192 THE WORLD-ENERGY 



Nevertheless, this transmission of motion from the 

 center outward by means of vibrations in a medium 

 involves something more than mere outward impulse. 

 Both center and medium must be highly elastic if any 

 movement is to be actually transmitted. For the out- 

 ward thrust of every single particle from the center of 

 radiation already, and necessarily, involves a tension 

 which can exist only through the inseparable union of 

 attraction and repulsion. 



The vibration, besides, is not merely an outward 

 thrust, but is also and equally a rebound. In other 

 words, vibrations can take place only in an elastic 

 medium. And elasticity is nothing else than the inter- 

 fusion of molecular attraction and repulsion. Thus the 

 undulatory theory of heat, light and electricity, only 

 brings into greater clearness the extreme simplicity of 

 the physical world. 



So, too, this simplicity of the physical world becomes 

 still more impressive when we further consider the 

 familiar fact, that like light, sound too, as an outer 

 physical fact, is nothing else than vibratory motion in an 

 elastic medium. It would indeed be only to repeat what 

 has been said of the former, were we to state the funda- 

 mental characteristics of the latter. 



There are, of course, important qualitative differences 

 in the media, as well as quantitative differences in the 

 vibrations which take place in these media. No doubt, 

 too, that the complexity of grouping of the particles 

 incident to the vibrations, in the one case, is greater 

 than in the other. But, after all has been said, the fun- 

 damental fact remains that sound and light, considered 

 simply as physical facts apart from their effects in 



