I'Jlo] on Paths of Particles Ejected from Atoms c?;') 



between the beta-rays produced by the action of X-rays and those 

 cinitted 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 

 aifected 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. 

 7 and 8. 



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 mure 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. 10 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 per centi- 

 metre amounts to about 188 pairs, when reduced to atmospheric 

 pressure. 



In taking the photograph shown in Fig. 11 the X-rays were made 

 to traverse the air before instead of after the expansion. The ions 

 hberated along the track of each cathode-ray were thus free to move 

 under the action of the vertical electric force maintained in the 

 cloud chamber, the positive travelling downwards, the negative 

 upwards. Each trail was thus divided into two portions, one consist- 

 ing of negative, the other of positive ions, before being converted 

 into visible cloudlets by expansion of the moist air ; the ions of 

 each trail have also had time to be considerably scattered by 

 diffusion. 



The representations of X-ray clouds shown thus far have all been 

 from photographs taken with the camera pointed horizontally and so 

 placed that a magnified image was obtained. The remaining photo- 

 graphs were obtained with the camera pointed vertically downwards, 

 the conditions being such that the whole visible contents of a 

 horizontal stratum of the cloud chamber, about 2 cm. in thickness, 

 were photographed just as in the case of the alpha-ray pictures. 

 Very intense illumination is required to make the cathode-ray tracks 

 visible in a picture taken in this way ; and it is only recently that I 

 have succeeded in photographing them. 



A thin sheet of copper was fixed in the centre of the cloud 



