200 CAST IRON. 



washed and dried. It is then very intimately mixed 

 with an equal weight of carbonate of potassa contain- 

 ing charcoal (calcined bitartrate of potassa), covered 

 with coarse charcoal-powder in a crucible, the cover of 

 which is then luted on, and exposed for an hour to a 

 moderate red heat. When cool, the mass, which contains 

 all the selenium as selenide of potassium, is quickly 

 powdered in a warm mortar, thrown on a filter, and 

 washed with well-boiled hot water, as long as the 

 washings are colored ; during this operation, the funnel 

 should always be kept full of water, so that the mass 

 may not come in contact with the air. 



The yellowish-red solution of selenide of potassium 

 begins immediately to deposit upon its surface a film 

 of selenium, the whole of which separates, after some 

 days, in the form of a thin reddish-black crust; only 

 a small quantity remains in solution in an oxidized 

 state. It may afterwards be precipitated by heating 

 the solution with sulphurous and hydrochloric acids. 



Since selenide of lead frequently contains selenide 

 of silver, the carbonaceous mass remaining after the 

 extraction of the selenide of potassium may be fused 

 with carbonate of potassa and some nitre. A metallic 

 button of argentiferous lead is thus obtained, from 

 which the silver may best be separated by cupellation. 



108. CAST-IEON. 



For the detection and estimation of the foreign sub- 

 stances, the total weight of which does not usually 

 exceed 5 per cent. ; it is best to employ separate 

 portions of iron. 



I. CARBON. The total amount of carbon may be 

 determined by burning the iron, in the state of very 

 fine filings, with the aid of a slow stream of pure 



