GALLIC ACID, C•118(OH)ICOOH, is an acid which exists in small quantity in gall-nuts, in valonia (the acorn-cup of Quercus rgilops), in dividivi (the pod of Cavalinnia coriona), in sumach, and other vegetables. It is usually pre pared from gall-nuts, which, in addition to gal lie acid, contain a large proportion of tannin (tannic acid or gallo-tannic acid). When the gall-nuts are digested with water for some weeks fermentation takes place, and the tannic acid is gradually converted into gallic acid. The same result is obtained more quickly if sulphuric acid be present. To obtain pure gallic acid the gall-nuts are boiled with water, and the hot liquor separated. On 'cooling gallic acid crystallizes out, and is further purified by solution in hot water and treatment with animal charcoal. It forms delicate, silky, acicular crystals, nearly colorless and having a sourish taste. It is soluble in three parts of boiling water, but only in 100 of cold water, and on this account it can be readily purified by re crystallization. With solution of iron salts
(ferric) it produces a blue-black color, and finally yields a black precipitate on exposure to the air. Hence it may be used in the produc tion of ink, for which purpose it has some ad vantages over tannin or gall-nuts. When the crystals are strongly heated pyrogallic acid (GH3(OH)s) is produced and sublimes over. Gallic acid is a useful astringent. As it does not coagulate albumen it is readily absorbed into the blood, and in this way it is efficacious in Bright's disease. Where a decided local astringent effect is desired tannic acid is much more powerful. It has been used in excessive sweating and is useful for sweating feet and as a local spray and gargle in tonsilitis, pharyn gitis arid similar affections of the nose and throat.