PREFACE. Vll 



additional tendency to promote it. J. J. Thomson (1888) was the first to 

 adduce theoretical reasons for the suspected condensational activity of the ion. 

 In the time since, so large in number and so important have been the researches 

 in which the precipitation of supersaturated water vapor on ions enters as an 

 argumentative premise, that insistence on the functions of the nucleus has 

 dwindled by comparison. I must nevertheless claim the right of an independent 

 investigator to interpret my work in a way which seems to me inevitable ; and 

 I have therefore ventured to believe that, so far as experimental evidence goes, 

 the occurrence of electrical excitation is quite without influence in promoting 

 condensation of moisture in supersaturated air. However ionization may be 

 produced in the laboratory, whether by X-rays, or by ignition, or by high 

 potential, by chemical means, or even by excessively vigorous trituration as in 

 jets, it is always accompanied by nucleation. The average size of the nucleus 

 resulting depends for a given medium on the time of exposure to the exciting 

 cause and its intensity; or, in general, upon the number of nuclei produced per 

 cubic centimeter per second. Roughly speaking, if the conditions producing 

 ionization are sufficient and if they are maintained, there will be continuous 

 growth in the number and size of the nuclei. On this question I have 

 already expressed myself at some length in Nature (vol. LXIX, 1903, p. 103), 

 believing that "out of all systems eventually issues a stable nucleation." 

 "Why," I ask, "may one condense on a nucleus from which the soul has fled, 

 and still be permitted to call it an ion? Why, indeed, does the nucleus persist 

 after the ionization has vanished; why does one not get back to dust -free air?" 

 I conclude that "electrification, if present simultaneously with nucleation, is an 

 incidental accompaniment with no immediate bearing on the condensation pro- 

 duced, and for this reason I have endeavored to account for the nucleus at the 

 outset, chemically." It is therefore merely necessary to summarize the point 

 of view in the following statement. Whenever ionization and nucleation are 

 associated in the outcome of any process, physical or chemical, the former is 

 generated proportionally to the latter, in such a way that each is produced at 

 its own rate, depending on incidental conditions. The subsequent life histories 

 of the nucleation and the ionization are distinct, nuclei being often surprisingly 

 persistent, ions by contrast characteristically fleeting. Hence it seems to me best 

 in keeping with all the data in hand, to regard the nucleation as the product 

 which owes its growth or origin to the expulsion (possibly also to the absorp- 

 tion) of the corpuscles representing the concomitant ionization. Moreover 

 ionization should be present only during the period in which the nucleation 

 varies, and a high order of nucleation may be associated with a very low or even 

 vanishing order of ionization. Many phenomena met with in the case of dust- 

 free air seem to be favorable to this view. Ignition and high potential nuclei, 

 X-ray and radiation nuclei in general, phosphorus and water nuclei, produced 

 throughout in dust -free air, all admit of this account of their occurrence and prop- 

 erties ; and there is no observable case of a process producing ionization free from 

 nucleation, although there are many cases of nucleation free from ionization. 



