The following are the officinal salts and preparations of magnesium, arranged according to their source:

Magnesiae Sulphas. - Sulphate of Magnesia.

Epsom Salt. MgSO4.7H1O.

Source. - Native.

Characters. - Minute colourless rhombic prisms, with a bitter taste. Solubility, 10 in 13 parts of cold water. Incompatible with alkaline carbonates, lime-water, acetate of . and nitrate of silver.

Impurities. - Lime, iron, and general impurities. Dose. - 60 gr. to 1/2 oz.

Preparations.

a. Enema Magnesiae Sulphatis. - 1 oz., in 1 oz. of Olive Oil, and 15 oz. of Mucilage of Starch; for one enema.

b. Mistura Sennae Composita. - 1 oz. in 5 fl.oz. See Senna.

From Magnesiae Sulphas are also made: c. Magnesias Carbonas. - Carbonate of Magnesia (heavy). (MgCO3)3.MgO.5H1O.

Source. - Made by mixing boiling solutions of Sulphate of Magnesia and Carbonate of Soda, evaporating, purifying, and drying. 4MgS04 + 4Na2CO3 + H1O = 3MgCO3,Mg2HO + 4Na2SO4 + CO2.

Characters. - A white granular powder, insoluble in water.

Impurities. - Lime, sulphates, metals.

Dose. - 10 to 60 gr.

Preparations.

a. Liquor Magnesias Carbonatis. - Solution of Carbonate of Magnesia. " Fluid Magnesia." MgCO3 + H1CO3 or MgH12CO3, in solution.

Source. - Made by passing an excess of Carbonic Acid gas through a mixture of Carbonate of Magnesia (freshly prepared) and water. 1 fl.oz. contains 13 gr. of carbonate.

Characters. - A clear fluid, sometimes effervescing slightly.

Dose. - 1 to 2 fl.oz.

. Trochisci Bismuthi. - 2 1/2 gr. in each.

From Magnesiae Carbonas is made:

Magnesia. - Magnesia (heavy). Magnesia Ponderosa. MgO.

Source. - Made by heating Carbonate of Magnesia in a crucible to expel the carbonic acid.

Characters. - A white powder, comparatively insoluble in water (1 part in 5,412 cold water, 1 in 36,000 hot water).

Impurities. - Those of the carbonates.

Dose. - 10 to 60 gr.

d. Magnesias Carbonas Levis, - Light Carbonate of Magnesia. (MgCO3)3.MgO.5H1O.

Source. - Made like magnesiae carbonas, but with cold dilute solutions instead of hot.

Characters. - A very light white powder, proving microscopically to be partly amorphous, partly prismatic crystals. Soluble in 2,493 parts of cold water, or in 9,000 parts of hot water; 3 1/2 times the bulk of the heavy carbonate.

Dose. - 10 to 60 gr.

From Magnesia Carbonas Levis is made:

Magnesiae Levis. - Light Magnesia. MgO.

Source. - Made by heating Light Carbonate of Magnesia in a crucible to expel the carbonic acid.

Characters. - A bulky white powder, 3 1/2 times the bulk of heavy magnesia.

Dose. - 10 to 60 gr.

Magnesiae Carbonis Levis is contained in Pulvis Rhei Compositus (6 parts in 9).

Action And Uses. 1. Immediate Local Action And Uses

Externally, magnesia has no action, and is not used.

Internally, magnesia is a valuable means of decomposing the contents of the stomach and intestines under various circumstances. The base and carbonates form comparatively insoluble or innocuous compounds with the mineral acids, oxalic acid, mercuric, arsenical, and cupric salts; in large quantities tin y prevent the absorption of alkaloids by rendering the contents of the stomach alkaline; whilst the sulphate precipitates insoluble sulphates of lead and baryta. Magnesia or its salts may therefore be employed as antidotes in cases of poisoning by any of these substances, the oxide being preferred to the carbonate, so as to prevent the evolution of carbonic acid, and care being taken to give it very freely.

By a similar process of decomposition, magnesia neutralises normal or excessive acidity in the stomach and bowels, and is itself converted into the chloride, lactate, and bicarbonate, this reaction removing irritant acid, and forming salts of magnesia, which have a stimulant or purgative action on the intestine. The carbonate is similarly decomposed, yielding carbonic acid, which exerts its specific action on the stomach. Both substances are therefore employed as local alkaline remedies in acidity of the stomach (heartburn, pyrosis, etc.), given with sal-volatile, after meals, a further laxative effect on the intestine being intended.

The chloride, bicarbonate, or lactate formed in the stomach, and the sulphate of magnesia directly given, having reached the intestine, are very slowly absorbed, and if in sufficient quantity, produce very marked local effects as saline purgatives, the sulphate being hydragogue in its action. The result is the free evacuation of a quantity of water by the bowel, and with it the whole, or almost the whole, of the magnesia. Sulphate of magnesia (Epsom salt) is our most common saline purgative, used in the form of Mistura Sennae Composita (black draught), of a simple solution in sulphuric acid and water with some carminative, and of several of the popular aperient waters, such as Friedrichshall, Pullna, Hunyadi Janos, of all of which it is an important constituent. Sulphate of magnesia is regarded as a mild, painless, non-nauseating purgative, less rapid in its action than the soda salt, to be used for completing the effect of purgative pills, for congestion of the portal system, for chronic constipation as an habitual laxative in combination with other salts in the above-named waters, and for feverish attacks with loaded bowels.

Magnesia and the carbonates, when used as purgatives, are chiefly given to children in diarrhoea with foul acid stools, very frequently in the form of Pulvis Rhei Composita (Gregory's Powder). In small doses, neither salt has any purgative action on the bowel, but enters the blood.

2. Action On The Blood And Its Uses

Entering the circulation as the chloride or lactate, magnesia increases the natural alkalinity of the plasma, of which it is a normal constituent, and helps to hold in solution any acid which may be in excess. It will therefore be useful in gout, lithiasis, and possibly in chronic rheumatism, to assist the more powerful alkalinisers of the blood with which it is combined in the waters of Ems, Baden-Baden, Aix-les-Bains, Carlsbad, etc.

3. Specific Action And Uses

Magnesia taken medicinally does not exert any appreciable effect upon the tissues or nutrition generally. Although an important constituent of bone, it cannot be said to be of any value in rickets or other diseases in which the osseous tissue is deficient in solid matter.

4. Remote Local Action

When magnesia does not purge, it is excreted chiefly by the kidneys, rendering the urine more abundant and less acid, and dissolving uric acid. Its diuretic and alkalinising effects contribute to the value of magnesia waters in gout and gravel.