HEAT IS MOTION
ART. 60. _Heat is Motion._--On the phenomena of Heat, Newton in his
eighteenth query in _Optics_ asks the questions: "Is not the heat of a
warm room conveyed through the vacuum by the vibrations of a much
subtler medium than air, and is not the medium the same as that medium
by which light is reflected and refracted, or by whose vibrations light
communicates heat to bodies? And do not the vibrations of this medium in
hot bodies, contribute to the intenseness and duration of their heat?
And do not hot bodies communicate their heat to contiguous cold ones by
the vibrations of this medium propagated from them into the cold ones?
And is not this medium exceedingly more rare and subtle than air, and
exceedingly more elastic and active?" Thus it can be seen that Newton
was of the opinion that heat consists in a minute vibratory motion of
the particles of bodies, and that such motion was communicated through
what he calls a vacuum by the vibrations of an elastic medium, the
Aether, which was also concerned in the phenomena of light.
One of the first experimental investigations into the real nature of
Heat was made in 1798 by Count Rumford.
While he was engaged in boring brass cannon in the arsenal at Munich, he
was struck with the degree of heat which the brass gun acquired, and
with the still more intense heat which the metallic chips, which were
thrown off, possessed. Of the phenomena he says: "The more I meditated
on these phenomena, the more they appeared to me to be curious and
interesting. A thorough investigation seemed even to bid fair to give us
a farther insight into the hidden nature of Heat." Rumford therefore set
himself to find out by actual experiments what the nature of Heat was.
For this purpose he constructed a cylinder, and mounted it so that it
could be made to rotate by horse-power. At the beginning of the
experiment the thermometer stood at 60° Fahrenheit, and after
half-an-hour, when the cylinder had made 900 revolutions, the
temperature was found to be 130° Fahrenheit, so that there had been an
increase in the temperature of the cylinder of 70° Fahrenheit. The
experiment was again repeated in another form with similar results.
Rumford in dealing with the results of his experiments said: "It appears
to me to be extremely difficult, if not quite impossible, to form any
distinct idea of anything capable of being excited and communicated, in
the manner the Heat was excited and communicated, in these experiments,
except it be Motion."
Only a year later, Davy gave to the world some results of experiments
which he had performed, by which he had arrived at a similar conclusion
to that of Rumford, viz. that "Heat is motion of some kind." His
experiment consisted of rubbing two pieces of ice together, and by so
doing showed the ice could be melted. He then caused two pieces of metal
to be rubbed together, keeping them surrounded by ice, and still he
found that the two pieces of metal when rubbed together, produced heat,
and melted the ice. He therefore rightly concluded that heat was
produced by friction, and of the experiment adds: "A motion or vibration
of the corpuscles of bodies must necessarily be generated by friction.
Therefore we may reasonably conclude that this motion or vibration is
Heat. Heat then may be defined as a peculiar motion, probably a
vibration of the corpuscles of bodies tending to separate them. It may
with propriety be called a repulsive motion. Now bodies exist in
different states, and those states depend upon the action of the
attractive and of the repulsive powers on their corpuscles, or in other
words, on their different quantities of repulsion and attraction." It
was not, however, till 1812 that Davy confidently stated that "The
immediate cause of the phenomena of Heat is motion, and the laws of its
communication are precisely the same as the laws of the communication of
motion."
The question therefore confronts us, if heat be motion, what is the
particular character of that motion? Is it a vibratory motion as Davy
suggested, or is it similar to the undulatory wave motion of light? I
need hardly point out, that we have evidence in favour of the hypothesis
that light is due to some form of periodic wave motion in the Aether,
the hypothesis being that known as the undulatory theory. We have also
similar evidence in favour of the hypothesis, that heat is also due to
some form of motion of the same aetherial medium. Indeed, it can be
shown that heat possesses all the properties of light, and is subject to
the same laws, with the exception that it cannot affect the sense of
sight.
Heat, then, is due to some motion in the universal aetherial medium,
that not only fills all space, but also forms an atmosphere around every
atom or particle of matter that exists in the universe, and that motion
is generally known as a vibratory or backward and forward motion.
Heat, then, may be said to be due to the vibrations of the Aether that
surrounds all atoms and molecules, and of which those very atoms are
composed, that is if we accept the aetherial constitution of all matter.
So that, whenever a body, whether it be an atom or a molecule, or a
planet or sun or star, is heated in any way whatever, such bodies excite
waves in the surrounding Aether, and these waves travel through the
Aether towards us from the heated body with the velocity of light. When
these waves fall upon any other body, they become more or less absorbed
by the body on which they fall, and cause corresponding vibratory
motions in the same, which give rise to the phenomenon of heat in that
particular body.
It has to be remembered that nothing definite is actually known as to
the character of this vibratory motion. It is called a vibratory motion
because it possesses a periodic vibratory movement, but as to its exact
character, that has not yet been discovered. I hope, however, to
indicate what the motion is that produces heat before the completion of
this work.
ART. 61. _Heat and Matter._--If it be true that heat is due to the
vibrations of the aetherial medium, the question now arises, as to how a
body may become heated, and by so doing be transformed into the three
stages in which matter is found. We have already seen (Art. 36), that
matter may be found in three forms, viz. solid, liquid, and gaseous, and
that all these different forms of matter are composed of minute parts
called atoms. In the case of the solid, the atoms are held closely
together by some strong attractive power, termed cohesion; in the case
of the liquid, the atoms have a greater freedom; while in the gaseous
form they have a greater freedom of movement than when in either the
liquid or the solid state. According to Young's Fourth Hypothesis (Art.
45), we find that all matter, and therefore all atoms have an attraction
for the Aether, by means of which it is accumulated within their
substance, and for a small distance around them in a state of greater
density, and therefore of greater elasticity. In other words, as Aether
is gravitative, every atom possesses an atmosphere of Aether in the same
way that the earth has its atmosphere of air; and further, the aetherial
atmosphere of each atom is densest nearest to the atom, gradually
getting rarer and rarer the further the atmosphere recedes from the
nucleus or centre, the elasticity or pressure being always proportionate
to the density. Professor Challis, in his Dynamical Theory of Light and
Heat, states that all the forces in Nature are different modes of
pressure under different circumstances of the universal Aether, and as
heat is a Force, and therefore a mode of motion, that also must be due
to some form of pressure due to the vibrations of the Aether.
Professor Challis[8] on this point says: "According to this theory, the
atoms of any substance are kept in position of equilibrium by attractions
and repulsions resulting from the dynamical action of the vibrations of
the Aether which have their origin at the atoms. Each atom is the centre
of vibration propagated equally from it in all directions, and that part
of the velocity of the vibration which is accompanied by change of density
(of the Aether) gives rise to a repulsive action on the surrounding
atoms. This action is the repulsion of heat, which keeps the individual
atoms asunder."
