174 Prof. E. Rutherford on some 



line. 12*5 thicknesses of foil absorbed the rays to the 

 same extent as 6*8 cms. of air, so that each thickness of 

 aluminium corresponded in absorbing power to *54 cm. of 

 air. For a screen of zinc sulphide, the phosphorescent action 

 ceased at a distance of air of 6*8 cms., showing that the 

 photographic and phosphorescent ranges of the a rays in air 

 were practically identical. 



The experiments with barium platinocyanide and wille- 

 mite were more difficult, as the ft and 7 rays from the active 

 wire produced a luminosity comparable with that produced 

 by the a rays. Fairly concordant results, however, were 

 obtained by introducing a thin sheet of black paper between 

 the active wire and the screen. If the luminosity was 

 sensibly changed, it was concluded that the rays still pro- 

 duced an effect, and in this way the point of cessation of 

 phosphorescent action could be approximately determined. 

 For example, with eight thicknesses of foil over the active 

 ware, the additional thickness of air required to cut off the 

 phosphorescent effect of the a rays was 2*5 cms. for willemite, 

 and 2*1 cms. for barium platinocyanide. The corresponding- 

 distance for zinc sulphide was 2*40 cms. ; a value,, intermediate 

 between the other two 



Since eight layers of foil are equivalent to 4'3 cms. of air, 

 the range in air of the phosphorescent effects for zinc sulphide, 

 barium platinocyanide, and willemite correspond to 6*7, 6'S, 

 and 6'4 cms. respectively. The differences in these values 

 are quite likely due to experimental error. 



Discussion of Results, 



We have seen that the ionizing, phosphorescent, and photo- 

 graphic actions of the a rays emitted from radium C cease 

 after traversing very nearly the same distance of air. This 

 is a surprising result when it is remembered ihat the a particle, 

 after passing through this thickness of air, still possesses a 

 velocity at least 60 per cent, of its initial value. Taking the 

 probable value of the initial velocity of the a particle from 

 radium C as 2*5 x 10 9 cms. per second, the ionizing, phos- 

 phorescent, and photographic actions cease when the velocity 

 of the a particle falls below 1*5 x 10 9 cms. per second, that 

 is, a velocity of about 1/20 of that of light. The particle 

 still possesses nearly 40 per cent, of its initial energy of 

 projection at this stage. 



These results show that the property of the a rays of 



