NATROL1TE. 141 



a gray powder. This method is not applicable to 

 graphic and foliated tellurium ore. It is very useful, 

 however, for the extraction of pure tellurium from the 

 crude tellurium of Vienna, which contains copper, iron, 

 lead, antimony and arsenic. For this purpose, also, a 

 mixture of 4 parts of dry carbonate of soda and 1 part 

 of powdered coal may be used ; 2 parts of this mixture 

 are taken for 1 part of the finely pulverized metal ; 

 coarsely powdered coal is placed in the bottom of the 

 crucible, the mass pressed down upon this and then 

 covered with the same kind of coal. 



The tellurium in the filtrate can be separated from 

 coal, &c., by using an excess of hypochlorite of soda. 

 After long digestion with this reagent, the mixture is 

 heated gradually to ebullition, the chloric acid decom- 

 posed by evaporation with hydrochloric acid, and then 

 the tellurium precipitated by sulphurous acid. 



75. NATROLITE * THOMSONITE,f &c. 



The amount of water is determined by igniting a 

 weighed quantity of the material, which has previously 

 been dried at 100. 



The finely-divided, unignited mineral, dried at 100, 

 is mixed in a porcelain dish with moderately strong 

 hydrochloric acid, and digested with it, with constant 

 stirring, until completely converted into a gelatinous 

 mass, and all the mineral is dissolved. 



This mass is then evaporated to complete dry ness, 

 being carefully stirred, in order to render the silica 



* A1 2 3 , 3 Si0 2 , NaO, 2 HO. 



t A1 2 3 , 2 Si0 2 , (fCaO+^NaO), 2 HO. 



