III., SOLIDS FROM SOLIDS.--As for grain machines, which are in
this class, it may be said that in centrifugal flour bolters, bran
cleaners, and middlings purifiers, though theoretically centrifugal
force plays an important part in their action, yet practically the real
separation is brought about by other agencies: in some by brushes which
rub the finer particles through wire netting as they rotate against it.

The principle exhibited in a separator of grains and seeds is very
neat. (Pat. 167,297.) See Fig. 32. That part of the machine with which
we have to do consists essentially of a horizontal revolving disk. The
mixed grains are cast on this disk, pass to the edge, and are hurled
off at a tangent. Suppose at A. Each particle is immediately acted on
by three forces. For all particles of the same size and having the same
velocity the resistance of the air may be taken the same, that is,
proportional to the area presented. The acceleration of gravity is the
same; but the inertia of the heavier grain is greater. The resultant of
the two conspiring forces R and (M_v_^{2})/2 varies, and is greater for
a heavier grain. Therefore, the paths described in the air will vary,
especially in length; and how this is utilized the drawing illustrates.

[Illustration: Fig. 32.]

ORE.--In ore machines there is one for pulverizing and separating coal
(Pat. 306,544), in which there is a breaker provided with helical
blades or paddles, partaking of rapid rotary motion within a stationary
cylinder of wire netting. The dust, constituting the valuable part of
the product, is hurled out as fast as formed. In this style of machine,
beaters are necessary not only for pulverizing, but to get up rotary
motion for generating centrifugal force. In the classes preceding, the
friction of the basket sufficed for this latter purpose; but here there
is no rotating basket and no definite charge. As the material falls
through the machine, separation takes place. Various kinds of ore may
be treated in the same manner.

An "ore concentrator" (Pat. 254,123), as it is called, consists of a
pan having rotary and oscillatory motions. Crushed ore is delivered
over the edge in water. The heavy particles of the metal are thrown by
centrifugal force against the rim of the pan, overcoming the force of
the water, which carries the sand and other impurities in toward the
center and away.

AMALGAMATORS.--The best ore centrifugal or separator is what is called
an