The Electrical Conductivity of Atmospheric Air. 699 



It is easy, however, to explain the discrepancy between 

 these results, if the theory which Mr. Dnrack has proposed 

 is the true explanation o£ the difference between his experi- 

 ments with radium and my experiments with ultra-violet 

 light. The velocity of the corpuscles will depend on the 

 thickness of the aluminium between the radium and the gas 

 which is examined. In my experiments the aluminium 

 through which the radiation had to pass was much thicker 

 than that used by Mr. Durack, so that the corpuscles which 

 passed through the gas must have been travelling with a 

 diminished velocity, and consequently they would each pro- 

 duce a larger number of ions per centimetre in the gas through 

 which they pass. 



If we accept this theory we might expect to obtain any 

 value less than 15 for the specific ionization by varying the 

 thickness of the aluminium through which the radiation has 

 to pass. 



Yours very truly, 



John 8. Townsend. 



LXXVII. Some Experiments on the Electrical Conductivity of 

 Atmospheric Air. By J. C. McLennan and E. F. Bukton, 

 University of Toronto*. 



I. Introduction. 



IN a paper by H. Geitelf reference is made to a gradual 

 increase observed in the conductivity of a mass of atmo- 

 spheric air after being confined in an air-tight chamber. This 

 effect was found to require from four to five days to reach its 

 maximum value, and was observed in localities where no 

 thorium compounds or other known radioactive substances 

 existed. 



In a subsequent investigation Elster and Greitel % found that 

 the air which had been confined for some time in closed caves 

 or house-cellars possessed an abnormally high conductivity. 

 This phenomenon, together with the observed increase in con- 

 ductivity mentioned above, they concluded, could not be due to 

 the presence of dust or water-vapour. They traced it rather, in 

 both cases, to the existence of some undetermined radioactivity 

 in the confining walls. 



More recently these physicists discovered that atmospheric 

 air possessed the property of exciting induced radioactivity in 



* Communicated by Prof. J. J. Thomson. Read before the meeting 

 of the American Physical Society in Washington, Dec. 31st, 1902. 

 t Phys. Zeit. ii. pp. 116-119 (1900). 

 X PJiys. Zeit. ii. pp. 560-563 (1901). 



