Societies and Academies

Abstract

LONDON. Royal Society, January 29.—P. M. S. Blackett: The ejection of protons from nitrogen nuclei, photographed by the Wilson method. Photographs have been taken of more than 400,000 alpha-ray tracks in nitrogen, using an automatic form of the Wilson condensation apparatus. A source of thorium B + Cj was used, giving a mixed beam of 8-6 and 5-0 cm. alpha particles. Among the tracks were found many normal forks due to the elastic collisions between alpha particles and nitrogen nuclei. In addition, eight forks were found of a strikingly different type. These abnormal forks represent the ejection of protons from nitrogen nuclei. Each track branches into two arms, one of which clearly represents the track of the proton. Since there is only one other arm to represent the tracks of both the residual nucleus and the alpha particle itself, the two particles must be bound together after the collision. When, therefore, a proton is ejected from a nitrogen nucleus by a fast alpha particle, the alpha particle itself is captured by the residual nucleus, forming a new nucleus which should have a mass of 17 and an atomic number 8.- R. E. Gibbs: The variation with temperature of the intensity of reflection of X-rays from quartz and its bearing on the crystal structure. Whilst the space group to which quartz belongs is known, the positions of the atoms in the molecule remain undetermined. The oxygen atoms cannot lie in the same basal planes as do the silicon, bub must interleave them at a distance d. Of all the four unknown parameters, the variation of d alone will affect the intensity of reflection from the basal plane. Reflection intensities measured from 0° to 8oo° C. show that marked changes occur for all the planes at the transition point.- R. W. Gurney: (1) Ionisation by alpha particles in monatomic and diatomic gases. In the monatomic gases - xenon, krypton, argon, neon, and helium- the amount of ionisation increases with increasing atomic number, a result to be expected from their decreasing ionisation-potentials. In the diatomic gases-hydrogen, oxygen; and nitrogen-ionisation is less than in any of the monatomic gases, in spite of the high value of the ionisation-potential of helium. The ratio of the ionisation in the gases to that in air varies with the velocity of the alpha particles. The question is discussed whether the value (33 volts) found by Geiger for the average expenditure of energy per pair of ions in air is- applicable to ionisation near the end of the range. (2) The stopping-power of gases for alpha particles of different velocities. Since the stopping-power of a substance varies with the velocity of the alpha particles traversing it, the value obtained for the stopping-power of a gas by a measurement made over the whole or a large part of the range, as has usually been done, is merely an average value. Small portions of the range are here selected, so that the relative stopping-power has been measured for alpha particles of high velocity, of low velocity, and of intermediate velocity, separately. The relative values of the atomic stopping-powers tend to converge at the end of the range.-W. E. Curtis: The Fulcher hydrogen bands. The Fulcher lines and Allen's additions to them have been examined with the view of finding a theoretical interpretation of them. The wave-numbers of two of the strongest lines require correction by about o -5 cm."1. The differences are then sufficiently regular to provide a criterion for the genuineness of the extra lines, which are in the main confirmed. The arrangement is consistent with the view that they originate from combinations of simultaneously occurring rotation and vibration changes. New values of the molecular moments of inertia concerned are obtained which probably refer to an "excited "molecule. The nuclear vibrations within the hydrogen molecule seem to be ver\r nearly simple harmonic, which would account, in conjunction with the small moment of inertia, for the unique structure of the system as compared with other band systems. The two sets of Fulcher triplets apparently originate from two molecules essentially similar in structure.-W. L. Webster: The magnetic properties of iron crystals. The magnetic properties may be accounted for by the Weiss theory of molecular fields. The magnitude of the molecular field is found for two crystals, giving respectively 620 and 479 gauss. The magnitude of the component along any one of the crystal axes varies as cos4 (f), (^) being the angle between the axis and the direction of magnetisation. The molecular field is a stable property of the crystal, and is affected considerably by the presence of impurities.-A. E. Ingham and J. E. Jones: On the calculation of certain crystal potential constants and on the cubic crystal Of least potential energy.-E. C. Stoner and L.' H. Martin: The absorption of X-rays. Two beams, defined by two slit systems, one vertically above the other, are reflected by the same crystal into two ionisation chambers. The beams are first balanced. A sheet of the absorbing material is then placed in the path of the upper beam, and the beams rebalanced by moving a wedge of aluminium across the path of the lower beam. The well-known law T/P = const. Z4\3 holds only on the long wave-length side, or sufficiently far away on the short wave-length side of the K absorption discontinuity. Neither the formula of de Broglie nor of Kramers gives correctly the variation of the magnitude of the K group with atomic numbers. Measurements on the absorption co-efficients of uranium on each side of the three L absorption discontinuities show that the number of electrons associated with the L3 level equals the sum of the numbers associated with the Lx and L2 levels. This is in agreement with Dauvillier's result for gold.- F. H. Schofield: The thermal and electrical conductivities of some pure metals. The maximum temperature used was 700° C. The thermal conductivity of aluminium increases with rising temperature, that of nickel decreases at first, and then above 5000 C. increases. Copper, magnesium, and zinc showed, on the whole, slight decreases of conductivity with temperature. The values of Lorenz's function for copper, magnesium, and zinc were practically constant at all temperatures; that for aluminium showed a rise with increasing temperature; that for nickel showed a rise to 300° C, above which temperature it remained nearly constant except for an abnormal value at 4000 C.-M. de Selincourt: On the effect of temperature on the anomalous reflection of silver. The existence of a well-defined band in the ultra-violet (about 40 A in width) at which the reflection co-efficient of silver is negligible, has been utilised to investigate the relation between the frequency of the free electrons which are responsible for the reflection and the mean distance between the particles of the metal. The point of minimum reflection has been determined by a photographic method at the four temperatures -183°, -79°, 160 and 1500; the band is displaced in the direction of decreasing wave-length as the temperature is lowered, and is at the same time rendered sharper and narrower.-T. L. Ibbs: Thermal diffusion measurements. Mixtures of each of the following pairs of gases were used: hydrogen and carbondioxide, hydrogen and nitrogen, nitrogen and carbon-dioxide, hydrogen and argon, helium and argon. The apparatus consists essentially of a small cold vessel maintained at uniform temperature, joined by a connecting tube to a larger vessel the temperature of which can be raised as required to about 300° C. Thermal diffusion produces a difference in the distribution of the components of the mixture on the hot and cold sides, and the resulting change in composition on the cold side is measured directly by means of a katharometer, the open cell of which forms part of the cold side. There is a general tendency for the gas with the heavier molecules to diffuse towards the cold side. The total separation is nearly proportional to log Tx/T2 (where T1 is the absolute temperature of the hot side, and T2 the absolute temperature of the cold) in all cases.