for drawing off the clear fluid.
In experiments, where the weight of the precipitate must be rigorously
ascertained, decantation is preferable to filtration, providing the
precipitate be several times washed in a considerable proportion of
water. The weight of the precipitate may indeed be ascertained, by
carefully weighing the filtre before and after the operation; but, when
the quantity of precipitate is small, the different proportions of
moisture retained by the paper, in a greater or lesser degree of
exsiccation, may prove a material source of error, which ought carefully
to be guarded against.
CHAP. V.
_Of Chemical Means for separating the Particles of Bodies from each
other; without Decomposition, and for uniting them again._
I have already shown that there are two methods of dividing the
particles of bodies, the _mechanical_ and _chemical_. The former only
separates a solid mass into a great number of smaller masses; and for
these purposes various species of forces are employed, according to
circumstances, such as the strength of man or of animals, the weight of
water applied through the means of hydraulic engines, the expansive
power of steam, the force of the wind, &c. By all these mechanical
powers, we can never reduce substances into powder beyond a certain
degree of fineness; and the smallest particle produced in this way,
though it seems very minute to our organs, is still in fact a mountain,
when compared with the ultimate elementary particles of the pulverized
substance.
The chemical agents, on the contrary, divide bodies into their primitive
particles. If, for instance, a neutral salt be acted upon by these, it
is divided, as far as is possible, without ceasing to be a neutral salt.
In this Chapter, I mean to give examples of this kind of division of
bodies, to which I shall add some account of the relative operations.
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