JAMES CLERK MAXWELL 321 



time the vehicle of electrical action. In imagination Ampere made 

 this stride ; but the illustrious physicist could not foresee that the se- 

 ducing hypothesis with which he was toying, a mere dream for him, 

 was ere long to take a precise form and become one of the vital con- 

 cerns of exact science. 



A dream it remained for many years, till one day, after electrical 

 measurements had become extremely exact, some physicist, turning 

 over the numerical data, much as a resting pedestrian might idly turn 

 over a stone, brought to light an odd coincidence. It was that the 

 factor of transformation between the system of electro-statical units 

 and the system of electro-dynamical units was equal to the velocity 

 of light. Soon the observations directed to this strange coincidence 

 became so exact that no sane head could longer hold it a mere coinci- 

 dence. No longer could it be doubted that some occult affinity ex- 

 isted between optical and electrical phenomena. Perhaps, however, 

 we might be wondering to this day what this affinity could be were it 

 not for the genius of Clerk Maxwell. 



DISPLACEMENT CURRENTS 



The reader is aware that solid bodies are divided into two classes, 

 conductors through which electricity can move in the form of a 

 galvanic current, and nonconductors, or dielectrics. The electricians 

 of former days regarded dielectrics as quite inert, having no part to 

 play but that of obstinately refusing passage to electricity. Had that 

 been so, any one nonconductor might be replaced by any other with- 

 out making any difference in the phenomena ; but Faraday found that 

 that was not the case. Two condensers of the same form and dimen- 

 sions put into connection with the same source of electricity do not 

 take the same charge, though the thickness of the isolating plate be 

 the same, unless the matter of that plate be chemically the same. 

 Now Clerk Maxwell had too deeply studied the researches of Fara- 

 day not to comprehend the importance of dielectrics and the impera- 

 tive obligation to recognize their active part. 



Besides, if light is but an electric phenomenon, when it traverses 

 a thickness of glass electrical events must take place in that glass. 

 And what can be the nature of those events? Maxwell boldly an- 

 swers, they are, and must be, currents. 



All the experience of his day seemed to contradict this. Never had 



