Using the term in its general application to a group of substances having similar chemical and physical properties, rather than in its limited application to a single definite chemical compound known commercially as "tannin," the _tannins_ are a special group of plant substances, mostly glucosides, which have the following characteristic properties. First, they are non-crystalline[4] substances, which form colloidal solutions with water, which have an acid reaction and a sharp astringent taste. Second, they form insoluble compounds with gelatine-containing tissues, as shown by the conversion of hide into leather. Third, they form soluble, dark-blue or greenish-black compounds with ferric salts, the common inks. Fourth, they are precipitated from their solutions by many metallic salts, such as lead acetate, stannous chloride, potassium bichromate, etc. Fifth, they precipitate out of solution albumins, alkaloids, and basic organic coloring matters. Finally, most tannins, in alkaline solutions, absorb oxygen from the air and become dark brown or black in color. FOOTNOTES: [4] The needle-like forms, in which commercial "tannin" comes on the market, are not true crystals, but are broken fragments of the threads into which the colloidal tannin is "spun-out" from the syrupy extracts of nutgalls, etc. OCCURRENCE Tannins occur widely distributed in plants. Practically every group of plants, from the fungi up to the flowering plants, contains many species of plants which show tannin in some of their tissues. Among the higher plants, tannins occur in a great variety of organs. Thus, they are found in the roots of several species of tropical plants; in the sterns, both bark and wood, of oaks, pines, hemlock, etc.; in the leaves of sumac, rhododendron, etc.; in many fruits, especially in the green, or immature, stages; and in the seeds of several species, either before or after germination. Tannins are also found in certain special structures, such as gland cells, cells of the pulvini, laticiferous tissues, etc. Further, they are especially abundant in the pathological growths known as galls, which often contain from 40 to 75 per cent of tannin and constitute the most important commercial source for these materials. The principal commercial sources of tannin, which is used in the manufacture of inks, in the tanning of leather, in certain dyeing operations, etc., are oak-galls, the bark and wood of oak, hemlock, acacia, and eucalyptus, the bark of the mangrove, the roots of canaigre, and the leaves of several species of sumac. CHEMICAL CONSTITUTION Tannins are either free phenol-acids or, more commonly, glucosides of these acids. Common "tannin," when hydrolyzed, yields from 7 to 8 per cent of glucose, which indicates that it is a penta-acid ester of glucose, i.e., each glucose molecule has five acid groups attached to it. The formula for such a tannin is, therefore, as follows,