Properties of the a. Rays from Radium. 171 



calculated on the above assumptions, are shown in the 

 following table. 



Number of layers Observed Calculated 



of aluminium foil. energy. energy. 



100 100 



5 73 75- 



8 58 61 



10 53 51 



12 41 41 



The experimental and calculated values agree within the 



limit of experimental error. We may thus conclude, as a 



first approximation, that the same proportion of the total 



energy is abstracted from the a particles in passing through 



successive layers of aluminium-foil. 



Velocity of the a. Rays from other products. 



Knowing the velocity of the a particle expelled from 

 radium C, the velocity of the a. particle emitted from the 

 other radioactive products can at once be deduced, provided 

 the maximum range of its ionization in air is known. 



This velocity can be determined from the data already 

 given of the decrease of the velocity of the « particle in 

 passing through screens of aluminium-foil. The ionizing 

 action of the « rays ceases after passing through 12*5 layers 

 of foil, which is equivalent to a thickness of 6" 7 cms. of air. 

 Each layer of foil corresponds to a depth of 0'54 cm. of air. 

 The velocity, for example, of the rays from radium itself 

 which have a range of 5 cms. in air, that is, which would be 

 stopped by 5'5 thickness of foil, corresponds to the velocity 

 of the rays from radium C which have traversed 12*5 — 5*5 

 = 7 layers of foil. For convenience, the velocity of the 

 « particle from different products which have ranges in air 

 from 1 to 6'7 cms. are given in terms of Y , the velocity of 

 the a particle from radium C. 



Maximum range 



of the 



Maximum 



velocity ot 



a particles in air. 



cc particle. 









•64 



V 



1 





•71 



V 



2 





•765 



55 



3 





•82 



55 



4 





•88 



;? 



5 





•92 



55 



6 





•96 



5? 



6-7 





1-00 



5? 



7 





1-01 



55 



8 





1-06 



55 



