106 ROYAL SOCIETY OF CANADA 



This result, surprising as it at first sight appeared, is exactly what 

 one would expect from a consideration of the values of Table XI, which 

 shew that practically the same value for the ionization was obtained over 

 water of depths ranging from 2.5 to 10 metres. Owing also to this fact 

 that complete absorption of the y rays from radium took place it is 

 clear that over the water of Lake Ontario, at least, there is no appre- 

 ciable secondary effect due to the earth's penetrating rays, such as has 

 been shewn to be emitted by a brick wall under bombardment by the 

 y rays from radium. 



Since, in addition, we know from an observation made by Prof. 

 McLennan, that the waters of Lake Ontario contain no appreciable 

 radioactive emanation, the conclusion is forced upon us that in the 

 case of the experiments described above with the cylinders of lead, zinc, 

 and aluminium, the water of the lake acted as a perfect screen for the 

 earth's penetrating radiation, and the values of " q " recorded in Table 

 XI for Observation Station No. 2 may be taken to represent the con- 

 ductivities impressed upon the air either by intrinsic radiations arising 

 from the metals of the receivers themselves, or else by radiations from 

 active impurities still remaining in them. 



Taking in the case of the lead cylinder the value of 8.6 ions per 

 cc. per sec, as due to intrinsic activity, we have a means then of cal- 

 culating the ionization in this cylinder, due to the soil alone in any 

 position on the surface of the earth. 



Prom the value of 11.1 ions obtained with the lead cylinder over 

 clay (given in Table XI) a calculation shews that 2. G ions per cc. per 

 second in such a cylinder is due to the soil alone. If now we assimie, 

 as seems justifiable from the experiments of Professor McLennan, loc. 

 cit., on the conductivity of air enclosed in lead receivers, that the 

 ratio of the ionization due to the 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 

 localities 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 agreement with the experimental result obtained in this 

 investigation. 



In passing, it might be noted that this calculation throws some 

 light on the results obtained (Table VI) for the regular daily variation, 



' strong, Phys. Zeit., 9, pp. 117-119, Feb. 15, 1908. 



