[Footnote: For fuller information on this subject, Grove's
"Correlation of the Physical Forces," or Tyndall's "Lectures on
Heat considered as a Mode of Motion," may be consulted.]
It has long been known that heat and light are closely connected
together. The accumulation of a certain amount of heat is always
accompanied by the appearance of light. But when it was found that
the light could be separated from the heat by various means, it
seemed possible that the two phenomena were simply associated. It
is now, however, ascertained that light and heat are identical in
their nature, and that a vast number of other phenomena--
electricity, galvanism, magnetism, chemical action, and
gravitation, as well as light and heat, are different
manifestations of one and the same thing, which is called force or
energy. In a great number of cases it is possible for us, by the
use of appropriate means and apparatus, to transform these
manifestations, so as to make the same force assume a variety of
forms. Thus motion suddenly arrested becomes heat. A rifle-ball
when it strikes the target becomes very hot. The heat produced by
the concussion against an iron shield is found sufficient to
ignite the powder in some of the newly invented projectiles. The
best illustration, however, is to be obtained from galvanism. By
means of the Voltaic battery we set free a certain amount of
force, and we can employ it at pleasure to produce an intense
light in the electric lamp, or to melt metals which resist the
greatest heat of our furnaces; it will convert a bar of iron into
a magnet, or decompose water into its constituents, oxygen and
hydrogen, or separate a metal from its combination with oxygen.
But in all these processes no new force is produced--the force
set free is unchangeable in itself, and we cannot increase its
amount. Owing to the imperfection of our instruments and our skill
a part of it will always escape from our control, and be lost to
us, but not destroyed. When, however, due allowance is made for
this loss, the results produced are always in exact proportion to
the amount of force originally set free. Thus, if we employ it to
decompose water, the amount of water decomposed always bears an
exact proportion to the amount of metal which has been oxidized in
the cells of the battery.
This force pervades everything which comes within the cognizance
of our senses. It exists in what are termed the elementary
substances of which the crust of the earth is composed. A certain
amount of it seems to be required to maintain them in the forms in
which we know them; for in many cases, when two of them are made
to combine, a certain amount of force is set free, which commonly
makes its appearance as heat. This seems to indicate that a less
amount of force suffices to maintain the compound body than was
requisite for its separate elements. Thus, when oxygen and
hydrogen are combined to form water intense heat is produced. If
we wish to dissolve the union, and restore the oxygen and hydrogen
to a gaseous state, we must restore the force which has been lost.
This, however, must be done by means of electricity, as heat
produces a different change--converting the water into vapour, but
not dissolving the union between its elements.
Force, in the shape of heat, determines the condition in which all
inorganic bodies exist. In most cases we can make any given
element assume the form of a solid, a fluid, or a vapour, by the
addition or subtraction of heat. Thus if a pound of ice at 32
degrees be exposed to heat, it will gradually melt--but the water
produced will remain unchanged in temperature till the last
particle of ice is melted--then it will begin to rise in
temperature; and, if the supply of heat be uniform, it will reach
a temperature of 172 degrees in exactly the same time as was
occupied in melting the ice. Thus then the force which was applied
to the ice as heat passes into some other form so long as the ice
is being melted--it is no longer perceptible by the senses--we
only see its effect in the change from the solid to the fluid
form. And this result is brought about by a definite quantity of
force. Each of the inorganic materials of which the crust of the
earth is composed seems thus to require in its composition a
definite amount of force.
The life of vegetables is developed in the formation of fresh
compounds of inorganic matter and force. No vegetable can thrive
without sunlight, either direct or diffused. This supplies the
force which the plant combines with carbon, hydrogen, and other
elements to form woody fibre, starch, oils, and other vegetable
products. When we kindle a fire, we dissolve the union which has
thus been formed--the carbon and hydrogen enter into simpler
combinations which require less force to maintain them, and the
superfluous force supplies us with light and heat.
The life of animals is developed by a process exactly the reverse
of vegetable life. It is maintained by the destruction of the
compounds which the vegetable had formed. These compounds are
taken into the body as food, and after undergoing certain
modifications and arrangements are finally decomposed. Of the
force thus set free a part makes its appearance as heat,
maintaining an even temperature in the body, and another part
supplies the power by virtue of which the muscles, &c., act. No
manifestation of animal life is possible except by force thus set
free. It seems all but certain that we cannot think a single
thought without the decomposition of an equivalent amount of the
brain. It must not, however, be concluded that force and life are
identical. Force seems to be only the instrument of which the
higher principle of life makes use in its manifestations.
Force then pervades the whole universe so far as it is cognizable
by our senses. But we cannot conceive of force as acting, without
at the same time conceiving of something on which that force acts.
That something, whatever it may be, we designate "matter." We have
not the slightest idea of what matter really is--no man has ever
yet succeeded in separating it from its combination with force.
Even if success were possible, which seems very improbable, it is
not likely that matter by itself would be discernible by any of
our senses. We know that two of them, sight and hearing, enable us
to perceive certain kinds of motion, i. e. manifestations of
force, and this is in all probability the case with the rest of
them. The existence of matter then is not known by scientific
proof but by inference. Our belief in it arises from something in
the constitution of our minds which makes it a necessary
inference.
There is one more point in reference to force which must be
noticed. It is indestructible, but it is capable of what is termed
"degradation." It may exist in various intensities and quantities,
and a small quantity of force of a higher intensity may be changed
into a larger quantity of force at a lower intensity. In the
instance above given of the union of oxygen and hydrogen, heat is
given out, but heat does not suffice to dissolve that union. The
force must be supplied in the more intense form of Voltaic
Electricity. But to reverse this process seems impossible for us.
As, however, this is clearly explained in a previous volume of
this series, [Footnote: Can we Believe in Miracles? p. 152.] it is
not necessary to dwell upon it at length.
We may conclude then that the whole material universe is built up
of matter and force in various combinations, but we can form no
conception of what these two things are in themselves; they are
only known to us by the effects produced by their union in various
proportions.