp.
One of the wires from the battery is connected with the extremity of the
compound bar, and as long as it remains cold, no curve or arch is
produced, but when heat is applied, the bar curves upwards, and touching
the other wire of the battery, the circuit is completed, and the
platinum lamp is immediately ignited. (Fig. 346.)

[Illustration: Fig. 346. A B. Compound bar resting on two blocks of
wood. The end A is connected with one of the wires from the battery. The
circuit is completed and the platinum lamp D ignited directly the bar
curves _upwards_ by the heat of the spirit lamp, and touches the wire C
C connected with the opposite pole of the battery.]

The expansion and contraction of liquids by heat and cold is also
another elementary truth which admits of ample illustration, and indeed
introduces us to that most useful instrument called the thermometer.

If a flask is fitted with a cork through which a long glass tube, open
[Page 359] at both ends, is passed, and then carefully filled with water
coloured with a little solution of indigo, so that when the cork and
tube are placed in the neck, all the air is excluded, a rough
thermometer is thus constructed, which, if placed in boiling water,
quickly indicates the increased temperature by the rising or expansion
of the coloured water inside the flask. (Fig. 347.)

[Illustration: Fig. 347. Expansion of liquids shown at A by the coloured
water rising in the tube from the flask, which is quite full of liquid,
and heated by boiling water. B. The expansion of the water heated by the
spirit-lamp is shown by the rising of the piston and rod C C. D
represents a retort filled up like A to show the expansion of a liquid
by heat.]

The thermometer embraces precisely the same principle as that already
described in Fig. 347, with this difference only, that the tube is of a
much finer bore, and the liquid employed, whether alcohol or mercury, is
boiled and hermetically sealed in the tube, so that the air is entirely
excluded. To make a thermometer, a tube with a capillary bore is
selected of the proper length; it is then dipped into a glass containing
mercury, so that the tube is filled to the length of half an inch with
that metal. The half-inch is carefully measured on a scale, and the
place the mercury fills in the tube marked with a scratching diamond;
the mercury is then shaken half an inch higher, and again marked, and
this proceeding is continued until the whole tube is divided into half
inches. The object of doing this is to correct any inequalities [Page
360]in the diameter of the bore of the glass tube, because if wider at
one part than another, the spaces filled with the mercury are not equal;
as the bore is usually conical, the careful measurement of the tube with
the half inch of mercury in the first place gives the operator at once a
view of the interior of his tube, and enables him to graduate it
correctly afterwards. (Fig. 348.)

[Illustration: Fig. 348. A B. Magnified view of the bore of one of the
thermometer tubes which are made by rapidly drawing out a hollow mass of
hot glass whilst soft and ductile, consequently the bore must be
conical, and larger at one end than the other.]

The next step is to heat one extremity by the lamp and blowpipe, and
whilst hot, to blow out a ball upon it; if this operation were performed
with the mouth, moisture from the breath would deposit inside the fine
bore of the glass tube, and injure the perfection of the thermometer
afterwards. In order to prevent any deposit of water, the bulb is blown
out, whilst red-hot, with the air from a small caoutchouc bag fitted on
to the other extremity of the tube. The operator now marks off the
intended length of his thermometer, and above that point the tube is
again softened with the flame and blowpipe, and a second bulb blown out.
(Fig. 349 _a_.)

[Illustration: Fig. 349 _a_.--No. 1. First bulb. The intended length of
the thermometer is shown at the little cross.--No. 2 is the second bulb
placed above the cross.]

The open end of the tube is now placed under the surface of some pure,
clean, dry quicksilver, and heat being applied to the upper bulb, the
air expands and escapes through the mercury, and as the tube cools a
vacuum is produced, into which the mercury passes. By this simple
method, the mercury is easily forced into the tube, as otherwise it
would be impossible to _pour_ the quicksilver into the capillary bore of
the intended thermometer. (Fig. 349 _b_.)

[Illustration: Fig. 349 _b_. Heating and expanding the air in the top
bulb, so that when cool the mercury in the glass A, may rise into the
tube and fill the bulb B.]

The tube is now taken from the glass containing the mercury, and simply
inverted; but in consequence of the very narrow diameter of the bore the
air will not pass out of the first bulb until heat is applied, when the
air expands, and the [Page 361] mercury, first stationary in the second
bulb, will now displace the air, and fall into the first bulb when the
tube is again cool.

The ball, No. 1 (Fig. 349 _a_), is now full of mercury, and there is
also some left in No. 2; in the next place, the tube is supported by a
wire, and held over a charcoal fire, when it is heated throughout its
entire length, and the mercury being boiled expels the _whole of the
air_, so that there is nothing inside the bulbs and capillary bore but
mercury and its vapour. (No. 1, Fig. 350.) The open end of the intended
thermometer is now temporarily closed with sealing-wax, and the whole
allowed again to cool with the sealed end uppermost, so that the ball
No. 2, Fig. 350, and the tube above it, are quite filled with
quicksilver.

After cooling, the tube is placed at an angle with the sealed end
uppermost, and, guided by experience, the operator heats the lower bulb
so as to expand enough mercury into the upper one to leave space for the
future expansion and contraction of the mercury in the tube, which has
now to be hermetically sealed. This is done by dexterously heating the
tube at the cross whilst the mercury in the first bulb is still
expanded; and by drawing it out rapidly with the help of the heat
obtained from the lamp and blowpipe, the second bulb is separated from
the first at the little cross (B, No. 3, Fig. 350), and the thermometer
tube at last properly filled with quicksilver, and hermetically closed.
(No. 4, Fig. 350.)

[Illustration: Fig. 350.--No. 1. Boiling quicksilver in the tube with
two bulbs.--No. 2. Tube cooled, with the sealed end uppermost.--No. 3.
Mercury in first bulb expanded by lamp A, and at the proper moment
hermetically sealed by the flame urged by the blowpipe at B. The upper
bulb and tube to the cross being drawn away and separated.--No. 4.
Thermometer tube containing the requisite quantity of mercury,
hermetically sealed, and now ready for graduation.]

[Page 362]

In order to procure a fixed starting-point, the thermometer tube is
placed in ice, with a scale attached; the temperature of ice never
varies, it is always at 32 degrees. When, therefore, the mercury has
sunk to the lowest point it can do by exposure to this degree of cold,
the place is marked off in the scale, and represents that position in
the graduated scale where the freezing point of water is indicated.

The tube is placed in the next place in a vessel of boiling water, care
being taken that the whole tube is subject to the heat of the water and
the steam issuing from it, and when the mercury has risen to the highest
position attainable by the heat of boiling water, another graduation is
made which indicates 212 degrees--viz., the boiling point of water. This
graduation should be made when the barometer stands at 30 inches,
because the boiling point of water varies according to the weight of the
superincumbent air pressing upon it.

Between the graduation of the freezing and the boiling point of water
the space is divided into 180 parts, which added to 32 make up the
boiling point of water to 212 degrees, being the graduation of
Fahrenheit, who was an instrument-maker of Hamburg. Why he divided the
space between the freezing and boiling point of water nobody appears to
know, unless he took a half circle of 180 degrees as the best division
of space. If the thermometer contains air the mercury divides itself
frequently into two or three slender threads, each separated from the
other in the capillary bore, and thus the instrument is rendered useless
until the threads again coalesce. If the thermometer has been well made,
and is quite free from air, it may be tied to a string and swung
violently round, when the centrifugal force drives the slender threads
of mercury to their common source--viz., the bulb containing the
quicksilver, and the whole is again united. The string must be attached,
of course, to the top of the thermometer scale.

When travelling on the Continent it is sometimes desirable to be able to
read the thermometers which are graduated in a different manner to that
of Fahrenheit. In France the Centigrade scale is preferred, and in many
parts of Germany Reaumur's graduation. The difference of the graduation
is seen at a glance.

  In the Centigrade the freezing point is 0, the boiling point 100°.
     "   Reaumur            "             0,          "         80°.
     "   Fahrenheit         "            32°,         "        212°.

The number of degrees, therefore, between boiling and freezing is 100 in
the Centigrade, 80 in Reaumur, and (212-32, that is) 180 in Fahrenheit.

If, then, the letters C, R, F, be taken to denote the _number_ of
degrees from the freezing point at which the mercury stands in the
Centigrade, Reaumur, and Fahrenheit thermometers, we have the following
proportions:--

  (1.) 100: 80 :: C: R, whence C = 5/4 of R, or R = 4/5 of C.
  (2.) 180:100 :: F: C, whence F = 9/5 of C, or C = 5/9 of F.
  (3.) 180: 80 :: F: R, whence F = 9/4 of R, or R = 4/9 of F.

[Page 363]

The following examples will show how to apply these formulæ:--

(1).--Suppose the Reaumur stands at 28°, at what height does the
Centigrade stand? We have C = 5/4 of R (in this case), 5/4 of 28 = 35:
that is, the Centigrade stands at 35°.

(2).--Suppose Fahrenheit to stand at 41°, what will Reaumur stand at? R
= 4/9 of (41-32) (that is, the number above freezing in Fahr.) = 4/9 of
9 = 4. Reaumur stands at 4.

(3).--Suppose Fahrenheit stands at 23°, what will the Centigrade stand
at? C = 5/9 of F = 5/9 of (32-23) = 5/9 of 9 = 5 below freezing (or-5).

(4).--If Fahrenheit stands at 4 below 0, what will Reaumur indicate? R =
4/9 of F = 4/9 of (32 + 4) = 4/9 of 36 = 16 below 0 (or-16).

The only liquid which has the exceptional property of expanding by cold
is water, and it will be seen presently that this curious anomaly is of
the greatest importance in the economy of nature.

If a box containing a mixture of ice and salt is placed round the top of
a long cylindrical glass containing water at a temperature of 60° Fahr.,
the intense cold of the freezing mixture, which is zero--that is to say,
32° below the freezing point of water--very soon reduces the temperature
of the water contained in the glass, and as it becomes colder it
contracts, is rendered heavier, and sinks to the bottom of the vessel,
and its place is taken by other and warmer water. This circulation
commencing downwards, proceeds till the water has attained a temperature
of about 40° Fahr., when the maximum density is obtained and the
circulation stops, because after sinking below 40° the cold water
becomes lighter, and continues to be so until it freezes, and of course,
being of a less specific gravity than the warmer water, it floats (like
oil on water) upon its surface; so that a small thermometer placed at
the bottom of the jar indicates only 40° Fahr., whilst the solid ice
enveloping the other or second thermometer placed at the top may be as
low as 29°, or even lower, according to the quantity of ice and salt
used in the box surrounding the top of the glass. (Fig. 351.)

[Illustration: Fig. 351. A B. Long cylindrical glass containing water
and two thermometers; the one at the bottom shows a temperature of 40°;
the other at the top 32°, or even lower, C C C C. Section of box
containing the ice and salt, and standing on four legs, two of which are
shown at D D.]

[Page 364]

The importance of this curious anomaly cannot be overrated. If water did
not possess this rare property, all the seas, rivers, canals, lakes,
&c., would _gradually_ become impassable from the presence of enormous
blocks of ice formed during the winter. The whole bulk of water
contained in them would have to sink below 32° before it could solidify
provided water increased in density or continued to contract by cold.
Having once solidified, the warmth of the rays from a summer's sun would
certainly melt a great deal of the ice, but not the whole, and winter
would come again before the solid masses had disappeared. The ocean
could not be navigated in safety even near our own shores, in
consequence of the vast icebergs that would be formed, and float about
and jostle each other even in the British Channel.

The earth has been wonderfully prepared for God's highest work--Man, and
in nothing is this supreme wisdom more apparent than in the fact that
water offers the only known exception to the law "that bodies expand by
heat and contract by cold."

The expansion of gases by heat and contraction by cold take place in
obedience to a law to which there is no exception, except in degree. It
was discovered in 1801 by M. Gay Lussac, of Paris, and also about the
same period by the famous English philosopher who established the atomic
theory--viz., by Dr. Dalton. Since these experiments and calculations
Rudberg, Magnus, and Regnault have made other researches, and their
successive experiments give the following results:--

                     Vols. of air.                      Volumes.

  Dalton, Gay Lussac 1000 heated from 32° to 212° became 1375
  Rudberg            1000   "       "           "        1365
  Magnus, Regnault   1000   "       "           "        1366.5

As a natural result, air at 32° Fahr, expands 1/491 part of its volume
for every degree of heat on the scale of Fahrenheit; and a volume of air
which measures 491 cubic inches at 32° will measure 492 at 33°, 493 at
34°, and so on. The exception is only in degree, and Magnus and Regnault
discovered by their searching experiments that the gases easily
liquified are more expansible by heat than air and those gases (such as
oxygen, hydrogen, and nitrogen) which have never been liquified.

The expansion of air is easily shown by placing the open end of a tube
with a large bulb blown at the other extremity, under the surface of a
little coloured water; on the application of heat the air expands and
escapes, and its place is taken, when cool, by the coloured liquid. Such
an arrangement represents the first thermometer constructed by Sanctorio
about A.D. 1600, which might certainly answer for rough purposes, but as
the ascent and descent of the fluid depend on the bulk of air contained
in the bulb, and as this is affected by every change of the height of
the barometer, no satisfactory indication of an increase or decrease of
temperature could be obtained with it, although the instrument itself is
interesting in an historical point of view, and in a [Page 365]
modified form as an air thermometer has been employed by Sir John
Leslie, under the name of the "Differential Thermometer," in his refined
and delicate experiments with heat.

[Illustration: Fig. 352 A. Sanctorio's original air thermometer; the
expansion and contraction of the air in the bulb indicate the rise or
fall of the temperature. The cork is merely a support, and is not fitted
into the bottle air-tight. B C. The differential thermometer. When both
bulbs are subjected to a uniform temperature, no movement of the fluid
shown at D occurs; but if the bulb B is put into any place warmer than
the position of the bulb C, then the air expands in B, and drives the
coloured liquid, which consists of carmine dissolved in oil of vitriol,
up the scale attached to the stem of the bulb C.]

Fire balloons are a good example of the expansion of gases, and the
levity of the air thus increases in bulk was taken advantage of by
Montgolfier in the construction of his famous balloon, which, with a
cage containing various animals, ascended, in the presence of the King
and royal family of France, at Versailles; and in spite of huge rents in
two places, it rose to a height of 1440 feet, and after remaining in the
air for eight minutes, fell to the ground at the distance of 10,200 feet
from the place whence it started, without injury to the animals. When it
is considered that a volume of air heated from 32° to 491° is doubled,
and tripled when heated to 982°, it will at once be understood how great
must be the ascending power of such balloons, provided the air within
them is kept sufficiently hot.

That gallant aëronaut, Pilate de Rozier, offered himself to be the first
aërial navigator; and having joined Montgolfier, they made three
successful ascents and descents with a large oval-shaped balloon,
forty-eight feet in diameter, and seventy-four feet high. On the fourth
occasion he ascended to a height of 262 feet, but in the descent a gust
of wind having blown the machine over some large trees of an adjoining
garden, the situation of the brave aëronaut was extremely dangerous, and
if he had not possessed the strongest presence of mind, and at once
[Page 366] given the balloon a greater ascending power, by rapidly
supplying his stove with some straw and chipped wood, he might on this
occasion have met with that untimely end which subsequently, in another
rash aëronautic adventure, befell this brave but foolhardy Frenchman.

On descending again, he once more, and without the slightest fear,
raised himself to a considerable height by feeding his fire with chopped
straw. Some time after he ascended, in company with M. Giroud de
Vilette, to the height of 330 feet, hovering over Paris at least nine
minutes, in sight of all the inhabitants, and the machine keeping all
the while perfectly steady.

The danger in using this method of inflating the balloon arises from the
possibility of generating gas, which escaping unburnt into the body of
the balloon, may accumulate and blow up, or burn afterwards.

[Illustration: Fig. 353. A B. Wessel's gas stove, with ring of gas jets
lighted inside; the air rushes in the direction of the arrows, C C, and
escaping at the top of the chimney, D D, soon fills the air or fire
balloon, which is usually made of paper.]

Fire balloons, as usually made, are very dangerous toys, and may
sometimes prove rather costly to the person who may send them off, in
consequence of their being blown by the wind on a hay or corn rick, or
other combustible substances. The safest mode of using fire balloons is
to fill them with hot air from a lighted gas stove (Wessel's, for
instance); the balloons may then be used in large rooms, or out in the
air, without fear of doing any harm to neighbouring property, as of
course the stove and the fire remain behind, and will fill any number of
air balloons. (Fig. 353.)

After all the fuss made about the novelty of the American hot-air
engine, it is somewhat amusing to look back to the records of civil
engineering, and in the "Transactions of the Institution of Civil
Engineers," to read Mr. James Stirling's account of his improved air
engine, in which the great expansion of air mentioned at p. 365 has been
successfully applied. The engine was constructed about the year [Page
367] 1843, and the principle, discovered thirty years before by Mr. R.
Stirling, will be comprehended by reference to the cut. (Fig. 354.)

[Illustration: Fig. 354. Stirling's air engine.]

Two strong air-tight vessels are connected with the opposite ends of a
cylinder, in which a piston works in the usual manner. About four-fifths
of the interior space in these vessels is occupied by two similar
air-tight vessels or plungers, which are suspended to the opposite
extremities of a beam, and capable of being alternately moved up and
down to the extent of the remaining fifth. By the motion of these
interior vessels, which are filled with non-conducting substances, the
air to be operated upon is moved from one end of the exterior vessel to
the other, and as one end is kept at a high temperature, and the other
as cold as possible, when the air is brought to the hot end it becomes
heated, and has its pressure increased; and when it is brought to the
cold end, its heat and pressure are diminished. Now, as the interior
vessels necessarily move in opposite directions, it follows that the
pressure of the enclosed air in the one vessel is increased, while that
of the other is diminished. A difference of pressure is thus produced
upon the opposite sides of the piston, which is thereby made to move
from the one end of the cylinder to the other, and by continually
reversing the motion of the suspended bodies or plungers, the greater
pressure is successively thrown upon a different side, and a
reciprocating motion of [Page 368] the piston is kept up. The piston is
connected with a fly-wheel in any of the usual modes; and the plungers,
by whose motion the air is heated and cooled, are moved in the same
manner, and nearly at the same relative time, with the valves of a steam
engine.

The pressure is greatly increased and made more economical by using
somewhat highly-compressed air, which is at first introduced, and is
afterwards maintained, by the continued action of an air-pump. The pump
is also employed in filling a separate magazine with compressed air,
from which the engine can be at once charged to the working pressure.
Mr. Stirling's chief improvement consists _in saving all or nearly all
the heat of the expanded air after it has done its work_, by passing it
from the hot to the cold end of the air vessel through a multitude of
narrow passages, whose temperature is at the beginning of the tubes
nearly as great as that of the hot air, but gradually declines till it
becomes nearly as low as the coldest part of the air vessel. The heat is
therefore retained by these passages, so that when the mechanism is
reversed, the cold air returns again through these hot pipes, and is
thus made nearly hot enough by the time it reaches the heating vessel to
do its work. Thus, instead of being obliged to supply at every stroke of
the engine as much heat as would be sufficient to raise the air from its
lowest to its highest temperature, it is necessary to furnish only as
much as will heat it the same number of degrees by which the hottest
part of the air vessel exceeds the hottest part of the intermediate
passages. This portion of the engine may be called the _economical
process_, and represents the foundation of all the success to which it
has attained in producing power with a small expenditure of fuel. No
boiler being required, of course the danger of explosions is much
lessened. The higher the pressure under which the engine was worked the
greater was the effect produced. A small engine on this principle was
worked to a pressure of 360 pounds on the square inch; and perhaps the
best popular notion of the novelty in the arrangement is that suggested
by Mr. George Lowe, who compared the economical part of the machine to a
"Jeffrey's Respirator" used by consumptive patients. The heat from the
air _expired_ being retained by the laminæ, and again used when cold air
is inspired or drawn into the lungs. Mr. Stirling states that the
consumption of fuel as compared to the steam engine which the air engine
had replaced was as 6 to 26; the same amount of work being now performed
by about six cwt. of coals which had formerly required about twenty-six
cwt., though he ought to have stated that the steam engine removed was
not of the best construction, nor had the boiler any close covering.
(Fig. 354.)


_Conduction of Heat._

This property of heat with reference to matter, and the consideration of
the curious manner in which it creeps, as it were, through solid
substances, brings the thoughtful mind at once to the bold question of
What is heat? Is it to be regarded as something real or material? or
[Page 369] must it be considered only as a property or state of matter?
These questions are not to be solved easily, and they demand a
considerable amount of experiment and reasoning even to appreciate their
meaning.

