R. MECKE. Bandenspektra und ihre Bedeutung für die Chemie. Bornträger. Berlin. 1929. Pp. 87.

R. Mecke. Banded Spectra and Their Significance for Chemistry.

The outstanding successes that spectroscopy owes to the quantum theory are especially striking in the field of the theory of banded spectra. The enormous complexity and confusion of these spectra, with their thousands of individual lines, and the absence of guiding principles (such as Ritz’s combination principle) for interpreting them—all this made the development of this field, before the introduction of the quantum theory into it, a very unrewarding task. But the very first attempts to apply the principles of Bohr’s theory to the chaotic heap of empirical material obtained in the study of banded spectra at once brought clarity here. At the present time there can no longer be any doubt that banded spectra are molecular spectra—their complexity is due to the appearance in the molecule of new degrees of freedom that are absent in the atom.

The enormous work on the study of banded spectra carried out in recent years has yielded a large number of important results. First of all, reliable criteria were found for establishing

the nature of the carriers of band spectra; moreover, it turned out that spectroscopically, under certain special conditions, one can detect the existence of molecules that are generally unknown to chemists under ordinary conditions. Such are the molecules CH, AlH, AlO, and even He₂. Further, the study of band spectra has made it possible to draw very substantial conclusions concerning the structure of molecules. It has proved possible, for example, to calculate with great accuracy such molecular constants as the distance between the nuclei of the atoms composing them, moments of inertia, frequencies of the natural vibrations of the nuclei, and—what is especially important—heats of dissociation.

Thus the study of band spectra is becoming one of the most powerful instruments for the knowledge of the chemical properties of matter.

All the more keenly felt up to now has been the absence in the literature of sufficiently complete and systematic surveys of the theory of band spectra suitable for acquainting chemists with this field. Mecke’s reviewed booklet admirably meets this need.

The booklet’s eight chapters consider the following questions: 1. Theory of band spectra; 2. Structure of band spectra; 3. Band spectra and the periodic system; 4. Valence and band spectra; 5. Determination of dissociation energy; 6. General isotopy of the elements and its spectroscopic detection; 7. Chemical kinetics; 8. Band spectra and specific heats.

In presenting the theory of band spectra the author makes use of the apparatus of the “classical” quantum theory and only incidentally indicates those minor changes in the final formulas to which the new quantum mechanics leads. In view of the elementary character of the booklet, such a procedure must be recognized as entirely legitimate. In the fourth chapter the author gives a very clear outline of the modern symbolism of spectral terms, for which chemists will undoubtedly be very grateful to him. In the last chapter are discussed the works of Dennison and Turov on anomalies in the molecular spectrum of hydrogen. Bonhoeffer’s effective works, which led to the separation of ordinary hydrogen into ortho-hydrogen and para-hydrogen, differing only in the relative arrangement of the moments of rotation of the hydrogen nuclei (in one case parallel, in the other antiparallel), were published in the very last months and therefore, unfortunately, could not be mentioned in the booklet under review.

The author’s exposition is distinguished by simplicity and clarity and requires of the reader only a small amount of knowledge from contemporary atomic theory.

D. Shpolskii.