cesses, we are directed to add to the fusible compound some porous
substance which is incapable of fusion, and will retain the whole in a
spongy form, thereby to facilitate the dissipation of the volatile
parts.

In order to know whether an alkali would lose a part of its air, and
acquire a degree of causticity, when exposed, with this precaution, to
the action of a strong fire, I mixed an ounce and a half of salt of
tartar with three ounces of black-lead, a substance of any the most
unchangeable by chemical operations. This mixture I exposed, for several
hours, in a covered crucible, to a fire somewhat stronger than what is
necessary to keep salt of tartar in fusion. When allowed to cool, I
found it still in the form of a loose powder; and taking out one half, I
diluted it with water, and by filtration obtained a ley, which, when
poured into a solution of white marble in _aqua fortis_, precipitated
the marble under the form of a weak quick-lime: for the turbid mixture
gave a green colour to the juice of violets, and threw up a crust like
that of lime-water; and the precipitated powder collected and mixed with
salt ammoniac immediately yielded the scent of the volatile alkali.

Lest it should here be suspected, that the alkaline qualities of this
mixture, and of the precipitated marble, were not owing to a lime into
which the marble was converted, but to the alkali itself which was
added, it is proper to observe, that I mixed so small a proportion of
the ley with the solution of marble as made me sure, from certain
experiments, that the whole of the alkali was spent in performing the
precipitation, and was consequently converted into a neutral salt by
attracting the acid. The properties therefore of the mixture can only be
referred to a lime, as is indeed sufficiently evident from the crust
which is peculiar to lime-water.

I was therefore assured by this experiment, that an alkali does really
lose a part of its air, and acquire a degree of causticity, by the
proper application of heat; but finding by several trials, that the
degree of causticity which it had thus acquired was but weak, and that
the quick-lime produced in this experiment was exhausted and rendered
mild by a small quantity of water, I exposed the crucible together with
that half of the alkali which remained in it to a stronger fire, in
order to expel a larger quantity of air, and render it more remarkably
caustic; but the whole of it was dissipated by