VI PREFACE. 



the much coarser ions, and show rapidly increasing numbers with 

 decreasing size until the molecular dimensions are reached or even sur- 

 passed in degrees of smallness, I have undertaken and have now in 

 progress, under the auspices of the Carnegie Institution of Washington, 

 a systematic research on the properties of filtered air. The fact that 

 the nucleation responds to very penetrating rays, like the gamma-rays 

 of radium, adds additional interest to the inquiry. 



In conclusion, it gives me pleasure to acknowledge my indebtedness 

 to Mr. Robinson Pierce, jr., for the efficiency and patience with which 

 he conducted the measurements of nucleation at Block Island, placed 

 in his charge under the very trying mid-winter conditions there encoun- 

 tered. His results are given in Chapter IV. I am further indebted to 

 Miss Lillie L. Scholfield, by whose skill in drawing and experience in 

 editorial work I have materially profited. 



My thanks are due finally to the Chief of the U. S. Weather Bureau, 

 for his kindness in placing suitable quarters for observation in the 

 Weather Bureau Building at Block Island at our disposal, and to Mr. 

 Day, the officer in charge of the station, for many courtesies throughout 

 the work. 



CARL BARUS. 



BROWN UNIVERSITY,, Providence, R. L, July, 1905. 



