Chemical Force in the Spectrum. 431 



In the examination of this subject I shall select two well- 

 known instances, presenting the fewest elements and the sim- 

 plest conditions. They are (1) the decomposition of silver iodide, 

 the basis of so many photographic preparations ; (2) the pro- 

 duction of hydrochloric acid by the union of its two contituents, 

 chlorine and hydrogen, a mixture of these gases being exceed- 

 ingly sensitive to light. 



1st. Of the decomposition of Silver Iodide, 



There are two forms in which the silver iodide has been used 

 for photographic purposes : — (1) when prepared by the action of 

 the vapour of iodine on metallic silver, as in the Daguerreotype 

 tablet; (2) when nitrate of silver is decomposed by iodide of 

 potassium or other metallic iodide. These preparations differ 

 strikingly in their actinic behaviour, the former furnishing by 

 far the most interesting series of facts. 



When a polished surface of silver is exposed at common tem- 

 peratures to the vapour of iodine, it speedily tarnishes, a film 

 of silver iodide forming. This passes through several well- 

 marked tints as the exposure continues and the thickness in- 

 creases. 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 radia- 

 tions they exhibit two phases of modification : — (1) an invisible 

 modification, which, however, can be made apparent or developed, 

 as Daguerre discovered, by exposure to the vapour of mercury — 

 the iodide turning white by the condensation of mercury upon 

 it wherever it has been exposed to the light, but remaining 

 unacted upon in parts that have been in shadow; (2) a visible 

 modification, which arises under a longer exposure, the iodide 

 passing through various shades of olive and blue, and eventually 

 becoming dark grey. 



But though all the variously tinted films of silver iodide are 

 impressionable, they diff'er 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 colours — an eff'ect readily accomplished by suitably un- 

 screening successive portions of the tablet during the process of 

 iodizing, and then exposing all at the same time to a conmion 

 radiation. It will be found, on developing with mercury vapour, 

 that the bands of a yellow colour 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 particularly remarked 

 that the second yellow, numbered 7 in the above series, is equally 

 sensitive as the first yellow, numbered 2. 



