SPECIAL SECRETIONS. TANNIN. 229 



more or less azotised matter, which adds to their value as articles 

 of nutriment. The use of these substances in the economy of 

 the Plant is not known. 



365. Next in importance to these, we are probably to rank 

 Tannin. This is the substance, by the chemical agency of which, 

 upon animal tissues containing gelatin (the material commonly 

 known as glue, which forms a large part of the skin of most 

 animals), leather is produced. Its chemical effect upon gelatin 

 may be shown, by steeping some oak-bark, or bruised gall-nuts, 

 in water ; and then adding some of this fluid to water in which 

 glue has been dissolved. A quantity of flaky matter will fall 

 down, which is, in fact, leather ; its particles being separate 

 from each other, on account of the liquid form in which the 

 elements were brought together. The process of tanning con- 

 sists in steeping the skins to be converted into leather, in a 

 solution of tannin ; this slowly penetrates their substance, con- 

 verting their gelatin, which would otherwise soon undergo putre- 

 faction, into the compound just mentioned, which is capable of 

 resisting decay. And, as no injury to the texture of the skin is 

 done by this process, it is converted into a substance which, from 

 its pliancy, combined with toughness and durability, is useful 

 for a great variety of purposes. 



366. In this country tannin is principally obtained from oak- 

 bark ; but as of late years the supply of that material has not 

 been equal to the demand, it has been necessary to look for some 

 other source, from which it may be procured. Several other 

 trees common in this country yield tannin ; such are the elm, 

 willow, elder, plum, sycamore, birch, cherry, poplar, hazel, and 

 ash ; but the proportion contained in all, save the two first of 

 these, is not sufficient to render their cultivation for this object a 

 source of profit. Even the common heath has been applied to 

 this purpose; the tannin being extracted from it by boiling. 

 There are, however, many trees of tropical climates, which con- 

 tain a larger proportion of tannin than that yielded by oak-bark. 

 One of these is a kind of Sensitive-plant (Acacia Catechu}, 

 which flourishes abundantly in the mountainous parts of Hin- 

 dostan, and yields the substance known as Catechu, or Terra 



