Magnesia. Lond. Magnesia

"Take of carbonate of magnesia, four ounces. Burn the carbonate in a very strong fire for two hours, or until no effervescence is excited when acetic acid is dropped on it." •

Edinburgh.

"Let (sub) carbonate of magnesia be exposed in a crucible to a red heat for two hours; after which preserve it in close-stopped bottles."

Magnesia, Dub. Calcined Magnesia.

"Take of magnesia, any quantity. Let it be put into a crucible, and subjected to a strong heat for two hours; and when it has cooled, preserve it in a well-closed glass vessel."

Syn. Magnesie (F.), Gebrannte Magnesia (G.), Gebrennte Bidderzoutard (Dutch), Magnesia (J.).

The carbonic acid and water are expelled by the heat, and the pure earth remains in the proportion of five twelfths of the weight of the carbonate employed: or, j. leaves 200 grs. of magnesia.1

Qualities. - It is inodorous and insipid; in the form of a white, very light, soft powder, having a specific gravity of 2.3. It has an alkaline reaction; does not effervesce with acids; is infusible; and requires for its solution 5142 parts of water at 60°, and 36,000 at 212°.2 It does not become hot when mixed with water, as lime does; and water filtered through it does not affect the vegetable blues. It is a hydrate, and when exposed to the air it attracts slowly carbonic acid. It is a compound of 60 parts of magnesium, and 40 of oxygen, in 100 parts; or of 1 eq. of magnesiums 12.7 + 1 of oxygen =8, making the equivalent 20.7 (Mg)

1 Black on Magnesia Alba, 28.

2 Fyfe.

Medical properties and uses. - The same as those of the carbonate. It sometimes contains lime, which is discovered by a precipitate falling when oxalate of ammonia is added to its solution in sulphuric acid. Its dose is from grs. x. to 3ss. taken in water or milk.

Magnesiae Carbonas. Lond. Carbonate of Magnesia

"Take of sulphate of magnesia, four pounds; carbonate of soda, four pounds eight ounces; distilled water, four gallons. Dissolve separately the carbonate of soda and the sulphate of magnesia in two gallons of the water, and filter; then mix the solutions, and boil, with constant stirring, for a quarter of an hour. Lastly, the liquor being poured off, wash the precipitated powder with boiling distilled water, and dry it."

Carbonas Magnesiae, Edin. Carbonate of Magnesia.

"Take of sulphate of magnesia, four parts; subcarbonate of potassa, three parts; boiling water, a sufficient quantity. Dissolve the salts separately in twice their weight of water, and strain, or otherwise free from impurities; then mix them, and instantly add eight times their weight of boiling water. Boil the liquor for a short time, stirring it; then let it remain at rest until the heat be a little diminished, and strain it through linen, upon which the carbonate of magnesia will remain. The carbonate, after being well washed with pure water, is to be dried with a gentle heat."

Magnesiae Carbonas, Dub. Carbonate of Magnesia.

"Take of sulphate of magnesia, twenty-five parts; carbonate of potassa, fourteen parts; boiling water, forty parts. Dissolve the sulphate of magnesia and the carbonate of potassa, each in two hundred parts of water. Mix together the defecated liquors; then boil the mixture for a short time, and strain it while it is hot through linen stretched in a proper manner for collecting the magnesia. Wash away the sulphate of potassa by repeated affusions of boiling water: and finally, dry the carbonate of magnesia.

Syn. Carbonate de Magnesie (F.), Kohlensaure Magnesia(G.), Carbonato di Magnesia (I.).

The product of these processes is a compound of magnesia;

1 Magnesiae subcarbonas, P. L. 1824.

in the carbonate, 11.48 of magnesia; in the hydrate, 38.42 of carbonic acid, and 15.76 of water.1 Mr. Phillips states the components to be carbonic acid 52.4, and magnesia 47.6, in 100 parts2; or 1 eq. of magnesia = 20.7+1 of carbonic acid =22.12; making the equivalent 42.82 (Mg. C.) In its production the salts are decomposed, and a double exchange takes place; the sulphuric acid separates from the magnesia, and unites with the soda of the carbonate, disengaging the carbonic acid, which in its turn combines with the magnesia. The success of the operation depends very much on the degree of attention which is paid to the following circumstances. The water employed in every part of the process must be very soft; either rain water, or pure distilled water: the carbonate of soda should be previously freed as completely as possible from any admixture of silica, by passing through the alkaline solution a current of carbonic acid, or exposing it to the air for some time before it be used, and the mixing the salts in small portions of water; and after boiling the mixture, throwing it into a large quantity of water.

Mr. Henry recommends to pour off the water by inclination, and to put the precipitate upon chalk stones for a little time; after which it is to be wrapped up in sheets of white paper, and dried before the fire.3

The greater part, however, of the carbonate of magnesia found in the shops is prepared, on a great scale, from bittern, the liquor remaining after the crystallization of common salt from sea water. The bittern is heated to 212°, a solution of impure carbonate of potassa instantly added to it, and the fire withdrawn. The other steps of the process resemble those above detailed. It is frequently adulterated with chalk, and sometimes gypsum: the former is detected by adding a little diluted sulphuric acid, which converts the magnesia into soluble sulphate, but produces an insoluble salt with the lime of the chalk. Gypsum is detected by boiling a portion of the magnesia in distilled water, and adding to the solution chloride of barium, which will produce an insoluble precipitate, if gypsum be present.

Qualities.-Carbonate of magnesia is inodorous and insipid; perfectly white, very light, smooth to the touch, nearly insoluble in water, and effervesces with acids. Its specific gravity is 0.295.4 It is decomposed by all the acids, the alkalies, the neutral and metallic salts, lime, baryta, alumina, and by a strong heat; these substances are therefore incompatible in prescriptions with it.

1 Henry's Elements, 9th edit. i. 594.

2 Trans. of the Pharm. p. 78.

3 Henry's Experiments on the Preparation, etc. of Magnesia, 8vo. Lond. 1773.

4 Hoffmanni Op. iv. 473.

Medical properties and uses. - Carbonate of magnesia is antacid. It is a useful remedy in acidity of the primae viae, particularly of children, in aphthous fever, and that which attends dentition. The compound formed by its union with an acid in the stomach is purgative; but if no acid be present, magnesia does not appear to increase in any degree the peristaltic motion of the bowels. It is preferable to chalk and other absorbents in heartburn, when the bowels are costive; and has been given with advantage in dysentery, combined with ipecacuanha and opium, and the dose followed by a draught of lemonade. In calculus, when the concretions are formed in the kidney, no remedy is so efficacious. The extrication of the carbonic acid in the gaseous state, when the carbonate is decomposed by acid in the stomach, sometimes proves inconvenient from the distension it occasions; but more generally it is beneficial. The usual dose is from 3ss. to 3 ij., taken in water or milk.1

Officinal preparations. - Magnesia, L. E. D. Mistura cam-phorae cum Magnesia, D. Hydrargyrum cum Magnesia, D.