If a red-hot ball is placed in the focus of a concave metallic speculum,
it gives out certain emanations that are quite invisible, but which are
reflected from the surface of the mirror in the same manner as visible
rays of light, and may be collected in the focus of another and second
concave speculum, when they can be concentrated on to a bit of
phosphorus, and will cause the combustion of that substance. If the air
from a pair of bellows is blown forcibly across the rays of heat as they
are being concentrated upon the phosphorus, the rays are not moved from
their course, they are no more blown away than a sunbeam darting through
an aperture in a cloud on a stormy, windy day. The heat has, therefore
nothing to do with the air, and is wholly independent of that medium in
its passage from one mirror to the other. Such an experiment as that
described would at once suggest the idea that heat is a matter _sui
generis_, a component part of all bodies, and given off from
incandescent matter, the sun, &c., and that it may be propagated through
space much in the same manner as light. (Fig. 355.) The mechanism may be
very much like the corpuscular movement of light as defined by Sir Isaac
Newton, and already explained in another portion of this book. Hence it
has been supposed that heat is propagated through the air, water, and
solid substances by a direct emission of material particles from the
heat-giving agent, and that these molecules of heat force their way
into, or along, or through them, according to circumstances.

[Illustration: Fig. 355. Heat reflected by mirror, but not blown away by
air from bellows.]

Certain bodies are almost transparent to heat rays, such as air, whilst
others take an intermedial position, and only stop a certain quantity of
the heat molecules, such as rock crystals, mirror glass, and alum. A
third class of bodies absorbs the heat plentifully, such as charcoal,
black cloth, &c.; and a fourth, when polished and placed at the proper
angle, reflects or throws off the heat, as in the case of polished
mirrors. The transparency or opacity of substances (so far as light is
concerned) [Page 370] does not affect the transmission of heat. Light of
every colour and from all sources is equally transmitted by all
transparent bodies in the liquid or solid form, but this is not the case
with heat.

The rays of heat emitted by the sun and other luminous bodies have
properties quite different to the rays of light with which they are
accompanied. From these statements it will be evident that the _material
theory_ of heat is surrounded with difficulties and anomalies that
cannot be reconciled the one with the other, or neatly adapted, fitted
in, and dovetailed with all the puzzling phenomena that arise. Our
knowledge of the theory of heat has been greatly assisted by the
researches of Melloni, who has demonstrated that different _species_ of
rays of heat are given off by the same body at different temperatures,
which may be distinctly sifted and separated from each other. Long
before the experiments of Melloni philosophers had endeavoured to weigh
heat; trains of the most delicate levers were exposed, without effect,
to the action of heat rays; and all attempts, experimental as well as
theoretical, to define heat by the _material_ theory, are imperfect,
crude, and unsatisfactory. We are perforce obliged to adopt another
theory, and the one that obtains the greatest favour, as offering the
best definition of heat, is the _dynamical_ theory, which is more or
less analogous to the undulatory theory of light. At pages 262, 328,
335, this theory has been partly explained, and in speaking of it again,
great care must be taken not to confuse the undulations of heat with
those of light. The sun and the stars swim in a molecular medium, and
39,180 vibrations or waves must occur in one inch to produce the
sensation of red light, and 57,490 undulations in the space of one inch
to produce a violet light. As vibrations of the ethereal molecules
affect the eye, so there may be other nerves in our bodies which are
peculiarly sensitive to the waves of heat. It requires eight vibrations
of the air to occur in a second to produce an audible sound; whilst if
the vibrations of the air amount to 25,000 per second they cannot be
appreciated by the human ear, although it is possible to conceive that
the ears of certain animals may be so susceptible of rapid vibrations
that they may be able, for certain wise purposes of the Creator, to
appreciate sounds which are inaudible to human ears.

Melloni exhibited a spectrum to a number of persons, and found that
there was more light apparent to some eyes than to others. Lubeck put a
scarlet cloth on a donkey, and found that the two were frequently
confounded together by the eyes of many spectators. These facts indicate
that there may be vibrations of molecules that produce the sensation of
heat, but which do not affect the nerves that are sensitive to the
action of light waves, and vice versâ; and it is also probable that all
these different undulations, some affording heat and some light, may be
generated and propagated through space, as from the sun; or through
shorter distances, as from burning lamps and fires, without in any way
interfering with or impeding each other's progress.

The dynamical theory seems to offer the best idea of the transmission
[Page 371] of heat which is carried, conducted, or propagated through
solids with variable rapidity, either by the vibration of the
constituent molecules of the body itself, or by the undulation of a rare
subtle fluid which pervades them. If a copper and iron wire of the same
length and diameter are bound together and heated at the point of union,
the waves of heat travel faster through the copper than the iron, and
the former is said to be the best conductor of heat; and the fact itself
is demonstrated by placing a bit of phosphorus at the end of each
metallic wire, and it will be found by experiment that the combustible
substance melts first and takes fire on the copper, and that a
considerable interval of time elapses before the phosphorus ignites on
the iron.

[Illustration: Fig. 356. C. Copper wire bound at A to I, an iron wire.
After the heat of the lamp has been applied for about five minutes the
heat travels to C first, and ignites the bit of phosphorus placed there.
After some time has elapsed the phosphorus at I also ignites.]

The same fact is exhibited in a most striking manner by inserting a
series of rods of equal lengths and thicknesses in the side of a
rectangular box, allowing them to pass across the interior to the
opposite side. The rods are composed of wood, porcelain, glass, lead,
iron, zinc, copper, and silver, and have attached to each of their
extremities, by wax or tallow, a clay marble. When the water placed in
the box is made to boil, the heat passes along the different rods, and
melting the wax or tallow, allows the marble to drop off. Consequently
the first marble would drop from the silver rod, the next from the
copper, the third from the iron, the fourth from the zinc, the fifth
from the lead, whilst the porcelain, glass, and wooden rods would hardly
conduct (in several hours) sufficient heat to melt the wax or tallow,
and discharge the marbles.


_Conduction of Metals._

  Gold       1000
  Silver      973
  Copper      898.2
  Iron        374.3
  Zinc        363
  Lead        179.6

[Page 372]

The experiment is made more striking if the marbles are allowed to fall
on a lever connected with the detent of a clock alarum, which rings
every time a marble falls from one of the rods. (Fig. 357.)

During a cold frosty day, if the hand is placed in contact with various
substances, some appear to be colder than others, although all may be
precisely the same temperature; this circumstance is due to their
conducting power: and a piece of slate seems colder than a bit of chalk,
because the former is a much better conductor than the latter, and
carries away the heat from the body with greater rapidity, and diffuses
it through its own substance.

[Illustration: Fig. 357. A B. Trough containing boiling water, heated by
gas jets below. C. The eight rods and marbles attached, one of which has
fallen. D. The tray to receive the marbles.]

The gradual passage of heat along a bar of iron as compared with one of
copper, is well illustrated by supporting the ends of the two bars on
the top of the chimney of an argand lamp, whilst the other extremities
are held in a horizontal position by little blocks of wood. If marbles
are attached by wax to the under side, they fall off as the heat travels
along the metallic bars, and more rapidly from the copper than the iron,
because the former is a better conductor of heat than the latter. (Fig.
358.)

[Illustration: Fig. 358. A. Section of an argand gas lamp, with a copper
chimney supporting the ends of the bars of copper and iron marked C and
I. The balls have fallen from C, the copper bar.]

From the experiments of Mayer, of Erlangen ("Ann. de Ch.," XXX.), it
would appear that the conducting powers of different woods are to a
certain extent to be regarded as in the inverse proportion to their
specific gravities--_i.e._, the greater the density of the wood the less
conducting power, and the contrary.

If a cylindrical bar or thick tube of brass, six inches long, and about
two inches in diameter, is attached to a wooden cylinder of the same
size, the conducting powers of the two substances are well displayed by
first straining a sheet of white paper over the brass, and then holding
it in the flame of a spirit lamp. The heat being conducted rapidly away
by the metal will not scorch the paper, until the whole arrives at a
uniform high temperature; whereas the paper is rapidly burnt when [Page
373] strained over the wooden cylinder, because the heat of the flame of
the lamp is concentrated upon one point, and is not diffused through the
mass of the wood. (Fig. 359.)

In the course of the highly philosophical experiments of Sir H. Davy,
which led him gradually to the discovery of the construction of the
safety lamp, he connected together, by a copper tube of a small bore,
two vessels, each containing an explosive mixture composed of fire damp
and air. When the mixture was fired in one vessel he found that the
flame did not appear to be able to travel, as it were, across the
bridge--viz., the copper tube--and communicate with the other magazine,
because it was deprived of its heat whilst passing through the tube, and
was no longer flame, but simply gaseous matter at too low a temperature
to effect the inflammation of the mixture in the second box.

[Illustration: Fig. 359. Cylinder, half brass and half wood. The paper
strained over the wood is taking fire. The other extremity, shaded, is
the brass portion.]

A mass of cold metal may be suddenly applied to a small flame, such as
that of a night light, and depriving it rapidly of heat (like the case
of the unfortunate Russian described at page 354), it is almost
immediately extinguished (fig. 360), not by the mere exclusion of the
oxygen of the air, but on account of the withdrawal of the heat
necessary for the maintenance of the combustion.

[Illustration: Fig. 360. A. Small flame from night light. B C. Large
mass of cold copper wire open at both ends to place over flame, and by
conduction of the heat to extinguish it.]

Sir H. Davy first thought of making his safety lamp with small tubes,
which would supply fresh air, and carry off the burnt or foul air, at
the [Page 374] same time they were to be so narrow that no flame could
pass out of his lamp to communicate with an outer explosive atmosphere;
and in speaking of his lamp with tubes he says:--"I soon discovered that
a _few apertures_, even of very small diameter, were not safe unless
their _sides_ were very _deep_; that a single tube of one-twenty-eighth
of an inch in diameter, and two inches long, suffered the explosion to
pass through it; and that a _great number_ of small tubes, or of
apertures, stopped explosion, even when the depths of their sides was
only equal to their diameters. And at last I arrived at the conclusion
that a _metallic tissue_, however thin and fine, of which the apertures
filled more space than the cooling surface, so as to be permeable to air
and light, offered a _perfect barrier_ to _explosion_, from the force
being divided _between_, and the heat communicated to an _immense number
of surfaces_. I made several attempts to construct safety lamps which
should give light in all explosive mixtures of fire damp, and after
complicated combinations, I at length arrived at one evidently the most
simple, that of _surrounding the light entirely by wire gauze, and
making the same tissue feed the flame with air and emit light_."

If a number of square metallic tubes of a fine bore are placed upright
side by side, and a section cut off horizontally, it would represent the
wire gauze which possesses such marvellous powers of sifting away the
heat from a flame, so that it is destroyed in its attempted passage
through the metallic meshes; and of this fact a number of proofs may be
adduced.

A gas jet delivering coal gas may be placed under a sheet of wire gauze,
the gas permeates the gauze, and may be set on fire at the upper side,
but the flame is cut off from the mouth of the jet by the cooling action
of the wire gauze. The same experiment reversed, by holding the gauze
over the gas burning from the jet, shows still more decidedly that flame
will not pass through the metallic tissue. (Fig. 361.)

[Illustration: Fig. 361. A A. A number of square tubes placed upright.
The arrow shows the direction of the section to obtain a figure like
wire gauze.]

Sir H. Davy again says: "Though all the specimens of fire damp which I
had examined consisted of carburetted hydrogen mixed with different
small proportions of carbonic acid and common air, yet some phenomena I
observed in the combustion of a _blower_ induced me to believe that
small quantities of olefiant gas may be sometimes evolved in coal mines
with the carburetted hydrogen. I therefore resolved to make all lamps
safe to the test of the _gas produced by the distillation of coal_,
which, when it has not been exposed to water, always contains olefiant
gas. I placed my lighted lamps in a large glass receiver through which
there was a current of atmospherical air, and by means of a [Page 375]
gasometer filled with coal gas, I made the current of air which passed
into the lamp more or less explosive, and caused it to change rapidly or
slowly at pleasure, so as to produce all possible varieties of
inflammable and explosive mixtures, and I found that iron gauze wire
composed of wires from one-fortieth to one-sixtieth of an inch in
diameter, and containing twenty-eight wires or seven hundred and
eighty-four apertures to the inch, _was safe under all circumstances in
atmospheres of this kind_; and I consequently adopted this material in
guarding lamps for the coal mines, when in January, 1816, they were
immediately adopted, and have long been in general use."

The remarkable conducting power of wire gauze is further shown by
placing some lumps of camphor on a piece of this material, and when the
heat of a spirit-lamp is applied on the under side of the gauze, the
camphor volatilizes, and as the vapour is remarkably heavy, it falls
through the meshes of the gauze, and takes fire; but the most curious
and further illustration of the conducting power of the wire meshes is
shown in the fact that the fire does not communicate through the thin
film of gauze to the lumps of camphor placed upon it.

The camphor may be ignited by applying flame to the upper side of the
gauze, showing that, although this substance is so exceedingly
combustible, it will not take fire even if placed at no greater distance
from flame than the thickness of the wire gauze, provided the latter
material is interposed between it and the flame.

A square box made of wire gauze, with a hole at the bottom to admit a
candle or spirit-lamp, may have a considerable jet of coal gas forced
upon it from the outside, or a large jug of ether vapour poured upon it;
and although the box may be full of flame, arising from the combustion
of the gas or ether, the fire does not come out of the wire box or
communicate with the jet or the ether vapour as it is poured from the
jug. (Fig. 362.)

[Illustration: Fig. 362. A box made of wire gauze, with a hole in the
bottom to admit a spirit lamp lighted. A hot jug full of the vapour of
ether may be poured on to the flame, but it only burns inside the box,
and does not communicate with that in the jug.]

Sir Humphrey Davy's safety lamp consists of a common oil-lamp, _f_, with
a wire through the cistern for the purpose of raising or depressing the
cotton wick without unscrewing the wire gauze; _b_ is the male screw
fitting the screw attached to the cylinder of wire gauze, which is made
double at the top. The entire lamp is shown at A, whilst the platinum
coil which Sir H. Davy recommends should be wound round the wick is
shown at _h_. The small [Page 376] cage of platinum consists of wire of
one-seventieth to one-eightieth of an inch in thickness, fastened to the
wire for raising or depressing the cotton wick, and should the lamp be
extinguished in an explosive mixture, the little coil of platinum begins
to glow, and will afford sufficient light to guide the miner to a safe
part of the mine. With respect to this platinum coil, Sir H. Davy gives
a careful charge, and says:--"The greatest care must be taken that no
filament or wire of platinum protrudes on the exterior of the lamp, _for
this would fire externally an explosive mixture_."

[Illustration: Fig. 363. Sir Humphrey Davy's safety lamp.]

Since the invention of the Davy lamp, a great number of modifications
have been brought forward, some of which for a short time have occupied
the public attention, but whether from increased cost or a sort of
inertia that arrests improvement, it is certain that the lamp originally
devised by Sir Humphrey Davy is still the favourite. It was perhaps
unfortunate that the lamp was called the _safety_ lamp, because it is
not so under every circumstance that may arise, unless it happens to be
in the hands of persons who have taken the trouble to study it and
understand how to correct the faults. The lamp might have escaped the
incessant attacks that have been made upon its just merits, if the name
had simply been that of its illustrious inventor--"a Davy lamp." No one
could carp at that, whilst "safety" was held to mean perfect immunity
from every possible and probable danger that might arise in the
coal-pits. The lamps are now usually placed under the charge of one man,
who trims them and ascertains that the wire gauze is in perfect order;
this latter is usually locked upon the lamp, and as it is a penal
offence, and punishable by a heavy fine and imprisonment, to remove the
wire gauze from safety lamps in dangerous parts of the mine, of course
the miners are being gradually brought to a sense of the obligations
they owe themselves and their brother-miners, and the rash, ignorant,
and foolhardy offences of breaking open safety lamps for more
illumination, or to light pipes, are becoming much less frequent than
formerly. One of the most ingenious "detector lamps" is that of Mr.
Symons, of Birmingham. (Fig. 364.) It consisted of the old-fashioned
Davy, but [Page 317] inside the rim of the wire gauze is placed a small
extinguisher and spring, which does not move so long as the gauze is
screwed _on_ to the lamp, but directly the gauze is unscrewed, the
reversed movement releases the detent, and the extinguisher falls upon
the light. In spite of the manifest ingenuity of this lamp, it is not
adopted, because it costs a trifle more than the ordinary "Davy." To
show the remarkable perfection of the wire gauze principle, some
turpentine may be poured upon a lighted safety lamp, when a great smoke
is produced by the evaporation of the spirit, but no flame passes
through to the outside, although the turpentine burns inside the lamp.
If some coarse gunpowder is laid upon two thicknesses of fine wire
gauze, it may be heated from below with the flame of the spirit lamp,
and the sulphur will gradually volatilize without setting fire to the
mass of powder. To show the security of the Davy lamp, it may be lighted
and hung in a large box with glass sides, open at the top, and a jet of
coal gas supplied at the bottom; as this rises and diffuses in the air,
the mixture becomes explosive, and the fact is at once evident by the
alteration in the appearance of the flame of the lamp, which enlarges,
flickers, and frequently goes out, in consequence of the suddenness with
which the explosion of the mixture takes place inside the lamp,
producing a concussion that extinguishes the flame. In this case the
utility of the platinum coil is very apparent, and it continues to glow
with a red heat until the explosive character of the air in the box is
changed.

[Illustration: Fig. 364. Symons' self-extinguishing Davy lamp.]

If a large washhand-basin is first warmed by some boiling water, which
is then poured away, and a drachm of ether thrown in, a
highly-combustible atmosphere is obtained, and when a lighted Davy lamp
is placed into the basin so prepared, the flame inside the lamp
immediately enlarges and flickers, but is not extinguished, and does not
communicate to the combustible vapour outside. The contrast between the
safety lamp and an unprotected flame is very striking; if a lighted
taper is thrust into the basin, the ether catches fire, and burns with a
very large flame. The solid conductors of heat, which are said to enjoy
this property in the highest degree, are the metals, marble, stone,
slate, and [Page 378] other dense and compact solid substances; whilst
the opposite quality of being non-conductors, or nearly so, is possessed
by fur, wood, silk, cotton, wool, eider and swansdown, paper, sand,
charcoal, and every substance which is of a light or porous nature. The
practical application of this knowledge is very apparent in the affairs
of every-day life. Thus we rise in the morning, and immediately after
the necessary ablutions, if it is winter time, proceed to encase the
body in non-conductors, such as flannel and wool. When we sit down to
the breakfast table to make tea, we may notice the contrivances for
preventing the handle of the top of the urn, or that of the teapot, from
becoming too hot for the fingers, by the interposition of ivory or wood.
If asked to place water in the teapot from the kettle, we instinctively
seek for the well-worn kettle-holder made of Berlin wool, and therefore
a bad conductor. As we cut our meat or fish at the same meal, we may
shiver with cold, but our fingers are not quite frozen by contact with
the steel knives, as we hold them by ivory handles; and we are agreeably
reminded that some metals are good conductors of heat, by the pleasant
warmth of the silver teaspoons, as we stir our tea or coffee.

Even the polish of the well-rubbed mahogany is protected from the heat
of the dishes by non-conducting mats, and plates are handed about, if
"nice and hot," with a carefully-wrapped non-conducting linen napkin.
Supposing we prefer a bit of fresh-made toast, the fork is provided with
a non-conducting handle; and should we peep out of window some wintry
morn whilst the baker delivers his early work in the shape of hot rolls,
we notice they come out of nicely-wrapped flannel or baize, which being
a bad conductor is employed to retain their heat. We read, occasionally,
in the military intelligence, statements respecting some
newly-constructed shells which are to burst and scatter melted iron
(!!); and of course the idea of the interposition of a good
non-conductor of heat between the bursting charge and the molten metal
must be realized in their construction.

The _central heat_ of our globe is a reality that cannot be disputed,
and after digging beyond a depth of twenty feet the thermometer
gradually rises at the rate of one degree of Fahrenheit's scale for
every fifteen yards. The bad conducting power of the crust of the earth
must, therefore, be apparent, as it is easy, knowing the diameter of our
globe, to calculate that the increase of heat downwards amounts to 116°
for each mile, consequently at a depth of thirty and a half miles below
the surface, there will be a temperature most likely equal to 3500°, or
a heat that might easily melt cast-iron, and would help to account for
the earthquakes and eruptions of volcanoes, which still remind us by
their terrible warnings, that we live only on the bad conducting upper
crust of a globe, the inside of which is still, perhaps, in a liquid and
molten state. Monsieur Fourier has demonstrated the non-conducting power
of this shell by calculating that, supposing the globe was wholly
composed of cast-iron, the central heat would require myriads of years
to be transmitted to the surface from a depth of 150 miles; and by
inverting the process of reasoning, we may come to the conclusion that
the [Page 379] internal heat must be excessive, because it is confined
and shut out from those influences that would carry off and weaken the
intensity.

