the force of the heat,
and the black lead, which still retained the form of a loose and subtile
powder, yielded little or nothing to water.

We learn then from the above experiment the reason why the alkali newly
obtained from the ashes of vegetables is generally of the more acrid
kinds of that salt. It never appears until the subject be converted into
ashes, and is supposed to be formed by the fire, and to be the result of
a particular combination of some of the principles of the vegetable; one
of which principles is air, which is contained in large quantity in all
vegetable matters whatever. But as soon as the smallest part of a
vegetable is converted into ashes, and an alkali is thus formed, this
salt necessarily suffers a calcination, during which it is kept in a
spongy form by the ashes, and shews a very considerable degree of
acrimony if immediately applied to the body of an animal but if the
ashes are for any time exposed to the air, or if we separate the alkali
from them by the addition of a large quantity of water and subsequent
evaporation, the salt imbibes fixed air from the atmosphere, and becomes
nearly saturated with it: tho' even in this condition it is generally
more acrid than salt of tartar, when this is prepared with a gentle
heat.

Borax has sometimes been referred to the class of alkalis, on account of
some resemblance it bears to those salts: but it has been demonstrated
by accurate experiments, that we should rather consider it as a neutral
salt; that it is composed of an alkali and of a particular saline
substance called the sedative salt, which adheres to the alkali in the
same manner as an acid, but can be separated by the addition of any acid
whatever, the added acid joining itself to the alkali in the place of
the sedative salt. As this conjunction of an acid with the alkali of
borax happens without the least effervescence, our principles lay us
under a necessity of allowing that alkali to be perfectly free of air,
which must proceed from its being incapable of union with fixed air and
with the sedative salt at the same time: whence it follows, that, were
we to mix the sedative salt with an alkali saturated with air, the air
would immediately be expelled, or the two salts in joining would produce
an effervescence. This I found to be really the case upon making the
trial, by mixing a small quantity of the sedative salt with an equal
quantity of each of the three alkalis, rubbing the mixtures