time, the whole of it is gradually saturated with air, to
which the water as gradually yields its place.

We have already shown by experiment, that magnesia alba is a compound of
a peculiar earth and fixed air. When this substance is mixed with
lime-water, the lime shews a stronger attraction for fixed air than that
of the earth of magnesia; the air leaves this powder to join itself to
the lime. And as neither the lime when saturated with air, nor the
magnesia when deprived of it, are soluble in water, the lime-water
becomes perfectly pure and insipid, the lime which it contained being
mixed with the magnesia. But if the magnesia be deprived of air by
calcination before it is mixed with the lime-water, this fluid suffers
no alteration.

If quick-lime be mixed with a dissolved alkali, it likeways shews an
attraction for fixed air superior to that of the alkali. It robs this
salt of its air, and thereby becomes mild itself, while the alkali is
consequently rendered more corrosive, or discovers its natural degree of
acrimony or strong attraction for water, and for bodies of the
inflammable, and of the animal and vegetable kind; which attraction was
less perceivable as long as it was saturated with air. And the volatile
alkali when deprived of its air, besides this attraction for various
bodies, discovers likeways its natural degree of volatility, which was
formerly somewhat repressed by the air adhering to it, in the same
manner as it is repressed by the addition of an acid.

This account of lime and alkalis recommended itself by its simplicity,
and by affording an easy solution of many _phaenomena_, but appeared upon
a nearer view to be attended with consequences that were so very new and
extraordinary, as to render suspicious the principles from which they
were drawn.

I resolved however to examine, in a particular manner, such of these
consequences as were the most unavoidable, and found the greatest number
of them might be reduced to the following propositions:

I. If we only separate a quantity of air from lime and alkalis,
when we render them caustic they will be found to lose part of
their weight in the operation, but will saturate the same quantity
of acid as before, and the saturation will be performed without
effervescence.

II. If quick-lime be no other than a calcarious earth deprived of
its air, and whose attraction for fixed air is stronger than that
of alkalis,