COLLODION PROCESS. 



361 



this operation requires a little practice to accomplish satisfactorily. 

 In a few seconds the ether mostly evaporates, and a semi-transparent 



Fig. 195. Pouring Collodion on Plate. 



Fig. 196. Glass Plate 

 Holder. 



film of soft collodion is obtained on the surface of the glass ; better 

 adherence of the film to the glass when dry is obtained if the glass 

 be coated with albumin beforehand. The glass is now dipped 

 into a watery solution of nitrate of silver (about 1 ounce of the 

 solid salt to 10 or 12 of water). Fig. 195 represents one way in 

 which the collodion may be poured over the glass plate so as to 

 form a uniformly thick film ; and fig. 196 represents the glass plate 

 supported on a slab of glass or por- 

 celain, with a ledge at the bottom so 

 as to enable it to be dipped into the 

 nitrate of silver bath (fig. 197). Double 

 decomposition (Expts. 11, 12) now takes 

 place between the iodide and bromide 

 of cadmium contained in the collodion 

 and the nitrate of silver, so that nitrate 

 of cadmium is formed, which dissolves 

 in the water, and iodide and bromide of 

 silver are precipitated in small particles 



disseminated through the mass of the F - 19 ^ gQ ver B^ 

 collodion, this salt being insoluble in 



water ; the action being very similar to that whereby chloride of 

 sodium and nitrate of silver give rise to soluble nitrate of sodium 

 and insoluble chloride of silver in Expt. 37. 



The glass covered with collodion should be then sensitised in 

 a dark room, i.e., one illuminated only with yellow light, such as a 

 sodium flame (Expts. 245, 355) or a candle, &c., placed in a lantern 

 with yellow glass sides, so that actinic rays may be prevented from 

 reaching the silver compound dispersed throughout the collodion. 

 If the nitrate of silver solution be perfectly fresh it should be 



