105b. Acidum Gallicum, U.S.—Gallic Acid. 105c. Pyrogallol, U.S.—Pyrogallic Acid.
105b. ACIDUM GALLICUM, U.S.—GALLIC ACID. Usually prepared from tannic acid. Also prepared by exposing moistened powdered nutgalls to the action of the air for a month or more; a peculiar fermentation sets in which converts the tannic acid into gallic acid; this is extracted by expression and purified by filtration and crystallization. It is in light, silky, acicular needles, colorless when pure, but as usually seen in the shops, of a more or less pale brownish color; inodorous; taste sourish and astringent. It differs from tannic acid in its sparing solubility in cold water, and in not precipitating gelatin or alkaloids from their solutions. It is less astringent than tannic acid, and inferior to it in all respects except where the astringent effect must be reached through the medium of the general circulation. When applied locally, gallic acid acts as a mild astringent, but does not cause coagulation of the blood, for which reason it is not used locally in the control of hemorrhage. Dose: 5 to 30 gr. (0.3 to 2 Gm.).
105c. PYROGALLOL, U.S.—PYROGALLIC ACID. A triatomic phenol, C6H3(OH)3, obtained chiefly by the dry distillation of gallic acid. It is in light, white, shining laminae, or in fine needles, becoming gray or darker when exposed to the air or light, and should therefore be kept in amber-colored bottles; inodorous; astringent. Soluble in water and alcohol. Used exclusively externally in the form of ointments, in lupus, psoriasis, and other skin diseases. Its absorption through abrasions in the skin has caused death by general poisoning.