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PREFACE. 



In Chapter I of the present publication the investigations on the 

 residual nuclei of pure water (compare Carnegie Institution of Washing- 

 ton Publication No. 96, Chapter V, 1908) have been resumed with regard 

 to the size and persistence of such nuclei. These are obtained by the 

 precipitation of water on the nuclei of pure water vapor, in a dust-free 

 fog chamber, by sudden cooling. The fog particles so produced evaporate 

 to water nuclei on compression, the number of the latter as compared 

 with the former being greater as the evaporation of fog particles is more 

 rapid and as their size is larger. In the extreme case nearly 50 per cent 

 of the fog particles were represented by these residual water nuclei. It 

 is a curious observation that whereas the relatively enormous fog particles 

 of pure water evaporate at once beyond the range of visibility, such 

 evaporation stops in cases of certain of the invisible water particles (0.5 to 

 50 per cent of the total number of fog particles) as the evaporation is more 

 rapid in the manner specified. The remaining fog particles evaporate 

 completely. It was impossible to detect any electrical effect due to rapid 

 evaporation. The cause of these phenomena is difficult to ascertain, but 

 it may be suspected that it is associated with the composite nature of the 

 molecule of liquid water. 



The first part of Chapter II contains further studies of the optics of 

 coronas. It was shown that the interference phenomenon superimposed 

 upon the diffraction phenomenon in the case of coronas may be treated in 

 a way similar to the lamellar grating, consisting of a uniform succession of 

 alternate strips of thin and thicker transparent glass. Given types of 

 coronas are reproduced in successively increasing size, when the respective 

 fog-particle diameters are in the ratio of . . . . 5, 4, 3, 2, i, o. The 

 ratio of fog-particle diameters d and interference plate thickness D for the 

 same color minimum in the interferences and a film of water is d/D = n 

 (n i ) , where n is the index of refraction of water. The experimental 

 value of d/D agrees well with this. It must therefore be possible to com- 

 pute the nucleation corresponding to a given corona purely from optical 

 considerations of diffraction and interference, as indicated. To further 

 verify the theory suggested, special study was made of the axial or inter- 

 ference colors of coronas by the aid of large drum-shaped chambers 2 

 meters long. 



The coronas obtained with electric light are almost too complicated 

 for practice, for which reason a part of the mantle of a Welsbach burner 



in 



