RESIDUAL WATER NUCLEI. 



93 



In 1903 J. J. Thomson* gave a general account of condensation nuclei, 

 at the end of which he formulates succinct reasons for the persistence 

 of water nuclei, even when derived from the evaporation of fog particles 

 of pure water. He says "on the view of the relation between surface 

 tension and the thickness of water films, to which Reinold and Rucker 

 were led by their experiments with very thin films, drops of pure water 

 of a definite radius might be in equilibrium with saturated water vapor 

 even if they were not charged," a proposition which is thereafter proved. 

 A further deduction of J. J. Thomson's which may be of use below is 

 that "the efficiency of an ion as a nucleus for condensation depends 

 upon the maximum size of the aggregation, while the velocity of the 

 electric field depends upon the average size." Thus the "average size 



of a negative ion may easily be less than that of a positive ion, " 



while the negative nucleus is larger than the positive, other things being 

 equal. I may also add that J. J. Thomson computes the radius of a vapor 

 nucleus to be io~ 7 /i .9 cm., whereas the radius of the ionized nucleus is 

 io~ 7 /3 i , so that the vapor nuclei are slightly larger than the ions. 



Furthermore, Thomson shows that vapor nuclei are probably aggrega- 

 tions of water molecules, and elsewhere that "in a space far from satura- 

 ted with water vapor drops will be formed, and that the size of 



these drops diminishes only very slowly as the quantity of water vapor 

 in the surrounding air diminishes . . . . " 



In 1905 the Transactions of the St. Louis International Electrical 

 Congress were published, which gave a review of the present state of our 

 knowledge of condensation nuclei by C. T. R. Wilson, f This contains 

 the most recent contributions relating to water nuclei. In view of the 

 investigations of Langevin and of E. Bloch| on the occurrence and 

 behavior of slow-moving ions, Wilson finally summarizes the results 

 bearing on nuclei as follows: 



(i) The ions proper, requiring a fourfold or sixfold supersaturation to cause water 

 to condense on them, and having a mobility exceeding i cm. per second in a field of 

 one volt per second. (2) Loaded ions, requiring little or no supersaturation to make 

 water condense on them, and having a mobility generally less than a thousandth 

 part of that of the ions proper. (3) Uncharged nuclei, resembling the second class and 

 requiring little or no supersaturation in order that visible drops may form upon them. 



59. Purpose, plan, and method. My purpose in the present paper 

 is to determine whether there is any difference in the sizes of residual 

 water nuclei obtained in the evaporation of fog particles under different 

 conditions; for instance, whether the fog particles of large coronas 



Conduction of Electricity through Gases, Chapter VII, Cambridge, 1903. 

 tTrans. of International Electrical Congress of 1904, p. 365 (cf. p. 378), St. 

 Louis, 1905. 



JRecherches sur la conductilite electrique de 1'air, etc., Paris, 1904 (quoted by Wilson). 



