otherwise too large a quantity would get below the pestle, and prevent
its operation. For this reason, likewise, too large a quantity of the
substance to be powdered ought not to be put into the mortar at one
time; and we must from time to time get rid of the particles already
reduced to powder, by means of sieves to be afterwards described.
The most usual method of levigation is by means of a flat table ABCD,
Pl. 1. Fig. 7. of porphyry, or other stone of similar hardness, upon
which the substance to be reduced to powder is spread, and is then
bruised and rubbed by a muller M, of the same hard materials, the bottom
of which is made a small portion of a large sphere; and, as the muller
tends continually to drive the substances towards the sides of the
table, a thin flexible knife, or spatula of iron, horn, wood, or ivory,
is used for bringing them back to the middle of the stone.
In large works, this operation is performed by means of large rollers of
hard stone, which turn upon each other, either horizontally, in the way
of corn-mills, or by one vertical roller turning upon a flat stone. In
the above operations, it is often requisite to moisten the substances a
little, to prevent the fine powder from flying off.
There are many bodies which cannot be reduced to powder by any of the
foregoing methods; such are fibrous substances, as woods; such as are
tough and elastic, as the horns of animals, elastic gum, &c. and the
malleable metals which flatten under the pestle, instead of being
reduced to powder. For reducing the woods to powder, rasps, as Pl. I.
Fig. 8. are employed; files of a finer kind are used for horn, and still
finer, Pl. 1. Fig. 9. and 10. for metals.
Some of the metals, though not brittle enough to powder under the
pestle, are too soft to be filed, as they clog the file, and prevent its
operation. Zinc is one of these, but it may be powdered when hot in a
heated iron mortar, or it may be rendered brittle, by alloying it with a
small quantity of mercury. One or other of these methods is used by
fire-work makers for producing a blue flame by means of zinc. Metals may
be reduced into grains, by pouring them when melted into water, which
serves very well when they are not wanted in fine powder.
Fruits, potatoes, &c. of a pulpy and fibrous nature may be reduced to
pulp by means of the grater, Pl. 1. Fig. 11.
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