_Total soluble matter._--The solution of which the preparation has been described, must now be filtered, the size and kind of paper, and exact method of filtration prescribed by the International Association being strictly adhered to. All papers and methods of filtration absorb traces of tanning matters, and but few will give a clear filtrate with such solutions as those of quebracho and hemlock extracts; and to obtain uniform results exact uniformity of method is essential.[156] Deviations from the exact method, in the case of quebracho, easily cause discrepancies of several units per cent. in the result. The object of rejecting the first portions of the filtered solution is to prevent, as far as possible, errors which would arise from the absorption of tannin by the paper, and to insure a clear filtrate. 50 c.c. of the clear filtrate is now measured by an accurate pipette, and evaporated to dryness in a weighed porcelain basin, on a steam-bath, in order to determine the “total soluble.” This and succeeding operations should be done in duplicate, even if this has not been the case in making up the original solution, which is certainly desirable. [156] Methods of correction for absorption of filter-papers have been worked out in the Author’s laboratory, and adopted by the last conference of the International Association. Cp. Collegium, pp. 145-158, 1902, and App. A, p. 477. Ordinary light porcelain basins, generally of about three inches diameter, are employed for evaporation, which takes place somewhat more rapidly if they are flat-bottomed (saucer-shaped). In place of porcelain, thin glass basins of hemispherical form may be used, and, but for the cost, platinum would be better than anything else. Aluminium and nickel basins have been tried, but are slightly attacked by some liquors, and hence are more liable to vary in weight, though they have the advantage in rapidity of evaporation. Evaporation takes place most quickly if the steam-bath can be placed in a draught of air, so as to rapidly carry away the vapour formed, but the basins must be protected from dust. Under favourable circumstances, evaporation of 50 c.c. in porcelain basins occupies one to one and a half hour. An ordinary pan fitted with a lid of thin copper perforated with holes of two and three-quarter inches in diameter, makes a useful water-bath; but where much work is done, it is desirable to have a rectangular bath of thin sheet copper, taking a single, or at most a double row of basins, and fitted with the usual appliance for keeping the water at constant level; or with a supply of steam from a boiler, and an overflow for condensed water.