Properties of the a. Rays from Radium. 



173 



is directly proportional to its density compared with air, 

 12 layers of aluminium;, each of thickness '00031 cm., cor- 

 respond to 8 cms. of air. The equivalent distance in air 

 found by experiment is 6 '5 cms., so that the absorption in 

 aluminium is somewhat less than the density law would lead 

 us to expect. 



Range of Phosphorescent Action in Air. 



Some experiments were also made to see whether the 

 action of the a rays in producing luminosity in substances 

 like zinc sulphide, barium platinocyanide, and willemite, 

 ceased at the same distance as the ionization property. 



A very active wire was taken and placed on a movable 

 plate, the distance of which from a fixed screen of phospho- 

 rescent substance could be quickly varied. The distance at 

 which the phosphorescent action ceased could be fairly 

 accurately determined. Different thicknesses of aluminium- 

 foil were then placed over the active wire, and the corre- 

 sponding distance at which the luminosity disappeared was 

 measured. The results are shown graphically in fig. 2, where 



'Fie. 2 



t 6 a 



Layelrs or Aluminium Foil. 



the ordinates represent the distance of the phosphorescent 

 screen from the active wire, and the abscissae the number of 

 layers of aluminium-foil, each *00031 cm. thick. 



It is seen that the curve joining the points is a straight 



