Ornstein, Leonard Salomon

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(b. Nijmegen, Netherlands, 12 November 1880; d. Utrecht, Netherlands, 20 May 1941)


At the suggestion of his teacher, H. A. Lorentz, Ornstein took as the subject of his doctoral dissertation (Leiden, 1908) the application of J. W. Gibbs’s general methods in statistical mechanics (published in 1902) to various concrete problems in molecular theory, including the determination of the equation of state of a nonideal gas.

Ornstein generalized Boltzmann’s definition of the probability of a macroscopically definable state of a gas to include systems in which the interactions between the molecules could no longer be neglected. He then calculated the probability that in a homogeneous system a given spatial distribution of density arises. This was the basis of virtually all of Ornstein’s subsequent work as a theoretical physicist, and, in particular, of his collaboration with F. Zernike from 1914 to 1917 on the formation of molecular “swarms.” They were thus able to add an important correction to the Einstein-Smoluchowski theory of opalescence of a fluid at its critical point, taking account of the correlation of the density fluctuations in volume elements separated by distances comparable to the wavelength of the scattered light.

From 1909 to 1914 Ornstein was lecturer in theoretical physics at the University of Groningen, where Hermanus Haga tried unsuccessfully to interest him in the experimental work of the physical institute. Late in 1914 Ornstein succeeded Peter Debye in the chair of theoretical physics at the University of Utrecht, where W. H. Julius provided him with a room in the physical institute. There, largely through contact with and in collaboration with W. J. H. Moll, Ornstein began to experiment on liquid crystals, regarded as an exemplification of his theory of molecular swarms.

It was, however, only in 1920, when Ornstein became acting director of the Utrecht physical institute (substituting for the ailing Julius, whom he succeeded officially in 1925), that his work shifted decisively into experiment, and his organizational talent began to unfold. In the preceding dozen years he had published about fifty papers; in the following twenty years over two hundred bore his own name, and almost five hundred additional papers were published from his institute, which underwent three substantial enlargements in this period. Among these publications were eighty-eight doctoral dissertations: about one per year in the early 1920’s, increasing to three per year in the middle and late 1920’s, and reaching a of about seven per year in the mid-1930’s.

With great acumen Ornstein had recognized around 1918 that the advance of atomic physics would require that the exceedingly precise measurements of spectral frequencies be supplemented by quantitative measurements of intensities. Immediately upon taking charge of the Utrecht institute, which had had under Julius a strong tradition in spectroscopy and radiation intensity measurements in the service of solar physics, Ornstein charted an ambitious, coordinated program for the systematic investigation of techniques of intensity measurement—especially the blackening of photographic plates—for exploitation in the service of atomic physics. Although generally regarded by his colleagues as a difficult man, within his own institute “the boss” maintained a harmonious collaboration of all the staff upon his tightly integrated program.

In the period 1923–1925 the publications from his institute on the simple integral relations (“sum rules”) between the intensities of spectral lines originating in transitions out of or into a complex spectral term, and also between the intensities of the several components into which a spectral line is split in a magnetic field, were an important stimulus and a unique source of data for theoretical atomic physicists seeking a quantum mechanics via Bohr’s correspondence principle.

In the late 1920’s and early 1930’s, after the development of a quantum mechanics whose arbitrary character he found unsatisfying, Ornstein and his co-workers turned these same techniques—for which his institute was then world-famous—to a wide variety of problems: the investigation of electric arcs; the Raman effect; liquid crystals; the determination of isotopic ratios; and purely technical problems of illumination engineering and light bulb lifetimes. From the mid-1920’s Ornstein, although an active Zionist, cultivated the closest relations with Dutch industry, providing space in his institute and his personal supervision for technical chores in electrical and heat, as well as light, engineering. “The physicist in society” was one of his watchwords.

In the late 1930’s, on the proposal and with the support of the Rockefeller Foundation, Ornstein moved into biophysics, concentrating upon bacterial luminescence and photosynthesis. Far more would have emerged from this effort had not the war and German occupation intervened. In November 1940 Ornstein was forbidden entrance to his laboratory; he died six months later.


A complete annotated bibliography of Ornstein’s publications and the publications from his institute to 1933 is given in L. S. Ornstein; A Survey of His Work From 1908 to 1933 Dedicated to Him by His Fellow-Workers and Pupils (Utrecht, 1933), reviewed by W. Gerlach in Naturwissenschaften, 22 (1934), 111–112. The text of this publication, pp. 1–86, gives semi popular expositions of the various areas of research conducted by Ornstein and his institute. The publications of the “Biophysical Group Utrecht-Delft,” 1936–1940, are listed in A. F. Kamp et al., eds., Albert Jan Kluyver. His Life and Work (Amsterdam, 1959), 548–553. Ornstein’s style in the direction of research is also described, pp. 30–31.

Valuable obituary notices are H. A. Kramers, “Levensbericht van L. S. Ornstein,“ in Jaarboek van het Koninkl. Akademie van Wetenschappen, Amsterdam (1940–1941), 225–231; F. Zernike, “Ornsteins Levenswerk,” 1 inNederlandsch Tijdschrift voor Naturkunde, 8 (1941), 253–265; and R. C. Mason, “Leonard Salomon Ornstein,” in Science, 102 (1945), 638–639.

Additional biographical data is given in Wie is dat, 4th ed. (The Hague, 1938), pp. 314–315. Some twentyfive letters by Ornstein are listed in T. S. Kuhn et al., Sources for History of Quantum Physics (Philadelphia, 1967), 71–72.

Paul Forman