Dr. J. Croll on Le Sage's Theory of Gravitation. 45 



and lie has not considered the propagation of electro-magnetic 

 disturbances in a conducting medium. But according to a 

 former paper of ours, on the "Viscosity of Dielectrics," no 

 dielectric can be assumed to be non-conducting, and the 

 charging of any condenser whatever is always accompanied 

 with absorption phenomena ; also absorption certainly increases 

 with conductivity. 



Hence although, if a method of experimenting were employed 

 in which a water-condenser of great internal resistance were 

 discharged through wires of less and less resistance for shorter 

 and shorter periods of time, the measured specific inductive 

 capacity might get less and less, and gradually approach a 

 value equal to the square of the index of refraction of water 

 for infinitely long luminous waves (the index of refraction for 

 air being called unity), still practically the measured specific 

 inductive capacity can never be even approximately equal to 

 the refractive index of water, since the absorbed charge is 

 immeasurably greater than the surface-charge. We therefore 

 need not expect to find the specific inductive capacity of water 

 in its variations with temperature consistent with Dr. Glad- 

 stone's results for index of refraction. When Prof. Clerk 

 Maxwell takes into account conductivity, his equations are not 

 generally integrable ; but even if they were they could not deal 

 with the real case, because he leaves absorption quite out of 

 account. 



July 30, 1877. 



VI. Le Sage's Theory of Gravitation. 

 By James Ckoll, LL.D., F.R.S* 



LE SAGE'S theory of gravitation is at present exciting a 

 good deal of attention among physicists. This is per- 

 haps to a considerable extent owing to the fact that some of 

 the conditions arbitrarily assumed by Le Sage in his hypo- 

 thesis have been proved, from the kinetic theory of gases, to 

 follow as necessary consequences. 



A clear and able account of this theory has been given by 

 Mr. Preston in the Philosophical Magazine for September and 

 November last. Mr. Preston has endeavoured to answer all 

 the objections which have been urged against the theory |. 

 There is one objection, however, which appears to me not to 



* Communicated by the Author. 



f Mr. W. B. Taylor,: in an interesting article on Kinetic Theories of 

 Gravitation, published in the Smithsonian Report for 1876, lays down 

 six fundamental characteristics of gravitation, with which every theory, he 

 says, must agree. Of these six requirements, Le Sage's theory, he main- 

 tains, satisfies but two — namely (1) that the direction of gravity is radial 