With all these facts before us, we are now in a position to account for
the changes of matter which take place when heat is applied to either
a solid or a liquid body. We have already seen (Art. 36) that it is by
the application of heat that matter in its solid form is changed into a
liquid, and from a liquid into a vaporous or gaseous form. It is now for
us to endeavour to form a mental picture as to how this is done.
For example, let us take an iron ball, and apply heat to it, either by
putting it in a furnace or suspending it in some way over an intense
heat. As the heat, which is vibratory motion of the Aether, begins to be
absorbed by the iron ball, it sets the atoms which compose the ball in
motion, urging them to separate, and thus cause the iron ball to expand
and increase in volume. As greater heat is absorbed, so greater motion
among the atoms is the result. So that the motion of heat is tending
all the time to expand the body, while they are held together by the
attraction of cohesion, whatever that may be. As the heat is further
increased, the iron ball begins to assume a liquid or molten form, its
atoms beginning to move about with greater freedom, though held together
by a decreased attractive power. In this condition we now say that it is
in the molten state. Now during all this time, what has the Aether been
doing, or what part has it played in the expansion and changing of the
solid to a liquid? We have to remember, from Art. 60, that wherever
there is motion of any kind or sort, there we have a capacity to do
work, and that the aetherial motion which we term heat is no exception
to this rule. We are now no longer dealing with a frictionless medium,
but with a medium which possesses weight, because it is gravitative, and
consequently possesses inertia also. So that whenever the Aether is set
in motion by flame or heat, its motion would be transmitted by waves of
some kind to the iron ball. These periodic waves, acting upon the mass
of the ball, attack the molecules of the ball and begin to set them in
motion. It is supposed that they are already in motion, as nothing is
absolutely cold, and the motion of the aetherial waves imparts a greater
motion still to the molecules, with the result that the agitation
becomes greater and greater, until at length the agitation becomes so
great, that the molecules break away from the power of attraction that
holds them together, and so begin to move about with greater freedom and
with greater rapidity. It is this state which we call molten. Now if
Aether be frictionless, as has hitherto been supposed, and if heat be
due to the vibratory motions of Aether, the problem confronts us, as to
how the motion of a frictionless medium can do work in expanding a body,
and urging the molecules of a body further and further apart. If the
Aether be frictionless, then the waves of Aether known as aetherial heat
waves ought to pass between the atoms as water passes through a sieve,
or wind passes through a forest. Yet it is assumed that the vibratory
motions of a hot body are caused by vibrations of the periodic waves of
the Aether, which act upon the molecules of the body; and, in order for
such an assumption to be consistent with the results, the only possible
conception that can be accepted of the Aether, is that it is
gravitative, and consequently possesses mass and inertia, and therefore
has a capacity not only to accept motion, but also to transmit motion to
another body, and impart the motion which it has accepted to a colder
body.
By imparting such motion, it increases the motion of the cold body, and
gradually changes its state from a solid to a liquid condition. Here,
then, from the realm of heat we have another argument in favour of the
fact that Aether is gravitative, and therefore possesses mass and
inertia.
In the experiment of reducing the iron ball from a liquid state, so to
speak, to a vaporous condition, we have practically a continuation of
the same process, only that greater heat or greater aetherial motion is
required, and whereas in the previous experiment the molecules of the
ball were acted upon, in this case the atoms are more directly acted
upon by the Aether waves. In all these processes it suggests itself to
me that the aetherial atmosphere must take its share in the expansion
and transformation of the liquid form into a gaseous form, or the solid
into a liquid form. Taking the analogy of our atmosphere in its relation
to the earth, we know that when heat is absorbed by it, it expands, the
result being that a greater pressure is exerted by the expanding
atmosphere, than would be exerted if it remained at the same temperature
all the time. If, therefore, each atom has an aetherial atmosphere,
which is capable of expansion, then the effect of the absorbed aetherial
motion of the heat waves on each atomic atmosphere must be to expand it,
and thus there will be a pressure _away from_ the atom, because of the
increased elasticity acquired by the heated aetherial atmosphere. So
that the expansion of the liquid is due to the increased elasticity of
the aetherial atomic atmosphere, which has been expanded by heat, and
which exerts an increased pressure on neighbouring atoms, thus seeking
to push them farther away from each other. There are other motions of
the atoms themselves in addition to this to be considered, but I am now
seeking to show only the effect of the aetherial atmosphere of each atom
upon the neighbouring atoms. This would give each atom a larger sphere
of freedom in which to move, and that state would then be called a
gaseous and not a liquid one. This assumption of the part which the
aetherial atmosphere plays in the expansion of a body is therefore in
agreement with Professor Challis' theory of heat already referred to, in
which he states that heat gives rise to aetherial vibrations which act
repulsively on the neighbouring atoms. In further confirmation of the
existence of these aetherial atmospheres that exist around atoms, I
would like to draw the attention of the reader to a theory of heat given
to the world by Rankine, _Phil. Mag._, 1851. His theory is known as the
"Hypothesis of Molecular Vortices."
He assumed that "each atom of matter consists of a nucleus or central
point, enveloped by an elastic atmosphere, which is retained in its
position by attractive forces, and that the elasticity due to heat
arises from the centrifugal force of those atmospheres revolving or
oscillating about their nuclei or centres."
Now in this assumption we find that he admits that each atom has an
atmosphere, such atmosphere evidently being an aetherial one, and in
that case the hypothesis would agree with the statement in Art. 46, that
every atom possesses an aetherial atmosphere. He further points out that
the atmosphere is retained in its position by attractive forces. This is
also in harmony with the hypothesis given in Art. 45, which proves that
Aether is gravitative, and therefore the atmosphere of the atom would be
held in its position by the attractive force of Gravitation, as
suggested by Young in his Fourth Hypothesis.
Further, he goes on to show that the elasticity of the atomic atmosphere
is proportionate to its density, which is also in conformity with the
statement made in Art. 47, and is also in accordance with Boyle's Law.
Then he goes on to prove that the quantity of heat in a body is measured
by the molecular revolutions of the vortices.
He does not clearly define the exact character of those molecular
vortices, but I take it to mean that each atmosphere is in a state of
revolution around its atomic centre, in the same way that the atmosphere
of a planet is in a state of revolution around its central body.
Such an assumption is entirely in harmony with experience, as there is
an analogy for its assumption from the planetary system; and if an atom
is a world in miniature, as I believe it to be, then the atmosphere of
the atom ought to revolve around its central nucleus in the same way
that the atmosphere of a planet revolves around its nucleus or central
body.
