t of a body has vanished from their
senses, notwithstanding the utmost care to retain it; and they have
always found, upon further inquiry, that subtile part to be air, which
having been imprisoned in the body, under a solid form, was set free and
rendered fluid and elastic by the fire. We may therefore safely
conclude, that the volatile matter, lost in the calcination of
_magnesia_, is mostly air; and hence the calcined _magnesia_ does not
emit air, or make an effervescence, when mixed with acids.

The water, from its properties, seems to contain a small portion of
volatile alkali, which was probably formed from the earth, air, and
water, or from some of these combined together; and perhaps also from a
small quantity of inflammable matter which adhered accidentally to the
_magnesia_. Whenever Chemists meet with this salt, they are inclined to
ascribe its origin to some animal, or putrid vegetable, substance; and
this they have always done, when they obtained it from the calcarious
earths, all of which afford a small quantity of it. There is, however,
no doubt that it can sometimes be produced independently of any such
mixture, since many fresh vegetables and tartar afford a considerable
quantity of it. And how can it, in the present instance, be supposed,
that any animal or vegetable matter adhered to the _magnesia_, while it
was dissolved by an acid, separated from this by an alkali, and washed
with so much water?

Two drams of _magnesia_ were calcined in a crucible, in the manner
described above, and thus reduced to two scruples and twelve grains.
This calcined _magnesia_ was dissolved in a sufficient quantity of
spirit of vitriol, and then again separated from the acid by the
addition of an alkali, of which a large quantity is necessary for this
purpose. The _magnesia_ being very well washed and dryed, weighed one
dram and fifty grains. It effervesced violently, or emitted a large
quantity of air, when thrown into acids, formed a red powder when mixed
with a solution of sublimate, separated the calcarious earths from an
acid, and sweetened lime-water: and had thus recovered all those
properties which it had but just now lost by calcination: nor had it
only recovered its original properties, but acquired besides an addition
of weight nearly equal to what had been lost in the fire; and, as it is
found to effervesce with acids, part of the addition must certainly be
air.

This air seems to have been furnished by the