stop-cock; and, as some of the water charged with salt necessarily
adheres to the straw and insoluble matters, several fresh quantities of
water are poured on. The straw serves to secure a proper passage for the
water, and may be compared to the straws or glass rods used in
filtrating, to keep the paper from touching the sides of the funnel. The
cloth which is laid over the matters under lixiviation prevents the
water from making a hollow in these substances where it is poured on,
through which it might escape without acting upon the whole mass.
This operation is less or more imitated in chemical experiments; but as
in these, especially with analytical views, greater exactness is
required, particular precautions must be employed, so as not to leave
any saline or soluble part in the residuum. More water must be employed
than in ordinary lixiviations, and the substances ought to be previously
stirred up in the water before the clear liquor is drawn off, otherwise
the whole mass might not be equally lixiviated, and some parts might
even escape altogether from the action of the water. We must likewise
employ fresh portions of water in considerable quantity, until it comes
off entirely free from salt, which we may ascertain by means of the
hydrometer formerly described.
In experiments with small quantities, this operation is conveniently
performed in jugs or matrasses of glass, and by filtrating the liquor
through paper in a glass funnel. When the substance is in larger
quantity, it may be lixiviated in a kettle of boiling water, and
filtrated through paper supported by cloth in the wooden frame, Pl. II.
Fig. 3. and 4.; and in operations in the large way, the tub already
mentioned must be used.
SECT. III.
_Of Evaporation._
This operation is used for separating two substances from each other, of
which one at least must be fluid, and whose degrees of volatility are
considerably different. By this means we obtain a salt, which has been
dissolved in water, in its concrete form; the water, by heating, becomes
combined with caloric, which renders it volatile, while the particles of
the salt being brought nearer to each other, and within the sphere of
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