| Description | The first half of this volume is concerned with thunderstorms and contains many suggestions regarding possible experiments to detect changes in the earth's field and for experiments with photography of falling drops, etc. After an apparent gap of about ten years in work on condensation, at least in so far as these records are concerned, Wilson returns to the subject with clearly indicated purposefulness,. Under a heading 'Present state of work on expansion apparatus', dated 24 December 1910, there is a detailed lay-out of the state of this technique at that time, followed by a series of suggestions and ideas for future chambers and of methods of photography and illumination. One finds here the sentence- 'Assuming that the difficulties of photographing the drops were overcome, one might see track of X-ray marked out by narrow core of drops formed on positive ions with more scattered cloud of those formed on negative ions round the,.' This section of the book is of great interest. It contains essentially the first detailed planning of the Wilson cloud track chamber. Under the heading 18 March 1911 (see plate 12), the use of such an apparatus under exposure to an X-ray beam is described and the observation is made: 'Cloud was discontinuous showing numerous knots. Are these cross sections of tracks of rays?'which marks, perhaps, the occasion of the first observations of tracks by this method. On 20 March 1911, Wilson writes '...rays showed up much better. They were extremely sharply defined...Looked at from above. The individual rays were seen, in many cases as extremely fine lines, chiefly radiating from the aperture.' On the same day photographs were taken. The rest of this volume contains many suggestions for improvements of the technique and discussions about the choice of expansion chambers for experiments with different types of radiation, together with methods of illumination and photography etc., in order to make the technique more efficient. |