42 ANIMAL PROTEINS 



finer constituent fibrils. Tannins, on the other hand, are 

 hydrophile colloids which in water form emulsoid sols, and 

 which may thus be expected to exhibit the phenomenon of 

 adsorption. A tan liquor usually contains several tannins 

 in addition to other closely similar substances, also in 

 colloidal solution, and is therefore a sol of considerable com- 

 plexity. The immersion of pelt into a tan liquor results in 

 an adsorption, which consists essentially in an inequality 

 of concentration in the sol, the greater concentration being 

 at the interface. This inequality between the surface con- 

 centration and the volume concentration of the sol, is due 

 primarily to considerations of surface tension and surface 

 energy, and exists before the immersion of the pelt. The 

 surface layer having excess over the volume concentration, 

 any considerable extension of surface in a fixed volume of sol 

 must produce a very considerable decrease in the volume 

 concentration. This is what occurs when pelt is immersed 

 in a tan liquor, the immersion being the considerable exten- 

 sion of surface. It should be especially remembered that the 

 inequality of concentration is in the sol, on the liquid side of 

 the interface. In adsorption, the substance adsorbed, i.e. the 

 excess at the surface, is too frequently regarded as bound to 

 the solid immersed. This is because the excess is in the layer 

 which wets the solid and remains wetting it when the solid 

 is removed. Thus the immersion of pelt produces primarily 

 only a change in the distribution of the tannins in the liquor. 

 It follows from this that the adsorption is an equilibrium, 

 and that if the sol be diluted, the equilibrium will become 

 the same as it would have been by immersing the pelt 

 directly into the dilute solution. Thus, if pelt be first 

 immersed in one tan liquor and then into a weaker one it 

 will yield tan to the latter solution. 



The chief object in heavy leather tanning is to obtain 

 the maximum possible adsorption in the minimum possible 

 time, or in other words, to obtain good weight quickly. 

 The amount adsorbed is proportional to the actual ex- 

 tension of surface, i.e. the adsorption is a function of 

 the specific surface of the adsorbent. Hence, to obtain 



