154 



TANNINS. 



The phlobaphenes mentioned in 48 may, as stated, have been 

 produced from tannins during manipulation, whilst those in 108 

 will probably have existed ready-formed in the material under 

 examination. 



But, although phlobaphenes are insoluble in pure water, they 

 are dissolved by solutions of tannic acid, sugar, and other sub- 

 stances, and small quantities may therefore have been extracted 

 by water. 



161. Proneness to Decomposition. The determination of the 

 glucosidal or non-glucosidal nature of a tannin is sometimes a 

 matter of considerable difficulty, because, on the one hand, it is 

 not always easily separated from any glucose with which it may 

 be contaminated, and, on the other hand, many tannins readily 

 decompose, yielding bodies resembling their mother-substances in 

 possessing a similar action on hide, gelatine, etc. A decomposi- 

 tion of this kind is observable with gallotannic acid, which, 

 especially on prolonged heating in aqueous solution, apparently 

 undergoes dissociation into a polygallic acid and sugar. Some 

 tannins, too, from barks, etc., appear to be capable of parting with 

 a portion of their glucose without completely losing their action on 

 gelatine, etc., which, however, rapidly diminishes, even at the 

 ordinary temperature, on allowing the aqueous solution to stand. 

 It is not surprising, therefore, that very varying statements are 

 met with as to the glucosidal ? .nature of a tannin, and especially 

 the amount of glucose certain members of the class can be made 

 to yield. And yet it is most important to determine whether 

 glucose can be obtained from a tannic acid or not. The produc- 

 tion of a sparingly soluble and possibly crystalline body from an 

 easily soluble amorphous tannin by the action of hydrochloric or 

 sulphuric acid ( 160) is insufficient proof of its glucosidal nature, 

 since non-glucosidal tannins undergo a similar decomposition. 



As a rule the action of dilute acids on a tannin results in the 

 formation, apart from glucose, of a single decomposition-product 

 belonging to the aromatic series (gallic acid, ellagic acid, phloba- 

 phenes, etc.), but the researches on the tannic acids of the 

 Nymphacese recently carried out in my laboratory by Griining 

 prove that two or more decomposition-products can be obtained 

 from one tannin. From Nymphsea alba and Nuphar luteum 

 non-glucosidal tannic acids were isolated, undergoing no further 

 separation by fractional precipitation with lead, and yielding, 



