30 VAPOR NUCLEI AND IONS. 



THE NUCLEATION OF FILTERED AIR IN THE LAPSE OF TIME. 



26. Method. In the above experiments with filtered air, it has been 

 carefully pointed out that all the curves tend to reach asymptotes, the 

 height of which depends on the other (larger) nuclei present. If, then, 

 the ionization of dust-free air is an essentially variable quantity, as 

 shown by the electrometer, the height of the asymptote in question 

 should be correspondingly variable. 



Again, it was shown that external radiation (gamma-rays of radium 

 for instance) are powerful nucleators, whether directly or secondarily. 

 Hence external radiation of this type, if variable, might be reasonably 

 sought for in a study of the nucleation of dust-free air. 



In relation to the ordinary intensities of radiation, this method of 

 varying asymptotes is exceedingly sensitive. Whether, however, it 

 will apply for cases of much weaker (cosmical) radiation must be left 

 to experiment. Moreover, whether the variable ionization of air can be 

 detected in the manner suggested, or whether it even exists in the fog 

 chamber, is similarly a question requiring experimental solution. 



27. Early data. The method here in question was carried out in a 

 series of experiments begun May 9 and continued until September 2, 

 1905. In the earlier part of the work it was shown that comparable 

 results could only be expected in cases of excessively slow filtration, 

 the filters for this purpose being 18 inches long, 2 inches or more in 

 diameter, and containing cotton very tightly packed. It was necessary, 

 moreover, to make observations after long intervals (12 to 24 hours) of 

 waiting, the object being to allow all nuclei which might have passed 

 through the filter time to decay. Apparent variations of considerable 

 interest were obtained in this way ; but the work below will show them 

 to have been untrustworthy. Neither changes of temperature nor of 

 the barometer produced any effect. 



To additionally safeguard the work, two fog chambers were eventually 

 installed side by side, drawn upon by the same exhaustion system 

 identically in every way. It was then found that the internal partitions 

 of wet cloth were essential, but it was nevertheless impossible to make 

 both chambers agree even when the partitions were increased four- 

 fold. Throughout the work, nucleations passing through maxima and 

 minima within the limits of about 400,000 and 160,000 were obtained, 

 apparently under trustworthy conditions. The data remained consistent 

 even when the fog chambers were exchanged. Small variations of the 

 rate of filtration were quite ineffective, showing that a limit has been 

 reached. 



