saturated with air, seems to be confirmed by the following experiment.

I exposed a small quantity of a pure vegetable fixed alkali to the air,
in a broad and shallow vessel, for the space of two months; after which
I found a number of solid crystals, which resembled a neutral salt so
much as to retain their form pretty well in the air, and to produce a
considerable degree of cold when dissolved in water. Their taste was
much milder than that of ordinary salt of tartar; and yet they seemed to
be composed only of the alkali, and of a larger quantity of air than is
usually contained in that salt, and which had been attracted from the
atmosphere: for they still joined very readily with any acid, but with a
more violent effervescence than ordinary; and they could not be mixed
with the smallest portion of vinegar, or of the sedative salt, without
emitting a sensible quantity of air.

As it now appeared that several alkaline substances have an attraction
for fixed air, I tried a few experiments to learn the relative strength
of their several attractions.

Twenty four grains of magnesia in fine powder were mixed with five
ounces of the caustic ley in a small vial, which was immediately corked
and shaken frequently for four hours. The ley was then poured off, and
the magnesia washed with repeated affusions of water, and dried. It had
lost about the half of its weight, and when reduced to a fine powder was
readily dissolved by acids with an effervescence which was hardly
perceivable: the alkali had therefore extracted its air. I also threw
some fresh magnesia into the ley which had been poured off, and thereby
rendered it perfectly mild and similar to a solution of salt of tartar;
so that it effervesced briskly with acids.

With an ounce of the mild spirit of salt ammoniac, I mixed a dram of
magnesia in very fine powder which had been previously deprived of its
air by fire; and observing that the magnesia had a tendency to concrete
into a solid mass, I shook the vial very frequently. After some days the
powder was increased to more than double its former bulk; and when the
vial was opened, the alkaline spirit emitted a most intolerably pungent
smell. It likewise floated upon water, but was not perfectly caustic;
for it still yielded some air when mixed with acids, and also rendered
lime-water turbid: neither of which would probably have happened if I
had used a greater quantity of magnesia, or had allowed the mixture to
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