_Ellagic acid_, which is an hydrolysis product of many of the pyrogallol tannins (see below) and which produces the characteristic "bloom" on leather tanned by this type of tannins, has the following formula, -----CO·O----- /\ /\ / \__________/ \ HO| | | |OH HO| |___CO·O___| |OH \ / \ / \/ \/ CLASSES OF TANNINS The tannins are divided into two general classes, known respectively as the _pyrogallol tannins_ and the _catechol tannins_. These differ in their characteristic reactions as follows: Pyrogallol variety Catechol variety Ferric salts Dark blue Greenish black Bromine water No precipitate Yellow or brown precipitate Leather Produce a "bloom" No "bloom" Conc. sulfuric acid Yellow or brown Red or pink Lime water Gray or blue ppte. Pink to brown ppte. Pyrogallol tannins contain approximately 52 per cent of carbon; while the catechol tannins usually contain 59 per cent to 60 per cent, the difference being due to the absence of glucose from the molecule in the latter types. The two types are distributed in plants as follows: pyrogallol tannins in oak-galls, oak wood, sumac, chestnut, divi-divi, and algaro billa; catechol tannins in the barks of pines, hemlocks, oaks, acacias, mimosas, cassia, and mangrove, in quebracho wood, canaigre roots, cutch and gambier. The so-called "pseudo-tannins" (i.e., compounds which do not tan leather but possess other properties like tannins) are found in hops, tea, wine, fruits, etc. SOME COMMON TANNINS Ordinary commercial "_tannin_," or "_tannic acid_," is a compound of one molecule of glucose with five of digallic acid. It is found in many plants, and is prepared commercially from the Turkish oak-galls and the Chinese sumac-galls. It exhibits all the characteristic properties which have been listed above for tannins in general and responds to all the characteristic reactions of a pyrogallol tannin. It is extensively used for the manufacture of blue-black ink, and in many technical processes. =Catechu tannin= and =catechin= are compounds of the catechol tannin type. The latter is obtained from acacia wood, mahogany wood, mimosa wood, etc. It is not a true tannin, since it does not convert hide into leather; but when heated to 120° or above, it is easily dehydrated, forming catechu tannin which is identical with that which is obtained directly from gambier and Bombay cutch (products made by evaporating water extracts from the bark of various tropical trees). This latter is a true tannin, which is much used in dyeing and other technical processes.