314 



Mr. 0. S. Wright on Variations in the 



afford a conclusive proof of the efficacy o£ Lake Ontario water 

 as a screen for the earth's penetrating radiation. 



The experiments made at stations 5 and 6 are of special 

 interest, as the conductivity obtained at these points was- 

 practically the same as that obtained in the experiments on 

 the surface of the Lake which shows that the sand was 

 entirely free from the radioactive substances which were 

 probably present in the clays and rocks at other points o£ 

 observation. 



6. Secondary Rays Induced by the 7 Rays from Radium.. 



To determine if this ratio of 1*1 to 1*23 to 1*0 for the 

 three receivers of Pb, Zn, and Al would be found to hold also 

 in the case of ionization due to the 7 rays from radium, a 

 series of measurements was undertaken with the electrometer 

 used before. 



A quantity of radium bromide encased in a lead box with 

 walls 2 cms. thick, was used as the source of the penetrating 

 rays, and the ionization measured in each receiver for different 

 distances between it and the radium. The difference between 

 the ionization before and after the radium was placed in 

 position, was recorded as due to the primary rays from the- 

 radium together with the secondary rays excited by them in 

 the receiver. From Table XII. it will be seen that " n," the 

 value of the ionization in each receiver due to the presence 

 of the radium, varied inversely as the square of the distance- 

 d from the cylinder, the same variation being shown graphi- 

 cally in fig. 8 (PI. V.). In this connexion it may be noted, 

 however, that for distances less than 3 metres a much larger 

 value for " n " was obtained than was demanded by the law 

 of the inverse square. 



Table XII. 





Voltage on Cylinder : 



4-250. 







d = distance of 



Lead. I. 



Zinc. 



1. 



Alwn 



inium. 



Radium from 

 Cvlindei 

























(metres). 



n. 



ml 1 . 



71. 



rid*. 



71. 



nd\ 



60 



223-5 



6847 



154-7 



5569 



135-4 



4874 



5-5 



262*6 



6933 



181-9 



5866 







50 



314-6 



7022 



2-29 6 



5740 



195-3 



4882 



4-5 



381-9 



7047 



279-8 



5665 



246-3 



4987 



40 



478-3 



6899 



3597 



5755 



317-1 



5074 



3-5 





7371 



462-0 



5668 



421-7 



5166 



3-0 











842-5 



5206 





Mean 



=6976 



Mean 



=5711 



Mean 



=4996 



