EILHARD MITSCHERLICH (1794-1863)

ON DECEMBER 1, 1894, a t the unveiling of the statue of Eilhard Mitscherlich before the University of Berlin, Wilhelm Ostwald, the speaker of the day, stated: "Mitscherlich m-as a prince in the realm of the mind, and a hero in the battle for the light." Nonetheless, few, except specialists, have a knowledge of the work of this scientist and its permanent value; even chemists vaguely remember little except his name. He himself was largely responsible for this semioblivion, because he had a pronounced shyness about publicizing his dis- coveries. When his "Collected Works" were published by his son1 it was necessary to preface the first, and probably the greatest of his works, with the remark: "This paper has never before appeared in German." For 75 years, only excerpts of it were h o w n in Ger- many, and only then, for the first time, was there avail- able a complete translation in his native language of the original (1821) Swedish text.2 This tendency against publishing became more pronounced from year to year so that Ostwald could truthfully declare: "Toward the last, the titles of the papers delivered by him before the ~ c ~ d e m ~ became the only trace which he allowed the outside world to obtain of his indefatigable a~t iv i ty ."~

Eilhard Mitscherlich was born on Januarv 7. 1794. at Neuende near Jever in Oldenburg. His father, the kil- lage pastor, wished to provide the best possible educa- tion for his bright son so he sent him to bever to live in the home of the historian, Friedrich Christoph Schlosser. At the latter's advice. Mitscherlich s~ecialized in orien- tal languages and history. ow ever, &ter securing his doctorate at Gottingen, with a dissertation on a Persian historical work, he abandoned this field and turned to

His Life and Achievements

RUDOLF WINDERLICH Oldenburg i. Oldenburg, Germany (Translofed by Rolph E. Oesper, University of Cincinnatz)

medicine. This led him to chemistry. In 1818 he went to Berlin and worked in the laboratory of the botanist H. F. Link. He chose to study phosphates and arsen- ates and was soon forcefully impressed by the similarity in the crystal forms of the corresponding salts. He therefore took lessons in crystallography from his friend Gustav Rose, the mineralogist at the University, and with the newly acquired hovledge discovered the law concerning "the relation which exists between rrys- talline form and chemical composition." In his words, "the same number of atoms combined in the same man- ner produce the same crystalline form; the crystalline

"Gesammelte Schriften von Eilhard Mitscherlich. Lebcns- bild, Briefweschsel und Abhandlungen." Published by Alexander Mit,soherlirh. Berlin. 1896. fi7X nares. ~ ~ . -~~

This translation, &ted by P. v. Groth, is available as No. 04 of "Ostwsld's Klassiker," Leipsic, 1894.

Mitscherlieh promised to write a Supplement on crystsllag- raphy for the Bersclius "Lehrbuch dcr Chemie," and the latter designated this promised contribution as something extraor- dinary. But he waited for it in vain, even though a, few of his many letters were snsuwed with a solemn promise to fulfill the obligation a t once. Finally, the "Lehrbuch" was published without the Mitscherlich supplement. The farce was repeated with the next edition. I n 1840, when Mitscherlich visited Berzelius, and again offered to make his promise good, he re- ceived the reply: "We are no longer negotiating on this point. At present, I am limiting my demands that a t least before you die, you yourself publish your crystallographic studies!' ~itesherlich's rath-

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form does not depend on the nature of the atoms, hut solely on their number and mode of combination." The announcement of this law of isomorphism (isos = like and morphe = form) aroused considerable atten- tion, and it made a particular impression on Berzelius during his 1819 stay in B e r h 4

Berzelius necessarily saw that this law was an iuvalu- able acquisition to chemistry, because it could be a de- cisive factor in determining the choice between two hotly contested possibilities. R. J. Haiiy, the founder of geometrical crystallography, had championed the view that crystal form determines the chemical struc- ture of a mineral, whereas Berzelius had taken as the basis for his mineral svstem the assumution that the crystal form is dependent on the chemical nature of the mineral. No one then could possibly have foreseed the full value of the law of isomorphism, because its real import was not revealed until the close of the 19th cen- turv. Since 1870 P. v. Groth had been enlralred in an attempt to elucidate the relation between crisial stmc- tnre and chemical stmcture. He started with Mit- scherlich's thought that crystallographically equal di- rections a+e also physically equal, and accordmgly, from 1895 on, regarded the chemical atoms of a crystal- line material as points of space lattices whose constants ~ ~ i ~ t i ~ . [MU~.I) in the ~ ~ d i t ~ ~ i ~ ~ of the ~ ~ ~ ~ . ~ i ~ ~

in Jever are determined by thekind, number, and position of the atoms. The space lattice of a crystal remains (almost) unchanged when an element of like valence is incorporated in place of one of the original elements. These bold postulates were brilliantly verified in 1912 by Max von Laue. By means of the interference patterns produced by the diffraction of X-rays on crystal lat- tices, he proved the manner in which the structural units are arranged. At the same time, he established the wave nature of X-rays, and provided scientists with a new tool for penetrating into the structure of ma- terials.

