given of boiling the mixture is not unnecessary; if it be
neglected, the whole of the _magnesia_ is not accurately separated at
once; and by allowing it to rest for some time, that powder concretes
into minute grains, which, when viewed with the microscope, appear to be
assemblages of needles diverging from a point. This happens more
especially when the solutions of the epsom-salt and of the alkali are
diluted with too much water before they are mixed together. Thus, if a
dram of epsom-salt and of salt of tartar be dissolved each in four
ounces of water, and be mixed, and then allowed to rest three or four
days, the whole of the _magnesia_ will be formed into these grains. Or
if we filtrate the mixture soon after it is made, and heat the clear
liquor which passes thro'; it will become turbid, and deposite a
_magnesia_.

I had the curiosity to satisfy myself of the purgative power of
_magnesia_, and of _Hoffman's_ opinion concerning it, by the following
easy experiment. I made a neutral salt of _magnesia_ and distilled
vinegar; choosing this acid as being, like that in weak stomachs, the
product of fermentation. Six drams of this I dissolved in water, and
gave to a middle-aged man, desiring him to take it by degrees. After
having taken about a third, he desisted, and purged four times in an
easy and gentle manner. A woman of a strong constitution got the
remainder as a brisk purgative, and it operated ten times without
causing any uneasiness. The taste of this salt is not disagreeable, and
it appears to be rather of the cooling than of the acrid kind.

Having thus given a short sketch of the history and medical virtues of
_magnesia_, I now proceed to an account of its chemical properties. By
my first experiments, I intended to learn what sort of neutral salts
might be obtained by joining it to each of the vulgar acids; and the
result was as follows.

Magnesia is quickly dissolved with violent effervescence, or explosion
of air, by the acids of vitriol, nitre, and of common salt, and by
distilled vinegar; the neutral saline liquors thence produced having
each their peculiar properties.

That which is made with the vitriolic acid, may be condensed into
crystals similar in all respects to epsom-salt.

That which is made with the nitrous is of a yellow colour, and yields
saline crystals, which retain their form in a very dry air, but melt in
a moist one.

That which is produced by means of spirit of salt, yields no c