JuLT 3, 1908] 



SCIENCE 



27 



(one quarter of the len^h 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 out- 

 stripped 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 nucleation is 

 seen where there is no radium. 



On moving the radium to the middle of the 

 chamber, the position of the maximum nuclea- 

 tion 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 ex- 

 treme nucleations being at least 200,000 to 

 100,000 per cubic centimeter in the cases ex- 

 amined. 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. There must, therefore, be 

 some sort of conflict (to use a word that has 

 not been preempted) between the primary and 

 secondary radiations which issue from the 

 ends and other parts of the chamber. The 

 solid walls both contribute nucleation; the 

 glass wall most when close (a few cm.) to the 

 radium, the metal wall at a greater distance 

 (20 cm.) from the radium; but no simple 

 hypothesis of the known properties of the 

 rays will account for the occurrence and 

 location of regions of maximum nucleation, 

 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 magnitude, but not in character. It fol- 

 lows that the gamma rays are chiefly con- 

 cerned. 



In a region of maximum nucleation there 

 must either be a larger rate of production or 

 a smaller coefficient of decay. The latter 

 may be expected if in the region of maximum 



nucleation the ions have largely the same sign. 

 The best conception of the phenomenon which 

 I can form at present is thus an explanation 

 in terms of Bragg's^ theory of neutral pairs 

 for the gamma rays. As such, a region of 

 primary rays may be regarded as devoid of 

 nucleation. On impact, however, these paired 

 rays separate into secondary cathode rays and 

 alpha rays, returning with unequal swiftness 

 from both ends of the fog chamber. In order 

 to exist as separate condensation nuclei, they 

 must, therefore, travel over a certain distance 

 to be recognized as distinct particles by the 

 fog chamber, the distance depending on the 

 intensity of the impact of the gamma rays; 

 depending, therefore, on the buffer, on the 

 strength and distance of the radium from the 

 buffer. In the above experiment this function 

 is performed by the ends of the fog chamber. 

 In case of very weak radium, a minimum of 

 nucleation in the middle of the chamber, and 

 coinciding in position with the radium, was 

 actually obtained, in contrast with the central 

 maxima for the strong radiations, as described 

 above. Possibly the frequent occurrence of 

 the ratio of 2 to 1 between the maximum and 

 minimum nucleations may be similarly in- 

 terpreted ; but much further work is necessary 

 before any definite conclusions can be reached. 

 I am now constructing a chamber about 1 

 meter long, with the object of ascertaining 

 whether more than one maximum of nuclea- 

 tion is producible ; in other words, to interpret 

 the stationary wave resemblances of the 

 phenomenon. 



3. A final element of interest is the be- 

 havior 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 ex- 

 haustion. 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 



' See Phil. Mag., May, 1908. 



