Section III, 1915 [113] Trans. R.S.C. 



Residual Ionisation in Gases. 



By Prof. J. C. McLennan, F.R.S., and Mr. C. L. Treleaven, M.A., 

 University of Toronto. 



(Read May Meeting, 1915.) 



I. INTRODUCTION. 



From observations made by Simpson and Wright,^ McLennan 

 and McLeod" and others it is now known that the ionisation in air 

 confined in air-tight clean zinc vessels is about 8 or 9 ions per c.c. 

 per second when the observations are made on land where the soil 

 contains only such minute traces of radio-active substances as are 

 found in ordinary clays or loams. It is also known that when the 

 observations are made on the Atlantic, the Indian, or the Antarctic 

 Ocean or on the surface of large bodies of water such as Lake Ontario 

 the ionisation in air confined in the manner indicated above drops to 

 approximately 4 ions per c.c. per second. Moreover this reduction in 

 the electrical conductivity of air has been shewn to be due to the ab- 

 sence of a penetrating radiation over the waters of oceans and lakes 

 which is known to be present at the surface of the earth on land made 

 up of rocks or of ordinary soils. Since this residual conductivity in 

 air of 4 ions per c.c. per second is found to persist when all known 

 external radiations are cut off from the vessel containing the air the 

 question naturally arises — to what is the residual conductivity due ? 



To this question there appear to be but four possible answers. 

 It may be due one may suppose either :— 



(i) to an extremely penetrating radiation present on the 



surface of the earth; or 

 (ii) to ionisation arising from molecular thermal agitation or 

 . (iii) to a real spontaneous ionisation such as one would have 

 with exploding atoms or molecules; or 



(iv) to a feeble radiation from the material forming the walls 

 of the containing vessel. 



The hypothesis of an extremely penetrating radiation being 

 present at the surface of the earth however does not appear to be 



1 Simpson and Wright, Proc. Roy. See. Ser. A vol. LXXXV, p. 175, 1911. 



2 McLennan and McLeod, Phil. Mag. Oct., 1913 p. 740 



