LONDON. Royal Society, January 12.—SirA. Geikie.K. C. B.,president, in the chair.—Prof. H. L. Callendar and H. Moss: The absolute expansion of mercury.—Dr. R. W. Gray and Sir W. Ramsay: The density of niton (radium emanation) and the disintegration theory.—Prof. J. S. Townsend: The charges on ions in gases, and some effects that influence the motion of negative ions. The experiments on charges on ions in gases which had previously been made with air only have been extended to oxygen, hydrogen, and carbonic acid. The value of the quantity Ne for the negative ions is in all cases very near the value 1.22 × 1010, which corresponds to a charge, e, equal to the charge on a monovalent atom. The ions were produced by secondary Röntgen rays, and it was found that when non-penetrating rays were used the value of Ne for the positive ions was practically the same as for negative ions, but is much larger when the penetrating rays are used, showing that in this case some of the positive ions have double charges. The motion of the negative ions is considerably changed by carefully drying the gases, and the results of the experiments may be used, in conjunction with the determinations of the velocities made by Mr. Lattey, to determine the apparent mass of the negative ion, which diminishes at low pressures as the electric force is increased. For a given force, the pressures at which the effect of drying becomes appreciable is higher in hydrogen than in oxygen, and much less in carbonic acid than in the other gases.—F. W. Aston: The distribution of electric force in the Crookes dark space. The method used in the investigation is one due to J. J. Thomson, and consists in shooting a beam of homogeneous kathode rays transversally through the discharge, and observing the deflection of the beam at various points. The results so obtained are free from the very serious objections which may be urged against the “sounding-point” methods used by previous observers. The electric force in the negative glow is found to be negligibly small, while within the Crookes dark space it is satisfied within experimental error by the simple formula μ(D - x), where D is the length of the dark space, x the distance from the kathode, and μ a constant. This result indicates the presence of a uniform charge of positive electrification within that region. The distribution is the same for all gases, pressures, and currents used. By integrating the forces so obtained, the potential fall across the dark space is calculated, and is found in all cases to agree within experimental error with the actual potential between the electrodes. The large and abrupt fall of potential at the surface of the kathode found by other investigators is probably a result of faulty methods, an explanation of which is suggested.—Dr. P. E. Shaw: The measurement of end-standards of length. A continuation of work published in Roy. Soc. Proc. (December 1, 1905). In recent years the authorities at the National Physical Laboratory have been required to measure and test end-standards with unprecedented accuracy. As a result, the faults of the standards and of the measuring machines have come to light. In this paper an account is given (1) of improvements in the planeness and parallelism of the standards; (2) of changes introduced in the author's measuring machine to cope with the more accurate standards. It is easy to get consistent readings of length provided the standard is not moved; but if, as is required, the standard is moved, it is a difficult mechanical problem to provide a movement so nearly parallel that the readings before and after movement shall be consistent. Curves and tables are given showing the degree of accuracy at present obtained. A great advance in refinement is expected on the present lines of work.