140 C. Barns — Nuclei and Ions in Dust-free Air. 



the chart, figure 1, where their relation to the usual order of 

 molecular size {in in the figure) is also indicated. Clearly 

 these values of r, the radius of the nuclei differing so little 

 from molecular radii (say 10" 8 cm ), can only indicate an order of 

 values ; for apart from the difficulties above enumerated in 

 computing 0, r depends on surface tension T, which has no 



Granting this, it is none 



meaning for molecular dimensions 



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the less remarkable that the values of r obtained should be so 

 nearly alike for water and alcohol, where different constants 

 {T, i?, s, etc.) occur throughout ; in other words, that at a given 

 temperature a given drop of pressure will condense both vapors 

 on nuclei of about the same size. 



In so far as these estimates are admissible, it follows that the 

 alcohol air nucleus is larger than the water air nucleus ; in the 

 former case coronal condensation begins at about Bp=16 cm , 

 where r = 10 ~ 7 , while in water vapor it begins at Sp — 26 cm , 

 where r = 4xl0~ 8 about, less than half as large. These rela- 

 tions once established are retained through all successions 

 of nuclei, as the following data for alcohol vapor in compari- 



