74 



NUCLEATION OF THE UNCONTAMINATED ATMOSPHERE. 



of i and 2 minutes produce about the same uucleation, which vanishes 

 with the lapse of i or 2 minutes after exposure to negligible residues. 

 In the second part of the table the radiation is stronger and maximum 

 nucleation appears after 3 seconds of exposure, so that the nucleation 

 is produced instantaneously. Initial nucleations obtained (ccet. par.} 

 in air which has been long stagnant are apt to be very low. The 

 mean nucleation after less than 3 seconds' exposure is about 90,000 

 per cubic centimeter. 



The effect of longer exposures is again investigated in the third part 

 of the table, but the possibility of a slight increase of the nucleation 

 in the lapse of time is negatived by the last observation. The fourth 

 part also shows that fog limit to be at Sp = 2o cm., and that there is 

 no accumulation of nuclei as time goes on. There is no appreciable 

 persistence. 



TABLE 45. Generation and decay of nuclei. D = 200. 3/> = 25 cm. Wood fog 



chamber. 



* After long waiting. Stagnant air. f Follows preceding high nucleation. 



As a whole the observations for D = 200 are irregular, for the usual 

 reasons instanced above. 



