PREFACE. V 



Throughout the whole research the important bearing of the solu- 

 tional or water nucleus 1 on the phenomena of condensation is manifest. 

 If the nucleus is soluble in water, vapor pressure decreases with the 

 continued evaporation of the fog particle, until the decrement of vapor 

 pressure due to increased concentration of the solution is equal to the 

 increment due to increased curvature. The result is a persistent solu- 

 tional nucleus (water nucleus), necessarily larger than the original 

 nucleus of solute. A great variety of puzzling phenomena like the alter- 

 nations of efficient nuclei in successive otherwise identical exhaustions, 

 the persistence of fleeting nuclei or ions on solution, the lowered fog- 

 limit of an evaporated corona, etc., thus find a satisfactory explanation. 



The final general result to be referred to here is the readiness with 

 which nuclei are produced by the gamma-rays, even after penetrating a 

 centimeter or more of lead, together with the distinction which is thus 

 drawn, experimentally, between these rays and the X-rays. The latter 

 show small penetration, but are so phenomenally active in producing 

 secondary radiation that to a wooden fog-chamber the distance effect 

 for a radius of over six meters between bulb and fog-chamber is 

 relatively neglible. The effect of the gamma-rays, on the contrary, in 

 spite of the remarkable penetration evidenced, for instance, by the 

 nucleation produced, is nearly vanishing when tested by the same 

 nucleation at a distance of but 50 centimeters. Again, within the 

 fog-chamber the distribution of nuclei along the axis is in both cases 

 uniform for all distances (50 cm.) within the range of observation, 

 except when the X-radiation is sufficiently intense to produce persist- 

 ent nuclei. In this case the curiously pronounced distribution detailed in 

 Chapter I is observed, which seems to show either that the nucleation 

 originates in the walls of the vessel or that, in consequence of second- 

 ary radiation, the density of ionization near the walls is such as to 

 promote rapid growth of nuclei in those parts to abnormal sizes. The 

 nuclei in question are over 200 times more persistent than the ions, and 

 if they decay by breaking into like fragments one may estimate that the 

 former are 5 or 6 times larger in diameter than the latter. Persistent 

 nuclei produced by the X-rays require, in fact, but a vanishing pres- 

 sure difference to induce condensation. They have, moreover, the 

 property of increasing in number if left without interference for a 

 short time after radiation ceases. 



In view of the interest which thus attaches to dust- free or filtered air, 

 the nuclear systems of which are throughout small as compared with 



: Structure of the nucleus, Smithsonian Contributions, No. 1373, 1903. 



