well in a
mortar, and adding a little water. It is however proper in this place
to observe, that, if the experiments be made in a different manner, they
are attended with a singular circumstance. If a small quantity of the
sedative salt be thrown into a large proportion of a dissolved fixed
alkali, the sedative salt gradually disappears, and is united to the
alkali without any effervescence; but if the addition be repeated
several times, it will at last be accompanied with a brisk
effervescence, which will become more and more remarkable, until the
alkali be entirely saturated with the sedative salt.

This _phaenomenon_ may be explained by considering the fixed alkalis as
not perfectly saturated with air: and the supposition will appear very
reasonable, when we recollect, that those salts are never produced
without a considerable degree of heat, which may easily be imagined to
dissipate a small portion of so volatile a body as air. Now, if a small
quantity of the sedative salt be thrown into an alkaline liquor, as it
is very slowly dissolved by water, its particles are very gradually
mixed with the atoms of the alkali. They are most strongly attracted by
such of these atoms as are destitute of air, and therefore join with
them without producing an effervescence; or, if they expel a small
quantity of air from some of the salt, this air is at the same time
absorbed by such of the contiguous particles as are destitute of it, and
no effervescence appears until that part of the alkali, which was in a
caustic form or destitute of air, be nearly saturated with the sedative
salt. But if, on the other hand, a large proportion of the sedative salt
be perfectly and suddenly mixed with the alkali, the whole, or a large
part, of the air is as suddenly expelled.

In the same manner may we also explain a similar _phaenomenon_, which
often presents itself in saturating an alkali with the different acids:
the effervescence is less considerable in the first additions of acid,
and becomes more violent as the mixture approaches the point of
saturation. This appears most evidently in making the _sal diureticus_
or regenerated tartar: The particles of the vegetable acid here employed
being always diffused thro' a large quantity of water, are more
gradually applied to those of the alkali, and during the first additions
are chiefly united to those that are freest of air.[10]

That the fixed alkali, in its ordinary state, is seldom entirely