Conductivity of Air enclosed in Metallic Receivers. 317 



Taking in the case of the lead cylinder the value of 8*6 

 ions per c.c. per sec. as due to intrinsic activity, we have a 

 means then of calculating the ionization in this cylinder due 

 to the soil alone in any position on the surface of the earth. 



From the value of 11*1 ions obtained with the lead cylinder 

 over clay (given in Table XI.), a calculation shows that 

 2*6 ions per c.c. per sec. in such a cylinder is due to the soil 

 alone. If now we assume, as seems justifiable from the 

 experiments of Professor McLennan, loc. cit., on the con- 

 ductivity of air enclosed in lead receivers, that the ratio of 

 the ionization due to secondary rays in a lead cylinder of the 

 dimensions of those used in this investigation, is twice that 

 due to the primary, it follows that approximately 0*9 ions 

 would be generated in free air over a clay soil by the earth's 

 penetrating radiation in the locality referred to above. 



On the basis of Strutt's determination of the radium 

 content of the rocks and soils, Strong * has recently deduced 

 a value of 0*8 ions as an upper limit for the ionization in free 

 air due to the penetrating rays from radioactive substances 

 in the soil ; and this number it will be seen is in good agree- 

 ment with the experimental result obtained in this investi- 

 gation. 



In passing it might be noted that this calculation throws 

 some light on the results obtained (Table VI.) for the regular 

 daily variation, which it will be remembered was found to be 

 inappreciable and which is exactly what should be expected 

 if only some two or three of the ions generated in the 

 cylinder per c.c. per sec. were due to a penetrating radiation 

 from the earth. 



8. Summary of Results. 



The results obtained in the preceding investigation may be 

 summed up as follows : — 



(1) No evidence of a regular daily variation was noted. 



(2) It has been shown that there is a penetrating secondary 

 radiation set up by penetrating rays such as those from radium, 

 in the brick wall of a room. 



(3) Proofs have been adduced to show that the water of 

 Lake Ontario acts as a perfect screen both for the earth's 

 radiation and, if a sufficient depth be taken, for the y rays 

 from radium. On this account and owing to the fact that 

 the water of Lake Ontario contains no active impurity, it has 

 been possible to determine what portion of the ionization in 



* Strong, Phys. Zeit. ix. pp. 117-119 (Feb. 15, 1908). 



