332 F. B. ©UTHRIE. 



+ 2 -2 Q 



TeZZjTe Te^zLTe 



If, however as seems more probable, tellurium follows 

 sulphur and selenium, and the molecule at lower tempera- 

 ture more nearly approaches the formula Te 8 , then we are 

 confronted with the question whether there are not more 

 than two allotropic forms of the element. If the molecule 

 is octatomic and the atom divalent, then as in the case of 

 sulphur and selenium we would require four allotropic 

 forms if the allotropy is conditioned by the direction of 

 valency. It is possible, of course, that the two recorded 

 colloidal forms are true allotropes, or that there are other 

 allotropic forms not yet isolated. Tellurium is still a some- 

 what rare element, and its properties have not yet been 

 studied with the thoroughness and care accorded to sulphur. 



Arsenic. 



The arsenic molecule like that of phosphorus is tetra- 

 tomic. At 860° the vapour density is 146*88 corresponding 

 to a molecular weight of 293*8 = As 4 . At higher tempera- 

 tures, 1600° - 1700°, the vapour density diminishes by nearly 

 one-half and the molecule only contains two atoms. 



Its molecular weight as determined by the elevation of 

 the boiling point of solutions of yellow arsenic in bisulphide 

 of carbon, corresponds also to the formula As 4 . We may 

 therefore assign to it a formula similar to that of the 

 phosphorus molecule. 



As = As 



With regard to its allotropes, the number of these has not 

 been definitely established. In addition to the steel-grey 

 crystalline form obtained by reduction of the oxide and 



