96 Dr. C. V. Burton on a 



forward by Challis *, which likewise assumed the existence of 

 compressional waves in the aether, although the function 

 attributed to these waves was very different. But the theory 

 now advanced is much more akin to that of Prof. Hicks, 

 which indeed suggested it, and of which it may be regarded 

 as a development. The introduction of aetherial compressional 

 waves of great wave-length to actuate the pulsatory move- 

 ments of atomic matter, and thus secure the necessary 

 agreement of phase amongst the pulsating centres, is one of 

 the main modifications suggested ; another being the manner 

 in which the capacity for pulsatory motion, assumed to be 

 associated with the electrons, is represented as mobile through 

 the aether. In this way we avoid the necessity for supposing 

 that any element of aetherial substance ever deviates by more 

 than a minute amount from its mean position, the motion 

 throughout being of: " stationary " type, while at the same 

 time the problem with which we have to deal is essentially 

 one of hydrodynamics, capable of being worked out to a 

 first order by means of a simple analysis. We are enabled 

 to treat the aether as if it were a fluid, not because we assume 

 it to yield freely to certain types of stress, but because the 

 distortions involved in the motions considered are so exces- 

 sively minute that no appreciable opposing stresses are called 

 up, save only the changes of hydrostatic pressure, which owe 

 their importance to the enormous value attributed to the 

 bulk-modulus of elasticity of the aether. Apart from the 

 simplicity thus attained, it appears to me most desirable that, 

 we should discard, if possible, the conception of matter, or of 

 any parts or properties associated with matter, as grossly 

 ploughing a course through a reluctant aether. All that we 

 know of aether and of matter seems to indicate that the 

 mobility of matter is absolute, and that the elastic properties 

 of the aether remain perfect, notwithstanding the motions of 

 material bodies, or even of detached negative electrons 

 travelling with velocities approaching that of radiation. And 

 the only way in which I have been able to conceive of matter 

 as travelling through the aether, without doing unwarrant- 

 able violence to the structure of the medium, is by supposing 

 the entire phenomenon of motion to be reducible to a trans- 

 ference of strain, so that no event in our universe involves a 

 progressive yielding of the aether. 



44. By Maxwell f it was felt to be an objection to such 

 hypotheses as those of Le Sage and of Challis, that they 



* See a review by Maxwell, ' Nature/ vol. viii. ; Scientific Papers, 

 vol. ii. p. 338. 

 | Loc. cit. 



