238 CHEMICAL RAYS AND RADIANT HEAT. [MEMOIR XVII. 



The plate from which the drawing of Fig. 37 is taken 

 gives from a to b ten parts, from to c seventeen parts, 

 from d to e twelve parts ; we perceive, therefore, how 

 large an amount of light is absorbed, and its effects ren- 

 dered latent, between the maximum of whiteness being 

 gained and solarization setting in. 



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

 ponderable material what rays shall be absorbed. 



I had prepared a number of observations in proof of 

 this, very much of the same kind as those which have 

 some time ago been published in the Phil. Trans, by 

 Herschel. These refer chiefly to the variable lengths of 

 the stains impressed by the prismatic solar spectrum on 

 different chemical bodies, and the points of maximum ac- 

 tion noticed in them. For the present I content myself 

 with referring to that Memoir for proofs substantiating 

 this proposition. 



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 

 depend on the condition of its surfaces and interior ar- 

 rangement. 



I took a polished silver plate, and having exposed it to 

 the vapor of iodine, found that it passed through the fol- 

 lowing changes of color : 1st, lemon yellow; 2d, golden 

 yellow; 3d, reddish yellow; 4th, blue; 5th, lavender; 6th, 

 metallic; 7th, yellow ; 8th, reddish ; 9th, green, etc., the 

 differences of color being produced by the differences of 

 thickness in the film of iodide, and not by any difference 

 of chemical composition. 



It is a common remark, originally made by Daguerre, 

 that of these different tints that marked 2 is the most 

 sensitive, and photogenic draughtsmen generally suppose 

 that the others are less efficient from the circumstance 

 of the film of iodide being too thick. Some suppose, 



