C. Barns — Vapor Nucleation in the Lapse of Time. 343 



at about 20° C. and for Sp/p = -345. Finally there is no ade- 

 quate reason why the effect of cooling below a higher sur- 

 rounding temperature should be more efficient than the 

 corresponding effect below a slightly lower temperature ; for 

 the rate of reheating would depend on the difference of tem- 

 peratures. 



2. Possible suggestions as to the temperature effects. — To 

 obtain a suggestion as to the reason of the apparent increase of 

 the size of "colloidal nuclei with rise of temperature (caet. par.), 



11 Jw, 



effectively therefore of their apparent increase in number at a 

 given supersaturation, it is expedient to recall the form of 

 Helinholtz's modification of Kelvin's vapor pressure equation. 

 If the ratio of pressures at a convex surface, r, and at a plane 

 surface be p r /p> x ? -R the gas constant of water vapor, 6 its 

 absolute temperature, s the density, and T the surface tension 

 of the liquid, 



Pr l2\ = z 2T/Rs6r 



whence it appears that the increments of 6 and R may recipro- 

 cally replace each other. A small radius at a high temperature 

 is as effective as a larger radius at a low temperature, 6; and 

 that is about what the above data have brought out. Naturally 

 the equation has been pushed beyond its limits for the mean- 

 ing of T, for particles not large as compared with molecular 

 dimensions is obscure ; but it appears in other cases and is 

 probably true here that the suggestions of the equation are 

 trustworthy in a general way. .Computing p r /p m by the aid 

 of the adiabatic equation, we may write 10V = 19*5/^ log 10 

 (Pr/pJ), where log^pr/p^ = "8, and 6 x r = 2/10 6 , nearly. But 

 B l = 262° if the gas is originally at temperature t = 20°, 

 whence r = 75/10 9 . Since dr/r — — ddjd^ an increment of 



