NITRATE OF SILVER. Ag. 0, = 170. This is one of the most important salts at present used in photography. , It is obtained in its purest form by dissolving pure silver in pure nitric acid, S. G. 1.25, evaporating and crystallizing, and then redissolv ing and recrystallizing. The salt is then neutral to test paper. Commercial nitrate of silver is made by dissolving silver coins, or silver plate, in nitric acid, evaporating and crystallizing, washing the crystals with nitric acid, and then redissolving and recrystalliz ing. The alloy of copper and other metals is not completely got rid of in this way. Commercial nitrate of silver is sometimes adulterated with the nitrates of potass, zinc, lead, &c. Impure nitrate of silver is one of the greatest evils with which the photo grapher has to contend, and his best remedy is to precipitate the chloride by adding salt to the solution of impure nitrate, then to reduc.e the chloride, by fusion in a crucible with twice of its weight of carbonate of soda, to a button of pure metallic silver, and re dissolve this in pure nitiic acid.
Nitrate of silver crystallizes in large, flat, nearly transparent, four or six sided tables. It is soluble in about an equal weight of cold water, and in four times its weight of boiling alcohol, but sparingly soluble in cold alcohol. It is anhydrous, and may be fused and run into moulds. It is slightly deliquescent in moist air. Its solution is perfectly colourless ; and neither the crystals nor the solution are affected by light, unless organic matter be present. It is a powerful caustic, in consequence of its ready decomposition by organic matter ; and therefore highly, poisonous, salt being the best antidote.
It has been recommended to use fused nitrate of silver in photo graphy, because all the free nitric acid which it may contain is driven off by the heat in the process of fusion ; but this is a bad plan when the nitrate of silver is adulterated with nitrate of potass, because nitrite of potass is first formed by heat, and this becomes oxidized at the expense of the nitrate of silver, and forms nitrite of silver, a most injurious substance to introduce into a nitrate bath. Fu.sed nitrate of silver is generally alkaline, probably because some oxide is formed which is combined with the nitrate ; the alkalinity should be neutralized with acetic acid for a negative bath. Fresh distilled water should always be used for dissolving nitrate of silver, as pump or river water contain salts which produce a cloudy precipitate with it. Rain water collected in leaden tanks should on no account be used, as it contains oxide of lead in solution, which is very likely to fog the negative. (See " Leaden Tanks.") Nitrate of silver may easily be fused in a porcelain capsule over a spirit lamp.
If a piece of copper be suspended in a solution of nitrate of silver, the silver is precipitated in beautiful crystals ; but a piece of polished iron or steel is not acted on. A little mercury poured into the solution occasions the precipitation of crystals in the form of a shrub, called the Arbor Dime.