AIR ENERGIZED,, BY RADIUM. 39 



Hence the gradation is effectively more even, finer, i. <?., with fewer 

 gaps, as the fog limit is low and the maximum size of nucleus larger, 

 while for sparse distribution the steps from any nucleus to the next in 

 order of size are relatively large. For a different medium, dust-free 

 air, for instance, as given in the summary, or figure 34, the gradation 

 is characteristically different. Later experiments (Chapter III) make 

 it probable that the curves for dust-free air, not energized, and dust- 

 free air energized by radium and X-rays, make a continuous series. 



36. Radium in sealed aluminum tube. The identical sample of 

 radium (o.oi gram, io,oooX) was now removed by cutting the glass 

 tube, put into an aluminum tube (walls about o.i mm. thick), and 

 again hermetically sealed. This tube was introduced into the inside 

 of the fog chamber, kept in place 15 minutes or more, and then 

 removed to an infinite distance. The results obtained are different 

 from the preceding case of the same radium in the thin glass tube ; 

 for whereas the fog limit of dust-free air was regained 15 minutes after 

 the removal of the glass tube (figs. 31 and 32), it took at least 30 hours 

 to restore the same fog limit after the removal of the aluminum tube. 

 Table 21 shows the results, where Si and s 2 are apertures obtained in 

 successive exhaustions about 2 minutes apart, dust-free air being 

 added to the fog chamber in the interval (fig. 35). 



The successive fog limits for radium sealed in the thin aluminum 

 tube are, therefore, in centimeters of mercury (fig. 36), 



Radium in place, Sp^ig 

 Radium tube removed: 



Fog limit j,Yz hours later 21 



" 21 " 22 



30 23 



provided time is allowed for the excited activity within the chamber 

 to saturate the air with nuclei (cases Si). When such time is not 

 allowed as in the succeeding exhaustions (cases s 2 ), the fog limit of 

 dust-free air is practically regained in 30 hours. Something like an 

 emanation seems here to escape from the aluminum tube rapidly and 

 from the glass tube slowly, in spite of their thickness, relatively speak- 

 ing, and induces radio-activity at the inner walls of the fog chamber; 

 or possibly this induced activity is a kind of phosphorescence produced 

 by the impinging /? and y rays. It seems probable that the life of 

 the excited activity may be prolonged at pleasure within limits, by 

 gradually decreasing the walls of the aluminum tube hermetically 

 sealing the radium exciter. 



The loss of the activity shown by the rise of the fog limits in the 

 lapse of time is naturally of an exponential character (fig. 36). It 

 rises quickly after the removal of the radium from 8^ = 19 to about 



