LEAD AND TIN 507 



The last set of charges consumes energy, while the first set 

 liberates energy. Both may be stated in a single equation: 



charge > 



2PbS0 4 + 2H 2 + Pb + 2H 2 S0 4 + PbO 2 . 

 < discharge 



In the Edison cell, when charged, one plate is of iron and the 

 other contains nickelic oxide Ni 2 O 3 . The cell liquid is a solution 

 of potassium hydroxide. When the cell operates, the nickelic 

 oxide is reduced to Ni(OH) 2 and the iron is oxidized to Fe(OH) 2 , 

 an action which delivers energy: 



Fe + 3H 3 + Ni 2 3 * Fe(OH) 2 + 2Ni(OH) 2 . 



When the cell is recharged, the nickel is reoxidized and the iron 

 reduced. 



TIN Sn 



Metallurgy. Tin is obtained from cassiterite or tin-stone 

 Sn0 2 (stannic oxide). The world's production averages 120,000 

 tons annually. Formerly the mines in Cornwall (England) con- 

 stituted the chief source of the metal, but now the largest supply 

 comes from the East Indies, the next largest from Bolivia. The 

 ore is roasted to expel sulphur and arsenic, and reduced with coal 

 in the reverberatory furnace. The melted metal is cast in blocks 

 (" block tin "). The metal was well known to the ancients (found 

 in Egyptian tombs). 



Properties. The metal is white, and markedly crystalline. 

 It is soft and malleable (tinfoil), and melts at 232. 



Tin does not tarnish in the air. With concentrated acids it 

 acts rapidly. Hydrochloric acid gives stannous chloride SnCl 2 and 

 hydrogen. Concentrated sulphuric acid gives stannous sul- 

 phate SnS04, sulphur dioxide (p. 271) and water: 



Sn + 2H 2 S0 4 -> SnS0 4 + S0 2 + 2H 2 0. 



