442 On the Distribution of Chemical Force in the Spectrum, 



3rd. If the substance is decomposable, those portions of 

 the radiation presented to it which are of a complementary tint 

 will be extinguished. The force thus disappearing will not 

 be expended in establishing vibrations in the arresting parti- 

 cles, but in breaking down -the union of those which have ar- 

 rested them from associated particles. No vibrations, there- 

 fore, are originated, no heat is produced, there is no lateral con- 

 duction. 



In actinic decompositions the effects may be conveniently 

 divided into two phases: — 1st, physical; 2nd, chemical. 



The physical phase precedes the chemical. It consists in a 

 preliminary disturbance of the group of molecules about to be 

 decomposed. Up to a certain ponit the dislocation taking place 

 may be retraced or reduced, and things brought back to their 

 original condition. But that point once gained, decomposition 

 ensues, and the result is permanent. 



I may perhaps illustrate this by a familiar example. If a 

 sheet of paper be held before a fire, its surface will 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 permanent change. One could 

 conceive that the laws of absorption 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 tempera- 

 ture or exposure gained, the paper scorches — that is, undergoes 

 chemical change ; and then there is no restoration, no recovery 

 of its original condition. 



Hence it may be said of such a sheet of paper that it exhi- 

 bits two phases, in the first of which a return to the original 

 condition is possible ; in the second such a return is impossible, 

 because of the supervening of a chemical change. 



An investigation of the effects produced by a ray presents, 

 then, these two separate and distinct phases — the physical and 

 the chemical. 



General Conclusions. 



The facts presented in the former 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. 



2nd. From the long-observed and unquestionable fact that 

 there is in the prismatic spectrum a gradual diminution in the 

 heat- measures, from a maximum below the red to a minimum 



