156 TANNINS. 



observed that gallic acid must be removed by shaking with ether 

 before chloride of sodium is added, and that certain tannins are 

 partially precipitated by saturating their aqueous solutions with 

 salt ; some also can be precipitated from their aqueous solutions 

 by sulphuric or other mineral acid, but this method seldom yields 

 them in a sufficient state of purity for our purpose. 



163. Tannins insoluble in Water. The tannic acids of alder 1 

 and hop, 2 together with some others, are stated to be insoluble in 

 water after isolation. In some instances the tannin may possibly 

 have been partially decomposed during the process of isolation 

 ( 48, 161). In any case in which tannins sparingly soluble in 

 water are anticipated, the lead precipitate should be decomposed 

 in the presence of spirit. 



There are also a number of bodies which resemble one or other 

 of the tannins in certain of their properties (as, for instance, in 

 being precipitated by acetate of lead), but are insoluble in cold 

 water. Paeniofluorescin might be placed in this class. 3 



164 It not unfrequently occurs that a single plant contaii 

 two or more tannins : for example, in addition to their own pecu- 

 liar tannins, both oak and willow bark contain a little gallotannic 

 acid ; 4 myrobalans and divi-divi contain gallotannic and ellag( 

 tannic acid. 5 If the presence of more than one tannin is antici- 

 pated, the method of fractional precipitation should be tried, or, 

 if that is unsuccessful, the examination of the products obtain* 

 by warming with acids may afford the information required, 

 was the case in the investigation of oak and willow bark ; gallk 

 acid can be removed by shaking with ether, whereas oak-i 

 cannot. 



165. Notes on some of the more important Tannins. In 

 following notes those tannins will be mentioned first that are not 

 glucosides, and yield principally pyrocatechin ( 43) by destructh 

 distillation. 



Catechu-tannic acid is probably produced from catechin (catechuic 

 acid) by loss of the elements of water. 6 In estimating catechu- 



1 Compare "Reichardt, Chem. Centralblatt, N. F. i. 12. 



2 See Etti, Polyt. Journ. cclxxxviii. 354, 1878 (Journ. Chem. Soc. xxxiv. 

 797) ; Bissell, Amer. Journ. Pharm. [4], xlix. 582, 1877. 



3 44 and 147. 



4 Compare Johanson, loc. cit. 



5 Compare Loewe, Zeitschr. f. anal. Chem. xiv. 35, 44, 1875. 



6 Compare Etti, Annal. d. Chem. und Pharm. clxxxvi. 327, 1878 ; Zeitschr. 

 f. anal. Chem. xii. 285, 1873 ; xiii. 113, 1874 ; Journ. f. prakt. Chem. cv. 



