NOTE.--All the apparatus named in this chapter can be obtained of Messrs. Reynolds and Branson, Commercial Street, Leeds; or of Messrs. Portway, Jamaica Road, S.E.; and of most other dealers in chemical apparatus. CHAPTER XXI. _GRINDING OF TANNING MATERIALS._ Before the tannin they contain can be extracted, most materials require to be ground, almost the only exceptions to this rule being divi-divi and algarobilla, in which the tannin is very loosely contained. Extracts, whether solid or liquid, merely require to be dissolved in water or liquor, in which they are, for all practical purposes, perfectly soluble. With the less soluble extracts it is generally preferable to dissolve at a temperature of 50° to 60° C. with vigorous stirring. The actual method of grinding, and consequently the machinery employed for the purpose, vary not only with the material to be ground, but with the method of leaching adopted, as it is essential that the mass of ground material should be completely permeated by the liquor employed in leaching; and if it be ground too finely, or subjected to too much pressure on account of the height to which it is piled in the leaches, it is apt to form a compact and clay-like mass, the interior of which remains unextracted. [Illustration: FIG. 68.--Cone-Mill.] In the laboratory, where thorough extraction must be completed in a few hours, the material can hardly be too fine; but on the larger scale a much coarser product must be used, and leaching requires days, or sometimes even weeks, and is then seldom successful in removing all the tannin. It is probable, however, that in the future these mechanical difficulties of extraction will be overcome; and the material will then be as finely divided, and as completely extracted on the large scale, as it is in the laboratory at the present time. One of the earliest methods of grinding oak-bark, and which is still used for sumach (p. 271) consists in crushing it under large circular edge-stones, frequently turned by a horse. This process was very slow and inefficient for barks, and both it and horizontal millstones similar to those used for wheat were long ago superseded by iron or steel mills on the same principle as the ordinary coffee-mill.