Atmosphere due to Radioactive Matter. 33 



This is the calculated value of the mean ionization at a 

 given locality, near the earth, due to the penetrating rays 

 from the radium C in the earth. It will be nearly constant at 

 any given place, but it will vary from place to place according 

 to the amount of radium present in the surface soils and 

 rocks of the more immediate neighbourhood. It may be 

 noted in particular that the ratio of N to N' is equal to that 

 of Q/\ to Q'/V, whose accented letters refer to the earth. 

 But A, and A/ are proportional to p and //, the densities of the 

 earth and soil. Hence 



N7N=Q>/Q/= l -4xlO-^x2-7x : 001.^ 1 1 xl 1 3 xl0Q = 23- 



The penetrating radiation from the earth is therefore 

 normally about 23 times as great as the penetrating radiation 

 from the atmosphere, at least so far as radium C is concerned. 

 I see no escape from this conclusion*, and it appears to 



* We may, however, consider an extreme case. Suppose that the 

 active deposit of radium and thorium were distributed uniformly, as 

 near the earth's surface, for a height of 5 kilometres ; and suppose, 

 further, that the whota of this active deposit were carried suddenly to 

 the earth by a, fall of rain. There would then be on or near the earth's 

 surface ox I0 1 "x8x 10~ 17 or 4x lO" 11 gramme of RaC (expressed in 

 terms of radium). This is equal to the amount of radium C in 30 or 

 40 centimetres of ihe soil, in each case per square centimetre. The thin 

 layer of RaC derived from the atmosphere would, however, be by far 

 the more efficient ionizer, because the y rays would not have to penetrate 

 through, and be absorbed by. the surface layers of the earth. The 

 increase of the penetrating radiation during rain has been observed by 

 Mache, and b} r G-ockel. Such variations, however, were not found by 

 McLennan at Toronto. The motion of RaC earthwards, not carried by 

 a water drop, would be only about 50 metres an hour, due to the 

 potential gradient. 



I have calculated the ionization at various altitudes due to this very 

 hypothetical surface layer of radium C, and obtain, 



whence 



N = 2tt 2 K 



Jo z 



Altitude. 

 lm. 

 10 

 100 

 1000 





Ions/cm. 3 sec. 

 1-8 

 1-05 

 •32 



negligible 



On comparing this table with result (18) it will be seen that, in 

 extreme cases, near the earth's surface the ionization due to the RaC 

 carried down by rain or snow might equal or exceed that due to the 

 RaC contained in the soil and rocks. In general, the active deposit 

 would doubtless enter the soil with the rain. 



Phil. Mag. S. 6. Vol. 21. No. 121. Jan. 1911. D 



