17 



combination with other bodies bj' the adoption of different modes of 

 reduction. In the main, however, and in general terms, these pro- 

 cesses have but one object in view, and the chemical principles involved 

 are analogous, although the results-vary very much, both in appearance 

 and in actual condition. They resemble, however, more or less the 

 products which would be obtained by the action of the reducing agents 

 under favoui'able circumstances upon the salts themselves, the portions 

 acted upon by light having only anticipated, in point of time, the result 

 which would follow over the whole surface were not the action arrested 

 at the proper moment. 



For example, in the daguerreotype process very finely divided iodide 

 of silver is obtained by submitting a highly polished silver plate to the 

 vapour of iodine. The light affects the structure of these crystals so 

 that when submitted to the vapour of mercury, reduction and amalga- 

 mation takes place, which the undecomposed iodide restrains from 

 spreading over the whole surface as would otherwise take place. Then 

 by immersion in an alkaline solvent, such as the hyposulphite of soda, 

 the iodide is removed and the silver beneath it exposed intact. This 

 white amalgam is obtained in a crystalline form when mercury is sus- 

 pended in a solution of nitrate or sulphate of silver, and is known as 

 the " silver tree." 



In the calotype process, paper is first saturated with iodide of potas- 

 sium, then with nitrate of silver, washed and exposed to the light: the 

 surface thus presented consists of iodide of silver, free nitrate of silver, 

 and organic matter; the latter materially assisting in the reduction. 

 The chemical agent employed to develope the image in this process is 

 gallic acid, which throws down metallic silver from its solutions in the 

 form of a black powder. In this state the silver is therefore reduced 

 in intimate adhesion to the pores and surface of the paper. The un- 

 decomposed iodide is dissolved out as before, and the negative rendered 

 more transparent by being saturated with wax. Among the processes 

 on glass the albumen process most nearly assimilates to the calotype. 

 Plates of glass being coated with albumen containing iodide of potas- 

 sium, are dried, saturated with nitrate of silver, exposed to the light, 

 then developed by gallic acid, and lastly, fixed by hj'posulphite of soda. 

 Thus the re-agents employed are the same as before, and effect the same 

 purpose, but the organic matter is very different; of course we have all 

 the advantage of additional translucency and great brilliancy of effect ; 

 but besides tliis we find the process greatly accelerated, and the tone of 

 the result much modified. How shall wc account for this except by 

 referring to the nature of albumen as a highly complex organic com- 



