being the finest. This process may likewise be used with advantage for
separating substances of different degrees of specific gravity, though
of the same fineness; this last is chiefly employed in mining, for
separating the heavier metallic ores from the lighter earthy matters
with which they are mixed.
In chemical laboratories, pans and jugs of glass or earthen ware are
employed for this operation; sometimes, for decanting the liquor without
disturbing the sediment, the glass syphon ABCHI, Pl. II. Fig. 11. is
used, which may be supported by means of the perforated board DE, at the
proper depth in the vessel FG, to draw off all the liquor required into
the receiver LM. The principles and application of this useful
instrument are so well known as to need no explanation.
SECT. III.
_Of Filtration._
A filtre is a species of very fine sieve, which is permeable to the
particles of fluids, but through which the particles of the finest
powdered solids are incapable of passing; hence its use in separating
fine powders from suspension in fluids. In pharmacy, very close and fine
woollen cloths are chiefly used for this operation; these are commonly
formed in a conical shape, Pl. II. Fig. 2. which has the advantage of
uniting all the liquor which drains through into a point A, where it may
be readily collected in a narrow mouthed vessel. In large pharmaceutical
laboratories, this filtring bag is streached upon a wooden stand, Pl.
II. Fig. 1.
For the purposes of chemistry, as it is requisite to have the filtres
perfectly clean, unsized paper is substituted instead of cloth or
flannel; through this substance, no solid body, however finely it be
powdered, can penetrate, and fluids percolate through it with the
greatest readiness. As paper breaks easily when wet, various methods of
supporting it are used according to circumstances. When a large quantity
of fluid is to be filtrated, the paper is supported by the frame of
wood, Pl. II. Fig. 3. ABCD, having a piece of coarse cloth stretched
over it, by means of iron-hooks. This cloth must be well cleaned each
time it is used, or even new cloth must be employed, if there is reason
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