_THE FORCES AND ENERGIES OF NATURE: THE LAW OF CONSERVATION._
64. In the last chapter we introduced our readers to two varieties of
energy, one of them visible, and the other invisible or molecular; and
it will now be our duty to search through the whole field of physical
science for other varieties. Here it is well to bear in mind that all
energy consists of two kinds, that of _position_ and that of _actual
motion_, and also that this distinction holds for invisible molecular
energy just as truly as it does for that which is visible. Now, energy
of position implies a body in a position of advantage with respect
to some force, and hence we may with propriety begin our search by
investigating the various forces of nature.
_Gravitation._
65. The most general, and perhaps the most important, of these
forces is _gravitation_, and the law of action of this force may be
enunciated as follows:--_Every particle of the universe attracts every
other particle with a force depending jointly upon the mass of the
attracting and of the attracted particle, and varying inversely as the
square of distance between the two._ A little explanation will make
this plain.
Suppose a particle or system of particles of which the mass is unity to
be placed at a distance equal to unity from another particle or system
of particles of which the mass is also unity--the two will attract each
other. Let us agree to consider the mutual attraction between them
equal to unity also.
Suppose, now, that we have on the one side two such systems with a mass
represented by 2, and on the other side the same system as before,
with a mass represented by unity, the distance, meanwhile, remaining
unaltered. It is clear the double system will now attract the single
system with a twofold force. Let us next suppose the mass of both
systems to be doubled, the distance always remaining the same. It is
clear that we shall now have a fourfold force, each unit of the one
system attracting each unit of the other. In like manner, if the mass
of the one system is 2, and that of the other 3, the force will be 6.
We may, for instance, call the components of the one system A_{1},
A₂, and those of the other A_{3}, A_{4}, A_{5}, and we shall have
A_{1} pulled towards A_{3}, A_{4}, and A_{5}, with a threefold force,
and A₂ pulled towards A_{3}, A_{4}, and A_{5}, with a threefold
force, making altogether a force equal to 6.
In the next place, let the masses remain unaltered, but let the
distance between them be doubled, then the force will be reduced
fourfold. Let the distance be tripled, then the force will be reduced
ninefold, and so on.
66. Gravitation may be described as a very weak force, capable of
acting at a distance, or at least of appearing to do so. It takes the
mass of the whole earth to produce the force with which we are so
familiar at its surface, and the presence of a large mass of rock or
mountain does not produce any appreciable difference in the weight of
any substance. It is the gravitation of the earth, lessened of course
by distance, which acts upon the moon 240,000 miles away, and the
gravitation of the sun influences in like manner the earth and the
various other planets of our system.
_Elastic Forces._
67. Elastic forces, although in their mode of action very different
from gravity, are yet due to visible arrangements of matter; thus,
when a cross-bow is bent, there is a visible change produced in the
bow, which, as a whole, resists this bending, and tends to resume its
previous position. It therefore requires energy to bend a bow, just as
truly and visibly as it does to raise a weight above the earth, and
elasticity is, therefore, as truly a species of force as gravity is.
We shall not here attempt to discuss the various ways in which this
force may act, or in which a solid elastic substance will resist all
attempts to deform it; but in all cases it is clearly manifest that
work must be spent upon the body, and the force of elasticity must be
encountered and overcome throughout a certain space before any sensible
deformation can take place.
_Force of Cohesion._
68. Let us now leave the forces which animate large masses of matter,
and proceed to discuss those which subsist between the smaller
particles of which these large masses are composed. And here we must
say one word more about molecules and atoms, and the distinction we
feel ourselves entitled to draw between these very small bodies, even
although we shall never be able to see either the one or the other.
In our first chapter (Art. 7) we supposed the continual sub-division of
a grain of sand until we had arrived at the smallest entity retaining
all the properties of sand--this we called a _molecule_, and nothing
smaller than this is entitled to be called sand. If we continue this
sub-division further, the molecule of sand separates itself into its
chemical constituents, consisting of silicon on the one side, and
oxygen on the other. Thus we arrive at last at the smallest body which
can call itself silicon, and the smallest which can call itself oxygen,
and we have no reason to suppose that either of these is capable of
sub-division into something else, since we regard oxygen and silicon as
elementary or simple bodies. Now, these constituents of the silicon
molecule are called _atoms_, so that we say the sand molecule is
divisible into atoms of silicon and of oxygen. Furthermore, we have
strong reason for supposing that such molecules and atoms really exist,
but into the arguments for their existence we cannot now enter--it is
one of those things that we must ask our readers to take for granted.
69. Let us now take two molecules of sand. These, when near together,
have a very strong attraction for each other. It is, in truth, this
attraction which renders it difficult to break up a crystalline
particle of sand or rock crystal. But it is only exerted when the
molecules are near enough together to form a homogeneous crystalline
structure, for let the distance between them be somewhat increased, and
we find that all attraction entirely vanishes. Thus there is little
or no attraction between different particles of sand, even although
they are very closely packed together. In like manner, the integrity
of a piece of glass is due to the attraction between its molecules;
but let these be separated by a flaw, and it will soon be found that
this very small increase of distance greatly diminishes the attraction
between the particles, and that the structure will now fall to pieces
from the slightest cause. Now, these examples are sufficient to show
that molecular attraction or _cohesion_, as this is called, is a force
which acts very powerfully through a certain small distance, but which
vanishes altogether when this distance becomes perceptible. Cohesion
is strongest in solids, while in liquids it is much diminished, and in
gases it may be said to vanish altogether. The molecules of gases are,
in truth, so far away from one another, as to have little or no mutual
attraction, a fact proved by Dr. Joule, whose name was mentioned in the
last chapter.
