COPPER, SILVER, AND GOLD 419 



Commercial silver generally contains copper, and, more rarely, other 

 metallic impurities also. Chemically pure silver is obtained either by 

 cupellation or by subjecting ordinary silver to the following treatment. 

 The silver is first dissolved in nitric acid, which converts it and the 

 copper into nitrates, Cu(NO 3 )2 and AgN0 3 ; hydrochloric acid is then 

 added to the resultant solution (green, owing to the presence of the 

 cupric salt), which is considerably diluted with water in order to retain 

 the lead chloride in solution if the silver contained lead. The copper 

 and many other metals remain in solution, whilst the silver is precipi- 

 tated as silver chloride. The precipitate is allowed to settle, and the 

 liquid is decanted off ; the precipitate is then washed and fused with 

 sodium carbonate. A double decomposition then takes place, sodium 

 chloride and silver carbonate being formed ; but the latter decomposes 

 into metallic silver, because the silver oxide is decomposed by heat : 

 A.g 2 CO 3 = Ag 2 + O + CO 2 . The silver chloride may also be mixed 

 with metallic zinc, sulphuric acid, and .water, and left for some time, 

 when the zinc removes the chlorine from the silver chloride and pre- 

 cipitates the silver as a powder. This finely-divided silver is called 

 1 molecular silver.' l5 



richness of these mines may be judged 'from the fact that one mine in the State of 

 Nevada (Comstock, near Washoe and the cities of Gold Hill and Virginia), which was dis- 

 covered in 1859, gave an output of 400 tons in 1866. In place of cupellation, chlpri- 

 nation may also be employed for extracting silver from its ores. The method of 

 chlorination consists in converting the silver in an ore into silver chloride. This is 

 either done by a wet or by a dry method, roasting the ore with NaCl. When the silver 

 chloride is formed, the extraction of the metal is also done by two methods. The first 

 consists in the silver chloride being reduced to metal by means of iron in rotating 

 barrels, with the subsequent addition of mercury which dissolves the silver,. but 

 does not act on the other petals. The mercury holding the silver in solution is distilled, 

 when the silver remains behind. This method is called amalgamation. The other 

 method is less frequently used, and consists in dissolving the silver chloride in sodium 

 chloride or in sodium thiosulphate, and then precipitating the silver from the solution. 

 The amalgamation is then carried on in rotating barrels containing the roasted ore mixed 

 with water, iron, and mercury. The iron reduces the silver chloride by taking up the 

 chlorine from it. The technical .details of these processes are described in works on 

 metallurgy. The extraction of AgCl by the wet method is carried on (Patera's process) 

 by means of a solution of hyposulphite of sodium which dissolves AgCl (see Note 28), or 

 by lixiviating with a 2 p.c. solution of a double hyposulphite of Na and Cu (obtained by 

 adding CuS0 4 to Na a S 2 O 3 ). The resultant solution of AgCl is first treated with soda 

 to precipitate PbCO 3 , and then with Na-jS, which precipitates the Ag and Au. The 

 .process should be carried on rapidly to prevent the precipitation of Cu 2 S from the solu- 

 tion of CuS0 4 and Na 2 S 2 O 3 . 



15 There is another practical method which is also suitable for separating the silver 

 from the solutions obtained in photography, and consists in precipitating the silver by 

 oxalic acid. In this case the amount of silver in the solution must be known, and 28 

 grams of oxalic acid dissolved in 400 grams of water must be added for every 60 grains 

 of silver in solution in a litre of water. A precipitate of silver oxalate, Ag a C 2 O 4 , is then 

 obtained, which is insoluble in water but soluble in acids. Hence, if the liquid contain 

 any free acid it must be previously freed from it by the addition of podium carbonate. 



