THE NATURE OF CHROME LEATHERS 133 



the writer, however, it seems that the facts are evidence 

 for the contrary proposition, that the tannages are funda- 

 mentally 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 goat- 

 skins are wet back from the crust and " re- tanned " 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 simi- 

 larly expect that vegetable-tanned leather would adsorb 

 the chrome sol. This, of course, is exactly the case of 

 semi-chrome leather. If, on the "chemical combination" 

 theory, the vegetable tan combines with the amino groups 

 and the chrome with carboxyl groups, it is natural to 

 inquire which groups the dyestuffs combine with. As 

 either tannage does not interfere with the adsorption of 

 dye, are we to conclude similarly that tanning and dyeing 

 are fundamentally different processes ? 



Those who favour this chemical combination theory, 

 and who offer equations for the formation of vegetable 

 and of chrome leather, should likewise suggest an equation 

 for the formation of leather from pelt by the action of 

 dyestuffs a practical though hardly an economic process. 



The remarks made earlier in this volume (Part I., 

 Section III.) as to the occurrence of what have been called 



