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PETER JOSEPH WILHELM DEBYE March 24, 1884-November 2, 1966 BY J. W. WILLIAMS VITAE PETER JOSEPH WILHELM DEBYE was born on March 24, 1884, at Maastricht, the Netherlands. His education began in the elementary and secondary schools there; it continued at the Technische Hochschule in Aachen. His first degree, achieved in 1905, was in electrical engineering. During the Aachen period Debye came under the influence of two exceptionally able physicists, Professors Max Wien and Arnold Sommerfeld, and with their encouragement and guidance remained there for a short additional period with an appointment as Assistant in Technical Mechanics. When Sommerfeld was called to Munich in 1906 as Professor of Theoretical Physics he invited Debye to accompany him as his assistant. Debye there com- pleted his doctoral program in July 1908 and was promoted to privatdozent in 1910. By this time it was abundantly evident that he was well on the way to an illustrious career in physics. In 1911 Debye received an appointment at the University of Zurich as Professor of Theoretical Physics. He returned to the Netherlands in 1912 to accept a position as Professor of Theoretical Physics at the University of Utrecht. The next invitation, two years later, was from Gottingen to take charge of the theoretical section of the Physics Institute. Within a 23

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24 BIOGRAPHICAL MEMOIRS short time he became director of the institute, and he lectured on experimental physics until after the end of World War I. Debye returned to Zurich in 1920, this time to become Professor of Physics and director of the Physics Laboratory at the Eidgenossische Technische Hochschule. An equivalent position at the University of Leipzig opened in 1927, and he was invited to fill it. From 1934 to 1940 he served as director of the Max Planck Institute of the Kaiser Wilhelm Institute for Physics at Berlin-Dahlem and Professor of Physics at the University of Berlin. The Berlin post turned out to be his last in Europe. Im- mediately following its termination (for political reasons) he became Professor of Chemistry and, later, also chairman of the Department of Chemistry at Cornell University at Ithaca, New York. The promotion to emeritus status came in 1950. It was during the Ithaca period that Debye became an American . . cltlzen. The quality of his scientific work gained him many honors and distinctions. A number of them have been listed to form an endpaper for this Memoir. Election to the National Academy of Sciences (U.S.) came first as a Foreign Associate (1931) and then as a Member (1947~. A different type of recognition came in 1939. A shoulder- length bust, a gift of the natives of his birthplace, Maastricht, was there unveiled in his honor to adorn the town hall. It has been noted by others that this distinction probably pleased Debye above all others. Professor Debye married Mathilda Alberer in 1913. There were two children, a son, Peter Paul Rupprecht (b. 1916), and a daughter, Mathilda-Maria (b. 1921~. He died on Novem- ber 2, 1966. THE SCIENTIST In the Nobel Prize citation to Debye ( 1936) one reads, "for his contributions to our knowledge of molecular structure

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PETER JOSEPH WILHELM DEBYE 25 through his investigations on dipole moments and on the dif- fraction of x-rays and electrons in gases." The structure of atoms and molecules was indeed a subject of major and continu- ing interest with Debye; it extended over the years from studies of the arrangements of the electrons in the simplest of the atoms to measurements of the average end-to-end distance in macromolecules of the "random-coil" type. One might have elected to consider in a single section those contributions that are related to the structure of matter, but here the attempt will be made to conform more closely to the outline Debye himself selected for his Collected Papers (1954), a volume that was pre- sented to him by his students and friends and by the publisher on the occasion of his seventieth birthday, in 1954. In this way there is retained to some degree a chronological order, another plan that might have been adopted. The articles presented in this compendium, fifty-one titles, constitute somewhat less than one quarter of the total number of his contributed papers. In even this portion one finds an impressive record of high-level achievement. The main subject areas are four in number: "X-Ray Scattering," "Dipole Moments," "Electrolytes," and "Light Scattering." A fifth unit is made up of "Miscellaneous Contributions." In the develop- ment and description of the researches, the reports are invari- ably replete with that same skill for which the author came to be known as a speaker and lecturer, namely, consummate proficiency in the description of a difficult and intricate subject in a lucid and well-organized fashion. A study of these and the other Debye contributions is indeed a rewarding experience. X-RAY SCATTERING The story has often been told of how, after learning about the progress of a study of the passage of light through crystals by Ewald and Sommerfeld at Munich in 1910-1912, Von Laue became interested in the passage of very short waves through such materials. He reasoned that if the wavelength of the