_Force of Chemical Affinity._
70. Let us now consider the mutual forces between atoms. These may be
characterized as even stronger than the forces between molecules, but
as disappearing still more rapidly when the distance is increased. Let
us, for instance, take carbon and oxygen--two substances which are
ready to combine together to form carbonic acid, whenever they have a
suitable opportunity. In this case, each atom of carbon will unite with
two of oxygen, and the result will be something quite different from
either. Yet under ordinary circumstances carbon, or its representative,
coal, will remain unchanged in the presence of oxygen, or of
atmospheric air containing oxygen. There will be no tendency to combine
together, because although the particles of the oxygen would appear to
be in immediate contact with those of the carbon, yet the nearness is
not sufficient to permit of chemical affinity acting with advantage.
When, however, the nearness becomes sufficient, then chemical affinity
begins to operate. We have, in fact, the familiar act of combustion,
and, as its consequence, the chemical union of the carbon or coal with
the oxygen of the air, carbonic acid being the result. Here, then, we
have a very powerful force acting only at a very small distance, which
we name _chemical affinity_, inasmuch as it represents the attraction
exerted between atoms of different bodies in contradistinction to
cohesion, which denotes the attraction between molecules of the same
body.
71. If we regard gravitation as the representative of forces that act
or appear to act, at a distance, we may regard cohesion and chemical
affinity as the representatives of those forces which, although very
powerful, only act or appear to act through a very small interval of
distance.
A little reflection will show us how inconvenient it would be if
gravitation diminished very rapidly with the distance; for then
even supposing that the bond which retains us to the earth were to
hold good, that which retains the moon to the earth might vanish
entirely, as well as that which retains the earth to the sun, and the
consequences would be far from pleasant. Reflection will also show
us how inconvenient it would be if chemical affinity existed at all
distances; if coal, for instance, were to combine with oxygen without
the application of heat, it would greatly alter the value of this fuel
to mankind, and would materially check the progress of human industry.
_Remarks on Molecular and Atomic Forces._
72. Now, it is important to remember that we must treat cohesion and
chemical affinity exactly in the same way as gravity has been treated;
and just as we have energy of position with respect to gravity, so
may we have as truly a species of energy of position with respect to
cohesion and chemical affinity. Let us begin with cohesion.
73. We have hitherto regarded heat as a peculiar motion of the
molecules of matter, without any reference to the force which actuates
these molecules. But it is a well-known fact that bodies in general
expand when heated, so that, in virtue of this expansion, the molecules
of a body are driven violently apart against the force of cohesion.
Work has in truth been done against this force, just as truly as, when
a kilogramme is raised from the earth, work is done against the force
of gravity. When a substance is heated, we may, therefore, suppose that
the heat has a twofold office to perform, part of it going to increase
the actual motions of the molecules, and part of it to separate these
molecules from one another against the force of cohesion. Thus, if I
swing round horizontally a weight (attached to my hand by an elastic
thread of india-rubber), my energy will be spent in two ways--first
of all, it will be spent in communicating a velocity to the weight;
and, secondly, in stretching the india-rubber string, by means of the
centrifugal tendency of the weight. Work will be done against the
elastic force of the string, as well as spent in increasing the motion
of the weight.
Now, something of this kind may be taking place when a body is heated,
for we may very well suppose heat to consist of a vertical or circular
motion, the tendency of which would be to drive the particles asunder
against the force of cohesion. Part, therefore, of the energy of heat
will be spent in augmenting the motion, and part in driving asunder the
particles. We may, however, suppose that, in ordinary cases, the great
proportion of the energy of heat goes towards increasing the molecular
motion, rather than in doing work against the force of cohesion.
74 In certain cases, however, it is probable that the greater part
of the heat applied is spent in doing work against molecular forces,
instead of increasing the motions of molecules.
Thus, when a solid melts, or when a liquid is rendered gaseous, a
considerable amount of heat is spent in the process, which does not
become sensible, that is to say, does not affect the thermometer. Thus,
in order to melt a kilogramme of ice, heat is required sufficient to
raise a kilogramme of water through 80° C., and yet, when melted, the
water is no warmer than the ice. We express this fact by saying that
the latent heat of water is 80. Again, if a kilogramme of water at
100° be converted entirely into steam, as much heat is required as
would raise the water through 537° C., or 537 kilogrammes of water
through one degree; but yet the steam is no hotter than the water, and
we express this fact by saying that the latent heat of steam is 537.
