TETRAHEDRITE. 95 



ratecT. The precipitated sulphide of copper is treated 

 as in No. 30, or, if lead be present, as in No. 56. 



The solution filtered off from the precipitate contains 

 part of the iron, and if too great a heat was avoided, 

 all the zinc; it is then heated to ebullition, and mixed 

 with some chlorate or hypochlorite of an alkali, in 

 order to convert the former metal into a higher oxide; 

 the iron and zinc are then separated and determined 

 as in No. 31. 



II. ANALYSIS OF THE VOLATILE PRODUCTS. This 

 solution contains the mercury, antimony, arsenic, a 

 portion of the iron, and the sulphur, partly in the form 

 of sulphuric acid, partly unoxidized. 



The sulphuric acid can be precipitated with chloride 

 of barium, and the excess of baryta again removed by 

 sulphuric acid. 



It is, however, much more convenient to determine 

 the amount of sulphur in a separate 'portion of the 

 mineral. For this purpose it is finely divided, mixed 

 with three times its weight of finely-powdered chlorate 

 of potassa and then with as much of dry carbonate of 

 soda, and the mixture very gradually heated in a pla- 

 tinum crucible (the bottom of which is previously 

 covered with carbonate of soda, as a precaution) until 

 all the chlorate of potassa is decomposed. When cold, 

 the mass is treated with water, the solution filtered, 

 slightly acidulated with hydrochloric acid, and the 

 sulphuric acid precipitated by chloride of barium as 

 in No. 3. 



For the determination of the other constituents, the 

 solution of the volatile chlorides is heated to about 60 

 and then a gentle current of sulphuretted hydrogen 

 passed through it until it is cold. When completely 

 saturated with sulphuretted hydrogen, the solution is 

 allowed to stand for twelve hours, and then the pre- 

 cipitate, which consists of the sulphides of mercury, 

 antimony, and arsenic; collected on a' filter which has 



