Magnesiae Carbonas Levis. Light Carbonate of Magnesia.
Prep. Sulphate of magnesia, twelve ounces; carbonate of soda, twelve ounces; distilled water, a sufficiency. Dissolve the carbonate and sulphate separately, each in a pint of water; then mix the solutions, and evaporate the whole to perfect dryness, by means of a sand-bath; digest the residue for half an hour with two pints of water, collect the insoluble matter on a calico filter, and wash till the washings cease to give a precipitate with chloride of barium; then dry at a temperature not exceeding 212°.
The light carbonate of magnesia is prepared by dissolving the same quantities of the sulphate and carbonate in half a gallon of water, mixing the two solutions cold, and boiling the mixture in a porcelain dish for fifteen minutes, then transferring to a calico filter, and washing and drying at a heat not exceeding 212°.
In these processes double decomposition takes place, sulphate of magnesia and carbonate of soda being converted into sulphate of soda and carbonate of magnesia.
Prop. & Comp. A white powder with scarcely any taste; insoluble in water; neutral, or very slightly alkaline in reaction; soluble in dilute mineral acids, yielding solutions which, when first treated with hydrochlorate of ammonia, are not disturbed by the addition of an excess of solution of ammonia, but yield a copious crystalline precipitate upon the addition of phosphate of soda. With excess of hydrochloric acid it forms a clear solution in which chloride of barium causes no precipitate. Another portion of the solution supersaturated with ammonia gives no precipitate with oxalic acid, indicating the absence of sulphates, and of lime. Fifty grains calcined at a red heat are reduced to 22. Composition (3 (Mg O, Co2 + HO) + MgO, 2 HO). It is not a simple carbonate; but a mixture of the carbonate and hydrate of magnesia; as magnesia when precipitated is incapable of retaining the whole of the carbonic acid. The light carbonate has the same composition, but is much lighter, and when examined under the microscope is found to be partly amorphous with numerous slender prisms intermixed.
Therapeutics. Acts the same as magnesia, with the exception of producing an evolution of carbonic acid when it meets with acid in the alimentary canal, and hence sometimes produces uncomfortable distension.
Dose. 10 gr. to 20 gr. as an antacid; 20 gr. to 60 gr. as a purgative.
Adulteration. Lime and some sulphate may be present, as in the last preparation, detected by the above tests.