Conductivity of Air enclosed in Metallic Receivers. 311 



This result is probably due to the fact that any radioactive 

 substances originally present in the sand have been washed 

 away by the action of water. 



In this connexion it is important to note that in making 

 these determinations of the conductivity of air enclosed in 

 lead receivers, it w T as frequently observed that in the measure- 

 ments on the surface of the lake, with newly cleaned lead 

 •cylinders filled with freshly filtered air, the drop in conduc- 

 tivity observed was invariably about 50 per cent, less than 

 the drop obtained with lead cylinders, which, after being- 

 well cleaned and filled with clean air, had been allowed to 

 stand long enough to reach the steady state. 



This difference in the drop in conductivity is well illus- 

 trated by the results obtained with the lead cylinder No. 1. 

 With this cylinder in the steady state, as the numbers in 

 Table X. show, the conductivity when measured in the 

 laboratory corresponded to the generation of 22*5 ions per 

 c.c. per second. With the same cylinder freshly cleaned and 

 filled with well-filtered air the conductivity, as measured in 

 the same room in the laboratory, always corresponded to the 

 generation of approximately 15'3 ions per c.c. per second. 

 In the measurements on the surface of the lake-water, 

 however, the conductivity corresponded to the generation of 

 13*9 ions per c.c. per second when the cylinder was in the 

 steady state; while, as will be seen from the results recorded 

 in the next section, it corresponded to the generation of only 

 8'6 ions per c.c. per second when the cylinder had been 

 freshly cleaned and filled with filtered air. 



This difference in the values obtained for the drop in con- 

 ductivity with the lead cylinder in the two conditions, can no 

 doubt be traced to differences in the secondary radiation 

 ■excited in the walls of the vessels by the penetrating radiation 

 from the earth. 



It is clear that the surface of the lead after being freshly 

 cleaned must have gradually become covered with a deposit 

 through oxidation and other causes, and it is reasonable to 

 conclude that the presence of this deposit would produce such 

 a modification in the intensity of the secondary radiation as 

 to bring about the results described. 



(3) Measurements made in different localities and with 



different receivers freshly cleaned. 



The preliminary measurements just described made it 



abundantly evident that the lake water acted as a very 



efficient screen for the earth's radiation, a maximum decrease 



in the value of u q " of as much as 9 ions being recorded in 



Y2 



