34 Proceedings of the Royal Irish Academy. 



Ill fig. 2 some of the curves corresponding to the slower ions have been 

 reproduced. The ions indicated are those of mobilities approximately 

 2, 1-8, 1-5, and 1-35. The agreement is again very good. In one practically 

 unique instance one of the four ions failed to appear. This curve has been 

 reproduced. Even in this case more observations taken about the range 

 14-15 volts would probably have shown up the missing ion. In general with 



VlO =23 



10 II e X5 /-5Z /5 16 17 le 19 20 



Fig. 2. 

 lonisation curves in saturated air, air-current method, showing positive and negative ions of 

 - mobilities 2-04, 1-79, l-o2, and 1-37. 



these ions the steps in the curve are more pronounced than in the case of the 

 more mobile groups, indicating the presence of greater quantities. There is 

 a certain gap between the regions dealt with by the two sets of curves given, 

 say, between voltages 8 and 10. This region has been carefully examined ; 

 but on only two occasions were any indications of an ion found. On both 



