Victor Henri. Study of the Absorption of Ultraviolet and Infrared Rays in Relation to Molecular Structure. Petrograd, 1919. II + 214 pp. Separate offprint from the journal of the Russian Physico-Chemical Society and the Proceedings of the Optical Institute.

The systematic study of the absorption of light, especially of the rays of the ultraviolet part of the spectrum, by various chemical individuals, after the work of Hartley, Baly, and others—chiefly English chemists—is becoming more and more fruitful, especially in its application to organic compounds. Recently, thanks chiefly to the work of Coblentz, the study of absorption in the infrared part of the spectrum has also begun to play an important role in America; and the work of the famous Russian physicist P. N. Lebedev brings forward, in this respect, the significance of still longer electrical waves.

Unfortunately, until recently the methodology for studying light absorption in the invisible parts of the spectrum either remained generally little developed in terms of the accuracy of the results attainable, or, because of its complexity, was not readily accessible to chemists.

The well-known physical chemist V. Henri, who has worked extensively on this question and who set forth the principal results of his interesting investigations, in the monograph under review, first of all dwelt on the development of a method as simple as possible and at the same time sufficiently accurate. Special attention was directed by him to the ultraviolet part

spectrum, for which Prof. Henri devised, instead of the crude qualitative methods of Hartley-Baly, his own original method, allowing one to determine directly the value of the absorption coefficient for each spectral line. The method is based on the systematic use of exposure time in successive photographs (on the same plate) and on comparison of the degree of blackening of the corresponding lines of the spark spectrum when light passes through the solution and through the pure solvent (practically non-absorbing), in which latter case the exposure time is not changed. To calculate the percentage of absorbed light and the absorption coefficient, the author uses the Schwarzschild formula, while the intensity of the blackening of the lines is compared directly by eye, with the aid of a binocular magnifier. In this last circumstance lies the weak side of Prof. Henri’s method. Evidently, far from all persons, even those experienced in photometry, are capable, without long preliminary training, of obtaining good results. Be that as it may, the author himself achieved an accuracy of up to 1–2%.

The numerous results of measurements obtained by this method, relating almost exclusively to organic compounds, as well as a series of measurements of absorption in the infrared part of the spectrum, collected in the monograph, constitute rich and valuable material, on the basis of which the author draws a number of interesting conclusions.

The brevity of this note does not permit us to dwell on the many interesting questions touched upon by Prof. Henri, on many of the generalizations he derives concerning the regular relation between the chemical structure of a substance, on the one hand, and its absorption, on the other; on such questions as the relation between infrared and ultraviolet absorption spectra, the relation of absorption to the stability of molecules, the simultaneous action of several atomic groups on the absorption of ultraviolet rays, etc.

In addition to his own investigations, the author cites and analyzes data obtained by other scientists who worked in the same field and in part dealt with the very same questions.

For the chemist, perhaps the most interesting result is the possibility, demonstrated by the author, of theoretically calculating spectra for a number of organic compounds from their structural formulas. The calculation is carried out with the aid of numerical coefficients, previously found from experiment, corresponding to individual chromophoric groups. In a number of cases, a good agreement was thus obtained between the experimentally observed absorption curve and the theoretically constructed one. In this way the absorption method is introduced into the circle of other physicochemical techniques that render substantial assistance, alongside purely chemical methods, in determining the molecular constitution of organic compounds.

The interest of the field treated in Prof. Henri’s monograph, from a more general point of view, lies in the possibility presented here of connecting experimental data with the newest theoretical conceptions of the structure of the atom (Bohr’s model) and with the quantum theory.

All who are interested in this question will read Prof. Henri’s work with great interest.

L. Chugaev.