sert in 1895 that in fifty years he had learned nothing 

 new regarding the nature of energy. 



This, however, must not be interpreted as meaning 

 that the world has stood still during these two genera- 

 tions. It means rather that the rank and file have been 

 moving forward along the road the leaders had already 

 travelled. Only a few men in the world had the range 

 of thought regarding the new doctrine of energy that 

 Lord Kelvin had at the middle of the century. The 

 few leaders then saw clearly enough that if one form of 

 energy is in reality merely an undulation or vibration 

 among the particles of ' k ponderable " matter or of ether, 

 all other manifestations of energy must be of the same 

 nature. But the rank and file were not even within 

 sight of this truth for a long time after they had partly 

 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 electrici- 

 ty and magnetism are but manifestations of various con- 

 ditions 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, 



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