Vlll PREFACE. 



What becomes of the ionization is a pertinent question: the ions probably 

 vanish by recombination, as they possess strong affinities for each other. Ejected 

 not by atomic but by molecular disintegration, we can scarcely attribute to 

 them phenomenal velocity. They may under favorable circumstances produce 

 fresh nuclei by absorption, by collision, but experiment does not show any ap- 

 preciable increase of nucleation during the period in which the ionization van- 

 ishes. If, however, the velocity of the ion is incremented by the presence of 

 an electric field, the production of fresh nuclei by collision may become per- 

 ceptible, and the result would then appear as if the nuclei themselves moved in 

 the electric field, whereas they are actually the inert residues left in the wake of 

 a fleeting electron. 



Finally, it should be noticed that to produce condensation on X-ray nuclei 

 after long exposure, less than a double supersaturation is needed, whereas in 

 Wilson and Thomson's case of condensation on ions, the supersaturation re- 

 quired is three- to four-fold. Thus the two views of condensation on nuclei 

 and condensation on ions would not in any case be mutually exclusive. Fur- 

 thermore, if initially (i. e., for short exposures, and nuclei in the extreme state 

 of fineness antedating growth) the nucleation is supposed to have ejected but 

 one electron per nucleus (an assumption which in one form or another must be 

 made in any other explanation), the present view is in no way incompatible 

 with J. J. Thomson's method of measuring the charge of one electron. 



If a nucleus like that of phosphorus, for instance, shows a continued ten- 

 dency to grow, until it finally appears as part of a visible smoke, there may be 

 continuous ejection of electrons within certain limits, as the growth matures. 

 In such a case, electric conduction through a gas freighted with these nuclei 

 would obey Ohm's law, as is actually the case for phosphorus. 1 



To return from this digression to the present volume : the contents of the 

 first two chapters bear on my "Experiments with ionized air." The second 

 chapter originally carried the working hypothesis into further development, but 

 as I have not been able to supply the requisite numerical detail, I have retained 

 the experimental parts only. These chapters, like the earlier work, show, I 

 think, that whereas ionization vanishes with characteristic rapidity, the nuclea- 

 tion has a long lease of life by contrast. At the same time the ionization and 

 nucleation produced in any given process are proportional quantities. The 



1 In the time elapsed since these experiments were made, I have carried them much fur- 

 ther than stated in the text (cf. Science, xxi., 1905, pp. 275 and 563; American fourn. of 

 Sci. (4), 1905, xix, pp. 175 and 349; Physical Review, July, 1905), showing among other 

 things that persistent X-ray nuclei pass into fleeting nuclei on removal of the X-ray tube to 

 greater distances from the outside of the fog chamber or on loss of intensity. Such fleeting 

 nuclei become persistent water nuclei on solution. For the case of radium in a sealed alumi- 

 num tube surrounded by a wall of lead i cm. in thickness, the nucleation is reduced by but 

 30 % of the value obtained when the lead envelope is absent. Hence the gamma rays which 

 produce but a few per cent, of the ionization are accountable for the greater part of the 

 nucleation. 



