9 o THE CHEMISTRY OF CATTLE FEEDING 



proteins, including gelatine (pp. 65, 78), and are used chiefly for 

 this purpose in the process of converting hides into leather. 

 The tannin of coffee beans is an exception to this rule. 



The tannins are all derivatives of gallic acid, a trihydroxy 

 benzoic acid. Some of them occur as glucosides, i.e. as glucose 

 esters of gallic acid. In others the glucose is replaced by 

 trihydroxy benzenes, C 6 H 3 (OH) 3 , pyrogallol [i, 2, 3], phloro- 

 glucin [1,3, 5], etc. The common tannin, or tannic acid, is a 

 digallic acid, and it occurs in oak galls in that form, not as 

 a glucoside or other compound of the acid. The relation of 

 this substance to gallic acid will be readily understood from 

 the formulae. 



H H H 



I I I 



O 00 



I I I 



H O C C O H H O C C O C C O H 



H C C H H C C H H C C H 



v \c/ \c/ 



I I 



.OH CO.OH CO.OH 



(Gallic acid.) (Digallic acid (Tannin).) 



co.< 



The constitution of the other tannins can be indicated in 

 a similar way, by replacing one of the gallic acid radicles by 

 the phloroglucin or glucose radicle. 



Colouring Matters. Colouring matters of various kinds 

 are of common occurrence in the organic world, both animal 

 and vegetable. To the former belong the characteristic 

 colours of blood, bile, yolk of egg, etc. ; to the latter, chloro- 

 phyll, indigo, alizarin. Some of the vegetable colours are 

 used as dyes, but they have been largely superseded by 

 aniline colours derived from coal tar. Even those which have 

 continued in use are now, to a large extent, manufactured by 

 chemical processes. 



They are all composed mainly of carbon, hydrogen, and 



