VEGETABLE TANNAGE 43 



good weight it is necessary to develop in the pelt its 

 maximum possible specific surface. This is one of the 

 objects of " plumping," which splits up the fibres. It is 

 attained also by the solution of interfibrillar substance in 

 limes and bates. 



The amount adsorbed is also a function of the volume 

 concentration in the sol after equilibrium is reached. Hence 

 the better weights are obtained with stronger liquors. 



The adsorption law is 



y l 



=ac n 

 m 



where y is weight adsorbed by the weight m of adsorbent, 

 and c the volume concentration after adsorption ; a and n 

 are numeral constants. Hence weight is determined by 

 the strength of the liquor which the goods finally leave. 

 The commencement of tannage is necessarily in weak 

 infusions, in order to secure the maximum diffusion into 

 interior of the fibres before they become heavily coated 

 on the exterior. As the equilibrium is being established 

 in such liquors the volume concentration diminishes, and 

 thus makes it less likely that good weight will be attained ; 

 hence it is necessary in practice to move the goods con- 

 stantly into fresh liquors of gradually increasing strength, 

 and so maintain the rate of adsorption and save time. A 



further consequence of the adsorption isotherm is that as 



j_ 



y varies as c n and n is >i, y is increased appreciably only 

 by a relatively large increase in c. Hence, though stronger 

 liquors give better weight, there is a limit beyond which 

 any further gain in weight is not justified by the enormous 

 increase in the concentration necessary to attain it. Such 

 great increase in c is impracticable not only on the ground 

 of expense, but also on account of the great viscosity of the 

 sol. 



The amount of adsorption depends also upon the exact 

 nature of the sol. It has been previously pointed out that 

 the tannins differ largely in their penetrating and weight - 

 giving powers. Some are readily adsorbable and are 



