§25 Mr. E. Marsden on the 



A comparison of the numbers shows that practically all 

 the scintillating particles originate in the passage of the a. 

 particles through the hydrogen. The small number obtained 

 with the aluminium foils near S only corresponds to one 

 scintillation in 8 minutes, and may be due to natural 

 activity. 



The following experiment was made to throw some light 

 on the direction of projection of the H particles. The source 

 was placed about 38 cm. from the screen, and between them 

 at a distance 16 cm. from the source was placed a card- 

 board stop with a hole 3' 2 cm. diameter at the centre 

 (rig. 1 a). With this arrangement 6*5 scintillations per 

 minute were observed on the screen. Replacing this stop by 

 its complement a disk 3*2 cm. diameter suspended centrally 

 by threads (fig. 1 b), the number was only 2'5 per minute. 

 When an aluminium foil, equal in stopping power of a 

 particles to 4 cm. of air, was placed near the zinc-sulphide 

 screen, these numbers were reduced to 4'3 and 0*42 scintil- 

 lations per minute, i. e. to 65 and 17 per cent, respectively. 

 In the second case, the relative angles between the K 

 particles and the a particles will be much greater than in 

 the first case ; and the experiment shows, in agreement with 

 theory, that they have a considerably smaller average 

 velocity and are consequently more easily absorbed. 



Experiments at High Pressure. 



The scintillations produced by the H particles differ little 

 from those produced by a particles except that they are 

 not so intense. In the above experiments, however, the 

 luminosity of the screen due to the /3 and y rays from the 

 source was considerable. Further, the counting was made 

 more difficult owing to the small rate at which the H particles 

 could be made to strike the screen, due to the large dis- 

 tance of the source necessitated by the long range of the 

 a particles. Experiments were therefore made in hydrogen 

 at high pressure so that the a particles had a smaller range, 

 and by placing the apparatus in a strong magnetic field the 

 /3 rays were prevented from striking the screen. The appar- 

 atus * used is shown in fig. 2. 



a particles from the source S were absorbed in the 

 hydrogen filling the apparatus, and the H particles produced 

 passed through a hole in the disk D and impinged on a zinc- 

 sulphide screen Z, where the scintillations were observed by 



* The high pressure apparatus v>-as purchased out of part of a grant 

 from the Royal Society. 



