ALLOTROPISM BASED ON THE THEORY OF DIRECTIVE VALENCY. 333 



sublimation, and the amorphous black variety formed when 

 arseniuretted hydrogen is strongly heated, the following 

 forms have been noticed by different observers. 



When arsenic is sublimed in hydrogen it is said to be split 

 up into a crystalline form, an amorphous form, and a yellow 

 vapour depositing grey crystals. These forms are deposited 

 at different distances from the source of heat. 



An extremely unstable, but, according to Erdmann and 

 Unruh, 1 a definite yellow crystalline variety is obtained 

 when arsenic vapour is strongly cooled and led immediately 

 into carbon bisulphide in which it is soluble. It is very 

 rapidly converted at ordinary temperatures and by light 

 into the black variety, and it is only possible to keep it 

 permanently in complete absence of light and at a tempera- 

 ture of -65° to -70°. 



Erdmann and Unruh (loc. cit.) also find that a reddish- 

 brown precipitate separates out very slowly from a solution 

 of the above yellow arsenic in carbon bisulphide. The 

 substance has not been obtained free from sulphur nor has 

 it been obtained in sufficient quantity to enable it to be 

 identified by a specific gravity determination. 



Reviewing the information available, there appear to be 

 certainly two well defined and distinct allotropic forms of 

 arsenic, the crystalline and the amorphous. 



The molecular structure of these may be regarded as 

 corresponding to those suggested for yellow and red phos- 

 phorus. There may be more than one crystalline form, but 

 the information on this point is not definite. The yellow 

 variety of arsenic is exceedingly unstable and might be 

 represented by one of the eight unsymmetrical forms which 

 are producible according to the hypothesis advanced. It 

 appears probable that the red-brown form is not anallotrope 

 at all. 



1 Erdmann and Unruh, Zeitsch. Anorg. Chemie, 1902, 32, 437. 