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26 BIOGRAPHICAL MEMOIRS radiation were of the same order as the distance between the structural units a diffraction effect should be obtained. For the experimental test he suggested that x rays be used; the result strongly supported the correctness of his anticipation of a dif- fraction of the x rays by the crystals. As a result of this experi- ment a whole new subject, x-ray analysis, had been created. Though the analysis came to be recognized as being simple in principle, there were certain complications in detailed ap- plication. Debye, well informed about the research activities at Munich, was quick to perceive that refinements of several kinds were necessary if the analyses were to have quantitative character. His treatments of two of them, the temperature effect (1914) and the atomic scattering factor effect (1915), are representative of great pioneering achievement. In the first of these efforts Debye made calculations of the influence of the thermal vibrations of solids on the x-ray diffrac- tion pattern. His earlier experience with the famous theoretical evaluation of the heat capacities of crystalline solids (1912) served him well in this endeavor. Using the same general idea, that the thermally induced atomic displacements in the crystal may be described as being elastic waves that are propagated through the material, he developed a mathematical expression to describe the temperature dependence of the x-ray structure amplitude factor. Introduced was the quantity now known as the Debye, or Debye-Waller, temperature factor. (Waller, later on, had made some adjustments.) Incidentally, this factor is essential to an understanding of the Mossbauer effect. A consideration of the atomic scattering factor, Debye's second of the two refinements discussed in x-ray analysis, is of vital importance in structure determination. For the analysis, observed intensities of the spectra are compared with those calculated for assumed electronic arrangements of the struc- tural elements. The calculations require a knowledge of the atomic scattering factor, a quantity that describes the result of interference effects within the scattering atoms.

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PETER JOSEPH WILHELM DEBYE 27 For atoms of different sizes and kinds the scattering power for the x rays varies. Further, the waves scattered from the different parts of the electron cloud that surrounds a nucleus will be diffracted with phase differences in the direction of observation. The total amplitude is thus a function of the scattering angle and the distribution of the electron density about the atom. The. atomic scattering factor, the quantity calculated, is defined as the ratio of the actual amplitude to that which would be produced by a single Thomson electron under the same experimental conditions. Debye was able to take these several factors into quantita- tive account (1915~. He demonstrated that as the angle of scattering increases, these phase differences become larger, so that the effective number of scattering units becomes smaller. The scattering factor, I, is now a quantity smaller than the total number of electrons in the atom. The factor depends on the wavelength, A, of the incident rays in such a way that it is a function of sin b/x, with ~ being the Bragg angle of diffraction. For example, it was possible for Debye to construct the curve for the distribution of diffracted x-ray intensity to be expected from Bohr atoms with their electrons arranged in circular orbits about their nuclei. It was at this time, 1915, that Debye first recorded his con- clusions that in matter of any state one never finds a completely random arrangement of atoms and molecules, and that perfect crystallinity is not required' for the diffraction of x rays. It was pointed out in this renowned article (1915) that even in gases the atoms are not completely random in their order. This obser- vation was the beginning of a whole sequence of experimental researches by Debye concerned with the scattering of x rays by gases, liquids, and amorphous solids. In such systems the curve of diffraction intensity versus angle of diffraction should show broad maxima and minima. However, Debye's first experimental test, conducted with Scherrer (1916), produced an unanticipated result. The test substance was finely powdered lithium fluoride,

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~8 BIOGRAPHICAL MEMOIRS but the x-ray diffraction pattern that was observed consisted of the sharp spots characteristic of diffraction by a crystal lattice. The formation of the sharp rings was properly explained as being due to the intersection on the photographic plate of a succession of conical beams from randomly oriented crystals. It was in this way that a new and useful method of x-ray analysis, the "powder method," was discovered. Debye persisted in his researches in x-ray optics. In an article published somewhat later (1925), his thoughts were refined, extended, and summarized. He reaffirmed that it should be possible to observe diffraction effects that are in- terpretable in terms of the structure of the atoms and the molecules, irrespective of their physical state. More definitively, the thought was still to the effect that certain arrangements of any given atom with respect to its neighbors are more probable than others; thus it should be possible to obtain information about them by an x-ray analysis, regardless of the state of matter. For liquids on exposure to x rays a small number of broad and diffuse halos are produced in scattering. Two factors determine the outline of these halos; Debye early called them "inner" and "outer" interferences, with those of the first kind being between waves scattered by atoms belonging to the same molecule, while those of the second kind derive from intermolecular interfer- ences. It is now known that this distinction cannot generally be made. For the molecular structure determination it was reasoned that the "outer" interferences should vanish if the system were "diluted," as in a gas. In this way, the mathematical analysis and interpretation should be greately simplified. The intensity factor, 1, scattered by the gas should be an average effect, one described by a well-known Debye formula n n I_ k ~ ~ f?fj 1 1 x~j

