Conductivity of Air enclosed in Metallic Receivers. 295 



absorbed, and light of the same kind as that emitted by the 

 photoelectric substance if it radiates. Thus zinc is rendered 

 photoelectric under the action of ultra-violet light, but zinc 

 when heated, whether by being used as spnrk-balls for an 

 electric spark or as arc terminals for an electric arc, radiates 

 much ultra-violet light. 



In the same manner, the photoelectric effect of potassium 

 appears to be due to the absorption of that violet ray which 

 potassium itself emits if heated. If, then, these atmospheric 

 particles absorb ultra-violet light, that would account for the 

 relatively small percentage of ultra-violet light found in 

 sunlight at the earth's surface, and also for the ionization 

 found to exist in the atmosphere. This suggests the need for 

 further observation on the number of ions present in the 

 terrestrial atmosphere at various heights above the sea-level. 



In conclusion, I have pleasure in mentioning the aid ren- 

 dered in these experiments by my assistant, Mr. Gr. B. Dyke. 



XXIII. On Variations in the Conductivity of Air enclosed in 

 Metallic Receivers. By 0. S. Wright, B.A., Exhibition 

 Scholar, University of Toronto, and Wollaston Student, 

 Gonville and Caius College, Cambridge*. 



[Plates IV. & V.] 



1. Introduction. 



IN a paper in the Phil. Mag. of December 1907, Professor 

 McLennan records some observations made on the 

 ionization of air enclosed in cylindrical receivers of lead, 

 zinc, and aluminium. For ' ; q" the number of ions gene- 

 rated per c.c. per sec. in these receivers he obtained the 

 value 15, w r hen they were made of zinc and aluminium; while 

 with lead, values were found ranging all the way from 23 to 

 160 ions per c.c. per sec, depending on the sample of lead 

 from which the cylinder was made. From these results 

 Prof. McLennan drew the conclusion that ordinary com- 

 mercial lead contained in general varying amounts of some 

 active impurity. 



From these and other experiments he pointed out also, 

 that four possible causes must be considered as contributing 

 to the ionization in the cylinders, viz. : — (1) penetrating 

 radiation from the earth; (2) secondary rays excited by this 



* Communicated by Prof. J. C. McLennan, and read before the Royal 

 Society of Canada, May 28, 1908. 



X2 



