from it, and this is
purified by two fresh solutions and cristallizations.

The deliquescent earthy salts which do not contain the nitric acid are
rejected in this manufacture; but those which consist of that acid
neutralized by an earthy base are dissolved in water, the earth is
precipitated by means of potash, and allowed to subside; the clear
liquor is then decanted, evaporated, and allowed to cristallize. The
above management for refining saltpetre may serve as a general rule for
separating salts from each other which happen to be mixed together. The
nature of each must be considered, the proportion in which each
dissolves in given quantities of water, and the different solubility of
each in hot and cold water. If to these we add the property which some
salts possess, of being soluble in alkohol, or in a mixture of alkohol
and water, we have many resources for separating salts from each other
by means of cristallization, though it must be allowed that it is
extremely difficult to render this separation perfectly complete.

The vessels used for cristallization are pans of earthen ware, A, Pl.
II. Fig. 1. and 2. and large flat dishes, Pl. III. Fig. 7. When a saline
solution is to be exposed to a slow evaporation in the heat of the
atmosphere, with free access of air, vessels of some depth, Pl. III.
Fig. 3. must be employed, that there may be a considerable body of
liquid; by this means the cristals produced are of considerable size,
and remarkably regular in their figure.

Every species of salt cristallizes in a peculiar form, and even each
salt varies in the form of its cristals according to circumstances,
which take place during cristallization. We must not from thence
conclude that the saline particles of each species are indeterminate in
their figures: The primative particles of all bodies, especially of
salts, are perfectly constant in their specific forms; but the cristals
which form in our experiments are composed of congeries of minute
particles, which, though perfectly equal in size and shape, may assume
very dissimilar arrangements, and consequently produce a vast variety of
regular forms, which have not the smallest apparent resemblance to each
other, nor to the original cristal. This subject has been very ably
treated by the Abbe Hauey, in several memoirs presented to the Academy,
and in his work upon the structure of cristals: It is only necessary to
extend generally to the class of salts the principle