674 Mr. C. T. R. Wilson [March 7, 



noticeable that it is practically straight. Another result of the high 

 velocity is that very few ions have lieen set free along this path ; for the 

 faster the particle traverses an atom the shorter is the time during 

 which the forces can act. The individual ions are readily dis- 

 tinguishable in the photograph ; the droplets appear mainly in pairs 

 (each representing a positive and negative ion) but there are, in 

 addition, here and there closely packed groups of twenty or 

 thirty. 



In addition to the alpha- and beta-particles, radioactive bodies 

 emit an extremely penetrating type of ionising rays — the gamma-rays 

 — having properties similar to those of Eontgen rays. If we expose 

 the cloud chamber to this radiation (cutting out the alpha- and beta- 

 rays by a lead screen), we see on expansion extremely fine threads of 

 cloud crossing the vessel in all directions. These are the tracks of 

 beta-particles emitted mainly from the walls of the vessel under the 

 influence of the gamma-rays. The whole of the ionization produced 

 by gamma-rays appears to he, as it were, secondary and due to the 

 beta-rays. 



The remaining pictures illustrate some of the properties of Rontgen 

 rays. 



In studying the nature of the process of the ionization of air by 

 X-rays by means of the expansion apparatus, it is convenient to use 

 an instantaneous flash of the rays produced by sending a single 

 Leyden jar discharge through the Crookes tube. The discharge is so 

 timed that the rays pass through the cloud chamber immediately 

 after the expansion of the air, so that they traverse it while it is 

 supersaturated with water vapour. The ions produced are thus at 

 once fixed by the condensation of water vapour upon them before 

 any appreciable diffusion has occurred ; the illuminating spark is 

 timed to pass a fraction of a second later and so give an instantaneous 

 photograph of the clouds condensed on the ions. 



Fig. 9 is a photograph showing the effect of such a flash of 

 X-rays— the radiation being confined to a narrow cylindrical beam 

 by lead screens provided with apertures. The photograph was 

 obtained with the camera pointed horizontally through the cloud 

 chamber in a direction at right angles to the beam of X-rays. 



In the light of knowledge furnished by other methods, we may 

 interpret the picture in the following way. Under the influence of 

 the X-rays, an atom here and there in the path of the cylindrical 

 beam of X-rays has emitted a corpuscle or beta-particle with sufficient 

 velocity to enable it to traverse several millimetres or even centi- 

 metres of air, ions being set free along its path. It is the paths 

 of these beta-particles or cathode-rays which are made visible in the 

 photograph. The X-rays do not appear to produce any ionization 

 other than that effected through the agency of the beta-rays excited 

 by them, as indeed Prof. Bragg has long maintained. 



The only room for difference — apart from their mode of origin — 



