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 versa. 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.



CHAPTER VII

LIGHT, A MODE OF MOTION

ART. 70. Light, a Mode of Motion.—No subject has in the past received greater attention from philosophers and scientists than that involved in the question as to "What is Light?" Indeed, it may truthfully be said, that even to-day its exact character is not positively known. That it is due like heat to some periodic wave motion in the Aether is known, but the exact character of that wave motion has yet to be determined. As in the case of heat, so in the case of light, there have been two theories which have contended with each other for supremacy in endeavouring to answer the question as to "What is Light?" Those two theories are known as the Emission or Corpuscular Theory, and the Undulatory or Wave Theory. The corpuscular theory was introduced and developed by Newton in his work on Optics, which ranks second only to the Principia as a work revealing masterly research and scientific genius. Newton supposed that a luminous or lighted body actually emitted minute particles, which were shot out from the body with the velocity of light, that is, at the rate of 186,000 miles per second. These minute particles he termed corpuscles. In the work just referred to regarding this matter, he asks the question, "Are not rays of light very small bodies emitted from shining substances?" These small particles or corpuscles were supposed by him to actually strike the retina of the eye, and so produce the sensation of Sight, in the same way that odorous particles entering the nostril, come into contact with the olfactory nerves and produce the sensation of Smell. In order, however, to account for certain phenomena of light, he was compelled to postulate an aetherial medium to fill all space, in which his luminous corpuscles travelled, and which would excite waves in that medium. In his eighteenth query on this point he asks: "Is not the heat of a warm room conveyed through the vacuum by the vibration of a much subtler medium than air, and is not this medium the same with that medium by which light is reflected or refracted, and by whose vibrations light communicates Heat to bodies, and is put into fits of easy reflection and easy transmission?" The corpuscular theory, however, received its death-blow when, in competition with the wave theory of light, as developed by Young, it was found that the latter theory satisfactorily accounted for certain phenomena as the refraction of light, which the corpuscular theory did not adequately account for. Even while Newton was developing his theory, Huyghens, a contemporary of Newton, was developing another theory which is now known as the undulatory or wave theory. Huyghens drew his conclusions from the analogy of sound. He knew that sounds were propagated by waves through the air, and from the region of the known, endeavoured to carry the principle into the region of the unknown, a strictly philosophical method, and one in accordance with the second Rule of Philosophy. He supposed that light, therefore, like sound, might be due to wave motion, but if it were wave motion, there must have been a medium to propagate the waves. In order to account for this wave motion, he supposed all space to be filled with a luminiferous Aether, which would be to his light waves what air is to sound waves. In this conception he was supported by Euler the mathematician, and in 1690 he was able to give a satisfactory explanation of the reflection and refraction of light, on the hypothesis that light was due to wave motion in the Aether. It was not, however, till the advent of Thomas Young, that the undulatory or wave theory reached its perfection, and finally overthrew its competitor the corpuscular theory. Young made himself thoroughly acquainted with wave motion of all kinds, and applied his knowledge and experience to the phenomena of light, and from the analogies so obtained, he gradually built up the undulatory theory, and gave to it a foundation from which it has not yet been moved. Young made use of the same aetherial medium in order to propagate the wave motion of light in the same way that Huyghens did. From that conception, the Aether has been gradually perfected, until we have the conception which has been presented to the reader in Chapter IV., in which I have endeavoured to show that this aetherial medium is matter, but infinitely more rarefied and infinitely more elastic, but notwithstanding its extreme rarefaction and elasticity, it possesses inertia, because it is gravitative. It is this Aether, then, that is concerned in the propagation of light, and is the universal medium which is to light what air is to sound. Young, therefore, having applied himself to the wave motion of sound, from such researches was able to explain the physical cause of colour, and that phenomenon termed interference.

We will therefore look at wave motion, in order to understand the wave theory of light.

