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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).