Eitner[155] states that the action of the bran drench cannot be considered a chemical one, and that the chief action is a dynamical one, resulting from the gases generated in the liquid and in the skin. While I do not deny that the gas has such action--indeed I pointed out in 1893[156] that the gas acted by floating and distending the skins, and so enabling them better to take up the acids--I maintain, however, that the chief action is a chemical one and due to the weak organic acids produced; this was confirmed experimentally. The researches on bran drenching commenced in 1887 by myself, and later in conjunction with Dr. W. H. Willcox, prove conclusively that the amount of acid produced, and especially the lactic acid, is sufficient to produce a very decided action on skin. In actual working we found from 1·07 to 2·34 grm. total acids per 1000 c.c., while in artificial cultures, with addition of calcium carbonate, inoculated with pure cultures of bacteria, obtained from drenches, we obtained from 4·53 to 11·44 grm. total acids per 1000 c.c. Now traces of lime in the skins act in just the same way as the calcium carbonate; they neutralize a portion of the acid produced by the bacteria, and so allow a greater amount of acid to be produced than if it accumulated in the liquid. By preparing solutions of the above acids of suitable strength and agitating skins in them, results were obtained in 1-1/2 hours equal to a drenching of 14 hours. The defect of such artificially prepared liquids in practice is due to the fact that they do not contain any particles of bran or flour; these take up dirt from the skin mechanically, and so produce a better-coloured leather. Where the skin has been bated in a perfectly clean bate, and then paddled in such an artificial drench, the leather is in every way equal to that prepared by means of the dung bate, followed by a bran drench. [155] Der Gerber, 24, 570. [156] Jour. Soc. Chem. Ind., 1893, p. 426. By far the most important action for the tanner is the solution of the last traces of lime not dissolved by the bate, and this is the chief reason for its use, where colour is a desideratum, and where sweet liquors are used; for some kinds of leather where it is desirable not to lose pelt, the drench is used alone for this purpose. A dung bate, such as most light leather tanners use, is seldom run off more than once a week, and contains a considerable amount of dissolved lime; the amount in a freshly made-up puer wheel is from 0·1 grm. to 0·3 grm. lime (CaO) per litre. After the wheel has been in use for a week, it rises to 0·5 grm. to 0·8 grm. per litre. It will be readily seen that a liquid which already contains such considerable quantities of lime is not an especially good medium to use for its complete removal from the skins, hence drenching is necessary to get rid of the lime left in the skins.