b. Konstanz, Germany, 13 January 1855;d. Karlsruhe, Germany, 17 June 1922)
Lehmann discovered liquid crystals; substances which behave mechanically as liquids but display many of the optical properties of crystalline solids.
Lehmann’s father had been a professor of science and mathematics at the Gymnasium in Freiburg im Breisgau and was particularly interested in the mathematical manifestations of organic nature. He sought to develop mathematical formulas for such phenomena as the geometric forms of the leaves of plants, and his interests stimulated the scientific bent of his son. Lehmann received his doctorate from the University of Strasbourg in 1876 and taught in secondary schools at Freiburg im Breisgau and Mulhouse from 1876 to 1883. In the latter year he became Dozent and in 1885 associate professor of physics at the Technische Hochschule at Aachen; in 1888 he was named associate professor at the Technische Hochschule at Dresden; and in 1889 he succeeded Heinrich Hertz as professor of physics at the Technische Hochschule at Karlsruhe. He remained in this post until his death.
Lehmann’s early scientific interest and experimentation were concerned with electric discharges in rarefied gases, but he soon turned his attention to the study of the fine structure of matter as revealed under the microscope. His first major work describing his studies was Molekularphysik, mit besonderer Berüchtsichtigung mikroskopischer Untersuchung und Anleitung zu solchen (1888-1889).
In 1888 the Austrian botanist Friedrich Reinitzer noticed that the solid compound cholesteryl benzoate seemed to have two distinct melting points, becoming a cloudy liquid at 145°C. and turning clear at 179°C. Reinitzer’s observation came to Lehmann’s attention, and he immediately began research on this and other organic substances displaying the same property. He determined in 1889 that the cloudy intermediate phase contained areas that possessed a molecular structure similar to that of solid crystals, and he called this phase “liquid crystal.” In 1922 G. Friedel suggested the term “mesomorphic” to include Lehmann’s liquid crystals as well as any state of matter intermediate between the amorphous and crystalline states; however, the term “liquid crystal” is still employed.
Lehmann published his results in two major works: Flüssige Krystalle (1904) and Die neue Welt der flüssigen Krystalle und deren Bedeutung für Physik, Chemie, Technik und Biologie (1911). His results astonished and perplexed the scientific world, since he demonstrated that the fluidity of many organic substances is not only equal to or greater than water but that they also display the double refracting properties of crystals, some being twice as birefringent as calcite.
Lehmann’s work stimulated much research in this area as well as studies to find technical applications of the phenomenon, and these efforts are still continuing.
I. Original Works. Lehmann’s chief publications are Molekularphysik, mit besonderer Berüchtsichtigung mikroskopischer Untersuchung und Anleitung zu solchen,, 2 vols. (Leipzig, 1888-1889); Krystallanalyse, oder die chemische Analyse durch Beobachtung der Krystallbindung mit Hülfe des Mikroskops (Leipzig, 1891); Elektrizität und Licht (Brunswick, 1895); Die elektrische Lichterschein (Halle, 1898); Flüssige Krystalle (Leipzig, 1904); and Die neue Welt der Flüssigen Krystalle und deren Bedeutung für Physik, Chemie, Technik und Biologie (Leipzig, 1911). In addition, he published approximately 120 articles in scientific journals.
II. Secondary Works. Obituary notices are in Physikalische Zeitschrift, 24 (1923), 289-291 and Zeutscgruft für technische Physik,4 (1923), 1.
John G. Burke