852 MAGNESIUM MAGNET and citrate. The oxide and carbonate are frequently used in indigestion, to neutralize an excess of acid in the stomach or to act as gentle laxatives, which they do after com- bining with any acid. If when administered they do not meet with an acid in the ali- mentary canal, and consequently do not cause a laxative action, it is desirable to follow their administration with that of lemonade or a similar acid. The simple oxide of magnesia differs from the carbonate in affording by its combination no carbonic acid. Magnesia fresh- ly prepared has been recommended as an an- tidote in arsenical poisoning, but is less effi- cient than the hydrated oxide of iron. These substances are used in pharmacy in the prepa- ration of aromatic waters, the fineness of the powders serving to divide minutely the vola- tile oil with which they are impregnated, and thus present to the menstruum a much larger surface than could be obtained by simple ad- mixture. The sulphate and citrate are consid- erably used as cathartics. The former is an exceedingly efficient member of the class, and acts without undue violence. It produces co- pious and rather watery discharges. ^It is oc- casionally combined with other more - drastic purgatives, to mitigate their violence. If these salts fail to act as cathartics, which may hap- pen if they are administered in small and re- peated doses, they are absorbed and pass out of the system through the kidneys. The effer- vescent preparations, both solid and liquid, known as citrate of magnesia and popularly used as laxatives, usually contain, in addition, citrate or tartrate of sodium or potassium. The average dose of sulphate of magnesia is an ounce. It acts best when it is freely diluted with water, and should be given on an empty stomach. In cases of nausea, it will often be retained when other cathartics are rejected. It is best suited to inflammatory conditions of the system, and is contraindicated by debility and prostration. A solution of the citrate of magnesia resembles lemonade in taste. Mag- nesia in combination with silica enters large- ly into the composition of many rocks and minerals, such as serpentine, steatite or soap- stone, asbestus, meerschaum, augite, horn- blende, and olivine. MAGNESIUM, the metallic base of magnesia ; symbol, Mg. ; chemical equivalent, 12 ; specific gravity, 1'74; hardness, that of calcareous spar. Davy proved its existence ; but Bussy in 1830 first obtained it in sufficient quantity to test its properties. He decomposed the chloride of magnesium by transmitting through it when heated the vapors of potassium. Since the discovery of large deposits of the double chloride of potassium and magnesium, known as carnallite, in the the salt mines of Stassfurt, it has been proposed to use this mineral in the preparation of the metal ; 1,000 parts of carnal- lite are fused with 100 parts of fluor spar and 100 parts of sodium. The metal resembles sil- ver in appearance ; it is malleable, ductile, and fuses at a dull red heat, and can be distilled like zinc. It does not change under water or in dry air, but in damp air it soon oxidizes. It consumes with a brilliant white flame when heated to redness, or when thrown into hydro- chloric acid. It burns brilliantly in chlorine or in the vapors of bromine, iodine, sulphur, &c. It was proposed by Bunsen to employ this metal in the form of fine wire for illumi- nating purposes. He found that it might be lighted by the flame of an alcohol lamp, and in burning gave a perfectly steady and very intense light. A wire -^ of an inch in di- ameter burns at the rate of about three feet in a minute, and gives a light equal to that of 74 stearine candles of five to the pound. The weight of such a wire three feet long is about two grains. Bunsen proposed to have the wire wound upon bobbins and furnished at a regu- lar rate to the lamp. Should the metal be pro- curable at a considerably reduced cost, as may very likely follow an increased demand for it, it may then prove an excellent method of fur- nishing light for domestic purposes, and more particularly for lighthouses and uses requiring a great intensity of light. It is now used in magic lanterns and for photographing in places inaccessible to the light of day. For the latter application it is especially adapted by reason of the extraordinary power of the photo-chem- ical property of its light. The alloy of silicon with magnesium, when decomposed by hy- drochloric acid, evolves a hydride of silicon, which is a spontaneously combustible gas. Metallic magnesium precipitates nearly all met- als from their neutral solutions. MAGNET (from Gr. M6os Mayors??, the Mag- nesian stone), the name applied to a mass of steel or iron, or of natural iron ore, that has the property of attracting to itself otherwise inert iron. Magnets probably first became known to the Aryan races through the discov- ery of natural magnets by the Greeks in the Thessalian district of Magnesia. The various phenomena presented in experiments with mag- nets have given rise to the modern branch- es of physical science known as magnetism, terrestrial magnetism, electro-magnetism, and magneto-electricity, which are treated under their own names. Natural iron magnets are exceedingly rare, but a magnetic iron ore is found in large quantities in Sweden and in the states of New York and New Jersey. The important property that a freely supported magnetic bar possesses of turning steadfastly toward the poles of the earth under the influ- ence of terrestrial magnetism is treated of in the article on that subject. (See also COMPASS.) END OF VOLUME TENTH.
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