IV 



CHALK, LIME, AND THE ALKALIS 63 



C. PREPARATION OF EARTHS. 



Black prepared magnesia alba (magnesium carbonate, 

 MgCO 3 ) by the action of alkalis on 



1. Mother of nitre (containing magnesium nitrate), 



Mg(NO 3 ) 2 +K 2 GO 3 - MgCO 3 + 2KNO 3 



2. Bittern (containing magnesium chloride), 



MgCl 2 +K 2 CO 3 -> MgCO 3 + 2KCl. 



3. Rpsom salts (magnesium sulphate), 



MgSO 4 +K 2 CO 3 -> MgCO 3 +K 2 SO 4 . 



This magnesia is easily decomposed by heat, liberating fixed 

 air and losing more than half its weight, 



MgC0 3 -> MgO + C0 2 . 



It effervesces with acids, but after it has been burnt it dis- 

 solves without effervescence, 



MgCO 3 +H 2 SO 4 -> MgSO 4 + H 2 O + CO 2 

 MgO + H 2 SO 4 -> MgSO 4 + H 2 O 



Magnesia, free from fixed air, was also prepared by igniting 

 the nitrate, 



2 Mg(N0 3 ) 2 -> 



