MEMOIR XVII. ] CHEMICAL RAYS AND RADIANT HEAT. 231 



3d. That, as when rays of heat fall on a mass of cold 

 ice its temperature rises degree by degree until it reaches 

 32 Fahr. and there stops until a certain molecular 

 change (liquefaction) is accomplished, and after that 

 rises again, so also the chemical rays impress certain 

 changes proportional to their quantity, up to a certain 

 point, and there a pause ensues a very large amount of 

 light being now rendered latent or absorbed, without 

 any indication thereof being given by the sensitive prep- 

 aration (as the heat of fluidity is latent to the thermom- 

 eter) ; a molecular change then setting in, the incre- 

 ments of the quantity of light are again indicated by 

 changes in the sensitive preparation. 



4th. That it depends on the CHEMICAL nature of the 

 ponderable material what rays shall be absorbed. 



5th. That while the specific rays thus absorbed de- 

 pend upon the chemical nature of the body, the absolute 

 amount is regulated by its OPTICAL qualities, such as de- 

 pend on the condition of its surfaces and interior ar- 

 rangement. 



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6th. It will be proved from this that the SENSITIVE- 

 NESS of any given substance depends on its chemical 

 nature and optical qualities conjointly, and that it is 

 possible to exalt or diminish the sensitiveness of any 

 chemical compound by changing the character of its op- 

 tical relations. 



7th. That, as when radiant heat falls on the surface of 

 an opaque body, the number of rays reflected is the com- 

 plement of those that are absorbed, so in the case of 

 a sensitive preparation, the number of rays reflected 

 from the surface is the complement of those that are 

 absorbed. 



I now commence with the proofs of the propositions 

 of this Memoir. 



1st. That the chemical action produced by the rays 