He then deals with temperature, and with the pressure of gases caused by
heat, showing the relation of elasticity and pressure to temperature in
a table of results given in the _Phil. Mag._ for 1851. I must refer the
reader to the paper itself for fuller details. Thus from one of the
greatest thinkers of modern times we have further testimony to the
hypothesis that Aether is matter and is therefore gravitative, and
because of its gravitating tendency, it forms around every atom and
molecule elastic envelopes or atmospheres, whose pressure is always
proportionate to their density.
[Footnote:8 _Phil Mag._, 1859.]
ART. 62. _Radiation and Absorption._--We have already seen (Art. 31)
that all matter is made up of atoms and molecules, each of which is
surrounded by its atmosphere of Aether. By means of the Aether, motion
in the form of light and heat may be transmitted from one atom and
molecule to another. The transmission of heat from one body to another
is termed Radiation, while the acceptance of heat is termed Absorption.
Tyndall defines Radiation as "the communication of molecular motion from
the heated body to the Aether in which it is immersed,"[9] and
Absorption, therefore, would be the acceptance of motion by the body
from the Aether. So that in Radiation, the atom, molecule, or body parts
with motion to the Aether, while in Absorption it gains motion from the
Aether.
Now in order for us to understand this theory of Radiation and
Absorption, it will be well for us if we look at a similar effect in the
sphere of music and sound. Let us suppose that we have two tuning-forks
of the same pitch, placed on a table at a distance of a foot from each
other. If we set one of the forks vibrating, the waves which it radiates
through the air will fall upon the other one, and will also set it in
vibration, because they are of the same period or size as those waves
which it would itself give off when sounded. Thus while one is losing
its motion, the other is gaining it, or while one is radiating motion,
the other is absorbing motion. This can readily be proved by stopping
the vibration of the first fork, when it will be found that the second
fork is now giving out a similar note to the first, although it was
silent at the commencement. Thus we have here an example of radiation
and absorption of sound, the success of the experiment depending upon
the fact that both forks shall have the same pitch. Again, it must be
noted, that if we have two tuning-forks both of which are of the same
pitch, and both vibrating at the same time, then, while one is radiating
sound and consequently losing motion to the other, yet at the same time
it is absorbing motion from the other. Because, if fork A can transfer
motion to fork B, the latter can equally transfer its motion to fork A,
and when both are vibrating together, each is the recipient of part of
the other's motion, while at the same time giving off motion in the form
of sound waves itself. So that the power of a fork to radiate sound
waves equals its power to absorb sound waves. If now we apply this
simile to the atomic and molecular world, we shall be able to form a
mental picture as to what takes place in radiation and absorption.
All atoms and molecules are ever in a state of ceaseless motion, ever
moving, never still. All are creating Aether waves which move away with
the velocity of light. If, in the transmission of the waves by the
Aether, they fall upon another atom which can emit a wave of similar
length, in the same way that two tuning-forks emitted sound waves of the
same length, then the atom upon which the waves strike will be set in
vibration, as the second tuning-fork was set in vibration by the first.
We shall look again at the principle of wave motion in the next chapter.
Further, from the simile of the two forks, which absorb sound at the
same time that they radiate sound, we learn that an atom or body
radiates heat waves at the same time that it is absorbing heat waves.
Suppose that we have two bodies at equal temperatures, it must not be
thought that the radiation or absorption has ceased, for, according to
the simile used, they both still continue to vibrate and emit the
aetherial heat waves; but where we get equality of temperatures, there
we get equality of radiation and absorption. Before this equality of
temperatures, however, is reached, the hotter body will radiate more
heat waves than it absorbs, while the colder body will absorb more heat
waves than it emits. All bodies, whatever their temperature, are
incessantly radiating heat waves. This may be proved experimentally with
proper apparatus, as for example with an instrument known as the
thermopile. When, however, the total heat waves radiated out by a body
are less than it absorbs, the body gets gradually colder, and the
temperature decreases. So long as this is continued, so long will the
body continue to get colder and colder, until it arrives at the same
temperature as the surrounding bodies, at which point the total heat
waves radiated out will equal the total heat waves absorbed, and at that
point the temperature of the body will remain constant.
This aspect of temperature was first introduced by Prevost of Geneva in
1792, in an article in which he tried to explain the radiation from a
cold body. According to his reasoning, a body is not simply regarded as
radiating heat when its temperature is falling, or absorbing heat when
it is rising.
What he tried to make clear was, that both radiation and absorption were
going on at one and the same time; the radiation depending upon the body
itself, but the absorption depending upon the nature of the body. While
radiation and absorption are thus reciprocal, which implies that a good
radiator is a good absorber, and a bad radiator is a bad absorber, it
does not follow that all bodies radiate and absorb alike.
The capacity of bodies to radiate and to absorb differ considerably. Dr.
Franklin made several simple experiments to prove the relative powers of
radiation and absorption with several pieces of cloth. These were put
out on the snow, and exposed to the heat of the sun. He found that the
pieces which were dark in colour sank deepest into the snow, while those
which were lightest in colour sank the least. From this he inferred that
the darkest pieces were the best absorbers, and therefore the best
radiators, while the light-coloured cloths were the worst absorbers, and
therefore the worst radiators.
Radiation, therefore, may be said to be the propagation of a wave motion
through the Aether; and, as all motion is a source of power or energy,
we have in the radiation of heat from one body to another by the
aetherial waves, the transmission of a motive power capable of doing
work, either internal work as increasing the temperature of the molecule
or body, or external work as separating the atoms, or driving them
further apart. It can readily be seen that if the Aether were
frictionless, as has generally been supposed, the Aether could not have
any motive power at all, and therefore could not transmit heat from one
body to another. Professor Tyndall[10] on this point says, referring to
the cooling of a red-hot ball: "The atoms of the ball oscillate in a
resisting medium, which accepts their motion and transmits it on all
sides with inconceivable velocity." Now in the previous quotation given
in this article from the same authority, he states that the atoms are
immersed in the Aether. So that evidently in his opinion the Aether and
the resisting medium are one and the same. So that our assumption of the
gravitative property of the Aether is perfectly in accord with Professor
Tyndall's conception of the Aether, in so far as it concerns the
propagation of heat waves; and, as will be shown later on, heat and
light waves are due to the same physical agent--that is, the Aether;
therefore, wherever we get heat and light, there, according to Professor
Tyndall's statement, we must have a resisting medium, and as Aether
fills all space, the resisting medium must fill all space. This is
perfectly in accord with our assumption that the Aether is gravitative
and possesses inertia--that is, the capacity to receive and to impart
motion, and being gravitative it possesses mass or weight, which is the
very quality necessary for the existence of a resisting medium.
