130 ANIMAL PROTEINS 



same sense, viz., to reduce the imbibition of the hide gel. Thus 

 the potassium sulphate in a chrome alum liquor has its 

 own specific action of this kind and contributes to the 

 leather formation. Unhydrolyzed chromium sulphate and 

 the sodium sulphate formed in " making basic " act also 

 in the same sense. 



The tanning sol is probably chromium hydrate, formed 

 by the hydrolysis of chromium sulphate : it is a lyophile 

 or emulsoid sol and is in consequence very strongly adsorbed 

 by the hide gel. This adsorption, involving a concentration 

 of lyophile sol, is the first stage in gelation, which occupies 

 a relatively more prominent place in chrome than in vegetable 

 tannage. Some diffusion into the gel also occurs, and both 

 the gelation arid diffusion of the sol are affected by lyotrope 

 influence, but to a greater extent than in the vegetable 

 tannage. Thus far the analogy is almost complete. 



There remains the question of the precipitation of the 

 tanning colloid at the interface. This is a point which has not 

 yet been thoroughly investigated, and which offers consider 

 able difficulty to a clear understanding, but the matter may 

 be probably summarized thus : the adsorbed chromium 

 hydrate is precipitated at the interface of gel and sol to 

 some extent, chiefly through the neutralization of its charge 

 by the oppositely charged ions of the electrolytes present, 

 but possibly also in the last stages of manufacture by the 

 mutual precipitation of oppositely charged gel and sol. 



To illustrate the matter, the case of a basic chrome 

 alum liquor will be considered. The chromium hydrate 

 sol is primarily a positive sol, just like ferric and aluminium 

 hydrate sols : i.e. in water they are somewhat exceptional 

 in that they adsorb H' rather than OH'. To cause pre- 

 cipitation therefore it is necessary to make the sol less 

 positive and more negative. The positive charge of the 

 sol, however, is greater than in water, because of the free 

 acid formed in the hydrolysis, which results in the adsorption 

 of more hydrions by the sol. Hence to ensure precipitation 

 steps must be taken to reduce the adsorption of hydrions 

 by the chromium hydrate sol. In practice such steps are 



