Gallic Acid, a product of the decomposition of tannic acid or tannin, obtained in slender, silky needles or crystals. When pure, these are colorless, without odor, sour, and astringent. They are soluble in 100 parts of cold or 3 parts of boiling water, very soluble in alcohol, and less so in ether. Their solution decomposes by exposure to the air. The decomposition of the crystals dried at 212° F. is supposed to be represented by the formula C7H6O5. Gallic acid is a useful reagent for detecting the presence of iron in solutions. It does not possess the property of the solution of galls of precipitating gelatine. The acid exists ready formed in the gall nut, in sumach, in valonia, and in a large number of other astringent vegetables, although the quantity in each is but small. Gallic acid is tribasic, and it forms three classes of salts; those of the heavy metals are generally insoluble. Several methods are in use for obtaining it, either directly from the galls or from the solution of tannic acid first extracted from them. The powdered galls are made into a paste with water, and exposed for some weeks to the air at a temperature of 70° to 75° F., water being occasionally added to keep the paste moist.

The residue, after expressing the paste to free it from the liquid portion, is boiled in pure water, and filtered while hot; the crystals of gallic acid separate as the solution cools. They should be purified by redissolving and boiling with a little animal charcoal or filtering through the same. As the presence of the smallest quantity of sesquioxide of iron will cause the crystals to be colored, the charcoal should be purified, and the filtering paper be washed with dilute hydrochloric acid. Gallic acid is obtained from solution of tannic acid by precipitation with sulphuric acid, the mixture being heated to the boiling point, and allowed to stand a few days. When gallic acid is heated to 410° F. it is wholly volatilized and converted into pyro-gallic acid and carbonic anhydride. Pyro-gallic acid is used to remove free oxygen from gaseous mixtures and as a developer in photographic operations.-When swallowed, gallic acid is rapidly absorbed from the stomach into the blood, and remains in the blood unchanged. When tannic acid is swallowed, it undergoes the decomposition indicated above, either before or after absorption, into gallic acid; so that tannic acid becomes gallic acid in the blood.

Hence gallic acid is used as an astringent internally in preference to tannic, and also because it is less irritating to the stomach and more agreeable to the taste. It is rapidly eliminated from the system, chiefly by the kidneys. Two or three hours after a dose of it has been taken the whole or nearly the whole of the amount has left the system, so that to keep the patient steadily under its influence, it should be administered every three or four hours. It may be given in large and frequently repeated doses, with advantage, to check haemorrhages, especially those from the chest or uterus. It has also been used with good results in diseases of the kidneys and bladder, the organs that are chiefly concerned in its elimination. It is of very little value as a local astringent or in cases of diarrhoea or dysentery. The dose of it varies from 5 to 15 grs. five or six times a day. It is best given dissolved in water. Those who prefer to do so may take it dry on the tongue.