The main application of Neradol D is in admixture with vegetable tanning materials; especially in the early stages of tannage is this substance of value, since by its use not only a light coloured leather surface is obtained, but its presence prevents a subsequent dead tannage when strong vegetable tan liquors are applied, and it also imparts strength to the grain layer. It is thus possible to shorten the time consumed by the tanning process by employing Neradol D in the manner described. A further explanation as to why the tanning process is considerably hastened by using Neradol D, either alone or in conjunction with natural tannins, is afforded by the fact that though Neradol D quickly penetrates the grain, it is but "loosely" fixed by the latter, _i.e._, it is not deposited to such an extent that it would prevent penetration of the vegetable tannins. In the case of a mixture of Neradol D and vegetable tannins, the former quickly diffuses into the pelt and fixes the fibres, thus facilitating penetration of the vegetable tannins. This assumption is justified in view of the speed with which Neradol D completely penetrates and tans the pelt, since Neradol D containing acids and salts exhibits effects similar to those of a pickle. 6. Reactions of Neradol D with Iron and Alkalies A special characteristic of Neradol D tannage is the sensitiveness of the latter to the action of iron and alkalies. The active principle of Neradol D being free dicresylmethanedisul-phonic acid, which is easily neutralised by lime, ammonia, and amino-acids and hence rendered inactive for tanning purposes, it is essential that the pelt prior to tannage with Neradol D should be completely delimed, bated, and freed from all constituents possessing alkaline reaction. It is, however, possible to regenerate Neradol D liquors contaminated with alkali or partly neutralised by the addition of small quantities of organic (formic, acetic, lactic, and butyric) or inorganic (hydrochloric or sulphuric) acids,_i.e._, the dicresyl-methanedisulphonic acid is again partly liberated, and this procedure is always preferred where the tanning process does not allow of a complete deliming of the pelt prior to introducing the latter into a Neradol D liquor. If, on the other hand, such liquors are kept properly, and the addition of acid referred to kept up, they will remain active for weeks and need only strengthening up with the requisite quantity of Neradol prior to introducing fresh pack.