C. Barus — Behavior of Nuclei of Pare Water. 409 



Art. XLIY. — The Behavior of Nuclei of Pure Water ; by 



C. Barus. 



1. For very small fog particles suspended in dust-free air 

 saturated with water vapor and left without interference, the 

 dissipation by evaporation is enormously more important than 

 that by subsidence. In my fog chambers the transition occurs 

 for the number of nuclei per cubic centimenter, n = 200,000, 

 or the diameter of fog particles, d, = 'OOOS 0111 , when about one 

 half evaporate and one half subside. 



Fog particles, precipitated on solutional nuclei (phosphorus) 

 evaporate to persistent water nuclei without other loss than is 

 attributable to subsidence and in a small degree to time losses 

 (diffusion). There is no loss by complete evaporation. 



Fog particles precipitated on the nuclei of water vapor in 

 dust-free air evaporate under the same circumstances and by 

 contrast, almost without residue, the yield of water nuclei 

 (after allowing for subsidence and in the absence of all inter- 

 ference) being but "004 of the number of fog particles when 

 d=-00016 cm , 1 increasing to -036 when d = '00032 cm . These 

 fog particles vanish into the wet air from which . they were 

 precipitated and the experiment may be repeated indefinitely. 

 Relatively more water nuclei persist as the fog particles evapo- 

 rated are larger. 



The persistence of water nuclei obtained in the last case 

 from the nuclei of water vapor is much increased by acceler- 

 ating the evaporation of the fog as soon as formed. Such 

 forced evaporation is produced by the rise of temperature, due 

 to the compression accompanying the influx of dust-free air 

 after the exhaustion which precipitated the fog. This result 

 cannot 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 rea- 

 sonably rapid evaporation of fog particles precipitated on 

 vapor nuclei and persisting within five minutes after the evap- 

 oration) may be as large as 5 per cent to over 20 per cent, 

 depending upon the size (<:Z = 19xlO _6 to <^=32xl0~ 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 — 19X10 -5 cm. or n — 10 6 . 

 If it were safe* to make use of more rapid evaporations this 



* Naturally the efficiency of the filter must be tested in connection with 

 each experiment and this efficiency must be perfect. 



