rystals;
and if evaporated to dryness, soon melts again when exposed to the air.

That which is obtained from the union of distilled vinegar with
_magnesia_, affords no crystals by evaporation, but is condensed into a
saline mass, which, while warm, is extremely tough and viscid, very much
resembling a strong glue both in colour and consistence, and becomes
brittle when cold.

By these experiments _magnesia_ appears to be a substance very different
from those of the calcarious class; under which I would be understood to
comprehend all those that are converted into a perfect quick-lime in a
strong fire, such as _lime-stone_, _marble_, _chalk_, those _spars_ and
_marles_ which effervesce with aqua fortis, all _animal shells_ and the
bodies called _lithophyta_. All of these, by being joined with acids,
yield a set of compounds which are very different from those we have
just now described. Thus, if a small quantity of any calcarious matter
be reduced to a fine powder and thrown into spirit of vitriol, it is
attacked by this acid with a brisk effervescence; but little or no
dissolution ensues. It absorbs the acid, and remains united with it in
the form of a white powder, at the bottom of the vessel, while the
liquor has hardly any taste, and shews only a very light cloud upon the
addition of alkali.[4]

The same white powder is also formed when spirit of vitriol is added to
a calcarious earth dissolved in any other acid; the vitriolic expelling
the other acid, and joining itself to the earth by a stronger
attraction; and upon this account the _magnesia_ of sea-water seems to
be different from either of those described by _Hoffman_. He says
expressly, that the solutions of each of his powders, or, what is
equivalent, that the liquors from which they are obtained, formed a
coagulum, and deposited a white powder, when he added the vitriolic
acid;[5] which experiment I have often tried with the marine bittern,
but without success. The coagulum thus formed in the mother of nitre may
be owing to a quantity of quick-lime contained in it; for quick-lime is
used in extracting the salt-petre from its matrix. But it is more
difficult to account for the difference between _Hoffman's_ bittern and
ours, unless we will be satisfied to refer it to this, that he got his
from the waters of salt springs, which may possibly be different from
those of the sea.

Magnesia is not less remarkably distinguished from the calcarious
earths, by jo