504 MAGNESIUM. 



and fuses at a red heat. It is oxidised superficially by moist 

 air, but undergoes no change in dry air or oxygen. Heated 

 to redness, it burns with great brilliancy, forming magnesia. 

 It is evidently more analogous to zinc than to the preceding 

 metals. 



Magnesia ; Mg O ; 258.3, or 20.69. This is the only known 

 oxide of magnesium. As usually prepared by calcining the 

 artificial carbonate of magnesia, it forms the magnesia usta of 

 pharmacy. Magnesia is a white soft powder, of density about 

 2.3, and highly infusible. It combines with water, but with 

 much less avidity than lime, forming a protohydrate. The 

 native hydrate of magnesia has the same composition, and so 

 has the compound, obtained by precipitating magnesia from 

 its soluble salts, when dried, either without heat, or at 212. 

 These preparations have a silky lustre and a softness to the 

 touch, characteristic of magnesian minerals, such as is observed 

 in asbestos and soapstone. According to Dr. Fyfe, magnesia 

 requires, for solution, 5142 times its weight of water at 60, 

 and 36,000 times its weight of boiling water. This earth, when 

 mixed with a blue infusion of cabbage, causes it to become 

 green, and is therefore alkaline. 



Chloride of magnesium, made by neutralising carbonate of 

 magnesia with hydrochloric acid, crystallizes with 6 HO, is very 

 soluble and highly deliquescent. When we attempt to make it 

 anhydrous by heat, hydrochloric acid escapes, and magnesia 

 remains. But the pure chloride, which is employed in pre- 

 paring the metal, may be obtained by dividing a quantity of 

 hydrochloric acid into two equal portions, neutralising one 

 with magnesia, and the other with ammonia, mixing and evapo- 

 rating these two solutions to dryness, when an anhydrous double 

 chloride of magnesium and ammonia is formed. On heating this 

 salt to redness in a porcelain crucible, sal-ammoniac sublimes, and 

 chloride of magnesium remains in a state of fusion, which 

 becomes a translucent crystalline mass on cooling. This chlo- 

 ride is decomposed by oxygen, which, at a high temperature, 

 displaces its chlorine, and magnesia is formed. 



Carbonate of magnesia. This salt occurs native, generally 

 as a white, hard, compact mineral of an earthy fructure, which 

 is known as magnesite, and sometimes in rhombohedral crystals, 

 similar to those of carbonate of lime. It is prepared artificially 

 by precipitating a soluble salt of magnesia, by means of carbo- 



