6 Proceedings of the Royal Irish Academy. 



during very heavy rains. In fact, reading over Aitken's earlier work, there is 

 no feature of it which could not be explained by considering the dust-particles 

 which he measures as being the same as the large ions, and including the 

 nuclei from which large ions are formed when they become charged ; and by 

 considering both as minute drops of a certain fixed stable size, formed, at 

 least for the most part, in the cooling gases rising from sources of combustion. 

 Other work at present being carried on in the laboratory may lead to direct 

 evidence as to whether the nucleus from which the large ion is formed is the 

 same as that occurring in the atmosphere normally as the centre of 



condensation. 



The Small Ions. 



Theoretical considerations regarding the large ion as a nucleus which has 

 attached to it a small ion, lead to the conclusion that for a given ionising 

 agency the number of small ions will be less, the greater the number of large 

 ions and nuclei from which they are formed. Now, in the previous paper, a 

 remarkable discrepancy is pointed out existing between the number of small 

 ions obtained in the atmosphere by observers at various centres using the 

 Ebert apparatus, and those obtained by Langevin in Paris, Pollock in Sydney, 

 and by us in Dublin. Pollock 1 first drew attention to this discrepancy. The 

 mean value obtained by him was 38 for the negative and 39 for the positive 

 ions. The value obtained by Langevin was about 200, while the mean value 

 obtained by us was 78. These numbers are in striking contrast with those 

 found by the Ebert apparatus, which range from 500 to several thousands. 

 The explanation seemed to be that the values given by the Ebert apparatus 

 are too great, inasmuch as it possibly collected not only all the small ions, but 

 a great number of large ions as well, the large ions being far more numerous 

 than the small ones. In order to obtain further information on this point, a 

 series of measurements of n were made at Dalkey as soon as it became apparent 

 that the value of N + n was much less than in Dublin. It will be seen from 

 the table that the values of n are much greater than those found in Dublin, 

 and are in fact of the same order as those found by observers with the Ebert 

 apparatus. The mean value of n is about 700, which is about the same as 

 that found by Simpson on the voyage of the " Terra Nova." It seems, there- 

 fore, that the true explanation of the small values of n referred to above is 

 that these observations were made in cities where there is an immense 

 number of large ions and of nuclei from which large ions are formed, the 

 effect of these being to diminish the number of small ions present at any time. 



In conclusion, I wish to thank Professor McClelland for suggesting the 

 investigation, and for his valuable interest in the work during its progress. 



1 Science, N. S. xxix, pp. 919-928. 1909. 



