MITSCHERLICH, EILHARD

(b. Neueden, Oldenburg, Germany, 7 January 1794; d. Berlin, Germany, 28 February 1863)

chemistry, mineralogy.

Mitscherlich was the son of a minister, also named Filhard Mitscherlich, and Laura Meier. He received his early education at Jever, in the school directed by the historian F. C. Schlosser, who encouraged him to apply himself to the liberal arts. In 1811 Mitscherlich entered the University of Heidelberg, where he studied Oriental languages; he continued this pursuit at the University of Paris, which he entered in 1813. He learned Persian with particular enthusiasm and hoped to be a member of the legation that Napoleon intended to send there. When Napoleon’s fall ended that prospect, Mitscherlich returned to Germany, where in 1817 he enrolled in the University of Göttingen to read science and medicine—a choice dictated by his determination to reach the Orient, as a ship’s doctor if not as a diplomat.

Simultaneously with his medical studies, Mitscherlich completed the research on ancient Persian texts for which he was awarded the doctorate. At the same time, his interest increasingly turned toward chemistry, which was taught at Göttingen by F. Strohmeyer, who, in addition to his lectures, gave his students the opportunity to carry out certain laboratory experiments.

In 1818 Mitscherlich went to Berlin to work in the laboratory of the botanist Heinrieh Link. There he began to study crystallography. He observed that the crystals of potassium phosphate and potassium arsenate appeared to be nearly identical in form and, his curiosity spurred, asked Gustav Rose to instruct him in exact crystallographic methods so that he could make precise measurements. He then applied spherical trigonometry to the data that he obtained, and was thereby able to confirm his first impression. He reported this finding in an article entitled “Ueber die Krystallisation der Saltze, in denen das Metall der Basis mit zwei Proportionen Sauerstoff verbunden ist,” published in the Abhandlungen der Preussischen Akademie der Wissenschaften for 1818–1819 and translated into French for publication in the Annales de chimie in the following year.

In this important article Mitscherlich discussed the crystals of the sulfates of various metals. He demonstrated that these sulfates—as well as the double sulfates of potassium and ammonium—crystallize in like forms, provided that they bind the same quantity of water of crystallization. Thus, for sulfates of copper and manganese, he found the ratio between the oxygen of the oxide and that of the water of crystallization to be 1:5; while for zinc, nickel, and magnesium, the ratio is 1:7. He further stated his hope “that through crystallographic examination the composition of bodies will be determined with the same certainty and exactness as through chemical analysis.”

Mitscherlich met Berzelius in 1819, when the latter was passing through Berlin. Berzelius had heard of Mitscherlieh’s work and recognized the significance of his findings. When the Prussian Ministry of Education olfered Berzelius the chair of chemistry at the University of Berlin, left vacant on the death of Klaprolh, Berzelius suggested appointing Mitscherlich in his stead. Mitscherlich was thought to be too young to fill the post, however, and a compromise was arranged whereby he would be sent to work with Berzelius in Stockholm for two years, in order to enlarge his knowledge of chemistry. In the course of this fruitful partnership Mitscherlich worked in Berzelius’ laboratory, visited and studied the mines and metallurgical works at Falun, and acquired further experience in chemical analysis and inorganic chemistry. Most important, he continued his work on isomorphism.

In his second article on his crystallographic researches,“Om Förhållandet einellan Chemiska Sammansättningen och Krystallformen hos Arseniksyrade och Phosphorsyrade Salter” (“On the Relation Between the Chemical Composition and the Crystal Form of Salts of Arsenic and Phosphoric Acids”), published in Kungliga Svenska vetenskapsakademiens handlingar in 1822, Mitscherlich reported on new observations that he had made with Berzelius. Among his findings were that

… each arsenate has its corresponding phosphate, composed according to the same proportions, combined with the same amount of water and having nearly equal solubilities in water and acids; in fact the two series of salts differ in no respect except that the radical of the acid in one series is arsenic, while in the other it is phosphorus.… Certain elements have the property of producing the same crystal form when in combination with an equal number of atoms of one or more common elements, and the elements, from this point of view, can be arranged in certain groups. For convenience I have called the elements belonging to the same group … isomorphous.

He then stated his conclusion that

… an equal number of atoms, combined in the same way produce the same crystal forms and the crystal form does not depend on the nature of the atoms, but only on their number and mode of combination.

Mitscherlich further noted that the hydrate crystals of NaO² + 2 PO⁵ (written today as NaH₂PO₄. H₂O) and NaO² + 2 AsO⁵ (NaH₂ASO₄. H₂O) ordinarily exist in two different forms; but since the phosphate crystal also exists in another form identical to the usual form of the arsenate crystal, the criterion for isomorphism is met. He was thus the first to recognize the phenomenon now called dimorphism. In his next paper,“Ueber die Kô, welche in zwei verschiedene Formen krystallisieren,” published in Abhandlungen der Preussischen Akademie der Wissenschaften for 1822–1823, he investigated this phenomenon in greater detail and presented a number of examples, including the rhombic and monoclinic forms of sulfur. (He thus refuted Haüy’s crystallographic axiom, whereby crystal angles, particularly the angles of cleavage, are characteristic of a given substance.)