Now in all wave motion, whether it be water waves or sound waves, that which is propagated or conveyed from place to place is energy, or motion. If a stone is thrown into water, a series of concentric circles of waves are generated, which spread out with increasing size, but decreasing power or motion, regularly on all sides. The water, however, does not move away from the generating source. There is a motion of the water, but it is simply a wave motion, so that the propagation of a wave is the propagation of motion, rather than the transference of the actual water which constitutes the wave. In the case of sound waves, we have again an illustration of the same principle. For example, suppose we strike a bell, and so set the particles of that bell in a state of vibration. These vibrations give the air in contact with the bell a forward movement, and then, owing to the elasticity and inertia of the air, a backward movement is set up, with the result that a series of waves are set in motion from the bell on every side, which gradually diminish in intensity the farther they recede from the generating body. According to the wave theory, therefore, we have to picture all heated and luminous bodies in a state of vibration, and the atoms of such luminous bodies imparting the vibrations to the atoms of the Aether, in the same way that the atoms of a bell impart their vibrations to the atoms of the air in contact with it. These vibrations are then propagated through the Aether in waves, which, entering the eye, impinge or strike upon the retina at the back of the eye, and being transmitted to the brain give rise to the sensation of sight. It must not be forgotten that the waves of Aether, as pointed out in Art. 64 in relation to heat, really form spherical shells which radiate out in all directions from the central body which gives rise to them. Thus it can be seen, that all points in the spherical wave which are at equal distances from the vibratory or luminous body, must possess the same intensity, and possess equal lighting powers. Light, therefore, like heat, is due to a periodic wave motion set up in the Aether by the vibrating atomic motion of heated or luminous bodies. It must be also noticed, that if we could see the air through which the sound waves are passing, we should see that each atom or particle of the atmosphere was vibrating to and fro in the direction of propagation. If, however, we could see an atom of Aether in vibration, accepting the principle that Aether is atomic, we should see that each aetherial atom is not vibrating in the direction of propagation, but across the line in which the wave is travelling. Thus the vibration of the air is said to be longitudinal, but the vibrations of the Aether are transversal. An illustration of the transverse motion of a light wave may be obtained by taking a rope and imparting to it a series of undulations by shaking it up and down, when it will be observed that the wave motion of the rope is transverse to the straight line in which it is propagated. The physical explanation of the transverse vibration of light will be dealt with in a subsequent article.

Now the question suggests itself to our mind, as to what effect the atomicity of the Aether has upon the undulatory theory of light. Does it establish it upon a firmer basis, or does it in any way destroy its truth as a theory? I venture to think that the atomicity of the Aether in no sense destroys any part of the undulatory theory of light, but rather tends to confirm and establish it upon a logical and philosophical basis.

For instance, as has been pointed out in Art. 47, in order for the undulatory theory to have any existence at all, it is essential that the Aether should possess the property of elasticity. But how the Aether possessed the property of elasticity while at the same time it was frictionless, and therefore possessed no mass, has been a problem that has taxed the ingenuity and resources of scientists for a century past, and up to the present is a problem which still remains unsolved. Now, however, with our atomic Aether, it is just as easy to conceive Aether transmitting a wave as it is for air to transmit sound waves, or water to transmit water waves.

Tyndall, in his Lectures on Light, seems to have appreciated the difficulty, and to avoid confusion, again and again refers to a particle of Aether. While Huyghens himself in speculating upon the elasticity of the Aether in his Traite de la Lumiere, 1678, makes a suggestion as to its origin, which practically amounts to the fact that the aetherial atom which gives rise to this elasticity is the core or centre of a vortex ring. Thus it can be seen that the elasticity of the Aether, so essential to the undulatory theory, is a problem that cannot be solved apart from recognizing the hypothesis of an atomic Aether.

Then, again, in the undulatory theory of light, the density of the Aether around molecules of bodies has to be taken into consideration to account for such phenomena as the refraction and reflection of light, but, as we have seen in Art. 46, such a property as density is inconceivable in connection with a medium which is neither atomic and possesses no mass. On the assumption, however, of an atomic and gravitative Aether, the difficulty is at once solved, and the density of the Aether, and different degrees of density are at once placed upon a logical and philosophical basis. So that in relation to the elasticity and density of the Aether upon which the transmission and reflection of wave motion depend, an atomic and gravitative Aether establishes and confirms the undulatory theory.

There is also another aspect of the subject that is worthy of notice. I refer to the effect of an atomic and gravitative Aether upon Newton's corpuscular theory of light. Newton's corpuscular theory failed in not being able to account for the relative velocity of light in rare and denser media, and if by an atomic Aether in conjunction with the undulatory theory, the phenomenon can be accounted for, as I believe it can, then our aetherial vortex atoms are analogous to Newton's corpuscles. This distinction will, however, have to be made, viz. that Newton supposed his luminous corpuscles to be emitted by the luminous body, whereas in the conception of our aetherial atoms, we conceive them to be stationary relatively in space, and only subject to those vibrations and oscillations that give rise to the aetherial waves recognized in the undulatory theory.

