156 TANNINS. 



observed ttiat gallic acid must be removed by shatiflg with ether 

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

 partially precipitated by saturating their aqueous solutions with 

 sa;lt; some alfeo can be precipitated irom their aqueous solution* 

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

 them in a sufficient state of purity for our purpose. 



§ 163 Tawiims! insoluble in Water.—r-The tannic acids of alder^ 

 and hop,^ togethw with some others,, are stated to be jnsolublein 

 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 stflabje in 

 water are anticipated, the lead pi'ecijiitatiB should be dacomposed 

 in the presence of spirit. 



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

 of the tannins in certain of their J)roperties (as, for instance, in 

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

 water. Paeniofluorescin might be placed in this class.^ 



§ 164. It not unfr6quently occurs that a single, plant contains 

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

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

 acid ; myrobalans and divi-drvi contain gallotannic and ellago- 

 tannic acid^^ If the presence of more tlian one tannin is antici- 

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

 if that is unsuccessfuli the examination of the products obtained 

 by warming with acids may afford the information required, as 

 was the case in the investigation of Oak and willow bark ; gallic 

 ftoid can be removed by shaking with ether, whereas oak-red 

 oannot. 



§ 165. Notes on some of the more important Tanmn»^~la- the 

 following notes thosa tannins will b? mentioned first that are hot 

 glucoKdes, an yield principally pyrocatechin (§ iS) by desitrutetive 

 distillation. 



Calechu-tannic acid is probably produced from catechin (catechuic 

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



1 Compare Iteiohardt, Chem. Centralblatt, N. F. i. 1?. 



^ See Eiti, Polyt. Joarn,. cclxxxviii. 354, 187'8 (Journ. Chem. Soe. xixiv. 

 797) ; Bissell, Atoer. Journ. Pharm. [4], slix. 582, 1877. 



« §§ 44 and 147. 



■* Compare Joha^son, loc, cU. 



" Compare Loewe, Zeitschr. f. anal, Obem. xiv. 35, 44, 1R76. 



« Compara EttJ, Annal. d. Chem, und Pharm. olxxxvi. 327,1878 ; ZeitBohr. 

 i. annl. ChejH. xii. 285, 1873'; xiii," IW, 1874 ; Journ. f. prakt. Chem , ov. 32. 



