March 23, 191 1] 



NATURE 



131 



But we can easily imagine them of sucli extremely low- 

 frequency that the molecules or atoms of matter cannot 

 respond to them — cannot vibrate in unison with them — 

 molecular resonance cannot be established ; hence no con- 

 version of the Esther's energy directly into heat in the 

 ordinary way can take place. 



We are familiar with the dissipation or degeneration of 

 •the higher forms of energy into heat, and the continual 

 degradation of heat to lower degree ; that is to say, less 

 violent molecular vibration and more general distribution. 

 As is well known, it is only through this degradation or 

 running down of natural energy that we are enabled to 

 -utilise some of it. Lord Kelvin called this function of 

 energy " motivity " (we now call it entropy), and said the 

 motivity of the universe tends to zero. 



We know that ordinary radiation waves in the Eether 



persist indefinitely and without change of frequency or 



direction until they encounter matter, when they are 



.-ih-;orbed and converted into heat, only to be radiated again, 



lally in longer waves, to some colder body. This 



gradation of wave frequency continues until we can no 

 1 .iiger follow it. I beg to suggest that the ultimate 

 1 sanation of this wave energy is that vast reservoir of 

 kinetic energy intrinsic to the aether. We may liken the 

 waves of radiant energy, which we apprehend as light and 

 hi at, to wind ripples on the surface of water, which con- 

 tinually degenerate in wave frequency until they are 

 absorbed into and become a part of the mighty swell of the 



OLlTin. 



Thus we may, perhaps, regard the aether's intrinsic 

 energy as energy in its lowest form — Kelvin's zero of 

 " motivity." But fortunately we may, and do, get some 

 of this energy back in available form in several ways, as, 

 for instance, when a falling body is arrested and develops 

 iieat ; some of our wind ripples are then returned to us. 



When two gigantic astronomical bodies collide under 



the influence of gravitation, as sometimes happens, we 



witness in far distant space the birth of a nebula. The 



inconceivably vast amount of heat developed by the 



collision converts both bodies into luminous vapour, which 



exjjands with incredible rapidity into the nebulous cloud. 



This heat energy must in course of time degenerate back 



into the aether whence it came, though billions of years 



may be required ; and during all this time the energy has 



motivity." We may picture the stupendous result of 



collision as only a local splash in the other's mighty 



an of energy. 



Having postulated that the aether is endowed with very 

 at intrinsic kinetic energy in wave form of some kind, 

 I the waves are propagated in straight lines in every 

 • iceivable direction, i.e. the wave energy is isotropic, 

 ,;nd that this energy is distributed uniformly throughout 

 I he universe except in so far as the distribution is dis- 

 hed by the presence of matter, I shall endeavour to 

 ilain my conception of the mechanism of gravitation, 

 lor illustration in terms of the known, let us imagine 

 a closed space having uniformly luminous walls of such 

 character that every point on their surface radiates light 

 in all internal directions. The enclosed space may be of 

 any shape, but for the sake of simplicity let it be spherical 

 Or cubical, and large, say as large as a lecture-room. 

 The space will be filled with isotropic radiant energy 

 uniformly distributed— any cubic centimetre of space con- 

 taining as much energy as any other. 



Next let us picture a small opaque body suspended any- 

 where in our luminous space. The body may be of any 

 shape we may imagine an atom or molecule to have ; but, 

 again for simplicity, let it be spherical — say a small grain 

 of shot, and let it be located near the centre of the space. 

 The small body will absorb the light which falls upon 

 and will cast a spherical shadow, the depth or intensity 

 which will vary inversely with the square of the distance 

 >m the centre of the body; and ihf sliadow will extend 

 the confines of the enclosure, liowv r large the latter 

 may be. We cannot perceive the shadow, but we know 

 it is there. It is true that the lx)dy will soon acquire the 

 t'lnperature of its surroundings, and radiate as much 

 rgy as it receives; but for the purjwse of this illustra- 

 ■n let us consider only the high-frequency light energy. 