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PETER JOSEPH WILHELM DEBYE 29 in which the magnitude xtj is proportional to the distance lid from atom i to atom j and ft and fj are their atomic scattering factors. The sums include the cases where i j. For the angle of scattering, 2b, of rays of wavelength A (of the primary radia- tion), sine x~j - 41 _. The formula is written for a molecule that consists of n atoms. The scattering curve is thus a composite of as many in- dividual curves as there are atomic distances in the molecule. Each such distance produces an intensity that increases and decreases as the angle of scattering is increased; the importance of the several interatomic distances is measured by the product of the scattering factors. The results of the first experiments with gases, those from Debye's laboratory, were reported from Leipzig (1929~. The reasoning had been correct; interference rings were produced by the scattering of x rays even from the simplest of molecules. For instance, from the photometered records of the rings, the chlorine-chlorine interatomic distances in carbon tetrachloride could be established with precision. Since the model for this molecule is taken to be a tetrahedron, this single distance suffices to define the complete structure. A more definitive and expanded account of similar researches, extended to certain other molecules, appeared within another year (1930~. It was at this time that Mark and Wierl ~ presented a pre- liminary description of their investigations showing that the Debye formula descriptive of the scattering of x rays by a gas, a brief outline of which appears above, could also be applied to describe the scattering of electron rays by gases. Physically, there is one difference. The electron interferences provide in- formation about the positions of the atomic nuclei themselves, while the x-ray interferences reveal the locations of the centers ~ H. Mark and R. Wierl, "Electron Diffraction, by a Single Molecule," Natur- wissenschaften 18 (1930): 205.

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30 BIOGRAPHICAL MEMOIRS of gravity of the electron clouds about them. What is really ascertained in either case is the position of the atom centers, the desired quantity. For reasons that need not be here described, electron dif- fraction became at once the preferred experiment. Though it is true that the actual number of molecules to which these methods may be applied remains small, still with modern computational devices and vastly improved equipment, electron diffraction has become a method of great utility and high pre- cision for the evaluation of molecular structure. Concurrently with the study of gaseous structure Debye, with Menke (1930), conducted experimental researches to determine the inner structure of liquids by x-ray analysis. The scattering pattern now represents the superposition of the two interference phenomena, an intramolecular part and an inter- molecular part. It was argued that if these two parts could be separated, it would become possible to draw conclusions about the structure of the liquid. Mercury, a monatomic liquid, was selected as being a suitable test substance. With this choice the separation of inner and outer effects becomes possible. The separation of the contributions to the scattering pattern was achieved, and, by using an analysis of the type that had been presented already, by Zernicke and Grins, it was possible to com- pute a distribution function to describe the probability of finding the molecules in the liquid at particular separations. This probability distribution curve for mercury demonstrated that even in the liquid there is found to be a quasi-crystalline state. The term "clustering" has been applied to short-range oraer situations of this general type; one finds it used in several other Debye discussions, in particular in his description of the underlying principles of electrolytic solution behaviors and in his treatments of the critical state. . .. .. ~ . . ~ F. Zernike and J. A. Prins, "The Bending of X-Rays in Liquids as an Effect of Molecular Arrangement'', 7~^~~` `^'~ Dl~.A:L A' 'l^~\ ADA ~t;zl~c~-rl~zJr fur rnysz~ g1 (lying/) 184.

