28 ANIMAL PROTEINS 



about 200 C. The tannins are soluble in water, alcohol, 

 acetone, ethyl acetate and acetic acid, but insoluble in 

 benzene, chloroform, carbon disulphide, petroleum ether, 

 dilute sulphuric acid and pure ethyl ether. The aqueous 

 infusions of the tannins are in reality colloidal solutions ; i.e. 

 heterogeneous systems of two phases. The systems are 

 lyophile, or, more particularly, hydrophile, i.e. there is an 

 affinity between the two phases. As usual with lyophile 

 systems the two phases may be considered as both liquid, 

 and an aqueous infusion of tannin forms an emulsoid sol, 

 which therefore is subject to the phenomenon of adsorption. 

 The tannins are all precipitated by solutions of basic lead 

 acetate and copper acetate, and many of them with varying 

 completeness by solutions of many other metallic salts and 

 hydroxides, of basic dyestuffs and of alkaloids. They give 

 dark colorations with ferric salts. 



The tannins are widely distributed in plant-life, but 

 only in a limited number of cases do the plants contain 

 sufficient tannin to render them of commercial importance. 

 Tannin is found in all parts of plants; but usually in greatest 

 amount in the bark or fruit. The tannins are classified 

 into " pyrogallol tans " and " catechol tans," according to 

 the parent phenol. This classification is confirmed by their 

 chemical, analytical and practical behaviour, and the 

 vegetable tanning materials may be classified into the 

 same two groups, for, although even the same plant contains 

 both pyrogallol and catechol tans, it is usual to find in any 

 one part of the plant that one group is predominant. 



Pyrogallol tans, which are oftenest obtained from 

 fruit or leaves, contain usually about 52 per cent, of carbon. 

 Used alone they produce a rather soft and porous leather. 

 Associated with them in many cases probably as decom- 

 position products are certain other substances of well- 

 known properties and constitution. These substances are 

 not only typical of the group, but also form the most valuable 

 clue to the chemical constitution of the group and the key 

 to their chemical behaviour. One of these substances is 

 gallic acid (3:4:5 trihydroxy-benzoic acid C 6 H^(OH) 3 - 



