774 Prof. J. C. McLennan on the 



manner to give satisfactory results. I£ this water proved to 

 be an efficient screen, it would be interesting to see whether 

 all ionization would disappear from the air confined in a 

 cylinder such as No. 1 of this investigation, if it were 

 immersed to a considerable depth in it ; for on the assumption 

 that the material used in the construction of the cylinder 

 contained no active impurity, this is what one should expect 

 to find. 



IV. On the Rise in Conductivity of Air confined in 

 Metallic Vessels. 



In the course of the experiments described above, it was 

 repeatedly found when one of the cylinders was filled with 

 fresh air filtered through cotton- and glass-wool, and after- 

 wards sealed up, that the conductivity of the enclosed air 

 steadily rose for a number of days, and finally reached a 

 steady value. 



This phenomenon, which has been described already by the 

 writer and E. F. Burton in the paper cited previously, has 

 also been observed by a number of experimenters, including 

 Elster and Geitel *, Evef, Wood and Campbell J, and 

 others, but up to the present has not received a satisfactory 

 explanation. 



During the present investigation special observations were 

 made on this effect in connexion with air confined in the 

 lead cylinders Nos. 1 and 2, on account of the great difference 

 observed in the values of the conductivity impressed upon 

 the air introduced into them. 



When cylinder No. 1 was thoroughly scoured and cleaned 

 in the manner described in the beginning of this paper, and 

 freshly filtered air blown through it for twenty minutes, a 

 reading of 7'7 divisions per minute, or a number within 1 or 

 2 per cent, of it, was regularly and repeatedly obtained 

 throughout the period, now nearly six months, during which 

 the observations have been carried on. If the air after being- 

 introduced into this cylinder was allowed to remain un- 

 disturbed for some time, and measurements made on its con- 

 ductivity at stated intervals, it was found that the ionization 

 steadily increased, and after a period of a week or ten days 

 reached a value of approximately 11 divisions per minute. 

 If when this stage was reached filtered air was blown through 

 the cylinder for twenty minutes, it was always found that a 



* Geitel, Phys. Zeit. ii. pp. 560-563 (1901) ; Elster and Geitel, ibid. 

 ii. pp. 116-119 (1900). 



t Eve, Phil. Mag-. j_6] xii. p. 189 (1906). 



j Wood and Campbell, Phil. Mag. Feb. 1907, p. 2C5. 



