EILHARDT

MITSCHERLICH (1794–1863)  , German chemist, was born on the 7th of January 1794 at Neuende near Jever, in the grand duchy of Oldenburg, where his father was pastor . His uncle, CHRISTOPH WILHELM MITSCHERLICH (176o-1854), professor at Gottingen, 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, 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 to send 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 Gottingen 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 Persian history, compiled from MSS. 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 he 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, con-firming the conclusions of J . J . Berzelius as to their composition; and his observation that corresponding phosphates and arsenates crystallize in the same foam 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 Mitscherlich a government grant to enable him to continue his studies in Berzelius's 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 . In the course of an investigation into the slight differences discovered by W . H . Wollaston in the angles of the rhombohedra of the carbonates isomorphous with calc-spar, he observed that the angle in the case of calc-spar varied with the temperature . On extending his 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 dimorphism . In 1833 he made a series of careful determinations of the vapour densities of a large Drawn from a photograph taken by Father J . Braun (reproduced in Die liturgiscite Gewandung) . By permission of B . Herder . Reproduced by kind permission of the Archbishop of Westminster . number of volatile substances, confirming Gay-Lussac's law . 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 he was able to carry on his academical work till December 1862 . He died at Schonberg near Berlin, on the 28th of August 1863 . Mitscherlich's published papers are chiefly to be found in the Abhandlungen of the Berlin Academy, in Poggendorff's Annalen, and in the Annales de chimie et de physique . The 4th edition of the Lehrbuch der Chemie was published in 1844—1847, a 5th was begun in 1855, but was not completed .