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PETER JOSEPH WILHELM DEBYE 31 In these ways it was proven that there is no absolutely sharp distinction between the amorphous and crystalline states. The general subject was again given definitive overall and more modern consideration in a critical review published rather recently (1960~. The volume in which this article appears pro- vides a good idea of the enormous amount of work that has been done in structural studies of various types of amorphous materials, an area in which Debye was the pioneer. DIPOLE MOMENTS It is as a consequence of their asymmetrical (electrical) structure that most molecules possess a permanent dipole mo- ment; the magnitude of this characteristic entity is a quantita- tive measure of the polarity of the molecule. The practical unit of dipole moment is 1 x 10-~8 e.s.u., now universally known as the debye, with symbol (D). Sixty years after the appearance of the original Debye con- tributions on the subject (1912; 1913), the measurement and interpretation of molecular dinole moments continues un- abated. Of the two articles, one was addressed to the problem of the behavior of a dielectric in a static electric field and the other to the case in which the electric field varies sinusoidally with time. The full significance of their teachings was not immediately recognized in chemical circles. The reports had appeared in journals for subjects in physics, and they were mathematical in character. This situation changed with the appearance of two more lengthy discourses by Debye: the re- nowned article in the Marx Handb?'ch der Radiologie (1925) and the record of a course of lectures presented at the Uni- versity of Wisconsin in early 1927 and published later in book form (1929) with the title "Polar Molecules." In these publica- tions the subject matter was superbly summarized, organized, and enriched, and it came quickly to the attention of the physicists, who in turn communicated their interest to friends

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32 BIOGRAPHICAL MEMOIRS in chemistry. In the United States Professors K. T. Compton and R. C. Tolman were of great influence in this way. To provide an indication of continuing interest we may note that beginning in 1955 at least three monographs that summarize advances in the subject have made their appearance: C. P. Smyth, "Dielectric Behavior and Structure," United States; N. E. Hill, W. E. Vaughan, A. H. Price, and M. Davies, "Dielectric Properties and Molecular Behavior," Great Britain; and V. I. Minkin, O. A. Osipov, and Yu. A. Zhdanov, "Dipole Moments in Organic Chemistry," the Soviet Union. An earlier Methuen pocket-size monograph, Dipole Moments, by R. J. W. LeFevre is now in its third edition; we consider this source of information to be an excellent introduction to the subject. Debye, in his treatment of the electrical case, made use of the Langevin statistical theory of (orientation for the permanent magnetic moments of paramagnetic molecules. In doing so, he took cognizance of the fact that matter is built up of electrically charged units. Prior to 1912 it had been recognized that many molecules, ammonia and water for example, showed abnormally high electrical susceptibilities, ones for which there was no explanation. By analogy with the magnetic problem Debye reasoned that such asymmetric molecules must possess finite and permanent electrical moments and that their total electrical polarizations result from two contributions, a displacement of electrons and atoms in the molecule and an orientation in the electrical field of the molecule as a whole. For the actual appli- cation in any given case it was necessary to devise means for the quantitative evaluations of each of these polarizations. The result, another well-known Debye equation that can be applied to polar gases at low pressures or (less exactly) to dilute solu- tions of polar substances in a nonpolar solvent, provides the means to compute the dipole moment, a. It is, in molar form, P 3 N ( OCR for page 22
PETER JOSEPH WILHELM DEBYE 59 Die magnetische Methode zur Erzeugung tiefster Temperaturen. Phys. Z., 35:923; also in Z. tech. Phys., 15:499. With H. Sack and F. Coulon. Experiences sur la diffraction de la lumiere par des ultrasons. Sciences, Paris, 198:922. Comptes rendus de l'Academie des 1935 Kernphysik. Angew. Chem., 48:381. Les propriet~s dielectriques du point de vue moleculaire. Rev. univ. mines (Liege), 11:176. Analyse des essaies de sedimentation. Rev. univ. mines (Liege), 1 1: 266. La rotation des molecules dans les liquides. Bull. sci. Acad. Roy. Belg., 21:166. Relations entre la constitution chimique et les proprietes dielec- triques. Bulletin de la Societe Chimique de Belgique, 44:167. Der Rotationszustand von Molekulen in Flussigkeiten. Phys. Z., 36:100; also in Bull. sci. Acad. Roy. Belg., 21:166. Dielektrische Sattigung und Behinderung der freien Rotation in Flussigkeiten. Phys. Z., 36:193. 1936 Dielectric properties of pure liquids. Chen~ical Reviews, 19:171. Der Weg zum absoluten Nullpunkt. Umschau, 40:41. Die tiefsten heute erreichten Temperaturen. Forschungen und Fortschrifte, 12:22; idem, English version. The lowest tempera- tures yet established. Research Progress, 2:89. Bemerkung zu dem Artikel von E. Gehrcke: "Wie die Energiever- teilung der schwarzen Strahlung in Wirklichkeit gefunden wurde." Phys. Z., 37:440. 1937 Das Kaiser-~7ilhelm-Institut fur Physik. 257. [ohann Diderik van der Waals. kunde, 4:257. With W. Ramm. Grundlagen der Strahlungsphysik. In: Die Welt der Strahlen, ed. by H. Woltereck. Leipzig, Verlag Quelle & Meyer. With H. Sack. Constantes dielectriques, moments electriques. Tables annuelles des constantes, Nr. 2. Paris, Hermann et Cie. Naturwissenschaften, 25: Nederlands tijdschrift voor natuur-

