LONDON. Royal Society, January 30.—Sir J. J. Thomson, president, in the chair.—Prof. J. C. McLennan and R. J. Lang: An investigation of extreme ultra-violet spectra with a vacuum grating spectrograph. In this investigation the vacuum grating spectrograph used was designed and constructed by the Adam Hilger Co. The grating had a ruling 2.5 cm. wide and 1.9 cm. in length, of 20,000 lines to the inch. Its radius of curvature was 120 cm. The vacuum are spectra of mercury, copper, iron, and carbon were investigated. With carbon, wave-lengths were observed and measured down to λ = 584 Å.U.—Prof. J. C. McLennan and J. F. T. Young: The absorption spectra and the ionisation potentials of calcium, strontium, and barium. In this paper it is shown that the wave-lengths constituting the series δ = (1.5, S) - (m, P), which are strongly absorbed by the vapours of calcium and strontium, are also strongly absorbed by the vapour of barium. The wave-length of frequency δ = (1.5, S) for barium has been shown to be λ = 2380.56 Å.U., and the wave-lengths of the two series δ = (2.5, S)-(m, P) and δ = (3.5, S)-(m, P) have been calculated. The wave-length of frequency δ = (1.5, S)-(2, p2) has been deduced as λ = 7901.11 Å.U. Assuming that the ionisation potential for barium is given by the relation Ve = hδ, where δ = (1.5, S), the value of this magnitude for barium has been calculated to be 5.21 volts.—Prof. J. C. McLennan, D. S. Ainslie, and D. S. Fuller; Vacuum arc spectra of various elements in the extreme ultra-violet. The experiments described were carried out with a fluorite spectrograph the optical train of which consisted of a 6o° prism and two biconvex lenses of 15 cm. focal length. The vacuum are spectra of copper, zinc, aluminium, carbon, thallium, tin, lead, iron, cobalt, nickel, and cadmium were investigated between λ = 2400 Å.U. and λ =1400 Å.U. The measurements obtained for the vacuum are spectra of copper, zinc, cadmium, and aluminium are well covered by the results for the spark spectrum of these metals, as obtained by previous workers. For tin, lead, and thallium, the results agree fairly well with those given bv Saunders from λ = 2400 Å.U. to λ=17oo Å.U. Below the region covered by Saunders's work many new lines were observed and measured. The measurements for the vacuum arc spectra of iron, cobalt, nickel, and carbon appear to be the first obtained for the arc spectra of these substances in the Schumann region. For these spectra nearly all the measurements between λ = 2400 Å.U. and λ=1850 Å.U., as given in the paper, are covered by previous work on their spark spectra. Within the region between λ=1850 Å.U. and λ=1400 Å.U. a number of new lines were photographed and measured.—R. C. Dearle: Emission and absorption in the infra-red spectra of mercury, zinc, and cadmium. In the investigation described in this communication the absorption spectra of mercury, zinc, and cadmium were studied with a Hilger infra-red spectrograph provided with a rock-salt prism and a linear thermopile, in conjunction with a Paschen galvanometer made by the Cambridge Scientific Instrument Co. With each of the vapours the range investigated lay between 1.0μ and 1.6μ. In studying the emission spectrum of mercury vapour bombarded by electrons, it was found that radiation of the wave-length λ= 10,140 Å.U. was emitted with impact voltages so low as 5, volts, and evidence was also obtained indicating that mercury vapour could be made to emit radiation of this wave-length with impact voltages less than 5 volts. The paper presents some considerations in support of the view that while mercury vapour has an ionisation potential of one type of 10.4 volts, it may also have an ionisalion potential of a second type of about 2.5 volts.—E. Wilson: The measurement of magnetic susceptibilities of low order. (1) An instrument which has been designed for the measurement of magnetic susceptibility of low order. It depends for its action upon the pull exerted by an electromagnet in accordance with the well-known Maxwell expression for the mechanical force exerted upon unit volume of the substance. This mechanical force is balanced against the force of torsion in a phosphor-bronze strip. (2) The instrumental constant is determined from data obtained directly with the instrument itself, and by the employment of substances the susceptibilities of which had been measured by other methods. A modified method of using a ballistic galvanometer has been devised which leads to greater sensitiveness. Rock specimens and other substances have been used, and some interesting results obtained. It is shown that the susceptibility of 13 per cent, manganese alloy is much smaller than is usually supposed. (3) The susceptibility of powdered rock specimens has been measured and compared with the solid. A very fair agreement has been obtained between the two, and the method has the advantage that powders can be rapidly made. (4) The susceptibilities of varieties of mica have been measured, and it is shown that in certain cases, in a direction parallel with the laminæ, the susceptibility may be more than fifty-fold that obtained in a direction at right angles thereto. (5) A series of light aluminium alloys has been tested, and it has been found that, whereas the susceplibility of commercial aluminium is increased by alloying with copper and manganese, it is diminished by alloying with cobalt. (6) It is shown that the balance could be used to determine rapidly the relative amounts of ferrous iron in different specimens of glass. (7) Certain specimens of tourmaline have been examined. The green and dark blue opaque varieties have susceptibilities in the direction of the principal crvslallo-graphic axis varying from 16 to 20 per cent, less than in a direction at right angles thereto. The susceptibility of rose-coloured tourmaline is very small in comparison. (8) The paper concludes with a note on the retentivity of rock specimens and its possible influence upon magnetic disturbances in magnetic survey work.