58 BRASS. 



the oxide of zinc previously in combination with car- 

 bonic acid, leaving the silicate untouched. 



33. BRASS. 



The alloy is dissolved in hydrochloric acid with 

 gradual addition of nitric acid, the solution diluted, 

 and the copper precipitated by sulphuretted hydrogen. 

 (See No. 30.) 



The excess of sulphuretted hydrogen is expelled 

 from the filtrate by boiling, and the oxide of zinc pre- 

 cipitated from the hot solution by carbonate of soda. 

 (See No. 31.) 



Oxide of zinc cannot be entirely separated from 

 oxide of copper by even a very large excess of caustic 

 potassa. 



Too little zinc is usually obtained by the above pro- 

 cess, because a portion is carried down with the sul- 

 phide of copper. The separation is more completely 

 effected by neutralizing the diluted solution of the 

 alloy with ammonia, and digesting with a slight excess 

 of solid hydrate of potassa until it has lost its color 

 and ammoniacal odor. The oxide of copper is then 

 filtered off, and washed with hot water. From the 

 alkaline solution, the zinc is precipitated by sulphide 

 of ammonium, or boiling with carbonate of soda. 



Another accurate method is the following: The so- 

 lution of both metals is saturated with sulphurous acid 

 and the copper precipitated as white subsulphocyanide 

 by sulphocyanide of potassium. After it has digested 

 for some time, the subsulphocyanide is filtered, a little 

 sulphur added and ignited in hydrogen gas,* when it 



* The precipitate with filter ash should be placed in a porcelain 

 crucible, and strongly ignited by a stream of hydrogen, by means 

 of the gas blowpipe. 



