This section is from the book "Materia Medica: Pharmacology: Therapeutics Prescription Writing For Students and Practitioners", by Walter A. Bastedo. Also available from Amazon: Materia Medica: Pharmacology: Therapeutics: Prescription Writing for Students and Practitioners.
These are a class of imperfectly defined astringent bodies of the aromatic group. They are all acids which form salts, and some of them are glucosidal in nature. They precipitate alkaloids, mercuric chloride, and other salts of the heavy metals, and also proteins and gelatin. With iron compounds they make ink (blue to black in some cases, green in others), and with the connective tissue, protein, and gelatinous material of hides they make leather. This suggests the unwisdom of administering a gelatin-coated pill or capsule at the same time as a tannin-containing drug. They are freely but slowly soluble in water, and readily soluble in alcohol and glycerin. They occur mostly in the bark of trees, and in the plant-galls which result from punctures of insects. The various tannins are given the names of the plants which yield them, e. g., that from cinchona is called cinchotannin, or cinchotannic acid, that from kino is kinotannic acid, etc. The official "tannic acid" is quercitannin, and is derived from oak-galls. It is Considered in Part II.