id, without, at the same time, saturating it with
air, we should then obtain it in a caustic form; but I am not acquainted
with an instance of this separation in chemistry. There are two indeed
which, at first sight, appear to be of this kind; these are the
separation of the fixed alkali from the nitrous acid by means of
inflamed charcoal, in the process for making _nitrum fixatum_, and of
the same alkali, from vegetable acids merely by heat; but, upon
examining the product of each process, we find the alkali either fully
or nearly saturated with air. In the first, either the charcoal or the
acid, or both together, are almost wholly converted into air; a part of
which is probably joined to the alkali. In the second, the acid is not
properly separated, but rather destroyed by the fire: a considerable
portion of it is converted into an inflammable substance and we learn
from Dr. _Hales_, that the bodies of this class contain a large quantity
of fixed air.

When we consider that the attraction of alkalis for fixed air is weaker
than that of the calcarious earths, and reflect upon the effects of heat
in chemistry, we are led to imagine, that alkalis might be entirely
deprived of their air, or rendered perfectly caustic, by a fire somewhat
weaker than that which is sufficient to produce the same change upon
lime; but this opinion does not seem agreeable to experience.

The alkalis do, however, acquire some degree of causticity in a strong
fire, as appears from their being more easily united with spirit of wine
after having been kept in fusion for some time. For that fluid, which
cannot be tinctured by a mild salt of tartar, will soon take a very deep
colour from a few drops of a strong caustic ley. The circumstances which
hinder us from rendering these salts perfectly caustic by heat, are
their propensity to dissipation in the utmost violence of the fire,
their extreme acrimony, and the imperfection of our common vessels. For
before the heat becomes very intense, the alkalis either evaporate, or
dissolve a part of the crucibles in which they are contained, and often
escape thro' their pores; which happens, especially as soon as they have
already acquired some degree of additional acrimony, by the loss of part
of their air.

The fusion also, which they so readily undergo, is well known by
Chemists, as a strong obstacle to the separation of the volatile from
the fixed parts of a compound by fire; accordingly, in several
pro