130 METALS AND THEIR COMBINATIONS. 



By filtering, washing, and drying the precipitate, it is ob- 

 tained in the form of a white, light powder; if, however, the 

 above-mentioned solutions are mixed, evaporated to dryness, 

 and the sodium sulphate removed by washing, the magnesium 

 carbonate is left in a more dense condition, and is then known 

 as heavy magnesia. 



Large quantities of magnesium carbonate are also manufac- 

 tured by treating dolomite (the double carbonate of magnesium 

 and calcium) with water and carbon dioxide under pressure. 

 Bicarbonate of magnesium is produced and dissolved in the car- 

 bonic acid. water, from which solution the carbonate is precipi- 

 tated by heating. 



Magnesium oxide, Magnesia, MgO = 40 (Calcined magnesia, 

 Light magnesia), is obtained by heating light magnesium car- 

 bonate in a crucible to a full red heat, when all carbon dioxide 

 and water are expelled : 



4(MgC0 3 ).Mg2H0.5H 2 O = 5MgO + 4CO 2 + 6H 2 O. 



It is a very light, amorphous, white, almost tasteless powder, 

 which absorbs gradually moisture and carbon dioxide from the 

 air; in contact with water it forms the hydrate Mg2HO, which 

 is very sparingly soluble in water. Milk of magnesia is the 

 hydrate suspended in water (1 part in about 15). 



The heavy magnesia of the U. S. P. differs from the common 

 or light magnesia, not in its chemical composition, but merely 

 in its physical condition, being a white, dense powder obtained 

 by heating the heavy carbonate of magnesium. 



Magnesium sulphate, Magnesii sulphas, MgS0 4 .7H 2 = 246 



(Sulphate of magnesium, Epsom salt], is obtained either from 

 spring waters or by decomposition of the native carbonate by 

 sulphuric acid : 



MgC0 3 + H 2 S0 4 = MgSO, + C0 2 + H 2 O. 



It forms colorless crystals, which have a cooling, saline, and 

 bitter taste, a neutral reaction, and are easily soluble in water. 



Magnesium sulphite, Magnesii sulphis, MgS0 3 6H 2 = 212, may 

 be obtained by adding sulphurous acid to magnesium car- 

 bonate : 



MgC0 3 + H 2 S0 3 = MgSO s + C0 2 + H 2 O. 



