Filterers of the second kind work on the following principle:  If a cylinder be rapidly revolved in a liquid in which solid particles are suspended, the liquid will be drawn into a like rotation and the heavy particles will be thrown to the outer part of the receptacle.  If a perforated cylinder is used as stirrer, the purified liquid will escape into it through the perforations and may be conducted away.  The impurities, likewise, after falling down the sides of the receptacle, are carried off.  The advantages of this method are that no filtering material is needed and the filtering surface is never in contact with anything but pure liquor.

Very fine sawdust is, to a considerable extent, employed in sugar refineries as a filtering medium.  By such use the sawdust becomes mixed with sand, fine particles of cane, etc.  As sawdust of such fineness is expensive, it is desirable to purify it in order to reuse it.  A centrifugal (Pat. 353,775—­J.V.V.  Booraem) built on the following principle is used for this purpose.  It has been observed that by rotating rather slowly small particles of various substances in water, the finer particles will be thrown outward and deposit near the circumference of the vessel, while the heavier and coarser particles will deposit nearer to or at the center, their centrifugal force not being sufficient to carry them out.  A mere rod, extending radially in both directions, serves by its rotation to set the water in motion.

Another form of filter of this second kind (Pat. 148,513) has a rotating imperforate basket into which the impure liquor is run.  Within and concentric with it is another cylinder whose walls are of some filtering medium.  The liquid already partly purified by centrifugal force passes through into the inner cylinder, thus becoming further purified.  Centrifugal filters are used also to cleanse gums for varnishes.

HONEY.—­The simplest form of honey extractor (Pat. 61,216) consists of a square framework, symmetrical with respect to a vertical spindle.  This framework is surrounded by a wire gauze.  The combs, after having the heads of the cells cut off, are placed in comb-holders against the wire netting on the four sides, the cells pointing outward.  The machine is turned by hand.  The honey is hurled against the walls of a receiving case and caught below.  But few improvements have been made on this.  The latest machines are still hand-driven, as a sufficiently high velocity can be obtained in this manner.  In one style the combs are placed upon a floor which rests upon springs.  The rotating box is given a slight vertical and horizontal reciprocatory motion, by which the combs are made to grate on the wire gauze sides, breaking the cells and liberating the honey.  Thus the labor of cutting the cells is saved.  Every comb has two sides, and to present each side in succession to the outside without removing from the basket, several devices have been patented.