GAMMA RAYS. 



145 



the bulb is to eliminate nearly all the rays (figs. 77, 78, Chapter III) ; 

 but if this is open, the number of nuclei instantly generated decreases 

 much more slowly than the first power of distance. The fact that to 

 the very pervious wooden fog chamber the medium within a sphere of 

 at least 6 meters in radius remains almost equally energized through- 

 out remains a result of importance. 



103. Possibility of two kinds of radiation from the X-ray tube. It has 



been shown that for very short exposures (sections 101 and 102) the 

 nucleation is the same, whether the bulb is placed at 6 cm. or 6 m. 

 from the fog chamber. But only in the former case (D=6 cm.) is the 

 effect cumulative ; only for very short distances will persistent or very 

 large nuclei appear if the exposure is prolonged several minutes. I 

 have, therefore, suspected that the radiation from the X-ray bulb is 

 twofold in character ; that the instantaneous effect (fleetingnuclei) is due 

 to a gamma-like ray, quick moving enough to penetrate several milli- 

 meters of iron plate appreciably even for D=6 meters ; furthermore, 

 that the cumulative effect (persistent nuclei) is due to X-light, properly 

 so called, which produces the usual effects subject to the laws of inverse 

 squares ; but it is noteworthy that while the penetration of X-rays 

 is relatively small, and the distance effect negligible (section 101), they 

 are both large for the radiation from radium (section 104). 



104. Nucleation due to gamma rays. To what extent nucleation is 

 producible by gamma rays may be tested by radium inclosed in a 

 thick chamber of lead. The results are strikingly comfirmatory. 

 (Figs. 87 to 89, Chapter III). For instance, in case of 10 mg. of 

 radium (io,oooX ) inclosed in a hermetically sealed aluminium tube and 

 placed outside but close to the end of the fog chamber (bottom nearly 

 i cm. thick, walls 0.3 cm. thick), the data were (fig. 89, Chapter III) : 



Radium in sealed aluminum tube ...... io~ 3 ^V=27 Transmission 100 



Radium in lead tube 0.5 cm. thick ..... 23 85 



Radium in lead tube i cm. thick ....... 18 69 



The nuclei are thus very largely due to this extremely penetrating 

 radiation. By using lead tubes, capped and not capped, 30 cm. and 

 60 cm. long, and placed parallel to the fog chamber and in contact 

 with its sides, no evidence of secondary radiation was discernible, the 

 effective radiation passing through the lead walls as specified. 



In comparison with the abundant nucleation after the penetration 

 specified, the decrease of nucleation observed when the tube is at dif- 

 ferent distances, D, from the fog chamber is remarkably large. For 

 example (fig. 90, Chapter III), 



D o 10 30 50 100 200 cm. 

 30 13 8 5 3 2 



Through 0.14 cm. of lead io~ 3 N= 3 ; through 0.05 cm. of iron io~ 3 JV== 44. 



