46 NUCLEATION OF THE UNCONTAMINATED ATMOSPHERE. 



Corresponding experiments given in the second part of the table 

 were therefore made with more powerful radiation (fig. 38). Here 

 there is a regular decrease of aperture from the beginning where pres- 

 sure differences are vanishing to the end where they are very high. 

 The nucleations nevertheless pass through a determined maximum at 

 about 8p=i6 to 20 cm. The endeavor was made to guard against 

 variation of efficiency of the X-ray bulb and other causes by repeated 

 redetermination of the fiducial pressures. The fog limit of dust-free 

 air is 8p = 24.7 here. Hence it is noteworthy that the computed nucle- 

 ation decreases at the highest pressure differences in spite of the acces- 

 sion of air nuclei to augment the number of X-ray nuclei present. 

 The figures (38, 39) show the changes of n ; but the reduction to N 

 (nuclei per cubic centimeter at normal pressure) adds nothing new to 

 the results. 



To determine the nucleations for the high exhaustions is precarious, 

 because the efficiency of the apparatus in producing truly adiabatic 

 conditions will rapidly grow less, and because the data needed for the 

 vapor pressure of water 20 or more below freezing are not forthcom- 

 ing. A method of quadratic extrapolation had to be used in the 

 present table. Indeed, with the possibility of temperatures below 

 freezing even after the condensation of the water vapor, the whole 

 phenomenon becomes very complicated. Hence the values of N in 

 table 27 are mere estimates for values of S/> exceeding 20 cm. 



It may be observed, in conclusion, that in all the above cases the 

 X-radiation was cut off some minutes before the exhaustion, so that 

 persistent nuclei are alone in question, to the exclusion of the fleeting 

 nuclei discussed in the next chapter. 



