Nolan — Ionic Mobilities in Air and Hydrogen. 75 



more closely, and to get accurate values for their mobility, it was found that 

 the ionization was more complex than had appeared at first sight.' Evidence 

 of the existence of separate groups of ions of very high mobility was obtained, 

 and it was also found that groups of ions existed having mobilities roughly 

 corresponding to some or other of the values commonly attributed to small 

 ions produced by the ordinary ionizing radiations. Thus for negative ions 

 the values 1'94, 170, 1-49, and 1-34 cm./sec./volt/cm. as corresponding to 

 the mobilities of four distinct groups were obtained. Almost identical values 

 were obtained for the positive ions. It was suggested that the ordinary 

 small ion in air was identical with these ions, and that it assumed one or 

 other of the four forms at any one time, the choice being determined by 

 questions of sign and humidity. It was not then suspected that the ordinary 

 ionization in air might be a mixture of all four. But this was the result at 

 once found when an attempt was made to obtain accurate measurements of 

 the mobilities of small ions produced in air by an ionizing radiation.' In 

 addition, evidence was obtained of the existence in small quantities of ions of 

 mobilities corresponding to the higher values previously found in the air 

 from a water-spray. As it is this work which has been the subject of 

 criticism, it would be well to deal with it somewhat fully. The method of 

 observation was devised with the intention of securing sharp values for 

 mobility, and was in principle identical with that employed previously for the 

 observation of the spray-ions. Air is drawn between two parallel plates, 

 the ionization being produced in a restricted region close to one of them. 

 The current to the other is read for various values of the field between them. 

 As the ions are produced at about the same place, they will travel along 

 almost identical paths (if they have the same mobility), and will all arrive 

 together at the upper plate for a certain field, or fail to reach it for a certain 

 smaller field. Thus, if a current-voltage graph is plotted, the air-stream 

 being steady, there will be a sharp increase in current at a certain point, 

 indicating the arrival of the ions. If more than one class of ion is present, 

 each class will show up as an abrupt upward inclination on the curve. The 

 mobility can be readily calculated as the velocity of the air-stream is known. 

 For, taking x along the air-stream and y at right angles to it and to the 

 plates, for any ion of mobility i6, 



dx va 

 dy Vu 



V being the velocity of the air, V the voltage difference, and a the distance 



1 Proo. Royal Society, Sect. A, vol. xciv, p. 112, 1918. 



■ Proc. Royal Irish Academy, vol. xxxv, Sect. A, p. 28, 1920. 



[8*] 



