140 NUCLEATION OF THE UNCONTAMINATED ATMOSPHERE. 



9T. Alternations of large and small coronas Periodic distributions 

 of efficient nuclei in dust-free air. The coronas in question may be 

 distinguished as superior and inferior coronas. They are obtained in 

 successive exhaustions of dust-free air, under conditions of experiment 

 which are quite identical, filtered air being introduced in the periods 

 between the exhaustions, after all the fog particles have subsided. 

 The efficient nuclei are therefore present in large and small number, 

 alternately, usually in the ratio of about 8 to i. Figures 14 and 15, 

 Chapter II, give an example of the changes of angular coronal diam- 

 eter, s, in the successive observations with dust-free air enumerated 

 by the abscissas. The pressure difference is 8p ^i cm. and the 

 time between the exhaustions 2 minutes. Twenty exhaustions are 

 recorded, but the experiment might have been prolonged indefinitely. 

 In figure 17, Chapter II, there are 3-minute periods between the 

 exhaustions. In figure 18 the periods are 5 minutes in length, but the 

 phenomenon here vanishes. All the graphs show that relatively high 

 inferior coronas (k) are followed by relatively low (/) superior coronas, 

 and low inferior coronas are followed by relatively high superior 

 coronas; furthermore, that coronas of mean aperture are followed by 

 coronas of the same kind, so that the periodicity ceases, as seen in 

 figures 1 6 and 18 of Chapter II. In figure 16 alternations and steady 

 aperture were obtained under otherwise like conditions. In figure 26 

 the same phenomenon is exhibited in case of dust-free air energized 

 by weak radium. The ordinates here show the number of nuclei per 

 cubic centimeter, so that the sweep of the alternations is more striking. 



An explanation of these phenomena may be given (sections 96, 98) 

 in terms of the occurrence of water nuclei produced by the evaporation 

 of the small fog particles to a size at which solutional decrement of 

 vapor pressure balances the increment due to increased curvature. 

 In figures 14 and 15, Chapter II, the average inferior nucleatious of 

 dust-free air are about 12,000, the average superior uucleations over 

 90,000, so that explanations in terms of negative and positive ions are 

 out of the question. 



To precipitate nuclei which, as is usual, are more or less graded in 

 size, in a single exhaustion, must be generally impossible for similar 

 reasons. While temperature after exhaustion approaches its original 

 isothermal value, the small particles caught at the end of the exhaus- 

 tion to the amount of about 10 per cent of the total number evaporate 

 to the water nuclei stage, to be precipitated in the next exhaustion. 

 This evaporation probably accounts for the permanence of coronas 

 throughout the period of subsidence of fog particles, during the early 

 stages of which temperature rapidly increases. 



