ALLOTROPISM BASED ON THE THEORY OF DIRECTIVE VALENCY. 329 



Saunders) show that they crystallise in different forms of 

 different stability, one of which is isomorphous with mono- 

 clinic sulphur. 



The third allotrope (metallic selenium) is the stable form 

 into which the red crystalline forms pass over on heating 

 to 170° -180°. The crystalline forms are produced from 

 the amorphous variety when the latter is dissolved in carbon 

 bisulphide. With certain solvents, such as quinoline, 

 amorphous selenium is converted directly into the metallic 

 modification. 



The vapour density of selenium diminishes very rapidly 

 with the temperature and its behaviour resembles very 

 closely that of sulphur. Selenium boils according to 

 Deville and Troost at 665° under 760 mm. The vapour 

 density, as determined by Szarvasy 1 at 774° corresponds to 

 a molecular formula between Se 2 and Se 3 . 



Between 900° and 1800° the vapour density is constant 

 at 78*6 and corresponds to the formula Se 2 . 



It is extremely probable that at temperatures closer to 

 the boiling point, the molecule would be very much larger 

 as in the case of sulphur, which selenium so closely 

 resembles. 



It is to be noted that in the case of sulphur the vapour 

 density observed at 606° (which is only 160° above its boil- 

 ing point), corresponds to the molecular formula S 4 , whereas 

 at 440° the molecule is S 8 . In the case of selenium, the 

 temperature (774°) at which the vapour density corresponds 

 to the molecular formula Se 25 is more than 100° above its 

 boiling point, and we are justified in expecting that at 

 lower temperatures the number of atoms in the molecule 

 would increase as in the case of sulphur. 



1 Szarvasy, Henchte, 1897, 30, 1214. 



8 Biltz, Zeitsch. Physik. Chemie, 1896, 19, 4, 15. 



