COPPER, SILVER, AND OO.LD 431 



substance be added to the alkali the chloride can easily be reduced to 

 metallic silver, the silver oxide being reduced in the oxidation of the 

 organic substance. Iron, zinc, and many other metals reduce silver 

 chloride in the presence of water. Cuprous and mercurous chlorides 

 and many organic substances are also able to reduce the silver from 

 chloride of silver. This shows the rather easy decomposability of the 

 halogen compounds of silver; Silver iodide is much more stable in this 

 respect than the chloride. The same is also observed with respect to 

 the action of light upon moist AgCL White silver chloride soon acquires 

 a violet -colour when exposed to the action of light, and especially 

 under the direct action of the sun's rays. After being acted upon 

 by light it is no longer entirely soluble in ammonia, ,but leaves 

 metallic silver undissolved, from which it might be assumed that the 

 action of light consisted in the decomposition of the silver chloride 

 into chlorine and metallic silver and in fact the silver chloride be- 

 comes in time darker and darker. Silver bromide and iodide are much 

 more slowly acted on by light, and, according to certain observations, 

 when pure they are even quite unacted on ; at least they do not change 

 in weight, 24 bi * so that if they are acted on by light, the change they 

 undergo must be one of a change in the structure of their parts and not 

 of decomposition, as it is in silver chloride. The silver chloride under 

 the action of light changes in weight, which indicates the formation of 

 a volatile product, and the deposition of metallic silver on dissolving 

 in ammonia shows the loss of chlorine. The change does actually 

 occur under the action of light, but the decomposition does not go 

 as far as into chlorine and silver, but only to the formation of a sub- 

 chloride of silver, Ag 2 01, which is of a brown colour and is easily de- 

 composed into metallic silver and silver chloride, Ag 2 Cl = AgCl + Ag. 

 This change of the chemical composition and structure of the halogen 

 salts of silver under the action of light forms the basis of photography, 

 because the halogen compounds of silver, after having been exposed to 

 light, give a precipitate of finely-divided silver, of a -black colour, 

 when treated with reducing agents. 25 



solution of Ag 2 F, obtained poly fluoride of silver, Ag 2 F, which is decomposed by water 

 into AgF + Ag. It is also formed by the action of a strong solution of AgF upon finely- 

 divided (precipitated) silver. 



24 bu The changes brought about by the action of light necessitate distinguishing the 

 photo-salts of silver. 



35 In photography these are called ' developers.' The most common developers are : 

 solutions of ferrous sulphate, pyrogallol, ferrous oxalate, hydroxylamine, potassium sul- 

 phite, hydroquinone (the last acts particularly well and is very convenient to use), &c. The 

 chemical processes of photography are of great practical and theoretical interest ; but it 

 would be impossible in this work to enter into this special branch of chemistry, which has as 

 yet been very little worked out from a theoretical point of view. Nevertheless, we .will pause 