[Footnote 9: _Heat, a Mode of Motion._]
[Footnote 10: _Heat, a Mode of Motion._]
ART. 63. _Heat is a Repulsive Motion._--Whatever be the particular
character of the vibratory motion of the Aether termed heat, there is
one fact regarding the same that is very patent and obvious to all; and
that is, that the vibratory motion of heat is essentially a repulsive
motion, or a motion from a centre and not one to a centre.
Professor Davy points this out (Art. 60) where he says of heat, "It may
with propriety be called a repulsive motion," while Professor Challis
(Art. 61) states that "Each atom is the centre of vibrations propagated
from it equally in all directions, which give rise to a repulsive action
on the surrounding atoms. This action (he adds) is the repulsion of heat
which keeps the individual atoms asunder."
There have been many experiments undertaken which go to prove that a
repulsive action between atoms and molecules is produced by heat. It has
been demonstrated that certain coloured rings, known as Newton's rings,
change their shape and position when the glasses between which they
appear are heated, thus indicating the presence of a repulsive power due
to the increased heat. If we consider the change of state that heat
induces in matter, as, for example, from solid to a liquid, or liquid to
a gaseous form, we are compelled to admit that heat possesses an
expanding and therefore a repulsive motion. It is almost an universal
law that heat expands and cold contracts, and the greater the heat
absorbed, the greater the expansion. In the case of a solid being
converted into a liquid, a much greater heat or repulsive motion is
required to separate the particles, on account of the power of cohesion
being greater in the solid than in the liquid. As Professor Tyndall[11]
states when dealing with the stability of matter from the molecular
standpoint: "Every atom is held apart from its neighbour by a force of
repulsion. Why then do not the mutually repellent members of the group
part company? The reason of this stability is that _two_ forces, the one
attractive and the other repulsive, are in operation between every two
atoms, and the position of every atom is determined by the equilibration
of these two forces. The points at which attraction and repulsion are
equal to each other is the atom's position of equilibrium. When the
atoms approach too near each other, repulsion predominates and drives
them apart; when they recede to too great a distance, attraction
predominates and draws them together." If, therefore, there are TWO
forces at work in the atomic world, viz. attraction and repulsion, then
the question arises, Can that repulsive power be increased in any way,
and if so, by what means? Such repulsive motion, as experiment and
experience teach us, can be increased, and such increase may be derived
from the absorption of heat which gives rise to increased atomic motion,
and so to increased aetherial motion away from the atom, by which the
repulsive action of one atom upon another is increased. Thus an atom's
repulsive power may be increased by heat; the greater the heat absorbed,
the greater the repulsive power that any atom or body exerts upon a
neighbouring atom or body. We can therefore understand how it is, that a
body when changed from a solid to a liquid condition occupies a larger
space in the latter condition than in the former; or why a body when
changed from a liquid to a gaseous condition occupies a still larger
volume in the latter than in its previous condition. The expansion in
both cases is essentially the result of the increased repulsive motion
that has been imparted to its atoms or molecules by the increased heat,
and this increased repulsive power has overcome the attractive power of
the atoms or molecules, with the result that they have been driven
further and further apart, until, in the gaseous state, the atoms may
be very far apart indeed. Wherever, therefore, we have heat of any kind,
there we have a repulsive motion, such motion being proportionate to the
heat radiated, that is, the aetherial waves propagated by the body. If,
therefore, in the atomic world we find a repulsive motion, which is due
to the vibratory motions of the Aether generated by heat, the question
now confronts us, as to whether in the solar system, and indeed all
through the universe, there is not the same repulsive motion from a
central body due to the wave motions of the Aether termed Heat.
May we not find in the repulsive power of heat in the atomic world, an
indication of that very power for which we are seeking in the solar
system--that is, a Centrifugal Force or motion which is the exact
opposite of the Centripetal Force or attractive power of Gravitation?
For if heat be a repulsive motion at all, then to be strictly logical it
must be equally repulsive in relation to large masses, the sun and the
planets for example, as it is in the atomic world, otherwise we have a
phenomenon in Nature which contradicts itself, which assumption would be
contrary to the simplicity which is to govern our philosophy, and also
contradictory to experience, which is the primary factor of
philosophical reasoning. Now what are the facts with reference to the
sun, which is the central body of our solar system, and the source of
all light and heat in that system? We will look at this aspect of the
question under the heading of Radiant Heat.
[Footnote 11: _Heat, a Mode of Motion._]
ART. 64. _Radiant Heat._--The source of all light and heat, not only of
our earth, but also of all the other planets, is to be found in the sun.
We have therefore to deal, not with an atom which is generating heat
waves on every side, but with a globe about 860,000 miles in diameter,
and with a circumference of over 2,700,000 miles. This huge orb consists
of a central body, molten or partly solid, with a temperature so hot
that it is almost impossible to conceive its intensity. The quantity of
heat emitted by the sun has been ascertained by Sir John Herschel from
experiments made at the Cape of Good Hope, and by M. Pouillet in Paris.
Sir John Herschel found that the heating power of the sun when it was
directly overhead was capable of melting .00754 of an inch of ice per
minute. According to M. Pouillet the quantity was .00703 of an inch,
which is equal to about half-an-inch per hour. From these results it has
been calculated that if the direct heat of the sun were received upon a
block of ice one mile square, 26,000 tons would be melted per hour by
the heat which would be absorbed. Again, as Herschel[12] puts it:
"Supposing a cylinder of ice, 45 miles in diameter, to be continually
darted into the sun with the velocity of light, the heat given off
constantly from the sun by radiation would be wholly expended in
liquefaction on the one hand, while on the other, the actual temperature
at the sun's surface would undergo no diminution." Sir John Herschel
further says: "All the heat we enjoy comes from the sun. Imagine the
heat we should have to endure if the sun were to approach us, or we the
sun, to a point the one hundred and sixtieth part of the present
distance. It would not be merely as if 160 suns were shining on us all
at once, but 160 times 160 suns according to the rule of inverse
squares--that is, 25,600. Imagine a globe emitting heat 25,600 times
fiercer than that of an equatorial sunshine at noonday, with the sun
vertical. In such a heat there is no solid substance we know of which
would not run like water, boil, or be converted into smoke or vapour."
Lockyer points out that the heat radiated from every square yard of the
sun's surface is equal to the amount of heat produced by the burning of
six tons of coal on that area in one hour. Now the surface of the sun
may be estimated at 2,284,000,000,000 square miles, and there are
3,097,600 square yards in each square mile; what therefore must be the
number of tons of coal which must be burnt per hour to represent the
amount of heat radiated from the sun into space? The approximate result
may be calculated by multiplication, but the figures arrived at fail to
give any adequate conception of the actual result.
