ScienceWeek

EILHARDT MITSCHERLICH (1794-1863) AND THE NATURE OF CRYSTALS

At the beginning of the 19th century, the prevailing view of the nature of crystalline matter was that the fundamental entities were tiny polyhedra ("integral molecules"), which could not be further subdivided. This view was the conception of Rene-Just Hauy (1743-1822), an eminent chemist and mineralogist of the time, so eminent that the view, although completely erroneous, was considered an "ipse dixit", a central dogma of mineralogy and crystallography. Since in science the destruction of central dogmas, often both common and pervasive, is of singular importance to conceptual progress, the details of this particular destruction make an interesting story.

For the most part, the destruction of the "integral molecule" view of crystalline substances was the result of the work of a young chemist and amateur mineralogist, Eilhardt Mitscherlich (1794-1863). Mitscherlich began studying crystals at the age of 24 in 1818, published what is now called "Mitscherlich's law of isomorphism" in 1821, was appointed to the Berlin Academy of Sciences in 1821 and to a chair in Berlin in 1825, and then went on to accomplish the first synthesis of benzene in 1834, and the confirmation, in 1842, of yeast as a microorganism. An interesting sidelight is that Mitscherlich began as a student of Oriental Languages at Heidelberg, and then apparently switched to medicine and chemistry only because the fall of Napoleon precluded continuing his studies in Paris.

Mitscherlich's "law of isomorphism" is simply stated: Substances that crystallize in isomorphous forms (i.e., have identical crystalline forms and form mixed crystals) have similar chemical compositions. The law can be used to indicate the formulae of compounds. For example, the fact that chromium oxide is isomorphous with Fe(sub2)O(sub3) and Al(sub2)O(sub3) implies that the formula of chromium oxide is Cr(sub2)O(sub3). This law became one of the central considerations in the atomic weight determinations of the great chemist J.J. Berzelius (1779-1848), who accurately determined more than 2000 relative atomic and molecular masses, devised the system of chemical symbols and formulae now in use, proposed oxygen as the reference standard for atomic weights, and who was the foremost proponent of the atomist theory of that time.

The following points are made by Robert W. Cahn (Nature 12 Aug 99 400:625):

1) The great crystallographer R-J. Hauy had convinced the mineralogy world of his time that crystals could not be understood in terms of the regular stacking of spherical atoms, and therefore there were no such entities as spherical atoms. So it was not surprising that Hauy attacked with sustained venom the work of the young Mitscherlich. It was Berzelius who took Mitscherlich under his wing and who persuaded the German authorities to appoint Mitscherlich to a chair in Berlin. Mitscherlich's isomorphism discovery was incompatible with Hauy's ideas about the fundamental entities in crystals. In his 1821 paper, Mitscherlich also recognized the existence of polymorphs (quite different crystal forms) of the same substance, and polymorphism was also incompatible with Hauy's idea of "integral molecules". Almost simultaneously with Mitscherlich's discovery of isomorphism, was the discovery by F. Beudant in France and W. Wollaston in England that isomorphous species can form a series of solid solutions with each other, the mixed crystals ("Mischkristalle"). This was also incompatible with Hauy's "integral molecules".

2) Mitscherlich and Berzelius, respectful of Hauy as a great experimental scientist, attempted for years to persuade Hauy of the validity of their findings. Hauy, however, was unmovable, and Berzelius finally decided that "one ought not to expect that a grey-haired scientist close to the end of an honorable life should give up a theory he erroneously considered to be the most important of his discoveries; this is perhaps too much to morally demand of any man." [*Note #1].

3) Berzelius declared Mitscherlich's discovery and interpretation of isomorphism, and the P-L. Dulong and A-T. Petit discovery that the specific heats of solids vary inversely with their presumed atomic weights, as the most important empirical proofs of the atomic hypothesis at that time. Yet for another century there was widespread skepticism about atoms -- until Jean Perrin's work on Brownian motion in 1926 produced the crucial experimental evidence that finally established the atomic nature of matter. [*Note #2].

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Notes:

Note #1: Berzelius, one of the most influential chemists of his era, had some other words about aging scientists: "God knows what happens to your time once you have begun to get old. You are busy all the time, you do important things, you work, and yet when you sum it all up the result is nothing."

Note #2: Both Mitscherlich and Berzelius, in collaboration, had much to do with yeast and fermentation, and it is an irony that whereas they were right on the mark with respect to the atomist theory of crystals, they were completely wrong in their analysis of the role of yeast in fermentation. Berzelius completely rejected the idea that fermentation required the intervention of a living organism. And although Mitscherlich recognized that yeasts were living organisms, he believed fermentation occurred only on the surface of yeast, the yeast cells acting only by contact, supporting the view of Berzelius that fermentation involved a "catalytic force". The eminent chemist Justus Liebig (1803-1873) also refused to believe that living yeast had anything to do with fermentation. It was Louis Pasteur (1822-1895), who started work on yeast fermentation in 1855, who began the modern understanding of the process.