not tell us the weight of the _osteocolla_ remaining in the retort,
and only says, that it was converted into quick-lime; but as no
calcarious earth can be converted into quick-lime, or bear the heat
which he applied without losing above a third of its weight, we may
safely conclude, that the loss in his experiment was proportional, and
proceeded chiefly from the dissipation of fixed air.

According to our theory, the relation of the calcarious earth to air and
water appeared to agree with the relation of the same earth to the
vitriolic and vegetable acids. As chalk for instance has a stronger
attraction for the vitriolic than for the vegetable acid, and is
dissolved with more difficulty when combined with the first, than when
joined to the second: so it also attracts air more strongly than water,
and is dissolved with more difficulty when saturated with air than when
compounded with water only.

A calcarious earth deprived of its air, or in the state of quick-lime,
greedily absorbs a considerable quantity of water, becomes soluble in
that fluid, and is then said to be slaked; but as soon as it meets with
fixed air, it is supposed to quit the water and join itself to the air,
for which it has a superior attraction, and is therefore restored to its
first state of mildness and insolubility in water.

When slaked lime is mixed with water, the fixed air in the water is
attracted by the lime, and saturates a small portion of it, which then
becomes again incapable of dissolution, but part of the remaining slaked
lime is dissolved and composes lime-water.

If this fluid be exposed to the open air, the particles of quick-lime
which are nearest the surface gradually attract the particles of fixed
air which float in the atmosphere. But at the same time that a particle
of lime is thus saturated with air, it is also restored to its native
state of mildness and insolubility; and as the whole of this change
must happen at the surface, the whole of the lime is successively
collected there under its original form of an insipid calcarious earth,
called the cream or crusts of lime-water.

When quick-lime itself is exposed to the open air, it absorbs the
particles of water and of fixed air which come within its sphere of
attraction, as it meets with the first of these in greatest plenty, the
greatest part of it assumes the form of slaked lime; the rest is
restored to its original state; and if it be exposed for a sufficient
length of