Olim, Magnesia Alba.
_Carbonate of Magnesia_, vulgo, _Common Magnesia_.
This preparation was formerly considered by Mr. Phillips to be a mixture
of carbonate and sub-carbonate of magnesia, an opinion which he has
lately retracted; it is, says he, evidently a _carbonate_, i. e.
magnesia combined with one proportion of carbonic acid, or forty-eight
of carbonic acid to forty-three of magnesia. Dr. Thompson entertains a
different opinion, he observes that it seems to be a mechanical mixture
of carbonate of magnesia, caustic magnesia, and perhaps of hydrated
magnesia; he found too great a diversity in its composition to permit
the conclusion that it was a definite chemical compound; in a specimen
purchased at Glasgow, he also found six per cent. of _sulphate of lime_.
I take this opportunity of stating that in some specimens which I have
examined, I have also detected portions of _gypsum_; and from the
experiments of Dr. Percival, it appears that if _hard water_ be employed
for its preparation it will be less light, and will contain a portion of
lime. Magnesia will be also liable to contain traces of silicious earth,
derived from the alkali used in producing it. INCOMPATIBLE SUBSTANCES.
_Acids, and acidulous salts; alkalies and neutral salts; alum; cream of
tartar; nitrate of silver; acetate of mercury; oxy-muriate of mercury;
acetate of lead; sulphates of zinc, copper, and iron._ MEDICINAL USES.
Antacid, and purgative. In cases of lithic calculi, carbonate of
magnesia, in doses of ℈j to ʒj, has been proposed by Mr. Hatchett, as a
valuable substitute for alkaline remedies. Its insolubility must render
its absorption equivocal; its beneficial operation must therefore
principally depend upon its neutralizing any excess of acid in the primæ
viæ, and in this way there can be no doubt of its lithonthryptic agency;
“but,” says Dr. Marcet, “such is the tendency which the public has to
over-rate the utility of a new practice, or to take a mistaken view of
its proper application, that there is every reason to believe that the
use of magnesia has of late years become a frequent source of evil in
calculous complaints.” _See_ page 124. OFFICINAL PREP. _Hydrarg. cum
Magnesia._ D. _Magnesia._ L.E.D. ADULTERATIONS. _Chalk_ may be detected
by adding dilute sulphuric acid to a suspected portion, when, should any
be present, the solution will be loaded with a white and insoluble
precipitate; _gypsum_, by boiling a sample in distilled water, and
assaying the solution by a barytic and oxalic test.[560]