ON DISTRIBUTIONS OF NUCLEI IN DUST-FREE WET 

 AIR, AND ON METHODS OF OBSERVATION/ 



By C. BARUS. 



r - {Read April 20, 1907.) 



i- 



1. Nuclei. — The author remarks that the experiments described 

 all refer to air, from which the ordinary or Aitken nuclei have been 

 removed by filtration. The air is carefully kept saturated with water 

 vapor, and examined in a plug-cock fog chamber by rapid exhaus- 

 tion, partly without further interference, partly when acted on by 

 the X-rays or the beta and gamma rays or radium, entering the 

 fo2f chamber from without. The radium was sealed in an aluminium 

 tube. Water nuclei when not themselves the subject of observa- 

 tion (as in § 6) were always scrupulously precipitated. 



The kind of nuclei to be considered are thus, first, the vapor 

 nuclei (colloidal nuclei) of dust free wet air, which are probably 

 aggregates of water molecules ; second, the ions produced by the 

 presence of the radiant field, natural or artificial ; third, water nuclei 

 produced in dust free wet air by the evaporation of fog particles. 

 A careful distinction is here to be made between water nuclei ob- 

 tained from the evaporation of fog particles precipitated on solu- 

 tional nuclei, on vapor nuclei, and on ions. 



2. Methods of Observation. — The number of nuclei was com- 

 puted from the angular diameter of the coronas of cloudy con- 

 densation. These were standardized, as shown in the author's earlier 

 papers, by successive exhaustions, each of the same amount, the 

 residual number of nuclei (after correction for subsidence) decreas- 

 ing in geometric progression with the number of exhaustions. New 

 experiments were deemed desirable for the present work and were 

 carried out at great length. 



To measure the angular diameters of the coronas the older 



^ Extract of certain investigations made by aid of a grant obtained from 

 the Carnegie Institution. 



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