.'500 



MR. C. T. II. WILSON OX TIIK F.FFU'IFXCY AS COXDKXSATIOK 



The tables show plainly that with the smaller expansions the number of drops on 

 the side which contains chiefly positive ions is very small compared with the number 

 of drops on the side containing mainly negative ions. No difference can be detected 

 between the fogs on the two sides when v. 1 jv l exceeds 1'35 ; the transition from the 

 one kind of result to the other begins when v t jv l is about 1'31. This is well shown 

 by the ratio of the times taken by the fogs on the negative and positive sides to fall 

 the same distance; increasing v 2 /v, from 1 - 30 to 1*31 brings down the ratio from 5'1 

 to T8 in the first series, and in the second series the ratio diminishes from 4*6 to 17 

 as v.,/Vi is increased from 1'30 to 1*32. The results are therefore in complete agree- 

 ment with those previously obtained. 



There can be no doubt that the drops which are formed when iv/u, is less than 

 1'31 on the side containing mainly positive ions are deposited on negative ions, of 

 which a considerable number must unavoidably be present, when the expansion is made 

 while the rays are acting. The number of these was exceedingly small when the rays 

 were turned off before the expansion was made. 



There is no evidence of any increase in the number of the negative ions caught as 

 Vt/Vi passes through the region in which the positive ions begin to be caught. The 

 following is an example of observations with expansions of different amount made in 

 rapid succession (so that the radiation might not have time to change in intensity) : 



The rate of fall on the negative side is the same when Vo/fi = 1'37 as when 

 it = T30. It is true that we would expect the drops if equally numerous to fall 

 slightly quicker with the greater expansion, since a rather larger quantity of water 

 would be condensed ; against this is to be set the fact that the positive minority on 

 the negative side are caught by means of the greater expansion, and not by the less. 



It will be convenient to consider here what light the new knowledge now acquired 

 throws upon phenomena previously observed in the course of experiments with 

 expansion apparatus. 



All the former measurements which I have published upon the least expansion 

 required to make condensation take place in presence of ions, are now seen to have 

 been concerned with negative ions only. One example of interest is the case of nuclei 

 produced by a zinc plate exposed to ultra-violet light. When the plate was negatively 

 charged, nuclei were found to be present, requiring an expansion, vjv t = T25, to make 



