PERSISTENCE ON SOLUTION. 137 



94. Fleeting nuclei become persistent on solution Origin of rain.- 



Let the fog chamber be exposed to radiation for a few seconds and 

 thereafter exhausted ($p= 25) as usual. Closing the exhaustion cock 

 and allowing only time enough to measure the first corona, let the 

 influx cock be opened and the fog chamber be refilled with dust-free 

 air. The (primary) corona observed is thus dispelled before much 

 subsidence of fog particles can take place, though the rain will natu- 

 rally drop out. If the fog chamber is now left without interference 

 (the radiation having been cut off immediately after the first exhaus- 

 tion) for one or more minutes or longer, a second exhaustion to the 

 stated limits will show a large (secondary) corona relatively to the 

 primary corona. In other words, relatively many of the fleeting 

 nuclei or ions caught in the first fog have persisted, whereas without 

 condensation they would have vanished at once after the radiation 

 was cut off. (Cf. figs. 56-59, Chapter III.) The following is an 

 example of data bearing on this point, / denoting the time elapsed 

 from the evaporation of the first corona to the precipitation of the 

 second, N l the number of nuclei in the first, and N* the number in 

 the second corona: 



Sees. Sees. Sees. 



t= 60 I2O 300 



Ni XicT 3 = 53 27 53 



yV"2Xicr 3 = 16 7 15 



The experiments are complicated by the variable X-ray bulb ; but 

 it is obvious that while all the nuclei would have vanished in a few 

 seconds without condensation, about one-fourth (in other experiments 

 more) persist indefinitely, if reevaporated after condensation from fog 

 particles. 



This result has an important bearing on the whole phenomenon of 

 condensation and nuclei. Clearly the latter, after the evaporation 

 specified, becomes solutional or water nuclei, in which the original 

 fleeting nucleus or ion behaves as a solute. The decreased vapor 

 pressure due to solution eventually compensates the increased vapor 

 pressure due to curvature, after which, at a definite radius, evaporation 

 ceases and a water nucleus results. Such a nucleus, however small, 

 must be large in comparison with the dissolved ion. Hence, on con- 

 densation, the water nuclei will capture the moisture soonest and grow 

 largest. Now, in any exhaustion about one-eighth of the fog particles, 

 i. e., those which are smallest and whose nuclei have been caught at 

 the end of the exhaustion, regularly evaporate into the larger parti- 

 cles to a residue of water nuclei. These are, then, the first to be 

 caught in a succeeding exhaustion. This is the explanation of the 

 rain which not only accompanies all coronas in dust-free air, but is 



