n a longer time in the vial. I now washed out the whole of the
mixture into a bowl, and dryed the magnesia until it lost all smell of
the alkali. It weighed a dram and fifty eight grains, effervesced
violently with acids, and therefore contained a large quantity of air,
which had been drawn from the alkali by a stronger attraction.

Having formerly shewn, that magnesia saturated with air separates an
acid from a calcarious earth, which it is not able to do after being
deprived of its air by fire; I now suspected that the air was the cause
of this separation, because I found that it was joined to the calcarious
earth at the same time that the acid was joined to the earth of
magnesia; and imagined that a pure calcarious earth might possibly have
a stronger attraction for acids than a earth of magnesia.

I therefore dissolved two drams of magnesia in the marine acid, and
thus obtained a compound of an acid and of the pure earth of this
substance; for the air which was at first attached to it, was expelled
during the dissolution. I then added thirty grains of strong quick-lime
in exceeding fine powder, shook the mixture well, and filtrated it. The
powder remaining in the paper, after being well washed, was found to be
a magnesia, which, as I expected, was destitute of air; for it was
dissolved by the vitriolic acid without effervescence. And the filtrated
liquor contained the lime united to the acid; for upon dropping spirit
of vitriol into it, a white powder was immediately formed.

We must therefore acknowledge a stronger attraction between the
calcarious earths and acids than between these and magnesia: but how
does it then happen, that, if magnesia saturated with air be mixed with
a compound of acid and calcarious earth, these two last, which attract
one another the most strongly, do not remain united; but the acid is
joined to the magnesia, and the calcarious earth to the air which it
attracts much more weakly than it does the acid? Is it because the sum
of the forces which tend to join the magnesia to the acid and the
calcarious earth to the air, is greater than the sum of the forces which
tend to join the calcarious earth to the acid, and the magnesia to the
air: and because there is a repulsion between the acid and air, and
between the two earths; or they are somehow kept asunder in such a
manner as hinders any three of them from being united together?

The first part of this supposition is favoured by our experim