MAGNESIUM AND ZINC. IONIC EQUILIBRIA 447 



with potassium chlorate, give flash-light powder. Signal lights 

 are made of shellac, barium nitrate and magnesium powder. 



Oxide MgO and Hydroxide Mg(OH) 2 . The oxide is made 

 by heating magnesium carbonate, and is therefore called calcined 

 magnesia. Being very infusible ("refractory") itr-is used in 

 lining electric furnaces. .On account of its poor heat conduc- 

 tance, it is also employed very extensively for insulating pipes 

 and boilers, and so reducing heat losses (85% Magnesia). It 

 combines slowly with water, giving the hydroxide Mg(OH) 2 , 

 which with water gives a mortar that hardens under the action 

 of the carbon dioxide of the air (see mortar, p. 386). The oxide 

 is basic, and with acids gives salts by double decomposition. 



Magnesium hydroxide Mg(OH) 2 , being insoluble, is easily pre- 

 cipitated by adding sodium hydroxide to a solution of a salt of 

 magnesium. When moistened and mixed with a little magnesium 

 chloride, it sets to a hard basic chloride of variable composition. 

 The mixture, to which sawdustis sometimes added, is used as a 

 plaster in house decoration. 



Salts of Magnesium. ^Jagnesium carbonate MgCQp is- 

 found in nature. That made by precipitation is a basic carbonate 

 3MgCO 3 ,Mg(OH) 2 , magnesia alba, which is used in tooth powder 

 and for polishing silver. 



Magnesium sulphate MgSO 4 is commonly sold as the hepta- 

 hydrate, MgS04,7H 2 O, Epsom salt. It is found in the salt depos- 

 its and in many aperient mineral waters. Thus Hunyadi water 

 contains little beside 47 g. Epsom salt and 52 g. sodium sulphate 

 (Na 2 S0 4 ,10H 2 O) and 1 g. sodium bicarbonate per liter. The salt 

 is used for loading cotton goods and as a purgative. 



Magnesium chloride MgCl 2 is found in sea water and in some 

 natural waters. It is very deliquescent (p. 118) and, being present 

 in impure table salt, causes the latter to cake or even become moist 

 in damp weather. Addition of a very little sodium bicarbonate 



