THE WOMBAT. 83 



of the developer. But, fortunately for the photographer, the 

 sub-bromide is more soluble than the bromide, and is therefore 

 reduced sooner, so that when the plate is brought into contact 

 with the developer the sub-bromide (which exists upon those 

 parts only which the light has affected) is reduced first, and 

 the picture is thus rendered visible. The chemical reaction is 

 as follows: - 3 (Fe C. Cb ) I 2 (Ag, Br) = 2 Ag 2 + Fe, 3 (C» Cb ) 

 I Fe Br_< i,e., three molecules of ferrous oxalate plus 

 two molecules of sub-bromide of silver = two molecules 

 of silver plus one molecule of ferric oxalate plus one molecule of 

 bromide of iron. If the developer acts very quickly, it will 

 reduce not only the sub-bromide, but the bromide also. It is 

 thus that there is produced what is called " chemical fog." 

 Now it is found that the bromide of silver is rendered still 

 more insoluble if a little bromide of potassium is added to the 

 solution, the sub-bromide being still amenable to the action of 

 the developer. This is why potassium bromide is used as a 

 restrainer. 



We now come to intensification. You will remember 

 that the black image in the negative consists of a deposit of 

 black metallic silver, each molecule being represented by the 

 symbol Ag-), that is, it contains two atoms of silver. To 

 intensify this image it is treated with a solution of bi-chloride 

 of mercury (Hg Cl-2 ). Thus we have : — Aga, ' 2 (Hg CI, ) = 

 2 (Ag Hg CL ) ; one molecule of silver plus two molecules of 

 bi-chloride of mercury = two molecules of chloride of mercury 

 and silver. This double chloride of mercury and silver is 

 white and the image now appears white ; it is said to be 

 bleached. It is then treated with a solution of ammonia 

 ,(NH 4 HO). 



AgHgCl, ! 2 (NH 4 HO)-Ag Hg O i 2 (NH 4 CI) + 

 Ho O i.e., one molecule of the double chloride of silver and 

 mercury plus two molecules of ammonia give one molecule of 

 black double oxide of silver and mercury plus two molecules 

 of chloride of ammonia plus one molecule of water. Thus 

 ■each molecule of silver is replaced by two molecules of black 

 <louble oxide of mercury and silver. 



Printing out Papers. As far as chemical reactions are 

 ■concerned, albumen and gelatino chloride papers are practi- 

 cally the same. They both contain chloride and nitrate of 

 silver, and also organic salts of silver. Of the two former 

 salts the chloride is the more easily reduced, giving a purple 

 colour to the print. If it be treated with ammonia the colour 

 is almost entirely reduced, as the dark deposit (sub-chioride of 

 silver) is almost soluble in ammonia. But if the paper be 

 still further exposed to the light, it becomes reddish brown 

 owing to the organic salts and the nitrate being reduced to 

 oxide of silver ; if treated now with ammonia or hypo, the 



