NUCLEI OF PURE WATER. 



II 



9. Exceptionally rapid evaporation. -In the third and fourth parts 

 of table 5 the influx of dust-free air was increased in a marked degree, 

 by withdrawing the influx of dust-free air from a large independent 

 reservoir. In this way the time of evaporation was reduced to about 1 5 

 seconds. Nevertheless it required about 1.5 minutes to reduce the pres- 

 sure from dp = 36 cm., to dp = 20 cm., so that some water nuclei vanished 

 by decay. The yield of water nuclei has been materially increased. Thus 

 even when n=io 6 or d = o. 00019 cm., rapid evaporation will convert at 

 least 1 8 per cent of the fog particles of pure water into persistent water 

 nuclei. The case is most pronounced for n = 300,000 to 400,000 nuclei 

 per cubic centimeter, or d = 0.0002 7 cm., when 48 per cent of the nuclei 

 (about one-half), persist. Beyond this, n = io 5 , the persistence decreases, 

 a result doubtless referable to the increasing importance of subsidence. 

 Within reasonable limits persistence increases as the original number 

 of nuclei decreases, which result is identical with the character of the 

 earlier series. 



Naturally all these data are lower limits. In appropriate apparatus 

 the time of evaporation, the interval of observation, etc., might be made 

 much shorter; but this would not change the general trend of the data. 

 Again, many particles must be washed out by contact with the sides of 

 the vessel or by coalescence, during the turbulent motion which accom- 

 panies the influx of air. The following is a digest of the data found: 



10. Conclusion. For very small fog particles suspended in dust-free 

 air saturated with water vapor and left without interference, the dis- 

 sipation by evaporation is enormously more important than that by 

 subsidence. In the above plug-cock fog chambers the transition occurs 

 when the number of nuclei per cubic centimeter, n = 200,000, or the 

 diameter of fog particles, d = 0.0003 cm -> when about half evaporate and 

 half subside. 



Fog particles precipitated on solutional nuclei (phosphorus) , evaporate 

 to persistent water nuclei without other loss than is attributable to sub- 

 sidence and in a small degree to time losses (diffusion) . There is no loss 

 by complete evaporation. 



