330 F, B. GUTHRIE. 



Beckmann and Pfeiffer 1 have determined the molecular 

 weight by observing the depression of the freezing point 

 of solutions of selenium in phosphorus, and find that this 

 corresponds to the molecular formula Se 8 . 



We are therefore justified in assuming that the selenium 

 molecule is of similar construction to the sulphur molecule 

 and consists of eight atoms. Like sulphur also, the selenides 

 of hydrogen and the metals have the formula Se"M 2 . If 

 therefore, we ascribe direction to the uniting bonds repre- 

 senting valency, we obtain as in the case of sulphur four 

 possible combinations, in which the electric charges on the 

 individual atoms are either neutralised or evenly distributed, 

 producing four symmetrical, more or less stable molecules, 

 corresponding to the four definite allotropic forms. 



If further speculation is permissible, one would be 

 tempted to suggest that the most stable form (metallic 

 selenium) might be represented by the neutralised molecule 

 (corresponding to Case 1 of sulphur) and that the less stable 

 crystallised modifications are represented by figures similar 

 to Nos. 3 and 4, in the case of the sulphur molecule. 



Tellurium. 



In the case of tellurium, the information is more meagre 

 and the conclusions less satisfactory. Apparently only two 

 allotropic forms of this element are recognised, namely the 

 crystallised form occurring in hexagonal riiombohedra, 

 with a silvery lustre, and the amorphous variety produced 

 when solutions of the oxide are reduced by sulphurous acid. 2 

 Gutbier 3 has obtained two colloidal modifications of tel- 

 lurium, a brown one obtained on reduction of solutions of 

 tellurium dioxide and telluric acid, and a bluish-green one 

 formed only on the reduction of telluric acid. Whether 



1 Beckmann and Pfeiffer, Zeit. Physik. Chemie, 22, 614. 



2 Fabre and Berthelot, C.R. 104, 1405. 



3 Gutbier, Zeitsch. Anorg. Chemie 1902, 32, 51. 



