Excitation of 7 Rays by /3 Rays. 



223 



accuracy required. This seems to disagree with Gray's con- 

 clusion, for he found that the higher the atomic weight of 

 radiator the larger the amount of secondary radiation *. 

 The disagreement between the results is probably due to 

 differences in the experimental arrangements : Gray mea- 

 sured the ionizing effect of rays which had passed through a 

 certain thickness of iron used to cut out the primary /3-rays ; 

 a large amount of soft 7 radiation was absorbed at the same 

 time and the effect of the lighter elements was therefore 

 largely decreased in favour of the heavier elements, which 

 give rise to harder rays. In addition Gray used a /3-ray 

 electroscope filled with air, whereas in the present investiga- 

 tion a very thin sheet of mica covered the face of the electro- 

 scope which contained the vapour of: methyl iodide. 



In order to examine the secondary radiation from silver 

 and tin, a different arrangement was found more satisfactory, 

 being simply an improvement of Gray's method as regards 

 sensibility. The soft primary 7 rays were cut out by a 

 plate of carbon 5 mm. thick ; this thickness was also suffi- 

 cient to stop all primary (3 particles, so that no magnetic 

 field was necessary ; but to get rid of the electrons excited 

 in the carbon and absorbers, it was necessary to cover the 

 face of the electroscope with some cardboard. The incident 

 secondary radiation was examined by placing the radiator 

 immediately behind the source. Subtracting the effect with- 

 out radiator from that with the radiator in position, when 

 different absorbing foils are placed in the path of the rays, 

 we obtain the values for the secondary radiation. The 

 results are shown in Table II., and are in fairly good agree- 

 ment with the values for the <w K" radiation from silver and 

 tin determined by Bark la. 



Table II. 



Atomic weight. 



y rays. 



X raj -. 



Silver 1079 



2-6(cn..r I AI 

 1-6 



2-5(c.n.)- 1 Al 

 L-57 



Tin 1190 



From the results of Barkla, another softer radiation 

 might be expected belonging to the " L " series, but this 



* Loc. cit. 



