The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Silver Nitrate
so sensitive to light as when excess of nitrate of silver is used, when the precipitated iodide is lemon-coloured. Iodide of silver is insoluble in water and dilute nitric acid, almost insoluble in ammonia, but soluble in all the other salts which dissolve bromide. When iodide of silver is dissolved in excess of solu-tion of an alkaline bromide, iodide, or chloride, a double salt is formed, and on addition of water the resulting solution im-mediately precipitates the iodide. It is used for making emulsions, giving extremely sensitive emulsions and great latitude of exposure, with great density of image, and is also used in the wet collodion process.
Silver Nitrate (Ger., Silbernitrat, Salftetersaures Silberoxyd; Fr., Azotate (Targent; Ital., Azotato d'argentd). AgN03= 170. Is prepared from pure silver by solution in nitric acid, and sub-sequent purification and crystallisation. Ordinary commercial nitrate is usually very acid, due to its not being absolutely freed from nitric acid; but that prepared for photographic purposes should be almost neutral, or at least show only a faint trace of acid. To the dry-plate operator nitrate of silver is hardly of so much interest as it was to the operator of the wet-plate process ; but to those who desire to make their own plates, a pure salt is a sine qua non; and as the testing of this salt is almost beyond the ordinary capabilities of the dry-plate workers, the only recommendation which the author can give is to buy the salt from a reputable firm. Solubility: 100 grains are soluble in 50 minims of distilled water, and will measure 80 minims : 1 in 15 of rectified spirit. When dissolved in common water, a thick curdy-white precipitate of carbonate and chloride of silver is formed. Boiling alcohol dissolves about one-fourth of its weight of nitrate of silver, but deposits it on cooling. It is soluble in ammonia, with the formation of a double salt. It is used for sensitising paper, and preparing all, or nearly all, the other salts of silver used in photography. When heated it melts, and forms, when poured into moulds, the lunar caustic of commerce ; and when heated higher still gives off some oxygen, and a mixture of nitrite (AgN02) and nitrate of silver (AgN03) is left. When exposed to the light, either in solution or in a pure dry state, no action takes place; but on contact with organic matter it darkens through purple to black.