It would indeed be a consummation to be desired, if, by an atomic Aether, it can be proved that Newton's Corpuscular Theory was made to harmonize with the Undulatory Theory, and that it can be I am profoundly convinced. Professor Preston is also of this view, for in his Theory of Light, writing on this subject, he says, page 19: "In conclusion, we may state that we believe an ingenious exponent of the emission theory, by suitably framing his fundamental postulates, might fairly meet all the objections that have been raised against it."

We will now apply the hypothesis of an atomic and gravitating Aether to Huyghens' principle of wave propagation, and see if this atomicity in any way destroys that principle, or whether it simplifies and confirms it.

Let us briefly review our conception of the Aether before making the application. In the first place, because Aether is gravitative, we learned from Art. 45 that it surrounds all bodies in the universe, from the smallest atom to the largest sun or star in the firmament of heaven. Our sun, then, which is to our system the source of all its light, will be surrounded by what are practically spherical aetherial envelopes or shells which decrease in density as they recede from the sun (Art. 46). These aetherial shells are, according to our conception, made up of minute aetherial spherical vortex atoms possessing polarity and rotation (Art. 43), and these atoms will be closer together the nearer they are to the central body, because of the increased density of the Aether due to the attractive influence of the sun. Thus, when a wave motion is set up in the Aether around the sun by the intense atomic activity of that incandescent body, each atom of that aetherial spherical shell or envelope participates in the motion or impulse received, at one and the same time, so that the wave is transmitted from envelope to envelope, by the elasticity of the aetherial atoms which compose the envelope or shell. Thus the light wave is always spherical in form, or nearly so, as the rotational and orbital motion of the sun affect the exact shape of the aetherial envelope as we shall learn more fully later on.

Further, the wave front always takes the form of a sphere, as the waves are radiated out from the luminous body in all directions, and we shall learn, in the next article, that the vibrations are always in the wave front, that is, take place on the surface of each of these envelopes, and these vibrations are also transverse to the propagation of the wave. As these aetherial envelopes extend right into space, the wave is transmitted from envelope to envelope by means of the aetherial atoms with the velocity of 186,000 miles per second, but as each succeeding envelope possesses a larger surface than the preceding one, the intensity of the light is proportionally decreased. The surface of such envelope is always proportionate to the square of the radius, the other quantities remaining equal. So that the intensity of the light waves, which are coincident with the surface of each spherical envelope, will always vary inversely as the square of the distance from the luminous body, which agrees with the law of inverse squares that governs light and heat.



We have considered the wave motion as a whole, that is, we have viewed it from the standpoint of the whole of the aetherial elastic envelope. Now we will look at the subject from the atomic standpoint, and see if it is in accordance with Huyghens' principle of wave propagation.

We will suppose that an undulatory movement is started by a luminous body at point A situated in the Aether, and surrounded by that medium. A may represent a part of any luminous body, as the sun or star, while B C and B' C' represent a segment of the aetherial envelopes already referred to, which exist around the sun. We will further suppose that the small dots surrounding the luminous body represent the aetherial atoms forming the envelope, which transmit the impulse or energy received from the atomic vibrations of the luminous body. As each aetherial atom is moved or pushed forwards, each atom directly in contact with it accepts and transmits the impulse. But each of these atoms stands in relation to those in front of them, as they did in relation to the first row of atoms, so to speak, and therefore exert a corresponding impulse on the front row.

But the third row stands in relation to the fourth row as the second row did to the third, and so on to infinity. Thus each atom being surrounded by other atoms may be looked on as the centre of a new wave system, so that every particle of the wave system is itself a centre of a new wave system which is transmitted in all directions. As these innumerable and minute wave systems co-operate with one another, they form a principal wave system which is coincident with the surface of the spherical envelope, part of which is represented by B C. Then if we conceive of all the aetherial atoms in part of the principal wave system B C, as themselves becoming the centre of wave propagation, by their wave systems the principal wave will be transmitted further on into space to another aetherial envelope B' C', which represents part of another principal wave, which again is coincident with the surface of one of the spherical aetherial envelopes. So that by the action of the aetherial atoms which exist on all sides of the luminous body, the aetherial wave can be transmitted from atom to atom in more or less spherical form.

