The choice of the different substances of which these instruments are
made is a matter of importance; brass or copper are unfit for operations
upon substances to be used as food or in pharmacy; and marble or
metallic instruments must not be used for acid substances; hence mortars
of very hard wood, and those of porcelain, granite, or glass, are of
great utility in many operations.
SECT. II.
_Of Sifting and Washing Powdered Substances._
None of the mechanical operations employed for reducing bodies to powder
is capable of producing it of an equal degree of fineness throughout;
the powder obtained by the longest and most accurate trituration being
still an assemblage of particles of various sizes. The coarser of these
are removed, so as only to leave the finer and more homogeneous
particles by means of sieves, Pl. I. Fig. 12. 13. 14. 15. of different
finenesses, adapted to the particular purposes they are intended for;
all the powdered matter which is larger than the intestices of the sieve
remains behind, and is again submitted to the pestle, while the finer
pass through. The sieve Fig. 12. is made of hair-cloth, or of silk
gauze; and the one represented Fig. 13. is of parchment pierced with
round holes of a proper size; this latter is employed in the manufacture
of gun-powder. When very subtile or valuable materials are to be sifted,
which are easily dispersed, or when the finer parts of the powder may be
hurtful, a compound sieve, Fig. 15. is made use of, which consists of
the sieve ABCD, with a lid EF, and receiver GH; these three parts are
represented as joined together for use, Fig. 14.
There is a method of procuring powders of an uniform fineness,
considerably more accurate than the sieve; but it can only be used with
such substances as are not acted upon by water. The powdered substance
is mixed and agitated with water, or other convenient fluid; the liquor
is allowed to settle for a few moments, and is then decanted off; the
coarsest powder remains at the bottom of the vessel, and the finer
passes over with the liquid. By repeated decantations in this manner,
various sediments are obtained of different degrees of fineness; the
last sediment, or that which remains longed suspended in the liquor,
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