410 0. Barus — Behavior of Nuclei of Pure Water. 



limit could unquestionably be much increased. Thus on the 

 rapid evaporation of fog particles by the influx of dust-free 

 air from a large independent reservoir into the fog chamber, 

 about 18 per cent of the fog particles were converted into 

 residual water nuclei when n — 10°, and actually 18 per cent 

 when n = 10 6 . All such values or lower limits, because the 

 loss of fog particles at, the walls of the vessel and by coales- 

 cence is not included ; but tests under most rapid evaporation 

 possible showed that a limit had been practically reached. 



2. 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 exhaustion, have no causal bearing on the produc- 

 tion of water nuclei by rapid evaporation. The} r merely 

 modify the number. Similarly the effect of subsidence is sec- 

 ondary. 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 parti- 

 cles during evaporation. It is a curious fact that whereas the 

 relatively enormous fog particle of pure water evaporates at 

 once beyond the range of visibility, this process stops in case 

 of certain of the invisible particles (about - 5 to 50 per cent of 

 the total number) as the evaporation is more rapid in the man- 

 ner specified. The remaining fog particles evaporate com- 

 pletely. 



3. J. J. Thomson, Langevin and Bloch, and others* have 

 referred the persistence of pure water nuclei of about 10 -6 cm. 

 in diameter, to the minimum of surface tension discovered by 

 Remold and Huckerf for thicknesses of films of about the same 

 value. Since all fog particles are so much larger than this 

 order of values, it is difficult to see why, under quiet evapora 

 tion without any interference, they do not all terminate in 

 water nuclei, allowance being made for subsidence. Yet under 

 these circumstances the yield of water nuclei is least, being 

 usually within one per cent. Whatever losses may be due to 

 coalescence should be increased when the rate of evaporation 

 is increased, because there is more motion of the air relatively 

 to the fog particles. Again precisely the reverse occurs, inas- 

 much as an increased rate of evaporation enormously increases 

 the yield of water nuclei. 



Moreover, the residual water nuclei may, on rapid evapora- 

 tion, exceed the order of 10~ 6 cm. in diameter, two or three 

 times; or on slow evaporation they may fall below 5X10 -7 cm. 



* Remold and Riicker, Proc. Roy. Soc, vol. xl, p. 441, 1886. 



f J. J. Thomson, Conduction of Electricity through Gases, p. 152, 1903 ; 

 C. T. R. Wilson, Trans. St. Louis Electrical Congress, vol. i, p. 374-8, 

 1904. 