From these facts it may be seen that the sun has a temperature far
exceeding any temperature that can be produced on the earth by
artificial means. All known elements would be transformed into a
vaporous condition if brought close to the sun's surface. It may readily
be seen, therefore, that the sun is constantly sending forth an
incessant flood of radiant heat in all directions, and on every side
into space. Now if heat be motion, and be primarily due to the vibratory
motion of Aether, what must be the volume and the intensity of the
aetherial waves, known as heat waves, generated by the sun? When we
remember its ponderous mass, with its volume more than 1,200,000 times
that of our earth, its huge girth of more than 2-1/2 millions of miles,
and this always aglow with fire the most extensive known--fires so
intense that they cover its huge form with a quivering fringe of
flames which leap into space a distance of 80,000 miles, or even
100,000 miles, or over one-third of the distance of the moon from the
earth,--remembering all these facts, what must be the volume and
intensity of the aetherial heat waves which they generate and send upon
their course into space on all sides! What a very storm of energy and
power must there be in this aetherial atmosphere which exists around the
sun's huge form, and with what volume of power must the aetherial heat
waves speed away from so great a generating source! Some idea as to
their velocity of motion may be gained by the fact, that these aetherial
heat waves traverse the distance of 92,000,000 miles between the sun and
our earth in the short space of 8-1/2 minutes. With such a velocity of
motion as that, and with the fact before us that all motion is a source
of energy or power, what must be the energy possessed by these heat
waves! There must, therefore, be a power in these aetherial heat waves
which is strictly proportionate to their intensity and flow. So that,
whenever they come into contact with any body, as a planet, as they flow
outwards from the sun, they must exert a power upon such a planet which
is directed _away_ from the sun, and therefore act upon that planet by
the energy of their motion away from the sun, the source of the
aetherial heat waves. Therefore, not only in the atomic world is heat a
repulsive motion, but equally in the solar world, which is but an atomic
world on a large scale, the same principle prevails, and the effect of
radiant heat is essentially a repulsive, that is, a centrifugal motion,
as it is always directed from the central body, the sun.
Further, it can be shown that the repulsive power of heat in the solar
system has already received the attention of scientists, especially in
France. This will be seen more fully when we come to deal with the
phenomena of comets' tails. One remarkable feature about comets' tails
is, that they are always directed away from the sun, and various
hypotheses have been advanced to account for that fact. Among them is
the hypothesis of M. Faye, in which he assumes that there is a repulsive
force which has its origin in the heat of the sun. This repulsive force
is not propagated instantaneously, but the velocity of propagation is
the same as that of a ray of light. By means of this repulsive power due
to the heat of the sun, M. Faye explains how it is that the tails of
comets are always turned away from the sun. Here, then, we have an
indication of the existence of this repulsive force of heat which we are
considering--a repulsive power which finds its source in the aetherial
waves, which give rise to the phenomena of Heat, and to which we must
look for the ultimate source of that repulsive power or Centrifugal
Force which is to form the complementary power to the attractive force
of Gravitation.
[Footnote 12: _Lectures on Scientific Subjects._]
ART. 65. _Direction of Ray of Heat._--The question as to the path which
a ray of heat takes may best be attacked by finding out what is the path
which a ray of light takes in its progress through the Aether. When we
come to deal with light, we shall find that it has been experimentally
proved that the path of a ray of light is that of a straight line
through space; so that if we have any body emitting light, the rays of
light will proceed from that body in straight lines, with decreasing
intensity, according to the law of inverse squares, the same as
Gravitation.
It can readily be shown, that wherever there is light there is heat. For
example, the radiant heat from the sun proceeds through space along with
the light from the sun, and when one set of waves, the light waves for
instance, are intercepted, the heat waves are also intercepted. Or, to
take another illustration, when the sun is eclipsed, we feel the sun's
heat as long as any portion of the sun is visible, but as soon as the
sun is totally eclipsed, then the light waves disappear, and with it the
heat waves. From this we can readily see, that not only do the heat and
light waves from the sun proceed in the same straight line, but that
they also travel at the same rate through space, at the rate of 186,000
miles per second. Then again the common lens, which is so familiar to
every one, will prove the same fact by concentrating the rays of light
to a focus, and by so doing will produce sufficient heat to burn a piece
of paper, or even set fire to wood. If, therefore, the path of a ray of
light be that of a straight line, proceeding from the luminous or
lighted body, and the path of a ray of heat coincides with the path of a
ray of light, the path of the ray of heat must also be in the direction
of a straight line from the heated or luminous body, which, as we shall
see in a subsequent article, also decreases in intensity according to
the law of inverse squares the same as Gravitation Attraction.
Professor Tyndall, on the direction of a ray of heat,[13] states his
opinion on the matter as follows: "A wave of Aether starting from a
radiant point in all directions in a uniform medium constitutes a
spherical shell, which expands with the velocity of light or of radiant
heat. A ray of light or a ray of heat is a line perpendicular to the
wave, and in the case here supposed, the rays would be the radii of the
spherical shell." From this it can be seen that a ray of light or heat
corresponds to what is known as the radius vector of a circle (Art. 20),
and therefore a ray of light and heat takes exactly the same path
through space (if we consider the sun as the source of the light and
heat) as the path of the attractive power of Gravitation. Collecting,
therefore, our results from the preceding articles of this chapter, we
learn that heat is due to vibrating wave motion of the Aether, and that
that motion is a motion which is always directed from the central body
which is the source of the heat; and further, that this motion amounts
to a repulsive motion acting in an opposite direction to the attractive
power of gravity or to the centripetal force of Gravitation. What is
more remarkable still, the path of a ray of heat corresponds with, and
takes up exactly the same direction through space, whether it be atomic
space, solar space, or interstellar space, as the attractive force of
Gravitation.
Looking at the subject from the standpoint of the solar system, with the
sun as the central body, we see that while we have the sun, which acts
as the controlling centre of the particular system of planets, holding
all the planets in their orbits by its attractive power, yet at the same
time it is also the source of all light and heat. Now heat being due to
the wave motion of the aetherial medium, such motion being always
exerted from the central body, we arrive at the only legitimate
conclusion that can be arrived at, viz. that the sun is also the source
of a repulsive motion, which motion coincides with the path that the
attractive power of Gravitation takes, that is, along the radius vector
of the circle, as shown in Art. 20.
ART. 66. _Law of Inverse Squares applied to Heat._--The law of inverse
squares which governs not only the Law of Gravitation Attraction (Art.
