it has been allowed to "weather" for a short time--some
finely-powdered magnetite the strongly hygroscopic constituents can be
made practically magnetic, because the magnetic impalpable dust
adheres to them, while it remains separate from the grains of the
other minerals.

Hardness--as well as magnetic attraction--is a property of ore which
has as yet been made available to only a very slight extent as the
basis of a system of separation. If a quantity of mixed fragments of
glass and plumbago be pounded together in a mortar with only a
moderate degree of pressure, so as to avoid, as far as possible, the
breaking of the glass, there will soon come a stage at which the
softer material can be separated from the harder simply by means of a
fine sieve. There are many naturally-existing mineral mixtures in the
crushing of which a similar result occurs in a very marked degree;
and, indeed, there are none which do not show the peculiarity more or
less, because the constituents of an ore are never of exactly the same
degree of hardness. When the worthless parts are the softer and
therefore have the greater tendency to "slime," the ore is very
readily dressed to a high percentage by means of water.

But when the reverse is the case, and the valuable constituents
through their softness get reduced to a fine pulp long before the
other parts, the ordinary operations of the ore-dresser become much
more difficult to carry out. Most elaborate ore-reduction plants are
constructed with the view to causing the crushing surfaces, whether of
rolls or of jaws, to merely tap each piece of stone so as to break it
in bits without creating much dust. This operation is repeated over
and over again; but the stuff which is fine enough to go to the
concentrator is removed by sieving after each operation of the kind;
and the successive rolls or other crushers are set to a finer and
finer gauge, so that there is a progressive approach to the conditions
of coarse sand, which is that specially desired by the ore-dresser.

Much of this elaboration will be seen to be needless, and, moreover,
better commercial results will be obtained when it is more clearly
perceived that the recovery of a valuable ore in the form of a fine
slime may be economically effected by the action of grinders specially
constructed for the purpose of permitting the hard constituents of the
ore to remain in comparatively large grains, while the other and
softer minerals are redu