_The Use of Extracts in the Tannery._--One of the great attractions of extracts is that they save the trouble and cost of leaching, and as the extract manufacturer makes this his specialty, he can often extract more tanning matter from a material than the tanner who has no means of concentrating his weak liquors. The extract manufacturer also can employ methods of decoloration which would be impracticable to the tanner, and so enable the latter to obtain better colour than if he employed the raw material. By the use of extracts a tanner can strengthen weak liquors without trouble, and with definite quantities of materials; and by using extracts for this purpose the tanner is enabled to use up the weaker liquors of his leaches and so employ more water and obtain better extraction of his solid materials than if he used them alone. In the case of very weak materials like oakwood, the difficulties of making liquors of sufficient strength for tanning without evaporation are so great as to render such materials useless to the tanner for his own extraction, and their carriage even for short distances may amount to more than their total value. Even with much richer materials, extraction effects a saving if the carriage is a long one, as it rarely pays to import any material containing less than about 25 per cent. of tanning matter. Even when the strength of the natural material is considerable, as in the case of quebracho, extraction may be profitable if from its hardness, or other reasons, the material is difficult for the tanner to handle. For long voyages, and especially from the tropics, solid extracts are more suitable than liquid, as the expense of casks is saved, and the danger of fermentation is lessened. As it is impossible for the tanner to judge by appearance or consistency of the strength or value of extracts, they should always be bought and sold on the analysis of the particular shipment or parcel by a competent chemist. For directions for sampling see pp. 301, 475. Extracts simply require to be dissolved in a suitable quantity of water or weak liquor at an appropriate temperature, to obtain a liquor of any required strength. Some extracts are completely soluble in cold water or liquor, but most dissolve better by the aid of heat. 40°-60° C. (100°-140° F.) is generally sufficient, and probably no advantage can arise from temperatures over 80° (180° F.). Boiling should be avoided, as it facilitates the formation of insoluble “reds” with consequent loss of tanning matter and darkening of colour. The extract should be run into the vat in a thin stream, and continuously plunged up; where large quantities of extract are to be dissolved, a mechanical agitator is advantageous. A “silent boiling jet” (p. 335) may be used, fitted into a small casing immersed in the liquor and open at both ends, and the extract run into the current it produces.