IO 



VAPOR NUCLEI AND IONS. 



If the radium is inclosed in a long, thick lead tube (60 cm. long, walls 

 0.5 cm. thick), the nucleation is but moderately reduced (see crosses in 

 fig. 6), showing that gamma-rays are in question. 



Table 4. Radium effect at high pressure differences. 8p =41.5 cm.; s (air) =7.5; D, 



distance from end of fog chamber. 



1 Radium tube in lead pipe walls 5 cm. thick. 



2 Lead radiation. 



Fig. 6. Efficient nucleation (N) near the middle of the fog chamber, for different dis- 

 tances (D) of radium from the end and for different exhaustions (dp). Table 4. 



8. Cause of the minimum and the maximum. This is easily explained, 

 since the ions are relatively large bodies and relatively few in number 

 as compared with the nuclei of dust-free air for the same dp. Hence 

 the ions capture the moisture more and more fully as their number, 

 with diminishing distance D, becomes greater. At D = 2$ cm. probably 

 the whole of the moisture is condensed on ions, and as their number 

 increases as D vanishes the minimum in question results. In fact, it 



