[mclennan-murray] residual IONISATION IN AIR 109 



As the numbers show, the readings obtained for the conductivity 

 with the Wolff Electrometer in the Library of the 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 Electro- 

 meter. The reading obtained with the ice receiver at the house on the 

 land, viz., 4-3 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 receiver were 

 a little more than 5 cm. in thickness. 



The low value obtained for the ionisation 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^ 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 radiation 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 if not lower 

 than the ionisation 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 receiver must have 

 been due to the existence of an ionising 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 ionisation in 

 air was found to be represented by the generation of but 2 • 6 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 conductivity 

 of air by enclosing it in an ice receiver it was but natural to seek for an 

 explanation of this final residual conductivity and the explanation 

 which most readily offered itself was that it was due to a feeble radio- 

 activity possessed by the ice. Part of it at least must have been due 

 to this cause for it is known that although the water of Lake Ontario 

 contains very little radium still traces of it in the water have been 



1 McLennan. Phys. Rev. Vol. XXVI. No. 6. June, 1908, p. 526. 



2 Wright. Phil. Mag. No. 17. p, 295, 1909. 



Sec. Ill, 1915—8 



