176 ABSORPTION OF LIGHT. SECT. XX. 



to our knowledge, for its explanation of the absorptive pheno- 

 mena. In attempting to explain the extinction of light on the 

 corpuscular doctrine, we have to account for the light so extin- 

 guished as a material body, which we must not suppose annihi- 

 lated. It may, however, be transformed ; and among the impon- 

 derable agents, heat, electricity, &e., it may be that we are to 

 search for the light which has become thus comparatively stag- 

 nant. The heating power of the solar rays gives a prima 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 luminous rays are not the most 

 calorific, but that, on the contrary, the calorific energy accom- 

 panies, in its greatest 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 appears to have been agitated among 

 the photologists of the last century, has been regarded as one of 

 considerable 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 dis- 

 tinct. 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 for ever. 

 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 abstracting 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 motion', in which every molecule continues 

 to be agitated by an indefinite multitude of internally reflected 

 waves, propagated through it in every possible direction, from 

 every 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 com- 

 plete the more irregular the figure of the body, and the greater 



