486 SCIENCE PROGRESS 



for its very small mass and its path may be sensibly straight for 

 distances of several centimetres, in spite of the very large number 

 of atoms which it must traverse. This is illustrated by the 

 next picture (fig. 7) in which is shown, in addition to the end of a 

 beta-ray, a portion of the trail left by a beta-particle while its 

 velocity was still very high ; it is noticeable that it is practically 

 straight. Another result of the high velocity is that very few 

 ions have been set free along its 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 distinguishable 

 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 Rontgen 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 ionisaation produced by 

 gamma-rays appears to be, 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 ionisation 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. 8 is a photograph showing the effect of such a flash 



