50 THE STBUCTOKE OK THE NUCLEUS. 



2/10* cm. ;is tbe diameter of tlie particles in the first exhaustions. The smaller size 

 here corresponds to less exhaustion. 



A final estimate is obtainable from the size of the normal coronas after, say, 

 twentv or thirty exhaustions. Estimating this as about twice as large in diameter 

 as the ordinary lycopodium corona, if the particles of the latter measuie .0032 cm., 

 one may rate the lesidual water globule at .001 cm. Indeed, Fraunhofer and 

 Kaemtz's measurements showed particles in lunai- coronas as large as .0017 to 

 .(»033 cm., depending on the season. Thus, if the final size is .001 cm., the initial 

 size must have been .0003 to .0002 cm., corresponding to twenty or thirty exhaus- 

 tions. Similar results are deducible from the drum and they are thus again in 

 ao'reemeut with the preceding estimates. Finally, the normal coronas may be 

 measured absolutely and the dimensions computed from the deviation on diffractinn. 

 This is the practical plan, which I will waive for further discussion in the next chapter. 



If the independent estimates Just stated be sun)marized, the data are clearly of 

 an order ten times greater than would follow if the axial colors of the drum or the 

 steam jet were produced by interference of thin plates, granting that the old vesic- 

 ular theory of atmospheric condensation is disproved. I admit that data as large 

 as those found for the w'ater particles are contiary to my exp)ectations. 



29. Eatimaied size of nuclei. — This has been variously estimated by the aid of 

 Kelvin's vapor pressure equation, successively modified by the younger Helmholtz,' 

 and by C. T. R. Wilson." Naturally, the nuclei are supposed to be of the same size 

 and the supersaturation carried far enough to condense water on each. Helmholtz 

 found ly/lO" to 26/108 ctn. as the size of his nuclei. Wilson finds S.T/IO^ for the 

 case of rain-like condensation, and 6.4/10^ and 5.9/108 di, for cloudy condensation, 

 foggy and colored. The above data for the globe, similarly interpreted, give 8/10^ 

 and 18/10^ as the smallest radii of the nuclei on which condensation would just 

 take place if the supersaturations used were essential to condensation, which they 

 are not. The actual dimensions must lie somewhere between Wilson's estimate, 

 which holds for watei- vapor free from artificial nuclei, and that of Helmholtz for 

 nucleated vapor. Of. Chapter V, § 2. 



The small size of the nucleus obtained in this way is startling, but as the 

 method involves a huge extrapolation from the radius of capillary action (say 

 5 X 10"^ cm.) almost as far as the molecular diameter, it cannot be received with 

 much confidence, and the size of the nucleus must be left in abeyance. 



On the other hand, the size of the water particle is sufficiently large to admit 

 of the application of Kelvin's equation. Moreover, there is here a mere accretion 

 of water upon water. If the change of pressure to pass from a given to a succeed- 

 ing corona, or to any recognizable change of corona, be determined by two suc- 

 cessive exhaustions, the particles of the first corona are the nuclei of the second, 

 and consequently the radius of the former should be determinable absolutely. 

 With this datum for the iiarticles of one corona, tables 9-12 and 19 would furnish 

 the diameteis of all. 



' R. V. Helmholtz, IVied. Ann., vol. xxvii, p. 526, 1886. 



'C. T. R. Wilson, Phil. Trans., London, vol. clxxxix, p. 306-307, 1897. 



