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



that some of the active deposit may have penetrated some 

 distance into the aluminium disk and there given rise to 

 particles in the forward direction of the a rays. To avoid 

 this uncertainty, one side only of a silver plate of 3*7 cm. 

 stopping-power was coated with active deposit by fixing it 

 at the end of a tube filled with emanation. This plate was 

 then used as a source with the active side facing away from 

 the zinc sulphide screen. The active face of the silver was 

 covered by an aluminium foil of 3'5 cm. stopping-power. 



The number and absorption of the particles which are 

 projected in a backward direction is shown in curve B of 

 fig. 5. The corresponding curve for particles which are shot 



Fig. 5. 











ULUMINi 



UM 











\ 







Ra 



nge Of o( Particles *7-o cms 

 Forward Particle 









\ 







A- 









\ 







6 ■ Baca ward Particle 











\a 





















\b 

















ABSORPTION IN CHS OF AIR 



in the forward direction for an equal thickness of aluminium 

 is shown in curve A. It is seen that the number in the 

 backward direction is of the same order of magnitude as for 

 the forward ; but the maximum range in the backward 

 direction for particles of range 7 cm. was smaller — viz., 

 67 cm. instead of the 90 cm. observed for those shat 

 forward. 



As previously pointed out, it is very difficult to fix the 

 exact ranges in these cases on account of the smallness of 

 the observed effects. A few experiments were made to test 

 whether the particles from nitrogen showed a similar effect. 

 The arrangement was the same as for aluminium, but the 

 ol particles were fired backwards into air instead of into 

 the aluminium plate. The number projected in a backward 

 direction, if there are any at all, is very small compared 



