Atmosphere due to Radioactive Matter. 31 



This value is smaller than that found by several observers 

 for the total ionization due to the penetrating radiation as 

 determined by complete screening with lead or water. This 

 fact suggests that a considerable part of the penetrating 

 radiation might come from the radioactive matter in the air. 

 Subsequently it will be shown, however, that such a con- 

 clusion appears altogether untenable, according to the present 

 state of our knowledge and information. 



Theoretical Consideration. 



If there be a quantity Q grammes of radium at a point 

 source, then the ionizing effect at a distance r in free air will 

 be proportional to Q/r 2 e Kr , where X is the coefficient of 

 absorption of the 7 rays in air. If N is the rate of production 

 of ions in a cubic centimetre, we may write N = KQ/rV w *. 

 This constant K was measured by me * with a fair degree of 

 accuracy, and its value inside a thin aluminium electroscope 

 is 3'4xl0 9 . Its value in free air at standard pressure and 

 temperature is therefore about 3*1 x 10 9 . I am hoping to 

 evaluate shortly this somewhat important constant with more 

 accuracy (6) 



Integrating from to so we can calculate the total number 

 of ions which one gramme of radium can produce in virtue 

 of the 7 rays from the equilibrium amount of RaC. We have 



K±Trr 2 drQe-^ or 4ttQK/\. . . , (7) 



1 



Now X /density is equal to *034 according to McClelland 

 and to *04 according to Soddy. Selecting, in the present 

 case, the former value, we have X for air equal to '000014. 

 Hence the total number of ions due to the 7 rays from a 

 gramme of radium in equilibrium is 8*5 x 10 14 per sec. . (8) 



But from Geiger'sf experiment on the ions produced by 

 a. rays, we know that RaEm, A and C in equilibrium with 

 one gramme of radium produce 2 x 10 It5 ions per second, or 

 about 23 times as many as the 7 rays, so far as the radium 

 products in the atmosphere aie concerned. Near the earth 

 the effect of the 7 rays must be halved, so that we get a ratio 

 of 1 to 46 or 



a rays Ra Em, A, C 1M53 



7 rays RaC '035 . . (9) 



The addition for the ionization of the primary /3 rays is ai 



* Phil. Mag. Sept. 1906. 

 t Proc. Roy. Soc. July 1909. 



