34 Dr. A. S. Eve on the Ionization of the 



negative the view that the penetrating radiation comes equally 

 from all directions. So far as our knowledge of the distri- 

 bution of thorium at present goes, we may also predict that 

 a similar statement will be found to hold good for the 7 rays 

 of thorium C. 



Returning to the result previously stated (13), we will next 

 assume that the work of Blanc, Joly, W. Wilson (Phil. Mag. 

 Feb. 1909), and others indicates that the penetrating radia- 

 tions from the radium and from the thorium in the earth 

 are about equal, and we can now set forth a complete table. 



Type. 



Whence. Ra. Th. Total. 



a rays 



;j , 



Air 1-63 1-00 2-63 



Air 035 -025 -06 



Air -030 -025 -06 



Earth -80 -80 160 









2-50 1-85 430 



1 



(15) 



This rate of production of ions per cubic centimetre, 

 namely 4*35, corresponds to the presence of 1660 ions per 

 cm. 3 , supposing that they were all small ions. The maximum 

 value of ions measured in clear weather by an Ebert ion- 

 counter is usually somewhat less than this. 



It must be remembered that the numbers quoted in the 

 preceding table are subject to considerable variations both 

 with time and place; and are only intended as a guide to the 

 mean values, which will no doubt in due course of time be 

 determined with an accuracy to which my estimates can of 

 necessity make no claim. 



It will be seen that the theoretical value found, 1*6 ions 

 per cm. 3 per sec. for the penetrating radiation from the earth, 

 is less than that obtained by Wright at Toronto, namely 2*5. 

 It is also considerably less than the number of ions per cm. 3 

 per sec. lost when an electroscope is well screened by lead, 

 amounting to about 6. It is true that my calculations do 

 not take account of the feeble and negligible 7 radiation from 

 actinium, the distribution of which is unknown. Further, 

 it has been shown by Soddy (Phil. Mag. Oct. 1909) that the 

 7 rays from uranium X are more penetrating than was at 

 first supposed. The addition which should on this account 





