84 



SCIENTIFIC THOUGHT. 



51. 



Corre- 

 spondence 

 between 

 velocities 

 oflight 

 and of 

 e ectricity. 



The idea of a medium of extreme rarity, pervading all 

 space and interpenetrating all matter, capable also of the 

 elastic reactions of a solid body, was not repugnant to 

 physicists at the time when Maxwell wrote. Though 

 violently opposed forty years earlier when proposed by 

 Fresnel and Young, it had gradually, through the de- 

 velopment of optical theories, become a well-recognised 

 instrument of scientific thought. In such a medium a 

 disturbance or displacement is propagated with a certain 

 velocity dependent on its elastic nature the so-called 

 constants of density and rigidity. Now, looking upon a 

 charge of electricity not as a material something an 

 imponderable but as a displacement of the medium, 

 the question arose, Does the velocity with which such a 

 displacement travels compare at all with the known 

 velocities of other elastic disturbances, such as light is 

 conceived to be ? It was known to electricians that 

 an amount or charge of electricity can be either station- 

 ary (called statical electricity) or in motion (called 

 an electric current) ; and Weber and Kohlrausch had in 

 1856 actually measured the number of units of statical 

 electricity which must flow through an electric circuit in 

 order to produce the known mechanical effect of a unit 

 of electric current. The quantity which they found, and 

 which corresponded to a velocity, was of the same order 

 as the velocity with which the elastic disturbance which 

 we call light is known to travel. Maxwell was the first 



are only partial is proved by the 

 divergence of the laws of the two 

 sets of phenomena in other respects. 

 We may chance to find, in the 

 higher parts of physics, instances of 



more complete coincidence which 

 may require much investigation to 

 detect their ultimate divergence " 

 (p. 188). 



