434 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX. 



gradually warm ; and if the exposure be not too long or 

 the fire too hot, on removing it the paper will gradually 

 cool, recovering its former condition without any perma- 

 nent change. One could conceive that the laws of ab- 

 sorption and radiation might not only be studied, but 

 again and again illustrated by the exposure and removal 

 of such a sheet. But a certain point of temperature or 

 exposure gained, the paper scorches that is, undergoes 

 chemical change and then there is no restoration, no re- 

 covery of its original condition. Hence it may be said 

 of such a sheet of paper that it exhibits two phases, in 

 the first of which a return to the original condition is 

 possible ; in the second such a return is impossible, be- 

 cause of the supervening of the chemical change. 



An investigation of the facts produced by a ray pre- 

 sents, then, these two separate and distinct phases the 

 physical and the chemical. 



General Conclusions. 



The facts presented in the preceding and the present 

 Memoir suggest the following conclusions: 



1st. That the concentration of heat heretofore observed 

 in the less refrangible portion of the prismatic spectrum 

 arises from the special action of the prism, and would not 

 be perceived in a diffraction spectrum. 



2d. From the long-observed and unquestionable fact 

 that there is in the prismatic spectrum a gradual dimi- 

 nution in the heat-measures from a maximum below the 

 red to a minimum in the violet, coupled with the fact 

 now presented by me (that the heat of the upper half of 

 the spectrum is equal to that of the lower half), it follows 

 that the true distribution of heat throughout the spaces 

 of the spectrum is equal. In consequence of the equal ve- 

 locity of ether-waves, they will, on complete extinction by 

 a receiving surface, generate equal quantities of heat, no 



