W. Gerlach. Die experimentellen Grundlagen der Quantentheorie, 1921, pp. 144 + VIII. Vieweg Verlag.

The title of the book is somewhat unfortunate; the content is considerably narrower than the title. Gerlach’s book collects the experimental foundations of Bohr’s theory and of the so-called Einstein equation. Experimental data connected with Planck’s theory (blackbody radiation, heat capacity, etc.) are completely omitted; the reader is referred to two books by Valentiner, devoted precisely to these questions and published by the same publisher. The main chapters of the book are as follows: 1) The quantum condition and the resonance and ionization potential. 2) Excitation of the continuous X-ray spectrum. 3) Absorption edge and excitation in the X-ray spectrum. 4) Quantum

laws of the photoelectric effect. 5) Photochemistry. The material collected is extremely recent; most of the papers abstracted belong to the period 1914–1920. This circumstance almost completely determines all the merits and shortcomings of the book. The need for thorough, yet concise, summaries of the data of the newest literature is at present especially keen. A need of this kind is caused, first, by the unusually intensive growth of the literature and, second, by the temporary, but nevertheless very prolonged (from the beginning of the war down to the present time) irregularity in the acquisition of foreign literature—and not only foreign literature, but simply literature appearing in different geographical points of the same country. This remark applies not only to Russia, where conditions in this respect have become truly unbearable, but also to the other European states, especially Germany. Original work is now often inaccessible to the European scholar. Gerlach’s book can, to a considerable extent, compensate for the absence of the original literature on the questions treated in it. Unfortunately, one must note the absence of references to some very important recent English works. Each chapter is accompanied by a very detailed index of the original literature; thus, for example, the list of original memoirs for the chapter on resonance and ionization potentials reaches 133 items. The exposition is concise and at the same time thorough; in the appropriate places numerous tables and curves are given. At the end of the book there is a long list of atomic constants. By the very nature of the fields of modern physics expounded by the author, the book under review may at the same time be regarded both as popular and as intended directly for the investigator. The quantum theory is so simple, from the formal point of view, in its consequences that it requires no mathematical apparatus of any degree of complexity beyond the four rules of arithmetic. The fundamental difficulties connected with the very foundations of the quantum theory are at present preferably passed over by the physicist, “leaving, in Warburg’s words, the final assault on this fortress to the future.”

Despite the dryness of the exposition and its complete factuality, the chapter on resonance and ionization potential is read with absorbing interest; there, in particular, a new method is opened for the study of the far-ultraviolet spectrum. The chapter on photochemistry is extremely interesting; it sets forth, chiefly, Warburg’s work and its interpretation from the standpoint of Nernst’s dissociation theory and Bohr’s theory. In 25 pages almost all the more or less quantitative results achieved in the study of photochemical processes are presented.

S. Vavilov.