There are no two words, says Tyndal, with which we are more familiar
than _matter_ and _force_. The system of the universe embraces two
things, an object _acted upon_, and an agent _by which_ it is acted
upon; the object we call matter and the agent we call force. Matter, in
certain respects, may be regarded as the _vehicle_ of force; thus, the
luminiferous ether is the vehicle or medium by which the pulsations of
the sun are transmitted to our organs of vision. Or, to take a plainer
case, if we set a number of billiard balls in a row, and impart a shock
to one end of the series in the direction of its length, we know what
will take place; the _last ball_ will fly away, the _intervening_ balls
having served for the transmission of the shock from one end of the
series to the other. Or we might refer to the conduction of heat. If,
for example, it be required to transmit heat from the fire to a point at
some distance from the fire, this may be effected by means of a
conducting body--by a poker, for instance; thrusting one end of a poker
into the fire, it becomes heated, the heat makes its way through the
mass, and finally manifests itself at the other end. Let us endeavour to
get a distinct idea of what we here call heat; let us first picture it
to ourselves as an agent apart from the mass of the conductor, making
its way among the particles of the latter, jumping from atom to atom,
and thus converting them into a kind of _stepping stones_ to assist its
progress. It is a probable conclusion, even had we not a single
experiment to support it, that the mode of transmission must, in some
measure, depend upon the manner in which those little molecular stepping
stones are arranged. But we must not confine ourselves to the molecular
theory of heat. Assuming the hypothesis, which is now gaining ground,
that heat, instead of being an agent apart from ordinary matter,
consists _in a motion of the material particles_; the conclusion is
equally probable that the transmission of the motion must be influenced
by the manner in which the particles are arranged. Does experimental
science furnish us with any corroboration of this inference? It does.
More than twenty years ago MM. De la Rive and De Candolle proved that
heat is transmitted through wood with a velocity almost twice as great
along the fibre as across it. This result has been recently expanded,
and it has been proved that this substance possesses three axes of
calorific conduction; the first and greatest axis being parallel to the
fibre; the second axis perpendicular to the fibre and to the ligneous
layers; while the third axis, which marks the direction in which the
greatest resistance is offered to the passage of the heat, is
perpendicular to the fibre and parallel to the layers.

If many solids are bad conductors of heat, they are at all events
greatly surpassed by fluids, and especially by water. The conduction of
heat by that fluid is almost imperceptible, so much so, that it has even
been questioned whether liquids do really conduct heat downwards at all.
It has, however, been found that liquid mercury will conduct heat
downwards, and therefore by analogy it may be assumed that other liquids
must possess a conducting power, although it may be exceedingly
limited.

[Page 380]

In order to prove that water is an exceeding bad conductor of heat, a
tube with a large glass bulb blown at one end is partly filled with
tincture of litmus, until it will just sink below the surface of water
placed in a tall cylindrical or open jar. If a copper basin, containing
burning ether, is now floated on the top of the water, so as to leave
about a quarter of an inch between the top of the air thermometer--viz.,
the bulb containing the coloured liquid--and the bottom of the copper
pan, it will be noticed that whilst the water surrounding the latter
almost boils, not the slightest effect arising from the conduction of
heat can be perceived in a downward direction. After the ether has burnt
out of the copper vessel, it may be removed, and the boiling water
stirred down and around the air thermometer, when the air within it
expands, drives out the colouring liquid, and the bulb becoming
specifically lighter, rises to the top of the containing glass. (Fig.
365.)

[Illustration: Fig. 365. A A. Cylindrical glass full of water. B. The
glass air thermometer containing the coloured liquid just standing
upright, the mouth of the tube at C being open. D D is the copper basin
containing the burning ether. E shows how the glass bulb and tube rise
after the upper basin is removed, and the hot water comes in contact
with and expands the air, making the thermometer light, and causing it
to rise.]

Again, if the tube of an air thermometer is placed through a cork in the
neck of a gas jar, inverted and standing on a ring stand, and the [Page
381] jar is then filled with water, and boiled at the top with a red-hot
iron heater, the heat does not pass downwards and affect the
thermometer. By introducing a syphon the water surrounding the
thermometer at the bottom of the jar may be drawn off, until the hot
water is within a fraction of an inch of the air thermometer, and still
no heat is conducted, and the liquid in the latter remains stationary.
(Fig. 366.)

[Illustration: Fig. 366. A A A. Inverted gas jar supported by the ring
stand. B. The red-hot urn heater. C C. The air thermometer, with the
coloured liquid stationary at C. D. The syphon for drawing off the cold
water, and bringing the hot down close to the bulb of C C.]

The diffusion of heat through water does not take place like that of
solids, but is effected by the motion of the particles of the water.
When heat is applied to the bottom of a vessel containing water, such as
an inverted glass shade, the first effect is to expand the layer of
water which is first affected by the heat; this expanded layer being
specifically lighter than the cold water above, it rises to the upper
part of the glass shade, and its place is immediately taken by other,
colder and heavier, water, which in like manner moves upwards, and is
again succeeded by a fresh portion. Now, the first and succeeding
strata [Page 382] of water all carry off so much heat, and thus by the
convective or carrying power of the water the heat is diffused finally
in the most perfect manner through the whole bulk of fluid; and indeed,
the movement itself of the particles of water may easily be watched by
putting a little paper pulp at the bottom of the inverted glass shade
containing the water. (Fig. 367.)

[Illustration: Fig. 367. A. A. Inverted glass shade containing water and
some paper pulp. B. Burning spirit lamp placed under _one_ side of the
glass; the pulp shows the rising of the heated water and the sinking of
the cold, in the direction indicated by the arrows.]

This bad conducting power is not merely confined to water, but is
likewise apparent with oil and other fluids, and if some water is frozen
at the bottom of a long test-tube by means of a freezing mixture, oil
may then be poured upon it, and some alcohol above the latter. If the
flame of a spirit-lamp is now applied to the alcohol at the top of the
tube it may be entirely boiled away, and no heat will travel down the
oil and communicate with the ice, and even after the alcohol has been
evaporated away the tube can be filled up with water; this may also be
boiled, and whilst demonstrating the bad conducting power of the oil,
the curious anomaly is observed of a vessel or tube containing ice at
the bottom and boiling water at the top, and further showing the wisdom
of the Supreme Creator in preventing the freezing of the water of lakes,
rivers, and seas, by the exceptional law of the expansion of water by
cold. It is evident from what has been stated that liquids acquire and
lose their heat by means of those currents and movements of the
particles of water which have already been partly explained. Whatever
interferes with this movement must prevent the passage of heat, and
consequently thick viscous liquids are always difficult to boil, and in
consequence of their motion being impeded they rise to too high a
temperature and are burnt. This fact is remarkably apparent in the
manufacture of nice white lump sugar; as the syrup is evaporated it
becomes very thick, and if boiled over a fire might frequently be burnt,
but it is boiled by the heat of steam, and under a vacuum produced by an
air-pump, and thus the sugar-boiler is enabled to avert all danger from
burning.

[Page 383]

It is, then, by a continual and perpetual motion, involving circulation
of the particles, that heat travels through water; and the fact already
described is still further elucidated by one of Professor Griffith's
simple but telling experiments. A glass tube, about three feet in length
and half an inch in diameter, is bent as at A (Fig. 368), and then being
filled with water, is suspended by a string attached to any convenient
support inside a copper dish containing water, so that the straight end
is at the top of the water, and the curved end at the bottom. Just
before it is used some ink or other colouring matter is poured into the
copper pan of water; and it should not be added till the moment the
experiment is to begin, as any rise of temperature in the room promotes
circulation, and interferes with the colourlessness of the water in the
tube, which is compared with the inky fluid in the basin. Directly heat
is applied the hot water rises to the top of the copper vessel, and
thence gradually up the tube; and this movement is rendered visible by
the hot coloured liquid matter creeping slowly up the tube, and
displacing the colourless water, which falls gradually into the copper
pan. (Fig. 368.)

[Illustration: Fig. 368. A. The bent glass tube full of water. B B. The
copper pan containing coloured water. The arrows show the circulation of
the water.]

The principle of the circulation of the particles of water being once
understood, it is easy to comprehend how it is applied to the heating of
buildings by what is called the "Hot Water Apparatus." A coil of pipe is
enclosed in a proper furnace, and the bottom end communicates with a
pipe coming from a second tube or set of coils, placed above it in
another apartment, whilst the top of the latter coil communicates with
the top pipe of the first coil. When the fire is lighted, the
circulation through the first coil of pipe commences, and is
communicated to the second, and from that back again to the first; so
that the "hot water system" [Page 384] involves an endless chain of
pipes of water, provided with proper safety valves to allow for the
escape of any expanded air or steam; and serious accidents have occurred
in consequence of persons neglecting to look after the perfection of
this safety valve. The fearful accident which occurred to the hot water
casing around one of the funnels of the _Great Eastern_ offers a painful
but memorable example of the heating of water, and of the dangers that
must arise if the pipe, casing, or other vessel which contains it, is
not provided with an escape or safety valve, which must always be in
_good working order_.

Mr. Jacob Perkins, in 1824, made his name remarkable for experiments
with the circulation of water through tubes, and his account of the
invention and improvement of the "Steam Gun," in which the improvement
consists chiefly in the circulation of water through coils of pipe, is
so important that we give it verbatim, with a drawing of the steam gun;
and the author is enabled to vouch for the accuracy of the statements
made in the description of the apparatus, as he purchased one of the
improved steam guns, and exhibited it at the Polytechnic Institution,
where it discharged three hundred bullets per minute.

[Illustration: Fig. 369. The charging tube and gun-barrel of steam gun.]

"The expansive power of steam has often been proposed as a substitute
for gunpowder, for discharging balls and other projectiles; the great
danger, however, which was formerly thought to be inseparably connected
with the generation and use of steam, at so extraordinary a pressure as
appeared necessary to produce an effect approximating to that of
gunpowder, prevented scientific men from testing the power of this new
agent by experiment. It was also apparent that the apparatus which was
ordinarily used for generating steam for steam-engines was wholly
inadequate to sustain the necessary pressure, and that one [Page 385]
of a totally different character must be contrived before steam could be
sufficiently confined to come into competition with its powerful rival.

"In the year 1824, Mr. Jacob Perkins succeeded in constructing a
generator of such form and strength, as allowed him to carry on his
experiments with highly elastic steam without danger, although subjected
to a pressure of 100 atmospheres. The principle of its safety consisted
in subdividing the vessel containing the water and steam into chambers
or compartments, so small, that the bursting of one of them was
perfectly harmless in its effects, and only served as an outlet, or
safety valve, to relieve the rest.

"Although Mr. Perkins' generator was originally intended for working
steam engines (it having long been evident to him that highly elastic
steam used expansively would be attended with considerable economy), the
idea occurred to him, in the course of his experiments, that he had
already solved the problem of safely generating steam of sufficient
power for the purposes of _steam gunnery_; and that the steam which
daily worked his engine possessed an elastic force quite adequate to the
projection of musket balls. He therefore caused a gun to be immediately
constructed, and connected by a pipe to the generator, the first trial
of which fully realized his most sanguine anticipations. Its
performance, indeed, was so extraordinary and unexpected, that it gave
rise to a paradox, which was difficult of explanation--viz., that
_steam, at a pressure of only forty atmospheres, produced an effect
equal to gunpowder_; whereas it was known that the combustion of
gunpowder was attended with a pressure of from 500 to 1000 atmospheres.

"Mr. Perkins gives the following explanation of this apparent
discrepancy, by referring to the small effect produced by fulminating
powder, compared to gunpowder, although many times more powerful; he
supposes that the action of fulminating powder, however intense, does
not continue sufficiently long to impart to the ball its full power. The
explosion of gunpowder, although not so powerful at the _instant of
ignition_, is nevertheless, in the aggregate, productive of greater
effect than that of fulminating powder, because the _subsequent
expansion continues_ in action upon the ball (but with decreasing
effect), until it has left the barrel. The action of steam differs from
either of these agents, inasmuch as it _continues in full force until
the ball has left the barrel_; and to this is assigned the cause of its
superiority.

"In the year 1826, Mr. Perkins had so perfected the mechanism of the gun
and generator that, at an exhibition and trial of its power, in the
presence of the Duke of Wellington and other distinguished officers of
the Ordnance Department, balls of an ounce weight were propelled, at the
distance of thirty-five yards, through an iron plate one-fourth of an
inch in thickness; also, through eleven hard planks, one inch in
thickness, placed at distances of an inch from each other. Continuous
showers of balls were also projected with such rapidity, that when the
barrel of the gun was slowly swept round in a horizontal direction, a
plank, twelve feet in length, was so completely perforated, that the
line of holes nearly resembled a groove cut from one of its ends to the
other.

[Page 386]

[Illustration: Fig. 370. Perkins's steam gun.]

[Page 387]

"A is an _iron furnace_, containing a continuous coil of iron tubing, 80
feet in length, 1 inch of external and 5/8th inch of internal diameter,
within which the fire is made; the upper end of this tube, B, called the
flow-pipe, is extended any required distance to the top of the
generator.

"The furnace is provided with a very ingenious _heat governor or
regulator_, by which the intensity of the fire is always proportionate
to the temperature which it may be requisite to maintain in the tubes.

"H is an iron box, containing a series of levers, _b b b_; _c_, a nut
screwed upon the flow-pipe, and in contact with the short arm of the
lowest of the levers. E. A lever, from one end of which is suspended the
damper _f_, and from the other end the rod _g_, which rests upon the
long arm of the highest of the levers, _b b b_. When the apparatus has
arrived at the required temperature, the nut _c_ is screwed down until
it bears upon the lever. Any farther increase of temperature will expand
or lengthen the flow-pipe, and depress the short arm of the lever, which
is in contact with the nut. The combined and multiplied action of the
levers will then elevate the rod _g_, and the damper _f_ will descend to
check the draught. When the fire slackens, and the apparatus cools, the
action of the levers will be reversed, and the damper will open. The
space through which the damper moves, compared with the nut _c_, is as
200 to 1.

"C is the _generator_, composed of a strong iron tube, 3 inches diameter
and 6 feet in length, within which are eight smaller tubes, having their
ends welded to the ends of the larger tube. These small tubes
communicate at the top with the _flow-pipe_ B, and at the bottom with
the _return-pipe_ D, which is continued to the bottom of the
furnace-coil of tubing. The circulation in the tubes is occasioned by
the difference in the specific gravities of the water composing the
ascending and descending currents; the portion contained in the
flow-pipe and fire coil becoming expanded by the heat, ascends by its
superior levity; while that contained in the small tubes of the
generator, having given off its heat, acquires increased density, and
descends through the return-pipe D to the bottom of the furnace-coil, to
take the place of the ascending current. When the hot-water current has
arrived at a temperature of 212° and upwards, cold water is injected
into the generator, and becomes converted into steam by its contact with
the small tubes; the rapidity of evaporation and the pressure of the
steam depending, of course, upon the temperature of the hot-water
current, which at 500° will cause a pressure within the tubes of 50
atmospheres, or 750 lbs. upon the square inch. The whole apparatus is
proved to be capable of sustaining a pressure of 200 atmospheres, or
3000 lbs. upon the square inch.

"G. A force pump for injecting water into the generator.

"I. The indicator for exhibiting the pressure of the steam in the
generator, and of the water in the boiler; it may be connected with
either by means of the valves attached to the levers.

"J. Valve to regulate the pressure of water.

"J 1. Valve to regulate the pressure of steam.

"K. The steam pipe.

"L. The gun.

"M. The discharging lever acting upon the valve N.

"O. The discharging cock, by a simple adjustment in which balls are
transferred from the charging tube P to the gun barrel, _singly_ or in a
_continuous shower_.]

"As the perfection and introduction of the steam gun was not a field for
private enterprise, and the British Government having declined to
institute experiments at its own expense, Mr. Perkins was reluctantly
compelled to leave the project, and to engage in others of a more
lucrative, although, perhaps, of a less important nature. He did not
suspend his operations, however, until he had constructed for the French
Government _a piece of artillery which discharged balls weighing five
pounds at the rate of sixty per minute_.

"The gun and generator exhibited at the Polytechnic Institution during
the time that Mr. Pepper was the Resident Director were the production
of Mr. A. M. Perkins, of London, who has invented an entirely _new
method of generating steam_, which has been successfully applied to
steam engines, and is at once so simple, safe, and economical, as to
leave little doubt that, with its aid, the steam gun will ere long rank
amongst the first instruments of warfare.

[Page 388]

"The gun, except in a few minor mechanical details, does not differ from
that originally constructed by Mr. Jacob Perkins.

"The novelty which distinguishes the generator from all others, consists
in the manner of conveying the heat from the fire to the water, _without
exposing the generator to the action of the fire_. This is accomplished
by means of the circulation, in iron tubes, of a current of hot water,
which is entirely separate from, and independent of, that to be
evaporated in the generator.

"The following are the principal advantages which this generator
possesses over all others: _Freedom from all wear or deterioration
consequent upon exposure to the fire_, an important quality in a
generator that is to be subjected to great pressure, inasmuch as its
original strength remains unimpaired; _no accident can arise from want
of water in the generator_, and the precautions indispensably requisite
when a generator is in contact with the fire are quite unnecessary, as
the water may be drawn off with impunity without producing the least
injurious effect, and the grossest neglect is followed by no worse
consequences than an inefficient supply of steam; _an explosion of the
generator is impossible_, as the temperature of the furnace-coil always
exceeds that of any other part of the apparatus, and consequently, being
the weakest part, is invariably the first to yield when the pressure is
carried beyond the strength of the pipes; _economy of fuel is also
obtained, with a small amount of fire surface_. The circulation of the
water has likewise the effect of preserving the fire-coil from the decay
to which boilers are liable; many such coils, which have been in
constant use for eight years, being apparently as good as when first
erected.

"The whole apparatus is exceedingly simple, and will be readily
understood by reference to the accompanying diagram. (Fig. 370.)

"The steam has often been raised to a pressure of 700 lbs. on the square
inch, but _one-third_ of that pressure is sufficient to completely
_flatten the balls_ when discharged against an iron target one hundred
feet distant from the gun; and a pressure of 400 lbs. per square inch,
at the same distance, _shivers the ball to atoms_, with the production
in a dark room of a visible flash of light. Steam guns are generally
mounted upon a ball and socket joint, which allows the barrel to move
freely in every direction."

       *       *       *       *       *

The conduction of heat through gases is also very slow when heat is
applied to the upper part of any stratum of air. Heat appears to be
diffused through air only by the circulation and rising of the heated
and lighter strata, and the sinking of the colder currents which take
their places; hence the danger of sitting in a room under an open
skylight. A current of cold air may descend upon the head of the
individual, whilst the warmer air takes some other opening to escape
from. No doubt the movement of heated volumes of air is subject to
definite laws, which apply themselves under every case, but are rather
difficult to grasp when the subject of ventilation is concerned. The
philosophical ventilator is often dreadfully teased by the inversion of
all that he had [Page 389] planned, or the total failure of his
apparatus. No specific mode of ventilation can be found to suit all
rooms and buildings; they are like the patients of a physician who
cannot be cured by one medicine only, but must have a treatment adapted
properly to each case. If the fires, candles, gas, or oil-lamps, doors,
windows, and chimneys, were always under the control of the scientific
ventilator, his task would be very simple, but it is well understood
that a ventilating system which answers well if certain doors
communicating with lobbies are closed, fails directly they are
accidentally opened. The watchful care of the ventilator must begin with
the lowest area door, and in his calculations he must study the effect
of every other door or window that may be opened, so that if a
scientific man undertakes to ventilate a house, he must have a
well-drawn plan hung up in the hall, and it must be clearly understood
by the inmates that any interference with that plan will prejudice the
whole.

There are a few common principles which will guide in ventilation, and
these are, first, the rise of hot and the fall of cold air; second, that
if an aperture is provided at the top of a room for the escape of hot
air, an equally large aperture must be left for the entry of cold air;
third, the aperture for the escape of hot air must be adapted in size to
the number of persons likely to enter the room, and the number of gas or
other lights burning in it. During the daytime, moderate apertures for
the exit and entrance of air may suffice, but these must be largely
increased at night, when the room is filled with people and lighted up.
Expanding and contracting openings are therefore desirable, and they are
to be regulated by rules stated on the plan of the ventilating system
(already alluded to as being hung up in the hall) of the house which has
submitted itself to a perfect system of ventilation, and no hall-keeper,
footman, or butler should be allowed to remain in his post unless he
undertakes to comprehend the system and work it properly by the written
rules.

