Bar us — Groups of Efficient Nuclei in Dust-Free Air. 299 



moisture is condensed on ions, and as their number increases 

 as D vanishes, the minimum in question results. 



In fact it was shown elsewhere, that below the fog-limit of 

 air, the nucleation observed and due purely to radium at differ- 

 ent distances D, is for example (8p = 22) 



z> = 



__ 



10 



30 



50 



100 



i^x io- 3 -. 



..120 



50 



32 



20 



12 etc. 



agreeing in character as far as may be expected with the data 

 here in question. These data multiplied by 4, i. e., 4 n X 10~ 3 , are 

 also given in figure 2 for comparison. Hence the ions caught 

 at Sp — 41-5 are about four times more numerous than at 

 hp = 22, and correspondingly smaller. They are, therefore, 

 markedly graded, but nevertheless, as a group, throughout 

 much smaller than the nuclei of dust-free air so long as the 

 radiant field is appreciable. Whether the latter are agglomer- 

 ated under the influence of radiation to make the ions (as 

 would seem more probable), or whether the ions are made from 

 the molecules themselves so that the ions and the nuclei of 

 dust- free air are present together, is a question beyond the 

 scope of the method. While the number of nuclei continually 

 grows smaller, with diminishing D, the efficient or capturing 

 nuclei may nevertheless increase again below a certain _D, 

 seeing that the nuclei in dust-free air are enormously in excess, 

 only a few of which are caught even in the absence of radium. 



4. Cause of the maximum. — It is more difficult to account 

 for the result that the same nucleation is observed wherever 

 the radium touches the elongated fog-chamber. In other 

 words, radium at the end of the chamber produces at least the 

 same nucleation as when at the top, although the distances 

 from the center of mass of the glass are as 3 to 1. The same 

 kind of explanation already given in § 3 may possibly hold. 

 The radium tube when placed on the top (Tin figure 3) and in 

 contact with thinner glass, may act with sufficient intensity to 

 admit of the formation in turn of a group of nuclei larger than 

 ions. This is what actually occurs in the case of X-rays. But 

 it is more probably connected with the uniform distribution of 

 nuclei within the chamber (§ 1) and in some way referable to 

 secondary radiation evoked within the chamber. Secondary 

 radiators added on the outside are quite without effect.. 



5. General Conclusion. — The occurrence of a continuous 

 succession of groups or gradations of nuclei in the curve of 

 figure 2, each of which groups constitutes a condition of chem- 

 ical equilibrium for the given radiating environment, is sugges- 

 tive. In the first place, it may be recalled that the nuclei of 

 dust-free air are an essential part of this body as much as the 

 molecules themselves. Such nuclei if withdrawn by precipi- 