Now, in both of these instances it is at least extremely probable that
a large portion of the heat is spent in doing work against the force
of cohesion; and, more especially, when a fluid is converted into a
gas, we know that the molecules are in that process separated so far
from one another as to lose entirely any trace of mutual force. We may,
therefore, conclude that although in most cases the greater portion of
the heat applied to a body is spent in increasing its molecular motion,
and only a small part in doing work against cohesion, yet when a solid
melts, or a liquid vaporizes, a large portion of the heat required
is not improbably spent in doing work against molecular forces. But
the energy, though spent, is not lost, for when the liquid again
freezes, or when the vapour again condenses, this energy is once more
transformed into the shape of sensible heat, just as when a stone is
dropped from the top of a house, its energy of position is transformed
once more into actual energy.
75. A single instance will suffice to give our readers a notion of
the strength of molecular forces. If a bar of wrought iron, whose
temperature is 10° C. above that of the surrounding medium, be tightly
secured at its extremities, it will draw these together with a force of
at least one ton for each square inch of section. In some cases where
a building has shown signs of bulging outwards, iron bars have been
placed across it, and secured while in a heated state to the walls.
On cooling, the iron contracted with great force, and the walls were
thereby pulled together.
76. We are next brought to consider atomic forces, or those which lead
to chemical union, and now let us see how these are influenced by heat.
We have seen that heat causes a separation between the molecules of a
body, that is to say, it increases the distance between two contiguous
molecules, but we must not suppose that, meanwhile, the molecules
themselves are left unaltered.
The tendency of heat to cause separation is not confined to increasing
the distance between molecules, but acts also, no doubt, in increasing
the distance between parts of the same molecule: in fact, the energy
of heat is spent in pulling the constituent atoms asunder against the
force of chemical affinity, as well as in pulling the molecules asunder
against the force of cohesion, so that, at a very high temperature, it
is probable that most chemical compounds would be decomposed, and many
are so, even at a very moderate heat.
Thus the attraction between oxygen and silver is so slight that at
a comparatively low temperature the oxide of silver is decomposed.
In like manner, limestone, or carbonate of lime, is decomposed when
subjected to the heat of a lime-kiln, carbonic acid being given off,
while quick-lime remains behind. Now, in separating heterogeneous
atoms against the powerful force of chemical affinity, work is done as
truly as it is in separating molecules from one another against the
force of cohesion, or in separating a stone from the earth against the
force of gravity.
77. Heat, as we have seen, is very frequently influential in performing
this separation, and its energy is spent in so doing; but other
energetic agents produce chemical decomposition as well as heat. For
instance, certain rays of the sun decompose carbonic acid into carbon
and oxygen in the leaves of plants, and their energy is spent in the
process; that is to say, it is spent in pulling asunder two such
powerfully attracting substances against the affinity they have for one
another. And, again, the electric current is able to decompose certain
substances, and of course its energy is spent in the process.
Therefore, whenever two powerfully attracting atoms are separated,
energy is spent in causing this separation as truly as in separating
a stone from the earth, and when once the separation has been
accomplished we have a species of energy of position just as truly as
we have in a head of water, or in a stone at the top of a house.
78. It is this chemical separation that is meant when we speak of coal
as a source of energy. Coal, or carbon, has a great attraction for
oxygen, and whenever heat is applied the two bodies unite together.
Now oxygen, as it exists in the atmosphere, is the common inheritance
of all, and if, in addition to this, some of us possess coal in our
cellars, or in pits, we have thus secured a store of energy of
position which we can draw upon with more facility than if it were a
head of water, for, although we can draw upon the energy of a head of
water whenever we choose, yet we cannot carry it about with us from
place to place as we can with coal. We thus perceive that it is not
the coal, by itself, that forms the source of energy, but this is due
to the fact that we have coal, or carbon, in one place, and oxygen in
another, while we have also the means of causing them to unite with
each other whenever we wish. If there were no oxygen in the air, coal
by itself would be of no value.
_Electricity: its Properties._
79. Our readers have now been told about the force of cohesion that
exists between molecules of the same body, and also about that of
chemical affinity existing between atoms of different bodies. Now,
heterogeneity is an essential element of this latter force--there must
be a difference of some kind before it can exhibit itself--and under
these circumstances its exhibitions are frequently characterized by
very extraordinary and interesting phenomena.
We allude to that peculiar exhibition arising out of the forces
of heterogeneous bodies which we call _electricity_, and, before
proceeding further, it may not be out of place to give a short sketch
of the mode of action of this very mysterious, but most interesting,
agent.
80. The science of electricity is of very ancient origin; but its
beginning was very small. For a couple of thousand years it made little
or no progress, and then, during the course of little more than a
century, developed into the giant which it now is. The ancient Greeks
were aware that amber, when rubbed with silk, had the property of
attracting light bodies; and Dr. Gilbert, about three hundred years
ago, showed that many other things, such as sulphur, sealing-wax, and
glass, have the same property as amber.
In the progress of the science it came to be known that certain
substances are able to carry away the peculiar influence produced,
while others are unable to do so; the former are called _conductors_,
and the latter _non-conductors, or insulators_, of electricity. To make
the distinction apparent, let us take a metal rod, having a glass stem
attached to it, and rub the glass stem with a piece of silk, care being
taken that both silk and glass are warm and dry. We shall find that the
glass has now acquired the property of attracting little bits of paper,
or elder pith; but only where it has been rubbed, for the peculiar
influence acquired by the glass has not been able to spread itself over
the surface.
