and
two grains of the same acid were required to compleat the solution, and
no weight was lost in the experiment.

As in the separation of the volatile from the fixed parts of bodies, by
means of heat, a small quantity of the latter is generally raised with
the former; so the air and water, originally contained in the
_magnesia_, and afterwards dissipated by the fire, seem to have carried
off a small part of the fixed earth of this substance. This is probably
the reason, why calcined _magnesia_ is saturated with a quantity of
acid, somewhat less than what is required to dissolve it before
calcination: and the same may be assigned as one cause which hinders us
from restoring the whole of its original weight, by solution and
precipitation.

I took care to dilute the vitriolic acid, in order to avoid the heat and
ebullition which it would otherwise have excited in the water; and I
chose a Florentine flask, on account of its lightness, capacity, and
shape, which is peculiarly adapted to the experiment; for the vapours
raised by the ebullition circulated for a short time, thro' the wide
cavity of the vial, but were soon collected upon its sides, like dew,
and none of them seemed to reach the neck, which continued perfectly dry
to the end of the experiment.

We now perceive the reason, why crude and calcined _magnesia_, which
differ in many respects from one another, agree however in composing the
same kind of salt, when dissolved in any particular acid; for the crude
_magnesia_ seems to differ from the calcined chiefly by containing a
considerable quantity of air, which air is unavoidably dissipated and
lost during the dissolution.

From our experiments, it seems probable, that the increase of weight
which some metals acquire, by being first dissolved in acids, and then
separated from them again by alkalis, proceeds from air furnished by the
alkalis. And that in the _aurum fulminans_, which is prepared by the
same means, this air adheres to the gold in such a peculiar manner,
that, in a moderate degree of heat, the whole of it recovers its
elasticity in the same instant of time; and thus, by the violent shock
which it gives to the air around, produces the loud crack or fulmination
of this powder. Those who will imagine the explosion of such a minute
portion of fixed air, as can reside in the _aurum fulminans_, to be
insufficient for the excessive loudness of the noise, will consider,
that it is not a large quantity of m