47 



The third column shows the values of A. calculated from the 

 results in the second column. The fourth column shows the 

 values of A for the primary ^ rays of radium, as given by 

 McClelland and Hackett. (Trans. Roy. Soc. Dub., March 22, 

 1907, p. 49.) A comparison of the two last columns shows 

 clearly that the secondary ^ rays excited by the y rays possess 

 nearly the same velocity, no matter in what substance they 

 are excited, and that this velocity is the same as that of the 

 primary )8 rays. Only hard y rays have been used to excite 

 the 13 rays in these experiments, while the values of McClel- 

 land and Hackett refer to 13 rays of ordinary heterogeneity. 

 It is scarcely possible in the present state of knowledge to 

 estimate what allowance should be made for this difference, 

 but it is clear that the main conclusion cannot be affected by 

 it. The value of A varies rapidly with the speed of the ^8 par- 

 ticle, and there is room for considerable alteration of the 

 values in the table without any important alteration in the 

 associated speed. Nor does much error arise from neglect of 

 the correction discussed above, which makes the emergence 

 radiations too small. The various curves of figs. 4 to 8 should 

 really run a little way past the point taken as origin of co- 

 ordinates. 



If we consider the corresponding values for the soft rays, 

 or rather, soft together with hard rays, we find, as expected, 

 that the $ rays which they produce are of a much less pene- 

 trating character. Also, the difference between the curves 

 .4 and B is most marked in the case of lead and tin, a re- 

 sult which is in agreement with what has gone before. For 

 these metals the thicknesses of screen required to give half 

 the full emergence radiation are "030 mm. and "048 mm. re- 

 spectively, which may be compared with the values '083 and 

 •141 obtained when hard rays were used. The amount of soft 

 radiation is so large that we may consider the effect of the 

 hard radiation mixed with it as a correction to be allowed 

 for. The effect of the hard radiation which has been passed 

 through the plug, 1*61 cm. of lead, is, of course, shown by 

 the curve .4. It is a little difficult to say precisely what it 

 would have been if the rays had not gone through the plug, 

 since the absorption co-efficient for y rays varies so much with 

 the hardness. From '8 cm. Pb to 1'25 cm. Pb, the value of A. 

 is given by McClelland as '44 cm.— ^ ; from 2*8 cm. Pb onwards 

 Wigger gives '241 cm.— ^. We have taken a value between 

 these, towards the harder side, and drawn the curve .4 ' for 

 each metal, derived from .4 by increasing the ordinates of A 

 by two-thirds, a factor calculated from the absorption co- 

 efficient chosen. Subtracting A' from B, we obtain^', a curve 

 which may be taken as representing approximately the beha- 



