Clouds are produced in an expansion chamber filled w ith air and w ater vapour by irradiating with ultra-violet light through a quartz window, and expanding shortly afterwards. I t is shown th a t the condensation does not occur upon charged particles, b u t on certain electrically neutral nuclei, not yet identified. By varying the wave-length of the ultra-violet light it is shown th a t the condensation is initiated by atomic oxygen, liberated by the action of the radiation on oxygen

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... Similar effects are produced when an intense beam of deuterons is passed into the chamber. I n t r o d u c t i o n I t is well known that, under certain conditions, an expansion made while a cloud chamber is illuminated by ultra-violet light, or shortly afterwards, gives rise to a dense fog; but no detailed explanation of the process has been given. I t now seems clear (although some authors have argued to the contrary) th at the nuclei around which the dro£s are formed are electrically neutral. Before reporting the experi ments, which concern the nature of the nuclei produced under particular experi mental conditions, it is necessary to recall the previous work on condensation which is relevant to the problem. Using air and water vapour in his expansion chamber, Wilson (1897, 1900) obtained condensation upon negative ions with expansion ratio 1-25, and upon positive ions with expansion ratio 1*31. In the absence of ions (and other adven titious nuclei), condensation occurred at expansion ratio 1-38; this point is called the cloud limit. The nuclei on which condensation occurs a t the cloud limit are random aggregates of about 40 water molecules. The thermodynamic theory of this process gives satisfactory agreement with experiment (Volmer & Flood 1934; Becker & Doring 1935). When the chamber is irradiated with ultra-violet light, Wilson (1899) obtained a cloud along the path of the radiation. In a series of experiments he showed th at the effect was not due to photo-electrons emitted from the walls of the chamber, and also th at it was not diminished by an electric field. Moreover, the expansion ratio required to produce this condensation was not a characteristic of the process (as in ionic condensation), but depended upon the duration of the illumination and its intensity. With a greater exposure, a smaller expansion ratio sufficed to produce a cloud. With very intense ultra-violet light, in fact, a cloud could be obtained without any expansion at all.