The analysis is performed in the following manner:--20 _c.c._ of indigo solution, and 5 _c.c._ of the infusion of tanning material is added, in a white basin as recommended by Kathreiner, to about 3/4 _litre_ good water, which it is best to measure approximately, so that if it contains any impurity which affects the permanganate it should be constant, and thus be eliminated with the indigo. Permanganate solution is then allowed to drop in, with constant stirring till the pure yellow liquid shows a faint pinkish rim, most clearly seen on the shaded side. This end-reaction, which is of extraordinary delicacy, is due to Kathreiner, and is quite different to the pink caused by excess of permanganate, being an effect common to all pure yellow liquids. It is not needful to make the titration so slowly as has been advised--the permanganate may be dropped in steadily with vigorous stirring, so long as there is large excess of indigo, but as soon as the bottom of the basin can be seen through the solution, it must be added very cautiously, one or two drops at a time, and with occasional pauses, to allow time for its complete mixture through so large a mass of fluid. The titration is repeated twice, and the results added together and denoted by _a_. Then take 50 _c.c._ of the infusion, and add 28·6 _c.c._ of the gelatin solution of Nelson's gelatin of 2 _grm._ to 100 _c.c._ After shaking, the mixture is saturated with salt, which brings the volume up to 90 _c.c._, and 10 _c.c._ of the dilute sulphuric acid (containing 1 vol. of concentrated acid in 10) and a teaspoonful of pure kaolin are added. It is best to do this in a flask in which it can be well shaken, after which, filtration may be at once proceeded with, although it is safer to let it stand an hour or two: (the flask may be cleansed with caustic soda solution). 10 _c.c._ of this filtrate (= 5 _c.c._ of the original infusion) are employed for a second pair of titrations, which are added as before, and the result denoted _b_. If, further, _c_ be the quantity of permanganate required to oxidise 10 _c.c._ of decinormal oxalic acid, and 10 _grm._ of the tanning material have been employed to make 1 _litre_ of infusion, _c_ : (_a_ - _b_) :: 6·3 : _x_, where _x_ is the percentage of tannin expressed in terms of crystallised oxalic acid. If it be desired to calculate the gallic acid and non-tannin substances contained in the infusion, the value in permanganate of the indigo alone must be determined. Calling this _d_, as _c_ is to (_b_ - _d_), so is 6·3 to the percentage of non-tannins in terms of oxalic acid, and for the present it is best invariably to calculate results in this way, since we do not actually know the relation of any single tannin to permanganate, even Neubauer's number for gallotannic acid being probably too high, according to the recent investigations of Councler and Schroeder,[M] and Oser's for quercitannic being at most only approximate. It happens, moreover, that