646 The Earth's Penetrating Radiation at Different Altitudes. 



the observations were made there. It should be possible, 

 however, to get more direct evidence o£ the existence of: this 

 /3 radiation from the soil by making observations at the 

 surface o£ the ground in a clear space and at a point directly 

 above in the free air about 15 metres from the ground. 

 Such measurements should not involve great difficulty, and 

 it is hoped shortly to undertake them. Additional obser- 

 vations will also be made to see if a /3 type of radiation is 

 emitted by the wails generally of structures which are exposed 

 to the earth's penetrating radiation. 



III. Summary of Conclusions. 



1. At Toronto the ionization obtained in air confined in a 

 thin-walled zinc receiver of about 30 litres capacity on the 

 surface of the bare ground of the university lawn is greater 

 than that obtained in the same air on the ice in Toronto Bay 

 by about 5'8 ions per c c. per second. 



2. Evidence has been presented which goes to show that 

 this same difference exists at Livorno, Italy, and near Cape 

 Town in South Africa, between readings taken on the open 

 sea and those taken on the neighbouring land. 



3. As Eve * and Simpson and Wright f have shown that 

 the penetrating radiation over the ocean from radium and 

 thorium in the sea is negligible, it follows that 5*8 ions per 

 c.c. per second represents the average effect of the penetrating 

 radiation emitted by the earth and any rays of the /3 type 

 which may accompany it. 



4. The observations at Toronto give evidence of the 

 existence of a radiation of the (3 type accompanying the 

 penetrating radiation from the soil which produced in the 

 air in the zinc receiver about 1*5 ions per c.c. per second. 



5. Assuming that Eve's value of 1*6 ions per c.c. per 

 second represents the direct effect of the penetrating radiation, 

 it follows that 2*7 ions per c.c. per second must be produced 

 by the secondary rays emitted by the walls of the zinc re- 

 ceiver, or by these and by other radiations which may be 

 emitted by the soil and which have not yet been detected. 



The Physical Laboratory, 



University of Toronto, 



May 10th, 1911. 



* Loc. cit. f Loc. cit. 



