480 ON THE ABSORPTION OF LIGHT 



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 for ever. No motion is, strictly speak- 

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

 parts made to oppose and, in point of ultimate effect, 

 counteract each other. 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-compensating motion, 

 in which every molecule continues to be agitated by an 

 indefinite multitude of internally-reflected waves, propa- 

 gated 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 counteraction, which will be 

 the more complete, the more irregular the figure of the 

 body and the greater the number of internal reflections. 

 (5.) In the case of a body perfectly elastic and of a 

 perfectly regular figure, the internal reflection of a wave 

 once propagated within it in some particular direction 

 might go on for ever without producing mutual destruc- 

 tion ; and in sonorous bodies of a highly elastic nature 

 we do in fact perceive it to continue for a very long time. 

 But the least deviation from perfect elasticity resolves our 

 conception of the vibrating mass into that of a multitude 

 of inharmonious systems communicating with each oilier. 