22), but also electricity and light, is equally applicable to the
phenomena of heat, so that we say the intensity of heat varies inversely
as the square of the distance. Thus, if we double the distance of any
body from the source of heat, the amount of heat which such a body
receives at the increased distance is one-quarter of the heat compared
with its original position. If the distance were trebled, then the
intensity of the heat would be reduced to one-ninth; while if the
distance were four times as great, the intensity of the heat would only
be one-sixteenth of what it would receive in its first position. This
may be proved from experiments as given by Tyndall in his _Heat, a Mode
of Motion_.
Let us apply the law of inverse squares in relation to heat to the solar
system, and see what the result gives. In our solar system, we have the
sun as the central body, the source of all light and heat, with the
eight planets, Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus,
Neptune, describing orbits around the central body, and at the same time
receiving from it the light and heat which the sun is ever pouring
forth into space. The mean distance of Mercury from the sun is about
36,000,000 miles, while that of the Earth is about 92,000,000 miles, so
that reckoning the distance of Mercury as unity, the distance of the
Earth is a little more than 2-1/2 times that of Mercury from the sun.
Now the square of 2-1/2 is 25/4, and that inverted gives us 4/25, so
that according to the law of inverse squares, the intensity of heat at
the Earth's distance from the sun is 4/25 of what the intensity of heat
is at the mean distance of Mercury. Again, the mean distance of Mars is
141,000,000 miles, while the mean distance of Saturn is 884,000,000
miles, and taking Mars' distance from the sun as unity, the distance of
Saturn would be represented by 6-1/4. Now the square of 6-1/4 is
(25/4)^{2} which gives 625/16 and the inverse of that is 16/625, so that
the intensity of heat at the distance of Saturn's mean distance from the
sun, in comparison with the intensity of heat at Mars' mean distance,
would be about 16/625; or in other words, the heat received by Saturn
would be only 16/625 of the intensity of heat received by the planet
Mars. In Art. 63 we have seen that heat is a repulsive motion, being a
wave motion of the Aether which is propagated from the heated and
central body, which in this case is the sun. Therefore, according to the
law of inverse squares from the standpoint of heat, we find in the solar
system a repulsive motion, due to the wave motion of the Aether, which
is always exerted away from the sun in the same path that the
centripetal force takes, and which like that force diminishes in
intensity inversely as the square of the distance. So that, wherever the
centripetal force, or the attractive force of Gravitation, is diminished
on account of the increased distance from the sun, the repulsive motion
due to heat is also diminished in exactly the same proportion and along
exactly the same path. If at any point in the solar system the
attractive force is doubled, then according to our repulsive theory of
heat, and the law of inverse squares, the repulsive motion is also
doubled. If the attractive force is halved, then the repulsive motion is
halved also, the repulsive motion being always and at all places exactly
proportional to the increase or decrease of the attraction of
Gravitation.
[Footnote 13: _Heat, a Mode of Motion._]
ART. 67. _First Law of Thermodynamics._--The Law of Thermodynamics is
based on two fundamental truths which have reference to the conversion
of Heat into Work, and Work into Heat. In Art. 54 we have already seen
that energy in the form of heat, light, electricity and magnetism is
capable of being converted into other forms of energy, while in Art. 59
we have seen that Joule gave us the exact relation in foot-pounds
between heat and work. He showed that when 1 lb. of water fell through
772 feet its temperature was raised one degree Fahr. Thus the principle
underlying the first law of thermodynamics states, that whenever work is
spent in producing heat, the amount of work done is proportionate to the
quantity of heat generated; and conversely, whenever heat is employed to
do work, a certain amount of heat is used up, which is the equivalent of
the work done. This principle is also in accord with the conservation of
Energy and Motion (Arts. 52 and 57), which assert that whenever energy
or motion disappears in one form, it is manifested in some other form.
Thus, from the first law of thermodynamics, we learn that wherever we
have heat we have the power to do work, and the amount of work so done
is proportionate to the heat used up. Heat, then, has a capacity to
perform work, and that power is known as the mechanical equivalent of
heat. Both Mayer of Germany, and Dr. Joule of Manchester, have worked
out this problem, and have given us the mechanical value of heat. By
experiments Mayer found out that a quantity of heat sufficient to raise
1 lb. of water one degree Fahr. in temperature was able to raise a
weight 771.4 lb. one foot high. Dr. Joule of Manchester, after making a
number of experiments which lasted over many years, came to the
conclusion that the mechanical equivalent of a unit heat was 772
foot-pounds, a unit of heat being the quantity of heat which would raise
1 lb. of water one degree Fahr. So that if a 1-lb. weight fell from a
height of 772 feet, an amount of heat is generated which would raise 1
lb. of water one degree Fahr.; and conversely, to lift 1 lb. 772 feet
high, one degree Fahr. of heat would be consumed.
Now if this law of thermodynamics is true, it must not only be true in
relation to terrestrial heat, or heat produced by artificial means on
our earth, but it must equally hold good in relation to the solar
system; and not only the solar system, but equally true throughout all
the systems of worlds that flood the universe. So that wherever we get
heat in the universe, in the solar system for example, there, according
to our first law of thermodynamics, we should have the capacity to do
work of some kind or other. That work may take either the form of
expanding a body, as the atmosphere of a planet for example, or it may
take a mechanical form, that is, actually moving a body by the increased
pressure due to aetherial heat waves generated by the sun. We have
already seen in Art. 64, on Radiant Heat, what a store of heat the sun
has. For thousands and millions of years the sun has been pouring forth
its heat rays into space, and yet its temperature does not seem to be
diminished. The great Carboniferous or coal period of past geological
times is an indication of the heat and light of the sun, which it must
have radiated out millions of years ago; and year by year, these
aetherial heat waves are still being poured forth by the sun on every
side into space, so that no matter where a planet may be in its orbit,
there it may be the recipient of these aetherial heat waves which break
upon its surface. Now if there be this quantity of heat existing in the
sun, and heat according to the first law of thermodynamics has a
mechanical value, which is that it can push or lift a body through
space, the question arises, as to what is the mechanical value of this
heat of the sun? Are we to suppose that if one unit of heat can lift 1
lb. 772 feet, the millions and millions of units of heat which are
constantly being poured out of the sun into space are doing no work at
all? Such an assumption is not only contrary to that simplicity which
governs our Philosophy, but is entirely opposed to experience, which is
the very foundation of all philosophical reasoning. If, therefore,
experience is to be any guide at all, we are compelled to come to the
conclusion that the heat poured forth into space does do work on the
bodies, as comets, meteors, planets, upon which the aetherial heat waves
fall. The problem is, what is the character of the work done? I have
already indicated part of the work, viz. in the expansion of the
atmosphere of the planets. Then there is also the reception of the heat
by the animal and vegetable life of the planet, but these do not account
for all the motive power of the aetherial waves, which break upon the
planet or its atmospheres.
