Magnesium

MAGNESIUM, a metallic chemical element of silvery white appearance, which is familiar in the form of ribbon (symbol Mg, atomic number 12, atomic weight 24.32, isotopes 24, 25, 26). The sulphate or Epsom salts was isolated in 1695 by N. Grew, while in 1707 M. B. Valentin prepared magnesia alba from the mother liquors obtained in the manufacture of nitre. Magnesia was confounded with lime until 1755, when J. Black showed that the two substances were entirely different ; and in 1808 Davy pointed out that it was the oxide of a metal, which, however, he was not able to isolate. Magnesium is found widely distributed in nature, chiefly in the forms of silicate, carbonate and chloride, and occurring in the minerals olivine, hornblende, talc, asbestos, meer schaum, augite, dolomite, magnesite, carnallite, kieserite and kain ite. The metal was prepared (in a state approximating to purity) by A. A. B. Bussy who fused the anhydrous chloride with potas sium; H. Sainte Claire Deville's process, which used to be em ployed commercially, was essentially the same, except that sodium was substituted for potassium and that carnallite, was used instead of magnesium chloride, the product being fur ther purified by redistillation. Electrolytic methods have entirely superseded the older methods; fused carnallite, KC1,MgC1,, is electrolysed at 760°C. in an iron pot, which acts as the cathode, the anode being a carbon rod surrounded by a porcelain tube to carry off the chlorine.

Magnesium is malleable and ductile. Sp. gr. 1.75. Its alloys with aluminium are known as "magnalium" and are useful for light castings. Magnesium preserves its lustre in dry air, but in moist air it becomes tarnished by the formation of a film of oxide. It melts at 632.7°C. and boils at about 1,100°C. Magnesium and its salts are diamagnetic. It burns brilliantly when heated in air or oxygen, or even in carbon dioxide, emitting a white light and leav ing a residue of magnesia, MgO. The light is rich in the violet and ultra-violet rays, and consequently is employed in pho tography. At high temperatures it acts as a reducing agent. It combines directly with nitrogen, when heated in the gas, to form the nitride (see ARGON). It is rapidly dissolved by dilute acids.

Magnesium Oxide,

magnesia, MgO, occurs native as the mineral periclase, and is formed when magnesium burns in air; it may also be prepared by the gentle ignition of the hydroxide or carbonate. It is a non-volatile and highly infusible white pow der, which slowly absorbs moisture and carbon dioxide from air, and is readily soluble in dilute acids. On account of its refractory nature, it is employed in the manufacture of crucibles, furnace linings, etc. It is also used in making hydraulic cements. Mag nesium hydroxide occurs native as the minerals brucite and nemalite. It is employed in the manufacture of cements.

When magnesium is heated in fluorine or chlorine or in the vapour of bromine or iodine there is a violent reaction, and the corresponding halide compounds are formed. With the exception of the fluoride, these substances are readily soluble in water and are deliquescent. The fluoride is found native as sellaite, and the

bromide and iodide occur in sea water and in many mineral springs. The most important of the halide salts is the chloride which, in the hydrated form, has the formula MgC12,6H20. The hydrated salt loses water on heating, and partially decomposes into hydrochloric acid and magnesium oxychlorides. To obtain the anhydrous salt, the double magnesium ammonium chloride, is prepared by adding ammonium chloride to a solution of magnesium chloride. Magnesium oxychloride when heated to redness in a current of air evolves a mixture of hydrochloric acid and chlorine and leaves a residue of magnesia, a reaction which is employed in the Weldon-Pechiney and Mond processes for the manufacture of chlorine.

Magnesium Carbonate,

normal salt is found native as the mineral magnesite, and in combination with calcium carbonate as dolomite, whilst hydromagnesite is a basic carbonate. It is not possible to prepare the normal carbonate by precipitating magnesium salts with sodium carbonate.

Other Magnesium Salts.

By adding sodium phosphate to magnesium sulphate and allowing the mixture to stand, hexagonal needles of are deposited. The normal phosphate, is found in some guanos, and as the mineral wagnerite. It is a white amorphous powder, readily soluble in acids. Mag nesium ammonium phosphate, is found as the mineral struvite and in some guanos; it occurs also in urinary cal culi and is formed in the putrefaction of urine. Magnesium nitrate, is a colourless, deliquescent, crystalline solid obtained by dissolving magnesium or its carbonate in nitric acid, and concentrating the solution. The crystals melt at 9o° C. Magnesium nitride, is obtained as a greenish-yellow amor phous mass by passing a current of nitrogen or ammonia over heated magnesium. Magnesium sulphate, occurs (with as kieserite. A hexahydrate is also known. The heptahy drate is known as Epsom salts (q.v.).

Organic Compounds.

The most important organic com pounds of magnesium are known as Grignard reagents (q.v.). They are of the composition RMgX (where R=-an alkyl or aryl group and X =a halogen).

Detection.

In the absence of barium, strontium, calcium and aluminium the magnesium salts may be detected by the colourless crystals formed by adding sodium phosphate (in the presence of ammonia and ammonium chloride) to their solutions. The same reaction is made use of in the quantitative determination of magnesium, the white precipitate of magnesium ammonium phos phate being converted by ignition into magnesium pyrophosphate.

Medicine.

The salts of magnesium may be regarded as the typical saline purgatives. Their aperient action is dependent upon the minimum of irritation of the bowel, and is exercised by their abstraction from the blood of water, which passes into the bowel to act as a diluent of the salt. Magnesium salts have a powerful depressant action, especially on nervous tissue.