CHAPTER XLIV 

 SILVER, GOLD, PLATINUM 



DESCENDING the activity list, we now reach the "noble" metals, 

 which are the least active. They do not displace hydrogen from 

 dilute acids. They do not combine with oxygen, even when 

 heated. 



SILVER Ag 



Occurrence. Native silver is found in many localities, usually 

 in small amounts. The chief supply of the metal is obtained 

 from the ores of lead and copper, which contain silver sulphide 

 Ag 2 S. The Rocky Mountains, Ontario and Mexico are the main 

 sources. 



Metallurgy. After the lead (bearing silver) has been ex- 

 tracted from the ore (p. 502), it is melted in large caldrons, a 

 small proportion of zinc is added, and the whole is vigorously 

 stirred (Parke's process). Zinc is only slightly soluble in lead, 

 but it combines with silver in several proportions. The zinc- 

 silver alloy rises to the surface, solidifies (while the lead is still 

 molten), partly as alloy and partly as a compound (usually Ag 2 Zn 5 ), 

 and is skimmed off. The most of the adhering lead is pressed out, 

 and the compound (or mixture) is placed in graphite retorts, in 

 which the zinc is removed by distillation. The silver and lead 

 which remain are heated in a blast of air (cupelled) to oxidize 

 the lead. The melted litharge flows off and the silver is then cast. 



The gold, which accompanies the silver through this treat- 

 ment, is separated electrolytically (see copper, p. 515). The 

 silver-gold alloy forms the anode and silver nitrate the vat-liquid. 

 The silver, being the more active metal, is ic-nized and deposited 



519 