 As is well known, the aether waves of light will exert 

 flight pressure on the body. But in the case supposed 



NO. 2160, VOL. 86] 



the pressure will be equal on all sides, and no effort 

 toward translation can result. 



Now let us introduce a second small body, similar to 

 the first, and some distance from it. This also will cast 

 a spherical shadow Hke the first. The two shadows will 

 intersect, and each body will lie within the shadow of the 

 other. In other words, each body will be partially shielded 

 by the other from the aether waves coming from that 

 direction. Hence the light pressure will be less on that 

 side of each body which faces tow'ard the other than on 

 the side w'hich is turned away, and the bodies will be 

 urged toward each other by the excess of light pressure 

 on the side turned away. This excess of pressure will 

 vary with the inverse square of the distance between the 

 centres of the bodies so long as the ratio of distance to 

 diameters remains large. 



The iether waves concerned in gravitation cannot, how- 

 ever, be like the light waves 1 have just used for illus- 

 tration, because light waves heat bodies on which they fall ; 

 and their pressure is almost wholly superficial, it does not 

 reach molecules much below the surface, and hence bears 

 little relation to mass. 



But let us substitute for the short and feeble waves of 

 light powerful waves, still of the radiant kind, but of such 

 great length and slow frequency that, as before explained, 

 they do not excite the molecular vibrations which we 

 appreciate as heat, and hence are not absorbed by matter ; 

 they pass freely through all bodies, bathing the interior 

 molecules as effectually as those on the surface. 



Under these conditions each molecule or atom or unit 

 of a gravitating body will have its own spherical shadow 

 or field of influence, and the gravitative force acting on 

 the body will vary directly with the sum of its units, i.e. 

 w'ith its mass. 



The spherical shadow which I have pictured as the field 

 of influence of each atom or material unit implies that the 

 atom has caused, principally in its immediate neighbour- 

 hood, a diminution of the aether's energy. Let us further 

 imagine this subtracted energy resident in the atom as 

 kinetic energy of translation in many paths, almost 

 infinitesimally short and in every direction, but without 

 collisions, because neighbouring atoms follow very nearly 

 parallel paths. We may then picture the collective atoms 

 or molecules of matter buffeted about in every direction 

 by the aether waves in which they are entangled, like a 

 suspended precipitate in turbulent water. 



Each atom or molecule may be regarded as a centre of 

 activity due to its kinetic energy of translation, with con- 

 tinual absorption and restitution of the aether's energy, 

 normally equal in amount. The manner in which this 

 molecular activity maintains, in effect, the supposed 

 spherical shadow, requires explanation, which I shall 

 attempt in a future paper. 



Of the several components into which the composite 

 motion of each atom can be resolved, that one lying in 

 the direction of an attracting body will be the greatest, 

 because the waves from that direction, being partially 

 intercepted by the attracting body, are weakest, and the 

 atom will be pushed in that direction by the superior waves 

 behind it. H free to fall, the atom will continually absorb 

 more energy from the stronger waves behind it than it 

 restores to the weaker waves in front, and will thus 

 acquire additional kinetic energy of translation in the line 

 of fall, measured directly by the number of waves in- 

 volved, i.e. by the distance moved. Conversely, if the 

 atom be forced away from the attracting body, restitution 

 of energy will exceed absorption, and the energy expended 

 in moving the atom against attraction will be transferred 

 to the a?ther. 



It will be seen that gi-avitation is a push toward the 

 attracting body, and not a pull. It is clear, also, that the 

 velocity which a falling body can acquire tends asymptotic- 

 allv to' a limit, which is the velocity of the aether waves 

 which push it — the velocity of light, if transverse waves 

 are involved. 



I have already intimaifd that any kind of a-ther waves 

 capable of imparting motion l_not internal vibration) to the 

 atoms of matter will fulfil the requirements of my theory, 

 but have thus far discussed only transverse waves. 



Let us now consider longitudinal waves — waves of com- 

 pression and rarefaction, like souivl waves in air and in 



F 