Of course, Berzelius could not anticipate any such radical effect of the law of isomorphism on present-day views of the fine structure of matter; for him the new law was valuable enough when it fvas applied in decid- ing the choice between atomic weights#and in demon- strating the correctness of his mineral system. Conse- quently he proposed to v. Altenstein, the Pmssian Minister of Education, that the promising young scien- tist be entrusted with the chemistry professorship at the University of Berlin. The chair had been vacant since the death (January 1, 1817) of Martin Heinrich Klap- roth, the distinguished analyst and discoverer of ura- nium, zirconium, titanium, and cerium. I t was offered to Berzelius who declined for patriotic reasons. After due deliberation v. Altenstein decided that Mitscherlich would need to he thoroughly trained for the post by Berzelius through a stay in the great chemist's private laboratory. Mitscherlich was in Stockholm, at the government's expense, for two years (December, 1819 to November, 1821). He worked so independently and

For an excellent review of Mitscherlich's work on isomorphism and dimorphism see Ida Freund, "A Study of Chemical Compo- sition," Cambridge, 1904, Chapter XV.

diligently that Berzelius was able to report to the Minister: "He has been more my student collaborator than my pupil." Such laudatory statements by Ber- zelius brought Mitscherlich not only election to the Pmssian Academy of Sciences, despite the opposition of Weiss (mineralogist) and Seebeck (physicist), but also this same year (1822) an appointment as extraordinary professor at the university. Three years later he was promoted to the,full professorship. His letter of thanks to Berzelius reveals not only his joy for the 800 talers, the annual salary, but still more the anticipation of hav- ing a good laboratory with adequate funds for its opera- tion. Freedom from financial worries was highly im- portant to him. He made this quite clear to Berzelius (letter of July 18, 1826) when he told his fatherly friend that he had taken a highly cultured wife of considerable means and "would never be forced to make a milch cow of science." The marriage turned out exceedingly

5Mit.9eherlich's handwriting was outstandingly poor. Bereeiius mote to him (February 2, 1828): "Your latest letter, provided with your variety of logiblo script, seemingly was sent to the Royal Office of Foreign Mail, from which I receive many of my letters from abroad. The clerk read the address a8 "Mademoiselle Hochschild," who frequently gets letters from her brother, who is Minister in Copenhagen, and accordingly t,he letter was sent to her. Fortunately, she is the sister-in-law of my good friend Poppius. One evening a t his home, they were talking about the stray letter, whose address could not be read by even President Poppius. They asked me to try. I recog- nized your writing s t once, and to the horror of everybody broke the letter open. They thought I had done this through a mis- understanding. How they laughed when I deciphered for them my name in the address. Consequently, from now on, have your wife write the addresses."

360 JOURNAL OF CHEMICAL EDUCATION

Closely related to his law of isomorphism, was his second (1821) great discovery, dimorphism. He found that "one and the same proportions, can assume two different forms." Aragonite, which he had found to be isomorphous with the naturally occurring lead carbo- nate (cerussite), is a variety of calcium carbonate, dif- fering in crystal form from the ordinary calcite, with which it is dimorphous. After Mitscherlich had recog- nized that the crystals of sulfur obtained from its solu- tions differ from those separating from its melts, he (1823) modified his statement to read: "that one and the same material, be it simple or compound, can as- sume two different crystalline forms." The fact did not escape his notice that heat is liberated when monoclinic sulfur changes into the rhombic form.

Subsequently, Mitscherlich expended much labor and care on the mental and experimental solution of various problems connected with crystal systems. He wrote to Berselius (June 14, 1823) that his objective was: "to arrive a t definite conclusions concerning the distribu- tion of matter itself in crystalline bodies, and also con- cerning the relation of heat and matter." He showed "that the expansion of crystals is exactly in the direction of the crystallographic axes, and, since the latter are connected with the optical axes, they stand in a certain

relation to these also, namely, that the shorter exes ex- pand relatively more than the longer." His inquiries into the regularity, magnitude, and cause of optical effects, especially double refraction, under the influence of heat, were eventually extended also to noncrystalline materials.

In 1827 he constructed a new type of goniometer but did not publish anything about it until 16 years later. With its aid he increased the accuracy of reading the re- flected angle to several seconds, as contrasted with the five minutes possible with the Wollaston instrument.

Extensive studies of the constituents of the slags in the dumps of Swedish copper smelters gave rise to suc- cessful attempts to produce minerals artificially. He was.thus able to refute the general belief "that chemis- try could indeed decompose minerals into their constitu- ents, but since forces of Nature were active at the for- mation of the natural compounds, they could never be reproduced artificially." In connection mith these stud- ies he developed a "theory of the smelting ont of cop- per," in which his chief objective was "to demonstrate clearly that in this process silica behaves entirely as an . - acid."

