12 



VAPOR NUCLEI AND IONS. 



180 



360 



320 



280 



240 



200 



9. Further experiments with radium. Incidental experiments corre- 

 sponding to those of the preceding section are given in table 6, showing 

 the reduction of the efficient nucleation of dust-free air when the radium 

 tube is gradually approached to the sides of the chamber. They are 

 shown graphically in fig. 7, both in relation to s, n, and N, and need 

 little discussion. The minimum position lies very near the chamber 

 and about 10 cm. from it. The action of radium is hardly perceptible 



beyond 1 meter, chiefly 

 because the green-blue- 

 purple corona happens to 

 be involved. Sharper re- 

 sults could therefore be 

 obtained at lower pressure 

 differences, where white- 

 red-green coronas are in 

 question. But on the 

 whole it is clear that ra- 

 diation which appreciably 

 lowers the asymptote cor- 

 responding to dust-free 

 non-energized air must be 

 of an intensity exceeding 

 the effect of 10 mg. of weak 

 radium (10,000 X), at a 

 distance of 1 meter. There 

 Nxio- 3 does not, therefore, seem 

 to be much hope of ob- 

 serving cosmical radiations 

 in this way. Finally the 

 second part of table 6 (and 

 other data below) shows 

 how soon the high nuclea- 

 tions of dust-free air (non-energized) is regained after the removal of 

 the radium tube. The rapidity with which ionized nuclei fall apart is 

 obtainable in this way and well worthy of special research. (Cf. Section 

 62 et seq.) 



160 



120 



Fig. 7. Efficient nucleation (A/) and coronal aper- 

 ture (S) for different distances (D) of radium from 

 end of fog chamber and for different exhaustions 

 (dp). Table 5. 



10. Distance effect of penetrating X=radiation. The question at issue 

 was whether the rays which have penetrated lead are characterized 

 by the same marked distance effect which is observed for radium. The 

 fog chamber was inclosed in a close-fitting lead casket with two narrow 

 side- windows for observation. The end of the casket toward the bulb 



