MITSCHERLICH, EILHARDT (1794-1863), was born January 7, 1794, at Neuende near Jever, in the grand-duchy of Oldenburg, where his father was pastor. He was educated at the gymnasium of Jever under the historian Schlosser. In 1811 lie went to Heidelberg, where he devoted himself to philology, giving special attention to the Persian language. In 1813 he went to Paris, partly for study, partly with the view of obtaining permission to join a French embassy to Persia. The political events of 1814 put an end to this scheme, and Mitscherlich returned to Germany. He then set to work on a history of the Ghurides and Kara-Chitayens, manuscript materials for which he found in the university library of Gottingen, and a portion of which lie published in 1815. Still anxious to visit Persia, he resolved to study medicine in order that lie 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 completely arrested his attention that he gave up the idea of the journey to Persia and the medical profession. In 1818 he went to Berlin, where he worked in the laboratory of Professor Link. He made analyses of phosphates and phosphites, arseniates and arsenites, confirming the observations of Berzelius as to their composition. In the course of these investigations lie observed that corresponding phosphates and arseniates crystallized in the same form.

This was the germ from which grew the theory of isomorphism. In order to follow out his discovery Mitscherlich set to work to learn crystallography. His teacher was a fellow student, Gustav Rose, to whose penetrating mind and profound knowledge of mineralogy have been due some of the most interesting developments and illustrations of the theory of isomorphism. Having measured the inclinations of the faces of a vast number of natural and artificial crystals, he established the principles of isomorphism very much as we now hold them.

It is right that we should remember that Mitscherlich was not the first to notice the fact that two different substances might have the same crystalline form, or that one element could partially replace another without great change of form. Romd de Elsie in 1772 mentions mixed vitriols containing variable proportions of iron and copper, and Leblanc in 1802 showed that the crystalline form remains the same although the proportions vary both in the case of these mixed vitriols and in that of mixed alums. Vauquelin had already, in 1797, proved that alum might contain variable quantities of ammonia without any corresponding variation of crystalline form.

The authority of Haiiy, who laid down as one of his principles that each compound has its own crystalline form, for a time kept these observations in the background. Further cases were, however, observed. Wollaston (1812) accurately measured the angles of the rhombohedral carbonates, and proved that the forms of these minerals, although nearly the same, are not absolutely identical. He showed that a similar close approximation to identity exists in the case of the vitriols. Fuchs in 1815 brought forward his theory of "vicarious constituents." Gay-Lussac proved that a crystal of common alum continues to grow when placed in a solution of ammonia alum, and cases of crystallized mixtures were pointed out by the French mineralogist Beudant. But notwithstanding these foreshadowings, of which we know, on the evidence of Gustav Rose, that Mitscherlich was wholly ignorant, there was at the timd of which we are now speaking no trace of a theory, but merely isolated observations. The theory of isomorphism is the work of .Mitscherlich. It was communicated to the Berlin Academy on December 9, 1819.

In that year Berzelius paid a visit to Berlin, and was so struck with Mitscherlich's ability that he suggested him to the minister Altenstein as the most fitting successor to Klaproth in the chair of chemistry in that university. It is not surprising that this idea was not carried out. It was only four years since Mitscherlich had begun to study chemistry ; he had never lectured, nor had he published anything on the subject.

Although Altenstein did not at that time carry out the proposal of Berzelius, he was so far impressed by it that he obtained for Mitscherlich a Government grant to enable him to continue his studies under Berzelius.

In 1820 lie went to Stockholm, where lie worked for a year in Berzelius's laboratory. In 1822 lie was appointed extraordinary and in 1825 ordinary professor in Berlin. In the course of an investigation into the slight differences discovered by Wollaston in the angles of the rhonibohedra of the carbonates isomorphous with calc-spar, Mitscherlich observed that the angle in the case of calc-spar varied with the temperature. On extending his inquiry to other non-isotropic crystals he observed a similar variation, and was thus led, in 1825, to the discovery that non-isotropic crystals, when heated, expand unequally in the direction of dissimilar axes. ID the following year he discovered the change, produced by change of temperature, in the direction of the optic axes of selenite. The discovery (also in 1826) that sulphur can be obtained in two absolutely distinct crystalline forms threw much light on the fact that the two minerals calc-spar and aragonite have the same composition but perfectly different forms. Other cases of this property, to which Mitscherlich gave the name of dimorphism, were arrived at not long after.

In 1833 he made a series of careful determinations of the vapour densities of a large number of volatile substances, and proved that Gay-Lussac's law as to the proportions by volume in which oxygen, nitrogen, hydrogen, and chlorine unite with one another holds generally for volatile elements, and that the simplicity of the relation of the volume of the compound to that of the component gases is also general In pure chemistry Mitscherlich's discoveries were mainly connected with isomorphism. Thus he obtained selenic acid in 1827, and showed the isomorphism of its salts with the sulphates, and examined with great care the manganates and permanganates, showing their isomorphism with the sulphates and with the perchlorates respectively. But he did much important work unconnected with this special subject. We may in particular refer to his discovery of ' the relation of benzene to benzoic acid, of nitro-benzene, and of a considerable number of the derivatives of benzene.

In 1833 he published his Lehrbuch der Chemie, a student's text-book of chemistry of the most thoroughly ' practical and yet rigidly scientific kind, from the study of which teachers of chemistry may still derive many a valuable hint. His interest in mineralogy led him to the study of 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, but since his death his notes have been arranged and published by Dr 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 SchiMeberg near Berlin on 28th August 1863.

Mitscherlich's published papers are chiefly to be found in the Abhandlungen of the Berlin Academy, in Poggendwi's Annalen, and in the Annales de Chimie et de Physique. The fourth edition of the Lehrbacle der Chemie was published in 1844 ; a fifth was begun in 1855, but was not completed. (A. C. B.)