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ROYAL SOCIETY OF CANADA 



the chalk quarries were found to be only about 42% of those obtained 

 on the surface of the ground. 



In this connection it will be recalled that Elster and GeiteP ob- 

 served a fall of 28% in the conductivity of air enclosed in an aluminium 

 receiver when the apparatus was taken from the surface down to the 

 bottom of a rock salt mine. It will be remembered, too, that C. S. 

 Wright, in his experiments at Toronto, found the conductivity of air 

 confined in closed metallic vessels in experiments on the ice of Lake 

 Ontario to be about 46% less than in similar ones made on the lawn in 

 the neighbourhood of the Physical Laboratory. It will be recalled 

 too that the ionisation values obtained by Wright on the sand bars on 

 the Island near Toronto, were but slightly higher than those obtained 

 by him on the ice. The experiments at Toronto coupled with those at 

 Valkenburg, and with those of Elster and Geitel, would seem to shew 

 that the earth and not the atmosphere is the source of the penetrating 

 radiation, but that certain waters and soils and salt deposits are com- 

 paratively free from radioactive substances, and can therefore act as 

 efficient screens. It also seems evident from these experiments that 

 the penetrating radiation, in some localities at least, does not come 

 from a very great depth in the earth's crust. 



The experiments made at Sestola, however, seem to point to the 

 atmosphere as the source of the radiation. 



In the following table the times of the day of the maxima and 

 minima ionisation periods observed by the different investigators cited 

 are collected. From this table it will be seen that very little connec- 

 tion exists between the times of the maxima and minima obtained in 

 the different localities mentioned. It seems evident, too, after sur- 

 veying all the recorded observations that the variations noted must 

 be ascribed to changes in atmospheric conditions rather than to solar 

 influences. No such regularity occurs in the variations as one should 

 expect if the penetrating radiation had its origin in the sun. 



Phys. Zeit. No. 1, 1905, p. 733. 



