e bitter saline liquor
called _bittern_, which remains in the pans after the evaporation of sea
water. But as that liquor is not always easily procured, I afterwards
made use of a salt called _epsom-salt_, which is separated from the
bittern by crystallization, and is evidently composed of _magnesia_ and
the vitriolic acid.

There is likewise a spurious kind of Glauber salt, which yields plenty
of _magnesia_, and seems to be no other than the epsom salt of sea water
reduced to crystals of a larger size. And common salt also affords a
small quantity of this powder; because being separated from the bittern
by one hasty crystallization only, it necessarily contains a portion of
that liquor.

Those who would prepare a _magnesia_ from epsom-salt, may use the
following process.

Dissolve equal quantities of epsom-salt, and of pearl ashes separately
in a sufficient quantity of water; purify each solution from its dregs,
and mix them accurately together by violent agitation: then make them
just to boil over a brisk fire.

Add now to the mixture three or four times its quantity of hot water;
after a little agitation, allow the _magnesia_ to settle to the bottom,
and decant off as much of the water as possible. Pour on the same
quantity of cold water; and, after settling, decant it off in the same
manner. Repeat this washing with the cold water ten or twelve times: or
even oftner, if the _magnesia_ be required perfectly pure for chemical
experiments.

When it is sufficiently washed, the water may be strained and squeezed
from it in a linen cloth; for very little of the _magnesia_ passes
thro'.

The alkali in the mixture uniting with the acid, separates it from the
_magnesia_; which not being of itself soluble in water, must
consequently appear immediately under a solid form. But the powder which
thus appears is not intirely _magnesia_; part of it is the neutral salt,
formed from the union of the acid and alkali. This neutral salt is
found, upon examination, to agree in all respects with vitriolated
tartar, and requires a large quantity of hot water to dissolve it. As
much of it is therefore dissolved as the water can take up; the rest is
dispersed thro' the mixture in the form of a powder. Hence the necessity
of washing the _magnesia_ with so much trouble; for the first affusion
of hot water is intended to dissolve the whole of the salt, and the
subsequent additions of cold water to wash away this solution.

The caution