30 NUCLEATION OF THE UNCONTAMINATED ATMOSPHERE. 



must be supposed to carry down more moisture than the small coronas, 

 but the difference need not be great. The hypothesis encounters a 

 serious obstacle, inasmuch as the coronas obtained from saturated air 

 which has been imprisoned for long intervals of time (section 8) are 

 usually an extremely small type of inferior corona, whereas they should 

 be large superior coronas. Long intervals of waiting between exhaus- 

 tions bring out not a superior corona but at best one of intermediate 

 size. Another precarious feature is suggested by computating the 

 rate at which saturation should be established in the most unfavorable 

 case of the middle air layer, between the wet top and bottom of the 

 fog chamber, for diffusion alone. 



In fact, if diffusion takes place from the wet top and bottom of the 

 rectangular trough of height a into a partially saturated atmosphere 

 of initial vapor pressure / 0) then at any time /, at the middle plate 

 x a/2 



4A~i / sin ^ e - + -sin^e- 37r + etc.) 



TT \ 2 2 / 



3 



where dpldt = k (cPpldx 2 }. Hence if a 1 1 cm. , as in the largest trough 

 (wood), and if = 0.23, the following values obtain : 



/ 30 jS = ,/=.28 /o = i/3. P= -52 /o = 2/3, ^=.76 



60 .59 .72 .86 



120 .87 .91 .96 



180 .96 .97 .99 



In the above tables a was usually less than 10 cm. (glass fog cham- 

 ber), making the condition correspondingly favorable. 



Hence by diffusion alone there should be saturation after 2 to 3 

 minutes even at the most distant (middle, x a\-2) plane, to within a 

 few per cent, for the central layer is probably always more than half 

 saturated at the outset. In addition to diffusion, however, there is 

 marked convection due to the lightness of water vapor. At the same 

 time there is no evidence that the more numerous but small drops of 

 the superior coronas carry down a sufficient excess of water ; nor are 

 the coronas, though blurred, otherwise distorted, as they would be 

 for a definite diffusion gradient. 



29. Undersaturation, continued. Assuming, however, that under- 

 saturation* does occur and is oscillatory as the result of successive 



* It will be shown below, Chapter VI, sections 97, 98, that the probable cause of 

 periodicity is not undersaturation but the production of water nuclei by the evapora- 

 tion of the small fog particles. The analysis of the phenomena given in section 14 

 applies, however, with obvious changes, to both cases, and has therefore been 

 retained in the text. 



