n6 TEE POPULAR SCIENCE MONTHLY 



this hypothesis, that light consisted of a motion of the nature of waves, 

 obtained the victory over the old notion that light consisted of small 

 particles shot out from the luminous object with great rapidity. The 

 laws of the propagation of these waves were thoroughly described by 

 Fresnel, but what the properties of the substance in which they were 

 propagated, and called the ether, might be, was long a matter of diffi- 

 culty. Green had shown that an elastic solid of a certain sort would 

 possess many of the properties of the ether, but this mechanical theory 

 was insufficient in certain ways. It was the conjecture of Faraday that 

 the actions of electrified and magnetic bodies upon each other, so thor- 

 oughly investigated by him, were transmitted to each other by means 

 of the ether, and it was the genius of Maxwell and his wonderful scien- 

 tific imagination, that enabled him to erect this conjecture into a 

 remarkably perfect theory. By means of the hypothesis that electrical 

 and magnetic actions are subject to the laws of mechanics, Maxwell was 

 able to apply to electric currents the equations of Lagrange for dealing 

 with the motion of the most general mechanical systems of bodies. In 

 this way Maxwell explained the laws of the induction of currents, the 

 difficulty of starting or stopping a current corresponding to the inertia 

 of a heavy body. Presently by the aid of an auxiliary assumption the 

 properties of the ether were described, and the remarkable result found 

 that electrical and magnetic effects would be propagated with the 

 velocity of light. From this it was a short step to declare light waves 

 to be electromagnetic waves. Such waves were not known at the time 

 of Maxwell's paper in 1865, and his theory waited long for acceptance. 

 In 1888, however, Heinrich Hertz, guided by his great master Helm- 

 holtz into acceptance of Maxwell's theory, succeeded, in a most brilliant 

 series of experiments, in producing the very waves predicted by Max- 

 well, and in showing that they traveled with the velocity of light. 

 These are the waves made use of by Marconi for transmitting intelli- 

 gence, and conquering the sea in peace and in war. But this is not all, 

 for Maxwell, in his description of the manner in which the ether trans- 

 mitted the electromagnetic waves, assumed that it was subject to cer- 

 tain stresses, so that a surface receiving a beam of light would experi- 

 ence a certain pressure, the amount of which he calculated. This result 

 awaited verification until 1900, when the Eussian physicist Lebedew, 

 and in 1903 still more exactly the Americans Nichols and Hull, now 

 president and professor, respectively, at Dartmouth College, gave it a 

 magnificent verification. Thus by faith Maxwell subdued kingdoms, 

 obtained promises, out of weakness was made strong, turned to flight 

 the armies of the aliens of ignorance. 



An interesting conclusion that may be drawn from the history of 

 scientific endeavor is that there is an accepted time for each discovery, 

 that is, that a certain stage of human progress the discovery is cer- 



