[ruthekfoed] properties OF THE a RAYS FROM RADIUM 



la 



the corresponding distance at which the luminosity disappeared was- 

 measured. The results are shown graphically in Fig. 2, where the 

 ordinates represent the distance of the phosphorescent screen from the- 

 active wire and the abscissœ the numljer of layers of aluminium foil 

 each .00031 cm. thick. 



It is seen that the curve joining the points is a straight 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 cms. of air. For a screen of zinc sulphide, the phosphorescent 

 action ceased at a distance of air of 6.8 ems., showing that the photo- 

 graphic and phosphorescent range of the a rays in air were practically 

 identical. 



Fig. 2. 



a t 6 e 'o I a. 

 LfKYELFiQ or Auur^iNiuM Foiu 



The experiments with barium platino cyanide and willemite were 

 more difficult, as the /? and y rays from the active wire produced a 

 luminosity comparable with that produced by the rays. Fairly 



concordai nt results, however, were obtained by introducing a thin sheet 

 of black paper between the active wire and the screen. If the lum- 

 inosity ^\ere sensibly changed, it was concluded that the a rays still 

 produced an effect, and in this way the point of cessation of phos- 

 phoresceiit action could be approximately determined. For example, 

 with 8 i tiicKnesses of foil over the active ware, the additional thiclcness 

 of air required to cut off the phosphorescent effect of the a rays was 

 2.0 cms. for willemite, and 2.1 for barium platino cyanide. 



