MAGNESIUM AND ZINC. IONIC EQUILIBRIA 451 



Other Compounds. Zinc sulphate, ZnSO 4 ,7H 2 O, is made 

 by the action of sulphuric acid on zinc or zinc oxide. It is used 

 in preserving hides and as a mordant in cotton printing (see 

 dyeing, p. 475). Zinc sulphide ZnS (white) is precipitated prac- 

 tically completely when a solution of ammonium sulphide (NH 4 ) 2 S 

 is added to a salt of zinc: 



ZnS0 4 + (NH 4 ) 2 S^ZnS J, + (NH 4 ) 2 S0 4 . 

 When hydrogen sulphide is employed instead 



ZnSO 4 + H 2 S <^ ZnS + H 2 S0 4 



only a small part of the zinc is precipitated. The cause of this 

 difference will come up for discussion in a subsequent section (p. 

 459). 



A mixture of zinc sulphide and barium sulphate BaSO 4 , pre- 

 pared in a special way, is called lithopone. Used as a white 

 pigment, it has greater covering power than has white lead and 

 is, besides, non-poisonous. 



Cadmium. Aside from the rare mineral greenockite CdS, 

 this element is found only in small amounts (about 0.5 per cent), 

 as carbonate and sulphide, in the corresponding ores of zinc. 

 During the reduction, being more volatile than zinc, it distils 

 over first (b.-p. 778). The metal is white, and is much more 

 malleable than is zinc. It melts at 320. 



It displaces hydrogen from dilute acids, but is itself displaced 

 from solutions of its compounds by zinc, since it is less electro- 

 positive. It is used in making fusible alloys. 



IONIC EQUILIBRIA 



In view of the importance of ionic actions in the chemistry of 

 the metals, we must now consider more closely the subject of 

 ionic equilibria. The whole basis for this exact consideration 

 has already been supplied, and only more specific application of the 



