210 C. Bar us — Changes of Colloidal Nucleation. 



the fog limits both for ions and for colloidal nuclei show no 

 variation of temperature between 10° and 30°. 



11. Conclusion. — Direct observation shows that the number 

 of nuclei caught in dust-free wet air at low barometer pressure, 

 is greatly in excess of the number caught (caet. par.) at high 

 barometer. This result may be accounted for as a necessary 

 consequence of the thermodynamics of the experiment, how- 

 ever large and unexpected the variations appear. 



The comparison of the nucleation of dust-free air with the 

 cotemporaneons changes of atmospheric ionization shows no 

 correspondence whatever. This is curious, because the ions, 

 though much fewer in number, are larger in size than even the 

 larger colloidal nuclei, and therefore capture much of the 

 moisture. One must conclude that the variations of the ioni- 

 zation are not sufficient to be detected in the presence of the 

 other nucleation. 



For the same reason, would it be unwarrantable to look for 

 effects due to variations of any external radiations ? In other 

 words, it is improbable that Wood and Campbell's phenomena 

 can be detected by the given fog chamber, and the results 

 which seemed at first in accord with it, are due to a rise of 

 temperature. 



The results as a whole show that the relative drop in pres- 

 sure $p/p is a suitable variable for the comparison of nuclea- 

 tions in the case of an apparatus like the above ; and that the 

 temperature variations obtained in this paper are directly 

 referable to the variable (8p — (ir — ir^/ip — tt) employed. 



The present, and a variety of other results since obtained, 

 will make it necessary to re-standardize the coronas in terms 

 of the number of nuclei represented, and the work is now in 

 progress. 



Brown University, Providence, R. I. 