Now let us compare this explanation of the transmission of light by an atomic Aether with the celebrated Huyghens' principle which is thus enunciated. "When an undulatory movement propagates itself through an elastic medium, every particle imitates the movement of the particle first excited. But every particle stands in relation to the adjoining ones in exactly the same relation that the first particle did to its neighbours, and consequently must exert upon those surrounding it, exactly the same influence as the first did. Every vibratory particle is therefore to be regarded as if it were the originally excited particle of the wave system; and as the innumerable and simultaneous elementary wave systems co-operate with one another at each instant, we obtain exactly that principal wave system by which the elastic medium appears at any moment to be moved." Now here, in this statement, we have the definite term particles used several times by Huyghens. But in the generally accepted theory of the Aether, such a term is unknown and unrecognized, with the obvious result that the definite and simple statement of Huyghens loses all its simplicity and meaning. Replace, however, the non-atomic Aether as at present recognized, by an atomic and gravitating Aether, and then Huyghens' exposition or principle stands out in all its simplicity and clearness, and finds in an atomic Aether its literal fulfilment and complete verification.

In conclusion on this point, viz. that light is a mode of aetherial motion, let us endeavour to form a mental picture of our atomic and aetherial world. We have to remember that every particle and atom of matter in existence are ever vibrating, and by their vibrations are ever creating and generating Aether waves in the aetherial medium. These waves, begetting others, the process is continued until they are either intercepted and brought to rest by other matter, or else speed away until they reach the boundary of space.

Now it is scarcely necessary for me to say, that if one atom can create and generate these Aether waves, a thousand atoms can create them in greater abundance still, and millions of atoms in even still greater abundance, and so on in proportion to the quantity or bulk of the matter vibrating. Further, as it is with quantity, so will it be with intensity, or activity of vibration. The more intensely an atom vibrates, the more intense would be the movement of the generated Aether waves, and the intensity would be in exact proportion to the intensity of the motion of the atoms vibrating. In regard to the power of atomic motions or vibrations, those are the greatest and most intense in energy or motion, which are produced by combustion or burning. The chemical activity by which the burning is brought about arouses and excites the atoms of matter subject thereto, into an intensity of motion, thousands, it may be millions of times greater than can be produced by any other known means. Therefore it can be readily seen, that the Aether waves generated by this means will be greater and more abundant, both in their volume and intensity, than the Aether waves produced merely by a cold body. For example, take a candle at night-time when the light has disappeared; look at it and feel it. Though its atoms are all in motion, generating Aether waves which are impressed with its own particular form and colour, yet it can scarcely be seen even at a short distance; but light it, and what a change takes place! We can both see it, and are enabled by its light to see other things also. By the power of combustion, its atoms have been excited into greater energy or motion, generating and speeding Aether waves on every side, and these Aether waves being reflected and re-reflected by the atoms of the air, and the walls of the house, give light to all that are in the house. I must now ask the reader to refer to Art. 64 on Radiant Heat, in order that we may recall facts regarding the heat of the sun. Remembering the intensity of the heat of the sun as calculated by Herschel and others, and remembering that the sun is 1,200,000 times larger in volume than our earth, the question naturally suggests itself to our mind, what must be the volume and intensity of the light waves as they flow from the sun into space? What a storm of fury and of motion must there be within the aetherial atmosphere around the sun, and with what volume and power must these light waves speed away from so mighty a source! Some idea may be gained from the fact that they speed away to the distant Neptune, a distance of nearly three thousand millions of miles, and impart to that planet the energy of light and heat which to the planet forms the physical source of all its life and activities. Thus from the sun, the centre of the solar system, there are ever being poured forth into space these aetherial light waves. The solar fires are ever glowing, and their flames ever burning, robing the solar disc with its quivering fringe, or madly leaping on every side to a distance of one hundred thousand miles, and by their madness lashing the aetherial atmosphere into fury, creating aetherial waves, myriads upon myriads, and sending them with lightning speed across the intervening space. As swift-footed messengers they come, the bearers of life and beauty to distant planets. They come to this our island home in space, these aetherial light waves, like rich argosies freighted with the treasures of light, of life, of beauty, and of glory, and the transmission of this life and beauty is effected by the incessant wave motion generated in the Aether by the central body of our solar system, the sun. Let us therefore endeavour to form a mental picture of this aetherial wave motion with its transverse vibrations.

