8 CONDENSATION OF VAPOR AS INDUCED BY NUCLEI AND IONS. 



6. Evaporation retarded as the diameter of fog particles decreases. 



A suggestive inference may be drawn from the results obtained. The 

 visible part of the fog vanishes within less than a minute after the com- 

 pression, or the (necessarily slow) influx of dust-free air begins. Here, 

 then, by far the greater bulk of the particle vanishes. In the ensuing 5 

 minutes or more, optically quite inappreciable water nuclei are still 

 present, to the extent of 5 to 20 per cent of the original charge of vapor 

 nuclei. In other words, whereas the tendency to evaporate increases 

 rapidly with the diameter of the fog particle, there must be, in case of the 

 fog particles in question, some counteracting tendency in action, by 

 which this evaporation is retarded much in excess of the accelerating 

 effect of convexity. All this occurs in rigorously dust-free air, in which 

 water vapor and the ions (less than 1,000 per cubic centimeter, whereas 

 there are from 25,000 to 50,000 water nuclei) inseparable from air and 

 due to natural causes, are alone present. It therefore becomes interesting 

 to endeavor to ascertain the reason of this complete inversion of the 

 behavior usually characteristic of these exceptionally small droplets. 



7. Time losses. The effect of lapse of time between the exhaustions 

 has already been shown to be of minor importance within intervals like 

 those of the above observations. It is important, however, to add 

 quantitative work, and table 5 supplies relevant data, dp being the drop 

 of pressure on exhaustion, n the nucleation, n' jn the ratio of successive 

 nucleations. Table 5 shows that the nucleation is spontaneously reduced 

 by diffusion to about one-half, in the interval between the second and 

 tenth minutes, and so far as may be observed the decrease is fairly 

 uniform. One may therefore estimate a time loss of about 6 per cent 

 per minute. Consequently the coronas obtained in the second and 

 third minutes (or even later) are liable to show no discernible difference 

 comparable with the other possible complications involved. Thus, for 

 instance, the loss of water nuclei proper can not begin before all the fog 

 particles have evaporated, a process which must consume a minute at 

 least if an influx of strictly dust-free air is to be assured. 



Table 5 also gives evidence to the effect that no nuclei come through 

 the filter. If the partial vacuum is gradually raised to dp = 36 cm. without 

 sudden exhaustion, i. e., without initial coronas, and filtered air is then 

 passed in the identical way through the filter into the fog chamber, no 

 water nuclei whatever are detected by the sudden exhaustion at dp = 1 8 

 cm. This is conclusive proof that the evaporation of fog particles is the 

 sole cause of nucleation. 



Again, table 5 shows that by keeping the influx cock open, nearly 9 

 per cent of the fog particles may be represented by water nuclei even 

 when nio 6 . It is not safe to admit a more rapid influx at the filter, 

 but the sudden introduction of previously filtered air suggests itself as a 

 means to the same end, and will be tried in turn. 



