680 Prof. McLennan and Mr. Mercer on Ionization 



stopper, K (fig. 2). When hydrogen was used the ionization 

 tracks in most cases extended completely across the cloud- 

 chamber. In the present investigation, however, it was the 

 ends of the trails which were to be specially examined, and 

 so the tracks in hydrogen were cut down by covering the 

 layer of polonium with sheets of very thin aluminium leaf. 

 This was done in all the experiments in which the photo- 

 graphs of tracks in hydrogen were taken which are described 

 in this paper. The lengths of the reduced tracks were 

 generally about 4 cm. or less. 



III. Experiments. 



Although the investigation was primarily directed to 

 obtaining, if possible, evidence of the production and of the 

 ionizing power of the "H" particles of hydrogen, and many 

 photographs were taken with that end in view, not one of 

 the photographs showed any trace of cloud tracks pointing 

 to their production or to ionization by them. Many of the 

 tracks showed abrupt bends similar to those obtained by 

 Wilson, and occasionally very short spurs were obtained at 

 these bends, but no spurs were obtained such as one should 

 expect to get with " H " particles travelling with velocities 

 such as those Marsden found they possessed. 



This absence of " H " particle cloud tracks in our experi- 

 ments cannot be taken, however, to mean that " H" particles 

 are not produced in hydrogen, or that they do not possess the 

 power to ionize a gas, but it goes to confirm, rather, what has 

 been already surmised, that when alpha rays traverse hydrogen 

 or hydrogen containing traces of air 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 pho- 

 tograph reproduced in fig. 4 (PL XIII.) is one taken of water- 

 cloud tracks of alpha rays in hydrogen, and enlargements 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 



