Abstract
LONDON. Royal Society, November 6.—T. R. Merton: On ultra-violet spectro-photometry. A “neutral wedge” of platinum can be used for measurements of radiations so short as 2100A. These “wedges” require to be calibrated, both as regards the thickness of the platinum as a function of the distance from the thin end of the wedge, and as regards the density gradient for a given change in the thickness of the platinum as a function of the wave-length. The density gradient is sensibly constant from X =4000A to X=2500A.—W. L. Bragg and S. Chapman: A theoretical calculation of the rhombohedral angle of crystals of the calcite type. The structure of calcite found by X-ray analysis is built on a series of rhombohedral cells, the edges of which meet at an angle of 1010-53. Other carbonates and sodium nitrate have a very similar form. The rhombohedral angle of about 1020 which is present in all cases is not fixed by the symmetry. The angle has been calculated by a consideration of the equilibrium of the calcite structure. It is assumed that the electrostatic forces correspond to those between a charge of +20 at the centre of the calcium atom, - 2e at the centre of the oxygen atom, and +42 at the centre of the carbon atom, and that the force of repulsion is directed from the electrostatic centre of each atom. The crystal is given a deformation which does not alter the distance between neighbouring atoms, so that the forces of repulsion do no virtual work. The electrostatic energy for each configuration is then calculated and a value for the rhombohedral angle found which makes the energy a minimum. This is the condition for equilibrium. The rhombohedral angle so calculated differs by 30 or 40 from the observed value. The alteration in rhombohedral angle, when one metal is substituted for another, is very exactly explained.—O. W. Richardson and T. Tanaka: (1) The striking and breaking potentials for electron discharges in hydrogen. There are three types of discharge from a thermionic electron-emitting cathode in hydrogen at pressures below 1 mm. The striking and breaking potentials for the discharge which sets in at the lowest voltages have the following properties: (a) They fall to a limit as the electron emission of the cathode increases; (b) these limiting values have a minimum for a pressure which depends on the electrode distance; (c) as the electrode distance is diminished the minimum values of the breaking potential approach a lower limit asymptotically. This limit coincides with the, strongest ionisation potential of hydrogen. Spectroscopic evidence is adduced in favour of the view that the type of ionisation which occurs at this point is the splitting of an electron from the hydrogen molecule with the formation of ionised H2. A considerable number of the lines in the secondary spectrum are selectively affected as regards intensity in the different types of discharge. (2) On a P, Q, and R combination in the many-lined spectrum of hydrogen. Starting from a study of the lines of the secondary hydrogen spectrum which are selectively weakened in a thermionic discharge tube at pressures less than 1 mm., we have been able to arrange 24 lines of this spectrum as members of series, with the designations?
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Societies and Academies. Nature 114, 737–739 (1924). https://doi.org/10.1038/114737b0
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DOI: https://doi.org/10.1038/114737b0