Mitscherlich's intense interest in minerals and crys- tals persisted throughout his life. He frequently de- voted himself to eeoloxical ~roblems, to whose solution he made notable contributions: the formation of ore veins; the source of mineral springs; volcanic processes; the metamorphosis of rocks by heat; etc. Other areas of inorganic chemistry were enriched by him. He dis- covered selenic acid, developed a simple method of de- tecting free phosphorus, and cleared up the nature of permanganic acid. In his "Jahrbuch," for 1834 Ber- selius commented admiringly th.?t the last of these stud- ies had yielded "the unexpected result that this metal has two acids, of which one, manganic acid, consists of 1 atom of manganese and 3 atoms of oxygen, while the other, which Mitscherlich calls permanganic acid, con- sists of 2 atoms of manganese and 7 atoms of ~xygen."~ The basic importance of the isomorphism of the perchlo- rates and permanganates was stressed by Mitscherlich. He recognized the prime importance of the vapor densi- ties of gasified substances. By numerous accurate analyses he extended the conclusion reached by Dumas "that sulfur gas (vapor) contains three times as many atoms as an equal volume of oxygen." This presented the physicists and chemists mith a troublesome situa- tion, because at that stage they were not able to dis- tinguish clearly between atoms and molecules and yet they did not wish to abandon Avogadro's hypothesis that under like conditions of temperature and pressure equal volumes of gases contain equal numbers of "atoms." This difficulty finally compelled the scien- tists to make a sharp differentiation between atom and molecule.

Mitscherlich's contributions to organic chemistry were no less important than his achievements in the in- organic field. His investigations (1834) on benzene and

6 According to the then prevailing oxygen theory of acids, MnOI and MnrO,, i. e. , anhydrides of H2Mn0, snd HMnO,.

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its compounds revealed the relation between "bicar- -i::::r., .?: .:.:-:-. .: .. . buret of hydrogen" (discovered in 1825by Faraday and studied by him with astounding exactness) and the acid obtained from benzoin resin. Dry distillation of cal- cium benzoate yielded an easily volatile liquid, which ... Mitscherlich recognized as identical with Faraday's hydrocarbon. Because of its origin, Mitscherlich pro- :::

posed the name "benzin." Liebig objected and changed it to "benzol," the form that is still used in German. Mitscherlich was the first to prepare nitro- benzene, azobenzene, benzenesulfonic acid, etc., and so, in a sense, laid the foundations of the dyestuff industry. His theoretical conclusions concerning the benzene de- rivative wrr~' nor approved by Jk.rrt.liu;, nnd i l l .Junr, 1835. 3I i t4fdivl1 wrote: "Tl~i.; work on lrenzin rml it. publication have lost all pleasure for me, and I wish I had never started it, since it is the first time that you do

decomposition~and~ombination through contact to a no longer disappearing, indispensable concept, which sheds i: lieht over manv thines" (letter of October 31. 18311. 8' ~ i t a l y t i c consic?eratio& la'tkr guided Mitscherlich in hk & ,,., studies of yeast fermentation and microorganisms.

Eilhard Mitscherlich must he classed among the lead- ing investigators. Beyond this, he possessed and freely ,,,v,,,,, , .,,..,:%,,,.,. , .. ,,,, . ., , . , , . ,,,. , . , .::

u t l I I t l i t i k i n I n t y r n i - nnmcly 111t. itllility to trnrh wll , Lotlt by mouth and by ..:. . t i ~ n w I 1rt1rl11111 1 h n i I l e e r : I ' ' i ? critical Berzelius that he wished to have it translated into Swedish. "You have a great talent, dear Mitscher- lich, of being interesting, terse, and instructive" (letter of August 27, 1832). Mitscherlich repeatedly stressed his love for teaching. He enlivened and reinforced his lectures with appropriate demonstrations, even though scarcity of the necessary equipment caused him cou- siderable difficulty. He complain,ed to Berzelius (July 14, 1826): "Of all that I anticipated here and had a right to expect from the Ministry, Phave obtained nothing." The promised funds for the purchase of in- struments were not granted, with the consequence that he had to dip into his own pocket. At that time the men in authority in Germany, especially in Berlin, were indifferent or even inimical to the natural sciences. When Ostwald stated that "Mitscherlich was a hero in the battle for the light," he was referring to the struggle against the natural philosophers, concerning whom

' Mitseherlich's papers on "Benein" are reproduced as No. 98 (editcd hy J. Wislicenus) of "Ostwald's Xlassiker," Leipsic, 1898.

A Hitherto Unpublished Likeness of Mitscherlich

Mitscherlich stated to Berzelius (May 30, 1827): "they declare war on the healthy human intelligence." The battle against these obscurantists was difficult, because the authorities "had set aside an annual sum of 4000 talers to support their activities." . On the other hand, the construction of a teaching labdratory vas not au- thorized. In an illuminating letter (May 20, 1837) he told Berzelius: "The installation of my collections and apparatus consumes practically all of the time which I can devote to scientific activities. In order to main- tain the building, and to spare myself other annoyances, I have taken upon myself a part of the expense of fitting up the laboratory and all the cost of the scientific fur- nishings."

A man who thought thus, acted thus, and who made such wide-reaching discoveries, deserves not to he for- gotten. He died on August 28, 1863.