46 Proceedings of the Royal Irish Academy. 



observing the ions at various short intervals or time after their fonnation 

 he finds the mobility value to decrease with time. Xow, in his ease the 

 "ageing" is produced by keeping ions of both signs in contact with one 

 another for various times before exposing them to the field in his measuring 

 vessel. J. Cabrera Felipe, using an alternating field method, finds that the 

 measured mobility of positive and negative ions is increased by increasing 

 the value of the accelerating field. On our ^dew this means that the average 

 time spent by an ion in contact with ions of the opposite sign has been 

 diminished, and that therefore mobile ions in greater proportion come into 

 the measuring part of the apparatus. 



It seems to us then reasonable to suggest that the alternating field 

 method fails to reveal the faster ions, because they have disappeared by re- 

 combination. The other method does reveal them, because the chances of 

 recombination have been reduced to a minimum. This does not necessaidly 

 mean that the ions in the air-cm-rent method do not change. It is possible 

 that they do. A moderate amount of change could oceui- in their passage 

 through the apparatus without obliterating the peaks in the cuiTent-voltage 

 curve. 



The remarkable efiect-s produced by drying had now to be considered. 

 The common negative ions have all disappeared, and in both positive and 

 negative, mobile ions are present in considerable niunbers, some of these being 

 identical with those found in moist air, while some are quite different. The 

 causes which rendered the altemating-cuiTent method ineffective for showing 

 up the mobUe ions in damp air are still operative. A veiy great increase in 

 the proportion of the faster ions must therefore have been produced by the 

 drying. Something necessary for the production of the slower or more 

 common type of ion must have been withdrawn ; and this fits in with the 

 view put forward by one of us previously :' that the common ion is a cluster 

 of wat^r-molecvdes. 



The work of Aitken and Barus on condensation has shown that a moist 

 gas contains a number of particles of graded sizes, the number of these 

 particles increasing as the size decreases towards the molecular size as a 

 limit. It is natural to suppose that these nuclei are water. It is possible 

 that the four common ions are four of these water-nuclei of different sizes. 

 These four ions, or ions closely resembling them in mobility, were found in 

 the ionisation produced by a water-spray. A calculatiou showed that their 

 mobility values would agree with the supposition that they were groups of 

 water-molecules containing numbers of molecules ranging roughly from 



• J. J. Xolan, Proc. Royal Society, Sec. A, vol. xciv, 1918. 



