EFFECTS OF MOLECULAR DISSYMMETRY 265 



or cube) the third term (which is the only one that Debye considers) should 

 also vanish. 



Debye then calculates that the average force which acts on a molecule B 

 when at a distance r from a similar molecule A is given by 



F = ?^ (I) 



where r is a function only of the "electric moments of inertia" (Sca--, S^y- 

 and 'Ees~) determined by the positions of the electrons and nuclei in the 

 molecule. The quantity a measures the deformability or ease of polarization 

 of the molecule in an electric field and can be calculated from the dielectric 

 constant, or better from the molecular refractive index. 



Debye then proceeds to test these conclusions by calculating from this 

 equation the coefficient a in the van der Waals' equation. 



which measures the attractive forces between molecules. He finds 



6k NW (3) 



5 rf^ 



where N is the Avogadro constant (6.o6 X lo^^) and d is the diameter of 

 the molecule. In deriving this relation Debye assumes that the force 

 between the molecules is given by Equation i for all distances from d to ^. 

 Considering the derivation of Equation ( i ) it is clear that we have no right 

 to apply it to distances comparable with the diameter of the molecule, for 

 the neglected terms in the expansion on which it is based, should then be as 

 important as the term actually considered. Debye, however, uses Equation 3 

 to calculate t from the experimentally determined values of a. 



He then compares the values of t obtained in this way with those cal- 

 culated by another method from the Sutherland constant c which is de- 

 termined from the temperature coefficient of the viscosity tj of gases by the 

 equation 



const VT 

 '^= C~ 



The Sutherland constant is a measure of the attractive forces between 

 molecules and thus Debye is able to express it in terms of t which occurs in 

 Equation i. 



The values of t calculated by these two independent methods agree very 

 well with one another (average error about 4 per cent) for the gases argon, 



