Myrobalans and mimosa-barks have proved especially troublesome to grind, the former from the hardness of the stones of the fruit, and a tendency to clog the mill, and the latter from their combined hardness and toughness. “Disintegrators” of various patterns are now made, which are capable of grinding both these materials satisfactorily, and but for their liability to cause fire, and the large proportion of fine dust which they make, are usually to be preferred to toothed mills. In spite of their disadvantages, however, they have come very largely into use, on account of their efficiency in grinding obstinate materials. Disintegrators work on the principle of knocking or beating the material to powder, by means of very rapidly revolving beaters, which, in the smaller machines, are driven at 2500 to 3000 revolutions per minute. The first disintegrator was made by Carr and consisted of two concentric cylinders or baskets of steel bars, rotating in opposite directions at a very high speed. The material was fed between these and was dashed to pieces by being thrown against the bars and the outer casing. [Illustration: FIG. 70.--Disintegrator.] A simpler form was soon introduced by Carter, in which only one axis was employed, carrying radial beaters which dashed the material against the serrated outer casing, a portion of the circumference of which was fitted with gratings, through which the ground material was thrown as soon as it was sufficiently reduced in size, the fineness of the grinding being regulated by changing the grates as required. This type of disintegrator is, with slight variations, made by all the leading makers of tanners’ machinery; and one form is shown in Fig. 70, and a similar but smaller machine, opened to show construction, in Fig. 71. In the more modern machines the sides as well as the circumference of the casing are frequently corrugated in order to increase the action on the material. Mills running at such high rates of speed as 3000 revolutions per minute will grind most hard substances, such as stone or brick, without injury, but pieces of iron among the tanning material are apt to cause damage, and various magnetic devices have been employed for separating this metal, but with only partial success. In the best mills, therefore, the beaters and inner casings are constructed so that they can be easily replaced, and the damage is then rarely serious. [Illustration: FIG. 71.--Disintegrator opened, showing construction.] In order to avoid vibration, the discs and beaters of all these high-speed mills must be balanced with great accuracy. This is best accomplished by removing the spindle from the mill, and allowing it to roll on two levelled straight-edges, and then filing or chipping the beaters on the heavy side until it will remain indifferently in any position.