ts a calcarious earth
to join itself to _magnesia_; but the exchange being performed slowly,
some of the _magnesia_ is still undissolved, and part of the calcarious
earth remains yet joined to the acid.

When a small quantity of _magnesia_ is thrown into a solution of the
corrosive sublimate of mercury, it soon separates part of the mercury in
the form of a dark red powder, and is itself dissolved.

Imagining that I perceived some resemblance between the properties of
_magnesia_ and those of alkalis, I was led to try what change this
substance would suffer from the addition of quick-lime, which alters in
such a peculiar manner the alkaline salts.

Twenty seven grains of _magnesia_ in fine powder were mixed with
eighteen ounces of lime-water in a flask, which was corked close and
shaken frequently for four days. During this time, I frequently dipp'd
into it little bits of paper, which were coloured with the juice of
violets; and these became green as soon as they touched the water, until
the fourth day, when their colour did not seem to be altered. The water
being now poured off, was intirely insipid, and agreed in every
chemical trial with pure water. The powder, after being perfectly well
dried, weighed thirty seven grains. It did not dissolve intirely in
spirit of vitriol; but, after a brisk effervescence, part of it subsided
in the same manner as the calcarious earths, when mixed with this acid.

When I first tried this experiment, I was at the trouble of digesting
the mixture in the heat of boiling water, and did not then know that it
would succeed in the heat of the air. But Dr. _Alston_, who has obliged
the world with many curious and useful discoveries on the subject of
quick-lime, having had occasion to repeat it, I learned from him that
heat is not necessary; and he has moreover added an useful purpose to
which this property of _magnesia_ may be applied; I mean the sweetening
of water at sea, with which lime may have been mixed to prevent its
putrefaction.

That part of the dried powder which does not dissolve in spirit of
vitriol, consists of the lime separated from the water.

Quick-lime itself is also rendered mild by _magnesia_, if these two are
well rubbed together and infused with a small quantity of water.

By the following experiments, I proposed to know whether this substance
could be reduced to a quick-lime.

An ounce of _magnesia_ was exposed in a crucible for about an hour to
such a heat as