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“If a gradually increasing voltage is applied to a pair of inert electrodes in solution and the current plotted against voltage we obtain a curve such as is shown in figure 77. It has often been supposed that if the straight part of the curve were extrapolated back to zero current the corresponding voltage would be the ‘decomposition potential,’ There seems to be no particular significance to this voltage, however.” Page 273. “A fast dye is a contradiction in terms.” Page 289. “It has been known for a long time that photographic plates could be sensitized to green and red light by certain dyes. The exact mechanism of this photosensitization is not known with certainty. Since fluorescent dyes do not show this effect especially, it seems that the mechanism is not one in which radiation is re-emitted by the sensitizing agent.” Page 339. “As we have learned more about the behavior of the atom we have been forced to the inevitable conclusion that the energy that is associated with lights exists in corpuscles that we call photons. . . . The number of photons in the beam is proportional t o the intensity. If we know the original intensity of the beam and the characteristics of the grating we can calculate by the wave theory the intensity in the diffracted beam; but all we shall ever observe in the diffracted beam is the number of photons transmitted per second. Now if no medium exists for the transmission of light waves and we never observe these waves directly, the question arises naturally as t o whether these waves really exist. This is a question, however, for the philosopher rather than the scientist. So far as the scientist is concerned, the electromagnetic theory of light is a mathematical theory which describes the behavior of light if one assumes a simple relation between the number of photons and the square of the amplitude of the wave; but the thing really observed is the number of photons.’* Page 335. The reviewer doubts whether the student will grasp the fact that the 2 in the phase rule equation refers to pressure and temperature, page 126. The paragraph on flotation, page 185, is surprisingly obscure. On page 249 the authors prove that there is no potential difference between two metals. This is proving too much because there would then be no thermopiles. Cannizzaro’s name is spelled wrongly on page 4. “Pure water will dissolve a considerable amount of ether. If sodium chloride is added to the solution the ether will separate out as a second layer. This ‘salting out effect’ is easily explained in terms of dielectric constant. An electrical charge attracts a medium of high dielectric constant in the same way that a magnet attracts material of high permeability, e.g., iron. Since the dielectric constant of water is so much greater than that of ether, the water is pulled into the field existing between the ions and the ether is thrown out.” Page 296. This explanation does not seem to harmonize with the fact t h a t if one added alcohol to a saturated sodium chloride solution, some of the salt is thrown out. The dielectric constant cannot always be on the job. WILDERD. BANCROFT. Recent Advances in Atomic Physics. By GAETANO CASTELFRANCHI. Translated by W. S. Stiles and J. W. T. Walsh. Vols. I and 11. 20 x 13 cm.; pp. Vol. I xii 372, Vol. I1 xii 412. Philadelphia: P. Blakiston’s Son and Co., 1932. Price: $4.00 each volume. This is a translation of the third Italian edition with omission of the chapters on Brownian movement, relativity and mass, and astrophysics. In the first volume, which deals with atoms, molecules, and electrons, the chapters are entitled: atoms and molecules in physical chemistry; light; the kinetic theory of gases; fluctuations; electrons and positive rays; isotopes; x-rays and the atomic number; crystals;
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radioactivity; the atomic nucleus. I n the second volume, which deals with quantum theory, the headings are: thermal radiation and quantum theory; spectroscopy, Bohr’s theory and the energy levels of the atom; Stark and Zeeman effects-multiplet lines-the spinning electron; specific heats; the photoelectric effect-the reverse effect-photoelectric cells and their applications; the Compton effect-light quants; magnetism and the quantum theory; wave mechanics and quantum mechanicsapplications; the new statistics-applications. “The spherical electron, with the radius assumed above, is a pure supposition based on the hypothesis originally made; a t present, however, i t seems less necessary to consider the spherical electron with a fixed radius, since modern tendencies seem to lead t o the conclusion-as we shall see in what follows-that the electromagnetic theory is no longer applicable to systems of atomic dimensions and therefore still less to electrons which are constituent parts of atoms; the structure of the electron is certainly one of the mysteries of contemporary physics.” Volume I, page 146. From Sommerfeld’s theory one can deduce that “the ratio of the thermal to the electrical conductivity of a metal is independent of the nature of the metal and is proportional t o the absolute temperature.” Volume I, page 178. “Thus, after the lapse of some years, the old hypothesis proposed by Prout a t the beginning of the nineteenth century, vis., that the different elements are composed of a single primordial material, has been restored once more to a place of honour; thus science, through a new discovery, and by a road quite different from t h a t followed for several decades, has found-and not for the first time-the confirmation of an idea which was originally put forward solely as a reasonable hypothesis.” Volume I, page 203. ‘‘Let us take a milligram of radium and observe its transformation. Why does one atom suddenly explode and die, why does another atom live for a day, another for a week, others for a month, a year, a century? If atoms are all alike, how is it that they are ‘healthy’ and ‘unhealthy’ ones? At the present time