A modification of Trouton's rule

Trouton's rule is modified to allow for the fact that the entropy of vaporization is generally a function of the Guldberg ratio and the parachor-critical volume ratio. The medification has the advantage that it is applicable to non-associated substances of low boiling-point, and furnishes a rational basis for their classification into normal and abnormal liquids. Moreover, a high value of the medified ratio is diagnostic of association in the liquid state.