hydrochloric, or sulphuric) to the liquor. If this be done, great care must be taken not to over-do it, and an acid free from iron must be used. The use of sulphurous acid for the purpose has been patented, and presents some advantages. Sulphites have been observed by the writer to give a pink or purple reaction even with very dilute infusions of valonia (see p. 112); but any coloration from this cause would probably disappear as the tannage proceeds. The difficulty can, however, often be remedied, either by altering the way of working the liquors, so as to bring more sour liquor down to the suspenders, or by using a larger proportion of materials capable of yielding acetic acid by fermentation, such as myrobalans. It is a common error to call all the free acid of sour liquors "gallic," as this is scarcely present in pure bark-yards, and at the best is a very feeble acid. The most abundant acid is usually acetic, though butyric, lactic, and other acids are frequently present in varying proportions, according to the tanning materials employed. In the English process, with its comparatively short layers, in which the butts almost float in strong liquors, but little souring takes place, and we have nothing comparable to the German "sour bark" and "sour liquor" from long layers with weak liquor, and much dusty material. These contain large quantities of acetic and lactic acids, and plump almost like vitriol. Though the American tanners generally use the latter, their hemlock liquors sour much more intensely than those of English yards. It must always be borne in mind, in comparing English with American and Continental tanning, that, in the first, the opening up of the fibre is effected by lime, and the swelling is maintained in the liquors, not so much by acids, which are only present in very small proportion, as by the careful and gradual working forward into infusions stronger and stronger in tannin; while in the two latter, lime, if used at all, is simply employed to loosen the hair, and the swelling and differentiation of the fibre is first accomplished in the liquors either by vegetable or mineral acids. Hence good results cannot be expected in English yards from such processes as sweating or painting with sodium sulphide, which does not plump, without a radical modification of the whole tanning process. This point has been ably treated by Eitner in a series of papers on _Extract-gerberei_, published during the last few years in 'Der Gerber,' which will well repay attentive perusal by English as well as German tanners. The butts should at first be brought into the weakest liquor; a circulation system, by which the liquors are all pumped in at one end of a set of suspenders, and run out at the other, the butts being moved forward in the opposite direction, seems to have much to recommend it. In this case, the top of one pit should be connected by a wooden box with the bottom of the next.