196 SELENI FERGUS DEPOSIT. 



The selenium is thus precipitated of a fine red color. 

 Its separation is promoted by digestion, and ultimately 

 by boiling for a quarter of an hour, when it becomes 

 black, and collects into a dense hard mass. It is well 

 washed and dried. 



The filtered liquid is boiled once more with hydro- 

 chloric acid, and again treated with sulphurous acid, 

 in case it should still contain selenium. 



The selenium thus obtained contains still small 

 quantities of lead, copper, and iron, and especially mer- 

 cury. On distilling it in a small retort or bent tube 

 closed at one end, the first- mentioned impurities are 

 left behind as selenides. 



In order to free it from mercury, the distilled sele- 

 nium is dissolved in aqua-regia, the greater excess of 

 acid evaporated, so that no nitric acid may remain, the 

 solution mixed with excess of carbonate of soda, eva- 

 porated to dryness, and the saline mass ignited to expel 

 the mercury. 



The mass is redissolved in water, the solution boiled 

 with hydrochloric acid, and the selenium again preci- 

 pitated by sulphurous acid. 



Or the ignited mass may be mixed with about an 

 equal weight of chloride of ammonium, and heated in 

 a retort till the greater part of that salt has sublimed, 

 when the selenium is reduced, and remains behind on 

 dissolving the saline mass in water. 



The selenium may also be at once extracted, and 

 obtained free from mercury, by fusing the deposit, 

 with an equal weight of carbonate of soda and about 

 of nitre, in a crucible. When the mass is in a state 

 of tranquil fusion, it is poured out, so as to leave the 

 oxide of lead, as far as possible, at the bottom of the 

 crucible. It is then dissolved in water, the solution 

 acidulated with sulphuric acid, the precipitated sul- 

 phate of lead filtered off, and the filtrate treated, as 

 above, with hydrochloric acid and sulphurous acid. 



