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ROYAL SOCIETY OF CANADA 



differences in their secondary y radiations, and I worked, therefore, 

 mainly with bricks, or with iron filings as radiator, and with a wooden 

 platform. 



The absorption of the secondary y rays by lead plates is shown 

 by the following: — 



The last three values of >^Tare not exact. 



The values of k and of Xj density for thicknesses between 2 to 

 3 mm. and 5 to 6 mm. were also found for various substances. 



In the right columns are values for the more easily absorbed 

 primary y rays as found by McClelland; these serve to illustrate 

 the relative softness of the secondary. 



The secondary ;- rays were next investigated by the method 

 of reversal of pairs of plates at A and B, precisely as described earlier 

 in this paper when using primary rays. Taking the mean values 

 for brick and iron radiators, on wood, iron and lead platforms, the 

 following results were obtained. 



