In the course of investigating the slight differences discovered by William Hyde Wollaston in the angles of the rhombohedra of the carbonates isomorphous with calcite, Mitscherlich observed that this angle in the case of calcite varied with the temperature. On extending this inquiry to other allotropic crystals, he observed a similar variation, and was thus led, in 1825, to the discovery that allotropic crystals, when heated, expand unequally in the direction of dissimilar axes. In the following year he discovered the change, produced by change of temperature, in the direction of the optic axes of selenite.