The basket itself may be either wire or perforated, but must have a
perfectly smooth bottom.

As the pressure of the liquor upon the filtering medium per unit of
surface depends entirely upon its radial depth, mere tubes, connecting
a central inlet with an annular compartment, will serve the purpose
quite as well as a whole basket. In this style of machine (Pat. 10,457)
the filtering material constitutes a wall between two annular
compartments. The outer one is connected with a vacuum apparatus.

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