7. Hide powder shall be of a woolly texture, thoroughly delimed, preferably with hydrochloric acid. It shall not require more than 5 c.c. or less than 2.5 c.c. of decinormal NaOH or KOH to produce a permanent pink colour with phenolphthalein on 6.5 gm. of the dry powder suspended in water. If the acidity does not fall within these limits it must be corrected by soaking the powder before chroming for twenty minutes in ten to twelve times its weight of water, to which the requisite calculated quantity of standard alkali or acid has been added. The hide powder must not swell in chroming to such an extent as to render difficult the necessary squeezing to 70-75 per cent. of water, and must be sufficiently free from soluble organic matter to render it possible in the ordinary washing to reduce the total solubles in a blank experiment with distilled water below 0.005 gm per 100 c.c. The powder, when sent out from the maker, shall not contain more than 12 per cent. of moisture, and shall be sent out in air-tight tins. The detannisation shall be carried out in the following manner:-- The moisture in the air-dried powder is determined, and the quantity equal to 6.5 gm. actual dry powder is calculated, which will be practically constant if the powder be kept in an air-tight vessel. Any multiple of this quantity is taken according to the number of analyses to be made, and wet back with approximately ten times its weight of distilled water. Two grammes per 100 of dry powder of crystallised chromic chloride, CrCl_3.6aq., is now dissolved in water and made basic with 0.6 gm. of Na_2CO_3 by the gradual addition of 11.25 c.c. of normal Na_2CO_3, thus making the salt correspond to the formula Cr_2Cl_3(OH)_3. In laboratories where analyses are continually being made, it is more convenient to employ a 10 per cent stock solution, made by dissolving 100 gm. of Cr_2Cl_6.6aq. in a little distilled water in a litre flask and very slowly adding a solution containing 30 gm. of anhydrous sodium carbonate, with constant stirring, finally making up to the mark with distilled water and well mixing. Of this solution 20 c.c. per 100 gm., or 1.3 c.c. per 6.5 gm. of dry powder, should be used. This solution is added to the powder, and the whole churned for one hour. At the end of the one hour the powder is squeezed in linen to free it as far as possible from the residual liquor, and washed and squeezed repeatedly with distilled water, until, on adding to 50 c.c. of the filtrate one drop of 10 per cent. K_2CrO_4 and four drops of decinormal silver nitrate, a brick-red colour appears. Four or five squeezings are usually sufficient. Such a filtrate cannot contain more than 0.001 gm. of NaCl in 50 c.c.