The transformation of polymorphs presents certain analogies to the solidification of a liquid.
At the same time there may be conditions of temperature and pressure at which polymorphs may exist side by side.
A comparison of the transformation of polymorphs leads to a twofold classification: (1) polymorphs directly convertible in a reversible manner - termed " enantiotropic " by O.
Lehmann and (2) polymorphs in which the transformation proceeds in one direction only - termed " monotropic."
The insufficiency of this argument, however, is shown by the data for arsenious and antimonious oxides, and also for the polymorphs of calcium carbonate, the more symmetrical polymorphs having a lower density.