520 SMITH'S INTERMEDIATE CHEMISTRY 



on the cathode, while the gold collects as a powder in a bag which 

 surrounds the cathode. 



An older method, still used, is to heat the silver with concen- 

 trated sulphuric or nitric acid, either of which will dissolve the 

 silver and leave the gold. From the solution the silver is dis- 

 placed by the action of plates of copper: 



Cu + 2Ag+ -* Cu++ + 2Ag J. 



The world's production of silver in 1920 was 170,000,000 ounces 

 (troy), of which the mines of Mexico were responsible for 65,000,000, 

 and those of the United States for 55,000,000. 



Properties. Silver is fairly hard, considering its great 

 ductility and malleability. It is the best conductor of electricity. 



When an electric discharge passes between the ends of two 

 silver wires, held under water, silver is dispersed at the points 

 and forms a colloidal solution. The color of the solution varies 

 from brownish to pink, according to the conditions. Colloidal 

 solutions of gold and platinum can be made in the same way. 



The metal is oxidized by ozone (p. 221), although not by oxy- 

 gen. Sulphur compounds in the air tarnish the surface (Ag2S, 

 see p. 255), as do also eggs, secretions from the skin and rubber 

 which contains sulphur. 



Cold nitric acid and hot concentrated sulphuric acid are at- 

 tacked by it, giving the nitrate and oxides of nitrogen, and the 

 sulphate and sulphur dioxide, respectively: 



3Ag + 4HN0 3 (dU.) -> 3AgN0 3 + 2H 2 O + NO f . 



Uses. For silver ware and coins the metal is alloyed with 

 copper. American coins contain 90 per cent of silver (" 900 

 fine "). British coins formerly contained 92.5 per cent, which is 

 the proportion in " sterling silver," but the rise in price of the metal 

 has recently necessitated a reduction to 50 per cent. Articles 

 of baser metal are plated with silver. The bath contains potassium 

 argenticyanide KAg(CN) 2| made by adding potassium cyanide 



