THE MOBILITY OF LARGE IONS IN THE AIR. 61 



THE MOBILITY of the LARGE IONS in the AIR. 



By J. A. Pollock, d.s c . 

 Professor of Physics in the University of Sydney. 



[Read before the Royal Society of N. S. Wales, June 2, 1909.] 



1. Introductory.— The feeble mobility of the large ions 

 in the air, discovered by M. Langevin 1 in 1905, indicates 

 that their structure is somewhat complex. In this connec- 

 tion the idea must have occurred to many, and is indeed 

 suggested by Professor Rutherford in his book on Radio- 

 active Transformations, that these electrified entities may 

 consist of water vapour molecules collected round small 

 ions, a notion which seems to receive support from Mr. 

 Lusby's 2 observation that they are absorbed by hygroscopic 

 substances. The suggestion is strengthened by the results 

 of the experiments to be described in the present paper, 

 which were undertaken to find if the mobility of these 

 slowly moving ions depends on the humidity of the air. The 

 observations show that a definite relation exists between 

 the two quantities, and incidentally, that if the humidity 

 considerably alters the ions are not in equilibrium with the 

 new vapour pressure conditions until after the lapse of a 

 few minutes. 



For help and encouragement derived from a discussion 

 of the results during the progress of the work I am greatly 

 indebted to Mr. William Sutherland of Melbourne ; I wish 

 also to acknowledge the great assistance I have received 

 from Mr. Carl Sharpe of the laboratory staff, both in con- 

 nection with the setting up and working of the apparatus 

 and with the observations. 



1 Langevin, C.E., t. 140, p. 232, 1905. 

 2 Lusby, This Journal, p. 55. 



