Mansel Davies
Edward Davies Chemical Laboratory University College of Wales Aberystwyth, U.K.
II
Peter 1. W. Debye (18844966)
Peter Josephus Wilhelmus Dehye (or Debije, as the name is spelled in Dutch) was horn on March 24,1884, a t lLlaastricht in The Netherlands and died on November 2, 1966, at Ithaca, N. Y. His life's work amounts to one of the greatest contributions made by an individual to the field of physicochemical studies: even a cursory summary of the items to which he made major contributions, such as is attempted here, becomes a wide ranging catalog of themes in molecular science. I t seems best to present them in chronological order. Maastricht is in the southeast Limburg corner of The Netherlands, on the Belgian border, and only some twenty miles from Aachen (Aix-la-Chapelle) in Germany. This location doubtless promoted the facility with European languages which Debye showed in many scientific conferences. His school education did not take him outside Maastricht, and his particular ability in mathematics and physical science did not result in his going further than Aachen for the equivalent of university training: he completed his degree course in electrical engineering at the Technische Hochschule there. Especially fortunate was the presence a t Aachen of physicists Max Wien and Arnold Sommerfeld. The latter became one of the leading theoretical physicists of his day hut, like many prominent scientists, he was inclined to feel, as Davy did of Faraday, that his greatest discovery was that of his pupil, Debye. It was wit>h Sommerfeld that Debye remained after graduation at Aachen, making a theoretical study of Foucault currents: this led to his first published work (1907), a paper which shows considerable mathematical competence in a young man of twenty-three years. Accordingly, it is not surprising that when Sommerfeld was elected to the professorship of theoretical physics a t the University of Munich he took Debye with him as (initially) his only assistant. Sommerfeld was succeeding to a chair which had been occupied by one of the greatest theoreticiansLudwig Boltzmann. This move launched Debye into the mainstream of scientific research and his promotions and progress were subsequently rapid. At Munich, Rontgen was the professor of experimental physics and maintained an active though small research group, of the highest standards, frequently concerned with X-ray problems. Joff6 and Pringsheim had recently completed courses with Rontgen. Despite his chair being that of theoretical physics, Sommerfeld had insisted that he must have facilities for experimental studies, if only as a means of promoting close contact with physical reality. This insistence and the astonishing success it was very soon to achieve almost certainly made a lasting impression on Debye. At Munich he completed his doctoral thesis on the radiation pressure experienced by spherical particles of varied electrical
properties. This work showed not only notable mathematical skill but an ability to make constructive developments in mathematical methods. Another publication of this early period, with Hondros [Ann.Physili, 32,465 (1910)l was some forty years ahead of its time in practical interest: it dealt with radiation problems of significance to radar and waveguide systems. The introduction to radiation and diffraction problems which Debye acquired in his early work with Sommerfeld was to form one of the main recurrent themes of his scientific career. Furthermore, a study closely related to Debye's was taken up by another pupil of Sommerfeld's. Paul Ewald's thesis problem was "to find the optical properties of an anisotropic arrangement of isotropic resonators-and was commenced in 1910. Already in 1909 Laue, although he has been described as Planck's favorite disciple, came from Berlin to join Sommerfeld as an assistant. It was Ewald's thesis in Laue's hands which led to the discovery at RIunich of X-ray diffraction in the experiment (June, 1912) by Friedrich and Ihipping. This settled the nature of X-rays and opened an enormous field of atomic and molecular structural research. On the status acquired by Dehye in these early years at Munich, Ewald (1) may he quoted Needless to say t o those who know of his later development, Debye was, even then, an outstanding physicist, mathematicisn, and helpful friend. He was, not less than Sornmerfeld himself, a centre for the senior students and graduates frequenting the Institute and the Physics Colloquium.
Peter Debye lecturing o t Cornell University.
Provided b y Cornell Photo
Science.
Volume 45, Number
7,July 1968 / 467
But Debye had left Munich in 1911. I n that year Einstein changed his professorship at Zurich University for one a t Prague: his successor a t Zurich in the chair of theoretical physics was Debye and, one can add, he in turn was succeeded by h u e . At Zurich, Debye took up not only Einstein's chair but also one of Einstein's many contributions to molecular dynamics. I n 1907 Einstein published a paper in which hc used the Planck quantum-concept to evaluate the specific heat of a crystalline solid. The essential novelty in the model was the assumption of a unique vibrational frequency for the atoms forming the latt,ice and a fixed energy (hv) associated with this lattice mode. I t was clcar that the Einstein treatment was correct in essence: it led to the Dulong-Petit law at high temperatures (atomic weight X specific heat = G cal [g atom]-' = 3R) and gave the proper general form of the decrease of atomic heat as the temperature fell. Nevertheless, significant deviations were established, especially a t low temperatures, where the conditions on approaching absolute zero were of significance to a number of questions of fundamental importance. Debye's contribution was to insist that a whole range of lattice frequencies would be found even for a simple atomic solid. The distribution density of these on the frequency scale he deduced and inserted this function in the calculation of the specific heat. The result was what can perhaps be listed as the first of the Debye equations:
Here x = (hv/lcT) and u, is the limiting maximum frequency of the lattice vibrations. Not only does this equation provide an excellent rcpresentatiou of the specific heat function but it has remained for fifty years the starting point from which further refinements of this problem have proceeded. There are, of course, very many other relations constantly referred to as "the Debye equation": only the context tells the reader which is involved. The specific heat equation incorporates two features of special significance: the "cube law" for the specific heats at the lowest temperatures; i.e., C , = aT3a.; T OoJ