Dr. Angus Smith, in a very able paper "On the Air of Towns," says--"One
of the conditions of health, and a most important, if not the most
important of all, is to be found in the state of the atmosphere. As to
the effect on the inhabitants, the question becomes exceedingly
complicated; but the Registrar-General's returns are an unanswerable
reply as to the results of the lethal influences of the district. Few
people seem clearly to picture to themselves the meaning of a decimal
plan in the percentage of death, and few clearly see that there are
districts of England where the deaths at least in some years, and when
no recognised epidemic occurs, are three times greater than in others.
When we hear of the annual deaths in some districts being 3.4 per cent.,
and in the whole of England 2.2, it is simply that 34 die instead of 22,
whilst even that is too slightly stated, as the whole of England would
show a lower death-rate if the towns were not used to swell it."

This quotation is given here to remind our readers of the important
question of a supply of pure air as well as pure water and pure food;
and if the agricultural labourer, with all his exposure to variable
[Page 390] weather, can take the first place in the scale of mortality,
and outlive the members of all other trades and professions, it is
evident that the importance of pure air is not overrated.

Every effort ought, therefore, to be made in large schools, hospitals,
and barracks, to enforce a rigid system of supply of fresh air, and a
sewage or removal of the impure; and in the use of a certain test
employed by Dr. Smith for the detection of organic matter in the air a
number of approximations were obtained, which clearly demonstrated that
1 grain of organic matter was detected in 72,000 cubic inches of air in
a room, and the same quantity in 8000 cubic inches taken from a
_crowded_ railway carriage.

[Illustration: Fig. 371. A B. The glass tube. C. The spirit lamp, with a
very large wick; if a little ether is mixed with the spirit in the lamp
it increases the length of the flame. D. The effect of the ascension of
air, increased by warming the top of the tube with the lamp D.]

To show the rising of heated air, a long glass tube, about
three-quarters of an inch in diameter, may be provided and held over the
flame of a spirit lamp at an angle of sixty degrees. As the tube warms,
the heated air rushes past the flame with great rapidity, and pulls it
out or elongates it so much, that the sharp point of the spirit-flame
[Page 391] will frequently be seen at the end of a tube ten feet six
inches in length. The flame is, as it were, the sign-post that indicates
the path or direction of the air. (Fig. 371.)

Upon the like principle, heated air may be dragged down the short arm of
a syphon, provided the other arm is sufficiently long to impart a strong
directive tendency to the upward current, and this mode of setting air
in motion has been frequently proposed in numerous schemes for
ventilation. In order to prove the fact that an inverted syphon will act
in this manner, an iron pipe of three inches diameter and six feet long
may be bent round during the construction into the form of a syphon, so
that the short length is about one foot long, and the long length the
remaining four feet, allowing one foot for the bend. If the interior of
the long arm is first warmed by burning in it a little spirits of wine
from a piece of cotton or tow wetted with the latter (which can be
easily done by dropping in such a wetted piece into the bend of tube, so
that it is just under the opening of the long part of the tube), the air
is soon set in motion up the long pipe, and as it must be supplied with
fresh volumes of air to take the place of that which rises, and as the
only entrance for the fresh air can be _down_ the short arm of the
syphon, the circulation soon commences, and it proceeds as long as the
upper arm is kept sufficiently warm. If a flame is held over the mouth
of the short arm, it is immediately dragged downward, whilst, if held at
the mouth of the long pipe, the motion of the air is seen by the
assistance of the flame to be in the contrary direction. (Fig. 372.)

[Illustration: Fig. 372. A B. Inverted sheet iron syphon. At C is seen
the piece of tow moistened with alcohol, which, being set on fire, warms
the tube B. D. A lighted torch of coloured spirit, the flame of which is
dragged down the tube at A by the descending current, and is impelled
upwards by the ascending current B.]

This plan of ventilation was proposed to be used in rooms in connexion
with the chimney and chimney-piece, and in order to give it an
ornamental appearance, the chimney-piece was supplied with two
ornamental hollow columns, the ends of which were open at the
mantel-shelf, and the tubes or columns were continued under the
hearthstone, proceeding up the back of the grate and entering the
chimney, in which there would be a constant current of heated air, and
it was expected that the [Page 392] syphon arrangement would keep a
current of air always in motion, and thus help to ventilate the room.
(Fig. 373.) This plan, however, does not appear to have been adopted,
and wisely so, because half the time the syphon arrangement might invert
itself, and vomit smoky air out of the chimney into the room; indeed it
is surprising what odd and contradictory freaks are performed by
currents of air. The author remembers a case where two rooms on the same
floor, the one a dining-room and the other a drawing-room, were always
exhibiting the most absurd phenomena of smoke. If the fire in one room
was lit, then the other, in a few moments, began to smell exactly like
the inside of a gas manufactory, and was, of course, more or less filled
with smoke, whilst the room in which the fire was actually burning
remained quite free from this annoyance. The smoke appeared to issue
from the wainscot or moulding which runs round at the bottom of the
wall, and was at first thought to be an escape from the chimney of the
kitchen beneath, the inside of which was duly examined and thoroughly
stopped with cement in every place likely to afford a channel to the
smoke, and [Page 393] the crevice whence the smoke issued was also
filled in neatly with cement. But it was all in vain; the smoke then
made its way out from another part of the cornice, and at last the rooms
exhibited a beautiful reciprocating action. If the drawing-room fire was
lighted the dining-room was full of smoke, and if the latter was lighted
the former had the agreeable visitation. At last the backs of the two
grates were examined, and in each was discovered a hole about one inch
in diameter; and it was also found that the spaces at the back of the
stoves had not been filled in properly, and, indeed, communicated with
the hollow space behind the cornice. When, therefore, the fire was
lighted, and coals heaped on just above the hole, the gas and smoke
distilled through the orifice and travelled on, where it found the most
convenient exit; and the fact is sadly at variance (_apparently_) with
theory, because it might be considered that cold air would rush towards
a fire, and that the draught ought to have been from the cornice to the
chimney instead of _vice versâ_. The fact seems to be that the coal in
all grates is, in the act of burning, distilling and giving off
inflammable gas; when the coal was, therefore, heaped above the orifice,
and was, possibly, caked hard at the top, the gas distilling from it
escaped more easily from the little orifice than elsewhere, and chance
determined that the channel or delivery pipe should be in the direction
of the drawing-room when the fire was burning in the dining-room, and in
the contrary direction when the fire was lighted in the latter chamber.
The nuisance was stopped by plugging the holes at the back of the grate
with clay, and putting a sheet of iron over the orifice.

[Illustration: Fig. 373. A B. Chimney-piece supported on two hollow
ornamental pillars corresponding with the short arm of a syphon. C C C.
The dotted line showing the pipes leading from each pillar under the
hearth, and terminating in a long pipe passing into the chimney. The
arrows show the path of the air descending from the chimney-piece and
ascending in the chimney.]

Before Dr. Faraday was appointed as a scientific counsellor to assist
the deliberations of the Trinity Board in connexion with lighthouses,
all the lamps were burnt in the lanterns with the smallest and most
imperfect arrangement for carrying off the heated air and products of
combustion; as a natural consequence, and particularly on cold nights,
the windows of the lantern of the lighthouse were covered with ice
derived from the condensation of the water produced by the combustion of
the hydrogen of the oil, whilst the carbon generated such quantities of
carbonic acid that the light-keepers were unable to stay in the lantern,
and if obliged to visit the latter (whilst looking to improving the
light of any single lamp that might be burning dimly), they were almost
overpowered with the excess of carbonic acid, and stated, in their
evidence, that it produced headache and sickness, and a tendency to
insensibility. Faraday immediately established a system of ventilation;
and by attaching a copper tube to the top of each lamp-chimney, and
centering them all in one large funnel passing to the top of the
lighthouse, the whole of the water which previously condensed on the
glass windows and impeded the light, besides injuring the brass and
copper fittings, was carried off, as also the poisonous carbonic acid
gas; and thus, as Dr. Faraday expressed himself, a complete system of
sewage was applied to the lamps of the lighthouses.

If any one of the numerous stories of ships saved by the Eddystone
Lighthouse could demonstrate more than another the value of this beacon
[Page 394] in mid ocean, it must be the graphic account in the _Times_
of the gallant conduct of the British Admiral with his fleet whilst
breasting the frightful storm of October, 1859, and endeavouring to
reach Plymouth Sound:--

"It was on Saturday, the 22nd October, that the _Hero_, the _Trafalgar_,
the _Algiers_, and the _Aboukir_, accompanied by the _Mersey_, the
_Emerald_, and the _Melpomene_, put to sea from Queenstown. Up to the
afternoon of Monday the squadron met with no remarkable adventure, but
about that time, just after the crews had been exercised at gunnery
practice, heavy storms of hail and sleet began to set in. Still there
was no immediate indication of the tempest at hand, and at sunset
topsails were double-reefed and courses reefed for the night, with no
particular character about the wind, except that of extreme variability.
As the morning broke on Tuesday--the day of the storm--the Land's-end
was sighted, and the rain and the wind continued to increase. About nine
A.M. the advent of the gale was no longer doubtful; topgallantyards were
sent on deck and topgallantmasts struck, and the signal was given from
the flagship, 'Form two columns; form line of battle; Admiral will
endeavour to go to Plymouth.' To Plymouth, accordingly, the course of
the fleet was shaped, but so terrifically had the wind increased that it
became very questionable whether the sternmost ships of the line could
possibly succeed in entering the Sound. Upon this the Admiral determined
to wear the fleet together, stand off, and face the storm, a manoeuvre
which, under circumstances of great difficulty, was most gallantly
executed. The ships were close upon the Eddystone Lighthouse, round
which they 'darted like dolphins' under the tremendous pressure of the
gale, the _Trafalgar_ stopping in the midst of the storm to pick up a
man who had fallen overboard. The whole squadron now stood off the land,
the _Mersey_ and _Melpomene_ furling their sails, and the former vessel
steaming along 'like an ocean giant.' Still the gale increased till
about three P.M., when there occurred that remarkable phenomenon by
which these rotatory tempests are characterized. The fleet had got into
the very centre of the storm, the 'eye' of the tornado, and, though the
sea towered up and broke in tremendous billows all around, the wind
suddenly ceased and the sun shone. When, however, the signal had been
given and obeyed for setting sail again, the ships soon encountered the
gale once more--not, as before, from the S.E., but the N.W.--and in
greater force than ever. It was now a perfect hurricane; and for three
hours the whole fury of the tempest was poured upon the squadron. When
it began, at length, to abate a little, the four line-of-battle ships
and one of the frigates were still in company, and all doing well. The
_Mersey_ and the _Emerald_ had steamed into Plymouth, but the five
remaining vessels kept in open order throughout that terrible night,
wore in succession by night signal at about one A.M., made the land at
daylight, formed line of battle, came grandly up Channel under sail at
the rate of eleven knots an hour, steamed into Portland, and 'took up
their anchorage without the loss of a sail, a spar, or a ropeyarn.'"

After making the important improvement in the ventilation of
lighthouses, many letters were addressed to the learned philosopher
by [Page 395] numerous light-keepers, one of which in plain but
striking language related that "_the enemy_ (alluding to the water and
carbonic acid) _was now driven out_."

[Illustration: The British fleet rounding the Eddystone Lighthouse
during the great storm of October, 1859. _p. 394_]

The ingenious invention alluded to was succeeded by another and equally
simple but philosophical arrangement, which Dr. Faraday presented to his
brother, and it was duly patented. It consisted of an arrangement for
ventilating gas burners, and it must be obvious that a necessity exists
for such ventilation, because every cubic foot of coal gas when burnt
produces a little more than a cubic foot of carbonic acid. A pound
weight of ordinary coal gas contains about 3/10ths of its weight of
hydrogen, which when burnt produces two pounds and 7/10ths of a pound of
water. A pound of ordinary coal gas also contains about 7/10ths of its
weight of charcoal, which produces when burnt rather more than two and a
half pounds of carbonic acid gas--viz., 2.56. In order to burn this
quantity of gas nineteen cubic feet and 3/10ths of a foot of atmospheric
air, containing 4.26 cubic feet of oxygen, are required.

[Illustration: Fig. 374. A B. Gas pipe and argand burner; the air
enters, as usual, up the centre of the argand. C C. The first glass
chimney open at the top. D D. The second glass chimney closed at the
top, with a disc of double talc, and fitting over C C, and leaving a
space between the two glasses, down which the air passes, and into the
ventilating tube, E E. H H. The ground-glass globe closed at the top,
and surrounding the whole.[I]]

[Footnote I: Mr. Faraday, of Wardour-street, supplies this ventilating
lamp.]

It is not therefore surprising that as common coal gas is sometimes
purified carelessly, and contains a minute trace of sulphuretted
hydrogen, with some bisulphide of carbon vapour, that it should produce
the most prejudicial effects in badly ventilated rooms, and especially
in some of those perched up glass boxes in large places of business,
where clerks are obliged to sit for many consecutive hours, lighted by
gas, and breathing their own breath and the products of combustion from
the gas light, thereby rendering themselves liable to diseases of the
lungs, and also to very troublesome throat attacks, when leaving their
close glass boxes, and passing into the cold night air. The dangerous
product of the combustion of ordinary coal gas is sulphurous acid--viz.,
[Page 396] the same gas as that generated when a sulphur match is
burnt; and if it will attack the bindings of books, and damage
furniture, goods in shops, curtains, &c., in consequence of the large
quantity of water with which it is accompanied, how much more is it not
likely to injure the delicate organism of the breathing apparatus of the
lungs? Dr. Faraday's lamp is therefore a great boon, but, like a great
many other clever things, it must be adapted to the currents of air and
draught from the room; and means must be taken to prevent the draught
becoming too powerful in Faraday's lamp, or else the illuminating power
is destroyed by the thorough combustion of the carbon of the coal gas,
and the heat generated is so intense that the glasses soon crack, and of
course become useless. The lamp will answer very well if (as has been
already stated) the draught in the ventilating pipe is not too great.

[Illustration: Fig. 375. Section showing the two air-shafts. A. The
downcast. B. The upcast. C C. One of the working galleries in connexion
with the upcast and downcast. D. The furnace at the bottom of the
_upcast_. In this sketch _one_ gallery only has been shown, to prevent
confusion and to show the principle.]

The system already explained and illustrated is likewise carried out on
a much larger scale in the ventilation of coal pits, where a shaft is
usually sunk into the ground for the admission of air, which, after
circulating through the intricate windings and mazes of the coal pit
workings, escapes at last from another shaft, at the bottom of which is
placed a powerful furnace, and this is kept burning night and day, so
[Page 397] that the movement of the air is maintained in one
direction--viz., from the outer air down the shaft called _the
downcast_, thence to the galleries, where the coal hewers are working,
to the second shaft, near which the furnace is placed, and up this
latter the air travels; the shaft, pit, or funnel being very
appropriately termed the _upcast_.

Should the furnace at the bottom of the upcast be neglected, the
ventilation may be just balanced, or set slightly towards the downcast;
under these circumstances the carbonic acid from the fire will begin to
circulate in the galleries, and poison those who are not aware of its
presence and take the proper means to escape. Such accidents, amongst
the host of others that occur in a coal pit, have actually been
recorded; and the firemen, whose duty it might be to attend to the
proper burning of the furnace, have had to pay the penalty of death for
their own carelessness in falling asleep and neglecting to maintain the
ventilation of the mine in one direction. (Fig. 375.)

These details are amply sufficient to demonstrate the manner in which
heat is diffused through air, whilst the rarefication of the air by heat
suggests the cause of those frightful storms of wind that rush from
other and colder parts of the surface of the globe, to supply the void
produced by the cooling and contraction of the enormous volumes of
gaseous matter.


_The Radiation of Heat._

When rays of heat are emitted from incandescent matter, they are not
necessarily visible, nay, they are generally invisible, and not
accompanied with a manifestation of light, and pass with great velocity
through a void or vacuum, also through air and certain other bodies.
From what has been stated respecting the manner in which air, by
continually moving, and by convection, carries off heat, it might be
thought that no proof existed that invisible rays of heat are really
thrown off from a ball filled with boiling water. But this question is
set at rest by the fact, that such a ball will cool rapidly when
suspended by a string inside the receiver of an air pump from which the
atmospheric air has been removed, so that no conduction of the particles
of air could possibly remove the heat.

In the year 1786, Colonel Sir B. Thompson examined the relative
conducting powers of air and a Torricellian vacuum--the latter being
used because, as the experimenter stated, it was impossible to obtain a
perfect vacuum, on account of the moist vapour which exhaled from the
wet leather and the oil used in the machine, for at that time carefully
_ground_ brass plates were not used in air-pumps, but plates only, with
a circular piece of wet leather upon them. In a paper which Colonel Sir
B. Thompson read before the Royal Society, he stated that "It appears
that the Torricellian vacuum, which affords so ready a passage to the
electric fluid, so far from being a good conductor of heat, is a much
worse one than common air, which of itself is reckoned among the worst;
for when the bulb of the thermometer was surrounded with air, and the
instrument was plunged into boiling water, the mercury rose from 18° to
27° [Page 398] in forty-five seconds; but in the former experiment,
when it was surrounded by a Torricellian vacuum, it required to remain
in the boiling water one minute thirty seconds to acquire that degree of
heat. In the vacuum it required five minutes to rise to 48°-2/10ths; but
in air it rose to that height in two minutes forty seconds; and the
proportion of the times in the other observation was nearly the same.

"It appears, from other experiments, that the conducting power of air to
that of the Torricellian vacuum, under the circumstances described, is
as 1000 to 702 nearly, for the quantities of heat communicated being
equal, the intensity of the communication is as the times inversely. By
others it appears that the conducting power of air is to that of the
Torricellian vacuum as 1000 to 603."

[Illustration: Fig. 376. The air-pump and receiver, containing at A the
electric light in the focus of a concave mirror, and at B a delicate
thermometer, also in the focus of a concave mirror.]

It is therefore very interesting to discover that the attention of
experimentalists was early directed to the fact that heat was
independent of the air, and passed either as waves of heat or molecules
of heat through space. The velocity with which heat moves through a
vacuum is very great, and in an experiment performed by M. Pictet, no
perceptible interval took place between the time at which caloric
quitted a heated body and its reception by a thermometer at a distance
of sixty-nine feet. It appears also, from the experiments of the same
philosopher, to be thrown off or radiated in every direction, and not to
be diverted (as shown at p. 369) by any strong current of air passing it
transversely. Sir Humphrey Davy ignited the charcoal points connected
with a battery in a vacuum, taking care to place the charcoal points at
the top of the jar, and a concave mirror, with a delicate thermometer in
its focus, at the bottom of the vessel placed upon the air-pump plate.
The effect of radiation was [Page 399] ascertained first when the
receiver was full of air, and next when it was exhausted to 1/120th
(_i.e._, 199 parts pumped out, leaving only one part of air in the
receiver). In the latter case, the effect of radiation was found to be
three times greater than in an atmosphere of the common density. The
greater rise of the thermometer _in vacuo_ than in air is to be ascribed
to the conducting power of the latter; for this conducting power, by
reducing the temperature of the heated body, has a constant tendency to
diminish the activity of radiation, which is always proportional to the
excess of the temperature of the heated body above that of the
surrounding medium. (Fig. 376.)

Count Rumford's experiments with a Torricellian vacuum gives the
proportion of five _in vacuo_ to three in air for the quantities of heat
lost by radiation, and by conduction or diffusion. It is not, perhaps,
departing very far from the truth, if it be stated that one half of the
heat lost by a heated body escapes by radiation, and that the rest is
carried off by the convective power of currents of air.

[Illustration: Fig. 377. Negretti and Zambra's terrestrial radiation
thermometer. The bulb of this instrument is transparent, and the
divisions engraved on its glass stem. In use it is placed with its bulb
fully exposed to the sky, resting on grass, with its stem supported by
little forks of wood, and protected from the wind.]

