312 C. Barns — Successive Cycles of Coronas. 



tributed similarly to my own so far as the slope of the curves 

 for the vapor nucleation of dust-free air is concerned. 



There is another way in which the estimate in question may 

 be made. Let the nucleations corresponding to the colors be 

 taken and reduction be made for the different drops of pres- 

 sure in question. This is merely a corroboration of the method 

 of computation. The coincidence is as close as may be ex- 

 pected, seeing that the methods of approach are widely differ- 

 ent and that the nucleation varies as the cube of the inverse 

 diameter of particle. 



Wilson's views of the nature of the phenomena are quite 

 different and lead to enormous nucleations, even as compared 

 with the improbable -710.1 • He says (lc:p. 301) "When all 

 diffraction colors disappear and th^ fog appears white from all 

 points of view, as it does when (the expansion.) ■vjv 1 amounts 

 to about 1*44, we cannot be far wrong in assuming that the 

 diameter of the drops does not exceed one wave length in the 

 brightest part of the spectrum, that is, about 5xl0 -5 centim. 

 That the absence of color is not due to the inequality of the 

 drops is evident from the fact that the colors are at their 

 brightest when vjt\ is only slightly less than I'll, and from 

 the perfect regularity of the color changes up to this point. 



Taking the diameter of the drops as 5XlO -5 centim., we 

 obtain for the volume of each drop about 6XlO~ 14 cub. cen- 

 tim., or its mass is 6XlO -14 grin. 



Now, we have seen that when the expansion is such as pro- 

 duces the sensitive tint (when v 2 /v 1 — 1'12), the quantity of 

 water which separates out is about 7*6 XlO -6 grm. in each cub. 

 centim. With greater expansion rather more must separate 

 out. We therefore obtain as an inferior limit the number of 

 drops, when v i /v 1 = 1'44, 7'6Xl0- 6 /6Xl0- 14 = 10 s per cub. 

 centim." 



In my data the smallest green coronas correspond to a 

 diameter of particles of about c7 4 = '00052 cm , the next to 

 d 3 = -00010 cm , the next to cL = -00023 ; the first (which I have 

 not been able to produce by any means whatever, however 

 large the nuclei) should correspond to c7 1 = •00013 cm , and even 

 this calls for particles nearly three times as large as Wilson's 

 estimate, '00005 c,n . In a small tube but 2 cm in diameter, like 

 Wilson's test tube apparatus, it is improbable that the <# 2 green 

 corona, which is about twenty-seven degrees in angular diame- 

 ter, could look otherwise than greenish white, whereas the 

 filmy disc of the large crimson coronas (the largest produci- 

 ble ci 1 = '00016 with an angular diameter of about 39 degrees) 

 would be regarded colorless. I shall venture to believe there- 

 fore that Wilson's large greenish-white coronas corresponded 

 to about - 9Xl0 6 rather than to 10 s nuclei per cubic centimeter, 

 that the maximum nucleation would not exceed 10 7 even if 

 colors of the unapproachable first order were produced. 



