IRON, NICKEL, COBALT 501 



moist air. Nickel-plating, deposited electrolytically on iron, has 

 the same qualities. The metal is used also in alloys such as monel 

 metal (copper, nickel, approximately 1:1), German silver (cop- 

 per, zinc, nickel, 2:1:1), and mckel coinage (copper, nickel, 

 3:1). Nickel steel is used for armor-plate. 



Compounds of Nickel. These salts are green, and give 

 green solutions (Ni ++ ). The sulphate NiSO 4 ,7H 2 is familiar, 

 as is also the double salt, ammonium-nickel sulphate (NH 4 ) 2 S0 4 , 

 NiS0 4 ,6H 2 0, used in nickel-plating. Nickel carbonyl Ni(CO) 4 is 

 a volatile, colorless liquid (b.-p. 43). It is formed by passing 

 carbon monoxide over warm, finely divided nickel, and is decom- 

 posed again, yielding nickel 99.6 per cent pure, by heating at 

 180. These actions are the basis of the Mond process for 

 separating nickel. 



Exercises. 1. Why does paint protect iron from rusting? 



2. Why does iron, in tune, turn completely into rust, while 

 zinc and aluminium receive only a slight film of tarnish? 



3. How could it be ascertained that ferric hydroxide is in col- 

 loidal suspension, and not dissolved? 



4. Make equations for : (a) the interaction of potassium f erro- 

 cyanide and cupric sulphate CuS0 4 , giving a brown precipitate of 

 cupric ferrocyanide; (b) the action of hydrochloric acid on ferric 

 hydroxide (or rust). 



