22 VAPOR NUCLEI AND IONS. 



is then near the point of transition from persistent to fleeting nuclei, 

 a result which will presently be made use of. 



The pressure difference (dp = i8 cm.) used throughout is (for the given 

 apparatus) far below the fog limit for dust-free air (dp = 21 to 26). 



In case of the fleeting nuclei or ions, the generation is instantaneous 

 and it will be necessary to represent the data (s, n, N) for a continuous 

 series of supersaturations for each of the five intensities of ionization 

 applied, beginning with non-energized dust-free air. These are given 

 for both increasing and decreasing values of dp, the latter being at times 

 larger (as would be expected from the removal of larger groups on con- 

 densation) ; but on the whole the data are very satisfactory. 



Remembering that the number is roughly as the cube root of s, the 

 region of ions (dp = 21 to 26) is particularly well represented. They 

 increase slowly but regularly in number until the coronal stages are 

 reached. Beyond these the increase is again slow; but that it is real is 

 shown by the top curve for X-ray nuclei, which fails to ascend. 



The lowest pressure at which the suggestion of a corona could be 

 observed in the given apparatus was dp = 21 cm. The coronal fog limit 

 is best found from the other curves. 



The succeeding curves (X-ray, D = 600; radium, D = o; X-ray, D = ioo, 

 50, 25 from end and 50 cm. from side) correspond to gradually increasing 

 ionization. It may be noticed that the fog limits slowly move to the 

 left into smaller supersaturations, while the initial slope of the curves 

 gradually rises. Persistence sets in when the slope is nearest the 

 vertical. All the curves eventually approach a limit, which for weak 

 radiation is practically reached near the fog limit of air, but for strong 

 radiation much beyond it. In the upper curves the green-blue-purple 

 corona first appears at dp = 30 cm. It is noteworthy that the asymptote 

 is reached later as the ionization is stronger. 



The relation of these results as to numbers of nuclei is best shown by 

 the w-curves, fig. 16, though the scale is now too small for the ions of 

 non-energized air. In this case (n) it is assumed that the nuclei are 

 reproduced faster than they can be removed by expansion. The 

 curves throughout have the same characteristic, being doubly inflected 

 and showing definite stages in which nucleation increases most rapidly. 

 If these branches are prolonged backwards the coronal fog limit may be 

 specified for each, as shown in the figures. This quantity decreases at a 

 retarded rate, while the ionization grows more intense. 



The effect of weak ionization (radium, or X-ray at D = 6oo) is to 

 increase the number of ions. It has been supposed above that the 

 result of this is to mask the corresponding increase of any of the col- 

 loidal nuclei originally present, should such an increase occur. 



