316 



Mr. C. S. Wright on Variations in the 



then enclosed in a tube o£ brass whose walls were 1 cm. in 

 thickness. 



The results (Table XIII.) showed that 2 metres of water 

 completely cut off all effect from the radium, both primary 

 and secondary, the same being illustrated by the curve in 

 fig. 9 (PI. V.). 



Table XIII. 

 Lead Cylinder 1 : 113 cms. above ice. 



Depth of water over 

 radium = (7. 



Number of ions due 

 to radium = " n." 



metres. 



4485 



1/2 



4472 



1 



16-11 



2 



•69 



3-65 at bottom. 



•62 



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 show 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 

 appreciable secondary effect due to the earth's penetrating 

 rays such as has been shown 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 XL for 

 Observation Station No. 2 may be taken to represent the 

 conductivities 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. 



