260 Berzelius on a neiv Mineral Body, [Oct. 



"When cold, it has the colour of iron, though rather darker, is 

 brittle, and has a crystalline fracture. 



17. Seletiruret of Arsenic. — Melted selenium dissolves metallic 

 arsenic hy degrees. If one of these bodies be in excess, it 

 sublimes, and we obtain a black and very fusible mass. When 

 heated to redness, it boils, and a mass sublimes which appears 

 to be perseleniuret of arsenic. The ebullition speedily terminates, 

 and the red mass remains without any internal movement. At a 

 heat almost reaching whiteness, it distils in drops ; when cold, 

 it is black, with a shade of brown. The surface has a vitreous 

 lustre ; the fracture is also vitreous. 



8. Alkaline, Earthy, and Metalline Selcniurets. 



Selenium has the same property with sulphur ; it combines 

 with the strongest bases, and forms a species of hepars. These 

 compounds have a taste and a smell so similar to those of the 

 compounds of sulphur with the same bases, that were it not for 

 their red colour, we should take them for sulphurets. Tellurium 

 does not combine with the alkalies in the moist way, unless it 

 has been in the first place united with hydrogen. Tellurium 

 unites with potash in the dry way, but the combination is decom- 

 posed by water. 



Selenluret of Potash. — If we boil selenium in powder in a 

 concentrated ley of caustic potash, it gradually dissolves, and 

 the liquid assumes the colour of strong ale so intensely that it 

 loses its transparency. Its taste is hepatic, and entirely analo- 

 gous to that of sulphuret of potash. The acids precipitate the 

 selenium ; but the filtered liquid still yields a portion of selenium 

 when treated with hydrogen gas. This shows that both selenic 

 acid and seleniuret of potash were formed. 



If we melt together in a glass vessel selenium and caustic 

 potash, they combine, and the alkali retains the selenium even 

 when heated to redness. The upper surface of the mass is 

 "brown ; but the portion in contact with the glass has a cinnabar 

 red colour. Seleniuret of potash dissolves readily in water, and 

 attracts a little moisture from the air. 



If we mix pulverized selenium with carbonate of potash like- 

 wise in a state of powder, and heat the mixture in an apparatus 

 proper for collecting gas, we find that the selenium drives off 

 the carbonic acid, and unites with the potash into a black porous 

 mass, which does not melt at an incipient red heat. This mass 

 yields a red powder. Treated with a small quantity of water, it 

 dissolves with a very deep ale colour. A larger quantity of water 

 precipitates a portion of the selenium in the form of red flocks. 

 If, in this operation, the selenium be in excess, the whole carbonic 

 acid is expelled, and the compound is decomposed by acids with- 

 out effervescence ; but if there be an excess of alkali, the 

 selenium is not precipitated, at least in the same degree, by the 

 addition of water- 