physics is unable to provide an answer to this question. “However, since every fact is connected with some other, from which it results, by the chain of cause and effect, according t o our rational line of thought (determination) we are led t o think that, in reality, there must be a complex universe in every atom, and, by reason of the intervention of causes not known to us, the death of an atom is occasioned from time to time. Exactly similarly, in considering the incidence of mortality in a city, we cannot know whichindividualswill die on the morrow, but we can know how munu, because the mathematics of probability has shown us how we may overcome our ignorance, our inability to take account of the extraordinary complication of elementary causes. We are here confronted with a practical problem which is insoluble, though perhaps not for ever.” Volume I, page 302. “Planck’s bold hypothesis, deduced by an involved process of reasoning from statistics and thermodynamics, received very careful consideration a t the hands of the more enlightened physicists, the more profound thinkers of the time; nevertheless in spite of the support of men like Einstein and Poincar6, it would probably have been relegated t o the philosophy of physics and buried for all practical purposes, if certain experimenters had not been led to examine other phenomena, more simple and less abstruse, which indicated the existence of a discontinuity in nature which, until then, had been entirely unsuspected. While Planck in 1900 had simply asserted that the electrons emitted and absorbed energy in fixed finite quantities and shortly afterwards revised this new conception by restricting i t to the process of emission, Einstein, in 1905, daringly proposed the theory that these fixed and finite quantities of radiant energy retained their separate identities throughout their journey from the instant of emission to the instant of absorption. I n chapter VI
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we shall examine this reasoning, but for the present we shall confine ourselves to mentioning the fact that the latest developments of physics have confirmed the idea that in all elementary phenomena, Le., those which concern a single atom, a single molecule, or a single electron, the discontinuity postulated by Planck is found to exist; in such elementary phenomena Nature proceeds by jumps or degrees; in macroscopic phenomena this discontinuity is hidden and it remained unsuspected by physicists for centuries.” Volume 11, page 28. “Niels Bohr, with whose work we shall, in the main, be concerned in this chapter, is not one of those specialists who multiply experiments in a small field without regard to the close relations between the different branches of science. He possesses in high degree the research spirit, coupled with a mental audacity which has enabled him t o throw off the old methods and worn-out conceptions, to find new approaches to a higher and more general view of phenomena. The experimental facts themselves are of course indispensable to the progress of science. They may in fact be regarded as the bony skeleton which resists the test of time. But, in the face of the mass and variety of observed phenomena, the intelligence would be overwhelmed and science would crack under the weight of material if it were not for the appearance from time to time, of a bold innovator with the theory which will harmonize and regroup the facts of experiment and predict new phenomena.” Volume 11, page 32. “When a gas is irradiated with light of frequency greater than the resonance frequency, the return of the atom from the excited state to the normal may occur in stages. This is the phenomenon of fluorescence.” Volume 11, page 99. “At low temperatures the atomic heat of solid substance is proportional to the cube of the absolute temperature, a result which Schrodinger has verified conclusively for various substances in a temperature range extending from 20” t o 50” absolute.” Volume 11, page 153. “Compton observed that, when a beam of X-rays fell on a body, the frequency of the scattered radiation remained unaltered in the direction of the incident rays, but became lower in other directions and depended on the angle of deviation.” Volume 11, page 196. “If the luminous intensity is very high, the electromagnetic theory provides a means for calculating exactly both the luminous intensity a t different points in space, and the variations produced by the superposition of the waves, variations which agree with those calculated directly from Planck’s formula which, in its turn, is based upon and is in accordance with experiment. I n the case of very low intensities, however, the formula giving the variations indicates that radiation must be regarded as composed of a collection of discrete particles, but nevertheless the electromagnetic theory-although of a kind which seems quite alien to the problem-enables the distribution of the light quants to be determined whether they be numerous or few. If, then, we abandon a n y attempt to determine the path of a single light quant between the moment and the place of its emission, and the moment and place at which i t arrives at the measuring apparatus (the sensitive plate), the electromagnetic theory provides us with the means for determining the average number of light quants which reach agivenpoint of an optical system however complicated. The luminous intensity, then, is simply the probable distribution of the photons.” Volume 11, page 222. “It appears at first sight, therefore, that the observed magnetic moments should be multiples of the Bohr magneton. I n reality they are much more complex, because the atom, in addition to the magnetic moment due to the orbital motion of the electrons, possesses also another magnetic moment which is the resultant of moments associated with the electrons themselves. We shall see, however, that the magneton theory is in fact valid and the magneton is obtained experimentally with the value just calculated.” Volume 11, page 245.
