SMITIISOXITE. 57 



32. SMITHSON1TE. 

 ZnO, C0 2 . 



This mineral generally contains small quantities of 

 protoxides of iron, manganese, lead, and cadmium, 

 together with lime, magnesia, and silicic acid. 



It is dissolved in hydrochloric acid, the solution 

 evaporated to dry ness, the mass digested with concen- 

 trated hydrochloric acid, diluted, heated, and the silicic 

 acid filtered off. 



The solution, which must be acid, is saturated with 

 sulphuretted hydrogen, which precipitates the lead 

 and cadmium. 



This precipitate is oxidized with concentrated nitric 

 acid, a little sulphuric acid being also added, evapo- 

 rated to dryness, and the sulphate of cadmium sepa- 

 rated from the sulphate of lead by water. (See Lead 

 and Bismuth.) 



The filtrate is boiled, to expel the sulphuretted hy- 

 drogen, and treated with chlorate of potassa to perox- 

 idize the iron. From the solution, which must still 

 contain free chlorine, the sesquioxides of iron and 

 manganese are precipitated by excess of caustic am- 

 rnonia, and separated as in No. 25. 



The zinc is precipitated from the filtered solution, 

 as sulphide, by addition of sulphide of ammonium, and 

 the precipitate treated as in No. 31. The solution is 

 rapidly filtered off) with as little exposure to air as 

 possible, and the lime precipitated by oxalate of am- 

 rnonia ; the magnesia is afterwards separated by phos- 

 phate of soda. 



IF a specimen of this mineral consist of carbonate 

 and silicate of zinc, their relative quantities may be 

 approximately determined by igniting the finely-pow- 

 dered mineral, and digesting it with a mixture of car- 

 bonate of ammonia and free ammonia, which dissolves 



