46 ANIMAL PROTEINS 



of the liquor. Other emulsoids are not electrically neutral, 

 but are electrically charged and exhibit considerable con- 

 ductivity. Into this class fall the tannins, and in tanning 

 it is thought that there is a mutual precipitation of the nega- 

 tive tannin sol with the positive hide gel, the precipitation 

 of the negative sol being favoured by the acid condition of 

 the liquor. The effect of increasing acidity soon falls off, 

 however, as a saturation limit is soon reached. This 

 mutual precipitation of colloids in tanning is in reality but 

 an extension of the adsorption theory, which explains the 

 predominant effect of H+, and OH~ on the electric charge 

 by stating that these ions are more readily adsorbed than 

 other ions, and that as OH~ is more readily adsorbed 

 than H+ most sols are negative to water. 



In addition to the adsorption phenomena described, 

 there are in vegetable tannage secondary changes which 

 are slow and " irreversible/' These changes are obscure and 

 are difficult to investigate. Oxidation, dehydration and 

 polymerization have all been suggested, but there is little 

 direct evidence. Certain it is, however, that time renders 

 the tannage more permanent. It perhaps should be pointed 

 out that in the very strongest tan liquors the viscosity of 

 the tannin sol is so great that adhesion would be a better 

 term than adsorption. There is no abrupt division between 

 the two phenomena. 



In the theory of vegetable tannage there is another factor 

 the importance of which has been strongly emphasized by 

 the author, viz., lyotrope influence. This has been most 

 conveniently discussed in connection with gelatin gels (pp. 

 200-219), but its effect on hide gels is analogous. It has 

 also an effect upon the diffusion and gelation of the tannin 

 and non-tannin sols (cp. pp. 129 and 174). 



Mechanical Operations. In the tanyard the liquors are 

 almost invariably divided up into sections, called "rounds" 

 or " sets," in which the mechanical operations are different 

 in aim and method. In the first pits entered by the goods 

 there is rapid adsorption in spite of the low concentration and 

 small astringency, and the great aim is to obtain evenness of 



