418 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX. 



tinic behavior, the former furnishing by far the most in- 

 teresting series of facts. 



When a polished surface of- silver is exposed at com- 

 mon temperatures to the vapor of iodine, it speedily 

 tarnishes, a film of silver iodide forming. This passes 

 through several well-marked tints as the exposure con- 

 tinues and the thickness increases. They may be thus 

 enumerated in the order of their occurrence : 1, lemon- 

 yellow; 2, golden -yellow; 3, red; 4, blue; 5, lavender; 

 6, metallic ; 7, deep yellow ; 8, red ; 9, green. 



All these films are sensitive. Under the influence of 

 radiations they exhibit two phases of modification : 1st, 

 an invisible modification, which, however, can be made 

 apparent or developed, as Dagtierre discovered, by ex- 

 posure to the vapor of mercury, the iodide turning white 

 by the condensation of mercury upon it wherever it has 

 been exposed to light, but remaining unacted upon in 

 parts that have been in shadow ; 2d, a visible modifica- 

 tion, which arises under a longer exposure, the iodide 

 passing through various shades of olive and blue, and 

 eventually becoming dark gray. 



But though all the variously tinted films of silver 

 iodide are impressionable, they differ greatly in relative 

 sensitiveness when compared with each other. This may 

 be very satisfactorily shown by producing on one silver 

 tablet bands of all the above-named colors an effect 

 readily accomplished by suitably unscreening successive 

 portions of the tablet during the process of iodizing, and 

 then exposing all at the same time to a common radiation. 

 It will be found, on developing with mercury vapor, that 

 the bands of a yellow color have been the most sensitive, 

 those of a metallic aspect have been scarcely acted on, 

 and those of other tints intermediately. It is to be par- 

 ticularly remarked that the second yellow, numbered 7 

 in the above series, is equally sensitive with the first 

 yellow, numbered 2. 



