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 extension of surface, _i.e._ the adsorption is a function of the specific surface of the adsorbent. Hence, to obtain 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/m = ac^(1/n) 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 constantly 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 y varies as c^(1/n) and n is > 1, 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.