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60 BIOGRAPHICAL MEMOIRS ~truct;ure in electrolytic solutions. journal of the Franklin Insti- tute, 224: 135. With W. Ramm. Hochfrequenzverluste und Struktur von Flussigkeiten. Ann. Phys., 28:28. quasikrystalline Die Untersuchung der freien Elektronen in Metallen mit Hilfe von Rontgenstrahlen. Phys. Z., 38: 161. \lethoden zur Bestimmung der elektrischen und geometrischen Struktur von Molekulen. Nobelvortrage. Angew. Chem., 50:3. English version. Nobel Lectures Chemistry 1922-1941. New York, N.Y., Elsevier Publ. Co. (1966~. 1938 A contribution. In: Physiq ue Generale. Wege der modernen Forschung in der Physik. wes, 58: 1. With M. H. Pirenne. Paris, Hermann et Cie. Stahl Eisenhuetten- . Uber die Fourieranalyse von interferome- trischen Messungen an freien Molekulen. Ann. Phys., 33:617. Die Geburt des M~irkungsquantums. Z. tech. Phys., 19: 121. Abkuhlung durch adiabatische Entmagnetisierung. Ann. Phys., 32:85. With W. Ramm. Dispersion und Absorption polarer Substanzen. Nuovo Cimento, 15:226. Die paramagnetische Relaxation. Phys. Z., 39:616. 1939 Die quasikrystalline Struktur von Flussigkeiten. Z. Elektrochem., 45:174. Uber den tiefsten heute erreichbaren Temperaturen. Schriften der Deutschen Akademie fur Luftforschung, No. 3, p. 8. Das Sektorverfahren l~ei der Aufnahme von Elektroneninterferenzen. Phys. Z., 40:507. Untersuchung eines neuen Vorschlags zur Fourier-Analyse von Elektronenaufnahmen. Phys. Z., 40:573. Zur Theorie des Clusiusschen Trennungsverfahrens. Ann. Phys., 36:284. 1941 The influence of intramolecular atomic motion on electron diffrac- tion diagrams. J. Chem. Phys., 9: 55.

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PETER JOSEPH WILHELM DEBYE 61 1942 Reaction rates in ionic solutions. 82:265. 1944 Trans. Am. Electrochem. Soc., Magnetic approach to the absolute zero of temperature. American Scientist, 32:229. Light scattering in solutions. J. Appl. Phys., 15:338. 1945 Angular dissymmetry of scattering and shape of particles. Tech- nical Report no. 637. Washington, D.C., Rubber Reserve Com- pany. 1946 The intrinsic viscosity of polymer solutions. l. Chem. Phys., 14:636. With R. H. Ewart, C. P. Roe, and I. R. McCartney. The determina- tion of polymeric molecular weights by light scattering in solvent-precipitant systems. l. Chem. Phys., 14:687. 1947 The structure of polymers in solution. ress, 8~1 /~: 1. Record of Chemical Prog- Molecular weight determination by light scattering. I. Phys. Col- loid Chem., 51: 18. 1948 The structure of polymers in solutions. Les Grosses molecules en solution. Homage national a Paul Langevin et Jean Perrin, p. 39. College de France, Paris. Light scattering in soap solutions. I. Colloid Sci., 3:407. With A. M. Bueche. Thermal diffusion of polymer solutions. In: High Polymer Physics, ed. by H. A. Robinson, p. 497. Brooklyn, Chemical Publishing Co., Inc. With A. M. Bueche. Intrinsic viscosity, diffusion and sedimentation rate of polymers in solution. J. Chem. Phys., 16:573.

