402 DISTRIBUTION OF HEAT IN THE SPECTRUM. [MEMOIR XXVIII. 



the conversion of motion into heat an illustration of 

 the modern doctrines of the conservation and transmu- 

 tation of force. 



From this point of view, the conception that there ex- 

 ist in an incident ray various principles disappears alto- 

 gether. We have to consider an incident ray as consist- 

 ing solely of ethereal vibrations, which, when they are 

 checked by an extinguishing substance, lose their vis viva. 

 The effect that ensues depends on the quality of the sub- 

 stance. The vibrations imparted to it may be manifest- 

 ed by the production of heat, as in the case of lamp- 

 black, or by chemical changes, as in the case of many of 

 the salts of silver. In the parallel instance of acoustics 

 clear views have long ago been attained, and are firmly 

 held. No one supposes that sound is one of the ingre- 

 dients of the atmosphere, and it would not be more in- 

 correct to assert that it is something emitted by the 

 sounding body than it is to affirm that light or heat, or 

 actinism, is emitted by the sun. 



The progress of actino-chemistry would be greatly ac- 

 celerated if there could be steadfastly maintained a clear 

 conception of the distinction between the mechanism of 

 a ray and the effects to which that ray may give rise. 

 The evolution of heat, the sensation of light, the produc- 

 tion of chemical changes, are merely effects manifesta- 

 tions of the motions imparted to ponderable atoms; and 

 these in their turn can give rise to converse results, as 

 when we gradually raise the temperature of a substance 

 the oscillating movements of its molecules are imparted 

 to the ether, and waves of less and less length are suc- 

 cessively engendered. 



In the title of this Memoir I have employed the phrase 

 "distribution of heat" in accordance with general usage; 

 but if the conclusion arrived at be true, it is plain that 

 this should be exchanged for " production of heat." The 



