Keesom, Willem Hendrik

(b. Texel, Netherlands, 21 June 1876; d. Oegstgeest, Netherlands, 24 March 1956)

Physics.

The son of a farmer, Keesom studied physics at Amsterdam University, where J. D. van der Waals was one of his teachers. An excellent student, he received the doctorate with a thesis on the isotherms of oxygen and carbon dioxide mixtures in 1904. He became a close collaborator of Kamerlingh Onnes at the University of Leiden, assisting him, for instance, in the liquefaction of helium (1908) and in the writing of a comprehensive treatise on the equation of state for the Enzyklopädie der mathematischen Wissenschaften (1912).

In 1917 Keesom became a teacher, and the next year a professor, of physics at the veterinary school in Utrecht (later incorporated into the university). In 1923 he returned to Leiden to occupy one of the two chairs of experimental physics, the other being occupied by W. J. de Haas in 1924.

At Utrecht, Keesom succeeded in finding a connection between the X-ray diffraction pattern and the intermolecular distance in liquids. This is a good example of his tendency to combine theoretical and experimental methods in order to clarify the picture of liquids and compressed gases without mathematical sophistications. (The introduction of the radial distribution function for the intermolecular distance is due to others.)

As a director of the Kamerlingh Onnes laboratory at Leiden, Keesom continued the tradition of lowtemperature research, especially (although by no means exclusively) on helium. He was the first to solidify it, by applying external pressure to overcome “repulsion” between the atoms, which in all other crystals is overcome by mutual attraction. Since the saturated vapor pressure is much lower than the pressure needed for crystallization, there is no triple point and the liquid state extends down to absolute zero. (See Figure 1.)

In the course of further work in 1927–1933 it gradually became clear that at very low temperatures the liquid (II) differed fundamentally in its behavior from ordinary liquids and from high-temperature liquid helium (I). The two liquid states are separated by the λ line indicated in Figure 1. At this line, for instance, the heat capacity changes abruptly (“second-order transition”) and the internal friction disappears, leading to “superfluidity” (more or less like the superconductivity in a number of metals). Theoretical explanations, founded on quantum statistics and the third law of thermodynamics, were presented later. The essence is that below a certain temperature both the liquid and the crystal become so fixed in the ground state that entropy is practically zero, and no further changes occur in it. Such a “thermally degenerate” system no longer behaves as an (irreversible) thermal system but as a (reversible) mechanical system.

In 1942 Keesom wrote a standard work on helium, and in 1945 he retired. Most of his work is to be found in Physica (The Hague), Proceedings of the Section of Sciences of the Royal Netherlands Academy of Sciences, and Communications from the Kamerlingh Onnes Laboratory of the University of Leiden. He received foreign as well as national honors and was elected a member of the Royal Netherlands Academy in 1924.

BIBLIOGRAPHY

Keesom’s writings include Isothermen van mengsels van zuurstof en koolzuur (Amsterdam, 1904), his thesis; “Die Zustandsgleichung,” in Enzyklopädie der mathematischen Wissenschaften, V, 1 (1912), 615–945, written with Kamerlingh Onnes; and Helium (Amsterdam, 1942.)

A biography is in Jaarboek van de K. Akademie van wetenschappen gevestigd te Amsterdam (1956–1957), p. 225.

J. A. Prins