816 Sir E. Rutherford and Dr. J. Chad wick on the 



was shown by the following experiment. A screen of silver 

 of stopping-power 3*7 cm. was placed 1 cm. distant from the 

 source of a rays of range S'6 cm. We know from experi- 

 ment that the ravs after traversing- the silver are ineffective 

 in producing long-range particles. Consequently the swift 

 atoms are produced only in the first centimetre of the air. 

 This absorption curve of the H atoms liberated by a particles 

 of range between 8' 6 and 7' 6 cm. is shown in curve F (fig. 3). 

 When a source of rays of range 7 cm. was substituted for 

 the 8" 6 cm. the effects observed were considerably smaller. 

 The effect due to the a rays of range 5 cm. emitted by ThC 

 was small and easily allowed for. 



Experiments with Aluminium. 



A plate of aluminium of stopping-power 3*5 cm. was 

 placed immediately in front of the source, whose distance 

 from the zinc-sulphide screen could be varied. The alu- 

 minium had been carefully heated in a vacuum furnace to 

 drive off occluded hydrogen and water- vapour. Dry oxygen 

 was circulated through the apparatus. By placing absorbing- 

 screens of silver and gold between the source and the 



Fio-. 4. 









! 



I 

 ALUMINIUM 











\ 







A* 8-6 °< Particle 



0=4-5 ■ - 











N 



\ 



\ 





I 

 i 









1 



x 



\ 

 \ 



\ 



V 



\ 



! 



X 



> 









i 



--. 



\c 





^ 1 \ 







" - — — . 



! 

 | 







- j r.ns or fti 



aluminium plate, the effect of velocity of the a particle on 

 the number and range of the emitted particles could be 

 examined as before. The results are shown in fig. 4, where 



