Although termed an _acid_, this substance is strictly _neutral_; it does not redden litmus-paper, and forms no salts. The addition of potash or soda decomposes pyrogallic acid, at the same time increasing the attraction for oxygen; hence this mixture may conveniently be employed for absorbing the oxygen contained in atmospheric air. The compounds of silver and gold are reduced by pyrogallic acid even more rapidly than by gallic acid, the reducing agent absorbing the oxygen, and becoming converted into carbonic acid and a brown matter insoluble in water. Commercial pyrogallic acid is often contaminated with empyreumatic oil, and also with a black insoluble substance known as _metagallic acid_, which is formed when the heat is raised above the proper temperature in the process of manufacture. Sel D'or. (_See_ Gold, Hyposulphite of.) Silver. Symbol, Ag. Atomic Weight, 108. This metal, the _luna_ or _diana_ of the alchemists, is found native in Peru and Mexico; it occurs also in the form of sulphuret of silver. When pure it has a sp. gr. of 10·5, and is very malleable and ductile; melts at a bright red heat. Silver does not oxidize in the air, but when exposed to an impure atmosphere containing traces of sulphuretted hydrogen, it is slowly tarnished from formation of sulphuret of silver. It dissolves in sulphuric acid, but the best solvent is nitric acid. The standard coin of the realm is an alloy of silver and copper, containing about one-eleventh of the latter metal. It may be converted into nitrate of silver, sufficiently pure for photographic purposes, by dissolving it in nitric acid and evaporating the solution to the crystallizing point: or, if the quantity be small, the solution may be boiled down to complete dryness, and the residue _fused_ strongly; which decomposes the nitrate of copper, but leaves the greater portion of the silver salt unaffected. (N. B. Nitrate of silver which has undergone fusion contains nitrite of silver, and will require the addition of acetic acid if used for preparing the collodion sensitive film.) Silver, Ammonio-Nitrate of. Crystallized nitrate of silver absorbs ammoniacal gas rapidly, with production of heat sufficient to fuse the resulting compound, which is white, and consists of 100 parts of the nitrate + 29·5 of ammonia. The compound however which photographers employ under the name of ammonio-nitrate of silver, may be viewed more simply as a solution of the oxide of silver in ammonia, without reference to the nitrate of ammonia necessarily produced in the reaction.