MAGNESIUM. 273 



water with liberation of hydrogen ; and when heated to a red 

 heat burns with a brilliant bluish-white light, which is extensively 

 used for photographic purposes. 



Magnesium carbonate, Magnesii carbonas. Approximately : 

 (MgC0 3 ) 4 .Mg(OH) 2 .5H,0 == 482.26 (Magnesia alba). The normal 

 magnesium carbonate, MgCO 3 , is found in nature, but the official 

 preparation contains carbonate, hydroxide, and water. It is ob- 

 tained by boiling a solution of magnesium sulphate with solution 

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

 dioxide evolved, and sodium sulphate remains in solution: 



5MgSO 4 + SN^COg + 6H 2 O == (MgCO 3 ) 4 Mg(OH) 2 5H 2 O + 5Na 2 SO 4 + CO 3 . 



By filtering, washing, and drying the precipitate, it is obtained in 

 the form of a white, light powder. 



Experiment 25. 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, Mag-nesii oxidum, MgO 40.O6 (Calcined 

 magnesia), is obtained by heating light magnesium carbonate in a 

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

 expelled : 



(MgC0 3 ) 4 .Mg(OH) 2 .5H 2 O = 5MgO -f 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). 



Heavy magnesium oxide, magnesii oxidum ponderosum, differs from the com- 

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

 physical condition, being denser and heavier. 



Experiment 26. Place 1 gramme of magnesium carbonate, obtained in per- 

 forming Experiment 25, 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? 

 18 



