FHARMACY. 
ing, cJutriation, decantation, filtration, des- 
pumation, expression. 
Sifting. From dry substances, which are 
reduced to the due degree of minuteness, 
the coarser particles are to be separated by 
sieves of iroti-warc, hair-cloth, or gauze, or 
by being dusted through bags of fine linen. 
For very light and valuable powders, or 
acrid substances, compound sieves, having 
a close lid and receiver must be used. The 
particles which are not of sufficient fineness 
to pass through the interstices of the sieve, 
may be again powdered. , 
Elutriation is confined to mineral sub- 
stances, on which water has no action. It 
is performed by separating them from foreign 
particles and impurities, of a different spe- 
cific gravity, in which case they are said to 
be washed ; or for separating tlie impalpable 
powders, obtained by trituration and levi- 
gation from the coarser particles. This 
process depends upon the property that 
very fine or light powders have of remaining 
for some lime suspended in water ; and is 
performed by diffusing the powder or paste 
formed by levigation through plenty of 
water, letting it stand a sufficient time, un- 
til the coarser particles settle at the bottom ; 
and then pouring off the liquid in wliich the 
finer or lighter particles are suspended. 
Fresh water may be poured on the residuum, 
and the operation repeated ; or the coarser 
particles, which fall to tlie bottom, may be 
previously levigated a second time. 
Decantation. The fine powder which is 
washed over with the water is separated 
from it, by allowing it to subside completely 
and by either decanting off the water very 
carefully, or by drawing it off by a syringe 
or syphon. These processes are very fre- 
quently made use of for separating fluids from 
solids which are specifically heavier, espe- 
cially when the quantity is veiy large, dr 
the solid so subtile as to pass through the 
pores of most substances employed for fil- 
tration, or the liquid so acrid as to corrode 
them. 
Filtration. For the same purpose of se- 
parating fluids from solids, straining and 
filtration are often used. These differ only 
in degree, and are employed when the pow- 
der either floes not subside at all, or too 
slowly and imperfectly for decantation. 
The instruments for this purpose are of 
various materials, and must in no instance 
be acted upon by the substances for which 
they are employed. Fats, resins, wax, and 
oils, are strained through hemp or flax spread 
evenly over a piece of wire-cloth or net 
stretched in a frame. For saccharine and 
mucilaginous liquors, fine flannel may be 
used : for some saline solutions, linen. Where 
these are not fine enough, unsized paper is 
employed ; but it is extremely apt to burst 
by hot watery liquors which dissolve its size ; 
and very acrid liquors, such as acids, are 
filtered by means of a glass-funnel, filled 
with powdered quartz, a few of the larger 
pieces being put into the neck, smaller 
pieces over these, and the finer powder 
placed over all. The porosity of this last 
filter retains much of the liquor ; but it may 
be recovered by gently pouring on it as 
much distilled water ; the liquor will then 
pass through, and the water be retained in 
its place. 
Water may be filtered in large quantities 
through basins of porous stone, or artificial 
basins of nearly equal parts of fine clay and 
coarse sand. The size of the filters depends 
OB the quantity of matter to be strained. 
When large, the flannel or linen is formed into 
a conical bag, and suspended from a hoop 
or frame ; the paper iS either spread on the 
inside of these bags, or folded into a conical 
form, and suspended by a funnel. It is of 
advantage to introduce glass rods, or quill- 
barrels, between the paper and funnel, to 
prevent them from adhering too closely. 
What passes first is seldom fine enough, and 
must be poured back again, until by the 
swelling of the fibres of the filter, or filling 
up of its pores, the fluid acquires the requi- 
site degree of limpidity. The filter is some- 
times covered with charcoal powder, which 
is a useful addition to muddy and deep- 
coloured liquors. The filtration of some 
viscid sultstances is much assisted by heat. 
Expression is a species of filtration, assist- 
ed by mechanical force. It is principally 
employed to obtain the juices of fresh ve- 
getables, and the unctuous vegetable oils. 
It is performed by means of a screw press 
with plates of wmod, iron, or tin. The sub- 
ject of the operation is previously beaten, 
ground, or bruised. It is then inclosed in 
a bag, which must not be too much filled, 
and introduced between the plates of the 
press. The bags should be of hair-cloth, or 
canvass inclosed in hair-cloth. Hempen 
and woollen bags are apt to give vegetable 
juices a disagreeable taste. The pressure 
should be gentle at first, and increased 
gradually. Vegetables intended for this 
operation should be perfectly fresh and 
freed from all impurities. In general they 
should be expressed as soon as they are 
braised, for it disposes them to ferment^ 
