CLAUSTHALITE. 



199 



If the metallic selenides are mixed or combined 

 with the metallic sulphides, as, for example, in the na- 



Fig. 21. 



tive selenide of mercury which contains sulphide of 

 mercury, the sulphuric acid and selenic acid formed in 

 the analysis are precipitated together by chloride 

 of barium, the precipitate ignited and weighed. A 

 weighed quantity is then heated in a bulb-tube, through 

 which a stream of dry hydrogen is passed, when the 

 selenate of baryta is reduced, with great facility, to the 

 state of selenide of barium, while the sulphate of baryta 

 remains unaltered. The selenide of barium is then ex- 

 tracted with dilute hydrochloric acid. 



In the same way the other metallic selenides which 

 occur as minerals may be analyzed, viz., the selenide 

 of silver and lead, the selenide of cobalt and lead, 

 and the selenide of mercury and lead. 



In order to obtain the selenium from the selenide of 

 lead occurring in many places in the Hartz, the mineral 

 is powdered, treated with dilute hydrochloric acid to 

 remove the calcareous spar and spathic iron-ore, well 



