human
In mineral-tanned leathers the metal is combined with carboxyl groups, while in vegetable-tanned leather the tannin is combined with the amino groups. This strongly suggests the possibility that the two methods of tanning are to some extent independent of one another, and that a piece of leather tanned by one method may remain as capable of being tanned by the other method as though it were still raw pelt (_Collegium_ (London), 1917, 110-111). To the writer, however, it seems that the facts are evidence for the contrary proposition, that the tannages are fundamentally of the same nature. On the adsorption theory, one would expect chrome leather to adsorb as much tan as pelt; the readily adsorbable tan is the same, and the chrome leather is an adsorbent of very much the same order of specific surface as pelt. The adsorption theory would find it difficult to account for chrome leather not adsorbing as much tan as pelt. It is quite conceivable that a chrome leather could adsorb more tan than pelt, owing to the more complete isolation of the fibrils by the chrome tannage and to their being coated over by a more adsorbent gel. Adsorption is often deliberately increased by a preparatory adsorption. Thus sumach-tanned goatskins are wet back from the crust and
gpt
in sumach before dyeing, to coat the fibres with a fresh and more adsorbent gel and so ensure the even and thorough adsorption of the dyestuff. Mordanting fabrics has a similar object,--the adsorption of colloidogenic substances which give rise to an adsorbent gel on the fibre. Unless vegetable-tanned leather is so much loaded with tan that its specific surface is effectively reduced, one would similarly expect that vegetable-tanned leather would adsorb the chrome sol. This, of course, is exactly the case of semi-chrome leather. If, on the