MAGNESIUM. 129 



22. MAGNESIUM. 



Mg" = 24. 



General remarks. Magnesium occupies a position intermediate 

 between the metals of the alkalies and the alkaline earths, of 

 which latter it was formerly classed as a member. To some 

 extent it also resembles the heavy metal zinc, with which it has, 

 in common, the volatility of the chloride, the solubility of its 

 sulphate, and the isomorphism of several of its compounds with 

 the analogously constituted compounds of zinc. 



Occurrence in nature. Magnesium is widely diffused in nature 

 and several of its compounds are found in large quantities. It 

 occurs as chloride and sulphate in many spring waters and in 

 the salt-mines of Stassfurt; as carbonate in the mineral mag- 

 nesite; as a double carbonate of magnesium and calcium in the 

 mineral dolomite (magnesia-limestone), which forms entire 

 mountains ; as silicate of magnesium in the minerals ser- 

 pentine, meerschaum, talc, asbestos, etc. 



Metallic magnesium may be obtained by the decomposition of 

 magnesium chloride by sodium: 



MgCl 2 + 2Na == 2NaCl + Mg. 



Magnesium is an almost silver- white metal, losing its lustre 

 rapidly in moist air by oxidation of the surface. It decom- 

 poses hot water with liberation of hydrogen : 



Mg + 2H 2 O = 2H + Mg2HO. 



When heated to a red heat it burns with a brilliant bluish- 

 white light, forming magnesium oxide. 



Magnesium carbonate, Magnesii carbonas, 4(MgC0 3 ).Mg2H0.5H 2 



= 484 (Carbonate of magnesium, Magnesia alba, Light magnesia.). 

 The neutral magnesium carbonate, MgC0 3 , is found in na- 

 ture, but the officinal preparation is a mixture of carbonate, 

 hydrate, and water. It is obtained by the addition of a solution 

 of magnesium sulphate to solution of sodium carbonate and 

 boiling, when the carbonate is precipitated, some carbon dioxide 

 evolved, and sodium sulphate remains in solution : 



6MgS0 4 + 5Na 2 C0 3 + 6H 2 = 4(MgCO 3 ).Mg2HO.5H 2 O + 5Na 2 SO 4 + CO r 



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