AMALGAMS. 71 



which has been dried at 100 and weighed, and its 

 weight determined after drying at 100. 



From the filtered solution the oxide of copper is pre- 

 cipitated, at a boiling heat, by caustic potassa. 



II. AMALGAM OF TIN (amalgam for mirrors). This 

 is dissolved in aqua regia, the solution mixed with am- 

 monia in slight excess, afterwards with an excess of 

 sulphide of ammonium, and digested for a long time 

 in a closed vessel. The bisulphide of tin which is 

 formed dissolves in the sulphide of ammonium, and 

 the black sulphide of mercury separates ; it is collected 

 upon a weighed filter, washed with weak sulphide of 

 ammonium, and dried at 100. 



From the solution in sulphide of ammonium the bi- 

 sulphide of tin is precipitated by dilute hydrochloric 

 acid, filtered off, washed, dried, and roasted, together 

 with the filter, in a porcelain crucible, with free access 

 of air. A gentle heat is at first applied, which is gra- 

 dually increased, till the whole of the precipitate is 

 converted into white binoxide of tin; a fragment of 

 carbonate of ammonia is held in the ignited crucible at 

 the end of the operation. 



III. AMALGAM OF SILVER.* The amalgam is dis- 

 solved, by the aid of heat, in nitric acid, so that the 

 solution may contain the whole of the mercury in th<j 

 state of protoxide; the acid solution is diluted with 

 water, and the silver precipitated by an excess of hy? 

 drochloric acid. 



The mercury in the filtrate from the chloride of 

 silver may be precipitated by phosphorous acid. 



* May be obtained in crystals by allowing a small quantity of 

 mercury to remain in a moderately diluted solution of nitrate of 

 silver, or by placing a thick bright copper-wire in the mixed solu- 

 tions of nitrate of silver and of subaxide f mercury, 



