Nolan — Mobilities of lon^ Produced bi^ Spraymg. 17 



degree of reliability. In classes '<!>, 4, and 5, however, some of the values were 

 obtained under conditions not specially suitable for accurate readings of these 

 particular mobilities. The numbers obtained specially imder good conditions 

 are marked with an asterisk. The others have been disregarded in obtaining 

 the mean values for these classes. 



It will be noted that a great number of observations of classes 1 and 2 

 have been made. As regards class 1 this is due to the fact that it is very 

 difficult to decide upon the exact saturation voltage. It was considered 

 specially important to make sure of this mobility. The same difficulty of 

 deciding on the exact point of bending of the curve is found with class 2. 

 But here there seems to be some real variation apart from experimental errors. 

 The values vary between 'OOISI and '00088, and no less than six observations 

 give the latter value. Then, again, disregarding one experiment about which 

 there is some doubt, we have two experiments giving a mobility -00065. 

 This is typical of a variation which tends to occur throughout these experi- 

 ments — more often in the form of an increase or decrease in the quantity of 

 some ion present, but occasionally in the occurrence of an ion of mobility 

 which does not fall within any of the divisions ordinarily observed. The 

 variation in quantity is very noticeable in the case of class 1. Occasionally 

 the quantity of this ion present is so reduced that it would be difficult to 

 assert that it was not absent. 



Summing up the results obtained so far, it may be said that five classes 

 of ions have been found, each class having a distinct mobility. That these 

 ions decrease rapidly with time, but do not alter in mobility — at least within 

 a considerable range. The mobilities are -00038, -00100, 0043, -013, and -046 

 cm./sec. in a field of 1 volt /cm. Taking the mobility of the slowest ion as 1, 

 these mobilities may be written 1, 2-6, 11'3, 34, and 121. 



Io7iS of Greater Mobility. 



In addition to the five classes dealt with above, seven other classes of 

 ions have been found. This division into two sets of five and seven groups 

 respectively was adopted arbitrarily at first for convenience ; but, as will be 

 shown later, there is some ground for believing that it corresponds to a 

 definite difference in the nature of the ionisation. All twelve classes are 

 present together; but the conditions of air-velocity and voltage which favour 

 the observations of the more mobile groups do nor yield observations of the 

 slower ions, and vice versa. 



In dealing with the more mobile ions, very rapid air-blasts were 

 generally used, and the measuring cylinder was of large diameter. Thus the 

 time-interval between the pi'oduction of the ions and their observation was 



K.I. A. PKOC, VOL. XXXIII., SECT. A. [3] 



