Nolan and Harris — Tonisation in Moist and Dry Air. 45 



plate, would have to pass through 5 cms. of air, and would therefore be very 

 liable to capture by a molecule, it is possible that what we measure is only a 

 mean velocity, and that we are not dealing with electrons free for the whole 

 of their path. 



Consideration of Bcsults. 



The results obtained so far may be briefly reviewed as follows : — 



I. In saturated air by the air-current method we find positive and 

 negative ions of mobilities 12-2, 6-62, 4-23, 3-0, 2-04, 1-79, 1-52, and 1-37, the 

 last four being present in large quantities. Amongst the negative ions we 

 sometimes find the mobilities 24 and (faintly marked) 2"2. 



II. In saturated CO^ by the same method we find a corresponding set of 

 ions, the fastest having a mobility of 10. 



III. In the ordinary iindried air of the atmosphere we find by the 

 alternating field method five ions. One of these is new (2*5) ; the others are 

 the four ions disclosed in large quantity by- the air-current method. In one 

 observation made with saturated air these four only were present. 



IV. In well-dried air we find by the alternating field method negative 

 ions of mobilities 24-6, 12-3, 8-2, 5-6, 4-4, 3-0, and 2-5. The four groups 

 previously so prominent have disappeared. The positive iouisation contains 

 the same groups as the negative from 12 to 2'5, and in addition shows the 

 four groups from 2 to 1'3. An ion of mobility 2'3 (corresponding to 2'2 

 noted above) has also appeared. In some cases a positive ion of mobility 

 24 is found. There are some indications of the presence of free electrons. 



iSTow, the first question that arises on considering these results is this : 

 Why does the alternating field method not show up the faster ions in moist 

 air when the other method sliows them so clearly ? We think that the 

 explanation of this lies in the one radical difference between the two 

 methods. In the air-current method the ions of one sign are almost 

 immediately separated from the others, and for practically all their path in 

 the measuring vessel are moving through air free from other ions. In the 

 alternating field method the ions are in contact with one another for some 

 time in the space above the perforated plate. The time that an ion spends 

 in this space will depend upon the value of the field there, the distance from 

 the place where it was formed to the perforated plate, and its mobility. 

 Eesults obtained by Erikson ^ and by J. Cabrera Felipe ° seem to support 

 this view. Erikson finds with positive ions an " ageing " effect, that is, 



'Erikson, " PhysicalReview," xvii, p. 400, 1921, and viii, p. 100, 1921. 

 ■J. Cabrera Felipe, R. Acad, de Ciencias, xviii, 1920. 



