RADIOACTIVITY VISUALISED 487 



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 velocity sufficient to enable it to traverse 

 several millimetres or even centimetres 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 photographs. The 

 X-rays do not appear to produce any ionisation 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 — between the beta-rays produced by the action of X-rays 

 and those emitted spontaneously by the radioactive substances 

 lies in their initial velocity ; for there is no lack of evidence 

 that all negatively charged corpuscles are alike, except in so 

 far as their properties are affected by their velocity. And in 

 fact, the tracks of the beta-particles or cathode-rays excited in 

 air by X-rays are indistinguishable from the end portions 

 of beta-ray tracks, such as are shown in figs. 6 and 7. 



The tracks are far from straight and as the particle approaches 

 the end of its course the deviation becomes generally more and 

 more marked, the particle being more easily deflected the 

 smaller its velocity. 



The departure from straightness is mainly of the nature of 

 a general curvature due to an accumulation of inappreciable 

 deflections at successive encounters ; sudden deviations through 

 large angles, the result of single encounters of a more effective 

 kind, also appear occasionally. 



The number of ions produced per centimetre is known to 

 increase rapidly as the velocity of the cathode-ray particle 

 diminishes. This is shown by the increased density of the 

 clouds towards the ends of the tracks. 



Fig. 9 is an enlargement of a portion of the track of a 

 beta-particle emitted in air exposed to X-rays. The individual 

 ions are clearly visible and may readily be counted ; the number 



