94 CONDENSATION OF VAPOR AS INDUCED BY NUCLEI AND IONS. 



evaporate to the same nucleus as the fog particles of small coronas; or, 

 more pertinently, whether the fog particles precipitated on ions evapor- 

 ate to the same nucleus as the fog particles precipitated on the vapor 

 nuclei of wet dust-free air. A number of allied questions will be treated. 

 A variety of methods were tested, as follows: 



I. The endeavor was made to find if from fogs characterized by 

 identical coronas the number of residual nuclei was the same after the 

 natural evaporation during subsidence, no matter whether the original 

 precipitate occurred on ions or on the vapor nuclei of dust-free air. 



II. Identical coronas were produced on ions and on vapor nuclei, 

 respectively; but the evaporation of fog particles was accelerated by 

 keeping the influx valve open by a definite amount. The number of 

 residual nuclei was then tested by a second exhaustion, the amount of 

 which was varied. This was done both by starting with different press- 

 ures in the vacuum chamber for full barometric pressure in the fog 

 chamber and by starting with different partial exhaustions in the fog 

 chamber for the same pressure in the vacuum chamber. 



III. The persistence of the residual nuclei was studied by measuring 

 their decrease in number in the lapse of time. Incidentally the loss due 

 to evaporation was estimated and the distribution of sizes considered. 

 Finally, in the second part of this chapter the method of successive 

 exhaustion, which is found to be most productive, is brought to a definite 

 conclusion. 



In all cases the ions were produced by a weak sample of radium in a 

 sealed aluminum tube, attached to the top of the fog chamber. This 

 was removed during the examination for water nuclei, inasmuch as the 

 ions are efficient in the presence of the latter. The corona obtained 

 from the radium was always the same, care being taken to precipitate all 

 residual water nuclei in these cases, and to have a pressure difference 

 sufficiently high to catch all the ions, or at least the same fraction of the 

 total number. To produce the same given corona with the vapor nuclei 

 of dust-free air is easily accomplished after a short preliminary trial. 

 Moreover, these coronas may be obtained at the same pressure when- 

 ever the asymptote for the ions has been reached. The eye is always 

 40 cm. and the source of light 250 cm. from the axis of the fog chamber. 



60. Residual water nuclei after the natural evaporation of fog par= 

 tides. The results obtained from these experiments are given in table 40. 

 Here p = "j6 and p 3p' a are the initial pressures of the fog and vacuum 

 chambers; p dp a the final pressure, when in communication after 

 exhaustion; s a /3o the angular diameter of the corona, observed for 

 ions and vapor nuclei in dust-free air, as specified. Again, p = op' b ; 

 p dp b denotes the initial pressure of the fog chamber and vacuum 

 chamber before exhaustion; pdp c the final common pressure after 