If, however, we take hold of the glass stem, and rub the metal rod,
we may, perhaps, produce the same property in the metal, but it will
spread over the whole, not confining itself to the part rubbed. Thus
we perceive that metal is a conductor, while glass is an insulator, or
non-conductor, of electricity.
[Illustration: Fig. 5.]
81. We would next observe that _this influence is of two kinds_. To
prove this, let us perform the following experiment. Let us suspend a
small pith ball by a very slender silk thread, as in Fig. 5. Next, let
us rub a stick of warm, dry glass with a piece of warm silk, and with
this excited stick touch the pith ball. The pith ball, after being
touched, will be repelled by the excited glass. Let us next excite, in
a similar manner, a stick of dry sealing-wax with a piece of warm, dry
flannel, and on approaching this stick to the pith ball it will attract
it, although the ball, in its present state, is repelled by the excited
glass.
Thus a pith ball, touched by excited glass, is repelled by excited
glass, but attracted by excited sealing-wax.
In like manner, it might be shown that a pith ball, touched by excited
sealing-wax, will be afterwards repelled by excited sealing-wax, but
attracted by excited glass.
Now, what the excited glass did to the pith ball, was to communicate to
it part of its own influence, after which the ball was repelled by the
glass; or, in other words, _bodies charged with similar electricities
repel one another_.
Again, since the pith ball, when charged with the electricity from
glass, was attracted to the electrified sealing-wax, we conclude that
_bodies charged with unlike electricities attract one another_. The
electricity from glass is sometimes called _vitreous_, and that from
sealing-wax _resinous_, electricity, but more frequently the former
is known as _positive_, and the latter as _negative_, electricity--it
being understood that these words do not imply the possession of a
positive nature by the one influence, or of a negative nature by the
other, but are merely terms employed to express the apparent antagonism
which exists between the two kinds of electricity.
82. The next point worthy of notice is that _whenever one electricity
is produced, just as much is produced of an opposite description_.
Thus, in the case of glass excited by silk, we have positive
electricity developed upon the glass, while we have also negative
electricity developed upon the silk to precisely the same extent.
And, again, when sealing-wax is rubbed with flannel, we have negative
electricity developed upon the sealing-wax, and just as much positive
upon the flannel.
83. These facts have given rise to a theory of electricity, or at
least to a method of regarding it, which, if not absolutely correct,
seems yet to unite together the various phenomena. According to this
hypothesis, a neutral, unexcited body is supposed to contain a store of
the two electricities combined together, so that whenever such a body
is excited, a separation is produced between the two. The phenomena
which we have described are, therefore, due to this electrical
separation, and inasmuch as the two electricities have a great affinity
for one another, it requires the expenditure of energy to produce this
separation, just as truly as it does to separate a stone from the earth.
84. Now, it is worthy of note that _electrical separation is only
produced when heterogeneous bodies are rubbed together_. Thus, if
flannel be rubbed upon glass, we have electricity; but if flannel be
rubbed upon glass covered with flannel, we have none. In like manner,
if silk be rubbed upon sealing-wax covered with silk, or, in fine,
if two portions of the same substance be rubbed together, we have no
electricity.
On the other hand, a very slight difference of texture is sometimes
sufficient to produce electrical separation. Thus, if two pieces of the
same silk ribbon be rubbed together lengthwise, we have no electricity;
but if they be rubbed across each other, the one is positively, and the
other negatively, electrified.
In fact, this element of heterogeneity is an all important one
in electrical development, and this leads us to conjecture that
_electrical attraction may probably be regarded as peculiarly allied to
that force which we call chemical affinity_. At any rate, electricity
and chemical affinity are only manifested between bodies that are, in
some respects, dissimilar.
85. The following is a list of bodies arranged according to the
electricity which they develop when rubbed together, each substance
being positively electrified when rubbed with any substance beneath it
in the list.
1. Cat’s skin.
2. Flannel.
3. Ivory.
4. Glass.
5. Silk.
6. Wood.
7. Shellac.
8. Resin.
9. Metals.
10. Sulphur.
11. Caoutchouc.
12. Gutta-percha.
13. Gun-cotton.
Thus, if resin be rubbed with cat’s skin, or with flannel, the
cat’s skin or flannel will be positively, and the resin negatively,
electrified; while if glass be rubbed with silk, the glass will be
positively, and the silk negatively, electrified, and so on.
86. It is not our purpose here to describe at length the _electrical
machine_, but we may state that it consists of two parts, one for
generating electricity by means of the friction of a rubber against
glass, and another consisting of a system of brass tubes, of
considerable surface, supported on glass stems, for collecting and
retaining the electricity so produced. This latter part of the machine
is called its _prime conductor_.
_Electric Induction._
[Illustration: Fig. 6.]
87. Let us now suppose that we have set in action a machine of this
kind, and accumulated a considerable quantity of positive electricity
in its prime conductor at A. Let us next take two vessels, B and C,
made of brass, supported on glass stems. These two vessels are supposed
to be in contact, but at the same time to be capable of being separated
from one another at their middle point, where the line is drawn in Fig.
