A HISTORY OF SCIENCE 



grasped the meaning of the doctrine of conservation. 

 When, late in the fifties, that marvellous young Scotch- 

 man, James Clerk-Maxwell, formulating in other words 

 an idea of Faraday's, expressed his belief that electric- 

 ity and magnetism are but manifestations of various 

 conditions of stress and motion in the ethereal medium 

 (electricity a displacement of strain, magnetism a whirl 

 in the ether) , the idea met with no immediate populari- 

 ty. And even less cordial was the reception given the 

 same thinker's theory, put forward in 1863, that the 

 ethereal undulations producing the phenomenon we call 

 light differ in no respect except in their wave-length 

 from the pulsations of electro-magnetism. 



At about the same time Helmholtz formulated a 

 somewhat similar electro-magnetic theory of light ; but 

 even the weight of this combined authority could not 

 give the doctrine vogue until very recently, when the 

 experiments of Heinrich Hertz, the pupil of Helmholtz, 

 have shown that a condition of electrical strain may be 

 developed into a wave system by recurrent interrup- 

 tions of the electric state in the generator, and that 

 such waves travel through the ether with the rapidity 

 of light. Since then the electro-magnetic theory of 

 light has been enthusiastically referred to as the great- 

 est generalization of the century ; but the sober thinker 

 must see that it is really only what Hertz himself 

 called it one pier beneath the great arch of conserva- 

 tion. It is an interesting detail of the architecture, 

 but the part cannot equal the size of the whole. 



More than that, this particular pier is as yet by no 

 means a very firm one. It has, indeed, been demon- 

 strated that waves of electro-magnetism pass through 



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