160. GLUCOSIDAL NATURE OF TANNINS. 153 



splitting up under the influence of dilute acids into glucose and 

 some second substance, but of a number it must be denied that 

 they possess any such glucosidal characters. 



160. Glucosidal nature. In enumerating the characters of a 

 newly-discovered tannic acid, it is, therefore, important to state 

 whether it has been found to be a glucoside or not ( 61). The 

 examination may be made by heating weighed quantities of the 

 tannin with 1 to 2 per cent, aqueous hydrochloric acid in sealed 

 tubes to 100 for several hours, allowing them to stand for some 

 time after being opened, in order to observe whether any spar- 

 ingly soluble decomposition-product separates out in the cold. If 

 this is the case, the substance may be filtered off, but at the same 

 time it is advisable to ascertain whether any portion that may 

 remain in solution cannot be removed by shaking with ether, 

 acetic ether or chloroform. After warming to expel dissolved 

 traces of those liquids, the solution may be examined for glucose 

 (^ 61, 83, et seq. ; 200, et seq.). 



The decomposition-products that are thus obtained, together 

 with glucose, are sometimes crystalline, as, for instance, gallic 

 acid, from the tannin of galls, sumach, myrobalans, divi-divi 

 (cf. 151), and the yellow ellagic acid from the tannic acid 

 of the pomegranate and bablah fruits. But they are generally 

 amorphous, difficultly soluble in pure or acidified water and in 

 pure ether; soluble in water containing ammonia, and freely 

 soluble in spirit ; they are, as a rule, deep in colour, and agree in 

 all essential properties with the phlobaphenes mentioned in 48 

 and 108. Some are so sparingly soluble that they may be of use 

 in the quantitative estimation of the respective tannins. This 

 is especially the case if, after the action of the acid, the liquid is 

 evaporated to dryness and treated with water, when they often 

 remain behind almost entirely insoluble. 



Substances of this description are yielded by the decomposition 

 of the tannic acid of oak, willow, elm, fir, birch, and acacia-bark, 

 as well as by that from rhubarb, male-fern, ledum, wine, and by 

 many others. Chemically the phlobaphenes approach many resins, 

 with which they share the solubility in alcohol and slight solubility 

 in water, differing from them in their solubility in dilute ammonia, 

 but resembling them again in the substances they yield when 

 fused with an alkali. (Cf. 42. ) Lignin and suberin also appear 

 to be connected with the phlobaphenes. 



