SILVER AND LEAD. 65 



sulphuretted hydrogen, set aside in a closed vessel for 

 twenty-four hours, after which the precipitated penta* 

 sulphide of arsenic is filtered off. 



The filtered solution is treated with ammonia which 

 precipitates the iron as sulphide, occasionally mixed 

 with a small quantity of sesquioxide of chromium. 



The phosphoric acid is precipitated from the filtrate, 

 after concentration, by sulphate of magnesia and 

 ammonia. 



In order to separate the sesquioxide of chromium, 

 the mineral is digested with a mixture of concentrated 

 hydrochloric acid and alcohol, the solution filtered) 

 evaporated to expel the alcohol, and the sesquioxide of 

 chromium precipitated from the hot solution by am*- 

 monia. It still contains a little phosphoric acid* 



40. SILVER AND LEAD, 



I. By cupellation. 



II. The solution of the two metals in nitric acid is 

 diluted with much water, heated nearly to the boiling- 

 point, and the silver precipitated as chloride of silver 

 by hydrochloric acid. (See No. 1.) 



The filtered solution is allowed to cool, the greater 

 part of the acid neutralized with ammonia, and the lead 

 precipitated by sulphuretted hydrogen. (See No. 39.) 



III. The diluted nitric solution of the two metals is 

 mixed with dilute hydrocyanic acid, which precipitates 

 the silver as cyanide. When this has accumulated, 

 leaving the solution clear, it is collected upon a filter 

 which has been dried at 120, washed, dried at that 

 temperature, and weighed. 



From the filtrate, after neutralizing the larger excess 

 of acid, the lead may be precipitated by sulphuretted 