If the process of radiation was not constantly proceeding, it can easily
be imagined that the temperature of our globe would become so elevated
by the regular accession of heat from the sun's rays, that the
vegetation would be parched up and destroyed, and consequently all
animals and the human race must become extinct. The best time to notice
the radiation of heat from the earth is at night and after a hot
summer's day. If the sky is clear, it will be noticed (with the help of
a thermometer,) that the ground is several degrees colder than the air a
few feet above it. (Fig. 377.) It is this reduced temperature that
causes the deposition of dew, and produces the earth-cloud which so
nearly resembles a sheet of water as to have been occasionally mistaken
for an inundation, the occurrence of the previous night. Mr. Luke Howard
has called this cloud, which is the lowest form of these draperies of
the sky, "The Stratus," or evening mist; but when permanent, and
increased to a depth so as to rise above our heads, it is then called
the morning fog, so peculiarly agreeable in London when incorporated
with the black smoke, making a fine reddish-yellow ochreous mist. By
placing a thermometer, standing at the ordinary temperature of the air,
cased [Page 400] with a good radiating material, such as filaments of
cotton, in the focus of a concave mirror, and by turning this
arrangement towards a clear sky in the evening, it will be noticed that
the temperature falls several degrees. Good radiators of heat are black
and scratched surfaces, filaments of cotton, grass, twigs, boughs, and
certain leaves, especially those with a rough surface.

Bad radiators of heat are bright and polished metallic surfaces, white
woollen cloth or flannel, hard and dense substances, such as a gravel
path and stone, or those leaves which have a polished surface, such as
the common laurel. It is the frozen dew and mist which produce the
beautiful effect of hoar-frost and icicles on the trees and bushes, the
primary cause being the radiation of heat from the various objects on
the surface of the earth, as well as from the latter itself. When the
wind is high, dew does not deposit, as it is necessary that the air
should be calm, in order to receive the cooling impression of the cold
earth, and to deposit the moisture, which it holds in solution as
invisible steam. When the wind blows, it mixes all parts of the air
together, and prevents that difference of temperature which causes the
deposit of dew. Hence the evening mist will be more generally observed
in the bosom of a valley surrounded by hills and screened from the winds
that may blow from either quarter. The continual presence of moisture in
the air is well shown by the condensation of water on the outside of a
glass of cold spring water, or especially on the outside of a jug
containing iced water. The invisible steam is always ready to bathe the
tender plants with dew, which would otherwise perish and be burnt up
during a hot summer, if they did not radiate heat at night, and thus
condense water upon themselves. The presence of watery vapour in the air
becomes therefore a matter of great importance, and hence the
construction of hygrometers or measurers of the moisture in the air.

Regnault's condenser hygrometer consists of a tube made of silver, very
thin, and perfectly polished; the tube is larger at one end than the
other, the large part being 1.8 in depth by 8.10 in diameter. This is
fitted tightly to a brass stand, with a telescopic arrangement for
adjusting when making an observation. The tube has a small lateral
tubulure, to which is attached an India-rubber tube with ivory
mouthpiece; this tubulure enters at right angles near the top, and
traverses it to the bottom of largest part. A delicate thermometer is
inserted in through a cork, or India-rubber washer, at the open end of
the tube, the bulb of which descends to the centre of its largest part.
A thermometer is attached for taking the temperature of the air; also a
bottle for containing ether.

To use the condenser hygrometer, a sufficient quantity of sulphuric
ether is poured into the silver tube to cover the thermometer bulb. On
allowing air to pass bubble by bubble through the ether, by breathing in
the tube, an uniform temperature will be obtained; if the ether
continues to be agitated by breathing briskly through the tube, a rapid
reduction of temperature will be the result. At the moment the ether is
cooled down to the dew-point temperature, the external surface of that
portion [Page 401] of the silver tube containing the ether will become
covered with a coating of moisture, and the degree shown by the
thermometer at that instant will be the temperature of the dew-point.

The most simple form of the hygrometer was formerly a very favourite
indicator of the state of the weather, and usually consisted of the
figure of a monk with his hood, which is attached to a bit of catgut;
this covering of paper, painted to represent the hood, falls over the
head on the approach of damp weather, and inclines well back during the
period that the air is dry or contains less moisture; and simple as it
is, this hygrometer, in conjunction with the reading of the barometer,
may assist _Paterfamilias_ in deciding the fate of a pet bonnet or
velvet mantle, which is or is not to be worn on a doubtful day. (Fig.
378.)

[Illustration: Fig. 378. The monk hygroscope, in which the hood, A B,
covers the head to dotted line C in wet weather, and takes various
intermediate positions, being quite back and on the shoulders in dry
states of the air. A thermometer, D, is usually attached.]

A decision on the possible changes of the weather requires considerable
experience, and it has been said that one of the most celebrated
marshals of France owed his invariable success in military combinations
and attacks to his attention to the signs of the weather, as indicated
by the state of the air during the phases of the moon. Inexperienced
persons (and by that we mean young persons) may, however, take a certain
position in the rank of "weather prophets" by consulting the
weathercock, the barometer, and the hygrometer, before committing
themselves to an opinion, if asked to say what the weather will be.

The dry and wet bulb hygrometer (as represented in the next engraving)
consists of two parallel thermometers, as nearly identical as possible,
mounted on a wooden bracket, one marked _dry_, the other _wet_. The bulb
of the wet thermometer is covered with thin muslin, round the [Page
402] neck of which is twisted a conducting thread of lamp-wick, or
common darning-cotton; this passes into a vessel of water, placed at
such a distance as to allow a length of conducting thread of about three
inches; the cup or glass is placed on one side, and a little beneath, so
that the water within may not affect the reading of the _dry bulb
thermometer_. In observing, the eye should be placed on a level with the
top of the mercury in the tube, and the observer should refrain from
breathing whilst taking an observation. The temperature of the air and
of evaporation is given by the readings of the _two thermometers_, from
which can be calculated the dew-point, tables being furnished for that
purpose with the instrument. (Fig. 379.)

[Illustration: Fig. 379. The dry and wet bulb hygrometer.]

The colour of the sky at particular times affords the most excellent
guidance to doubting members of pic-nic or other out-of-door parties.
Not only does a rosy sunset presage fine weather, and a ruddy sunrise
bad weather, but there are other tints which speak with equal clearness
and accuracy. A bright yellow sky in the evening indicates wind; a pale
yellow, wet; a neutral grey colour constitutes a favourable sign in the
evening, an unfavourable one in the morning. The clouds, again, are full
of meaning in themselves. If their forms are soft, undefined, and
feathery, the weather will be fine; if their edges are hard, sharp, and
defined, it will be foul. Generally speaking, any deep, unusual hues
betoken wind or rain, while the more quiet and delicate tints bespeak
fine weather.

The principle of radiation of heat is employed by the Indian natives in
the neighbourhood of Calcutta for the purpose of obtaining small [Page
403] quantities of ice. In that climate, the thermometer during the
coldest nights does not indicate a lower temperature than about 40°
Fahr. The sky, however, is perfectly cloudless, and as heat radiates
with great rapidity from the surface of the ground, the Indian natives
ingeniously place very shallow earthenware pans on straw, which is a bad
conductor of heat, and hence insulates the pans from communication with
the parched earth. In a few hours, the water in the pans is covered with
a thin sheet of ice, and there can be no doubt of its production by an
absolute loss of heat by radiation, because the plan does not succeed on
a windy night, and succeeds best even when the pans are sunk in trenches
dug in the earth. A windy night prevents that difference of temperature
between one portion of the surface of the earth and another, which is so
essential to a steady and uniform loss of heat, as it must be evident
that the continual mixture of warmer portions of air with that which is
colder would tend to prevent the desired lowness of temperature being
attained.

The manner in which heat is observed to be radiated has suggested
another theory to the fertile brain of philosophical observers, and it
has been supposed that the conduction of heat may be nothing more than a
radiation from one particle of matter to another, as through a bar of
copper, in which the particles, though packed closely together, are not
supposed to be in actual contact, so that it is possible to conceive
each separate atom of copper receiving and radiating its heat to the
neighbouring particle, and so on throughout the length and breadth of
the metal. By this theory the radiation of heat through a vacuum is
brought into close connexion with that of the radiation of heat through
the air and other solid and liquid bodies.

Some of the most interesting phenomena of heat are those discovered by
Leslie, who has proved in a very satisfactory manner that the rapidity
with which a body cools, depends (like the reflection of light) more on
the condition of the surface than on the nature of the material of which
the surface is composed. With a globular and bright tin vessel it was
observed that water of a certain heat contained in it, required 156
minutes to cool; but when the latter vessel was covered with a thin
coating of lamp-black and size, the water fell to the same degree as
that noticed in the first experiment in the space of eighty-one minutes.

By very careful observations made with a differential air thermometer,
Leslie determined that the power of radiating heat in various substances
was as follows:--

  Lamp-black    100
  Writing paper  98
  Sealing wax    95
  Crown glass    90
  Plumbago       75
  Tarnished lead 45
  Clean lead     19
  Iron, polished 15
  Tin plate      12
  Gold           12
  Silver         12
  Copper         12

[Page 404]

As in the reflection of light, it was noticed that a piece of charcoal
covered with gold leaf, partook of the nature of the precious metal so
far as its power of throwing off or scattering the rays of light was
concerned, so a piece of glass covered with gold-leaf appears to possess
the same power of radiating heat as that of any brilliant metal.

Radiant heat, like light, can be propagated through a great variety of
substances, but is stopped by the larger number; and it can be
reflected, refracted, polarized, absorbed, or it may undergo a secondary
radiation.

The intensity of radiant heat follows the same law as that of light, and
decreases as the square of the distance from its source. The same law
that governs the reflection of light, also prevails with that of heat;
and it may be found by experiment that the angle of incidence is equal
to the angle of reflection, so that the heat is disposed of in the same
manner as light when it falls upon bright polished planes, convex and
concave surfaces; hence the use of bright tin meat screens and Dutch
ovens, and of all those simple pieces of culinary furniture which are
employed in the kitchen for the purpose of arresting the cold currents
of air that set towards burning matter, as also to reflect the heat upon
whatever viands may be cooking before the fire. A bright silver teapot
retains its heat better than a dirty one, and the fact is determined
very readily by pouring boiling water into two teapots, the one being
made of bright tin and the other of black japanned tin. A thermometer
inserted into each vessel will soon show that the latter radiates, and
therefore loses its heat quicker than the former; the relative radiating
powers of bright and blackened tin being as 15 to 100. Pipes for the
conveyance of hot water or steam should be kept bright, if possible,
although this trouble is avoided usually by packing them in bad
conductors of heat, whilst the polish of the cylinder of a steam-engine
is of great importance as a means of economizing heat.

[Illustration: Fig. 380. A B. The cone of paper, gilt inside. C. The
red-hot ball. D. Stand with wood supporting a slice of phosphorus, which
is brought into the focus of the rays of heat reflected through the
cone.]

When the finger is approached within an inch or so of a red-hot ball,
the heat radiated from the latter is so intense that it cannot be held
there [Page 405] for more than a few seconds. If, however, the finger
is coated with gold leaf it may be kept near the iron ball for some
considerable time, because the radiant heat is reflected from the
surface of the gold. If the word heat is written upon a sheet of paper
and the letters afterwards gilt, the whole of the white surface is
rapidly toasted and scorched when held before a fire, whilst the surface
of the paper under the gold leaf remains perfectly white, which can be
ascertained by turning the paper round and observing the other side. A
sheet of paper gilt inside and turned round as a cone, being left open
at both ends, may be employed as a reflecting surface; and if a bit of
phosphorus, placed on paper, is held, say at two feet from a red-hot
ball of about two inches diameter, the radial heat from the latter has
not sufficient intensity at that distance to set it on fire quickly; if,
however, the cone of gilt paper is used between the two, and the
phosphorus brought into the focus of the rays of radial heat, it very
quickly takes fire. (Fig. 380.)

Dr. Bache has determined by experiments that the radiation of heat from
a body is not affected by colour, so that in winter all coloured clothes
are alike in that respect, and radiate heat without any appreciable
difference. The power of _absorbing_ heat, however, is greatly dependent
on colour; and as a general rule, good radiators of heat (such as a
black cloth, or indeed any surface covered with lamp-black), are also
excellent absorbents of heat. Dr. Hooke and Dr. Franklin placed pieces
of cloth of similar texture and size on snow, allowing the sun's rays to
fall equally upon them. The dark specimen always absorbed more heat than
the light ones, and the snow beneath them melted to a greater extent
than under the others; and they both remarked that the effect was nearly
in proportion to the depth of the shade, as in the following
order:--After black, the maximum absorbent quality was possessed by,
first, blue; second, green; third, purple; fourth, red; fifth, yellow.
The minimum absorbent power was observed to belong to white.

When radiant heat is allowed to pass through glass, the latter substance
is not found to be transparent to heat rays as it is to those of light,
but a considerable proportion of heat is arrested and stopped;
consequently glass fire-screens are to be found in the mansions of the
wealthy, because they obstruct the heat but do not exclude the cheerful
light and blaze of the fireside.

Melloni's researches on the nature of the rays of heat, and also on the
media which affect them, would demand and merit a chapter to themselves;
want of space, however, obliges us to omit the consideration of
thermo-electricity, and the refined and beautiful experiments of
Melloni, whose labours are a model for the imitation of all original
seekers after truth.

[Page 406]




CHAPTER XXVIII.

THE STEAM-ENGINE.


[Illustration: Fig. 381. Hancock's steam omnibus, which ran on the
common roads.]

It must be apparent to those who read popular works on science, that
they possess, at all events, one point of utility--viz., that they are
_indicative_ of the various subjects that may be selected in science for
special, searching and exhaustive study. The subject of steam and the
steam engine is not one that could be thoroughly treated of in the
narrow space allowed in this volume, but enough may be said to give some
instruction and to impart common principles, whilst the minute details
are better examined and learnt in the works of Bourne, Rankine, and
other authors who devote themselves specially to the important
commercial question of steam.

The first truth to be comprehended is, that all matter contains within
its substance the power of creating heat--or as it may be expressed more
plainly, solids, fluids, and gases contain what is termed _latent_ or
insensible heat, in contradistinction to the heat which is apparent when
we touch a vessel containing warm water or approach a cheerful fire;
this latter is termed _sensible_ heat, and has formed the subject of the
preceding chapters.

If a cold horse-shoe nail is applied to a thin dry slice of phosphorus
laid on a sheet of paper, no combustion of the phosphorus ensues,
because the temperature of the iron is not sufficiently high to affect
that combustible substance; but if the horse-shoe nail is vigorously
hammered on an anvil, the particles of the metal are brought closer
together, and if it is applied to the phosphorus, so much heat has been
generated, thrust or squeezed out by the hammering or _condensation_ of
the iron, that it is now sufficiently warm to set fire to it.

[Page 407]

The reverse or antithesis to this experiment--viz., the production of
cold--would be shown if it were possible to expand a mass of metal
suddenly, and this can be effected by first melting together

  207 parts by weight of lead.
  118     "      "       tin.
  284     "      "       bismuth.

When these metals are in the liquid state and perfectly mixed, they are
poured from a sufficient height into a pail of cold water, for the
purpose of _granulating_ or dividing them into small fragments.

If the granulated compound metal is now mixed with 1617 parts by weight
of quicksilver, it becomes suddenly liquefied and expanded: liquefaction
is the reverse of solidification, and hence cold is produced from the
natural heat of the compound metals being rendered latent by the change
from the solid to the liquid state; so that a small quantity of water
placed in a glass tube, and surrounded with the metals whilst liquefying
in the mercury, becomes rapidly converted into ice, the fall of the
temperature, as shown by a thermometer, being from 60° Fahr. to 14°,
which is 18° degrees below the freezing point of water. In the former
case, by hammering the iron the _latent heat_ is made _sensible_; whilst
in the latter case, by the liquefaction of the compound metal in
mercury, the _sensible_ heat is rendered _latent_. The heat rendered
latent by melting different substances is not a constant quantity, but
varies with every special body employed, and the Drs. Irvine have proved
this fact by the following experiments:--

                            Ditto, reduced
               Heat            to the
                of          specific heat
             fluidity.        of water.

  Sulphur      143.68° Fahr.   27.14
  Spermaceti   145      "       --
  Lead         163      "       5.6.
  Bees'-wax    175      "       --
  Zinc         493      "      48.3.
  Tin          500      "      33.
  Bismuth      550      "      23.25.

Every one of these substances requires more heat to bring them into the
liquid condition than ice, for which 140° of heat are sufficient, or are
rendered latent during its conversion into water.

In coining at the Mint, the cold blank pieces of gold, silver, or copper
become hot directly they have sustained the violent and sudden pressure
of the coining press, and they must be heated again, or annealed, to
restore the equilibrium of the heat disturbed by the violent blow, or
else they remain hard and unfit to sustain the finishing process of
milling.

The condensation of water when it assumes a smaller bulk by union with
sulphuric acid, is easily proved by measuring a pint of water and a pint
of acid, and mixing them together, when a very great increase of
temperature may be perceived; and by placing into the mixture a cold
copper wire that previously could not ignite phosphorus, it becomes
[Page 408] very hot, and when removed and wiped it will cause phosphorus
to fire directly it touches that substance. When the mixture of
sulphuric acid and water is measured after it has cooled, it has no
longer a bulk of two pints, but is found to have lost bulk equal to one
or more ounces by measure. The heat evolved by a mixture of four parts
of strong sulphuric acid and one part water is shown by the thermometer
to be 300° Fahr., and this mode of obtaining heat has been used by
aeronauts for the purpose of obtaining artificial warmth without the
danger of setting fire to the gas in the balloon.

[Illustration: Fig. 382. Aeronauts in the car warming their hands by a
bottle containing sulphuric acid and water.]

When alcohol and water are mixed a change of density occurs, and heat is
produced; and if equal measures of alcohol of a specific gravity of
.825, and water, each at 50° Fahr., are mixed, a temperature of 70°
Fahr. is obtained; if the mixture is made in a glass vessel, as shown in
the annexed cut, the combination is very apparent. To perform the
experiment properly, water is poured into the lower tube and bulb, and
alcohol into the top one; when this is done, the stopper is inserted,
and the whole thoroughly shaken and mixed together; the warmth which is
[Page 409] thus obtained is apparent to the hand, whilst the contraction
is shown after the mixture is cold, as it no longer fills the two bulbs
of the instrument. (Fig. 383.)

[Illustration: Fig. 383. Glass bulbs and tube to show the contraction in
bulk of a mixture of alcohol and water.]

The latent heat of gases is easily shown by suddenly condensing air in a
small syringe or pump, of which the piston contains a minute fragment of
amadou (a species of fungus, _Polyporus igniarius_; this, according to
Simmonds, after having been beaten with a mallet, and dipped in a
solution of saltpetre, forms the spunk or German tinder of commerce; it
is also used as a styptic, and made into razor strops), which takes
fire, and before the invention of vesta and other matches,
tobacco-smokers were in the habit of obtaining a light for their pipes
and cigars in this manner--viz., by the latent heat obtained from the
contraction or compression of air. Then, again, an instructive though
opposite parallel is afforded by suddenly expanding or rarefying air in
a glass receiver provided with a delicate thermometer. By pumping out
some of the air, a considerable diminution of the temperature occurs,
and equal to several degrees of the thermometer. Every child knows that
steam direct from the kettle will scald, but if it issues from a
high-pressure boiler, say at fifteen pounds on the square inch, the hand
may be held with impunity in the escaping steam, as it merely feels
gently warm, and not scalding. This is due partly to the loss of heat
rendered latent by the expansion of the high-pressure steam directly it
passes into the air, and partly to the currents of air that are dragged
into an escaping jet of steam. This tendency of the air to rush into a
jet of steam was discovered by Faraday, and explains those curious
experiments with a jet of steam by which balls, empty flasks, and
globular vessels are sustained and supported either perpendicularly or
horizontally.

[Illustration: Fig. 384. A. Jet discharging high-pressure steam B B.
Lighted torch held round the escaping steam the flames from the former
all rush into the latter.]

If steam at a pressure of about sixty pounds per inch is allowed to
escape from a proper jet, and a large lighted circular torch composed of
tow dipped in turpentine held over it, the course of the external air is
shown, by the direction of [Page 410] the flames, which are forcibly
pulled and blown into the jet of steam with a roaring noise, indicating
the rapidity of the blast of air moving to the steam jet. (Fig. 384.)

Egg-shells, empty flasks, india-rubber or light copper and brass balls,
are suspended in the most singular manner inside an escaping jet of
high-pressure steam; and before the explanation of Faraday, reams of
paper were used in the discussion of the possible theory to account for
this effect; and what made the explanation still more difficult, was the
fact that the jet of steam might be inclined at any angle between the
horizontal and perpendicular, and still held the ball, egg-shell, or
other spherical figure firmly in its vapory grasp. (Fig. 385.)

[Illustration: Fig. 385. A. Ball and socket jet at an angle, and
discharging steam. The egg-shells are supported by the enormous current
of air moving into the jet in the direction of the arrows.]

