the Radiations from Radioactive Substances. 11 



substances examined, viz. aluminium and Dutch-metal, tin- 

 foil, and paper. 



The same order, as will be shown later, holds also tor the 

 penetrating power of" radiations in air. 



Absorption of the Radiation in Air. 



The method employed of determining the absorption of the 

 radiation in air was similar to that explained in a previous 

 paper (Phil. Mag. Jan. 1899, p. 124). 



Two insulated parallel plates kept a fixed distance of 2 cm. 

 apart, could be moved by means of a screw to different 

 distances from the parallel radioactive surface. The radia- 

 tion from the active surface passed through a circular opening 

 in the lower plate, covered with thin aluminium-foil, and 

 was stopped by the upper plate. The current . between the 

 two fixed plates for a voltage sufficient in all cases to give 

 the maximum or saturation current, was determined for 

 different distances from the radioactive plate. If the radius 

 of the active surface is large compared with the distance of 

 the lower of the pair of plates from it, the current between 

 two fixed plates for a distance as of the lower plate from the 

 radioactive surface, can be readily shown from the theory of 

 ionization (loc. cit.) to vary as e~ Kx , where X is the coefficient 

 of absorption of the radiation in the gas. 



The results of the experiments are given in curves fig. 5. 

 For each radiation the maximum rate of leak between the 

 plates at a distance of about 2 mm. from the active surface is 

 taken as 100, for the purpose of comparison. It will be 



Fig. 5. 



20 Zl ?4 tb e8 30 11 34 36 38 40 42 



observed that the rate of leak, which is a measure of the 

 intensity of the radiation, falls off, approximately in G. P. 