ART. 71. Transverse Vibration of Light.—In the previous article we saw that the vibration of light was transverse to the line of propagation. If we could see the particles of air which are vibrating when sound waves are produced, we should find that each particle or atom is vibrating backwards and forwards in the direction of propagation.

In the case of an aetherial atom, however, which, according to our own theory, participates in the vibration, we have to try to conceive of each atom as vibrating across the line of propagation. So that if A B represents a ray of light proceeding from a luminous body, as the sun (Fig. 5), then the vibration must be across the line, as up and down and across that line as shown in the figure, each phase of the vibration being at right angles to the line of propagation—that is, to A B. How can we form a physical conception of this phenomenon? There must be some physical explanation to it, for if it be an effect there must be a cause for its existence and production. Up to the present, however, no physical explanation has been forthcoming, so that for over 200 years a frictionless medium has failed to account for, or to explain, the transverse vibration of light as suggested by Fresnel.

If, therefore, by the hypothesis of an atomic and gravitative Aether, we succeed in accomplishing a result that a frictionless Aether has failed to accomplish, then the explanation will be a most important factor in proving the atomicity and consequent gravitative property of the Aether.

Let us therefore revert to our hypothesis of the Aether as given in Art. 45. From that we learn, because Aether is atomic, it is also gravitative, and therefore forms around every atom and molecule, every satellite, planet, sun and star, an aetherial atmosphere—such aetherial atmosphere being doubtless proportionate to the mass of the atom or molecule or planet as the case may be, in accordance with the Law of Gravitation. We shall consider this view of the subject later on.



Thus we learn that every particle of matter, and every body in the universe has its aetherial atmosphere so to speak, to which it is held bound by the universal Law of Gravitation. In the case of a satellite or planet or sun or star, that atmosphere will be more or less spherical in shape, decreasing in density as it recedes from the attracting body. As we saw in the previous chapter, Tyndall stated that the waves of light really formed spherical shells which surrounded the luminous body. In the conception of an atomic and gravitating Aether we can form a physical conception of these aetherial shells, which can be pictured as elastic envelopes, or rather series of envelopes surrounding each particle of matter, and also surrounding each satellite, planet, sun, and star; each envelope getting gradually less and less dense as the distance from the central body is increased.

Now we learn from experiments that the vibration is always in the wave front, but the wave front is coincident with the surface of each aetherial spherical shell, therefore the vibration must be in, and coincide with, the surfaces of the spherical shells that are formed around every body in the universe.

We are now, however, dealing specially with one body which is the source of light, viz. the sun, and have therefore to picture the sun as being surrounded by these aetherial elastic envelopes, which gradually get less and less dense as they recede from it. What, therefore, will be the effect of the heat of that body as it is poured forth into space? We have already learned (Art. 63) of the untold quantity of heat that is continually being poured forth into space from the sun with its diameter of 856,000 miles, and its circumference of over 2-1/2 million miles. What intense activity it must generate in the Aether near its surface! and what must be the direct effect of that heat upon the aetherial elastic envelopes or shells which surround it?

Perhaps the answer can be best illustrated by a simple experiment. Let us take an ordinary toy balloon, with its elastic envelope, and fill it moderately full with air, and observe what the effect on it is when we put it near the fire. Gradually, as heat is imparted to the air in the balloon, the air which is also elastic expands, with the result that the envelope of the balloon is extended, and its size enlarged. Now withdraw it from the fire and note what happens.

As the air inside gets cold again, the elastic envelope of the balloon gradually shrinks, until it has been reduced to its former size. What has been taking place during this experiment with regard to the elastic envelope and the atoms thereof? May we not say that there has been a vibration or oscillation, among the particles which go to make up the elastic envelope, that forms the surface of the balloon? Certainly there has been some form of motion, and that motion took first the form of an expansion, and then contraction of the individual particles; and we have only to conceive of this process being repeated quickly and continuously, to form a mental picture of what takes place in any aetherial elastic envelope or shell that surrounds the sun.

