The same sort of precautions are required in drawing the still smaller sample required for analysis from the larger original sample, but these are sufficiently detailed in the directions of the I.A.L.T.C. given in the Appendix. As materials usually require finer grinding than can be managed with the mills employed in the tannery, a suitable mill must be provided, and one of the simplest, at a moderate price, is a No. 4 drug-mill made by A. Kenrick and Sons, Limited, West Bromwich, Fig. 64. Coffee mills are seldom strong enough for the purpose, but if nothing better is available, the sample must be _thoroughly_ dried before grinding, and its loss of weight noted, and taken into account in calculating the analysis, care being taken that the sample after grinding is so preserved that it cannot re-absorb moisture. Valonia, myrobalans and even barks, may before grinding be broken with a flat-faced hammer, on a thick cast-iron plate, with raised edges to prevent loss from flying fragments. [Illustration: FIG. 64.--Kenrick’s Drug-Mill.] _Preparation of solution for analysis._--As the method of analysis only gives satisfactory results when the quantity of tanning matter in the solution is within certain limits, the International Association prescribes that it must be such as to contain between 3·5 and 4·5 grms. of tanning matter per liter, or as near as possible, on the average, to 4 grms. If, as will rarely happen, the strength of a material is quite unknown, it may be necessary to make a trial test to ascertain the quantity of substance to be used, but the following table gives the quantity with sufficient accuracy for most ordinary materials. TABLE SHOWING THE AMOUNT OF DIFFERENT MATERIALS TO BE WEIGHED OUT FOR ANALYSIS TO MAKE UP ONE LITER OF SOLUTION. _Barks, etc._ Grams. Algarobilla 9 Canaigre 15 Divi-divi 9 Hemlock bark 16 Mimosa bark 11 Myrobalans 15 Oak-bark 30 Oak-wood 100 Quebracho wood 20 Sumach 15 Valonia 15 Valonia beard 11 _Extracts._ Oak-wood, sp. gr. 1·2 or over 15 Chestnut ditto 14 Quebracho (solid) 6 „ (liquid) 9 to 13 Gambier (block) 10 „ (cube) 7