432 Prof. McLennan and Mr. Murray on Residual 



Physics Building and at the house mentioned above were 

 slightly greater than those obtained at the same stations with 

 the Wilson instrument. On the ice of: the Bay, on the other 

 hand, the reading obtained with the Wolff electrometer was 

 a little less than that given by the Wilson electrometer. The 

 reading obtained with the ice receiver at the house on the 

 land, viz. 4*37 ions per c.c. per second, was much less than 

 the mean reading, 7'39 ions per c.c. per second, obtained at 

 the same place with the Wolff and Wilson instruments when 

 the receivers were of zinc. This may have been due in part 

 to differences in the absorption of the earth's penetrating 

 radiation by the walls, for the zinc vessels were only from 

 1 to 3 mm. thick, while the walls of the ice receivers were a 

 little more than 5 cm. in thickness. 



The low value obtained for the ionization in air with the 

 ice receiver at the land station cannot be entirely accounted 

 for by the absorbing power of the walls, for it is known 

 from experiments made by McLennan * and Wright f that 

 it requires from 2 to 3 metres of water to entirely cut off 

 the gamma rays from radium, and it is probable therefore 

 that it would be necessary to use a screen of water or ice of 

 the same thickness to entirely absorb the penetrating radia- 

 tion present at the surface of the earth. Besides, it will be 

 noted that the reading 4*37 ions obtained with the ice 

 receiver on the land is as low as if not lower than the ioniza- 

 tion obtained for air in the zinc vessels on the ice of the Lake 

 where it is known that the intensity of the earth's penetrating- 

 radiation is practically negligible. It seems rather that the 

 difference in the readings obtained with the ice and the zinc 

 receivers must have been due to the existence of an ionizing" 

 radiation emitted by the zinc walls which was not emitted by 

 the ice. Moreover, the low reading obtained with the ice 

 receiver on the Bay confirms this view, for it will be seen 

 from the table that under these conditions the ionization in 

 air was found to be represented by the generation of but 

 2'Q ions per c.c. per second. This, it will be recalled, is the 

 lowest value ever obtained for the electrical conductivity 

 of air. 



After having obtained this striking decrease in the con- 

 ductivity of air by enclosing it in an ice receiver, it was but 

 natural to seek for an explanation of this final residual con- 

 ductivity, and the explanation which most readily offered 

 itself was that it was due to a feeble radioactivity possessed 

 by the ice. Part of it at least must have been due to this 



* McLennan, Phys. Rev. vol. xxvi. No. 6, June 1908, p. 526. 

 t Wright, Phil. Mag. xvii. p. 295 (1909). 



