[MCLENNAN- IONISATION TRACKS OF ALPHA RAYS IN HYDROGEN 215 



mercer] 



alpha rays traverse hydrogen exceedingly few of them, collide with 



the hydrogen atoms in such a way as to expel the "H" particles. 



Although the experiments were disappointing in this regard they 

 served to bring out some points of minor interest and a few of the 

 photographs taken are reproduced in the present communication to 

 illustrate these points. The photograph reproduced in Fig. 4 is one 

 taken of water cloud-tracks of alpha rays in hydrogen and enlarge- 

 ments of portions of this photograph are shown in Figs. 5 and 6. 



In Fig. 5 there is shown a sharp bend near the end of one of the 

 tracks and a less abrupt one near the middle of a second track. Fig. 

 6 also shows a very sharp bend near the end of one track but 

 as will be seen there is no sign of a spur of any appreciable length 

 associated with it. The tracks shown in these photographs are typical 

 of many which we obtained but as mentioned above none of them 

 showed spurs such as we expected to get with expelled "H" particles. 

 In taking the photograph shown in Fig. 4 a field of 70 volts was applied 

 in the cloud chamber and a Zeiss Planar lens F 4- 5, 5 cms focus was 

 used. The photograph was taken on an Ilford Monarch plate. 



The reproduction shown in Fig. 7 also shows a sharp bend in one 

 of the tracks but there is no indication of any spur at the bend. It 

 was obtained with hydrogen and condensed water vapour. The 

 field applied to the cloud chamber was 8 volts. 



Figs. 8, 9 and 10 are reproductions of enlargements of the ends 

 of some alpha ray tracks in hydrogen the clouds of which were formed 

 from water vapour. The field applied to the cloud chamber when 

 they were taken was 8 volts. The interest in these photographs lies 

 in the fact that the ends of the tracks and the curves they exhibit are 

 somewhat more extended than those obtained by Wilson with air. 

 In some cases it will be seen they resemble the curve at the end of a 

 hockey stick while in others the bends are more like the ends of a 

 shepherd's crook. 



Some of the tracks shown in Fig. 8, as will be seen, appear to be 

 double while others again are single and very distinct. The double 

 tracks are clearly out of focus and more or less indistinct. From their 

 general appearance the doubling would appear to be entirely an op- 

 tical effect but it is just possible that it represents an electrical separa- 

 tion of the ions in those tracks which were produced a short time before 

 the more distinct ones. Against this latter view, however, there is 

 the fact that the separation shown on the photograph appears to be 

 lateral while if it had been produced by the electrical field the displace- 

 ment in all probability would have been in a vertical plane or very 

 close to it. 



