CHAPTER V. 



RESIDUAL WATER NUCLEI. 

 PROMISCUOUS EXPERIMENTS. 



58. Historical. A nucleus obtained from a partial evaporation of fog 

 particles will be called a residual water nucleus or, briefly, a water nucleus. 



C. T. R. Wilson,* in his experiments with ultra-violet light, found 

 that nuclei were spontaneously producible on long exposure of dust-free 

 air saturated with water vapor to the radiation. He explained this as 

 being due to the probable production and solution of hydrogen peroxide, 

 wherefore the vapor pressure at the surface of the minute solvent water 

 droplets would be diminished. Such droplets would therefore grow in 

 the saturated environment. Wilson also encountered water nuclei pro- 

 duced by evaporation, but he expressed no opinion of their nature, merely 

 treating them as an impurity to be removed to make the air dust-free. 



J. J. Thomson,! in his famous experiments, encountered similar dif- 

 ficulties with water nuclei. He states that 



When .... the number of ions is large, experience shows that they are not all 



brought down by the first cloud formed by sudden expansion after the 



first cloud has subsided, [and] another expansion be made, a second cloud 



is formed 



On page 531, moreover, 



The first expansion .... though it does not bring all the ions down, seems to 

 increase the size of those left and makes them more permanent, .... these modified 

 ions are able to cause a cloud to settle with an expansion of less than 1.25... . 

 secondary clouds .... are but little affected by the electric field 



From this it seems that Thomson did not regard these secondary 

 clouds as precipitated upon water nuclei derived from the evaporation 

 of the fog particles of the first cloud. 



In lyos.J and more at length in my memoir on the structure of the 

 nucleus, I gave a detailed account of the behavior of the residual water 

 nuclei and showed by direct experiment that the merest trace of solute 

 in the fog particle evaporated left a persistent water nucleus behind. 

 The water nuclei of pure water seem by comparison to be evanescent. 

 The reduction of vapor pressure due to solution compensated the in- 

 creased vapor pressure due to curvature. 



*Phil. Trans., p. 428, vol. 192, 1889. 



tPhil. Mag. (5), 1898, vol. 46, p. 528 (cf. pp. 529 and 531). 



JPhil. Mag. (6), iv, pp. 262-269, 1902. 



Structure of the Nucleus, Smithsonian Reports, No. 1373, 1903, Washington. 



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