To prevent the escape of dust, the machine is provided with an air-circulation chamber, which maintains the flow of air in continuous circulation, the resulting strong draught also drawing the fine material through the screen and keeping the meshes open. By this means the capacity of the pulveriser is considerably increased. The interchange of the crushing organs and screens, and also the cleaning of the machine, can be effected without difficulty or loss of time. The charge is introduced through a feed hopper at the side, and may vary, according to the size of the machine, from nut size to lumps twice as large as a man’s fist. If necessary, suitable mechanical feed devices can be applied. [Illustration: FIG. 12.] _Disintegrators_ (Figs. 12 and 13).--This type of machine is used for reducing medium-hard or soft materials, especially where it is desired to obtain a comparatively large output of a gritty product. In the patterns shown, the main shaft is of steel, with dust- and dirt-proof red-brass bearings with pad or ring lubrication. The spindle case draws out to facilitate cleaning. Mechanical feeding attachments can be provided. According to local conditions, the disintegrator can be mounted either on a brick foundation, with lateral discharge outlet into a storage bin, or on a raised grating of iron joists. If the product is to be conveyed to a distance, it is advisable to have a hopper-shaped collector leading directly to a worm conveyor or bucket elevator. [Illustration: FIG. 13.] The arrangement shown in Fig. 13, in which the disintegrator is mounted on a dust-proof cast-iron collector, has been found very suitable for colour works of various kinds (aniline, lead, mineral and other colours), particularly on account of the suppression of dust; whilst the automatic charging worm greatly increases the capacity as compared with charging by hand. LEVIGATION The effect of levigation is based on the circumstance that bodies of greater density than water remain longer in suspension in that medium in proportion as the fineness of their particles increases. This treatment consequently enables the finer portions of a substance to be mechanically separated from the coarser. Levigation is extensively practised in colour works because it furnishes powder of finer grain than can be obtained by even the most careful grinding.