SUBLIMATION CURVE FOR SELENIUM CRYSTALS 



511 



are treated on the basis of a diatomic vapor, in absence of knowl- 

 edge as to the real state of aggregation of atoms in the molecule of 

 selenium vapor at the temperatures of the experiment. Such treat- 

 ment of the data gives a vapor pressure curve, figure 127, that, 

 empirically at least, and for the present range of temperatures, can 

 be expressed by a simple exponential relation, p^Cie[Co/T]. For 

 selenium vapor treated as diatomic this becomes, on the basis of the 

 present measurements, p=4.92xl0^^ 10[ — 7644/T]. 



The pressures for six temperatures as computed from this empirical 

 equation are given in Table II, and the corresponding points on the 

 curve are indicated by the large circles, enclosing crosses. 



If the relation is either exactly exponential or very nearly so, the 

 only effect of regarding the vapor molecule as other than diatomic 

 will be to change the magnitude of the constant C^, which change 

 will not destroy the characteristic exponential shape but only affect 

 the computed pressure values, by decreasing them as the number of 

 atoms taken per molecule is increased. Thus, the lower, p^, and the 

 upper, p,, limiting pressure values for six different values of the 

 number of atoms, m, assumed per molecule, are given in the follow- 

 ing Table III, in absolute units. 