6. Now let us cause B and C to approach A together. At first, B and C
are not electrified, that is to say, their two electricities are not
separated from each other, but are mixed together; but mark what will
happen as they are pushed towards A. The positive electricity of A will
decompose the two electricities of B and C, attracting the negative
towards itself, and repelling the positive as far away as possible.
The disposition of electricities will, therefore, be as in the figure.
If we now pull C away from B, we have obtained a quantity of positive
electricity on C, by help of the original electricity which was in A;
in fact, we have made use of the original stock or electrical capital
in A, in order to obtain positive electricity in C, without, however,
diminishing the amount of our original stock. Now, this distant action
or help, rendered by the original electricity in separating that of B
and C, is called electric induction.
88. The experiment may, however, be performed in a somewhat different
manner--we may allow B and C to remain together, and gradually push
them nearer to A. As B and C approach A, the separation of their
electricities will become greater and greater, until, when A and
B are only divided by a small thickness of air, the two opposite
electricities then accumulated will have sufficient strength to rush
together through the air, and unite with each other by means of a spark.
89. The principle of induction may be used with advantage, when it is
wished to accumulate a large quantity of electricity.
[Illustration: Fig. 7.]
In this case, an instrument called a _Leyden jar_ is very frequently
employed. It consists of a glass jar, coated inside and outside with
tin foil, as in Fig. 7. A brass rod, having a knob at the end of it,
is connected metallically with the inside coating, and is kept in its
place by being passed through a cork, which covers the mouth of the
jar. We have thus two metallic coatings which are not electrically
connected with one another. Now, in order to charge a jar of this kind,
let the outside coating be connected by a chain with the earth, while
at the same time positive electricity from the prime conductor of an
electrical machine is communicated to the inside knob.
The positive electricity will accumulate on the inside coating
with which the knob is connected. It will then decompose the two
electricities of the outside coating, driving the positive electricity
to the earth, and there dissipating it, but attracting the negative
to itself. There will thus be positive electricity on the inside,
and negative on the outside coating. These two electricities may be
compared to two hostile armies watching each other, and very anxious
to get together, while, however, they are separated from one another
by means of an insurmountable obstacle. They will thus remain facing
each other, and at their posts, while each side is, meanwhile, being
recruited by the same operation as before. We may by this means
accumulate a vast quantity of opposite electricities on the two
coatings of such a jar, and they will remain there for a long time,
especially if the surrounding atmosphere and the glass surface of the
jar be quite dry. When, however, electric connection of any kind is
made between the two coatings, the electricities rush together and
unite with one another in the shape of a spark, while if the human body
be the instrument of connecting them a severe shock will be felt.
90. It would thus appear that, when two bodies charged with opposite
electricities are brought near each other, the two electricities rush
together, forming a current, and the ultimate result is a spark.
Now, this spark implies heat, and is, in truth, nothing else than
small particles of intensely heated matter of some kind. We have here,
therefore, first of all, the conversion of electrical separation into a
current of electricity, and, secondly, the conversion of this current
into heat. In this case, however, the current lasts only a very small
time; the discharge, as it is called, of a Leyden jar being probably
accomplished in ¹⁄₂₄₀₀₀th of a second.
_The Electric Current._
91. In other cases we have electrical currents which, although not so
powerful as that produced by discharging a Leyden jar, yet last longer,
and are, in fact, continuous instead of momentary.
We may see a similar difference in the case of visible energy. Thus we
might, by means of gunpowder, send up in a moment an enormous mass of
water; or we might, by means of a fountain, send up the same mass in
the course of time, and in a very much quieter manner. We have the same
sort of difference in electrical discharges, and having spoken of the
rushing together of two opposite electricities by means of an explosion
and a spark, let us now speak of the eminently quiet and effective
_voltaic current_, in which we have a continuous coming together of the
same two agents.
[Illustration: Fig. 8.]
92. It is not our object here to give a complete description, either
historical or scientific, of the voltaic battery, but rather to give
such an account as will enable our readers to understand what the
arrangement is, and what sort of effect it produces; and with this
object we shall at once proceed to describe the battery of Grove, which
is perhaps the most efficacious of all the various arrangements for the
purpose of producing an electric current. In this battery we have a
number of cells connected together, as in Fig. 8, which shows a battery
of three cells. Each cell consists of two vessels, an outer and an
inner one; the outer vessel being made of glass or ordinary stone ware,
while the inner one is made of unglazed porcelain, and is therefore
porous. The outer vessel is filled with dilute sulphuric acid, and a
plate of amalgamated zinc--that is to say, of metallic zinc having
its outer surface brightened with mercury,--is immersed in this acid.
Again, in the inner or porous vessel we have strong nitric acid, in
which a plate of platinum foil is immersed, this being at the same time
electrically connected with the zinc plate of the next outer vessel,
by means of a clamp, as in the figure. Both metals must be clean where
they are pressed together, that is to say, the true metallic surfaces
of both must be in contact. Finally, a wire is metallically connected
with the platinum of the left-hand cell, and a similar wire with the
zinc of the right-hand cell, and these connecting wires ought, except
at their extremities, to be covered over with gutta-percha or thread.
The loose extremities of these wires are called the _poles_ of the
battery.
