Symbol, C{8}H{4}O{4} (Stenhouse). Atomic weight. 84.
The term pyro prefixed to gallic acid implies that the new substance is obtained by the action of heat upon that body. At a temperature of about 410° Fahr., gallic acid is decomposed, and a white sublimate forms, which condenses in lamellar Crystals; this is pyrogallic acid.
Pyrogallic acid is very soluble in cold water, and in alcohol and ether; the solution decomposes and becomes brown by exposure to the air. It gives an indigo blue color with protosulphate of iron, which changes to dark green if any persulphate be present.
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.