Now that a large variety of other materials besides bark are required by tanners, the mills just described are not always sufficient for the purpose. Myrobalans and mimosa-bark have proved specially troublesome, the former from its very hard stones and clogging character, and the latter from its combined hardness and toughness. "Disintegrators" of various makes have proved admirably adapted for grinding both of these materials, their advantage being the universality of their reducing powers, ranging from oak-bark to bones or brick-dust, and their disadvantages, the somewhat considerable power they consume, and the rather large portion of fine dust they make. Their principle is that of knocking the material to powder by rapidly revolving beaters, which, in the smaller mills, are driven at so high a speed as 2500-3000 rev. a minute. Wilson's is shown in Fig. 34, as an example. It is one of the oldest tanners' disintegrators, and probably still one of the best. In the figure, it is opened, showing the disc with its steel beaters attached. When myrobalans are only required roughly crushed, a machine with fluted or toothed rollers (Fig. 35) acts better than a disintegrator, making less dust, and requiring less power. Such a machine also crushes valonia very satisfactorily. [Illustration: Fig. 34.] [Illustration: Fig. 35.] In England, the tanning material is generally carried from the mill, to the pits where it is exhausted, in baskets or barrows; in America, this is frequently accomplished by a "conductor," or horizontal spout, in which a double belt, or malleable iron "drive chain,"[U] with wooden cross-pieces, carries the bark forward, on the same principle as the elevators of corn-mills. Fig. 36 shows the conveyors used in a Chicago tannery. Another American plan is to use circular tubs for extraction. These are mounted on wheels, and are worked on a railway, coming up to the mill to be filled, and thence under a series of sprinklers like those used by brewers, and finally "dumping" their contents before the boilers, which are heated solely by wet bark, burnt in a peculiar furnace with brick chambers. This furnace for burning wet bark seems worthy of extended adoption in Europe, as spent tan is frequently not only valueless, but costly to get rid of. Full details and scale drawings may be found in Jackson S. Schultz's book on 'Leather Manufacture' and in Fig. 37 is shown a modification of it, patented by Huxham and Brown, which has been very successfully used in burning wet tan, either alone or with a portion of coal. In American sole-leather tanneries, where the bark is resinous and almost unlimited in quantity, sufficient steam may be raised with tan wet from the leaches; but in England, where material is more sparingly used, it is advisable partially to dry it before burning. This is accomplished by powerful roller-presses, as shown in Fig. 38. Gläser, of Vienna, constructs