93. Let us now suppose that we have a battery containing a good many
cells of this description, and let the whole arrangement be insulated,
by being set upon glass supports, or otherwise separated from the
earth. If now we test, by appropriate methods, the extremity of the
wire connected with the left-hand platinum plate, it will be found to
be charged with positive electricity, while the extremity of the other
wire will be found charged with negative electricity.
94. In the next place, if we connect these poles of the battery with
one another, the two electricities will rush together and unite, or,
in other words, there will be an electric current; but it will not be
a momentary but a continuous one, and for some time, provided these
poles are kept together, a current of electricity will pass through the
wires, and indeed through the whole arrangement, including the cells.
The direction of the current will be such that _positive electricity
may be supposed to pass from the zinc to the platinum, through the
liquid; and back again through the wire, from the platinum at the
left hand, to the zinc at the right_; in fact, to go in the direction
indicated by the arrow-head.
95. Thus we have two things. In the first place, before the two
terminals, or poles, have been brought together, we have them charged
with opposite electricities; and, secondly, when once they have been
brought together, we have the production of a continuous current of
electricity. Now, this current is an energetic agent, in proof of which
we shall proceed to consider the various properties which it has,--the
various things which it can do.
_Its Magnetic Effects._
96. In the first place, _it can deflect the magnetic needle_. For
instance, let a compass needle be swung freely, and let a current of
electricity circulate along a wire placed near this needle, and in the
direction of its length, then the direction in which the needle points
will be immediately altered. This direction will now depend upon that
of the current, conveyed by the wire, and the needle will endeavour to
place itself at right angles to this wire.
In order to remember the connection between the direction of the
current and that of the magnet, imagine your body to form part of the
positive current, which may be supposed to enter in at your head, and
go out at your feet; also imagine that your face is turned towards
the magnet. In this case, the pole of the magnet, which points to the
north, will always be deflected by the current towards your right
hand. The strength of a current may be measured by the amount of the
deflection it produces upon a magnetic needle, and the instrument by
which this measurement is made is called a _galvanometer_.
97. In the next place, _the current is able_, not merely to deflect
a magnet, but also _to render soft iron magnetic_. Let us take, for
instance, the wire connected with the one pole of the battery, and
cover it with thread, in order to insulate it, and let us wrap this
wire round a cylinder of soft iron, as in Fig. 9. If we now make a
communication between the other extremity of the wire, and the other
pole of the battery, so as to make the current pass, it will be found
that our cylinder of soft iron has become a powerful magnet, and that
if an iron keeper be attached to it as in the figure, the keeper will
be able to sustain a very great weight.
[Illustration: Fig. 9.]
_Its Heating Effect._
98. _The electric current has likewise the property of heating a wire
through which it passes._ To prove this, let us connect the two poles
of a battery by means of a fine platinum wire, when it will be found
that the wire will, in a few seconds, become heated to redness. In
point of fact, the current will heat a thick wire, but not so much as a
thin one, for we may suppose it to rush with great violence through the
limited section of the thin wire, producing in its passage great heat.
_Its Chemical Effect._
99. Besides its magnetic and heating effects, _the current has also the
power of decomposing compound substances_, under certain conditions.
Suppose, for instance, that the poles of a battery, instead of being
brought together, are plunged into a vessel of water, decomposition
will at once begin, and small bubbles of oxygen will rise from the
positive pole, while small bubbles of hydrogen will make their
appearance at the negative. If the two gases are collected together in
a vessel, they may be exploded, and if collected separately, it may
be proved by the ordinary tests, that the one is oxygen and the other
hydrogen.
_Attraction and Repulsion of Currents._
100. We have now described very shortly the magnetic, the heating, and
the chemical effects of currents; it remains for us to describe the
effects of currents upon one another.
In the first place, suppose that we have two wires which are parallel
to one another, and carry currents going in the same direction; and
let us further suppose that these wires are capable of moving, then it
is found that they will attract one another. If, however, the wires,
although parallel, convey currents going in opposite directions, they
will then repel one another. A good way of showing this experimentally
is to cause two circular currents to float on water. If these currents
both go either in the same direction as the hands of a watch, or in
the opposite direction, then the two will attract one another; but if
the one goes in the one direction, and the other in the other, they
will then repel one another.
_Attraction and Repulsion of Magnets._
101. Ampère, who discovered this property of currents, has likewise
shown us that in very many respects a magnet may be likened to a
collection of circular currents all parallel to one another, their
direction being such that, if you look towards the north pole of a
freely suspended cylindrical magnet facing it, the positive current
will descend on the east or left-hand side, and ascend on the west or
right-hand side. If we adopt this method of viewing magnets, we can
easily account for the attraction between the unlike and the repulsion
between the like poles of a magnet, for when unlike poles are placed
near each other, the circular currents which face each other are then
all going in the same direction, and the two will, therefore, attract
one another, but if like poles are placed in this position, the
currents that face each other are going in opposite directions, and the
poles will, therefore, repel one another.
[Illustration: Fig. 10.]