The illustration is not, however, perfect, because we have made the source of heat to be outside instead of inside the elastic envelope, as is the case with the sun and its aetherial atmosphere or envelope. We will therefore slightly modify the experiment, and take two balloons, A, B, one smaller than the other, and put the smaller one A into the interior of the larger one, inflating the smaller one, so that it can be situated in the middle of the larger one, the latter having twice the diameter of the smaller one, as in the diagram (Fig. 6). To the neck of the smaller balloon A we will attach an india-rubber tube which ends in a closed bulb C. We have now the two balloons inflated. Let us press the bulb C and notice what happens. The effect will be exactly the same as it was when we brought the balloon in contact with the heat of the fire in the first experiment—that is, the elastic envelope will be again expanded. As soon as we take the pressure from the bulb C the envelope, being elastic, seeks to recover its original position, with the result that it springs back to its original size. If we pressed the bulb C 20 times per minute, we should get 20 vibrations of the particles of the envelopes per minute, and if we pressed it 1000 times per minute, we should get 1000 vibrations among the particles of the elastic envelope, so that the number of vibrations would correspond to the number of times we pressed the bulb. Now how did this vibration reach the elastic envelope of the balloon B from the balloon A?



The reply is, by means of the particles, or atoms of air that exist between the two surfaces of the balloons, and that transmission would take the form of a wave propagated from particle to particle, so that we might put dots on the right side of A to represent the atoms of air which transmit the wave from A to B.

But the vibration which takes place in the surface of the envelope of the outer balloon is across this line of propagation, because as the wave proceeds from A to B, the elastic envelope expands and stretches always across the line of propagation—that is, it stretches up and down, left and right, as it is expanded outwards, so that the vibration or oscillation of the particles always takes place in the surface of the elastic envelope across the line of propagation. Let us therefore apply the result of this simple experiment to our solar system and the Aether, and see if it can be made to explain the transverse vibration of light. Let A represent the sun (Fig. 7) and B an aetherial elastic envelope surrounding the sun. In this case we dispense with the bulb C, as the sun possesses within itself the power to generate heat, and so to produce the required expansion of the elastic aetherial envelopes B, G, H, etc.



Instead, however, of having air particles between A and B, we will put in their place our aetherial atoms which we have conceived according to Art. 44. These surround the sun, represented by A, forming elastic spherical shells or envelopes. As the sun radiates its heat into space, it urges the aetherial atoms against each other, with the result that they transmit the energy from atom to atom, or particle to particle, till they come to the elastic aetherial envelopes of H, G, B.

The effect on B, or on any other aetherial envelope, is to expand it outwardly, and thus set the atoms of which it is composed into vibration. The wave, which is now an aetherial wave travelling with a velocity of 186,000 miles per second, may be represented by the line D E. But while it is travelling from D to E the same energy is being radiated out in all directions, so that a wave reaches the whole surface of the elastic envelope B at the same time, with the result that the whole of the shell or envelope is set in vibration as it expands outwardly.

Thus the vibration is always in the wave front, and the wave front is always coincident with the surface of one of these envelopes, and as these aetherial envelopes are themselves formed by aetherial atoms, the wave is spread outwardly from any central point in a spherical form as proved by experiment. Not only, therefore, is the vibration in the wave front, but it is always transverse to the line of propagation, for the simple reason that the surface of the spherical shell or envelope is always at right angles to the radius vector or straight line which joins any centre to the surface of a spherical envelope.

As soon as the aetherial atom which forms the spherical aetherial envelope has reached the limit of its expansion, it seeks to recover its former position because of its elasticity, with the result that the whole envelope contracts again, and arrives at its original position in space ready to accept motion again and transmit it onwards in the same manner as before.

Thus, by the acceptance of an atomic and gravitating Aether, we may form a physical conception of one of the greatest problems in optical phenomena, viz. the transverse vibration of light which always takes place in the wave front, and across the line of propagation. Whether this explanation is exactly correct in detail, or not, I am convinced that the true physical explanation of the problem is to be found in an atomic and gravitating Aether, as hitherto a frictionless Aether has failed even to suggest to any scientist how such a transverse vibration can take place.

