VEGETABLE TANNAGE 45 



tan liquors give in the long run more weight, but tan more 

 slowly. Pelt tanned whilst thus plumped forms naturally 

 a thicker and less pliable leather. This occurs in tanning 

 sole leather, to a less extent with heavy dressing leather, 

 and to a very small extent in the case of softer dressing 

 leathers. 



In addition to adsorption, there is another phenomenon 

 of colloid chemistry in operation, viz. the mutual precipita- 

 tion of the sols in the liquid by the gels in the hide. In 

 most sols the disperse phase is electrically charged. The 

 sol therefore possesses electric conductivity, and migration 

 occurs in the electric field to the cathode or anode according 

 to the nature of the charge. Oppositely charged sols pre- 

 cipitate one another, the precipitate containing both colloids. 

 The maximum precipitation occurs when the + charge of one 

 sol exactly equals and neutralizes the charge of the 

 other. There is thus an electrical equivalence ; an amount 

 of sol which is equivalent to a given amount of the other. 

 This is not a chemical equivalence, however, and the 

 precipitate is not a chemical compound in spite of its fairly 

 constant composition. The composition of the precipitate, 

 indeed, is not quite constant, for the optimum precipitation 

 may not correspond exactly with the electrical equivalence, 

 being influenced by the number of particles required, their 

 size (dispersity), the rate of mixture, and the relative con- 

 centrations of the sols. This mutual precipitation is ex- 

 hibited by emulsoids as well as suspensoids, but the charge 

 (+ or ) on an emulsoid is in many instances largely an 

 accidental matter, being determined by the medium in 

 which it happens to be, its normal condition being electrical 

 neutrality. Gelatin and pelt are such emulsoids, and 

 a positively charged gelatin sol has been observed to pre- 

 cipitate a negatively charged gelatin sol. It is thought, how- 

 ever, that gelatin is primarily a positive sol. Pelt (whether 

 delimed or not) is rapidly acidified by the quickly penetrating 

 and strongly adsorbed organic acids of the old tan liquors 

 and becomes positively charged before the tannins are 

 adsorbed. The positive charge increases with the acidity 