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“Whilst de Broglie and Schrodinger had tried to discover the laws of atomic phenomena in the light of classical methods, and to bridge the gap separating the quantum mechanics of atoms from classical physics, Heisenberg-a few months before the appearance of Schrodinger’s first paper-started out from the opposite idea. He considered that only a definite renunciation of the deeply-rooted classical representation could lead to a proper understanding of the rbgime obtaining in the atom. He was convinced that, as a fundamental principle in atomic physics, any attempt a t explanation was pointless and without meaning, and that all magnitudes not susceptible to direct observation should be excluded from atomic theory.’’ Volume 11, page 314. “One of the fundamental characteristics of the history of Science in recent years has been, strangely enough, to show that every explanation founded on the classical mechanics is quite untenable throughout the whole range of electricity, magnetism and optics, which latter has been proved by Maxwell and Hertz to be a branch of electromagnetism. “The process of unification, although checked by this discovery, has not been entirely destroyed, but it has had to proceed in the opposite direction; relativity showed that it was not the simplest phenomena of mechanics (those that were studied and classified first of all) which were the most fundamental in character; after Einstein’s genius had elucidated matters the opposite was seen to be the case: electrical and magnetic phenomena are the simplest and these lead to a mechanics which is more precise and more complete than the old mechanics. “Another new and unexpected development of recent years is the discovery of discontinuity in every branch of atomic physics; it is the mysterious law of quanta which reigns supreme, and the discontinuity of atomic phenomena lies a t the root of optics, electricity, magnetism and heat; Planck’s constant, appearing as it does in the most diverse phenomena, has brought out new connections between different branches of physics. Through Heisenberg’s principle it shows what is the region of error, of inaccuracy inevitable in human observation, when we are investigating atomic phenomena.” Volume 11, page 393. WILDERD. BANCROFT. Neure Forschungen iiber die optische Aktivitat chemischer Molekiile. By G. KORTUM. 25 cm. x 16 cm.; pp. 118. Stuttgart: Ferdinand Enke, 1932. Price: R.M. 11.10. After more than a century of effort, the physical theory of optical rotation is still in the melting-pot. The fundamental basis for this theory was provided in 1824, when Fresnel showed that the optical rotatory power of media such as quartz or turpentine could be explained b y postulating that the velocity of transmission of circularly-polarized light in the medium was not the same for circular polarizations of opposite signs. The unequal refraction of right and lest circularly-polarized light, which results from these unequal velocities, was detected by direct measurements with a quartz prism of wide angle, compensated b y two prisms of glass; and it is now a well-established practice that, in constructing optical apparatus, d and 1 quartz prisms must be used in pairs, in order to avoid the double images which would be produced if the circular double refraction were not compensated in this way. The influence on optical rotatory power of variations in the wave-length of the Iight was investigated in the earliest experiments of Biot, who propounded an inverse square law, a = k/?? as long ago as 1818. A theoretical foundation for these observations was, however, not provided until 1898,when Drude invented a model in which the electrons in an asymmetric medium were assumed to vibrate in spiral instead of linear paths. On this basis he deduced his well-known equation, a = ukn/(XLXt),
