NUCLEATION CONSTANTS OF CORONAS. 65 



DISTRIBUTION OF VAPOR NUCLEI AND OF IONS IN DUST-FREE WET 

 AIR. CONDENSATION AND FOG LIMITS. 



37. Introductory. It will, in the first place, be desirable to gather cer- 

 tain of the older data together for the comparison of fog limits. There 

 is, in fact, quite a serious discrepancy between Mr. Wilson's results and 

 mine when reduced to the same scale. Mr. Wilson's supersaturations 

 for negative ions and cloud are distinctly higher, which seems to mean 

 nothing less than that my fog chamber, instead of being inferior, is in 

 these regions superior to his own. Thus, in moderately ionized air my 

 condensations begin at a drop of about 18.5 cm. from 76 cm. as com- 

 pared with 20. 5 in Wilson's apparatus; similarly, my fogs begin at the 

 drop 20.3, Wilson's at 27.7. Furthermore, at low ionization even the 

 vapor nuclei of dust-free wet air become efficient in the presence of ions. 

 It seems impossible .therefore, that any positive ions should fail of capture. 



38. Notation. The whole case may best be represented graphically, 

 but the tables will also be given. In my apparatus, however, the adia- 

 batic volume expansion v 1 /v is a troublesome datum to compute accu- 

 rately; it appears as 



v l P ( 



where p and p' are the pressures in the fog and vacuum chambers before 

 exhaustion, p 3 their common pressure when in communication after 

 exhaustion, always at the same temperature. The volume ratios of the 

 chambers is [v/V] =0.064; the TT'S denote the different vapor pressures 

 and k and c the specific heats. With a large vacuum chamber the 

 approximation 



may be used, so that if dp=pp 3 , the convenient variable for the com- 

 parison of exhaustions is the relative drop dp/p a . This is used in the 

 diagram with the approximate equivalent of the volume expansion v 1 /v. 

 (Cf. Chapter I.) 



39. Data. In table 25 results are given for the conditions observed 

 near the fog limits of dust-free air, and of dust-free air weakly ionized 

 by the beta and gamma rays (coming from a closed tube containing 

 radium placed on the outside of the fog chamber) and strongly ionized by 

 the X-rays (at a distance D from the fog chamber) . The data for ionized 

 air are nearly coincident, but dust-free air requires higher supersatura- 

 tion. The notation is as above, p, pdp' being the pressures of the fog 

 and vacuum chambers before, p Sp 3 the common pressure after ex- 

 haustion. The relative drop in pressure is x, the angular diameter of the 

 coronas 5/30, the number of nuclei per cubic centimeter n, the volume 



