CH. vii.] BY RONTGEN RAYS T5 



Condensation of Moisture Experiments. 



C. T. R Wilson investigated the amount of expan- 

 sion required by both positive and negative ions to 

 act effectively as nuclei in condensing moisture. The 

 ions were produced by Kontgen rays, and electrolytic 

 terminals were inserted to effect a separation of 

 the ions. He found that with an expansion such 

 that the ratio of the final to the initial volume was 

 1*25, there was a fog in the half of the vessel which 

 contained negative ions, and hardly any condensation 

 in the half containing positive ones, but that when 

 the expansion-ratio was as much as 1*31 there was 

 little or no difference to be seen between the two 

 halves. Thus proving that the negative ions are 

 more efficient, as centres of condensation for water- 

 vapour, than the positive ions. 



It may be doubted whether electrons themselves 

 are able to act as nuclei and condense vapour round 

 them, and it appears unlikely that they can do that 

 without the aid of one or more atoms of matter. 

 On this subject Mr. C. T. K. Wilson has favoured me 

 with the following opinion : 



" I certainly think that in practically all cases the 

 electron has already been loaded to form a negative 

 ion, before the expansion by which the necessary 

 supersaturation is brought about has been effected. 

 In air which contains less moisture than corresponds 

 to a four- fold supersaturation, equilibrium is probably 

 reached when the electron has collected around itself 

 a group of molecules not much larger than the ion 

 in dry air. If however a four-fold supersaturation 

 is exceeded, the conditions become unstable and the 

 cluster of molecules increases to a visible drop. If 

 electrons enter the expansion chamber immediately 



