42 CONDENSATION OF VAPOR AS INDUCED BY NUCLEI AND IONS. 



When the radium is moved about 12 cm. (one-quarter of the length of 

 the fog chamber) from the glass end toward the metal end, the maximum 

 nucleation, moving at a greater rate toward the brass end, has already 

 outstripped the position of the radium and now lies near the middle of 

 the chamber. The coronas and the corresponding nucleations, therefore, 

 fall off rapidly toward both ends. In other words, the maximum nucle- 

 ation is seen where there is no radium. 



On moving the radium to the middle of the chamber, the position 

 of the maximum nucleation coincides with the brass end, over 20 cm. 

 beyond the radium. The coronas now fall off from left to right, to a 

 uniform size near the glass end of the chamber, the ratio of the extreme 

 nucleations being at least 200,000 to 100,000 per cubic centimeter in the 

 cases examined. Finally, when the radium is placed in the brass cap of 

 the chamber, the maximum still lies there and the nucleation falls off 

 toward the glass end, but all nucleations are reduced throughout about 

 one-half. 



It is clear that the two ends of the chamber behave differently, but 

 no simple hypothesis of the known properties of the rays, at least, will 

 account for the occurrence and location of regions of maximum nuclea- 

 tion, nor for the high nucleation ratios specified. Moreover, plates of 

 lead placed outside over the glass end of the chamber to modify the 

 secondary radiation are quite without effect. Covering the aluminum 

 tube with a thick lead pipe the phenomenon is slightly reduced in magni- 

 tude, but not in character. It follows that the gamma rays are chiefly 

 concerned. 



34. Behavior after removal of radium. A final element of interest 

 is the behavior of the axial aluminum tube after the radium (in small 

 sealed glass or aluminum tubes) has been removed. The internally- 

 sealed aluminum tube is distinctly radioactive for several hours, even 

 though gamma rays alone have passed through it. The activity vanishes 

 gradually, and more quickly if the ions are continually precipitated by 

 exhaustion. The behavior of this residual nucleation is very peculiar; 

 if the aluminum tube is pushed into the fog chamber, axially, from the 

 glass end as far as the middle, the part of the chamber around the tube 

 shows strong coronas on exhaustion, while the other half (toward the 

 brass cap) is blank. Something, consisting of very slow-moving particles, 

 gradually diffuses radially out of the aluminum tube. Of course it is 

 difficult to deny with assurance that merest traces of emanation decaying 

 within the aluminum tube may not possibly account for the activity; 

 but what is remarkable in any case is the existence side by side of a region 

 with nucleation and a region without it, in the absence of anything like 

 a partition. The fog chamber itself must at all times be scrupulously free 



