ALLOTROPISM BASED ON THE THEORY OF DIRECTIVE VALENCY. 331 



these differences in colour are due to the state of suspension 

 or to variety of allotropic forms he leaves an open question 

 in view of the present state of our knowledge as to the 

 colloidal condition of the elements. On passing an electric 

 current through solutions of telluric acid in the presence of 

 potassium cyanide, Gutbier and Resenscheck 1 obtain a 

 brown violet liquid hydrosol. If ammonium oxalate is used 

 instead of potassium cyanide the solution becomes steel- 

 blue. If the brown solution is hydrolised before metallic 

 tellurium is produced the solution of hydrosol is permanent. 



With regard to the molecular weight of tellurium there 

 is an equal difficulty. The only determinations of the 

 vapour density quoted are those of Deville and Troost, 3 

 who found the vapour density at 1390° to be 9, (Air = l 



Biltz 3 finds this number, (which corresponds to the 

 formula Te 2 ) to be correct at 1800 degrees. At the same 

 time Biltz considers that Deville and Troosts' conclusions 

 are certainly incorrect, and that at lower temperatures, 

 tellurium vapour has a much higher specific gravity, and like 

 sulphur and selenium, the molecule contains a greater 

 number of atoms, splitting up into smaller molecules at 

 higher temperatures. There are no determinations, as far 

 as I have been able to ascertain, of the molecular weight 

 of tellurium by other methods. The data therefore, in the 

 case of tellurium both in respect to the number of allotropic 

 forms and the molecular weight are not sufficiently reliable 

 to enable us to speculate on the connection between the two. 



If we assume that only two allotropic forms exist, and 

 that the formula is correctly represented by the vapour 

 density at 1390° and above, and is Te 2 , then we have the 

 very simple explanation that under these circumstances 

 two and only two modifications are possible, namely: — 



1 Gutbier and Resenscheck, Zeit. Anorg. Chemie, 1904, 40, 264. 



2 C.R., 1863, 56, 891, 



3 Biltz, Zeifschr. Physik. Chemie, 1894, 19, 415. 



