EFFECTS OF MOLECULAR DISSYMMETRY 261 



distance, with a force which varies inversely with the ninth power of the 

 distances. This result is obtained by considering the deformation of each of 

 two molecules in the electric field of the other. Under the influence of an 

 electric field the electrons become displaced with respect to the nucleus by 

 an amount which may be estimated from the dielectric constant, and thus 

 the molecule becomes polar to a degree which depends on the proximity of 

 other molecules. If the molecule is more symmetrical than a quadrupole 

 (as neon probably is), the force will vary inversely with a power of the 

 distance which may be much greater than the 9th. 



In general, non-polar molecules do not become oriented in weak electric 

 fields, but when the field becomes very strong, the dipole moment, which 

 may differ for different orientations of the molecule, may cause orientation. 

 However, thermal agitation will nearly always make such orientation 

 negligible for molecules of this type. 



2. Polar, 



The molecules of hydrochloric acid gas and water vapor are extreme 

 examples. The center of gravity of all the electrons of the molecule does not 

 coincide with that of the positive charges, so that the molecule acts as an 

 electric dipole. If the separation of the centers of gravity of the electric 

 charges is comparable with the distance between the atoms in a molecule 

 the electric forces are much more intense than in the case of non-polar 

 molecules. The attractive force due to deformation is thus greater than for 

 non-polar molecules. The polar molecules become oriented in an electric 

 field in so far as the thermal agitation will permit. Thus the dielectric con- 

 stant, or rather that part of it which is due to orientation, varies inversely 

 proportional to the absolute temperature.^ Such orientation of the polar 

 molecules in the field of neighboring molecules, if it occurs to any large 

 degree, causes a very great increase in attractive force between molecules. 

 The amount of orientation increases rapidly as the molecules are brought 

 closer together, and also increases slowly as the temperature is lowered. 

 The force of attraction between two dipole molecules which are oriented in 

 each other's fields is Ge'^h^/r'^ where e is the charge and 6 the displacement 

 of the dipole {eh = the electric moment) and r is the distance between the 

 the molecules. The force due to orientation thus varies inverselv with the 

 fourth power of the distance. 



3. Ionic. 



Electrolytic ions such as Na\ SO4"", etc., are familiar examples. 

 Gaseous ions must also be included in this group. The molecule, which in 

 this case is an ion, has an unequal number of electrons and protons, or, in 

 other words, the number of electrons is not equal to the positive charges of 



