This section is from the book "Materia Medica And Therapeutics Inorganic Substances", by Charles D. F. Phillips. Also available from Amazon: Materia medica and therapeutics.
The two oxides, identical in composition, differ in the arrangement of their molecules, and, consequently, in their weight.
By strongly heating the heavy and light carbonates respectively, until all carbonic acid is driven off: hence the name "calcined magnesia."
Magnesia and light magnesia occur as white powders, almost tasteless: the heavier form is called simply "magnesia" and is smoother than "magnesia levis," and more readily miscible with water. A given weight of the light variety occupies three and a half times the bulk of the same weight of the condensed magnesia. (There is no advantage in retaining both in the Pharmacopoeia.) Both forms are almost insoluble in water, but their solubility is increased by heat; they absorb water, and if kept long in it, may form a concrete mass of "hydrate." They are soluble in acids.
By dissolving in boiling water and then mixing sul-phate of magnesia, and carbonate of soda, evaporating, and then washing and drying the precipitate. To prepare the light variety, the first solution is effected in a large quantity of cold water, which is afterward boiled. The result of the decomposition is an oxycarbonate, which is hydrated, and sulphate of soda is removed by washing. Thus:
4MgSO4 + 4Na2CO3+5H1O=3MgCO3MgO5H1O+4Na4SO4 + CO2.
The carbonates are white powders, soluble in acids with effervescence. The light form appears under the microscope partly amorphous, with slender prisms intermixed. Their solubility in plain water is slight, but it is much increased by carbonic acid, which converts them into bicarbonates.
By evaporating sea-water or saline springs - also from dolomite, by treating with sulphuric acid: soluble sulphate of magnesia is dissolved out and crystallized, insoluble lime sulphate is left.
Occurs usually in small acicular opaque or whitish crystals, but may be obtained in large, transparent, rhombic prisms. The pure crystals are somewhat efflorescent; but if they contain chloride of magnesium they are moist or deliquescent. Iron is an occasional impurity, and gives a reddish tint to the solution.
The small acicular crystals resemble those of zinc sulphate, with which, indeed, they are isomorphous: they may be distinguished (1) by the taste, magnesia sulphate being bitter and nauseous, zinc sulphate astringent; (2) ammonium sulphide gives with magnesia no precipitate, but with zinc a white one of sulphide (ZnS); (3) caustic potash gives with magnesia a white precipitate insoluble in excess, with zinc, a white precipitate soluble in excess. The rhombic prisms resemble those of oxalic acid: the latter are markedly acid to the taste, and are colored a purplish brown by common ink; magnesia sulphate is not affected by it (except blackened where touched).
Magnesia and its carbonates and neutral salts, such as the citrate and tartrate, are changed into chlorides in the stomach, and absorbed either wholly or in part according to the amount taken and the condition of the gastric fluids (Buchheim and others): not more than 15 gr. at a time is changed (Rabuteau); the un-absorbed portion passes on into the intestine, and under the influence of albuminous secretions, or of carbonic or other acids, especially in the large intestine, an additional amount becomes absorbed, and any residue passes unchanged with the faeces, or under certain circumstances accumulates in the bowel, and forms concretions. Absorption varies with the degree of acidity of the intestinal tract, and if this be not marked, lemonade or other acidulous drinks will be required to secure solution. We need scarcely say that absorption varies also with the nerve-condition (v. p. 237). Part of the absorbed magnesia appears in the urine as a triple phosphate.
The sulphate of magnesia, given in small doses, is wholly absorbed without producing definite physiological effects. Of large and purgative doses, part only is absorbed, and passes out by the urine or other emunc-tories. Part of the sulphuric acid of the sulphate is withdrawn by potash and soda salts met with in the bowel, and the magnesia is almost wholly excreted with the motions combined, more or less, with effete bile-products (Buchheim).