This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
This is the variety of tannic acid which precipitates the salts of sesquioxide of iron of a bluish-black colour. Besides galls, it is found in different products of the oak, and in other vegetable astringents. Other varieties of tannic acid precipitate the salts of iron of a greenish-black or grayish-black colour, and are apt to be associated with a reddish or reddish-brown colour in the medicine containing them, as in kino, catechu, and rhatany. These all differ from that now under consideration, in not being convertible into gallic acid, upon exposure in the state of infusion to the atmospheric air. They probably differ among themselves, and may be distinguished from the tannic acid here referred to, and from one another, by designative epithets derived from the medi-cine Thus, the variety derived from kino may be called kino-tannic acid, that from catechu, catechu-tannic acid, etc. Tannic acid is extracted from galls by means of ether containing a little water. The liquid is passed through powdered galls, and after passing separates into two layers, the lower of which holds the tannic acid in solution, and yields it on evaporation.
Sensible and Chemical Properties. Gallo-tannic acid is solid, spongy, light, white with frequently a yellowish or greenish-yellow tint, inodorous, and of a strongly astringent taste, without bitterness. It is freely soluble in water, less so in alcohol and ether, and insoluble in the fixed and volatile oils. Its incompatibilities are the same with those of galls (sec page 112), which owe their chemical relations with other substances chiefly to this principle. Exposed in solution to the air, it is slowly changed into gallic acid, with the escape of carbonic acid.
Tannic acid, being the peculiar active principle of the vegetable astringents, must affect the system in the same manner as these medicines, so far as their astringency is concerned, with this difference only, that it exceeds them all greatly in power. In reference to topical effect alone, it is probably the most energetic astringent known. In its general action, it is inferior to some of the mineral substances belonging to the class. In consequence of its strong affinity for some of the proximate constituents of the tissues and of the blood, as albumen, gelatin, and fibrin, with which it forms insoluble compounds, it is probably never taken unchanged into the circulation. Indeed, it could scarcely exist as tannic acid in the blood. Experiments with tannic acid upon dogs have shown that, when it is given freely to these animals, the urine becomes darker, and yields evidences, upon chemical examination. of the presence of gallic and pyrogallic acids. Into these acids, then lore. or into Something intermediate between them and itself, the tannic acid is probably converted on entering the circulation. This presumption is rendered more plausible by the fact, that gallic acid does not, like the tannic, form insoluble compounds with the animal principles above mentioned. Now, as this acid is far less astringent than the tannic, we can readily account for the much greater power of the latter as a local than as a general astringent. Among its local effects are of course those which it produces in the bowels; for with the mucous membrane of these it is brought into as close contact as with the skin.
In moderate doses, tannic acid is said to produce warmth in the stomach, and somewhat to excite the appetite. It undoubtedly tends to diminish the number and quantity of the stools. Largely given, it causes a feeling of constriction in the epigastrium, not unfrequently nausea, and sometimes obstinate constipation. But it is asserted that a drachm or more may be given, in the course of a day, not only with impunity, but with advantage, if it be administered in the liquid form, or followed, if in the pilular form, by food or drink; and, in these large doses, it is said greatly to increase the secretion of urine. (P. Gamier, ArchivesGenerales, Janv. 1859, pp. 28-35).