6 



L. F. Nilson, 



seleniferous mass, when this, by means of washing has been freed from 

 sulphuric acid. This will best take place in large porcelain-bowls, heated 



by steam. 



The cyanide of potassium very soon exerts a dissolving action upon 

 the uncombined selenium and sulphur of the mass, yielding selenio- and 

 sulphocyanate of potassium, the latter only in small quantities, whilst the 

 sulphate of lead is little or not at all attacked by it; in all cases, cyanide 

 of lead is insoluble even in cyanide of potassium, so that the lead is in no 

 danger of being dissolved. The red colour of the mass disappears in a 

 few minutes; and when it has become quite grey without a tint of red, the 

 whole amount of selenium is dissolved, of which I convinced myself by 

 testing the residue for selenium. In fact, by observing this discolouration, 

 one may avoid employing too large a quantity of cyanide of potassium, 

 which would only be unnecessarily lost. The undissolved residue is now 

 allowed to deposit itself and the solution separated by means of decantation 

 and reiterated washing with warm Avater, as long as any remarkable quan- 

 tity of selenium is to be observed in the washing-water, or, in other words, 

 until, on adding hydrochloric acid and boiling, only a very slight opale- 

 scence in redish tint is discernable. If the solution thus obtained, after 

 being filtered, is precipitated with hydrochloric acid, the whole amount of 

 selenium contained in it is yielded in cherry-coloured flakes, under a brisk 

 evolution of hydrocyanic acid; the precipitation ought therefore to be under- 

 taken in the open air. The sulphur remains in the solution as sulphocyanic 

 acid. The selenium is now obtained as a red voluminous precipitate which, 

 when heated under water, cakes together in black, considerably smaller 

 masses. Selenium obtained in this manner is still impure with small quan- 

 tities of iron and copper, in form, no doubt, of ferrocyanide of copper. The 

 cause of the presence of these compounds is undoubtedly to be looked for 

 in the action of cyanide of potassium on the compounds of the above-mean- 

 tioned metals existing in the deposit. Double salts of the cyanide of po- 

 tassium and the cyanides of the two other metals enter into the solution, 

 wherefrom, on adding hydrocloric acid, ferrocyanide of copper, being insoluble 

 in this acid, is precipitated J ) and thus mixes with selenium precipitated at the 



') Experiments, made by me, have pointed out that these solutions do not 

 affect each other, before hydrochloric acid is added, when ferrocyanide of copper 

 instantly subsides. Information on this subject being, to the best of my information, 

 still wanting, it may be convenient here to observe, that ferrocyanide of potassium 

 may be employed for the detection of copper in the double cyanures, merely by 

 adding hydrocloric acid at the same time. 