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62 BIOGRAPHICAL MEMOIRS 1949 Light scattering in soap solutions. emy of Sciences, 51:575. Annals of the New York Acad- Light scattering in soap solutions. l. Phys. Colloid Chem., 53:1. With R. V. Nauman. The scattering of light by sodium silicate solutions. I. Chem. Phys., 17: 664. With A. M. Bueche. Scattering by an inhomogeneous solid. I. Appl. Phys., 20:518. With A. M. Bueche. Light scattering by inhomogeneous solids. India Rubber World, 119:613. With W. M. Cashin. Determination of molecular weights and sizes by absorption. Phys. Rev., 75: 1307. 1950 With A. M. Bueche. Scattering by inhomogeneous materials. Col- loid Chemistry, 7:33. With A. M. Bueche. Light scattering by concentrated polymer solutions. J. Chem. Phys., 18:1423. Estructura de altos polimeros estudiada por metodos opticos. Anales de la Real Sociedad Espanola de Fisica y Quimica (Madrid) Ser. B. 46:343. 1951 With R. V. Nauman. Light scattering investigations of carefully filtered sodium silicate solution. l. Phys. Colloid Chem., 55:1. With F. Creche. Dielectric constant of polystyrene solutions. I. Phys. Colloid Chem., 55:235. With E. W. Anacker. Micelle shape from dissymmetry measure- ments. I. Phys. Colloid Chem., 55:644. With C. W. Tait, R. i. Vetter, and i. M. Swanson. Physical char- acterization of cellulose xanthate in solution. I. Polym. Sci., 7:261. With W. M. Cashin. Effect of small refractive-index differences be- tween solution and solvent on light scattering. I. Chem. Phys., 19:510. With F. Bueche. Electric moments of polar polymers in relation to their structure. l. Chem. Phys., 19:589. With F. Bueche and Id. M. Cashin. Expressions for turbidities. J. Chem. Phys., 19:803.

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PETER JOSEPH WILHELM DEBYE 63 With F. Bueche. A study of crystallite sizes in polymers by a light scattering method. Phys. Rev., 81:303. 1952 With J. O. Edwards. Long-lifetime phosphorescence and the dif- fusion process. I. Chem. Phys., 20:236. With F. Bueche. Distribution of segments in a coiling polymer molecule. l. Chem. Phys., 20: 1337. With F. Bueche and W. M. Cashin. The measurement of self- diffusion in solid polymers. l. Chem. Phys., 20:1956. With I. O. Edwards. A note on the phosphorescence of proteins. Science, 116:143. 1954 Equilibrium and sedimentation of uncharged particles in inhomo- geneous electric fields. In: Ion Transport Across Membranes, p. 273. New York, Academic Press, Inc. With P. P. Debye, B. A. Eckstein, W. A. Barber, and G. I. Arquette. Experiments on polymer solutions in inhomogeneous electrical fields. If- Chem. Phys., 22:152. With P. P. Debye and B. H. Eckstein. Dielectric high frequency method for molecular weight determinations. Phys. Rev., 94: 1412. With W. A. Barber, P. P. Debye, and B. H. Eckstein. A field-in- duced-diffraction method for molecular-weight determinations. Phys. Rev., 94:1412. The Collected Papers of Peter I. W. Debye. New York, Interscience Publishers Inc. 1955 With N. T. Notley. The extension of polystyrene chains; de- pendence on molecular weight and solvent. J. Polym. Sci., 17: 99. Structure of gel-catalysts by low angle x-ray scattering. American Chemical Society Directory of Petroleum Chemistry, General Papers, No. 33, p. 35. 1957 With H. R. Anderson, Jr., and H. Brumberger. Scattering by an inhomogeneous solid. II. The correlation function and its ap- plication. I. Appl. Phys., 28:679.

