Conductivity of Air enclosed in Metallic Receivers, 315- 



This variation from the law appeared at first sight some- 

 what difficult of explanation. On consideration, however,, 

 of the results obtained in the previous section, it at once 

 seemed evident that the variation of " n " from the value for 

 it demanded by the law of the inverse square was but another 

 manifestation of the same secondary radiation excited by the 

 presence of the radium in the neighbourhood of the brick 

 wall, against which the electrometer was set up. 



In Table XII. the value of the constant " nd 2 " has been 

 calculated, giving for the receivers of Pb, Zn, and AI 

 respectively the numbers 6976, 5711, and 4996, which are 

 thus a measure of the ionization in the different cylinders due 

 to the 7 rays from radium plus the secondary rays induced 

 by them in the enclosing metals. These numbers we see are 

 in the ratio of 1*4 to 1*14 to 1/0 for the three metals Pb, 

 Zn, and Al ; whereas the ratios arrived at from the figures 

 of Table XI. for the corresponding effects due to the earth's 

 radiation were 1*1 to 1*23 to 1*00. The considerable differ- 

 ence between these ratios would thus seem to indicate a 

 difference in penetrability between the y rays from radium 

 and the penetrating radiation from the earth. It is possible, 

 however, that the discrepancy may have had an entirely 

 different origin, and further measurements should be made to 

 ascertain the cause of it before a satisfactory explanation can 

 be offered. 



7. Experiments on Absorbing Power of Water. 



From the foregoing experiments with radium in con- 

 junction with the effect noticed in 4 (c), we see that i there 

 was in each case some additional effect inside the cylinder 

 which must be considered as due to the presence of the wall. 

 From these experiments the idea presented itself that possibly 

 the earth's penetrating radiation was the same at all points 

 on its surface, and that the differences observed in the values 

 for " q " for the land and water experiments were due not so 

 much to differences in the absorbing power of the different 

 soils, as to differences in a secondary radiation induced in the 

 crust of the earth by this penetrating radiation. 



To determine if any effect of this kind could be noted in 

 the case of water for the penetrating rays from radium, the 

 sample used in the investigation described in Section 4 

 was lowered under the ice on Grenadier Pond, and the 

 ionization noted in a lead cylinder placed above it for different 

 depths of the radium. In making these measurements the 

 radium was hermetically sealed in a glass tube, which was 



