this last equivalent (62·36 _grm._ of quercitannic acid = 63 _grm._ of crystallised oxalic acid) does not differ from that of oxalic acid more than the ordinary limits of error of such estimation, and the substitution is therefore of no commercial importance, while it is surely better to employ a standard which is easily and exactly verified than one which is certain to be modified by further research, and so to run the risk either of having our results made useless for future comparison, or of establishing a false or arbitrary equivalent. What is wanted for practical purposes is not the absolute weight of tannins in the various materials, but only a means for the relative comparison of two samples of the same material; cross comparisons of different tannins being simply delusive. If, however, it is necessary at any time to give actual percentages of gallotannic acid, it is probably best to stick to Neubauer's number for the present, as it is in general use. Neubauer states that 63 _grm._ of oxalic acid consume as much permanganate as 41·37 _grm._ of gallotannic acid. Tshekawa found 41·688 as the equivalent for tannin from Japanese gall nuts (Chem. News, xlii. 274). Councler and Schroeder on the other hand give only 34·3 _grm._ Simand gives 61·1 _grm._ as the equivalent of quercitannic acid. Commercial "pure tannin" always gives results higher than the truth, as the gallic acid which it contains consumes more permanganate than an equal weight of tannin, or even than the tannin which would yield it if boiled with acid. When this is done the equivalent used should be definitely stated, or it will certainly lead to confusion. Neubauer's equivalent is only properly applicable to gall nuts, and possibly to sumach and myrabolans. For oak bark Oser's number or that of oxalic acid is most likely nearly correct; and this may also be approximately true of oak wood and valonia, but as respects all other materials we have no information whatever, and the oxalic equivalent is as likely to be right as any other. (Compare note, p. 128.) [Footnote M: From researches by von Schroeder, published since the above was penned, it seems that the permanganate consumed by tannin is largely influenced by the way in which the titration is conducted, see p. 128.] A few results are given below, not as showing the relative values of the materials, which, of course, cannot be directly compared by any analytical process, but for comparison with those obtained by other methods and modes of calculation:--