MAGNESIA. L. Magnesia Usta. D.
Calcined Magnesia.
Qualities. Form, a white, very light, soft powder; Specific gravity, 2·3; it turns to green the more delicate vegetable blues. Solubility. Although it requires 2000 times its weight of water to hold it in solution, yet it has the property of considerably increasing the solubility of camphor, opium, and resins in the same fluid; it is soluble in solutions of the alkaline carbonates, but not in those of caustic alkalies. Chemical Composition. It is an oxide of a peculiar metal. Medicinal Uses. Antacid, and when acidity prevails, purgative; it is preferable to the carbonate whenever the bowels are distended with flatus, (Form: 150): in other respects, its virtues are the same. See Magnes. Carb. The Medicinal Dynameter will shew the equivalent doses of the pure earth and its carbonate; it will be seen for instance that 12 grains of the former will be as efficient, as an antacid, as 25 grains of the latter. Incidental Impurities. It ought not to effervesce with acids, and if magnesia and muriatic acid be placed at one time in separate cups, in a scale of a balance, no diminution of weight should take place on mixing them. Lime is detected by its solution in dilute sulphuric acid affording a precipitate with oxalate of ammonia; the sulphuret of lime betrays itself by yielding, when moistened, the smell of sulphuretted hydrogen.[[559]]
MAGNESIÆ SUB-CARBONAS. L.
Carbonas Magnesiæ. E. Magnesia. D.
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]]
MAGNESIÆ SULPHAS. L. Sulphas Magnesiæ. E.D.
Magnesia Vitriolata. Sal catharticum amarum.