12 CONDENSATION OF VAPOR AS INDUCED BY NUCLEI AND IONS. 



Fog particles precipitated on the nuclei of water vapor in dust-free air, 

 in contrast with the preceding case, evaporate under the same circum- 

 stances almost without residue, the yield of water nuclei (after allowing 

 for subsidence and in the absence of all interference) being but 0.004 

 when d = 0.00016 cm., increasing to 0.0036 when d = 0.0003 2 cm - These 

 fog particles evaporate into the wet air from which they were precipi- 

 tated, and the experiment may be repeated indefinitely. Relatively more 

 water nuclei persist as the fog particles evaporated are larger. 



1 1 . The same, continued. The persistence of water nuclei obtained 

 in the last case from the nuclei of water vapor is much increased by 

 accelerating the evaporation of the fog as soon as formed. Such forced 

 evaporation is produced by the rise of temperature due to the com- 

 pression accompanying the influx of dust-free air after the exhaustion 

 which precipitated the fog. This result can not be associated with losses 

 due to subsidence. 



When the rate of evaporation is increased by compression, moreover, 

 the number of water nuclei (derived from the reasonably rapid evapor- 

 ation of fog particles precipitated on vapor nuclei and persisting within 

 5 minutes after the evaporation) may be as large as 5 per cent to over 20 

 per cent, depending upon the size (d=i9Xio~ 5 to d = 32Xio~ 5 cm., 

 respectively) of the fog particles evaporated. Again, relatively more 

 water nuclei persist when the fog particles evaporated are larger within 

 limits given. By keeping the influx cock for dust-free air slightly open 

 on sudden exhaustion, 10 per cent of the fog particles evaporated may be 

 represented by persistent water nuclei, even when d 1 9 X io~ 5 cm. or n = 

 io 6 . If it were safe* to make use of more rapid evaporations, this limit 

 could unquestionably be much increased. Thus on the rapid evaporation 

 of fog particles by the influx of filtered air from a large independent reser- 

 voir into the fog chamber, about 18 per cent of the fog particles were 

 converted into residual water nuclei when n io 6 and actually 48 per cent 

 when w=io 5 . All such values or lower limits, because the loss of fog 

 particles at the walls of the vessel and by coalescence is not included, 

 but tests under most rapid evaporation possible showed that a limit had 

 been practically reached. 



The loss of nuclei by decay (diffusion) in the lapse of time (say 6 per 

 cent per minute within the given interval of observation) and the effect of 

 changes in the drop of pressure on sudden exhaustions have no causal 

 bearing on the production of water nuclei by rapid evaporation. They 

 merely modify the number. Similarly the effect of subsidence is second- 

 ary. Hence the cause of the production of persistent water nuclei in 

 rigorously dust-free air must be associated with the speed of evaporation 

 or with the motion of the fog particles during evaporation. It is a curious 



*Naturally the efficiency of the filter must be tested before each experiment. 



