56 CHEMICAL PHYSICS. 



3. LIGHT. 



Light a form of energy. It has been shown in the preceding 

 chapter that a heated body sends forth waves through the ether, 

 which in striking a cooler body cause its molecules to vibrate more 

 quickly i. e., heat it to a higher temperature. But an increased 

 molecular motion may produce other effects than heat, as we may 

 observe by heating an iron bar, which, when sufficiently hot, will 

 emit light. That light is different in effect from heat, though both 

 come from the same hot body, may be observed in the action of 

 luminously hot bodies on certain chemicals ; for example, the silver 

 salts, which undergo a change in their chemical composition. The 

 action of sunlight on photographic plates is an excellent illustration. 



The explanation of these phenomena is that ether waves of diverse 

 characters produce different effects. Thus, the waves coming to us 

 from the sun may affect the sense of touch, and we call the effect 

 heat ; they may affect the sense of sight, and we term it light ; or 

 they may affect the composition of matter, when we speak of it as 

 chemical action. But in all these results we have simply different 

 manifestations of some form of radiant energy. Light is that form 

 of energy which may be appreciated by the organ of vision. 



Color. Both heat- and light-producing waves or oscillations are 

 propagated through ether with the same velocity, viz., at the rate of 

 300,000 kilometers, or 186,000 miles, per second ; but the waves 

 differ in regard to length and in the amplitude of vibration. Waves 

 of a particular limited range of frequency (477,000,000 millions to 

 699,000,000 millions per second) when falling upon the eye produce 

 a sensation of light. Of these, the slowest i. e., the waves with 

 the least frequency and the greatest wave-length produce a sensa- 

 tion of red light ; as the frequency increases, the sensation produced 

 by them is successively that which is termed orange, yellow, green, 

 blue, indigo, and violet light. 



Ether waves of frequency too limited to be visible are called infra- 

 red waves ; they have, however, great heating power when falling 

 upon a substance. On the other hand, there are waves of greater 

 frequency than those which produce a sensation of violet ; they are 

 also invisible, but have the power of producing chemical action ; they 

 are called ultra-violet or actinic waves. There is no inherent difference 

 between any of the different kinds of waves here mentioned ; indeed, 

 they all may be produced at the same time by the same body. 



In what is called daylight there is a mixture of waves of different 

 frequencies, affecting our eye simultaneously in such a manner 



