e
same quantity; but I allowed it to remain with the chalk, and shook it
frequently, for two days. This, after being filtrated, formed a cream or
crust upon its surface when exposed to the air; changed the colour of
the juice of violets into green; separated an orange-coloured powder
from a solution of corrosive sublimate; became turbid upon the addition
of an alkali; was entirely sweetened by magnesia; and appeared so
strong to the taste, that I could not have distinguished it from
ordinary lime-water. And when I threw some salt ammoniac into the lime
which remained, the vapour of the volatile alkali immediately arose from
the mixture.

In this experiment therefore the air is first driven out of the chalk by
an acid, and then, in order to separate this acid from it, we add an
alkali which has been previously deprived of its air; by which means,
the chalk itself is also obtained free of air, and in an acrid form, or
in the form of slaked lime.

We have also several processes for obtaining the volatile alkali in a
caustic form, which seem to be only so many methods of obtaining it in
its pure state, and free of fixed air. The first of these is the
separation of the alkali from an acid, merely by heat; an instance of
which we have from Mr. _Margraaf_.[9] He prepared from urine an
ammoniacal salt, the acid of which is the basis of the phosphorus, and
is of such a peculiar nature, that it endures a red heat without being
dissipated. Sixteen ounces of the neutral salt were subjected by him to
distillation. The acid remained in the retort, and he found in the
receiver eight ounces of an alkaline spirit, which, he tells us, was
extremely volatile, very much resembling the spirit of salt ammoniac
distilled with quick-lime; and no crystals were formed in it, when
exposed to the cold air.

A caustic volatile alkali may also be obtained, by mixing salt ammoniac
with half its weight of a caustic fixed alkali, or of magnesia which has
been previously deprived of its air by fire; and then submitting these
mixtures to distillation: Or merely by adding any ordinary volatile
alkali to a proper quantity of a caustic ley; for in this case the air
passes from the volatile to the fixed alkali, by a superior attraction
for the last, and, by a gentle heat, the compound yields a spirit
similar to that prepared from salt ammoniac and quick-lime.

It is therefore probable, that, had we also a method of separating the
fixed alkali from an ac