122 NUCLEATION OF THE UNCONTAMINATED ATMOSPHERE. 



former curves dip down to meet the latter, while the nucleation may 

 be 50 or more times greater. The difference must be due (since it is 

 exaggerated in the winter months) to the originally ionized products 

 of combustion. The two curves give evidence of a rapid self-purifica- 

 tion (probably due to dilution) of the atmosphere, from which it follows 

 that the nuclei observed in Providence are to a large extent generated 



27&tc 28 29 30 31 

 100 



10 



10 Jan II 



12. 16 U 15 16 17 IS /9 20 21 

 FIG. 97. Showing daily record for December 27 to January 24. 



there. It would be hazardous to assume, however, that all the nuclei 

 are artificially generated, and Mr. Pierce is of the opinion that there 

 is an outstanding nucleation in the Block Island results which can 

 not in this manner be explained away. 



The early part of January is subject to rains, and the sustained 

 maximum at Providence following January 4 is but obscurely repre- 

 sented at Block Island. On the other hand, the maximum following 



