172 METALS AND THEIR COMBINATIONS. 



29. SILVER MERCURY. 



Silver, Ag = 107.7 (Argentum}. This metal is found some- 

 times in the metallic state, but generally as a sulphide, which is 

 nearly always in combination with large quantities of lead sul- 

 phide, such ore being known as argentiferous galena. The lead 

 manufactured from this ore contains the silver, and is separated 

 from it by roasting the alloy in a current of air, whereby lead is 

 oxidized and converted into litharge, while pure silver is left. 



A second method by which silver is obtained from ores con- 

 taining it in the metallic state (or in such a form that the com- 

 pounds are easily decomposed with liberation of metallic silver) 

 is the so-called amalgamation process, which depends upon the 

 solubility of silver in mercury ; the ores are treated with this 

 metal, and the silver-amalgam upon being distilled leaves silver 

 behind, the mercury being of course condensed to be used over 

 and over again. 



Silver is the whitest of all metals and takes the highest 

 polish ; it is the best conductor of heat and electricity, and 

 melts at about 1000 ; it is univalent, and forms but one series 

 of salts ; it is not affected by the oxygen of the air at any tem- 

 perature, but is readily acted upon by traces of hydrosulphuric 

 acid, which forms a black film of sulphide upon the surface of 

 metallic silver. Hydrochloric acid scarcely acts on silver, nitric 

 and sulphuric acids dissolve it. 



Silver is too soft for use as coin or silverware, and is there- 

 fore alloyed with from 5 to 25 per cent, of copper, which causes 

 it to become harder, and consequently gives it more resistance 

 to the wear and tear by friction. 4 



Pure silver may be obtained by dissolving silver coin in nitric 

 acid, when a blue solution, containing the nitrates of copper and 

 silver, is formed. By the addition of sodium chloride to the 

 solution a white precipitate of silver chloride is formed, while 

 cupric nitrate remains in solution. The silver chloride is washed, 

 dried, mixed with sodium carbonate and heated in a crucible, 

 when sodium chloride is formed, carbon dioxide escapes, and a 

 button of silver is found at the bottom of the crucible : 



2AgCl + Na 2 CO 3 = 2NaC'l + CO, + 2Ag + O. 



