Gallotannic acid as obtained from plants invariably yields traces of glucose, as well as of ellagic acid, when boiled with dilute acids. It is still an open question whether the glucose exists in the plant as a glucoside of tannic acid or is always the product of some impurity (as is shown by Etti to be the case with oak bark, where lævulin is always present). It seems most probable however that natural gallotannic acid is really a glucoside of digallic acid, or possibly, according to the theory of Hlasiwetz, a gummide, or compound of dextrin, which, by the action of acids, is easily converted into glucose. What has been said of gallotannic acid in this respect, applies to many other tannins, which like it give glucose by treatment with acids. Gallotannic acid is met with, in commerce, in the form of light buff-coloured scales, with a faint peculiar odour and a powerfully astringent taste. It is soluble in 6 parts of cold water or glycerin, and very readily in hot. It is also very soluble in alcohol containing water, but much less so in absolute. It is moderately soluble in washed, but scarcely at all in anhydrous ether, chloroform, benzene, or petroleum spirit. The commercial acid usually contains more or less of gallic acid, which may be detected by dissolving in water, shaking with ether, and after decanting and evaporating the ether, applying the tests described under gallic acid (p. 70). It may also be frequently distinguished in the simple aqueous solution of the tannic acid by the tests given. Its quantity may be determined (in the absence of other impurities) by the Löwenthal method (p. 121), the gallic acid forming the "not-tannin," by comparison with a solution of pure gallic acid. Commercial tannic acid is sometimes adulterated with starch, which is left undissolved on treating the sample with ordinary alcohol. For the estimation of gallotannic acid see pp. 118 _et seq._ For its principal reactions, Table, p. 113. _Ellagitannic acid_, C_{14}H_{10}O_{10}, is contained in divi-divi, myrabolans, and as a glucoside in pomegranate rind. When boiled with dilute acids, or treated with water at 230° F. (110° C.) in a sealed tube, it yields its anhydride, ellagic acid (see p. 71), C_{14}H_{8}O_{9}. In its reactions ellagitannic acid closely resembles gallotannic acid. _Quercitannic acid._ Oak-bark tannin.--This tannin may be prepared from oak bark by agitating an alcoholic extract with ethyl acetate, and separating and evaporating the ethereous layer. It is still contaminated with a brownish-green terpene resin, and with some of the higher anhydrides of the tannin. The resin may be removed by treating the dried extract with ether or benzene, in which it is readily soluble; and the phlobaphenes, or higher anhydrides, by dissolving the tannin in ether-alcohol, or probably to a great extent, by simple solution in cold water in which the phlobaphenes are scarcely soluble.