214 RECENT STUDIES IN GRAVITATION. 



But this unlikeness, this independence of ^gravitation of Siny quality 

 but mass, bars the way to any explanation of its nature. 



The dependence of electric forces on the medium, one of Faraday's 

 grand discoveries forever associated with the Royal Institution, was 

 the first step which led on to the electromagnetic theor}- of light, now 

 so splendidly illustrated b}^ Hertz's electromagnetic waves. The 

 quantitative laws of electrolysis, again due to Faraday, are leading, I 

 believe, to the identification of electrification and chemical separation — 

 to the identification of electric with chemical energy. 



But gravitation still stands alone. The isolation which Faraday 

 sought to break down is still complete. Yet the work I have been 

 describing is not all failure. We at least know something in knowing 

 what qualities gravitation does not possess, and when the time shall come 

 for explanation all these laborious and, at first sight, useless experi- 

 ments will take their place in the foundation on which that explanation 

 will be built. 



