Photochemical Reactions in Liquids and Gases. A general Discussion held by the Faraday Society. — Transactions of the Faraday Society. No. 63, Vol. XXI, part 3, 1925. German translation: Zeitschrift für Physikalische Chemie. Vol. 120, 1926, VIII + 360 pp.

Photochemical reactions in liquids and gases. Discussion of the Faraday Society, October 1–2, 1925, in Oxford.

The international discussion conferences of the Faraday Society, together with the well-known Solvay congresses, point to a new method of summing up the results of science. The number of scientific investigations is increasing each year at such a pace that the traditional congresses of natural scientists threaten a new Babylonian confusion, where the builders of the “pillar of science” will altogether cease to understand one another, uselessly pouring out a sea of reports in speeches and on printed pages. Even special congresses of chemists, physicists, etc., with their diversity, thereby force one to raise the question of expediency. Their advantage is the personal contact of the assembled scientists, and the center of gravity is transferred from the official sessions to the sidelines of the congresses. The Solvay congresses and the discussions of the Faraday Society are built on a new principle. 1) The subject of the congresses is comparatively narrow, for example: quantum theory, valence theory, the photographic process, etc. 2) The reports are of broad interest for the given narrow subject, or are of a survey character. 3) The active participants in the congress are the leading workers in the field under discussion. The proceedings of such congresses give a very vivid picture of the state of science. Instead of a schematic drawing, a monograph, or a textbook, before us is an unretouched photograph, in which often contradictory currents are irreconcilably interwoven; in the discussions, indications of new, unexplored paths break through in hints, and everywhere one sees that unformed quality which is most enticing for the researcher. Reading

such discussions are the best stimulus to work, both for the seasoned researcher and for the beginner. One would very much like as many fields of science as possible to find a similar reflection in discussion.

The subject of the discussion under review is photochemical reactions. About 30 scientists from all over the globe took part in the discussion; the number of reports was 34. The German edition has been substantially altered by the participants in the congress, as compared with the English one. Very substantially supplemented, for example, is one of the most interesting reports, by Franck: “On the Elementary Processes of Photochemical Reactions (Remarks on Homeopolar Bonds).” Not all the reports satisfy the requirement of breadth of subject; for example, Halban’s communication, interesting in itself, “On the Absorption of Light in Electrolyte Solutions,” is almost out of place. It must be noted that the general plan of the discussion is rather chaotic. It is divided into two parts: I. The photochemical equivalence law of Einstein; II. The mechanism of photochemical reactions. Meanwhile, a considerable portion of the first part is devoted to the question of the photochemical theory of thermal chemical processes, to which Einstein’s law has only a remote relation. Stern’s communication, “On the Transformation of Atoms into Radiation,” which in general is not connected with the topic of the discussion, also found its way into the same section. To criticize the views expressed at the discussion would mean adding one more voice to its already very discordant chorus. We shall nevertheless allow ourselves to remark that the extensive discussion on Einstein’s law resembles the siege of an open door—on the one hand, and an incomprehensible desire to lock it—on the other. The thermodynamic derivation of the equivalence law by Einstein, about which much was said at the congress, has retained only historical interest; the equivalence law for the primary photochemical process is an obvious lemma from the postulates of quantum theory. Beautiful in form, Berthelot’s attempt to reconcile the contradictions between classical and quantum conceptions in photochemistry gives nothing new, apart from the scarcely needed new words “theron” and “radion.”

The reply by Lindemann and Christiansen to the defenders of the photochemical theory of chemical processes may, it is to be hoped, put an end to this strange “heresy,” or direct it onto a new path.

In the second part of the discussion a considerable share is occupied by the question of the permanent photochemical problem—the combination of chlorine with hydrogen. The “water” theory of Chapman and the “adsorption theory” of Norrisch equally failed to find sympathy, and the problem remained unresolved as before.

Einstein’s law gave the optical key to photochemistry. Beyond that, the problem becomes purely chemical, and first of all there arises the question of the influence of light on chemical bonds. The fundamental problem of chemistry—homeopolar bonds—also becomes the next task of photochemistry. This is especially vividly and substantively emphasized in Franck’s report.

S. Vavilov.