The true solution of the first law of thermodynamics, in its relation to
the solar system, seems to me to be found in the fact already stated in
Art. 63, viz. that heat is a repulsive motion, and the law of
thermodynamics confirms that statement, and shows that the work done on
a planet by the aetherial heat waves is that of pushing it, or urging it
by their very energy and motion away from their controlling centre, the
sun. This would practically amount to a repulsive force which had its
home in the sun, and this conception would bring our Philosophy into
harmony with our experience, which teaches us that wherever there is
heat there is the capacity of doing work, the amount of work being
proportionate to the heat generated and consumed.
ART. 68. _Second Law of Thermodynamics._--This law was enunciated by
Sadi Carnot in 1824, when he wrote an essay on the Motive Power of Heat.
Previous to the time of Carnot no definite relation seems to have been
suggested between work and heat; Carnot, however, discovered what were
those general laws which govern the relation between heat and work. In
arriving at his conclusion, he based his results on the truth of the
principle of the conservation of energy already referred to (Art. 52).
Carnot started his reasoning on the assumption that heat was matter, and
therefore indestructible. The two great truths in relation to heat and
work, enunciated by Carnot, are known as, first, a Cycle of operations;
and, secondly, what he termed a Reversible Cycle. In order to be able to
reason upon the work done by a heat-engine, say a steam-engine for
example, Carnot stated we must imagine a cycle of operations, by which,
at the end of such operations, the steam or water is brought back to
exactly the same state in which it was at its start. He calls this a
cycle of operations, and of it he says, that only at the conclusion of
the cycle are we entitled to reason upon the relation between the work
done and the heat spent in doing it. His other idea of the reversible
cycle implies that an engine is reversible when, instead of using heat
and getting work from it, the engine may be driven through the cycle of
operations the reverse way, that is, by taking in work, it can pump back
heat to the boiler again. Carnot showed that if you can obtain such a
reversible engine, it is a perfect engine. All perfect engines, that is
all reversible engines, will do exactly the same amount of work with the
same amount of heat, the amount of work being strictly proportionate to
the amount of heat consumed. I need hardly point out that the reversible
engine, or the perfect engine of Carnot, is only the ideal one, as there
is no engine in which all the heat is converted into work, as a great
deal of the heat is radiated away and not converted into work at all.
Again, working from the standpoint that heat is matter, Carnot reasoned
that in the heat-engine the work is performed, not by the actual
consumption of heat, but by its transportation from a hot body to a cold
one. Thus, by the fall of heat from a higher to a lower temperature,
work could be done in the same way that work could be done by allowing
water to fall from a higher to a lower level. The quantity of water
which reaches the lower level is exactly the same as that which leaves
the higher level, as none of the water is destroyed in the fall. He
argued, therefore, that the work produced by a heat-engine was produced
in a similar manner, the quantity of heat which reaches the condenser
being supposed to be equal to that which left the source. Thus the work
was done by the heat flowing from a hot body to a cold one, and, in
doing this work, it lost its momentum like falling water, and was
brought to rest. One of the most important points noted by Carnot is
the necessity that, in all engines which derive work from heat, there
must be two bodies at different temperatures, that is, a source and a
condenser, which correspond to a hot and cold body, so that there may be
the passage of heat from the hot to the cold body. In order to get work
out of heat it is absolutely necessary to have a hotter and a colder
body. From this reasoning we learn, therefore, that work is obtained
from heat by using up the heat of the hotter body, part of which is
converted into actual work, while part is absorbed by the colder body.
So that wherever we have two bodies at different temperatures, according
to the second law of thermodynamics, there we have the power of doing
work by the transmission of heat, from the body of higher to the one of
lower temperature.
That Carnot ultimately came to believe in the dynamical theory of heat,
is proved by the following passage taken from his notes on the Motive
Power of Heat: "It would be ridiculous to suppose that it is an emission
of matter, while the light which accompanies it could only be a
movement. Could a motion produce matter? No! undoubtedly, it can only
produce a motion. Heat is then the result of motion. It is plain then
that it could be produced by the consumption of motive power, and that
it could produce this power. Heat is then simply motive power, or rather
motion which has changed its form. It is a movement among the particles
of bodies. Wherever there is a destruction of motive power, there is at
the same time production of heat in quantity exactly proportional to the
quantity of motive power destroyed. Reciprocally, whenever there is
destruction of heat there is production of motive power."
Let us apply this principle to the solar system, and endeavour to find
out whether in that system we have, in relation to the heat thereof,
either a cycle of operations or a reversible cycle. We have again to
consider the sun as the source of all light and heat in the solar
system, radiating forth on every side, year by year, the countless units
of heat which go to form the continuance of all planetary life and
existence. One of the problems that has confronted scientific men for
many years is this, Where does the sun get its supply of heat from? When
we remember the incessant loss of heat which the sun suffers through its
radiation of heat into space, we are compelled to ask, How is that
supply maintained, and how has it been kept up through the countless
ages of the past? Several suggestions have been made, and several
theories advanced to account for the fact. Mayer, of Germany, suggested
that the heat is partly maintained by the falling into the sun of
meteors, which, like comets, pursue a path through the heavens, and are
subject to the attractive influence of the sun. In the combustion of
these meteorites, or meteors, he contended there were the means by which
the light and heat of the sun might be maintained. Whatever theory,
however, may be suggested as to the maintenance and the source of the
continuity of the sun's heat, I do not think it has been suggested by
any scientist that the heat emitted and radiated by the sun is ever
returned in any way back to the sun from infinite space, whether by
reflection or by any other method. So far as I can learn, there are no
facts in connection with the solar system which would lead us to make
that assumption. On the contrary, experience and experiment teach us
that radiation implies loss of heat, and that the body, which so
radiates, ultimately becomes cold, unless its internal heat is kept up
by some means or other. So that the terms introduced by Carnot in the
second law of thermodynamics, viz. that of a Cycle of Operations and of
a Reversible Cycle, do not apply to the solar system, and the solar
system, viewed from the standpoint of a machine, with the sun as the
source of the heat, does not represent a perfect engine, that is, all
the heat is not used up in doing work, some of it being radiated out
into space. Wherever, however, the heat, that is the aetherial heat
waves generated by the sun, comes into contact with a planet, as
Mercury, Venus, or Jupiter, then, in accordance with Carnot's reasoning,
work is done. Carnot points out that, in order for work to be done, we
must have a source and a condenser, that is, two bodies at different
temperatures, a hot body and a cold one. Now these conditions of work
are satisfactorily fulfilled in the solar system, and as a result work
is performed. We have the sun with its huge fires, and its intensity of
heat, representing the source or the hot body, while every planet and
every meteor and comet, that come under its influence, represent the
cold body, and between the two work is always going on. That work is
represented by the repulsive power of heat, which I have already
indicated, so that, viewed from Carnot's standpoint with relation to the
motive power of heat, we find that there are in the solar system those
conditions which govern work, and by which, from a mechanical
standpoint, work is performed; further, that work takes the form of a
repulsive power on every planet or other body upon which the aetherial
heat waves fall. Therefore, from the second law of thermodynamics we
have another proof of this repulsive power of heat already indicated and
referred to in Art. 63.