ART. 72. Reflection and Refraction.—A ray or wave of light is said to be reflected when it meets with an obstacle which opposes its free passage and turns it back. We have illustrations of this law of reflection in the case of water waves striking against a breakwater, or a sound wave striking against the wall of a room. In either case the wave is turned back, and reflection is the result. A ray or a wave of light is said to be refracted when, in passing from one medium into another, it is turned from the straight path in which it was going before it entered the refracting medium. An illustration of the refraction of light is to be found in the case of the glass lens, so often used to converge the light waves into one focus. We have up to the present dealt with only two theories of light, the Corpuscular theory and the Undulatory or Wave theory. We have seen how both harmonize with Huyghens' principle, and the question arises as to whether both can be made to harmonize with the phenomena of reflection and refraction.

In the Corpuscular theory we have luminous particles emitted by luminous bodies. These particles we have learned are practically synonymous with our aetherial atoms.

In the Wave theory it is impossible to conceive of a wave without conceiving of particles which transmit the wave; even Huyghens refers to particles of Aether, and so does Tyndall in his Notes on Light.

In the Electro-magnetic theory of light we have again to think of atoms, which are termed electrons by Dr. Larmor and Sir William Crookes; while Professor J. J. Thompson calls them corpuscles.

So that in all three theories we have the same fundamental idea of atoms, either expressed or imagined, underlying all the three theories. Now what is the property of the Aether on which all reflection and refraction is based? Is it not the property of density? Fresnel assumes that reflection and refraction of light are dependent upon different degrees of density of the Aether associated with any body, and has given a mathematical formula, which decides the index of refraction, such formula being entirely dependent upon the relative density of the Aether in association with the refracting medium.

But with a frictionless medium, it is an absolute impossibility to conceive of different degrees of density of the Aether in association with matter.

If the Aether does possess different degrees of density which decide the refractive index of the substance, then of a certainty there must be some law to govern and decide the density, and that law can only be the Law of Gravitation.

As Young points out in his Fourth Hypothesis, every particle of matter has an attraction for the Aether by which it is accumulated around it with greater density. Now on the basis of our conception of a gravitative Aether, every atom and molecule, and indeed every body in the universe, possess aetherial atmospheres, which possess varying degrees of density, the denser layers being nearest to the nucleus of the atom or molecule as the case may be, the elasticity of each layer or envelope being always proportionate to its density.

When we apply the corpuscular theory to the reflection of light we find that it satisfactorily accounts for the phenomenon.

According to Newton's corpuscular theory, each luminous particle travels in a straight line through a homogeneous medium. When, however, it comes almost into contact with a reflecting surface, which in our case we conceive to be a layer of one of the aetherial elastic envelopes surrounding the atoms or molecules of the reflecting body, then, according to Newton, the light particle is repelled, or reflected by the medium; the angle of reflection or repulsion being always equal to the angle of incidence. So that the emission theory harmonizes with the wave theory in regard to reflection.

When, however, we come to deal with the refraction of light, the corpuscular theory apparently breaks down, and it was in relation to this phase of the phenomena of light that the undulatory theory overthrew the corpuscular theory.

According to the corpuscular theory, when a luminous particle or corpuscle is nearing the surface of a denser medium, as glass or water, it was attracted by the denser medium, with the result that the velocity of the particle in the denser medium was greater than its velocity in air. But direct experiments prove exactly the opposite, as it is found that when light passes from a rare into a denser medium, the velocity of light in the denser or more refracting medium is less than it was in the air. Here then was a test to decide the respective merits of the two theories. As the undulatory theory was able to give a satisfactory explanation of the phenomenon, the corpuscular theory was rejected, and the undulatory theory was accepted. Now the question suggests itself, as to whether it is possible to reconcile the two theories in relation to the refraction of light by our conception of an atomic and gravitative Aether. I believe it is possible. Let us look at the case for a moment. We have, according to our theory of the Aether, to conceive of all atoms and molecules, of all planets and suns and stars, being surrounded by aetherial elastic atmospheres, or envelopes, which, like the atmosphere in association with the earth, are always the densest nearest the nucleus of the atom, getting gradually less and less dense the further they recede from the central point. Further, according to our theory, with regard to the elasticity or pressure of these elastic envelopes, they exert a pressure proportionate to their density. So that the nearer the aetherial atmosphere or envelope is to the central point or nucleus of the atom, the greater will be the elasticity or pressure.