MAGNESIUM. 151 



of sodium carbonate, when the carbonate is precipitated, some carbon 

 dioxide evolved, and sodium sulphate remains in solution : 



5MgSO 4 + 5Na,CO 3 + 6H 2 O = (MgCO 3 ) 4 Mg(OH) 2 .5H 2 O -f 5Na 2 SO 4 + CO 2 . 



By filtering, washing, and drying the precipitate, it is obtained 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 magnesium carbonate. 



Experiment 21. Dissolve 10 grammes of magnesium sulphate in hot water 

 and add a concentrated solution of sodium carbonate until no more precipitate 

 is formed. Collect the precipitated magnesium carbonate on a filter and dry it 

 at a low temperature. (How much crystallized sodium carbonate is needed 

 for the decomposition of 10 grammes of crystallized magnesium sulphate?) 

 Notice that the dried precipitate evolves carbon dioxide when heated with 

 acids. 



Magnesium oxide, Magnesia, MgO = 40.3 ( Calcined magnesia, 

 ia) , is obtained by heating light magnesium carbonate 

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

 are expelled : 



(MgCO 3 ) 4 .Mg(OH) 2 .5H 2 = 5MgO + 4CO 2 + 6H 2 O. 



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

 absorbs moisture and carbon dioxide gradually from the air; in con- 

 tact with water it forms the hydroxide Mg(OH) 2 , which is almost 

 insoluble in water, requiring of the latter over 50,000 parts for solu- 

 tion. Milk of magnesia is the hydroxide suspended in water (1 part 

 in about 15). 



The heavy magnesia, magnesia ponderosa 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 ? rlpngg powder 

 obtained by heating the heavy magnesium carbonate. 



Experiment 22. Place 1 gramme of magnesium carbonate, obtained in per- 

 forming Experiment 21, into a weighed crucible and heat to redness, or until 

 by further heating no more loss in weight ensues. Treat the residue with 

 dilute hydrochloric acid and notice that no evolution of carbon dioxide takes 

 place. What is the calculated loss in weight of magnesium carbonate when 

 converted into oxide, and how does this correspond with the actual loss deter- 

 mined by the experiment ? 



), Magnesii sulphas, Mg-SO 4 .7H 2 O = 246.3 

 from spring waters, from the mineral 



