S. C. Lind. The chemical Effects of alpha-particles and Electrons. Second revised and enlarged Edition. American Chemical Society Monograph Series.—The Chemical Catalog Co. New York. 1928. Pp. 252.

Lind. Chemical effects of $\alpha$-particles and electrons.

The special interest and significance of photochemical reactions for understanding the nature of chemical processes lies in the fact that, in the case of the action of light of a definite wavelength, we are dealing with entirely unambiguous initial conditions. Indeed, the frequency of the acting light determines the magnitude of the quantum absorbed by the reacting molecule. Thus we know in advance the excess of energy received by each reacting molecule. On the other hand, so long as we remain in the region of optical frequencies, the magnitude of this excess energy is commensurate with the work of dissociation of the molecule. Therefore all that the absorbed quantum can produce is to cause the molecule to undergo decomposition, either directly or as a result of a subsequent collision. Even in those cases where the magnitude of the quantum exceeds the work of ionization of the molecule, the excess energy imparted to the ejected electron is not sufficiently great for it, in turn, to activate further molecules. Precisely for this reason, in the absence of secondary chemical processes occurring after activation of the absorbing molecule, the number of molecules that have reacted must be equal to the number of absorbed light quanta, as is asserted by the photochemical law of equivalence. Thus, in a photochemical reaction, the excess energy of each “active” molecule and the number of “active” molecules are exactly determined by the magnitude of the light quantum.

BIBLIOGRAPHY

Somewhat different conditions are present under the action of fast electrons and α-particles. Here the store of energy carried by each particle exceeds, by hundreds of thousands of times, the work of dissociation and ionization of molecules. Therefore there can be no question of each such rapidly moving particle being absorbed at its first encounter with a molecule. On the contrary, before losing its colossal velocity, the particle (whether an electron or an α-particle) is capable of activating an enormous number of molecules. In all cases this activation, apparently, must consist in the ionization of the molecule, since experiments with the critical potentials of molecules have shown that in all cases where the energy of the electrons considerably exceeds the work of dissociation, an ionized molecule arises in the primary act.

All these considerations—which, incidentally, are not given in the book under review, which is based on purely empirical material—show that if one attempts, by analogy with the photochemical law of equivalence, to formulate the fundamental law of the action of corpuscular rays, then it is most natural to compare the number of reacting molecules with the number of ions that have arisen in the system. And indeed, in a long series of works Lind established a relation which he calls the “law of ion-chemical equivalence.” It turns out that the ratio of the number of reacting molecules \((M)\) to the number of ions formed \((N)\), in a very large number of cases, is close to unity. This law is also laid at the basis of the systematization of the material presented in the book under review.

The determination of the quantity \(M/N\) for many reactions leads, however, to the result that in a number of cases this quantity is not equal to unity, but to two or four. Lind explains these apparent deviations from the law of ion-chemical equivalence as follows. A molecule ionized by the impact of an α-particle does not decompose immediately, but first attaches to itself one or several neutral atoms, forming an ionic complex. Decomposition occurs only upon the subsequent collision of this positively charged ion complex with one of the detached electrons or with negative ions that have had time to form. This explanation, on the basis of all the facts known to us from the field of gas ionization, must be regarded as very plausible.

From the kinetic point of view, reactions under the action of rapidly moving corpuscles are distinguished by a number of peculiar, but at the same time readily accessible to investigation, features. Thus, for example, the rate of a reaction caused by the addition to a reacting gaseous mixture of radon (radium emanation) is inversely proportional to the square of the radius of the spherical reaction vessel. It is easy to see why precisely such a dependence occurs: 1) an increase in the radius of the vessel increases the path of the fast particles in the gas, and therefore the number of reacting molecules must be proportional to the diameter of the sphere; 2) but for a given amount of radon, the amount falling on each \(\mathrm{cm}^3\) of reacting gases is inversely proportional to the volume of the vessel, i.e. to the cube of the radius. The combination

these two factors, acting in opposite directions, and gives an inverse proportionality to the square of the radius.

The contents of the book under review are as follows. Chapters 1–3 are devoted to the presentation of preliminary information (radioactivity, ionization); chapters 4–6 to the presentation of qualitative observations on the chemical reactions of α-, β-, and γ-rays; in chapters 7–16 the law of iono-chemical equivalence is examined in detail through a long series of examples; in chapter 17 the chemical effects of discharge in gases are considered, in chapter 18—the photochemical law of equivalence, and finally, in chapter 19—the most energetic “reaction” caused by α-particles—the artificial destruction of elements. It is to be regretted that the author did not dwell on the effects of slow electrons of a definite velocity and on works devoted to establishing the nature of the ions arising under the action of such slow electrons (Smith, Kallman, and Bredig, et al.). Perhaps these works are closer to the main subject of the book than is the artificial destruction of atoms.

In conclusion it should be noted that, by compiling this monograph, the author has rendered a great service to all those interested in photochemistry and the physical problems connected with it. By a strange accident, before the appearance of the first edition of Lind’s book (in 1921), the works set forth in this book were very little known. Meanwhile, the results of these works are very interesting and important.

E. Shpolsky.