GALLIC ACID occurs in the form of colorless silky needles which lose their water of crystallization at 212°; they dissolve slightly in cold water, but require only three parts of boiling water for their solution, and they are freely soluble in alcohol. Solutions of G. A. have an acid reaction and a sour astringent taste; with the persalts of iron they yield a deep-blue color, and no apparent reaction occurs when they are mixed with a solution of gelatine. The gallates of the alkalies, especially if an excess of the base be present, speedily absorb oxygen, and become brown when exposed to the.air ; and hence they may be usefully employed in eudiometry. G. A. possesses the property of reducing the salts of gold and silver, and it is on this account that it has been employed in photography.
G. A. exists ready formed in small quantity in gall-nuts, in valonia (the acorn-cup of guereus ogilops), in divi-divi (the pod of Casalpina coriaria),.in sumach, and other vege tables. It is formed in association with glycose from gallotannic acid (q.v.), when the latter is boiled with dilute sulphuric or hydrochloric acid; it is likewise produced by boiling a solution of gallotannic acid with caustic alkalies, or (more slowly) by simply exposing a solution of gall-nuts to the air, the process of oxygenation being appar ently favored by the presence of a ferment contained in the gall-nut.
To obtain G. A.., we mix powdered gall-nuts with water, and expose them freely and for a long time to the air at a temperature of 70° or 80'. The tannin or gallotannic acid becomes gradually converted into gallic acid. We pour away the supernatant brown fluid, and take up the G. A. from the residue with boiling water, decolorize with animal charcoal, and crystallize.
When G. A. is exposed to a temperature of from 410° to 420°, it is' converted into carbonic acid and pyrogallic acid (q.v.) (C1211.0.), which is sublimed, 31 or 32, parts of the latter acid being yielded by 100 of gallie acid. The reaction is represented by the formula— Gallic Acid. Carbonic Acid. Pyrogallic Acid.