The statement of the law of isomorphism, made early in his career, marks Mitscherlich’s most important contribution to chemistry—indeed, Berzelius considered Mitscherlich’s discovery to be the most significant since that of chemical proportions. Berzelius himself found Mitscherlich’s work to be of great use; he was at this time concerned with the determination of the atomic weights of the elements and the law of isomorphism provided him with a valuable tool. Since the relative atomic weight of an element could bedetermined only through a knowledge of how many atoms are contained in the molecule, Berzelius’ task was simplified by the application of Mitscherlich’s law—once he had established the atomic composition of one of the isomorphic compounds, those of the others could be assumed to correspond to it. He was thus able to check the atomic weights that he had set out in his Lärbok i kemien of 1814 and presented corrected values for twenty-one elements in the second edition, which was published in 1826.

Mitscherlich refined his work on isomorphism from time to time throughout his scientific life. When it became clear that his original formulation of the law was too broad, he modified it (in 1832) to state more precisely that only certain elements can substitute for each other in crystal form. During the following years, too, Mitscherlich established the isomorphism that exists between a number of specific compounds, including sulfates, metallic selenates, potassium chromate and potassium manganate, and potassium perchlorate and potassium permanganate. All of his later work was conducted in Berlin, where he returned in 1822 to take up the post of assistant professor of chemistry at the university. He became full professor three years later. He was also a member of the Berlin Academy of Sciences and director of its laboratory, located in the observatory. He made extensive use of this installation for teaching as well as for research, since the university offered no facilities for practical instruction in chemistry.

Besides his sojourn in Sweden, Mitscherlich made other trips abroad to work with foreign scientists. In 1823–1824 he was in Paris, where he collaborated with Fresnel in investigating the alteration of the double refraction of crystals as a function of temperature; he also met Thenard and Gay-Lussac. In 1824 he visited Humphry Davy, Faraday, Wollaston, and Dalton in England, where he inspected a number of factories. Back in Berlin, he worked in a number of areas of both organic and inorganic chemistry, in addition to his studies of isomorphism.

In inorganic chemistry Mitscherlich investigated the higher compounds of manganese, including the mixture of manganate and permanganate that Glauber, in the seventeenth century, had called the “chameleon mineral.” Mitscherlich olfered an explanation for the transformations of this substance, establishing that its red and green salts are the derivatives of two different (manganic and permanganic) acids; he determined their chemical composition in 1830. Aschoiff produced the anhydride of permanganic acid in Mitscherlich’s laboratory, and Mitscherlich himself was the first to obtain iodine azide and selenic acid.

During the same period Mitscherlich was also concerned with vapor-density determinations. He modified Dumas’s apparatus by employing a metal bath for measuring higher temperatures; he was thus able to determine the vapor densities of bromine, sulfur phosphorus, arsenic, mercury, sulfur trioxide, phosphorus pentachloride, calomel, and arsenic oxide. His results were highly accurate in most instances. He further measured the pressure of water vapor over Glauber’s salt, in response to a suggestion of Berzelius, who had hoped—erroneously—that a numerical indication of the affinity of water for various substances might be determined from the differences between the pressure of water vapor over those substances.

In organic chemistry, Mitscherlich in 1834 obtained benzene by the dry distillation of the calcium salt of benzoic acid. He found the product of the distillation to be identical with the “bicarburet of hydrogen” that Faraday had isolated from compressed oil-gas five years earlier. From his observation that benzoic acid might be a compound of benzene and carbon dioxide, Mitscherlich concluded that all organic acids must consist of hydrocarbons plus carbonic acid—a misconception that was long perpetuated.

By vapor-density measurements, he reached the formula C₃H₃ (the present C₆H₆)for the composition of benzene, a quantity that corresponds in volume to one atom of hydrogen.

Mitscherlich went on to conduct experiments on various benzene derivatives. He obtained nitrobenzene from the reaction of benzene with fuming nitric acid (ordinary nitric acid does not react with benzene) and benzenesu1fonic acid from the reaction of benzene with fuming sulfuric acid. He also obtained azobenzene, trichlorobenzene, hexachlorobenzene, and their corresponding bromine derivatives.