_Induction of Currents._
102. Before closing this short sketch of electrical phenomena, we must
allude to the inductive effect of currents upon each other. Let us
suppose (Fig. 10) that we have two circular coils of wire, covered with
thread, and placed near each other. Let both the extremities of the
right-hand coil be connected with the poles of a battery, so as to make
a current of electricity circulate round the coil. On the other hand,
let the left-hand coil be connected with a galvanometer, thus enabling
us to detect the smallest current of electricity which may pass through
this coil. Now, it is found that when we first connect the right-hand
coil, so as to pass the battery current through it, a momentary current
will pass through the left-hand coil, and will deflect the needle of
the galvanometer, but this current will go in an opposite direction to
that which circulates round the right-hand coil.
103. Again, as long as the current continues to flow through the
right-hand coil there will be no current through the other, but at
the moment of breaking the contact between the right-hand coil and
the battery there will again be a momentary current in the left-hand
coil, but this time in the same direction as that of the right-hand
coil, instead of being, as before, in the opposite direction. In other
words, when contact is _made_ in the right-hand coil, there is a
momentary current in the left-hand coil, but in an opposite direction
to that in the right, while, when contact is _broken_ in the right-hand
coil, there is a momentary current in the left-hand coil in the same
direction as that in the right.
104. In order to exemplify this induction of currents, it is not even
necessary to make and break the current in the right-hand coil, for we
may keep it constantly going and arrange so as to make the right-hand
coil (always retaining its connection with the battery) alternately
approach and recede from the other; when it approaches the other, the
effect produced will be the same as when the contact was made in the
above experiment--that is to say, we shall have an induced current in
an opposite direction to that of the primary, while, when it recedes
from the other, we shall have a current in the same direction as that
of the primary.
105. Thus we see that whether we keep both coils stationary, and
suddenly produce a current in the right-hand coil, or whether, keeping
this current constantly going, we suddenly bring it near the other
coil, the inductive effect will be precisely the same, for in both
cases the left-hand coil is suddenly brought into the presence of a
current. And again, it is the same, whether we suddenly break the
right-hand current, or suddenly remove it from the left-hand coil, for
in both cases this coil is virtually removed from the presence of a
current.
_List of Energies._
106. We are now in a position to enumerate the various kinds of
energy which occur in nature; but, before doing so, we must warn our
readers that this enumeration has nothing absolute or complete about
it, representing, as it does, not so much the present state of our
knowledge as of our want of knowledge, or rather profound ignorance, of
the ultimate constitution of matter. It is, in truth, only a convenient
classification, and nothing more.
107. To begin, then, with visible energy. We have first of all--
_Energy of Visible Motion._
(A.) Visible energy of actual motion--in the planets, in meteors, in
the cannon ball, in the storm, in the running stream, and in other
instances of bodies in actual visible motion, too numerous to be
mentioned.
_Visible Energy of Position._
(B.) We have also visible energy of position--in a stone on the top of
a cliff, in a head of water, in a rain cloud, in a cross-bow bent, in
a clock or watch wound up, and in various other instances.
108. Then we have, besides, several cases in which there is an
alternation between (A) and (B).
A pendulum, for instance, when at its lowest point, has only the
energy (A), or that of actual motion, in virtue of which it ascends a
certain distance against the force of gravity. When, however, it has
completed its ascent, its energy is then of the variety (B), being
due to position, and not to actual motion; and so on it continues to
oscillate, alternately changing the nature of its energy from (A) to
(B), and from (B) back again to (A).
109. A vibrating body is another instance of this alternation. Each
particle of such a body may be compared to an exceedingly small
pendulum oscillating backwards and forwards, only very much quicker
than an ordinary pendulum; and just as the ordinary pendulum in passing
its point of rest has its energy all of one kind, while in passing its
upper point it has it all of another, so when a vibrating particle is
passing its point of rest, its energy is all of the variety (A), and
when it has reached its extreme displacement, it is all of the variety
(B).
_Heat Motion._
110. (C.) Coming now to molecular or invisible energy, we have, in
the first place, that motion of the molecules of bodies which we term
heat. A better term would be _absorbed heat_, to distinguish it from
_radiant heat_, which is a very different thing. That peculiar motion
which is imparted by heat when absorbed into a body is, therefore, one
variety of molecular energy.
_Molecular Separation._
(D.) Analogous to this is that effect of heat which represents
position rather than actual motion. For part of the energy of absorbed
heat is spent in pulling asunder the molecules of the body under the
attractive force which binds them together (Art. 73), and thus a store
of energy of position is laid up, which disappears again after the
body is cooled.
_Atomic or Chemical Separation._
111. (E.) The two previous varieties of energy may be viewed as
associated with molecules rather than with atoms, and with the force
of cohesion rather than with that of chemical affinity. Proceeding now
to atomic force, we have a species of energy of position due to the
separation of different atoms under the strong chemical attraction
they have for one another. Thus, when we possess coal or carbon and
also oxygen in a state of separation from one another, we are in
possession of a source of energy which may be called that of chemical
separation.
_Electrical Separation._
112 (F.) The attraction which heterogeneous atoms possess for one
another, sometimes, however, gives rise to a species of energy which
manifests itself in a very peculiar form, and appears as electrical
separation, which is thus another form of energy of position.
_Electricity in Motion._
113 (G.) But we have another species of energy connected with
electricity, for we have that due to electricity in motion, or in
other words, an electric current which probably represents some form
of actual motion.
