64 NUCLEATION OF THE UNCONTAMINATED ATMOSPHERE. 



It is particularly interesting that if the nuclei or dust-free nonener- 

 gized air become the solute of water nuclei, that the new nuclei require 

 pressure differences much below the fog limit of the original air for 

 condensation, and the solutional nuclei are stable. 



52. Cause Of periodicity. Very many of the occurrences which accom- 

 pany condensation may be explained by the result just announced. 

 In particular the alternations of large and small coronas (cf. Chapter II, 

 sections 24, 27) so frequent throughout the present work are fully 

 accounted for. The small fog particles, i. e., those condensed on the 

 smaller nuclei and caught near the end of the exhaustion, evaporate 

 into the larger particles and become the water nuclei available in the 

 next exhaustion.* Hence if the other condensation nuclei are all 

 small, like those due to weak radiation or those normally present in 

 air, the next condensation will take place on the water nuclei only. 

 Thus a small inferior corona follows the large superior corona of the 

 primary exhaustion. In the next exhaustion water nuclei will be 

 absent, relatively speaking, and the condensation now occurs on all 

 the small nuclei available for producing a large superior corona ; and 

 so on in succession. In general there are thus three groups of nuclei, 

 x, y, z, concerned in any given condensation a group, x t of water 

 nuclei from the preceding exhaustion ; a group, y, of nuclei belonging 

 to the condensation in question and corresponding to the observed 

 superior coronas ; a group, z, of nuclei belonging to the same conden- 

 sation, but which will evaporate their water and make the water nuclei 

 for the next condensation. In case of periodicity the successive 

 exhaustions would run, 



1. j'i z\ Superior corona on y\. 



2. x%=2i 22 = Inferior corona on .T2. 



3. .TS = .eg = o ; va+.vs ^3 Superior corona on j2 +3 '3. 



4. Xi = .?3 Zi = o Inferior corona on .\~i. 



5. .Y5 = 2i = o; j'4+>'5 ^5 Superior corona on 



etc. If the nuclei are fleeting the superior corona of the n-th exhaus- 

 tion is condensed on _y n only, and under these circumstances, jFn+^n, the 

 total nucleation may often be replaced by x n -\-y n , or the total nuclea- 

 tion is the sum of the values belonging to the superior and inferior 

 coronas. In this way the analysis of Chapter II, sections 28, 29, admits 

 of easy interpretation. 



53. Rain. In any exhaustion the group z n =x n+l are accountable 

 for the rain which almost always accompanies coronal display. In 

 such cases the water nuclei are comparable in smallness with the other 

 nuclei, and the former are not able to capture all the available water. 



* Incidentally the medium is kept saturated while temperature rises after exhaustion. 