In 1834 Mitscherlich also showed that a mixture of ether and water distills out of a mixture of alcohol and diluted sulfuric acid; he suggested that in this case the sulfuric acid acts as a dehydrating agent. From this observation he developed his contact theory, whereby certain chemical reactions can take place only in the presence of certain other substances. Mitscherlich’s theory was a direct predecessor of Berzelius’ catalyst theory, which was, in fact, a refinement of it.

Mitscherlich further sought to explain fermentation by this theory, the “contact” in this process being yeast, which is necessary for the conversion of sugar into alcohol. He observed that if a test tube filled with yeast is dipped into a sugar solution, no fermentation occurs, whereas if the sugar is introduced directly into the tube that contains the yeast—or is brought into contact with it—fermentation does take place. Since it is not necessary that a contact agent be a chemical substance in Mitscherlich’s theory, he was able to accept Cagniard de La Tour’s assertion (of 1842) that yeast is a microorganism; indeed, Mitscherlich was the first chemist to do so.

In his experiments on fermentation Mitscherlich further discovered that yeast does not act directly on cane sugar; instead, an invert sugar, a kind of levorotatory “modified cane sugar” identical to the sugar formed by the action of acids on cane sugar, is formed first. He also established that 0.001 percent acid is sufficient to invert sugar solutions. He gave impetus to the sugar industry both by developing the first practical polarization apparatus and by devising a method to control polarization through polarimetric analysis.

Mitscherlich worked to improve the methods and accuracy of both organic and inorganic analytical chemistry. In 1855 he developed a toxicological detection index for white phosphorus, by which the substance to be tested was distilled with steam and the presence of phosphorus determined by luminescence in the condenser of the distilling apparatus. He was also the first to employ a mixture of potassium carbonate and sodium carbonate to produce fusion. For analyzing organic compounds, Mitscherlich constructed a combustion apparatus that differed from those of Berzelius and Liebig in that the combustion tube was heated by a spirit lamp, rather than by burning charcoal. The oxygen produced by the potassium chlorate was used to regenerate cupric oxide. Liebig, who was never on very good terms with Mitscherlich, pronounced the apparatus to be of little value.

Mitscherlich’s early interest in geology and mineralogy continued throughout his life. He was particularly concerned with the production of artificial minerals through the fusion of silica with various metallic oxides, and achieved some valuable results in such experiments. In his last years he made a number of journeys to the most important European volcanoes to gather data toward a general theory of volcanoes, the subject of his last, posthumously published, articles. (It must be noted, however, that his work in volcanology produced little of significant value.)

Mitscherlich was perhaps most successful as a writer of textbooks. His Lehrbuch der Chemie was first published in 1829; by 1847 it had had four new editions in German, as well as two editions in French and one in English. The work contained Mitscherlich’s lectures on all aspects of pure and applied chemistry, as well as a considerable amount of material on physics, all illustrated with a number of beautiful woodcuts. The lectures themselves are characterized by their exemplary clarity and ingenious experiments; the book was highly praised by Mitscherlich’s contemporaries, including Berzelius and Liebig. As a teacher, Mitscherlich was aware that his students needed practical instruction; although his efforts to this end were in fact little more than perfunctory, he did take them on visits to factories.

Mitscherlich married and had five children, of whom the youngest, Alexander, also became a chemist. It was he, rather than his father, who discovered the Mitscherlich process for extracting cellulose from wood through boiling with calcium bisulfite, upon which discovery the German cellulose industry was based.

BIBLIOGRAPHY

I. Original Works. A more complete list of Mitscherlich’s writings can be found in Poggendorff, II, cols. 160–162. His major book is Lehrbuch der Chemie, 2 vols. (1829; 4th ed., 1847), also trans, into French (1835) and into English by S. L. Hammick as Practical Experimental Chemistry Adapted to Arts and Manufactures (1838). Many of his shorter writings were brought together as Gesammelte Schriften von Eilhard Mitscherlich. Lebensbild, Briefwechsel und Abhandlungen (1896)

II. Secondary Literature. See G. Bugge,“Mitscherlich,” in Das Buch der Grossen Chemiker, I (Berlin, 1929);F. Heinrich,“Zur Erinnerung an Eilhard Mitscherlich,” in Chemische Zeitung, 37 (1913), 1369, 1398; H. Kopp, Geschichte der Chemie, I (Brunswick, 1843), 414; and Die Entwicklung der Chemie in der neueren Zeit (Munich, 1873), p. 417; A. Mitscherlich, in E. Mitscherlichs Gesammelte Schriften (Berlin, 1896); J. R. Partington, A History of Chemistry, IV (London, 1964); W. Prandtl, Deutsche Chemiker (Weinheim, 1956); G. Rose,“Eilhard Mitscherlich,” in Zeitschrift der Deutschen geologischen Gesellschaft, 16 (1864), 21; and Williamson,“Eilhard Mitscherlich” in Journal of the Chemical Society, 17 (1864), 440.

F. SzabadvÁry