from the crust and "retanned" 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 "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 "irreversible changes" subsequent to the mutual precipitation of oppositely charged gel and sol, are equally applicable to the chrome tannages. Chrome tannage was once thought to embrace such irreversible changes, but the process can now be "reversed" with ease. The reversibility of the chrome tannage is an easier proposition than that of vegetable tannage, partly because the leather is comparatively much less tanned, and partly because the acidity or alkalinity of the stripping agent may be adjusted, as desired, without the oxidation trouble. In approaching this question from the theoretical side one must consider mainly whether to solate the tanning agent to a positive or to a negative sol. Our imperfect knowledge of the electrical forces in operation in the chrome tannage is thus a serious drawback, but the evidence on the whole points to the precipitation being effected by a negative sol near its isoelectric point but in faintly acid solution. Hence, we should theoretically expect that reversion should take place into a negative sol in nearly neutral or even faintly alkaline solution. Thus, suitable stripping agents for chrome leather would be the alkali salts of organic acids (especially if multivalent). Now, Procter and Wilson have recently accomplished this stripping of chrome leather by the use of such salts. They approached the question from an empirical and practical point of view and found that Rochelle salt, sodium citrate, and sodium lactate would strip the chrome tannage with ease. This important and very creditable achievement will have great practical and commercial importance. Procter and Wilson have deliberately and carefully refrained