206 JAMES CLERK MAXWKLL 



body by electric force. Fontaua, Govi, and Dutcr 

 had all observed that changes take place in tho 

 volume of the dielectric of a condenser when it is 

 charged. Quincko had taken up the work, and the 

 first of his classie papers on this subject was published 

 in 1SSO, the year following Maxwell's death. Maxwell 

 himself was fond of shewing an experiment in which 

 a charged insulated sphere was brought near to the 

 surface of parattin ; the stress on the surface causes a 

 heaping up of the parattin under the sphere. 



Kerr had shewn in 1S75 that many substances 

 become doubly refracting under electric stress; his 

 complete determination of the laws of this action was 

 published at a later date. 



As to direct measurements on electric waves, there 

 were none ; the value of the velocity with which, if 

 Maxwell's theory were true, they must travel had 

 been determined from electrical observations of quite 

 a different character. Weber and Kohlrausch had 

 measured the value of 1C for air, for which p. is unity, 

 and from their observations it follows that the value 

 of the wave velocity for electro-magnetic waves is 

 about 31 x 10 centimetres per second. The velocity 

 of light was known, from the experiments of Fizeau 

 and Foucault, to have about this value, and it was the 

 near coincidence of these two values which led Max- 

 well to write in 1864: 



"The agreement of the results seems to show that 

 light and magnetism are attentions of the same 

 substance, and that light is an electro-magnetic 

 disturbance propagated through the Held according 

 to electro- magnetic laws." 



