BIBLIOGRAPHY1

J. FRANCK und P. JORDAN. Anregung von Quantensprüngen durch Stösse. J. Springer, Berlin, 1926. Pp. VIII + 312.

Franck and Jordan. Excitation of quantum transitions by collisions.

As the third book in the series “Struktur der Materie,” a long-awaited book by Franck has appeared, written with the participation of Jordan. The title may give rise to a certain misunderstanding, suggesting that what is involved is a description of a group of works prompted by the famous investigations of Franck and Hertz. In reality, this group of works has developed into an entire research methodology that has made it possible to penetrate deeply into the mechanics of atomic structure. Its results, closely intertwined with the data of optical investigations, form the basis of all theoretical investigations of the atom. And in the book under review the question is posed precisely in this way. Devoting a large part of the description—especially from the standpoint of experimental technique—to the method of electron impacts, the authors discuss the results of optical investigations in the most careful manner. As might have been expected, Franck nowhere confines himself to a simple presentation of results. Everywhere a practical description is given, and not infrequently a different interpretation of the results is put forward from that given by the authors of the investigations and from that given by Franck himself in his already published works. In a living and continuously developing field, temporary and preliminary interpretations are inevitable, and what is required of the investigator is not the absence of errors, but the ability to abandon in time conceptions that have proved untenable and to replace them with new ones, without fear of falling into error again. “The fact that some working hypotheses previously put forward (both by others and by ourselves) proved in part incompatible with the facts did not prevent us from putting forward others in their place. For even the hypotheses that have now already been abandoned contributed to success, for they stimulated further investigations,” the authors write in the preface.

Both the book as a whole and each chapter open with a brief theoretical introduction that systematizes the necessary information. The factual material and the experimental methodology are then set forth with exhaustive completeness. It is constantly indicated in what respects the available material is insufficient and what questions or details require additional experimental investigation. The result is a book of first-rate merits, equally valuable both for persons specially engaged in the field under consideration and for all other physicists.

The first chapter is devoted to describing the kinetics of slow electrons in gases and vapors, beginning with the work of Lenard and Ramsauer and ending with the investigations of Ilponer and Minkowski, carried out under the direction of Franck himself,

and Brode’s work. The second chapter describes in detail the methods for determining critical potentials by means of electron impacts and the parallel spectroscopic observations. The third contains a comparison of the data obtained concerning critical potentials and the spectral terms of atoms; here the systematic description (by columns of the periodic system) reconstructs the picture of the distribution of electrons among atomic shells, i.e. develops the theory of the periodic system of the elements according to Bohr and Stoner.

The fourth chapter treats the important and little-investigated question of the probability of excitation and ionization by means of electron impacts. The fifth concerns effects produced by collisions with positive ions and atoms (thermal excitation of ionization). The sixth, perhaps the most interesting chapter, is devoted to the question of the transformation of excitation energy, i.e. to the doctrine of collisions of the first and second kind, the natural continuation of the excited state, and the metastable states introduced into science by Franck himself. Finally, the seventh chapter considers the question of the structure of molecules, i.e. sets forth facts concerning band spectra and electron affinity—facts obtained almost exclusively by optical methods. In this field the electron-impact method has not yet yielded any special results, although work on determining the critical potentials of molecules is beginning gradually to be systematized and to give definite results (for example, the investigations of Sponer, who, on the basis of her own and others’ data concerning nitrogen, was able to give a fairly plausible picture of the energy levels and to estimate the heat of dissociation of the nitrogen molecule; to this group also belong the works of Smith and N. N. Semenov, who attempted to determine directly the products of ionization of molecules). The last, eighth chapter speaks briefly about the connection of quantum transitions with chemical reactions, expounding chiefly the quantum theory of photochemical reactions and chemiluminescence and the theory of Franck and Born’s triple collision.

The appearance of the book is impeccable, but its price (22 marks) makes one think with apprehension that in the same series another dozen and a half books are planned, promising to be almost equally interesting.

Ir. Landsberg.

1. Owing to circumstances beyond the editors’ control, the remaining part of the Bibliography will appear in issue 6. ↩