_Estimation of matters soluble in water._--This is important both to detect weighting, and to draw conclusions as to the materials used in tanning. Fine raspings or shavings may be exhausted with warm water in a percolator, or roughly a weighed piece (20 _grm._) of leather, air-dry, may be well kneaded and worked in 100 _c.c._ of warm water in a basin. 50 _c.c._ of this may be evaporated to dryness in a light basin over the water-bath (or under a paper hood on a steam boiler), and the gain of weight will give the amount dissolved from 10 _grm._ This is more accurate and quicker than redrying the leather and weighing loss. The residue will contain tannins and their products, often in considerable quantities, and may be examined by the table of reactions, p. 112, though these are as yet very imperfect. It will also contain glucose, dextrin, and soluble salts, if these have been used to give weight and firmness. The absolute proof of weighting with glucose or dextrin is difficult, since tanning materials naturally contain these and analogous principles. The residue may be powdered and exhausted with cold water, and the tannins and colouring matter removed by shaking with magnesia (p. 108) or lead carbonate. Fehling's solution[J] is then added and the mixture is heated nearly to boiling. A rapidly formed and considerable precipitate of red cuprous oxide indicates weighting with glucose or dextrin. Leather extracts, however, invariably reduce Fehling's solution more or less, and a conclusion can only be drawn after some experience and comparative tests. Gallotannic acid and pyrogallol reduce it when heated, but not cane sugar or gum arabic. If a solution of cane sugar be heated to 68° C. for 1/4 hour with 10 per cent. of fuming hydrochloric acid, it is "inverted," and then after neutralising the acid with potash or soda, will reduce Fehling's solution when heated. [Footnote J: 4 _grm._ cryst. cupric sulphate are dissolved in 20 _c.c._ of water; and 16 _grm._ of neutral potassic tartrate and 13 _grm._ of fused sodic hydrate are dissolved in 60 _c.c._ The two are mixed, made up to 100 _c.c._, and boiled for some minutes. It should always be tested before use by boiling a portion, which should remain perfectly clear.]