ATTEMPTS TO EXPLAIN GRAVITATION 257 



have not yet been detected. However, this is no final argument against 

 their existence as new and striking discoveries are being made every year. 

 These longitudinal waves force us to conclude that the ether is at least 

 slightly compressible. If compressible we are inclined, from our knowl- 

 edge of matter to think of the ether as discrete or molecular. If dis- 

 crete the particles are elastic and we have to postulate a second ether 

 to explain the elasticity of the molecules of the first. This destroys the 

 simplicity of the ether. 



J. J. Thomson in his Silliman Lectures (page 160) enunciates a 

 view somewhat similar to that given by Brush. In place of longitudinal 

 waves he used short ether-pulses something like the Eontgen and gamma 

 rays are supposed to be. This view presents the aberrational difficulty, 

 as do other views which attempt to explain gravitation by a type of 

 radiation. On this view any change in gravitation would be propagated 

 with the velocity of light ; and certain phenomena in astronomy require 

 the gravitational effect to be propagated much faster than light. 



Near the beginning of the paper we showed that the gravitative 

 attraction between small charged bodies is very small compared with 

 the electrical effects. Now if we assume that the linear equations of the 

 ether are only approximate we may account for this relatively small 

 gravitational effect from terms involving differentials of second or 

 higher orders. Larmor 3 opposes this view because the introduction of 

 higher order term not only disturbs the simplicity of the ether scheme, 

 but also leads to optical dispersion in free space. If such dispersion 

 exists it must be very minute, as bodies emerging from eclipses show no 

 appreciable change in color. However, when we compare the gravita- 

 tional force with the electrical force we see the former is so very small 

 that the higher order term introduced to account for gravitation might 

 give us a dispersion too minute for observation. But, as usual, we meet 

 insurmountable difficulties. On this view the velocity of transmission 

 would be of the same order as that of light ; and while we do not know 

 whether the speed is finite at all, we do know that if it is finite it greatly 

 exceeds that of light. 



Hence at present gravitation seems to be precluded from the electro- 

 magnetic scheme, owing to speed of propagation. Since in the last ten 

 years we have tried to unify, it is unfortunate that it rebels against 

 admission into our scheme. Many have pronounced it a mode of 

 activity in the ether not specified but entirely different from the electro- 

 magnetic mode. 



Beplace Brush's long waves or Thomson's short electromagnetic pulse 

 by rapidly moving corpuscles and we have Le Sage's theory. According 

 to Le Sage space is filled with minute particles, ultramundane cor- 

 puscles, moving rapidly in all directions; hence a single body experi- 



3 See "Ether and Matter," p. 187. 



VOL. LXXIX. — 18. 



