Eilhard Mitscherlich

Eilhard Mitscherlich (1794-1863) was a German chemist, who is perhaps best remembered today for his law of isomorphism (1819), which states that compounds crystallizing together probably have similar structures and compositions. This relationship was used by Berzelius in early attempts to assign relative masses to the elements.

Early life and work
Mitscherlich was born on January 7, 1794 at Neuende near Jever, in the grand duchy of Oldenburg, where his father was pastor. His uncle, Christoph Wilhelm Mitscherlich (1760-1854), professor at Göttingen, was in his day a celebrated scholar. He was educated at Jever under the historian F. C. Schlosser, when he went to Heidelberg in 1811, and devoted himself to philology, giving special attention to the Persian language. In 1813 he went to Paris to obtain permission to join the embassy which Napoleon I was sending to Persia.

The events of 1814 put an end to this, and Mitscherlich resolved to study medicine in order that he might enjoy that freedom of travel usually allowed in the East to physicians. He began at Göttingen with the study of chemistry, and this so arrested his attention that he gave up the journey to Persia. From his Gottingen days dates the treatise on certain parts of Eurasian history, compiled from manuscripts in the university library and published in Persian and Latin in 1814, under the title Mirchondi historia Thaheridarum historicis nostris hucusque incognitorum Persiae principum.

In 1818 Mitscherlich went to Berlin and worked in the laboratory of H. F. Link (1767-1851). There he made analyses of phosphates and phosphites, arsenates and arsenites, confirming the conclusions of J. J. Berzelius as to their composition; and his observation that corresponding phosphates and arsenates crystallize in the same form was the germ from which grew the theory of isomorphism, which he communicated to the Berlin Academy in December 1819. In that year Berzelius suggested Mitscherlich to the minister Altenstein as successor to M. H. Klaproth at Berlin. Altenstein did not immediately carry out this proposal, but he obtained for Mitscheriich a government grant to enable him to continue his studies in Berzelius' laboratory at Stockholm. He returned to Berlin in 1821, and in the summer of 1822 he delivered his first lecture as extraordinary professor of chemistry in the university, where in 1825 he was appointed ordinary professor.

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 aelotropic crystals, he observed a similar variation, and was thus led, in 1825, to the discovery that aelotropic 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 sulphur 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.

Later work and last years
In 1833 he made a series of careful determinations of the vapour densities of a large number of volatile substances, confirming the law of Gay-Lussac. He obtained selenic acid in 1827 and showed that its salts are isomorphous with the sulphates, while a few years later he proved that the same thing is true of the manganates and the sulphates, and of the permanganates and the perchlorates. He investigated the relation of benzene to benzoic acid and to other derivatives. In 1829-1830 he published his Lehrbuch der Chemie, which embodied many original observations. His interest in mineralogy led him to study the geology of volcanic regions, and he made frequent visits to the Eifel with a view to the discovery of a theory of volcanic action. He did not, however, publish any papers on the subject, though after his death his notes were arranged and published by Dr. J. L. A. Roth in the Memoirs of the Berlin Academy (1866).

In December 1861 symptoms of heart disease made their appearance, but Mitscherlich was able to carry on his academic work until December 1862.

He died at Schadenberg near Berlin, on the 28th of August 1863, and was buried in the St Matthäus Kirchhof Cemetery in Schöneberg, Berlin, close to the (eventual) gravesites of Gustav Kirchhoff and Leopold Kronecker.

Papers
Mitscherlich's published papers are chiefly to be found in the Ablsendlungen of the Berlin Academy, in Poggendorff's Annalen, and in the Annales de chemie et tie physique.

The fourth edition of Mitscherlich's Lehrbuch der Chemie was published in 1844-1847; a fifth was begun in 1855, but was never completed.