In consequence of the great rush of air towards a jet of escaping
high-pressure steam, Mr. Goldsmith Gurney has patented the application
of this principle in his ventilating steam jet, which he has already
successfully applied; in one case especially, where a coal-mine had been
on fire for several years, and the whole working of the coal-measures in
the pit was jeopardized by the spreading of the combustion to new
workings; the fire was first extinguished by carbonic acid gas, pulled,
as it were, into the coal-mine by a jet of steam blowing into the
_downcast_, but placed in connexion with a furnace of burning coke; and
the circulation of the carbonic acid, called _choke-damp_, through the
pit workings was further assisted by a jet of high-pressure steam
blowing upwards, and placed over the mouth of the _upcast_ shaft.

The experiment succeeded perfectly at the South Sauchie Colliery, near
Alloa, about seven miles from Stirling, where a fire had raged for about
thirty years over an area of twenty-six acres in the waste seam of coal
nine feet thick. (Fig. 386.)

For the general purpose of ventilating the coalmine, Mr. Gurney's plan
was tried at the Ebbw Vale Colliery, and very economically, the waste
steam alone being used. Experiments have also been satisfactorily made
with it for blowing a cupola for smelting iron, and with dry
steam--_i.e._, steam of a very high pressure--escaping through a warm
tube, the results were perfectly successful.

[Page 411]

[Illustration: Fig. 386. Gurney's steam jet. A. Furnace. B. Water tank.
C. Downcast stopping. D. Upcast stopping. E E E. Steam jets. F F.
Galleries from shaft to shaft.]

[Page 412]

With this digression from the subject of latent heat derived from the
compression of air, we return again to the subject with another case in
point, furnished by the Fountain of Hiero, as it is called, at
Schemnitz, in Hungary, described by Professor Brande; and it may be
observed that all the phenomena related would apply to the great
pressure of the water from the water-towers at the Crystal Palace, if
fitted with a similar air-vessel.

"A part of the machinery for working these mines is a perpendicular
column of water 260 feet high (the Crystal Palace water-towers are each
284 feet high), which presses upon a quantity of air enclosed in a tight
reservoir; the air is consequently condensed to an enormous degree by
this height of water, which is equal to between eight and nine
atmospheres; and when a pipe communicating with this reservoir of
condensed air is suddenly opened, it rushes out with extreme velocity,
instantly expands, and in so doing it absorbs so much heat as to
precipitate the moisture it contains in a shower of snow, which may
readily be gathered on a hat held in the blast. The force of this is so
great, that the workman who holds the hat is obliged to lean his back
against the wall to retain it in its position."

The best examples of latent heat are furnished by ice, water, and steam,
and we are indebted chiefly to Dr. Black for the elegant and conclusive
experiments demonstrating the important truths connected with the latent
heat of these three conditions of matter. When various solids are
heated, they frequently pass through certain intermediate conditions of
softness, terminating in perfect liquidity; but ice and many other
bodies change at once to the liquid state on the application of a
sufficient quantity of heat. The process of melting ice is very slow,
because every portion must absorb or render latent a certain quantity of
heat before it can take the liquid state--hence the difficulty of
melting blocks of ice when they are surrounded with non-conducting
materials; and this fact the author has proposed to take advantage of in
keeping water cool which is to be supplied to the ova of salmon whilst
taking them to stock the rivers of Australia.

In order to prove that heat is rendered latent by the liquefaction of
ice, it is only necessary to weigh a pound of finely-powdered ice and a
pound of water at 212° Fahr. (_boiling water_), and mix them together;
when the ice is all melted, the resulting temperature is only 52°,
therefore the boiling water has lost 160° of temperature, of which 20°
can be accounted for, because the resulting temperature of the melted
ice is 52°; but in the liquefaction of the pound of ice, 140° have
disappeared or become latent, or, as Dr. Black termed it, have become
_combined_.

  1 lb. of ice at 32° + 20° = 52°, the resulting temperature.
  1 lb. of water at 212° - 52° = 160° - 20° = 140°, rendered latent.

140° represents the result obtained from innumerable experiments made by
mixing equal parts of ice and boiling water, and it is this large
quantity of latent heat required by ice and snow that prevents their
sudden liquefaction, and the disastrous circumstances that would arise
from the floods that must otherwise always be produced.

[Page 413]

To put the fact beyond all doubt, it is advisable to mix together equal
weights of water at 32° and boiling water at 212°, and the result is
found by the thermometer to be the mean between the two, because half
the extremes are always equal to the mean; and if the two temperatures
are added together and divided by two, the result is a temperature of
122°, as shown below:--

  1 lb. of ice water at 32° + 1 lb. of water at 212° = 244° ÷ 2 = 122°.

From similar experiments Dr. Black deduced the important truth, "that in
all cases of liquefaction a quantity of heat _not indicated by, or
sensible to_, the thermometer, is _absorbed_ or disappears, and that
this heat is _withdrawn_ from the _surrounding bodies_, leaving them
_comparatively cold_." At p. 79 it is shown how the sudden solution or
liquefaction of certain salts produces cold, and hence numerous freezing
mixtures have been devised. In olden times, when officials in authority
did what they pleased, without being troubled with disagreeable returns,
and colonels clothed their men, and were merchant tailors on the grand
scale, gun cartridges were not confined to practice on the enemy, but
they did duty frequently in the absence of ice as refrigerators of the
officers' wine, in consequence of the gunpowder containing nitre or
saltpetre; as a mere solution of this salt finely powdered will lower
the temperature of water from 50° Fah. to 35°; whilst a mixture of four
ounces of carbonate of soda and four ounces of nitrate of ammonia
dissolved in four ounces of water at 60°, will in three hours freeze ten
ounces of water in a metallic vessel immersed in the mixture during the
liquefaction or solution of the salts.

Fahrenheit imagined he had attained the lowest possible temperature by
mixing ice and salt together, and it is by this means that confectioners
usually freeze their ices, or ice puddings; the materials are first
incorporated, and being placed in metallic vessels or moulds, and
surrounded with ice and salt placed in alternate layers, and then well
stirred with a stick, they soon solidify into the forms which are so
agreeable, and so frequently presented at the tables of the opulent. The
temperature obtained is Fahrenheit's _zero_--viz., thirty-two degrees
_below_ the freezing point of water. According to the very wise police
regulation observed in London, all householders are required to sweep or
remove the snow from the pavement in front of their houses, and this is
frequently done with salt; should an unfortunate shoeless beggar, tramp
past whilst the sudden liquefaction is in progress, the effect on the
soles of his feet is evidently very disagreeable, and the rapidity with
which he retires from the _zero_ affords a thermometric illustration of
the most lively description.


_Heat the Cause of Vapour._

Every liquid, when of the same degree of chemical purity, and under
equal circumstances of atmospheric pressure, has one peculiar point of
temperature at which it invariably boils. Thus, ether boils at 96°
Fahr., and if some of this highly inflammable liquid is placed carefully
in a [Page 414] flask, by pouring it in with a funnel, and flame
applied within one inch of the orifice, no vapour escapes that will take
fire; but if the flame of a spirit lamp is applied, the ether soon
boils, and if the lighted taper is again brought near the mouth of the
flask, the vapour takes fire, and produces a flame of about two feet in
length. This fire only continues as long as the flame of the spirit-lamp
is retained at the bottom of the flask, and on removing it the vessel
rapidly cools. The length of the flame is reduced, and is gradually
extinguished for the want of that essence of its vitality, as it
were--viz., heat. (Fig. 387.) If a thermometer is introduced into the
flask, however rapid may be the ebullition or boiling of the ether, it
is found to be invariably at 96°. The heat carried off by evaporation is
most elegantly displayed by placing a little water in a watch glass, and
surrounded by charcoal saturated with sulphuric acid, in the vacuum of
an air-pump. The rapid evaporation and condensation of the water by its
affinity for the sulphuric quickly produces ice; and the pumps and other
apparatus of Knight and Co., Foster-lane, City, are greatly to be
recommended for this and other illustrations.

[Illustration: Fig. 387. Heat the cause of vapour.]

The illustration of the determination of the fixed and invariable
boiling point belonging to every liquid is further carried out by
introducing some water into a second flask standing above a lighted
spirit-lamp, with a small thermometer, graduated, of course, properly to
degrees above the boiling point of water; when the water boils, it will
be found to remain steadily at a temperature of 212°. And however
rapidly the water may be boiled, provided there is ample room for the
steam to escape, the heat indicated by the thermometer is like the law
of the Medes and Persians, which altereth not, and it remains standing
at the number 212°. The only exception (if it may be so termed) to this
law is brought about by the shape and nature of the containing vessel;
under a mean pressure the boiling point of water in a metallic vessel is
generally 212°; in a glass vessel it may rise as high as 214° or 216°,
but if some metallic filings are dropped in, the escape of steam is
increased, and the temperature may then drop immediately to 212°.

When a thermometer is inserted in a flask containing water in a state
[Page 415] of ebullition or boiling, so that the bulb does not touch the
fluid, but is wholly surrounded with steam, it will be found that the
temperature of the latter is exactly the same as that of the former; and
if the liquid boils at 96°, the vapour will be 96°, if at 212°, the
steam is 212°. Steam has therefore exactly the same temperature as the
boiling water that produces it. (Fig. 388.)

[Illustration: Fig. 388. Thermometer in the steam escaping from boiling
water.]

Whilst performing the last experiment, it may be noticed that the steam
inside the neck of the flask is invisible, and that it only becomes
apparent in that kind of intermediate condition between the vaporous and
liquid state called _vesicular vapour_--a state corresponding with the
"earth fog," and called by Howard the _stratus_. When a flask containing
boiling water is placed under the receiver of an air pump (as soon after
the ebullition has ceased as may be possible), and the air pumped out,
it will be noticed that the water again begins boiling as the vacuum is
obtained, showing that the boiling point of the same fluid varies under
different degrees of atmospheric pressure, and according to the height
of the barometer.

  Height of     Boiling point
  barometer.     of water.

    26            204.91°
    26.5          205.79
    27            206.67
    27.5          207.55
    28            208.43
    28.5          209.31
    29            210.19
    29.5          211.07
    30            212
    30.5          212.88
    31            213.76

Alcohol and ether confined under an exhausted receiver boil violently at
the ordinary temperature of the atmosphere, and in general liquids boil
with 124° less of heat than are required under a mean pressure of the
air; water, therefore, in a vacuum must boil at 88° and alcohol at 49°.

On ascending considerable heights, as to the tops of mountains, the
boiling point of water gradually falls in the scale of the thermometer.
Thus, on the summit of Mont Blanc water was found by Saussure to boil at
187° Fahr. In Mr. Albert Smith's delightful narrative of his ascent of
Mont Blanc, he mentions the violent commotion and escape of the whole of
the champagne in froth directly the bottle was opened at the summit of
this king of mountains.

Dr. Wollaston's instrument for measuring the heights of mountains by
[Page 416] the variations of the boiling point of water has long been
known and used for this purpose.

If a Florence flask is first fitted with a nice soft cork, and this
latter removed, and the former half filled with water, which is then
boiled over a gas or spirit flame, the same fact already mentioned and
illustrated in the preceding table may be rendered apparent when the
flask is corked and removed from the heat. If it is now inverted, and
cold water poured over it, an ebullition immediately commences, because
the cold water condenses the steam in the space above the hot water in
the flask, and producing a vacuum, the water boils as readily as it
would do under an exhausted receiver on an air-pump plate. (Fig. 389.)

[Illustration: Fig. 389. The paradoxical experiment of water boiling by
the application of _cold_ water.]

Water may be heated considerably higher than 212°, if it is enclosed in
a strong boiler, and shut off from communication with the air; by this
means steam of great pressure is obtained.

Dr. Marcet has invented a very instructive form of a miniature boiler,
supplied with a thermometer and barometric pressure gauge, which can be
purchased at any of the instrument makers, and is figured and described
in nearly every work on chemistry.

The reason water boiled in an open vessel does not rise to a higher
temperature than 212° is because all the excess of heat is carried off
by the steam, and is said to be rendered latent in the vapour. The
fixation of caloric in water by its conversion into steam may be shown
by the following experiment. Let a pound of water at 212° and eight
pounds of iron filings at 300° be suddenly mixed together. A large
quantity of steam is instantly generated, but the temperature of the
water and escaping steam are still only 212°; hence the steam must
therefore contain all the degrees of heat between 212° and 300°, or
eight times 88. When the water is heated in the hydro-electric machine
or other boiler, to 322.7°, it very quickly drops to 212° when the steam
is allowed to blow off; yet if the latter is collected, it represents
but a very small quantity of water which constituted the steam, and it
has carried off and rendered latent the excess of heat in the
boiler--viz., the difference between 212° and 322.7°, or 110.7°

If steam can carry off heat, of course it may be compelled, as it were,
[Page 417] to surrender it again; and this important elementary truth is
shown by adapting a tube, bent at right angles, and a cork, to a flask
containing a few ounces of water, and when it boils, the steam issuing
from the end of the pipe may now be directed into and below the surface
of some water contained in a beaker glass; in a very short time the
water in the latter will be raised to the boiling point by the
condensation of the steam and the latent heat arising from it. (Fig.
390.) The amount of latent heat is enormous, when it is remembered that
water by conversion into steam has its bulk prodigiously enlarged--viz.,
1698 times, so that _a cubic inch_ of water converted into steam of a
temperature of 212°, with the barometer at thirty inches, occupies a
space of _one cubic foot_, and its latent heat amounts, according to
Hall, to 950°; Southeron, 945°; Dr. Ure, 967°. When we come to the
consideration of the steam-engine, it will be noticed that the question
of the latent heat of steam is one of the greatest importance.

[Illustration: Fig. 390. A. Flask for generating steam. B. Glass pipe
bent at right angles to convey the steam into the fluid containing some
cold water.]

  Temperature of        Elasticity in inches        Latent Heat.
    Steam.                 of Mercury.
    229°                        40°                     942°
    270                         80                      942
    295                        120                      950

The same weight of steam contains, whatever may be its density, the same
quantity of caloric, its latent heat being increased in proportion as
its sensible heat is diminished; and the reverse. In consequence of the
enormous amount of latent heat contained in steam, it is advantageously
employed for the purpose of imparting warmth either for heating rooms or
drying goods in certain manufacturing processes. The wet rag-pulp
pressed and shaken into form on a wire-gauze frame or _deckle_, passes
gradually to cylinders containing steam, and is thoroughly dried before
the guillotine knife descends at the end of the paper machine, and cuts
it into lengths. In calico stiffening and glazing, also in calico
printing, steam-heated cylinders are of great value, because they impart
heat without the chance of setting the goods on fire. The elementary
principles already described with reference to heat, will prepare the
youthful reader for the application of the expansion of water into
steam, as the most valuable _motive power_ ever employed to assist the
labour of man.


[Page 418]

CHAPTER XXIX.

THE STEAM-ENGINE--_continued_.


[Illustration: Fig. 391. The first steam-boat, the _Comet_, built by
Henry Bell, in 1811, who brought steam navigation into practice in
Europe.]

  "So shalt thou instant reach the realm assign'd
  In wondrous ships, _self-mov'd_, instinct with mind.

       *       *       *       *       *

  Though clouds and darkness veil the encumbered sky.
  Fearless, through darkness and through clouds they fly,
  Tho' tempests rage,--tho' rolls the swelling main,
  The seas may roll, the tempests swell in vain;
  E'en the stern god that o'er the waves presides,
  Safe as they pass, and safe repass the tides,
  With fury burns; while careless they convey,
  Promiscuous, ev'ry guest to ev'ry bay."


These lines, from Pope's translation of the "Odyssey," were very aptly
quoted twenty-five years ago by Mr. M. A. Alderson, in his treatise on
the steam-engine, for which he received from Dr. Birkbeck, the [Page
419] originator of Mechanics' Institutions, the prize of 20_l._, being
the gift of the London Mechanics' Institution, and these lines seem to
indicate some sort of rude anticipation by the ancients of that free
passage of the ocean by the agency of steam which has rendered ships
almost independent of wind and weather.

Homer's description, as above, of the Phoenician fleet of King
Alcinous, in the eighth book of the "Odyssey," is certainly an ancient
record of an _idea_, but nothing more. In a work written by Hero of
Alexandria, about a hundred years B.C., and entitled "Spiritalia seu
Pneumatica," a number of contrivances are mentioned for raising liquids
and producing motion by means of air and steam, so that the first
steam-engine is usually ascribed to Hero; and the annexed cut displays
the apparatus. (Fig. 392.)

[Illustration: Fig. 392. Hero's steam-engine. A. The boiler in which
steam is produced, and then passes through the hollow support B, from
which there is no outlet but through the two apertures, C C. The
reaction of the air on the issuing steam produces a rotatory motion in
the jets, C C, attached to a centre but hollow axle.]

It is a remarkable circumstance that Sir Isaac Newton applied the same
principle in a little ball, mounted on wheels, containing boiling water,
and provided with a small orifice; and in his description he says: "And
if the ball be opened, the vapours will rush out violently one way, and
the wheels and the ball at the same time will be carried the contrary
way." From the time of Hero, there does not appear to be any record or
mention made of steam apparatus till the year 1002, when, in a work
called "Malmesbury's History," mention is made of an organ in which the
sounds were produced by the escape of air (query, steam) by means of
heated water. It is strange that, in these days of steam application,
the Calliope, or steam organ, should be an important feature at the
present moment at the Crystal Palace; and it only shows how the same
ideas are reproduced as novelties in the ever-recurring cycles of years.

On the revival of classical learning throughout Gothic Europe, the work
of Hero began to attract attention, and it was translated and printed in
black letter, and most likely first from the Arabic character, as in the
year 1543 the first fruits appeared in Spain, where Blasco de Garay, a
sea captain, propelled a ship of 200 tons burden, at the rate of three
miles per hour, before certain commissioners appointed by the Emperor
Charles the Fifth. Alas for inquisitorial Spain! had she looked deeper
into the matter, and performed her _auto-da-fées_ on the boilers of
[Page 420] steam engines instead of the bodies of poor human beings,
what lasting glories would have been her reward. The invention made its
_début_ in Spain, the commissioners reported, the worthy inventor was
rewarded, but the mighty giant invoked was put to sleep again for at
least 150 years. The steam giant was disturbed with dreams; one Mathias,
in 1563, gave him a nightmare; Solomon de Caus, in 1624, nearly woke him
up; Giovanni Bianca, in 1629, did more; and the Marquis of Worcester, in
the middle of the seventeenth century, as the evil genius of Spain,
carried off the giant bodily and made him the slave of England; at
least, he experimented, and wrote such wondrous tales of his new motive
power, that in 1653 we read of steam being fairly tethered to its work,
and set to draw water out of the Thames at Vauxhall; and Cosmo de
Medici, a foreigner who inspected the apparatus in 1653, says, "It
raises water more than forty geometrical feet by the power of one man
only, and in a very short space of time will draw up full vessels of
water through a tube or channel not more than a span in width, on which
account it is considered to be of greater service to the public than the
other machine near Somerset House, which last one was driven by _two
horses_."

What would the Marquis of Worcester and Cosmo de Medici have thought of
Blasco de Garay on the ocean, and ruling 12,000 steam horses? Write the
name of the brave and prudent Captain Harrison, in the good ship _Great
Eastern_, date 1859, instead of that of the gallant Spaniard, and our
brief history is finished.

The first really useful steam-engine was made, not by a plain Mr., but
again by a captain--namely, Captain Savery, who appears to have been the
first inventor who thoroughly understood and applied the _vacuum_
principle. (Fig. 393.)

[Page 421]

[Illustration: Fig. 393. Savery's engine.]

A A. The furnaces which contain the boiler. B 1 and B 2. The two
fireplaces. C. The funnel or chimney, which is common to both furnaces.
In these two furnaces are placed two vessels of copper, which I (Savery)
call boilers--the one large as at L, the other small as D. D. The small
boiler contained in the furnace, which is heated by the fire at B 2. E.
The pipe and cock to admit cold water into the small boiler to fill it.
F. The screw that covers and confines the cock E to the top of the small
boiler. G. A small gauge cock at the top of a pipe, going within eight
inches of the bottom of the small boiler. H. A large pipe which goes the
same depth into the small boiler. I. A clack or valve at the top of the
pipe H (opening upwards). K. A pipe going from the box above the said
clack or valve in the great boiler, and passing about one inch into it.
L L. The great boiler contained in the other furnace, which is heated by
fire at B 1. M. The screw with the regulator, which is moved by the
handle Z, and opens or shuts the apertures at which the steam passes out
of the great boiler at the steam-pipes O O. N. A small gauge cock at the
top of a pipe, which goes half way down into the great boiler. O 1, O 2.
Steam pipes, one end of each screwed to the regulator; the other ends to
the receivers P P, to convey the steam from the great boiler into those
receivers. P 1, P 2. Copper vessels called receivers, which are to
receive the water which is to be raised. Q. Screw joints by which the
branches of the water-pipes are connected with the lower parts of the
receivers. R 1, 2, 3, and 4. Valves or clacks of brass in the
water-pipes, two above the branches Q and two below them; they allow the
water to pass upwards through the pipes, but prevent its descent; there
are screw-plugs to take out on occasions to get at the valves R. S. The
forcing-pump which conveys the water upwards to its place of delivery,
when it is forced out from the receivers by the impelled steam. T. The
sucking-pipe, which conveys the water up from the bottom of the pit to
fill the receivers by suction. V. A square frame of wood, or a box, with
holes round its bottom in the water, to enclose the lower end of the
sucking-pipe to keep away dirt and obstructions. X is a cistern with a
bung cock coming from the force-pipe, so as it shall always be kept
filled with cold water. Y Y. A cock and pipe coming from the bottom of
the said cistern, with a spout to let the cold run down on the outside
of either of the receivers, P P. Z. The handle of the regulator to move
it by, either open or shut, so as to let the steam out of the great
boiler into either of the receivers.

