Researches on the Salts of Selenious Acid. 



7 



same time. The above mentioned are the only metals to be observed; the 

 others are no doubt contained in the deposit in such compounds as are not 

 attacked by cyanide of potassium. 



In order to separate the selenium from the above-mentioned admix- 

 tures, I made an attempt to dissolve it in caustic soda, by which operation, 

 as is known, selenide and selenite of sodium are formed. From this mix- 

 ture, all selenium was precipitated as an extremely fine powder, hydroselenic 

 and selenious acid mutually decomposing each other. Selenium treated in 

 this way yielded, after oxidation with nitric acid, a solution green-coloured 

 by copper; that copper could not, however, be precipitated with excess of 

 soda. The strongly alkaline solution assumed an intense green colour and 

 resembled a concentrated solution of nickel. On adding hydrosulphuric acid, 

 it was discoloured and a green precipitate obtained, that blackened on 

 boiling and, when submitted to the blow-pipe, gave the reactions of copper. 



The dissolution in caustic soda having led to the above-mentioned 

 result, it became obvious that the method of purifying, that I intended by the 

 above proceeding, to avoid because it seemed to be attended with too much 

 difficulty, viz. that of transforming selenium into selenious acid and subli- 

 mating the latter, must be resorted to. For this purpose selenium was 

 dissolved in nitric acid and the solution evaporated to dryness in the water- 

 bath ; the selenious acid was then converted into anhydride, which was 

 obtained in cakes of fine, satiny prisms, slightly green-coloured by copper. 

 If the evaporation takes place too rapidly and with too great an excess of 

 nitric acid, not insignificant quantities of selenious acid are also carried 

 away with the fumes. The anhydride thus obtained was then laid in a 

 tolerably wide tube of hard-melted glass, divided in the middle by an in- 

 serted stopple of asbestos, with or without a contraction of the glass at that 

 point, and drawn out at both ends, so that the one part, wherein the im- 

 pure acid was placed, might, by means of a wide caoutchouc-tube, be con- 

 nected with an apparatus for drying and filtering through cotton-wool the 

 current of air that was to be sucked through the tube during the opera- 

 tion; this was necessary, partly in order that the anhydride might be brought 

 to sublimate into the other part, with which an aspirator, applied for the 

 purpose, was connected, and partly that the sublimate might not obstruct the 

 glass-tube. In order not to lose any selenious acid, which, in case of a 

 rapid current of air, might easily pass into the aspirator, it will be con- 

 venient to place, between this and the sublimation tube, a Liebig's bulb- 

 apparatus, filled with water. The heating is undertaken on a combustion- 

 furnace; and by regulating the heat and the rapidity of the current of air, 



