﻿of the a, Rays of Actinium. 



253 



good agreement with one another, the range at which the 

 « particles of the actinium emanation cease to ionize the air 

 (including the correction of 0'3 cm. for the electrical method 

 and 1*2 cm. for stopping power of the mica screen) was found 

 to be about 5*8 cms. 



The ranges of the a. ray products of actinium are shown in 

 the following table. For completeness the ranges of the 

 products of thorium, determined by the writer, and of 

 radium, determined by Bragg and Kleeman, are added for 

 comparison. 



With the exception of the a rays from uranium, the range- 

 in air of the a particles expelled from all the products of the 

 radioelements have now been determined accurately. 



Table I. 





Range of 

 a Particles in 











Product. 



cms. in air for 

 atmospheric 

 pressure and 



temperature*. 



Product. 



Range. 



Product. 



Range. 



Radioactinium ... 



4"8 cms. 



Radiothorium ... 



30 cms. 



Radium 



350 cms. 



Actinium X 



G'55 ,, 



Thorium X 



5'7 „ 



Emanation... 



433 „ 



Emanation 



5-8 „ 



Emanation 



5-5 „ 



Radium A ... 



±•83 „ 



Actinium B 



5-50 „ 



Thorium B 



SO „ 



Radium C ... 



7-06 „ 







Thorium C 



S-6 „ 



Radium F t 



3-86 „ 



* The pressure during the period of this investigation did not vary to such 

 an extent as to produce marked differences in the ranges. 



t Determined by M. Levin, Americ. Journ. Science. July 1906. 



It is seen that the ranges of the a particles from thorium 

 differ among each other to a greater extent than those of 

 radium, and the latter to a greater extent than those of 

 actinium. For example, the extreme difference in the ranges 

 for the thorium products is -±'7 cms., for radium products 

 3*56 cms., while for actinium it is only 1*75 cm. 



A comparison of the ranges of the a particles from thorium 

 and actinium shows a very interesting relationship. In the 

 table below the products are arranged in the order of 

 magnitude of the ranges of their a particles. 