[Page 422]

This is Savery's own description (taken from the "Miner's Friend,"
printed in 1702), of his water-engine, which differs from that suggested
by the Marquis of Worcester, in the fact that he made the _pressure of
the air_ carry the water up the first stage. Savery's patent was "for
raising water and occasioning motion to all sorts of mill-work by the
impellant force of fire;" and the patent was granted in the reign of
King William the Third of glorious memory.

Thus Savery overcame, as he remarks, the "oddest and almost insuperable
difficulties," and introduced a steam apparatus or engine, a good many
of which were constructed, and employed for raising water. The
mechanical skill required to construct the boiler, the very _heart_ (as
it were) of the iron engine, had not been acquired in the time of
Captain Savery, and hence the weakness of the boilers, and the danger of
working them. As the pressure required was very considerable to overcome
the resistance of a lofty column of water, these engines were gradually
relinquished for those of another clever mechanician--viz., for those of
Thomas Newcomen, an ironmonger of Dartmouth, who, about the year 1705,
constructed and introduced the _cylinder_, from which the transition was
gradually made to the mode of condensing by a jet of cold water, the use
of self-acting valves, and the construction of self-acting engines by
Smeaton, Hornblower, and finally by the illustrious Watt, whose portrait
heads the first chapter on Heat in this book.

Newcomen was assisted in his work by one Cawley, a glazier; and their
persevering labours were crowned with a successful result of the most
memorable importance in the history of the steam-engine.

In the engine by Savery, the operation of the steam was twofold--namely,
by the direct pressure from its elasticity, and by the indirect
consequence of its condensation, which affords a vacuum. This last may
be said to be the only principle used by Newcomen, who employed a boiler
for the generation of steam, and conveyed it by a pipe to the bottom of
a hollow cylinder, open at the top, but provided with a solid piston,
that moved up and down in it, and was rendered tight by a stuffing of
hemp, like the piston of a boy's common squirt. It can readily be
understood, that if the jet of the latter was connected with a tight
little boiler, and steam blown into it, that the piston of the squirt
would rise to the top of the barrel in which it works, being thrust up
by the pressure or force of the steam; but unless the steam was cut off,
and cold water applied to the interior of the barrel, the piston could
not descend again. As soon, therefore, as Newcomen had thrust up the
piston by the action of steam, he introduced a jet of cold water,
supplied from an elevated cistern beneath the piston, when the steam was
condensed into water, and a vacuum or void space obtained. The piston
being free to move either up or down, was now forced in the latter
direction by the pressure of the air, which is a constant force equal to
fifteen pounds on the square inch; and thus the piston in Newcomen's
engine was raised by _heat_--viz., by steam, and thrust down by
_cold_--i.e., by the condensation of the steam producing a vacuum. The
void obtained in this manner was very considerable, because one cubic
_foot_ of [Page 423] steam at 212° condenses into one cubic _inch_ of
water. The production of a vacuum with the aid of steam is quickly
effected by boiling some water in a clean camphine can, and when the
steam is issuing freely from the mouth of the latter it is then corked,
and cold water thrown over the exterior. Directly the temperature is
lowered, the steam inside the tin vessel is condensed suddenly into
water, and a void space being suddenly obtained, the whole pressure of a
column of air of a breadth equal to the area of the vessel, and of a
height of forty miles, is brought suddenly down like a sledge-hammer
upon the sides of the tin vessel, and as they are not sufficiently
strong to offer a proper resistance, they are crushed in like an
egg-shell by the giant weight which falls upon them.

The barometer, or measurer of the weight of the air, consists of a glass
tube about thirty-three inches in length, hermetically sealed at one
end, and containing mercury that has been carefully boiled within it,
and being perfectly filled the tube is inserted in a cistern of clean
mercury, when it gravitates to a height equal to the pressure of the
air, leaving a space at the top called the torricellian vacuum. As the
atmospheric air decreases in density by admixture with invisible steam
or vapour, any given volume becomes specifically lighter: hence the
column of mercury falls to a height of about twenty-eight inches; whilst
if the aqueous vapour diminishes, the weight of the air becomes greater,
and the barometer may rise to a height of about thirty-one inches.

Having thus secured a "reciprocating motion," Newcomen applied it to the
working of a force-pump by the intervention of a great beam or lever
suspended on gudgeons (an iron pin on which a wheel or shaft of a
machine turns) at the middle, and suspended like the beam of a pair of
scales; and, in fact, he invented that method of supporting the beam
which is in use to the present day. Supposing we compare Newcomen's beam
to a scale beam, he attached to the extremities (instead of scale pans)
a water pump and his steam cylinder--the latter being at one end, and
the former at the other. The beam played at "see-saw:" by the primary
action of the steam on the bottom of the piston in the _cylinder_ it was
pushed up at this end, and of course suffered an equal fall at the
other, to which the pump piston was attached; and when the motion was
reversed by the condensation of the steam, down went the piston again by
the pressure of the air, whilst that of the water pump was again raised,
and being provided with proper valves, the water was pumped slowly out
of the mine, although the steam power used was very moderate, and only
just sufficient to counterpoise the weight of the atmosphere. Newcomen
made the end attached to the water pump purposely heavier than the steam
piston of the other end of the beam, and by this means the work of the
steam, by its elasticity, was very moderate, whilst the actual lift of
the water from the mine was performed by the pressure of the air, equal
(as already stated) to fifteen pounds on every square inch of the
surface of the steam piston. This engine is called the atmospheric
engine, and in the next cut we have a picture taken from a photograph by
the "Watt Club" of the actual model of the Newcomen engine in the [Page
424] Hunterian Museum of the University of Glasgow; the dimensions
being--length, 27 in.; breadth, 12 in.; height, 50½ in.; from which, "in
1765, _James Watt, in seeking to repair this model_, belonging to the
Natural Philosophy Class in the University of Glasgow, _made the
discovery of a separate condenser_, which has identified his name with
that of the steam-engine." (Fig. 394.)

[Illustration: Fig. 394. Model of the Newcomen engine, in which the
furnace and boiler, the steam cylinder, beam, water-pump, and elevated
cistern of water, are apparent.]

In Newcomen's engine, the opening and shutting of the cocks required the
vigilant care of a man or boy, and it is stated on good authority that a
boy who preferred (like nearly all other boys) _play_ to work,
contrived, by means of strings, a brick, and one or two catches on the
working beam, to make the engine self-acting.

This poor boy's ingenious contrivance paved the way for the improved
[Page 425] methods of opening and shutting the valves, which were
brought to a great state of perfection by Beighton, of Newcastle, about
1718. Between that time and the year 1763, we find honourable mention
made of Smeaton in connexion with the steam-engine, but the name of the
great James Watt at this time began to be appreciated, and by a series
of wonderfully simple mechanisms, he at last perfected the machine whose
origin could be traced back not only to the time of Blasco de Garay, in
1543, but even to the days of the ancient mechanicians, such as Hero,
who lived 130 B.C.

In 1763, James Watt was a maker of mathematical instruments in Glasgow,
and his attention was drawn to the subject of the steam-engine by his
undertaking to repair a working model of Newcomen's steam-engine, which
was used by Professor Anderson, who then filled the Chair of Natural
Philosophy, and subsequently founded the Andersonian Institution. The
repairs required for this model induced Watt to make another, and by
watching its operation, he discovered that a vast quantity of heat, and
therefore fuel, was wasted in the constant and successive heating and
cooling of the steam cylinder. About two years after, when Watt was
twenty-nine years of age, he had made so many experiments, that he was
enabled to put into a mechanical shape his original ideas, which are
embodied in his patent of 1769, as follows:--

"My method of lessening the consumption of steam, and consequently fuel,
in fire-engines, consists of the following _principles_:

"First: That vessel in which the powers of steam are to be employed to
work the engine, which is called the cylinder in common fire-engines,
and which I call the steam-vessel, must, during the whole time the
engine is at work, _be kept as hot as the steam that enters it_--first,
by enclosing it in a case of wood or any other materials that transmit
heat slowly; secondly, by surrounding it with steam or other heated
bodies; and thirdly, by suffering neither water nor any other substance
colder than steam to enter or touch it during that time.

"Secondly: In engines that are to be worked wholly or partially by
condensation of steam, the steam is to be condensed in vessels
_distinct_ from the steam-vessels or cylinders, although occasionally
communicating with them; _these vessels_ I call _condensers_; and whilst
the engines are working, these condensers ought at least to be kept as
cold as the air in the neighbourhood of the engine, by application of
water or other cold bodies.

"Thirdly: Whatever air or other elastic vapour is not condensed by the
cold of the condenser, and may impede the working of the engine, is to
be drawn out of the steam-vessels or condensers by means of pumps
wrought by the engines themselves, or otherwise.

"Fourthly: I intend in many cases to employ the expansive force of steam
to press on the pistons, or whatever may be used instead of them, in the
same manner as the pressure of the atmosphere is now employed in common
fire-engines. In cases where cold water cannot be had in plenty, the
engines may be wrought by this force of steam only, by discharging the
steam into the open air after it has done its office.

"Lastly: Instead of using water to render the piston or other parts of
[Page 426] the engines air and steam-tight, I employ oils, wax, resinous
bodies, fat of animals, quicksilver, and other metals in their fluid
state.

"And the said James Watt, by a memorandum added to the said
specification, declared that he did not intend that anything in the
fourth article should be understood to extend to any engine when the
water to be raised enters the steam-vessel itself, or any vessel having
an open communication with it."

"About the time he obtained his patent, Watt commenced the construction
of his first real engine, the cylinder of which was eighteen inches in
diameter, and after many impediments in the details of the work he
succeeded in bringing it to considerable perfection. The bad boring of
the cylinder, and the difficulty of obtaining a substance that would
keep the piston tight without enormous friction, and at the same time
resist the action of steam, gave him the most trouble, and the
employment of a piston rod moving through a stuffing-box was a new
feature in steam-engines at that time, and required great nicety of
workmanship to make it effectual. While Watt was contending with these
difficulties, Roebuck's finances became disarranged, and in 1773 he
disposed of his interest in the patent to Mr. Boulton, of Soho. As,
however, a considerable part of the term of fourteen years, for which
the patent was granted, had already passed away, and as several years
more would probably elapse before the improved engines could be brought
into operation, it was judged expedient to apply to Parliament for a
prolongation of the term, and an Act was passed in 1775 granting an
extension of twenty-five years from that date, in consideration of the
great merit of the invention." (Bourne's "Treatise on the
Steam-engine.")

In Fig. 395 (p. 427) we give an illustration of a low-pressure
condensing engine and boiler of eight-horse power, constructed on the
principle of Boulton and Watt, as the latter had fortunately united his
skill, learning, originality, and experience with Mr. Boulton, of Soho,
near Birmingham, whose metal manufactory was already the most celebrated
in England.

During the explanation of this eight horse-power engine, the opportunity
may be taken to discuss occasionally the special improvements effected
by Watt. The steam-pipe A conveys the steam generated in the boiler B to
the slide-valve C, which is kept close to the surface, against which it
works by the pressure of the steam.

Here we notice some of the valuable improvements of Watt in the
admission of steam _above_ as well as _below_ the piston, by which he
increased the power of his engine, and no longer confined it to the
force of the atmospheric pressure. It is also necessary to remark the
beautifully simple mechanism of the slide-valve, by which steam is
admitted alternately above and below the piston. Want of space prevents
us tracing out the gradual improvements effected by Watt, and therefore
we take his invention as it stood in the year 1780, and refer our
readers to Bourne's "Treatise on the Steam-engine" for the full and
minute particulars of the improvements to that date.

[Page 427]

[Illustration: Fig. 395. An eight-horse power condensing steam-engine,
after the principle of Boulton and Watt, and explained in pages 426 to
432.]

[Page 428]

At that time it occurred to Watt that the _condensation_ of the steam
from the _cylinder_ after it had done its work, might be made more
perfect if a _perpetual vacuum_ was maintained beneath the piston, while
an alternate steam-pressure and vacuum were produced above it. (Fig.
396.)

[Illustration: Fig. 396. "E E is the cylinder. J. The piston, _a._ The
steam-pipe. _b._ The regulating or throttle valve, _e._ The eduction and
equilibrium single valve, performing the functions of both. _c._ The
upper, and _f_ the under, portholes, by which passages only the steam
can enter and pass away. _d, j, g._ The eduction-pipe by which the steam
passes from above the piston during every returning stroke to the
condenser, a perpetual exhaustion being maintained beneath it."--From
BOURNE _on the Steam-engine_.]

Instead of obtaining a specific advantage the contrary occurred, and
Watt was obliged in this case to return to the ponderous Newcomen
counterweight to balance the difference in the vacuum above and below
the piston, consequently this form of the cylinder and valves was
abandoned. The juvenile reader will perceive in the above drawing that
the superior arrangement of Watt's cylinder to that of Newcomen arises
from the steam operating above and below the piston, and that the piston
rod works air-tight in a _stuffing box_ at the top of the cylinder. A
most important improvement in the employment of steam as a motive power
has been discovered in the mode of using it "expansively," by which the
steam, at a pressure say of sixty pounds on the square inch, is admitted
below the piston, and then cut off and allowed to expand and drive up
the latter without the expenditure of any more fuel, and leaving, after
lifting the piston to a height say of three feet, an average or mean
power of thirty pounds on the square inch.

Returning to the eight-horse condensing engine, D is the steam cylinder
surrounded by a case to prevent the steam cooling and to maintain in
the [Page 429] cylinder the same, or nearly the same, temperature as
that of the steam in the boiler, according to the condition of Art. I.
of Watt's Patent, quoted at p. 425 of this book. The same outer case is
apparent around the cylinder in Fig. 396; E, the piston, which, by
stuffing with hemp or other proper material, fits the interior of the
cylinder in the most accurate manner, and prevents the escape of steam
by its sides: _e_ is the piston rod attached to the parallel motion.
This clockwork-like piece of mechanism has often been quoted as one of
the masterpieces of Watt, and in its greatest perfection is called the
_complete_ parallel motion, and may be found in all the best land beam
steam-engines. The object of the parallel motion is to cause the piston
and pump rods to move always in straight lines, never deviating to
either side. (Fig. 397.)

[Illustration: Fig. 397. A B is half the beam, A being the main centre,
B E. The main links connecting the piston-rod F with the end of the
beam. G D. The air-pump links, from the centre of which the air-pump rod
is suspended. C D and E D produce the parallelism, because C D is
moveable only round the fixed centre C, whilst E D is not only moveable
round the centre D, but the centre itself in the arc described by C D,
and by this action E D corrects the distorting influence of its own
radius. The dotted lines and letters above enable the observer to see
the effect of the movement of the beam on the parallel motion.]

In the eight horse-power engine shown in page picture, _e_ is also
attached to the piston E, which moves the beam F, and the other end of
this beam, by the connecting rod _g_, gives motion to the heavy fly
wheel G, by means of the crank _h_.

H is an eccentric circle on the axle of the fly wheel G, it gives motion
to the slide valve, which admits the steam alternately above and below
the piston. The slide valve and its seat are contained within an oblong
box or case, large enough to permit the easy motion of the valve within
it, and usually forming an enlargement in the course of a pipe.

The valve rod by means of which the valve is opened and shut, passes out
through a stuffing box; or, instead of such a rod, a valve of moderate
size often has a nut fixed to it, within which works a screw on the end
of an axle which passes out through a bush, and has shoulders within and
without to prevent it from moving longitudinally, and a square on the
outer end on which the key fits that is used in turning it. I is the
throttle valve inside the steam pipe and lever connected with a governor
for regulating the admission of steam into the cylinder.

Here, again, we pause in the description of our eight horse-power engine
to illustrate more particularly this admirable contrivance of [Page
430] Watt, which remains to the present day without any material
alteration even in the best steam-engines. (Fig. 398.)

[Illustration: Fig. 398. A. The seat of the throttle valve, Z. The valve
itself turning on a spindle, which passes through its centre. _a_ is the
steam pipe. _w._ The throttle valve lever on which the rod H, proceeding
from the governor, acts. D D. The spindle of the governor revolving by a
belt acting on the pulley _d_. E E. The balls hung on the ends of the
arms, which cross each other at _e_ like a pair of scissors. When D D is
set in motion, the balls fly out by centrifugal motion, and in doing so
draw down the collar into which the lever F works by means of the links
_f h_. When F is depressed, of course H rises, and the valve Z is partly
closed, and the supply of steam reduced.]

In the eight-horse engine already partly explained, _k_ is the cylinder
of an air-pump to remove any air, and the water which condenses the
steam, from the condenser L. There is also the eduction pipe, which
conducts the steam from the cylinder to the condenser L. O is the pump
that supplies cold water to the cistern S, in which the condenser and
air-pump stand, P is a rod connected with the injection cock for
admitting a jet of water into the condenser from the cistern, and which
is continually flowing during the working of the engine, Q Q, cast-iron
columns, four of which support the principal parts of the engine.

We now come to the boiler of the steam-engine, which is of course of
almost equal importance with the engine itself; and the one in our
page-picture is a good type of one of the favourite boilers used by
Messrs. Boulton and Watt, and is called the "Wagon boiler." The boiler
is made of wrought-iron plates rivetted together, and properly
strengthened where necessary; and the steam-pipe A conveys the steam to
the engine. It may be remarked here that the cylindrical [Page 431]
boiler--consisting of two cylinders, one within the other, of which the
former contains the fire, whilst the furnace-draught circulates outside
the latter, and the space between the two cylinders being filled with
water--is the form of boiler which is most highly approved of, and is
employed in the famous economical steam-engines of the Cornish mines.

As the water evaporates in the form of steam, the boiler must be
continually supplied with fresh water, which comes (as will be noticed
by inspecting the page picture) from the _hot well_ S, by means of the
_hot-water pump R_, attached to the beam F. The water is pumped to the
top of a column rising above but connected with the boiler. There is a
cylindrical float, inside the column of water, connected with the
boiler, suspended ever a pulley by a chain passing to the damper of the
furnace. The damper and float balance each other, and when the water in
the boiler rises to too high a temperature, it causes the float to rise
in the column of water, which lowering the damper or shutter that stops
the draught of the chimney of the furnace T, diminishes the intensity of
the heat, and reduces the formation of steam. On the other hand, as the
temperature diminishes, the float descends and the damper rises, and
permitting more air to rush to the burning fuel in the fire, a greater
quantity of steam is generated.

There is likewise a stone float inside the boiler, for regulating the
supply of water by the feed pipe, or column of water, which latter must
always be sufficiently lofty to press with greater force than the steam
produced in the boiler, or else the power of the steam might, under
certain circumstances, eject or blow out the water from the top of the
column. The stone is suspended by a brass wire which works through a
stuffing box, and is connected with a lever, to which is attached a
heavy counterpoise, so adjusted that when the stone is immersed to a
certain depth in water (according to the principle of a solid body
losing weight in a fluid, explained in the article on specific gravity,
page 48), it shall exactly balance the latter, but when the water sinks
in the boiler, and the stone is no longer surrounded with water, it
becomes heavier, and sinking down opens a conical plug, ground so as to
fit water-tight into a hole in the bottom of the column of water or feed
pipe, and directly the plug opens, water rushes into the boiler; being
cut off again as the stone rises when immersed or surrounded with the
proper height of water. Unless our juvenile readers refer to the article
on specific gravity, they will not understand the otherwise seeming
anomaly of a _stone float_.

