The centrifugal hydro-extractor consists, therefore, of a vessel in rapid rotation; and if a liquid be introduced into such vessel, it is projected with considerable force against the peripheral walls. If the peripheral surface be perforated, the liquid portion of a charge consisting of liquid and solid matters will be ejected through the perforations, while the solid matter remains inside. As a rule, a few minutes’ treatment in a hydro-extractor is sufficient to separate the water from a thin pulp so completely that the solid residue is in an almost completely dry state. A hydro-extractor which, though of an old pattern, is well adapted for the purposes of the colour-maker, is shown in Fig. 16. [Illustration: FIG. 16.] The drum A, which revolves easily on a vertical axis, is of metal, and is provided with a large number of fine perforations on its peripheral surface. It can be rotated at high speed by means of the crank _f_ and pinions _d_, _e_, or by the fast-and-loose pulley _a b_ connected with a source of power. To prevent any of the charge from being projected over the rim of the drum, the upper edge is turned over so as to leave only a comparatively small opening at the top. The lower end of the drum shaft carries a strong steel spindle, which must be carefully machined and enable the drum to revolve as easily as possible. This is essential, because even small machines require a comparatively large amount of motive power--which is not surprising in view of the high speed at which the drum has to revolve in order to perform its functions. The drum is enclosed in a casing of somewhat larger diameter, which may be of any convenient material. The bottom of the casing is preferably tapered slightly downward, and is covered, at its lowest part--below the bearing of the drum--with a sieve communicating with a pipe through which the ejected liquid is drained off. When a liquid, containing solid matter, is fed into the drum, which is already running at high speed, the liquid is thrown, by the centrifugal force, against the peripheral surface of the drum and escapes through the perforations, leaving the solid matter behind. Where large crystals are in question, as for instance in sugar factories, the centrifugal machine can be employed without any additional precautions, the liquid being expelled and the crystals being practically dried by keeping the machine running a short time longer. In the case of the pulp obtained by levigating colours, however, this procedure would result in failure, because the fine solid particles would be ejected along with the liquid and the drum would be left quite empty.