_Radiant Energy._
114 (H.) It is well known that there is no ordinary matter, or at
least hardly any, between the sun and the earth, and yet we have a
kind of energy which we may call radiant energy, which proceeds
to us from the sun, and proceeds also with a definite, though very
great velocity, taking about eight minutes to perform its journey.
Now, this radiant energy is known to consist of the vibrations of an
elastic medium pervading all space, which is called ether, or the
_ethereal medium_. Inasmuch, therefore, as it consists of vibrations,
it partakes of the character of pendulum motion, that is to say, the
energy of any ethereal particle is alternately that of position and
that of actual motion.
_Law of Conservation._
115. Having thus endeavoured, provisionally at least, to catalogue our
various energies, we are in a position to state more definitely what
is meant by the conservation of energy. For this purpose, let us take
the universe as a whole, or, if this be too large, let us conceive, if
possible, a small portion of it to be isolated from the rest, as far as
force or energy is concerned, forming a sort of microcosm, to which we
may conveniently direct our attention.
This portion, then, neither parts with any of its energy to the
universe beyond, nor receives any from it. Such an isolation is, of
course, unnatural and impossible, but it is conceivable, and will,
at least, tend to concentrate our thoughts. Now, whether we regard
the great universe, or this small microcosm, the principle of the
conservation of energy asserts that the sum of all the various energies
is a constant quantity, that is to say, adopting the language of
Algebra--
(A) + (B) + (C) + (D) + (E) + (F) + (G) + (H) = a constant quantity.
116. This does not mean, of course, that (A) is constant in itself, or
any other of the left-hand members of this equation, for, in truth,
they are always changing about into each other--now, some visible
energy being changed into heat or electricity; and, anon, some heat or
electricity being changed back again into visible energy--but it only
means that the sum of all the energies taken together is constant. We
have, in fact, in the left hand, eight variable quantities, and we
only assert that their sum is constant, not by any means that they are
constant themselves.
117. Now, what evidence have we for this assertion? It may be replied
that we have the strongest possible evidence which the nature of the
case admits of. The assertion is, in truth, a peculiar one--peculiar
in its magnitude, in its universality, in the subtle nature of the
agents with which it deals. If true, its truth certainly cannot be
proved after the manner in which we prove a proposition in Euclid.
Nor does it even admit of a proof so rigid as that of the somewhat
analogous principle of the conservation of matter, for in chemistry we
may confine the products of our chemical combination so completely
as to prove, beyond a doubt, that no heavy matter passes out of
existence that--when coal, for instance, burns in oxygen gas--what we
have is merely a change of condition. But we cannot so easily prove
that no energy is destroyed in this combination, and that the only
result is a change from the energy of chemical separation into that of
absorbed heat, for during the process it is impossible to isolate the
energy--do what we may, some of it will escape into the room in which
we perform the experiment; some of it will, no doubt, escape through
the window, while a little will leave the earth altogether, and go
out into space. All that we can do in such a case is to estimate, as
completely as possible, how much energy has gone away, since we cannot
possibly prevent its going. But this is an operation involving great
acquaintance with the laws of energy, and very great exactness of
observation: in fine, our readers will at once perceive that it is much
more difficult to prove the truth of the conservation of energy than
that of the conservation of matter.
118. But if it be difficult to prove our principle in the most rigorous
manner, we are yet able to give the strongest possible indirect
evidence of its truth.
Our readers are no doubt familiar with a method which Euclid frequently
adopts in proving his propositions. Starting with the supposition
that they are not true, and reasoning upon this hypothesis, he comes
to an absurd conclusion--hence he concludes that they are true. Now,
we may adopt a method somewhat similar with regard to our principle,
only instead of supposing it untrue, let us suppose it true. It may
then be shown that, if it be true, under certain test conditions we
ought to obtain certain results--for instance, if we increase the
pressure, we ought to lower the freezing point of water. Well, we make
the experiment, and find that, in point of fact, the freezing point of
water is lowered by increasing the pressure, and we have thus derived
an argument in favour of the conservation of energy.
119. Or again, if the laws of energy are true, it may be shown that,
whenever a substance contracts when heated, it will become colder
instead of hotter by compression. Now, we know that ice-cold water,
or water just a little above its freezing point, contracts instead
of expanding up to 4° C.; and Sir William Thomson has found, by
experiment, that water at this temperature is cooled instead of heated
by sudden compression. India-rubber is another instance of this
relation between these two properties, for if we stretch a string of
india-rubber it gets hotter instead of colder, that is to say, its
temperature rises by extension, and gets lower by contraction; and
again, if we heat the string, we find that it contracts in length
instead of expanding like other substances as its temperature increases.
120. Numberless instances occur in which we are enabled to predict
what will happen by assuming the truth of the laws of energy; in other
words, these laws are proved to be true in all cases where we can put
them to the test of rigorous experiment, and probably we can have no
better proof than this of the truth of such a principle. We shall
therefore proceed upon the assumption that the conservation of energy
holds true in all cases, and give our readers a list of the various
transmutations of this subtle agent as it goes backwards and forwards
from one abode to another, making, meanwhile, sundry remarks that may
tend, we trust, to convince our readers of the truth of our assumption.