Sources. - The chief source is dolomite, or mountain limestone, which consists of carbonates of magnesium and calcium. Magnesium is also found native as carbonate and silicate.

General Reactions of Magnesium Salts. - They give a gelatinous white precipitate with potash, soda, or ammonia, insoluble in excess, but soluble in a solution of ammonium chloride. They likewise give a white precipitate with potassium and sodium carbonates, but none with ammonium carbonate.

The characteristic test of magnesium is the formation of a precipitate of triple phosphate on the addition of ammonia and a soluble phosphate to a solution of a magnesium salt. Caustic ammonia itself throws down a precipitate of magnesium hydrate insoluble in excess, but soluble in ammonium chloride. As it is easier to prevent the precipitation of hydrate than to re-dissolve it when down, it is usual to add ammonium chloride first, then the ammonia, and lastly the phosphate of sodium.

General Prepaeation Of Salts Of Magnesium

Is prepared

From

By

Magnesium sulphate, B. and U.S.P.

Dolomite . .

Dissolving in sulphuric acid; when soluble magnesium sulphate and insoluble calcium sulphate are formed.

Magnesium carbonate (heavy), B.P.

Magnesium sulphate

Precipitating with sodium carbonate, using hot concentrated solutions.

Ditto (light), B. and U.S.P.

Ditto . .

Ditto, using dilute solutions in the cold.

Magnesia (heavy), B. and U.S.P.

Magnesium carbonate (heavy) .

Calcining until all the carbonic acid is driven off, as shown by some taken from the centre of the crucible no longer effervescing on the addition of acid.

Ditto (light), B. and U.S.P.

Ditto (light). . .

Calcining like the heavy magnesia.

Granulated citrate of magnesium, U.S.P.

Ditto . . .

Mixing with citric acid and water, drying and powdering. The powder is mixed with sugar, sodium bicarbonate, and citric acid, damped with alcohol, passed through a sieve, so as to form a coarse powder, and dried.

Magnesium sulphite, U.S.P.

Magnesia. . .

Suspending in water and adding excess of sulphurous acid.

General Impurities. - The chief impurities in the sulphate are the calcium and iron from dolomite. Other alkaline earths and alkalis may also be present. The sulphuric acid employed may be impure, or the sulphate may have been prepared by a process in which hydrochloric acid is used, and thus chlorides may occur. In the carbonate prepared from the sulphate the same impurities may occur, as well as unchanged sulphate. In magnesia these may all occur, and carbonate as well.

Tests. - The absence of iron and other metals is ascertained by the aqueous solution giving no colour or precipitate with ferrocyanide of potassium, hydrogen sulphide, or ammonium sulphide. Chloride of ammonium prevents the precipitation of magnesium by ammonia and ammonium carbonate, but it does not prevent the precipitation of other alkaline earths, and their absence is ascertained by the solution remaining clear after the addition of these three reagents.

General Action of Magnesium. - When administered by the mouth the difference between absorption and excretion (p. 39) is not great enough to allow magnesium salts to accumulate in the blood sufficiently to produce any toxic effects. When injected into the blood, sulphate of magnesium, in doses of about 5 grs. per pound of body weight, abolishes reflex action, and paralyses the respiration and heart in cats (Hay), and has a similar effect in other animals also.