68 J. A. POLLOCK. 



and just before entering the testing pipe it passed, without 

 bubbling, over water contained in a covered shallow 

 tray. 



Owing to the extreme variability of the natural ionisa- 

 tion, it does not seem possible, in such an investigation as 

 that described, to obtain measures sufficiently accordant 

 for the purpose of a discussion of small corrections, but 

 following a suggestion made by Mr. Sutherland, all the 

 observations have been reduced to a pressure of 760 mm., 

 on the assumption that the mobility varies inversely as the 

 density of the air. The results do not show any indication 

 of a difference between the mobilities of the positive and 

 negative ions. 



Included in the table are the values of the number of 

 large ions per cubic centimetre at the time of each obser- 

 vation, calculated from the value of the ionic charge, 4*65 

 x 10~ 10 electrostatic units, lately determined by Professor 

 Rutherford and Dr. Geiger. 1 The discussion of the results 

 is reserved for a following paper. 



Rutherford and Geiger, Proc Roy. Soc, A. 81, p. 162, 1908 