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64 BIOGRAPHICAL MEMOIRS With N. T. Notley. Dimensions of linear polystyrene molecules in solution: molecular weight dependence for low molecular weights. J. Polym. Sci., 24:275. With H. Brumberger. Low-angle scattering of x-rays by glasses. J. Phys. Chem., 61:1623. 1958 With P. Dorefuss and N. T. Notley. penylstyrene. J. Polym. Sci., 28:611. With W. Prins. Polymerization of isopro- Micellar dispersion of oe-monoglycerides in benzene and chlorobenzene. l. Colloid Sci., 13:86. 1959 Rontgenstreuung in Korpern mit regelloser Struktur. Z. Phys., 156: 256. With R. L. Cleland. Flow of liquid hydrocarbons in porous Vycor. J. Appl. Phys., 30:843. Angular dissymmetry of the critical opalescence in liquid mixtures. [. Chem. Phys., 31:680. With L. K. H. van Beek. Effect of adsorbed water on the optical transmission properties of isotropic powders. J. Chem. Phys., 31:1595. With I. Daen. Stability considerations on non-viscous jets exhibit- ing surface or body tension. Physics Fluids, 2:416. Strukturbestimmung von Korpern mit regelloser Struktur mit Hilfe van Streustrahlung. In: Physikertagung Berlin 1959. Mos- bach, Baden, Physik Verlag. 1960 Scattering of radiation by non-crystalline media. In: Nonc~ystal- line Solids, ed. by V. D. Frechette, p. 1. New York, John Wiley & Sons. Paul Scherrer und die Strenung von Rontgenstrahlen. Basel-Stutt- gart, Birkhauser Verlag, GmbH. Die Winkelverteilung der kritischen Opalezenz und die Messung molekularer Wechselwirkung. Makromolekulare Chemie, 35A: 1.

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PETER JOSEPH WILHELM DEBYE 65 With A. Prock and G. McConkey. Inhomogeneous field method for the study of large polarizable particles. I. Chem. Phys., 32:234. With H. Coll and D. Woermann. Critical opalescence of poly- styrene solutions. l. Chem. Phys., 32:939. With H. Coll and D. Woermann. Critical opalescence of polystyrene in cyclohexane. i. Chem. Phys., 33: 1746. Arnold Sommerfeld und die Uberlichtgeschwindigkeit. Physika- lische Blatter, 16: 568. With H. Coll. Non-ionic detergents in non-aqueous solvents. PB 146,513. Washington, D.C., U.S. Department of Commerce, Office of Technical Services. With H. Coll. Non-ionic detergents in non-aqueous solvents. II. Critical opalescence of binary liquid mixtures: the system poly- styrene-cyclohexane. PB 149,895. Washington, D.C., U.S. De- partment of Commerce, Office of Technical Services. 1961 With R. N1. Nauman. The slow change in turbidity of sodium sili- cate solutions. l. Phys. Chem., 65:5. With R. V. Nauman. The refractive indices of sodium silicate solu- tions. l. Phys. Chem., 65:8. With R. V. Nauman. A light scattering study of the aggregation of acidified sodium silicate solutions. l. Phys. Chem., 65:10. With B. Chu. Critical opalescence of polystyrene in cyclohexane: transmission measurements. AD 264,359. Washington, D.C., U.S. Department of Commerce, Office of Technical Services. With B. Chu. Critical opalescence of polystyrene in cyclohexane: range of molecular forces and radius of gyration. AD 264,360. Washington, D.C., U.S. Department of Commerce, Office of Technical Services. 1962 Molecular forces. In: International Symposium on Electrolytes, ed. by B. Pesce, p. 1. Proceedings of a conference, Trieste, 1959. Oxford, Pergamon Press, Inc. Interatomic and intermolecular forces in adhesion and cohesion. In: Symposium on Adhesion and Cohesion, ed. by Philip Weiss, p. 1. Proceedings of a conference, Warren, Michigan, 1961. New York and Amsterdam, Elsevier Pub. Co.

