C. Bar us — Behavior of Nuclei of Pure Water. 411 



and yet persist for half an hour or more. Under any circum- 

 stances they are graded. They appear to diminish in size 

 with extreme slowness in the lapse of time, so that an appro- 

 priate interval of waiting will yield any size. 



4. Since the fog particles in question are absolutely pure 

 water (water condensed on water vapor), it is tempting to sug- 

 gest electrical charge as the cause of the observed persistence, 

 such charge being acquired either by friction during the 

 motion of particles undergoing rapid evaporation (influx of 

 air), or by the mere act of evaporation. The latter, like the 

 minimum of surface tension, would require the same persist- 

 ence of all fog particles under conditions of quiet evaporation. 

 As has frequently been shown, this "is not the case. A fric- 

 tional mechanism, suggested in view of the occurrence of con- 

 vection during the period of evaporation and influx of air, if 

 in action, would account for the discrimination between fog 

 particles as to survival. Thus drops of larger size are stirred 

 about for a longer time before completed evaporation and they 

 are therefore more favorably circumstanced to persist, as they 

 have been found to do ; water nuclei should not be of the 

 same size and they are not ; etc. But all my experiments have 

 failed to detect the amount of charge commensurate with the 

 persistence of nuclei. An effective charge would have to be 

 latent. 



If the radius of residual water nuclei be taken as 10 -6 cm., 

 the charge needed would be roughly e = 6*3 X 10~ 8 electro- 

 static units per particle, and its potential would be about 18 

 volts. If about two hundred thousand of these droplets or 

 residual water nuclei are present per cubic centimeter (as 

 were found above), the charge would be about 4 coulombs for 

 a cube each side of which is 100 meters. Finally the electric 

 contents of my fog chamber should be about 30 electrostatic 

 units of quantity and ought thus, in spite of the moisture 

 present, to be easily determinable. The experiments showed 

 only about •50X10" 6 electrostatic units per cubic centimeter, 

 less than the contents (-88X10 -6 ) in the room air without at 

 the time; that is, the average charge per nucleus was about 

 5XlO -12 electrostatic units, or less than 1 electron. Hence 

 the electrical hypothesis must be abandoned. It would in 

 any case be improbable for the charge to show so small a 

 coefficient of decay as do the water nuclei. 



5. Under the circumstances it seems permissible to suggest 

 an hypothesis of a statistical character ; namely, that the mole- 

 cule of liquid water is composite, consisting of virtually more 

 volatile and less volatile constituents. Such a view is quite 

 compatible with the composite molecule observed in water 

 vapor, where millions of nuclei may be captured long before 



