332 METALS AND THETR COMBINATIONS. 



remains in solution. The silver chloride is washed, dried, mixed 

 with sodium carbonate, and heated in a crucible, when sodium chlo- 

 ride is formed, carbon dioxide escapes, and a button of silver is found 

 at the bottom of the crucible : 



2AgCl + Na 2 C0 3 = 2NaCl + CO 2 + 2Ag + O. 



Experiment 43. Dissolve a small silver coin in nitric acid, dilute with water, 

 and precipitate the clear liquid with an excess of solution of sodium chloride. 

 The washed precipitate of silver chloride may be treated with sodium carbon- 

 ate, as stated above, or may be converted into metallic silver by the following 

 method : Place the dry chloride in a small porcelain crucible and apply a 

 gentle heat until the chloride has fused ; when cold, place a piece of sheet 

 zinc upon the chloride, cover with water, to which a few drops of sulphuric 

 acid have been added, and set aside for a day, when the silver chloride will be 

 found to have been decomposed with liberation of metallic silver and forma- 

 tion of zinc chloride. 



A simpler method is to heat the silver chloride with a solution of formal- 

 dehyde and a few drops of alkali, or with glucose and alkali. Metallic silver 

 in a finely divided state is obtained, which, after washing, may be fused into 

 a lump. 



Silver nitrate, Arg-enti nitras, AgNO 3 = 168.69. Pure silver is 

 dissolved in nitric acid : 



3Ag + 4HN0 3 = NO + 2H 2 O + 3AgNO 3 . 



The solution is evaporated to dryness with the view of expelling all 

 free acid, the dry mass dissolved in hot water and crystallized. 



If the silver used should contain copper, the latter may be elimin- 

 ated from the mixture of silver and cupric nitrate by evaporating to 

 dryness and fusing, when the latter salt is decomposed, insoluble 

 cupric oxide being formed. The fused mass is dissolved in water, 

 filtered, and again evaporated for crystallization. 



When silver nitrate, after the addition of 4 per cent, of hydro- 

 chloric acid, is fused and poured into suitable moulds it yields the 

 white cylindrical sticks which are known as moulded silver nitrate, 

 caustic, lunar caustic, or lapis infernalis. 



When fused with twice its weight of potassium nitrate and formed 

 into similar rods, it forms the mitigated or diluted silver nitrate (mit- 

 igated caustic) of the U. S. P. 



Silver nitrate forms colorless, transparent, tabular, rhombic crys- 

 tals, or, when fused, a white, hard substance; it is soluble in less 

 than its own weight of water, the solution having a neutral reaction. 

 Exposed to the light, especially in the presence of organic matter, 

 silver nitrate blackens in consequence of decomposition ; when 



