PENETRATION OF GAMMA RAYS. 



TABLE 54, continued. Summary. 



TABLE 54, continued. Tubes of lead 60 cm. long often capped. Wall, 0.5 cm. thick. 



* No induced radio-activity. 



69. Continued Effect Of distance. Radiations from radium are in 

 curious contrast with the corresponding results for the X-rays, inas- 

 much as the corresponding nucleating power falls off rapidly with the 

 distance of the sealed aluminum tube from the fog chamber. At 200 

 cm. the effect is but just appreciable above the nucleation of nonener- 

 gized dust-free air, as shown in table 55. 



Marked excess of nucleation is observed within loocrn., apparently 

 increasing as the distance from the fog chamber decreases ; but it is 

 difficult to make definite statements here, because all the effects are 

 small and successive exhaustions show marked periodicity. The 

 mean values are estimates. Though 5 minutes of exposure was at 

 first allowed, the effect is probably instantaneous and the succeeding 

 sections show that the effect is very penetrating. 