A large hole, called the man-hole, covered with an iron plate and
securely fastened with screws, is provided for the purpose of allowing
the engineer to enter the boiler, when cold, for the purpose of clearing
out the incrustation and dirt arising from the water. To prevent the
incrustation of lime and other earthy matters, it is sometimes usual, on
the principle "_that prevention is better than cure_" to put a large log
of "logwood" inside the boiler, as it is found that the colouring matter
curiously prevents the earthy matter, so well known as the "fur" in iron
"tea-kettles," sticking to the sides of the boiler. Sal ammoniac [Page
432] and other salts also have the same property, but neither are much
used, the mechanical labour of chipping out the boiler and stopping its
work for a day or so, being preferred to the _prevention plan_ already
described.

There is also a valve opening inwards to prevent the consequences of a
sudden condensation in the boiler, and also a safety valve and lever
with weights opening outwards, and allowing the steam to escape when it
reaches a dangerous excess, and in order to look as it were at the state
of the pressure inside the iron boiler, a proper steam gauge is
provided, also two cocks--viz., a water and steam cock, to enable the
engineer to ascertain if the water is up to, and does not exceed, the
proper height, because when turned, supposing that all is going on
properly, the former, No. 7, should eject water, the latter, No. 8,
steam.

It is truly wonderful, considering the number of safeguards and warnings
provided, that accidents ever happen to boilers, but the statistics of
deaths and annual destruction of property show that science is
powerless, nay, absolutely dangerous, when handled by ignorant and
careless persons. The great fly-wheel, which is usually such an
awe-inspiring and marvellous exhibition of strength in an engine of any
great power, is employed for the purpose of storing up force, so that if
any parts of the engine work indifferently (they all work with
resistance), it shall equalize the wants of the whole, and by its
inertia it will continue to move until its motion is stopped by a
resistance equal to its momentum.

In starting an engine, the engineer may sometimes be observed labouring
to move the "fly-wheel," and when once he succeeds in getting it to
move, the resistance of the other parts of the machinery is soon
overcome. Mr. Alderson, in his prize essay, remarks that "it is in the
property which the steam-engine possesses of regulating itself, and
providing for all its wants, that the great beauty of the invention
consists. It has been said that nothing made by the hand of man
approaches so near to animal life. Heat is the principle of its
movement; there is in its tubes circulation, like that of the blood in
the veins of animals, having valves which open and shut in proper
periods; it feeds itself, evacuates such portions of its food as are
useless, and draws from its own labours all that is necessary to its own
subsistance. To this may be added, that they are now regulated so as not
to exceed the assigned speed, and thus do animals in a state of nature.
That the safety valves, like the pores of perspiration, open to permit
the escape of superfluous heat in the form of steam. The steam gauge, as
a pulse to the boiler, indicates the heat and pressure of the steam
within; and the motion of the piston represents the action and the power
of which it is capable. The motion of the fluids in the boiler
represents the expanding and collapsing of the heart; the fluid that
goes to it by one channel is drawn off by another, in part to be
returned when condensed by the cold, similar to the operation of veins
and arteries. Animals require long and frequent periods of relaxation
from fatigue, and any great accumulation of their power is not obtained
without great expense and inconvenience. The [Page 433] wind is
uncertain; and water, the constancy of which is in few places equal to
the wants of the machinist, can seldom be obtained on the spot where
other circumstances require machines to be erected. To relieve us from
all these difficulties, the last century has given us the steam-engine
for a resource, the power of which may be increased to infinitude: it
requires but little room; it may be erected in all places, and its
mighty services are always at our command, whether in winter or summer,
by day or by night, on land or water; it knows no intermission but what
our wishes dictate."

The _high-pressure_ steam-engine appears to have been first brought into
general use by Trevethic and Vivian, although the primary notion of such
a modification of the Newcomen or water-engines did not originate with
them. As the name implies, the steam is brought to a much higher
temperature and pressure than is required in the condensing engines of
Boulton and Watt. It consisted, in the first place, of a cylinder open
at the top, and provided with a piston. To save heat the cylinder was
fixed _inside_ the boiler, and was provided with a two-way cock worked
by a crank, for the purpose of supplying and cutting off the steam. The
downward stroke was produced by the atmosphere, and the steam having
done its work, was simply blown away and wasted in the air.

The engine was provided with a fly-wheel, to which the piston-rod was at
once attached, producing a continuous rotatory movement without the
assistance of the heavier parallel motion, or hot and cold water pumps.

This form of engine was soon adopted for pumping work--such as that of
draining fens; and in 1804 Mr. Richard Trevethic used it for propelling
the first carriage on the Merthyr Tydvil rail or tram way, and it was
then speedily adopted in all the coal districts where the levels were
moderate. Stephenson the elder, succeeded by the late lamented Robert
Stephenson, followed with inventions and improvements of the locomotive
steam-engine; and we are told in "Once a Week" that,

"One of those best qualified to speak to the latter's contributions to
the development of the locomotive engine, states that from about five
years from his return from America, Robert Stephenson's attention was
chiefly directed to its improvement. 'None but those who accompanied him
during the period in his incessant experiments can form an idea of the
amazing metamorphosis which the machine underwent in it. The most
elementary principles of the application of heat, of the mode of
calculating the strength of cylindrical and other boilers, of the
strength of rivetting and of staying flat portions of the boilers, were
then far from being understood, and each step in the improvement of the
engine had to be confirmed by the most careful experiments before the
brilliant results of the Rocket and Planet engines (the latter being the
type of the existing modern locomotive) could be arrived at.'

"Stephenson's time was not, however, so fully taken up during the above
interval as to preclude attention to his other civil engineering
business, and he executed within it the Leicester and Swannington,
[Page 434] Whitby and Pickering, Canterbury and Whitstable, and Newton
and Warrington Railways; while he also erected an extensive manufactory
for locomotives at Newton, in Lancashire, in partnership with the
Messrs. Tayleur. About the middle of the above period, also, the first
surveys and estimates for the London and Birmingham Railway were framed,
leading eventually to the obtaining of the Act. Then followed the
execution of that line, and here Robert Stephenson had an opportunity of
showing his great talent for the management of works on a large scale.
This was the first railway of any magnitude executed under the contract
system; perfect sets of plans and specifications (which have since
served as a type for nearly all the subsequent lines) were prepared--no
small matter for a series of works extending over 112 miles, involving
tunnels and other works of a then unprecedented magnitude.

"Many other railways in England and abroad were executed by him in rapid
succession; the Midland, Blackwall, Northern and Eastern, Norfolk,
Chester and Holyhead, together with numerous branch lines, were executed
in this country by him; and among railways abroad may be enumerated as
works either executed by him or recommended in his capacity of a
consulting engineer, the system of lines in Belgium, Italy, Norway, and
Egypt, and in France, Holland, Denmark, India, Canada, and New Zealand.

"Robert Stephenson first saw the light in the village of Willington, at
a cottage which his father occupied after his marriage with Miss Fanny
Henderson--a marriage contracted on the strength of his first
appointment as "breaksman" to the engine employed for lifting the
ballast brought by the return collier ships to Newcastle. Here Robert
was born on the 17th of November, 1803. As the cottage looked out upon a
tramway, the eyes of the child were naturally familiarized from infancy
with sights and scenes most nearly connected with his future
profession."

In locomotive steam-engine boilers, the principal object is to generate
steam with the greatest rapidity; hence the boiler consists of two
parts--viz., a square box containing the fire, and around which a thin
stratum of water circulates, whilst the draught for the fire rushes
through a number of copper tubes placed in the second or cylindrical
part of the boiler. By the use of these tubes an immense _surface_ of
water is exposed to the action of the fire, and the steam is not only
generated with amazing rapidity, but is also maintained at a very high
pressure.

Within the last few years "superheated steam" has been favourably
mentioned, and employed economically for driving certain engines. The
principle consists in first generating steam, and then passing it
through coils of strong wrought-iron pipe, by which it acquires
additional heat, and we have therefore combined in steam the ordinary
principle of evaporation of water with the heated-air principle of
Stirling, described at p. 367. We give a drawing of Scott's patent
generator and superheated steam engine. (Fig. 399.)

The apparatus is used as follows:--A fire is made in the furnace, and so
soon as a pyrometer connected with that indicates about 800 degrees,
[Page 435] a little water is pumped into the coils by hand, which is
immediately converted into steam. The donkey engine is then started,
which maintains the necessary feed of _air_ and water. The generator
produces a copious supply of elastic mixed gaseous vapour, at a pressure
of 250 pounds on the square inch; and it is stated that this engine
works satisfactorily, and is started in the incredibly short time of
from three to five minutes, so that for marine engines in war vessels,
expecting to be ordered out suddenly, no fuel need be burnt till the
moment required.

[Illustration: Fig. 399. Scott's patent generator, or new _versus_ old
steam.]

Experiments with superheated steam have already been tried most
successfully on board the Peninsular and Oriental Company's ship the
_Valetta_, whereby it is stated that a saving of thirty per cent. in
fuel [Page 436] is obtained. The engine to which the superheated steam
was adapted was constructed by Penn and Sons, and the vessel attained a
speed of nearly sixteen knots per hour, and under the most adverse
circumstances had an abundance of steam to spare.

"A most important experimental improvement in steam machinery was on
Thursday last tried for the first time down the river, on board the
Peninsular and Oriental Company's ship, the _Valetta_. The actual nature
of the improvement may be described in a few words as consisting of a
simple apparatus for working marine engines by means of superheated
steam; but it is not too much to say that in the success or failure of
this experiment are involved results so important as to affect
materially all ocean-going steamers, and, indeed, steam machinery of all
kinds. To be able to work machinery with superheated steam, means to
command increased power with a thirty per cent. reduction in the
consumption of fuel. A principle which can effect such important changes
in the universal application of steam has not remained undiscovered to
the present day. The want of superheated steam has long been felt, and
the enormous comparative advantages of working engines on such a plan
have long been known. A simple and effective working of the principle,
however, has been an engineering difficulty which various
expedients--all, however, sufficiently successful to show the value of
the improvement--have failed to obviate entirely. This obstacle has now,
we believe, been effectually overcome by Mr. Penn, and the value of the
improvement so clearly demonstrated, that the general application of the
principle to steam machinery of every kind may now be regarded as
certain.

"The idea of working engines by superheated steam, and the immense
saving of fuel and increase of power it would effect, was, we believe,
first started many years ago by Mr. Howard, and subsequently by Dr.
Haycraft. The difficulties, however, in the way of its adoption at that
time, and the undue estimate of the importance of the principle,
prevented those gentlemen from realizing very great practical results.
At a later period the matter was again taken up by an American
engineer--Mr. Weatherhead--who, however, only superheated a portion of
the steam and mixed it with common steam in its way to the cylinders.
The success which attended even this partial application of the process
again revived the idea, and encouraged other engineers to turn their
attention to the subject. The result of these renewed efforts is that
several methods of securing the great economy to be effected by
superheating the steam are now under trial, and there is no doubt that a
most important step in the progress of steam, especially as applied to
ocean navigation, is now at last on the point of being successfully
accomplished.

"The value of the improvement on the score of economy in working may be
best illustrated by a single fact--namely, that the Peninsular and
Oriental Company's bill for coal annually amounts to the enormous sum of
700,000_l._, and that by working their vessels with superheated steam
properly applied, it is become almost certain that, without any [Page
437] detriment to the machinery, from 28 to 30 per cent. of this
gigantic outlay can be saved. As to the various proposed methods of
superheating steam, it may be briefly explained, that the conditions
required to be fulfilled are perfect simplicity of arrangement with
ready control over the apparatus; that it should be so placed as not to
be liable to accidental injury in the engine-room; and that the heat
employed for superheating the steam should be waste heat which has
already done its duty in the boilers and is passing away.

"All these conditions have been most satisfactorily fulfilled by Mr.
Penn in the new engines on board the _Valetta_, which were tried down
the Thames for the first time on Thursday. The _Valetta_, as our readers
may remember, was for many years the mail-boat between Marseilles,
Malta, and Constantinople. While thus employed, she had Penn's engines
of 400 horse-power, and to work these up to an average speed of 15 miles
an hour required a consumption of fuel of from 70 to 75 tons of coal per
day. At no time was it less than from 45 to 55 tons. These engines have
now been removed to a vessel nearly double the tonnage of the _Valetta_,
and the latter fitted with engines by Mr. Penn on the superheating
principle. We may mention that, besides this alteration, the _Valetta_
has been considerably improved. A poop and forecastle have been added,
increased accommodation given to passengers, and the whole vessel fitted
up in the richest style. The saloon is one of the simplest and
handsomest things of the kind we have seen, sufficiently lofty and
capacious, and above all, admirably ventilated on the system which is
now being adopted on all sea-going steamers, and the merit of devising
which belongs to Mr. Robinson, of the Peninsular and Oriental Company.

"To return, however, to the engines. Mr. Penn, at the repeated request
of Mr. Allen, the Managing Director of the Peninsular and Oriental
Company, undertook to apply to them the principle of superheating, to
which his attention had many years before been seriously directed by Dr.
Haycraft. His method of doing this is to place in the smoke-box of the
boiler, through which the hot air from the furnace first passes, as
large a number of small pipes as is consistent with allowing a free
draught from the furnaces. Through these all the steam from the boilers
passes in its way to the cylinders. By this plan an immense heating
surface in the pipes is secured, the steam is in a subdivided form, so
as to be readily acted on, and the waste heat from the furnace is
utilized at the point where its intensity is greatest, and where the
greatest conveniences exist for applying the apparatus. By means of
three ordinary stop-valves, the whole contrivance can be shut in or off
from the engines at pleasure. In ordinary engines steam leaves the
boilers at about 250°, but declines from this temperature in its way to
the engines to 230°, undergoing from condensation a still greater and
more serious diminution of heat in the cylinders. From these causes, and
also from the immense quantity of waste heat which escapes through the
smoke-box and up the funnels, there has always been a theoretical loss
of steam power amounting to forty per cent., as [Page 438] compared
with the coal consumed. It is this loss of power and waste of heat which
the superheating process is intended to prevent, and which will, of
course, allow a reduction of from twenty-eight to thirty per cent. on
the fuel now consumed. By the superheating process the steam is raised
in passing along the pipes in the smoke-box (where the heat is about
650°) from a temperature of 250° to 350°, and so enters the cylinders at
100° in excess of the temperature due to its pressure. This extra heat
is, of course, rapidly communicated to the metals, and prevents the
condensation in the cylinders or other parts of the engines, which would
otherwise, of course, take place. Singularly enough, a smaller amount of
cold water is required to condense the steam at this high temperature of
350° than when at the ordinary heat of common steam.

    "The trial trip of the _Valetta_ on Thursday was most satisfactory,
    not only as regards the engines, but still more so as to the
    application for the superheating process. At the measured mile at
    the Lower Hope, near the Nore, the result of repeated runs gave an
    average speed of nearly 14½ knots per hour, thus realizing with
    engines of 260 horse-power, and a small consumption of fuel, the
    same rate of speed as had been gained with her previous engines of
    400 horse-power, and a consumption of seventy-five tons of coals per
    day. The superheating apparatus evidently effected a most important
    saving in fuel, but until an average of many days' working can be
    obtained, it would be difficult to estimate the exact amount
    economized. There seems, however, every reason to believe that an
    average of fourteen knots an hour can be obtained with a consumption
    of only from twenty-four to twenty-six tons per diem. The
    thermometer during the trial indicated in the steam pipes an
    addition to the ordinary temperature of 100°, which Mr. Penn
    believes to be enough for all practical purposes of superheating.
    Even when making from thirty-three to thirty-four revolutions per
    minute, and driving the vessel against a strong head wind and tide,
    it was impossible to consume all the steam generated, which was
    blowing off from both boilers all the trip. The engines are
    remarkable for the extraordinary beauty and simplicity of their
    proportions, qualities well known in all engines from Penn and Sons,
    and which, combined with the strength of the materials and
    perfection of the workmanship, make this firm the foremost in the
    world for machinery of this description. Both cylinders are
    oscillating, of sixty-two inches diameter, and with a stroke of four
    feet six inches. The paddles are on the feathering principle, and
    the boilers of Lamb and Co.'s patent. During the whole course of the
    trials, and when going at one time nearly sixteen knots, there was
    no perceptible vibration, even at the end of the saloon nearest to
    the engines. When it is remembered that the superheating process
    which can effect such important results is capable, as we have said,
    of application to steam machinery of every kind, including even
    locomotives, it cannot be doubted that the trial of Thursday and its
    great success is one of the most important events for the progress
    of steam which we have had to chronicle for many years." (_The
    Times_, April 23rd. 1859.)

[Page 439]

Whilst speaking of the application of this somewhat novel condition of
steam, it may be observed that many inventors, who have paid little or
no attention to _first principles_, have proposed to apply the vapours
of alcohol, ether, or turpentine, instead of that of water; and they
have founded their notions on the idea that in consequence of the less
latent and sensible heat of alcohol, ether, and turpentine vapour, and
of the small quantity of fuel required to boil them, that they would
compete advantageously with steam. This view of the case, however, is
soon proved to be a very shortsighted one, because the _amount_ of
_expansion_ has been quite overlooked; and if it was desirable, by way
of comparison, to produce a cubic foot of steam, alcohol, ether, or
turpentine, the steam would stand first for cheapness, and would require
the least quantity of fuel to produce it, so that if the more expensive
of combustible liquids could be obtained for nothing, it would still be
cheaper to employ water.

                                                         Latent heat, or
                                                           equivalent
                                                            for fuel.

  A cubic foot of water yields 1700 cubic feet of steam      = 1000°
  A cubic foot of alcohol produces 493 cubic feet = 457°.
    Then, by rule of proportion, 493 cubic inches : 457
    :: 1700 :                                                  1575°
  A cubic foot of ether yields only 212 cubic feet of
    vapour = 312°, and 212 : 312° :: 1700 :                    2500°
  A cubic foot of the oil of turpentine affords 192 cubic
    feet of vapour = 183°, and 192 : 183 :: 1700 :             1620°

It will therefore be seen that water, when converted into steam, expands
eight times as much as sulphuric ether, and nearly three times and a
half as much as alcohol.

The application of steam for the purpose of propelling vessels has
already been mentioned in connexion with the Spanish inventor, Blasco de
Garay, in the year 1543. The first patent in this kingdom granted for
that purpose was that of Mr. Jonathan Hull in 1773. In 1787, Mr. Miller
tried a number of important experiments in the propulsion of vessels by
steam-engines, and it would appear that Lord Cullen advocated his ideas,
and endeavoured to secure the co-operation of the great firm of Boulton
and Watt, who, occupied with their land engines, could not pay attention
to it; and twenty years elapsed after the reply of Watt to Lord Cullen's
application, before the real novelty appeared of a first successful
experiment with a steam-boat in "the open sea," by Henry Bell, in 1811.
A picture of this boat, called the _Comet_, which was afterwards
wrecked, is shown at p. 418. Henry Bell's _novelty_ was _success_, and
he is fairly entitled to the merit of first introducing steam navigation
into Europe.

In 1811, the public stared with mingled astonishment and satisfaction at
the realization of that which was called a fable. Only forty-seven years
afterwards another generation spontaneously exhibits the liveliest
interest in the gigantic private speculation of the _Great Eastern_.
[Page 440] Henry Bell's vessel of 1811 was 40 feet keel, 10 feet 6
inches beam, and 25 tons burthen! The _Great Eastern_ of 1859 is 692
feet long, 83 feet wide, 60 feet deep, and 24,000 tons burthen!! The
whole nation with one voice wish her God speed in her projected voyage
across the Atlantic, as the embodiment of that great goodwill which
every generous-hearted Englishman feels towards the enlightened
free-born people of the United States.

Should the author's little vessel, with its humble freight of science,
meet with the approbation of his good friends, the boys and their
advisers, another and another, if health permits, shall be launched for
their benefit. _Vale._

[Illustration]

    THE END.




Transcriber's Notes.


Words and phrases in italics are indicated with
underscores thus:- _italics_.

Chapter XV. Experiment one is not indicated by a title heading.

Page 99. "the pulse is raised forty or fifty beats per second" changed
to "the pulse is raised forty or fifty beats per minute"

Page 148. "it is allowed to dry spontaneously, and being coated with
amber varnish (a solution of amber in chloroform) is now ready to print
from. It is, perhaps, hardly necessary to add, that the
sensitizing and developing processes must be performed in a dark room."
Fig. 123. is irrelevant to this section. The reference has been deleted.

Page 365. "an air thermometer has been employed by Sir John Leslie,
under the name of the "Differential Thermometer," in his refined and
delicate experiments with heat. (Fig. 401.)" Ref. to (Fig. 401.)
removed. Fig. 401. not in original hard copy version.





End of Project Gutenberg's The Boy's Playbook of Science, by John Henry Pepper