416 Prof. J. C. McLennan and Mr. C. L. Treleaven 



on 



electrical conductivity of air has been shown to be due to 

 the absence 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 ionization arising from molecular thermal 



agitation ; or 

 (iii.) To a real spontaneous ionization, 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 tenable; for in some experiments by McLennan 

 & McLeod * it has been shown when working with clean 

 air contained in an air-tight Wolff electrometer of zinc 

 placed in a water-tight box of aluminium-bronze that a 

 value of 4'81 ions per c.c. per second was obtained for the 

 conductivity of the air at the surface of Lake Ontario, while 

 -a value of 4' 7 7 ions per c.c. per second was obtained when 

 the box containing the electrometer was immersed to a depth 

 of 8 metres. This would go to show that the conductivity 

 of the air was practically the same in the two positions, 

 which would indicate that air and water had the same 

 coefficient of absorption for this hypothetical extremely 

 penetrating radiation. Since from two to three metres f of 

 water will entirely cut off all known gamma radiations 

 emitted by radium, the penetrating powers of this radiation 

 would have to be so extraordinarily great that, in the absence 

 of any collateral corroborative evidence, one hesitates to 

 believe in the reality of its existence. In considering the 

 hypothesis of ionization by collisions due to thermal agita- 

 -tion, Langevin & Key J have shown from theoretical con- 

 siderations on the basis of exceptional collisions that the 



* McLennan & McLeod, Phil. Mag. Oct. 1913, p. 749. 



T McLennan, Phys. Tiev. vol. xxvi. no. 6, June 1908, p. 530. 



X Langevin & Key, Le Radium, April 1913, p. 142. 



