MEMOIR XVII.] CHEMICAL KAYS AND RADIANT HEAT. 243 



sively sensitive, because it absorbs all the chemical rays 

 that can disturb it, while the lavender is insensitive, be- 

 cause it reflects them. Under this point of view, sensi- 

 tiveness therefore is directly as absorption and inversely 

 as reflection. 



The superiority of Daguerre's preparation over com- 

 mon sensitive paper may now be readily understood. It 

 absorbs all the rays that can affect it, but the chloride 

 of silver, spread upon paper, reflects many of the active 

 rays. The former, when placed in the camera, gives rise 

 to no reflections that can be injurious; the latter fills it 

 with active light, and stains the proof all over. Hence 

 the daguerreotype has a sharpness and mathematical ac- 

 curacy about its lines, and a depth in its shadows, which 

 is unapproachable by the other. Moreover, the translu- 

 cency of the white chloride of silver, as well as its high 

 reflecting power, permits of particles lying out of the 

 lines of light being affected, the light becoming diffused 

 in the paper. 



The fact, therefore, that a given compound remains un- 

 changed even in the direct rays of the sun is no proof 

 that light cannot decompose it ; it may reflect or trans- 

 mit the active rays as fast as it receives them. It results 

 from this that optical conditions can control and even 

 check the play of chemical affinities. While thus it ap- 

 pears that there are points of analogy between this chem- 

 ical agent and radiant heat, we must not too hastily infer 

 that the laws which regulate the one obtain exclusively 

 also with the other. As is well known, there are strik- 

 ing analogies between radiant heat and lisjht. but there 



o o o / 



are also points of difference, the convertibility of heat of 

 one degree of refrangibility to another does not occur 

 with light ; there are also dissimilitudes in the phenom- 

 ena of radiation and its consequences. 



From the phenomena of the interference of these rays, 



