216 TANNINS 



to account for the discrepancies above mentioned, but in view 

 of E. Fischer and Freudenberg's work, already referred to 

 above, these suggestions are valueless. 



Incidentally it may be mentioned that two other digallic 

 acids are known in addition to SchirTs acid, namely : 



1. /9-digallic acid, C 14 H 10 O 9 + 2H 2 O, obtained by heating 

 ethyl gallate with pyroracemic acid, and 



2. Fischer's * digallic acid, C 14 H 10 O 9 or C 14 H 10 O 9 + C 7 H 6 O 5 , 

 obtained by the action of tricarbomethoxygalloylchloride with 

 dicarbomethoxy gallic acid. 



ELLAGITANNIC ACID. 



This tannin, which is commonly found together with 

 gallotannic acid, is important as being the mother substance 

 of ellagic acid, which is responsible for the bloom characteristic 

 of pyrogallol tannins. The quantity of this substance present 

 in different plants varies considerably ; it is greatest in divi- 

 divi. Amongst the other tannins giving ellagic acid bloom 

 may be mentioned algarobilla, myrobalans, chestnut tannin, 

 pomegranate tannin, valonia, etc. 



Ellagitannic acid, unlike ellagic acid (p. 207), is soluble 

 in water or alcohol ; prolonged boiling with water converts it 

 into ellagic acid. It has been variously described by different 

 authors as a glucoside, as a hydrated soluble form of ellagic 

 acid, or as a condensation product of ellagic acid with gallic acid.f 



PYROCATECHOL TANNINS. 

 CATECHU TANNIC ACID. 



Catechu tannic acid is the name given to the tannin con- 

 tained in gambier catechu and in Bombay catechu or cutch,* 

 a substance obtained by evaporating an aqueous extract of 

 the bark of various trees (see below). A similar tannin is 

 also contained in kino. 



Little is known as to its constitution, but it is believed to 

 be an anhydride of catechin. 



* Fischer: " Ber. deut. chem. Gesells.," 1908, 41, 2890; "Annalen," 1911, 

 384, 225. 



t Cf. Nierenstein : " Ber. deut. chem. Gesells.," 1907, 40, 4575 ; 1909, 42, 

 353 ; 1910, 43, 1257. 



I This substance is used largely for dyeing. 



