| Description | This volume is a laboratory record which follows straight on after volume A1. Between 16 August 1895 and 7 September 1895, no fewer than twelve separate determinations of the critical expansion ration for drop formation, practically all of which lie within 1 per cent of 1.250 are recorded. On 17 February 1896, experimental results are given for the first investigations with X-rays in connexion with cloud formation, and the basic discovery is summarized by Wilson as follows- 'From the above it is seen that no effect is produced by the X-rays unless the expansion is great enough to produce condensation in any case. When it is sufficient to cause condensation without the rays, they produce a very great increase in the number of the drops.' It is strange that in the neighbourhood of these results and summary there is no precise association of the observation of enhancement of condensation at the critical limit by the action of X-rays with the conception that the nuclei responsible were ions. It is however certain that this explanation was appreciated by Wilson as volume A1 contains a calculation (8 May 1895) of the charge necessary upon a droplet to 'neutralize' the surface tension. (A more careful calculation on 15 August 1896, in volume A4, yields the value of 3.14 X 10-10 e.s.u.!)
Many diverse phenomena, including of course effects of cosmic rays, were involved in these experiments and one feels impelled to quote here a few sentences from the brilliantly clear and logical exposition of this work which Wilson made in this broadcast lecture, two days after his ninetieth birthday.
'I found that when all the dust particles, as Aitken called them, or what we should now call Aitken nuclei, had been removed, one still was able to produce condensation in the form of drops provided a certain quite definite degree of supersaturation was exceeded...I was very much interested in the comparatively small number of drops that one got with a certain range of expansion, between 1.25 and 1.35, and I could not help wondering what these few nuclei could be. They were obviously always being reproduced and at a very early stage I began to wonder if they could be charged atoms, very soon to be called ions, and if one was thus make visible the individual ions.'
'....In 1896, very soon after the discovery of the X-rays by Roentgen, I was able to use a primitive form of X-ray tube made in the laboratory by Everett, Professor Thomson's assistant, and I was delighted to find on exposing the expansion chamber to these rays that I got a dense fog instead of the two or three drops, still requiring the same definite expansion to bring the nuclei from which the drops were formed into action. This in itself was sufficient to make it extremely likely that the condensation nuclei that I had been studying were ions, the ions to which the conductivity of a gas exposed to X-rays was attributed, especially by Thomson and Rutherford who were working together at that time'.
The rest of this volume contains further experimental results upon cloud formation in oxygen, carbon dioxide and hydrogen, together with investigations upon the influences of electrical discharges
and X-rays.
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