170 ABSORPTION 01' LIGHT SECT. XX. 



become thus comparatively stagnant. The heating 

 power of the solar rays gives a primd facie plausibility 

 to the idea of the transformation of light into heat by 

 absorption. But when we come to examine the matter 

 more nearly, we find it encumbered on all sides with 

 difficulties. How is it, for instance, that the most lu- 

 minous rays are not the most calorific ; but that on the 

 contrary, the calorific energy accompanies, in its great- 

 est intensity, rays which possess comparatively feeble 

 illuminating powers ? These and other questions of a 

 similar nature may perhaps admit of answer in a more 

 advanced state of our knowledge ; but at present there 

 is none obvious. It is not without reason, therefore, 

 that the question ' What becomes of light ?' which ap- 

 pears to have been agitated among the photologists of 

 the last century, has been regarded as one of consider- 

 able importance as well as obscurity by the corpuscular 

 philosophers. On the other hand, the answer to this 

 question, afforded by the undulatory theory of light, is 

 simple and distinct. The question, ' What becomes of 

 light ?' merges in the more general one, ' What becomes 

 of motion ? ' And the answer, on dynamical principles, 

 is, that it continues forever. No motion is, strictly 

 speaking, annihilated ; but it may be divided, and the 

 divided parts made to oppose and, in effect, destroy one 

 another. A body struck, however perfectly elastic, 

 vibrates for a time, and then appears to sink into its 

 original repose. But this apparent rest (even abstract- 

 ing from the inquiry that part of the motion which may 

 be conveyed away by the ambient air) is nothing else 

 than a state of subdivided and mutually destroying mo- 

 tion, in which every molecule continues to be agitated 

 by an indefinite multitude of internally reflected waves, 

 propagated through it in every possible direction, from 

 eveiy point in its surface on which they successively 

 impinge. The superposition of such waves will, it is 

 easily seen, at length operate their mutual destruction, 

 which will be the more complete the more irregular the 

 figure of the body, and the greater the number of inter- 

 nal reflections." Thus Sir John Herschel, by referring 

 the absorption of, light to the subdivision and mutual 

 destruction of the vibrations of ether in the interior of 



