Nolan and Hakei.s — lonisation in Moist and Dnj Air. 47 



eighteen to thirty-six. From this point of view we could explain the dis- 

 appearance of the common types of negative ion by saying that drying had 

 removed the water-nuclei of those sizes, were it not for the fact that the 

 same ions are still found with a positive chai'ge. In whatever way this 

 difficulty is to be surmounted, the importance of water as an element in the 

 formation of the ordinary ion is quite clear. It must, however, be remembered 

 that the role of the water-molecule might possibly be that of an inter- 

 mediary for the grouping of other molecules. 



Turning to the more rapid ions, the most prominent is the ion of mobility 

 about 12-3. There is hardly any doubt that this is a single molecule — a 

 molecule of oxygen or nitrogen. On the hypothesis of elastic collisions, a 

 molecule of oxygen or nitrogen with a single electronic charge should have a 

 mobility of 12 approximately. This ion is therefore, if positive, a molecule of 

 oxygen or nitrogen from which a negative electron has been removed ; if 

 negative, a molecule of oxygen or nitrogen to which an electron has become 

 attached. With regard to the slower ions, a possible view is that, in the 

 presence of sufficient water-vapour, the original molecular ion passes rapidly 

 through various stages of growth to one or other of the four stable sizes, 

 which correspond to the ordinary ion. On this view the intermediate ions 

 (6'6, 4'2, etc.) would represent simply stages in growth. A more probable 

 view which fits in better with the experimental facts is this : that stable 

 water-nuclei of different sizes are always present in air, the numbers of each 

 size present depending on the degree of humidity ; that the original 

 molecular ion becomes attached to one or other of these, and that no further 

 change occurs except loss by recombination, which will remove all the faster 

 ones first. The advantage of this view is that it explains why the original 

 ion (mobility 12) is so readily found even in saturated air by the air-current 

 method. The appearance of two new ions in very dry air, 8 and 5"6 in place 

 of 6"6 found in moist air, remains to be explained. We hope to be in a 

 better position to put forward an explanation when we have acquired more 

 accurate numerical data. 



Eegarding the ion of mobility 12 as a molecule of oxygen or nitrogen with 

 a single charge, we were not surprised to find negative ions of mobility 24. 

 These are clearly doubly charged molecules. The appearance of a positive 

 ion of this mobility was .more surprising. This ion has been found four 

 times — twice with the first alternating field apparatus and twice with 

 the second apparatus. As already mentioned, there is something hap- 

 hazard about its appearance in the first apparatus; but we believe that 

 this is simply a question of the magnitude of the accelerating field. It 

 has appeared with the other apparatus on the two occasions on which 



R.I.A. PEOC, VOL. XXXVI, SECT. A. [4] 



