nted in the plate, immediately below the mortars to which they
respectively belong, and are made of hammered iron or brass, of wood,
glass, porcellain, marble, granite, or agate, according to the nature of
the substances they are intended to triturate. In every laboratory, it
is requisite to have an assortment of these utensils, of various sizes
and kinds: Those of porcellain and glass can only be used for rubbing
substances to powder, by a dexterous use of the pestle round the sides
of the mortar, as it would be easily broken by reiterated blows of the
pestle.

The bottom of mortars ought to be in the form of a hollow sphere, and
their sides should have such a degree of inclination as to make the
substances they contain fall back to the bottom when the pestle is
lifted, but not so perpendicular as to collect them too much together,
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 fi