SILVER MERCURY. 195 



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 

 w r hite 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 diluted silver nitrate (mitigated 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 

 brought in contact with animal matter, it is readily decomposed into 

 free nitric acid and metallic silver, which produces the characteristic 

 black stain ; it is this decomposition, and the action of the free nitric 

 acid, to which the strongly caustic properties of silver nitrate are 

 due. 



Nitrate of silver is used largely in photography, and also in the 

 manufacture of various kinds of indelible inks and hair-dyes. 



Silver oxide, Argenti oxidum, Ag" 2 O = 231.4. Made by the 

 addition of an alkali hydroxide to silver nitrate : 



2AgN0 3 + 2KOH = 2KNO 3 + H 2 O + Ag 2 O. 



A dark-brown, almost black powder, but very sparingly soluble 

 in water, imparting to the solution a weak alkaline reaction. It is a 

 strong base, and easily decomposed into silver and oxygen. 



Silver iodide, Argenti iodidum, Ag-I 234.3. Made by the 

 addition of potassium iodide to silver nitrate : 



AgNO 3 + KI = KNO 3 -f Agl. 



A heavy, amorphous, light yellowish powder, insoluble in water, 

 and but slightly soluble in ammonium hydroxide. 



Antidotes. Sodium chloride, white of egg, or milk, followed by an emetic. 



