Gallic Acid is produced by the _decomposition and oxidation_ of Tannic Acid when powdered galls are exposed for a long time in a moist state to the action of the air. By boiling the mass with water and filtering whilst hot, the acid is extracted, and crystallizes on cooling, on account of its sparing solubility in cold water. Gallic Acid occurs in the form of long silky needles, soluble in 100 parts of cold and 3 of boiling water; they are also readily soluble in Alcohol, but sparingly in Ether. The aqueous solution becomes mouldy on keeping, to obviate which, the addition of Acetic Acid or a drop or two of Oil of Cloves is recommended. Gallic Acid is a feeble acid, scarcely reddening litmus; it forms salts with the alkaline and earthy bases, such as Potash, Lime, etc., but not with the oxides of the noble metals. When added to Oxide of Silver the metallic element is separated and the Oxygen absorbed. b. _Pyrogallic Acid._--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 colour 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. CHEMISTRY OF THE PROTOSALTS OF IRON. The combinations of Iron with Oxygen are somewhat numerous. There are two distinct Oxides which form Salts, viz. the Protoxide of Iron, containing an atom of Oxygen to one of metal; and the Peroxide, with an atom and a half of Oxygen to one of metal. As _half atoms_ however are not allowed in chemical language, it is usual to say that the Peroxide of Iron contains three equivalents of Oxygen to two of metallic Iron. Expressed in symbols, the composition is as follows:-- Protoxide of Iron, Fe O. Peroxide of Iron, Fe{2}O{3}.