P. DEBYE. Polare Molekeln. — Leipzig, S. Hirzel 1928. Pp. VIII + 200, Preis geh. R. M. 15.50.

P. Debye. Polar Molecules.

The scientific literature on questions of molecular physics has been enriched by a new, very interesting book. Debye’s monograph arose as a re-

as the result of a series of lectures and reports delivered by the author during his repeated trips to the United States of North America, and was originally published in English (Polar Molecules. New York, The Chemical Catalog Co., 1929). Although only a few months separate the appearance of the German edition from the English one, for the German edition under review the book was revised and substantially expanded by the author.

The contents of the book amount to the following. In the first three chapters Debye expounds, in the classical spirit, the polarization effects in a dielectric and establishes the significance of these effects for the chemical structure of substances. Among other things, in these same chapters extensive tabular material is given for the values of \(\mu\)—the measure of the polarity of a substance, which is related to the molecular polarization \(P\) by the relation

\[ P=\frac{4\pi}{3}N\left(\alpha+\frac{\mu^2}{3kT}\right). \]

Here \(N\) is Avogadro’s number, \(k\) is Boltzmann’s constant, and \(T\) is the absolute temperature. In the fourth chapter an attempt is made at a model description of such polar molecules as HCl, H\(_2\)O, NH\(_3\). The fifth chapter concerns the anomalous dispersion of electric waves. Here Debye’s views are presented, first published as early as 1913 in Ber. d. Deutsch. Phys. Ges. (Band 15); a generalization of the Maxwell-Boltzmann distribution is given; the concept of relaxation time for polar liquids is introduced; the relation between the molecular polarization \(P\) and the frequency of the electromagnetic wave is given; and, finally, a certain experimental characterization of the theory is presented. The sixth chapter is devoted to the very interesting question of the dielectric constant of electrolyte solutions. The dependence of the dielectric constant of a solution on concentration is derived; the result is compared with experimental data. However, this question, from the experimental side, is rather confused and uncertain. Debye feels this and emphasizes that this chapter was written also in order to arouse the interest of theory and experiment in this important question.

In the last four chapters Debye dwells on what the new quantum mechanics provides in the theory of dielectrics. Here, in a few pages and in a fairly complete form, the author gives the foundations of Schrödinger’s theory; the exposition is conducted so clearly that a physical chemist (for whom Debye’s book is of special interest), even if not versed in playing with mathematical symbols, can nevertheless grasp the most serious and important aspects of the new, distinctive doctrine of matter. As for the results in general of the penetration of wave mechanics into the theory of the dielectric, they are as yet not very significant. Debye’s book, in this exceptionally interesting question, can serve only as a stimulating factor for further theoretical and experimental investigations.

The book is written with Debye’s characteristic mastery: the physical aspect is not obscured by excessive mathematical subtleties, but at the same

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at the same time bears the stamp of a sufficiently serious, mathematically rigorous exposition. Physicists and chemists alike will find in this book an objective and, to the necessary extent, complete treatment of all questions connected with the electrical properties of liquids and solutions. The exposition is not a dry retelling of past and current literature; the book belongs to that very small class of books in which the material presented awakens new thoughts and impulses toward research.

A. Predvoditelev.