Eilhard Mitscherlich - Isomorphism

Isomorphism

In the course of investigating slight differences discovered by William Hyde Wollaston in the angles of the rhombohedra of the carbonates isomorphous with calc-spar, Mitscherlich observed that the angle in the case of calc-spar 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. His investigation, also in 1826, of the two crystalline modifications of sulfur threw much light on the fact that the two minerals calc-spar and aragonite have the same composition but different crystalline forms, a property which Mitscherlich called isomorphism.