chapter Twelve 



THE EFFECTS OF MOLECULAR 

 DISSYMMETRY ON PROPERTIES OF MATTER 



The kinetic theory of gases during the last century led to extremely 

 valuable conceptions and even quantitative laws, notwithstanding the fact 

 that in the development of the theory, it had been necessary to make many 

 simplifying assumptions which frequently were not actually correct. For 

 example in most derivations it was assumed that the molecules were 

 infinitely hard elastic spheres which acted on one another only at the instant 

 of contact. In this way it was possible to account quantitatively with fair 

 accuracy not only for the effects of pressure and temperature on the 

 volume, but for the viscosity, heat conductivity and diflFusion of gases, and 

 other phenomena involving the free paths of the molecules. Maxwell, on 

 the other hand, developed a very complete theory based on the assumption 

 that the molecules are point centers of force repelling one another according 

 to the inverse fifth power of the distance between them. This assumption 

 was made not because it was supposed to correspond to the actual forces 

 around molecules, but because it lent itself particularly well to mathematical 

 treatment. The results obtained by Maxwell differed from those based on 

 the assumption of elastic spheres mainly in slight differences in numerical 

 coefficients of the equations obtained. To handle the problem mathe- 

 matically it was necessary to make assumptions as to the forces between 

 the molecules, but to get most of the essential truths of the kinetic theory 

 it did not much matter exactly what these assumptions were. 



To account for the main features of the transition from the gaseous to 

 the liquid state Van der Waals found it necessary to consider attractive 

 forces, but again the particular law of force did not need to be known to 

 obtain this useful result. Sutherland also took into account attractive forces 

 in dealing with the temperature coefficient of the viscosity of gases. 



In considering other properties of gases or liquids it may be necessary 

 to analyze the forces around molecules in more detail. For example, 

 Debye ^ showed that the variation of the dielectric constant of a substance 

 with temperature depends on an orientation of the molecules in an electric 

 field, thus indicating that these molecules are electric dipoles. 



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