140 TELLURIUM ORE. 



The filtered solution is concentrated by evaporation, 

 and the tellurium precipitated by sulphurous acid. 



The tellurium thus obtained is not quite pure. In 

 order to purify it, it is distilled in a tube of hard glass, 

 at a strong red heat, in a current of hydrogen. The 

 traces of lead and copper remain behind as tellurides, 

 and a slight mixture of selenium is carried off by the 

 gas. 



Tellurium containing selenium may be easily sepa- 

 rated from it completely by fusing it for some time in 

 hydrogen gas. If the two are in solution they are 

 precipitated by sulphurous acid and the selenium, 

 separated by a solution of cyanide of potassium, in 

 which it is soluble. The tellurium remains undissolved. 

 Selenium is precipitated from this solution by acids. 



The pulverulent tellurium may be fused easily into 

 a button, if covered with a mixture of 9 parts of chlo- 

 ride of potassium and 7 parts of chloride of sodium. 



Accurate quantitative analyses of pure graphic tel- 

 lurium and foliated tellurium are best made by means 

 of chlorine gas, as in the case of tetrahedrite. Tellurium 

 is volatilized in this process as' chloride of tellurium ; 

 it requires, however, a very wide tube, on account of 

 its being very bulky. 



From the telluride of bismuth (tetradymite) of 

 Schemnitz, in Hungary, the tellurium is best obtained 

 by the following process: The finely-divided ore is 

 intimately mixed with 3 times its weight of ignited 

 cream of tartar, and exposed to a moderate red heat 

 in a covered crucible during one hour, when all the 

 tellurium is converted into telluride of potassium, and 

 the bismuth separated. The cold mass is reduced to 

 powder, thrown on a filter, and completely washed 

 with cold water which has been freed from air by 

 boiling. The dark-red solution of telluride of potas- 

 sium passing through the filter, when exposed to the 

 air, soon deposits all the tellurium in the form of 