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66 BIOGRAPHICAL MEMOIRS With D. Woermann and B. Chu. Critical opalescence of poly- styrene in cyclohexane: transmission measurements. I. Chem. Phys., 36:851. Critical opalescence and the range of molecular interaction. Ponti- ficiae Academiae Scientiarum, Scripta Varia, 22:53. With B. Chu and D. Woermann. Critical opalescence of poly- styrene in cyclohexane: range of molecular forces and radius of gyration. l. Chem. Phys., 36:1803. With B. Chu and H. Kaufmann. Critical opalescence of binary liquid mixtures: methanolcyclohexane and aniline-cyclohex- ane. i. Chem. Phys., 36: 3378. With B. Chu. Spectrophotometry and light scattering on supported platinum. J. Phys. Chem., 66:1021. With H. Coll. The association of cx-monoglycerides in non-aqueous solvents. [. Colloid Sci., 17: 220. With H. Kaufmann, K. Kleboth, and B. Chu. Angular dissymmetry of critical mixtures: anilinecyclohexane: aniline-l-hexene. Transactions of the Kansas Academy of Sciences, 66:260. With B. Chu. Critical opalescence of polystyrene in ethylcyclo- hexane. AD 266,258. Washington, D.C., U.S. Department of Commerce, Once of Technical Services. Topics in Chemical Physics, ed. by A. Prock and G. McConkey. Harvard lectures. New York, Elsevier Pub. Co. 1963 Structure determination by radiation scattering. Chemical Engi- neering News, 41:92. With B. Chu and D. Woermann. Viscosity of critical mixtures. J. Polym. Sci. Ser. A, 1:249. With D. Woermann and B. Chu. Critical opalescence of poly- styrene in ethylcyclohexane. J. Polym. Sci. Ser. A, 1 :255. With B. Chu and H. Kaufmann. Molecular configuration of poly- styrene in benzene. J. Polym. Sci. Ser. A, 1:2387. Light scattering and molecular forces in electromagnetic scattering. In: Interdisciplinary Conference on Electromagnetic Scattering, ed. by Milton Kerker. Oxford, Pergamon Press, Inc. 1964 The early days of lattice dynamics. In: Lattice Dynamics, ed. by

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PETER JOSEPH WILHELM DEBYE 67 R. F. Wallis. Proceedings of Copenhagen conference, August 1963. London, Pergamon Press, Inc. Flussigkeiten, Gase, Makromolekule: kritische Streuung und die Reichweite der Molekularkrafte. Zeitschrift fur Kristallo- graphie, Kristallgeometrie, Kristallphysik, Kristallchemie, 120: 113. Light scattering as a tool. Official Digest of the Federation Society of Paint Technology, 36:518. With D. Caulfield and I. Bashaw. Critical opalescence of binary mixtures: perfluorotributylamine-isopentane. I. Chem. Phys., 41:3051. NVith K. Kleboth. An electrical field effect on the critical opales- cence. AD 604,494. Washington, D.C., U.S. Department of Commerce, Office of Technical Services. 1965 Hans Falkenhagen zum 70 Geburtstag am 13 mai 1965. Z. Phys. Chem., 1965: 228, 289. Spectral width of the critical opalescence due to concentration fluctuations. Physical Review Letters, 14:783. Title K. Kleboth. Electrical field effect on the critical opalescence. J. Chem. Phys., 42:3155. Static homogeneous electrical field effect on critical opalescence. Ithaca, N.Y., Cornell University Report No. TR-9. NASA N65- 1 1285. Surface determination by x-ray scattering. In: Coloqulo sobre Quimica Fisica de Procesos en Superficies Solidas, pp. 1-11. Madrid, Consejo Superior de Investigaciones Cientificas. 1966 Light-scattering as a tool. Pure and Applied Chemistry, 12:23. With J. Bashaw, B. Chu, and D. M. Tancredi. Critical opalescence of the polystyrenecyclohexane system: small-angle x-ray scat- tering. I. Chem. Phys., 44:4302. With C. C. Gravatt. The behavior of non-ionic detergents in non- polar solvents. AD-642604, N67-16150. Washington, D.C., U.S. Department of Commerce, Office of Technical Services. With C. C. Gravatt and M. Ieda. Electric field effect on the critical opalescence. II. Relaxation times of concentration fluctuations.

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68 BIOGRAPHICAL MEMOIRS Report AD-642606, N67-16129. Washington, D.C., U.S. Depart- ment of Commerce, Office of Technical Services. With C. C. Gravatt. Behavior of non-ionic detergents in non-polar solvents. AD-642604. Washington, D.C., U.S. Department of Commerce, Office of Technical Services. 1967 With C. C. Gravatt and M. Ieda. Electric field effect on the critical opalescence. II. Relaxation times of concentration fluctuations. J. Chem. Phys., 46:2352. With C. C. Gravatt. Measurement of relaxation times of concen- tration fluctuations by the electric field effect on critical opales- cence. AD-657208, N67-38297. Washington, D.C., U.S. De- partment of Commerce, Office of Technical Services. Molecular Forces. Baker Lectures, Cornell. (Book: B. Chu) New York, Elsevier Pub. Co. 1968 With R. T. Jacobsen. Direct visual observation of concentration fluctuations in a critical mixture. I. Chem. Phys., 48:203.

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