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Volume 42, 1909
First Meeting: 8th June, 1908.

E. V. Miller, Esq., President, in the chair.

New Members.—M. Aldis, A. Clark, M.B., A. J. Cottrell, W. G. Guinness, M.D., S.E. Lamb, M.A., T. Simson, H. G. Stringer, A. Wyllie, C. E.

The President delivered the anniversary address.

The address was mainly devoted to an account of the scientific work which had been done during the last two or three years in connection with the discharge of electricity through gases, and of radio-activity; and more especially as these two bear on the nature of material atoms and of electricity. After a brief preliminary account of older scientific work, in the course of which the nature of ionisation, of radio-activity, and of the more important phenomena connected with the discharge of electricity through a vacuum tube were touched on, the lecturer proceeded to refer to more recent work on the same subject. An account was given of the method employed by J. J. Thomson to measure the velocity and the electro-chemical equivalent of the particles constituting the canal rays, and the remarkable nature of his results was pointed out. The work of Rutherford in 1906 in measuring the velocity and electro-chemical equivalent of the Α particles from different radio-active sources was then described, and it was noted that the electro-chemical equivalent found by Rutherford for the Α particles was identical with one of the two values found by Thomson for the canal ray particles. This coincidence, in conjunction with the already proved identity of the β rays with cathode rays, γ rays with Röntgen rays, brings the phenomena of the vacuum tube into the domain of radio-activity. By means of the discharge in a vacuum tube various ordinary materials, otherwise non-active, may be rendered strongly radio-active at will, and the elements disintegrated and transmuted.

Existing views were discussed as to the share which the corprscles in an atom take in making up its total mass.

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The “range” of the Α particle, and the sudden cessation of its ionising power when its velocity falls below a critical value, were then alluded to, and the aid which this property gives in radio-active analysis was pointed out.

An account was then given of the method employed by Rutherford and Geiger to detect the ionisation produced by a single Α particle; to estimate by actual count the number of Α particles emitted per minute from a known weight of a radio-active body, and thereby to determine the charge on an Α particle.

Reference was made to the methods adopted to discover the immediate forerunner of radium in the uranium radio-active series, and to the consequent discovery of ionium by Boltwood.

The address corcluded with some reflections concerning the place which physical hypothesis should occupy in a truly philosophical system of thought.

During the course of the address several points were illustrated either experimentally or by means of diagrams. By the aid of a Wilson tilted electroscope and a lantern to make the movement of the leaf visible on the screen, the ionisation due to uranium, thorium, and thorium emanation, together with the rapid decay of the latter, were exhibited.