Table of contents

A normal coordinate analysis of the planar vibrations of tetra-chloro-ethylene is carried out. For the B_1g, B_2u and B_3u modes the analysis is complete, including the calculation of the coefficients in the transformation to normal coordinates, the components in the displacements, the directions of the displacement vectors and the distribution of the potential energy, as well as the force constants, for a range of solutions; in each case, two alternative assignments are employed. For the A_g vibrations, several ranges of possible solutions are obtained. Curves are given in every case, and corrections for variable structural parameters calculated for selected solutions. The results for the various symmetry classes are discussed and correlated, and the most likely set of force constants in the valence-force potential function deduced. Details of the method of analysis are given.

The method of molecular orbitals has been applied to find the bond orders and the bond lengths of ovalene C₃₂H₁₄.

A comparison with the preliminary experimental values of Donaldson and Robertson shows general agreement.

By the use of a liquid-air-cooled hollow-cathode tube with very low current densities it was possible to resolve hyperfine structure in the arc spectrum of yttrium. The value of the nuclear spin was found to be I=½ and the sign of the magnetic moment to be negative. This agrees with recent results of Crawford and Olson, who determined the nuclear spin from the intensity ratio of a hyperfine-structure doublet in the second spark spectrum of yttrium.

Experiments are described in which a differential ionization chamber was used in the measurement of angular distributions of disintegration particles. Results are given for seven groups of protons from the (d, p) reaction with aluminium in which deuterons having energies of 4.6, 5.8 and 7.5 MeV. were used. The distribution curves are of complicated form, with a general tendency for the protons to be emitted in the forward direction.

The photo thresholds for the emission of neutrons by the following elements have been determined. The threshold in MV. is given in the brackets following the symbol: Pt (6.1±0.1); Au (8.1±0.1); Hg (6.6±0.2); Tl (7.3±0.25); Pb (6.9±0.1); Bi (7.2±0.1); Th (6.0±0.15); U (5.8±0.15).

The neutrons were detected by the Szilard and Chalmers method of chemical concentration and the γ-rays obtained from a small betatron. Rough values of the total cross sections can be estimated from the results.

Proportional counters operating in a ` condensing ' type of magnetic field can be used as β- or γ-ray spectrometers up to at least 2 MeV. Results for P32 and annihilation γ-radiation are given.

Recently described developments have led to greatly improved agreement between theory and experiment for the magnetization curves of single crystals of iron and iron-silicon in the region where magnetization proceeds by rotation of domain vectors against the action of magnetocrystalline forces, i.e. in the range 10-500 oersteds. On the basis of the assumption that in polycrystalline material the magnetization is approximately uniform from grain to grain, which is rendered plausible by consideration of internal demagnetizing effects, a method is developed for deriving the magnetization curves of such material by averaging the fields required to produce a given magnetization in the different constituent grains, rather than by averaging the magnetizations produced in the different grains by a given field. The application of this method to specimens with known distributions of grain orientations is shown to give results in fair agreement with experiment.

The paper describes the investigation of the orientation of magnetite inclusions in mica using electron diffraction by transmission through composite films. The diffraction pattern obtained indicates that the magnetite is a single crystal with its (111) planes parallel to the cleavage plane of mica and one of the face diagonals parallel to the b axis of mica. The technique for preparing the thin films is described in some detail.

An account is given of the experiments on the transition effects of local and extensive penetrating cosmic-ray showers at Colombo (geomagnetic latitude 4° S.). A comparison of the present results with those reported for higher latitudes indicates that there is no observable latitude effect of the local penetrating showers. Contrary to the findings of Broadbent and Jánossy, the transition effect of the extensive showers in paraffin appears comparable in magnitude with the effect in lead.

Heitler and Jánossy have calculated the probability that n mesons are produced due to nucleon-nucleon collisions by a fast nucleon in its passage through matter of a given thickness. In this note the results of Heitler and Jánossy are obtained by the direct method of writing the integro-differential equations of the problem using the Markoff nature of the process.

Showers have been examined in the density range 5-500 particles/m² using two trays of Geiger counters 5 m. apart, the discharges of individual counters being recorded using neon lamps. The observed showers are compared with a theoretical analysis based on the usual assumptions that the incident densities are uniform over the extent of the apparatus and distributed according to the spectrum N(x) = Bx ^{- γ} per hr. per m². The comparison shows (i) that in the density range examined the above spectrum applies only for a vanishingly small area of observation, with γ = 1.425 ± 0.022, B = 620, and with no evidence for a change of γ with density, and (ii) that for the separation of 5 m. employed, the density can not be regarded as uniform over the extent of the apparatus. It is shown that this second conclusion is capable of explaining many of the differences found between the results of other authors obtained on the assumption of uniformity of density: in particular, it is concluded that there is yet no significant evidence for a change of γ with altitude.

The photographic plate method has been used to investigate the angular distribution of deuterons recoiling after neutron impact. The deuteron recoils were produced in a thin target of heavy wax placed in contact with a C2 Ilford nuclear emulsion.

For neutrons of energy 2.6-3.1 MeV., produced in the d-d reaction, 2,000 deuteron recoils were measured. Owing to the short range of the recoils produced when neutrons of low energy are scattered through small angles, the technique is only applicable to the study of the angular distribution in the range 90°-180° in the centre-of-mass system. For angles between 120° and 180° the results of the present work are in satisfactory agreement with those of previous experimenters using different methods, and they agree with the theoretical form of the distribution calculated by Buckingham and Massey on the assumption of ordinary forces between nucleons. This agreement is not decisive, however, since in this region the calculated distributions for ordinary and exchange forces do not lie far apart. For smaller scattering angles where a decisive test could have been made, the method is too inaccurate.

For neutrons of energy from 4.0 to 6.0 MeV. and from 6.0 to 9.0 MeV., obtained from a Ra-Be souice, the angular distribution over a much larger range of scattering angle has been obtained from the measurement of 1,200 deuteron recoils. The angular distribution for neutrons scattered through angles near 180° appears to be rather steeper than for slower neutrons; but the ratio of the differential cross section at 90° to that at 180° is found to be about 0.35-0.40, of the same order as for neutrons in the lower energy range.

Measurements of 1,000 proton recoils from the d-d neutrons showed an angular distribution for neutron scattering angles between 90° and 180° that was constant within the limits of the statistical error.

The properties of the potential derived from the mixed pseudoscalar-vector meson field are investigated and compared with the experimental data on the two-nucleon system. An attempt has been made to use methods which are more accurate than those formerly applied to this problem, and some of these may be useful in calculations with other potentials. It is found that agreement with experiment cannot be obtained for any choice of meson masses and coupling constants.

An experiment is described in which the free surface of rotating liquid helium II is studied. It is shown that the results, which are the same as for any ordinary liquid, are not in accordance with the hypothesis that the superfluid component remains stationary. On this basis an estimate of the critical velocity for superfluidity is made and is compared with previous determinations.