It is shown that to a high degree of accuracy the complex relativity permittivity for a gas dielectric or for a liquid dielectric in very dilute solution in a nonpolarsolvent is given theoretically for symmetric polar molecules by the modification of a relation, which essentially goes back to Rocard. When the molecules are linear or spherical, the modified relation is particularly simple. There is no simple relation for complex permittivity when the polar molecules are asymmetric. Thus in comparing the theory of rotational Brownian motion with the results of dielectric relaxation experiments it would seem advisable at the present stage to focus attention on experiments performed with symmetric polar molecules, preferably spherical or linear.
