Temperature of Sublimation. 13 



there unless the temperature is finally raised to about 1200° 

 or even higher. The actual decomposition o£ the mineral 

 is often defined by the appearance of deflagration at 

 some lower hob-temperature, varying from about 600° 

 to 700°. 



All the substances 2 to 9 contain the molecule Bi 2 S 3 . 

 These appear to break up at temperatures varying around 

 6*00° ; lillianite alone of these bodies showing no low tempe- 

 rature sublimate. 



Selenium. 



Only one oxide. Se0 2 is recorded. It forms a diffusive, 

 white, sublimate possessing a more or less acicular structure 

 as seen under the microscope. It is very volatile, clearing 

 rapidly off the upper glass just over the heated platinum ; 

 and leaving selenium behind it if any of the element has been 

 volatilized without oxidation. The sublimate of the oxide 

 is very hygroscopic, the crystals melting when breathed upon 

 and being replaced by w r et patches. 



Compounds in which selenium is loosely attached give 

 much red sublimate of selenium at low temperatures. With 

 native selenium it is difficult to get the oxide in any quantity. 

 The red sublimate begins at about 300° and the white at about 

 440°. Presence of oxygen favours the formation of the red 

 sublimate, although this does not appear to contain anv 

 appreciable quantity of oxygen. In an atmosphere of oxygen 

 selenium may be sublimed entirely in the form of the red 

 sublimate without any gain in weight being detectable. 

 Thus 14*5 mgrms. of selenium sublimed at 460° in oxygen 

 showed not the smallest increase in weight. In coal-gas 

 selenium sublimes as a rich black sublimate. 



The first appearance of a sublimate from a selenium com- 

 pound is often that of a pale reddish film, best seen bv 

 placing the glass on white paper. The film is dichroic 

 passessing a fine green sheen, like certain aniline dyes. To 

 the touch this film feels sticky : gentle rubbing appears to 

 intensify the sheen. 



In quantitative work careful regulation of the temperature 

 will enable nearly all the selenium to be got off as the oxide. 

 Thus clausthalite yields at 340° a faint trace of red sub- 

 limate, but if raised to 600° the white sublimate begins to 

 form, and between 700° and 850° all selenium is got off as 

 Se0 2 , massicot only being left, which does not sublime till 

 about 100 degrees higher. 



