Metabolism; Digestion and Translocation 263 



151. Tannins. The tannins are bitter, astringent, 

 water-soluble, amorphous substances widely distributed in 

 the leaves, bark, and fruits of plants. All of these sub- 

 stances, which are of commercial importance, may be em- 

 ployed in the process of tanning skins and hides, since they 

 form insoluble compounds with various nitrogenous bodies, 

 giving a toughness and durability to the skin which con- 

 stitutes the differences between leather and natural skin. 



The tannins are alike in certain physical and chemical 

 properties, but there may be dissimilarity in chemical 

 composition. The tannin (glucoside) used in the making 

 of leather is usually derived from the bark of various trees, 

 including that of hemlock and oak, so extensively employed 

 in the United States. The bark of hemlock may yield 

 from 8 to 10 per cent of its dry weight of tannin and the 

 leaves of tea may contain'as much as 15 per cent. 



Tannin is also extensively used as a mordant in the 

 process of dyeing, for it produces colored products with 

 various dye-stuffs ; and it has long been employed in the 

 manufacture of ink. The characteristic purplish brown 

 color of the trunk of the cork oak from which the bark 

 has been removed is due to this substance. The chief 

 source of tannic acid (digallic acid) is a gall-nut produced 

 upon an oak (Quercus infectoria), a product obtained for 

 the most part from Turkey. Tannic acid constitutes 

 more than one half of the dry weight of this gall. A 

 similar product is yielded by a gall upon the sumac, Rhus 

 semi-alata, which occurs in China. 



Upon heating with sulfuric acid, tannic acid is 

 hydrolyzed, yielding two molecules of gallic acid, thus, 

 C 14 H 10 O 9 + H 2 O = 2 C 7 H 6 O 5 . This process is also ac- 



