PREFACE. Vll 



produced by any eiuanation or by radio-activity in hydrocarbon vapors, for in- 

 stance, nuclei which sticlc to the air and differentiate it gravitationally, the question 

 of diffusion is a vital one. The positive answer of the chapter is, 1 believe, war- 

 rantable. 



The quaint theory that nuclei which vanish are absorbed on contact with the 

 walls of the vessel and that no other loss of nuclei occurs, has been accepted liei'e 

 as in my earlier volume. I have found none at once so sinq)le and so gracious. 

 Nuclei are always sparsely distributed (10- to 10'' per cub. cm.) in comparison 

 with molecules. I have conceived them to be lai'ger tlian molecules but not so 

 large as to receive symmetrical molecular l)oml)ardment when suspended in a gas. 

 In other words, there are not quite molecules enough surrounding the nucleus to 

 insure a virtually persistent uniform pressure upon it in all directions. The mo- 

 tion of the nucleus is the result, and it moves faster in pi'oportion as it is smaller 

 and the asymmetry due to paucity <>f bombarding molecules below the statistical 

 limit, is accentuated ; and vice veisa. So conceived, the velocity with which the 

 nucleus dift'uses is at once its own and only velocity. 



If the nuclei vanished by subsidence, however slowly, this would appear in 

 the coronas. Not only is subsidence of nuclei absent, but they actually diffuse 

 ao-ainst gravity, while the coronas give evidence (so far as the motion of the un- 

 loaded nuclei are concerned) of permanently unifoiiu distribution. This narrows 

 down the question of loss to the possibility of a marked coalescence of nuclei, and 

 to the hypothesis of dynamic diffusion selected, with absorption at the boundaries 

 (if any) of the region. But the amount of loss by dift'usion actually found by 

 direct observation in the last chapter is of the same order of values computed by 

 the hypothesis in the two preceding chapters. Thus far then, there seems to be 

 no reason for introducing any source of decay of luiclei other than one involving 

 the motion of the nucleus. Moreover, if there are no boundaries to the region 

 there will be no loss of nuclei. 



A final word is due as to the bias of the volume. Recently it has become 

 customary to refer the phenomena of condensation too glibly, I think, to the 

 occurrence of ionization. I am well aware of the precision with which C. T. R. 

 Wilson' has stated his results, but others have not been so cautious, and the^ im- 

 pression is that negative ions embrace the whole story of condensation. The 

 elasticity and scope of the electronic hypothesis, the ease with which it lends itself 

 to the correlation of hitherto isolated an.l anomalous facts, may well account for 

 the enthusiasm with which it has been generally applied. Under these circum- 

 stances, it has seemed to me fitting that somebody should seriously endeavor to 

 see how much of what is known in relation to condensation, would follow from 

 the older theories of Coulier, Kelvin, and Aitken,'' and apart from ionization. As 

 this was the position I took in 181)8 shortly after an electronic hypothesis had been 



■ C T R Wilson: Phil. Trans., London, vol. 189, pp. 265-307, 1897; ihicL, vol. 192, pp. 403- 

 4=3 .899; ibid., vol. .93, PP- 289-30S, .899. The last paper is chiefly referred to. 



= An account of the earlier researches on the subject will be found ui my Bulletin of 1S93 

 (1. c), Chapter II. 



