CHAPTER VI. 



PERIODIC COLOR DISTRIBUTIONS IN RELATION TO THE CORONAS OF CLOUDY 

 CONDENSATION, WITH A REVISION OF THE CONSTANTS OF CORONAS. 



INTRODUCTION. 



i. Purpose and plan. The growing importance of cosrric dust ' in rela- 

 tion to geophysic phenomena suggested the need of developing a method by 

 which the atmospheric dust contents could be speedily and systematically 

 determined. An appropriate method for this purpose was tested in a number 

 of my earlier papers * which gave promise of being in a measure independent of 

 merely local or accidental dust distributions. It is based on the measurement 

 of the angular apertures of the coronas produced on suddenly cooling moist 

 atmospheric air under definite conditions. Observations of atmospheric nu- 

 cleation made in this way for about two years show results of considerable 

 interest. 



There is some difficulty, however, in reducing these data to absolute values 

 (number of nuclei per cubic centimeter), inasmuch as the coronas obtained 

 with lamp light very frequently pass beyond the ordinary white centered normal 

 type into the more complex forms corresponding to very small particles. I 

 have therefore been obliged to make an extended study of coronas. 3 The 

 method pursued consisted in highly nucleating the air stored within a given 

 receiver over water (with adequate provision for continued saturation), and 

 then withdrawing definite amounts of it by successive partial exhaustions. If 

 the nucleated air is replaced by filtered air free from nuclei, the residual number 

 of nuclei in the receiver must decrease in geometric progression with the number 

 of partial exhaustions. The latter, moreover, produce the sudden cooling by 

 which the coronas are obtained. Let m be the moisture precipitated per cubic 

 centimeter, in any exhaustion, n the number of cloud particles contained, d the 

 diameter of each: then n = 6m/7rd } . .Since for the successive partial exhaus- 

 tions m is constant, n follows from d, and vice versa. 



Two methods are available for the absolute measurement of d. One may 



1 The pioneering work of Aitken is well known and cited in my earlier papers. 



1 Science, xvi, p. 948, 1902; Physical Review, xvi, p. 193, 1902; ibid., xvn, p. 233, 1903. 



3 Phil. Mag. (6), iv, p. 24, 1902; American Journ. of Science (4), xm, p. 81, 1902; 

 ibid., xv, p. 335, 1903; Physical Review, 1. c. ; Smithsonian Contributions to Knowledge, No. 

 1373, xxix, pp. 1-176, 1903. 



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