Secondary 7 Radiation. 365 



the extra-radiation from lead is approximately constant in 

 amount. This supports the view that this secondary radia- 

 tion is a characteristic radiation. The tin radiator is 

 found to act in a slightly anomalous manner, indicating the 

 first appearance of an extra radiation, or a radiation charac- 

 teristic of tin. In certain arrangements there is a gradual 

 change in the quantity of secondary radiation emitted from 

 an element of low atomic weight, such as carbon, to an 

 element of high atomic weight, such as lead ; but this is 

 probably due to the variation in absorbing power of the 

 different materials. 



It has been shown by Barkla * that for elements of low 

 atomic weight the scattering is proportional to the atomic 

 weight, and that the number of scattering electrons per atom 

 is about half the atomic weight of the element. Bragg t has 

 also drawn attention to the fact that in the case of X rays 

 there are not the abnormal variations in the amount of scat- 

 tering when we pass from elements of lower to those of 

 higher atomic weights as there are in the case of the 

 absorption coefficients, and he concludes from his experi- 

 ments that the scattering power of an element is proportional 

 to the atomic weight. For an element of atomic weight 

 higher than 32, Barkla (loc. cit.) found that the amount of 

 scattering for equal masses increased considerably, and in 

 the case of silver it was about six times as great as for 

 an element of low atomic weight, and that the scattering- 

 was independent of the penetrating power of the X ray- 

 examined. This law docs not appear to hold if the range of 

 penetrating power is extended beyond the hard X rays to the 

 very penetrating 7 rays of radium (J. In the case of 7 rays 

 equal masses of all elements scatter to the same extent. 

 It has also been pointed out by Gray and the writer that 

 the scattering of 7 rays is less than that of X rays. The 

 experimental results of Barkla show that for the range of 

 penetrating power of X rays at his disposal there is no 

 diminution in scattering. This is in agreement with the 

 theory of Sir J. J. Thomson J and with the later theory 

 developed by Darwin § . These theories do not account for 

 the variation in scattering of 7 rays, nor for the well-known 

 property of scattered 7 rays, namely the decrease in pene- 

 trating power of the scattered radiation the greater the 

 angle through which scattering has taken place. 



* Barkla, Phil. Mag. May 1911 ; Phil. Mag. 1904, vol. vii. 

 t Bragg, Proc. Roy. Soc. lxxxix. p. 430. 

 X ' Conduction through Gases.' 

 § Darwin, Phil. Mag. Feb. 1914. 



