Atmosphere due to Radioactive Matter. 29 



emanation, A and 0, in the atmosphere, namely, 



80 x 10- 12 x 3-4 x 10 10 (1*74 + 1'87 + 2'37)10 5 per rn. 3 , 

 or 8 X 3-4 x 5-98 x 10 4 per m. 3 , 



or I 63 ions per cm. 3 per sec, (2) 



Thorium and Alpha Rays. 



We are not in a position to make any such accurate calcu- 

 lation for thorium. It is known, however, that the active 

 deposit due to thorium on a negatively charged wire is, on an 

 average, about 60 per cent, of that due to radium ; and we 

 may perhaps guess, for it is little better than a guess, that 

 the a rays from the thorium products in the atmosphere will 

 contribute about, and not more than, 



1*00 ion per cm. 3 per sec. .... (3) 



Gamma Rays from RaC in the Earth. 



Next the reasonable assumption will be made that the 

 penetrating radiation, discovered by Rutherford, Cooke, and 

 McLennan, is y radiation due to the presence of uranium, 

 radium, and thorium. It is noteworthy that the amount of 

 the radioactive products in the soil is of the right order 

 of magnitude to account for the ionization effects due to the 

 penetrating radiation, at least near the earth's surface; also 

 that the amount of radium emanation is of the right order to 

 account for the active deposit which may be collected on a 

 negatively charged wire. These relations encourage the 

 belief that we are dealing with a true cause. It has also been 

 shown repeatedly that the natural leak of an electroscope 

 may be decreased in rate by surrounding it with screens of 

 lead or water. Thus McLennan and Wright at Toronto have 

 found that the water and ice of Lake Ontario form an efficient 

 screen from the rays from the radioactive matter in the land 

 beneath. F. W. Bates has verified this result over the ice 

 on the River St. Lawrence near Macdonald College, and 

 Gockel also has repeated the observation in a boat over a 

 Swiss lake. There is therefore abundant and certain evidence 

 that at least a part of the ionization of the air in an electro- 

 scope is due to 7 rays from the radioactive matter in the 

 earth. When we come to examine this question in more 

 detail there are several points of considerable importance 

 and interest. 



We wall first quote C. S. Wright's figures (Phil. Mag. Feb. 



