Mitscherlich relationship

Mitscherlich, E. Sur la relation qui existe entre la forme cristalline et les proportions chimiques, I. Memoire sur les arseniates et les phosphates, [On the relationship that exists between crystal form and chemical proportions. I. Note on arsenates and phosphates.] Ann. Chim. Phys. 19, 350-419 (1822). 

Mitscherlich, E. Sur la rapport qui existe entre les proportions chimiques et la forme cristalline. III. Memoire sur les corps qui affectent deux formes cristallines differentes. [On the relationship between chemical composition and crystalline form. III. On the substances that affect two different crystalline forms.] Ann. Chim. Phys. 24, 264-271 (1823). 

That solids with different compositions can adopt identical crystal shapes was documented in 1819 by Mitscherlich, who called the phenomenon isomorphism (Mitscherlich 1819, Melhado 1980). Isomorphism can describe phases with similar atomic architectures but unlike constituents, such as NaCl and PbS, and it also can refer to members of a continuous solid solution series, such as the olivine group with formula (Mg,Fe)2Si04. Three years later, Mitscherlich documented the complementary property of polymorphism, whereby phases with identical compositions occur as different structures (Mitscherlich 1822). Although mineralogists of the nineteenth century recognized the important inter-relationship between crystal structure and composition, the crystallographic probes available for structure determination did not keep pace with advances in wet chemical analysis. Consequently, understanding the effects that chemical modifications exert on crystal structures could be revealed only by careful measurements of subtle variations in habit. 

In 1834, Eilhardt Mitscherlich correctly determined benzene s molecular formula (CgHg) and decided to call it benzin because of its relationship to benzoic acid, a known substituted form of the compound. Later its name was changed to benzene. 

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