ART. 69. _Identity of Heat and Light._--We have seen from the preceding
articles of this chapter, that heat is due to a periodic wave motion of
the Aether, and in the succeeding chapter we shall also see that light
is due to some kind of periodic wave motion in the Aether. So that not
only heat, but light also, it would appear, is due to certain periodic
wave motions that are set up in the Aether by the vibrations of hot or
luminous bodies. The question therefore arises, how many wave motions
are there in the Aether? Are there different wave motions which in one
case produce light, and in the other case produce heat, or are light and
heat both produced by the same set of aetherial waves? The identity of
light waves with heat waves is manifested by the fact that wherever we
get light we get heat, as can be proved in many ways. One of the
simplest proofs is found in the common lens or burning-glass, by which
the light waves are brought to a focus, and as a result, heat is
manifested. Although there is this close identity between light and heat
waves, yet there must be some distinction between the heat and light
waves, because while light waves affect the eye, heat waves do not.
There is actually a difference between the two kinds of waves, and that
difference is one of period or length. It must not, however, be thought
that there are really two classes or sets of waves in the Aether, one of
which could be called light waves, and the other heat waves, but rather
the same wave may be manifested in two different forms because of its
different wave lengths. In one case the waves may affect the eye, and we
have the sensation of sight, but in the other case they affect the body,
and we experience the sensation of warmth. An analogy from the waves of
sound may make these facts much clearer. We know that sound travels
about 1100 feet per second. If, therefore, we have a bell which vibrates
about 1100 times per second, we should have a wave one foot long. If it
vibrated 100 times per second the waves would be 11 feet long, while if
it vibrated only 11 times per second, the waves would be 100 feet long.
Now the impression made upon the ear depends upon the number of
vibrations the bell makes per second, and from the rate of vibration we
get the idea of pitch. If the vibrations are very rapid, then we get a
note of high pitch, and if the vibrations are slow, then we get a note
of low pitch. A note of high pitch, therefore, will correspond to waves
of short length, while a low note will correspond to waves of a greater
length; so that the greater the rapidity with which a sounding bell
vibrates, the shorter will be the length of the sound waves which it
generates, and _vice versâ_. The range of the ear however for sound
waves is limited, so that if the vibrations be too rapid or too slow,
the ear may not be able to respond to the vibrations, and so no distinct
impression of the sound will be conveyed to the brain. It need hardly be
pointed out, that both the very short and long waves are of exactly the
same character as those of a medium length, which the ear can detect,
the only difference being one of rapidity. We do not therefore suggest
that in the case of sound, where the vibrations lie outside the compass
of the ear, those which lie outside are not sound waves, or that they
are different from those which lie within the compass of the ear, and
which the ear can detect. Whether the sound waves are long or short,
whether they can be detected by the ear or not, we still say that all
are sound waves, and that all are due to the vibrations of the sounding
body, which vibrations are transmitted through the air, in waves, that
fall upon the tympanum or drum of the ear, and set that vibrating, which
vibrations are transmitted to the auditory nerve and so give rise to the
sensation of hearing. In a similar manner, every atom and every particle
of matter, every planet, every sun and star, is constantly in a state of
vibration, sending off aetherial waves on every side. Nothing in Nature
is absolutely cold, nothing is absolutely still. Therefore all matter,
whether in the atomic form, or in the planetary or solar world, is
constantly generating aetherial waves, which travel from their source or
origin with the velocity of light. If these aetherial waves so generated
fall within certain limits, then they affect the eye, and we get the
sensation of sight. To do this they must vibrate 5000 billion times per
second, and if they fail to do this, they fail to give rise to the
sensation of sight. If the aetherial waves fall below this limit, then
they affect the body, and give rise to the sensation of heat. For it
must be remembered, that as the ear has a certain compass for sound
waves, which may vary in different individuals, so the eye has also a
certain compass for aetherial waves, with the result that some waves may
be too slow or too rapid to affect the eye, and consequently fail to
give rise to the sensation of sight. When that is so, the sensation of
warmth helps us to detect these longer waves, so that the longer waves
would warm us and make their presence felt in that manner. We shall see
in the next chapter that there are both shorter and longer waves, which
may be detected in other ways. From these facts it can be readily seen,
that we have a common origin for both light and heat, and that they are
both due to periodic waves in the Aether, and therefore all the laws
that govern heat should also govern the phenomena of light. Further, if
heat possesses a dynamical value, and if there be such a truth as the
motive power of heat, then there ought equally to be a motive power of
light; and further, if heat possesses a repulsive motion, then because
of the identity of light and heat, light should equally possess this
repulsive power, because it is due to similar periodic wave motions in
the Aether. With regard to the same laws governing both light and heat,
we shall see that this fact also holds good. We have already seen (Art.
66) that the intensity of heat is inversely as the square of the
distance, and we shall also see in the succeeding chapter that the same
law holds good in relation to light. We have seen (Art. 65) that the
path of a ray of heat is that of a straight line; we shall see in the
succeeding chapter that the path of a ray of light is that of a straight
line also.
Indeed, there is no law applicable to heat which is not applicable to
light. The law of reflection and refraction of heat equally holds good
in relation to light; and further, Professor Forbes has shown that heat
can be polarized in a similar manner to the polarization of light. This
last fact is considered the most conclusive argument as to the identity
of light and heat, and proves that the only difference between the two
is simply the difference corresponding to the difference between a high
note and a low note in sound. That being so, I hope to be able to show
that as heat possesses a dynamical value, so light equally possesses a
dynamical value, and that as heat is a repulsive motion, then light must
equally possess a similar repulsive motion, that motion always being
directed from the central body, being caused by the same agency, viz.
the waves of the Aether, the common source of both light and heat. I
purpose to address myself to this subject in the following chapter,
which I have termed Light, a Mode of Motion.