n6 



VAPOR NUCLEI AND IONS. 



Fig. 54. Supersaturations (S) of media air-water 

 and alcohol -water, and estimated radius (r cm.) 

 of smallest efficient nuclei, at different exhaus- 

 tions (dp), m, molecular radius. Table 45. 



Since p T is the adiabatically 

 reduced vapor pressure (with- 

 out condensation) in the vol- 

 ume expansion due to the 

 drop of pressure (dp) and p M 

 the normal vapor pressure at 

 the same temperature (d = 

 2 7 3 -f- / 2 in table 1), r follows 

 from the equation. The 

 values of p T lp m and r so found 

 are both given in table 45, 

 and have been constructed in 

 the chart (fig. 54), where their 

 relation to the usual order of 

 molecular size is also indi- 

 cated. Clearly these values 

 of r, the radius of the nuclei 

 differing so little from molec- 

 ular radii (say io~ 8 ), can only 

 indicate an order of values; 

 for apart from the difficulties 

 above enumerated in com- 

 puting d, r depends on sur- 

 face tension (T), which has no 

 meaning for molecular dimen- 

 sions. Granting this, it is 

 none the less remarkable that 



the values of r obtained should be so nearly alike for water and alcohol 

 where different constants (T, R, 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, since in the former case 

 coronal condensation begins at about dp = 15 cm. where r io~ 7 cm. 

 and in water vapor it begins at dp = 26 cm. where r= 4 X io~ 8 cm. about, 

 less than half as large. These relations once established are retained 

 through all successions of nuclei, as the following data for alcohol vapor 

 in comparison with water vapor show. It is a little difficult to under- 

 stand why the ionized nuclei in alcohol vapor should, like the colloidal 

 nuclei, be larger than the corresponding cases for water vapor, unless the 

 ions are aggregated vapor nuclei, a point of view tentatively advanced 

 elsewhere; but this larger alcohol nucleus suggests that it is primarily 



