HELMHOLTZ ON THE CONSERVATION OF FORCE. 129 



The vis viva of elastic waves is only destroyed by such pro- 

 cesses as we denominate absorption. The absorption of sonorous 

 waves we find to be chiefly effected by concussion against yielding 

 unelastic bodies, for example, curtains and coverlets ; they may 

 therefore be regarded as a communication of motion to the 

 bodies in question, in which the motion is destroyed by friction. 

 Whether motion can be destroyed by the friction of the air- par- 

 ticles against each other is a question which cannot yet perhaps 

 be decided. The absorption of rays of heat is accompanied by a 

 proportional development of heat ; how far the latter corresponds 

 to a certain equivalent of force, we will consider in the next 

 section. The conservation of force would take place if the 

 quantity of heat radiated from one body appeared again in the 

 body into which it was radiated, provided that none was lost by 

 conduction, and no portion of the rays escaped elsewhere. The 

 theorem is certainly assumed in investigations upon radiant heat, 

 but I am aware of no experiments which furnish the proof of it. 

 As regards the absorption of light by imperfectly transparent or 

 totally opake bodies, we are acquainted with three peculiarities. 

 In the first place, phosphorescent bodies absorb the light in such 

 a manner that they yield it up again afterwards. Secondly, most 

 luminous rays, perhaps all of them, appear to excite heat. The 

 obstacles to the belief in the identity of the luminous, calorific, 

 and chemical rays of the spectrum have been lately disappearing 

 more and more j the heat-equivalent of the chemical and luminous 

 rays appears to be very inconsiderable in comparison to their 

 intense actions upon the eye. If, however, the similarity of these 

 differently acting rays does not permit of being established, then 

 the end of the motion of light must undoubtedly be declared to be 

 unknown. Thirdly, in many cases the light absorbed developes 

 chemical action. We must here distinguish two species of 

 action; first, where the mere incitement to chemical activity 

 is communicated, as in the case of those bodies which induce 

 catalytic action, for example, the action upon a mixture of 

 chlorine and hydrogen; and secondly, those in which it is 

 opposed to chemical action, as in the decomposition of salts of 

 silver and the action upon the green portions of plants. In 

 most of these processes however, the effect of light is so 

 little known, that we are able to form no judgement regarding 

 the magnitude of the forces developed. The latter appear to be 



