NUCLEI OF PURE WATER. 



7 



5. Rapid evaporation of fog particles. Vapor nuclei. Finally, the 

 fog particle precipitated on vapor nuclei may be rapidly evaporated by 

 compression (influx of air). In order that all results may be comparable, 

 it is necessary to allow the same time interval between the first ex- 

 haustion (capture of vapor nuclei) and the second (capture of residual 

 water nuclei). Since filtered air enters in the same way here as in the 

 foregoing section 4, its effect (if any) is eliminated from the results. Sub- 

 sidence, however, is essentially reduced by the present method, since the 

 time for subsidence is but one-third to one-fifth as large as in section 4. 

 The results are given in table 4 on the same plan as in table 3. In part i 

 two control experiments with gradual exhaustions are introduced to 

 indicate the difference. The nucleations are as a rule low. In part 2 they 

 are high and there is one control experiment. The curve is shown in Nos. 

 6 and 7 in fig. i. 



The data taken from smoothed curves will therefore be about as 

 follows, d being the approximate diameter of fog particles, subsidence 

 being ignored and the exhaustion loss 0.25. 



The apparently greater persistence of water nuclei on rapid evapor- 

 ation is thus sustained. Subsidence is in excess in section 4; and though 

 of the same order as the number of persistent nuclei, it always refers to 

 the total charge of nuclei. As the number to be accounted for in the 

 present instance can not well be estimated, it is safe to conclude that 

 about 95 per cent of the fog particles precipitated on vapor nuclei evapo- 

 rate without residue, when about io 6 fog particles are suspended in each 

 cubic centimeter ; and that this percentage decreases with the number of 

 fog particles or vapor nuclei caught, or as their size increases. Conversely 

 the number of residual water nuclei persistent within 5 minutes increases 

 as the number of fog particles decreases (i. e., as their size increases) from 

 a persistence of about 0.054 at n=io* to over 0.17 at n = 2Xio 5 . One 

 would thus be tempted to conclude that larger fog particles take a longer 

 time to evaporate completely ; but that the case is far more subtle will 

 appear in the next paragraph. 



