594 Mr. J. Chadwick on the 7 Rays 



character of the X rays. Whiddington * has shown that 

 the maximum energy of the emitted electrons is equal to 

 that of the slowest cathode rays which will excite the charac- 

 teristic Rontgen rays. He has measured this quantity in 

 equivalent volts in a number of cases and has shown that, 

 in general, for Barkla's K series of X rays the energy is 

 proportional to the square of the atomic weight of the metal 

 of which they are characteristic. These results are consistent 

 with the formula T m = hv — w for the energy of the emitted 

 electrons provided v , the frequency of the characteristic 

 Rontgen rays, is given by 



v = 6-55xM 2 xl0 14 , (22) 



where M is the atomic weight of the metal of which they 

 are characteristic. This holds only for the rays which con- 

 stitute Barkla's K series. The values for the L series should 



be given by substituting (^ — 25) for M in (22), to accord 



with Whiddington's conclusions. Thus the frequency of the 

 Rontgen rays characteristic of copper would be, since they 

 belong to the K series, 



j/ = 2-64xl0 18 sec." 1 . 



In the case of all these rays w is negligible in comparison 

 with hv . This accounts for the energy being independent 

 of the metal of origin, as found by Sadler. 



Palmer Physical Laboratory, 

 Princeton, N. J. 



LIV. The 7 Rays excited by the ft Rays of Radium. 

 By J. Chadwick, M.Sc.'t 



IT has been shown by Gray J that when the ft rays of 

 radium E impinge on any material they excite 7 rays. 

 The amount of the excited 7 rays is large compared with the 

 amount of primary 7 rays. He has found that the excited 

 7 rays increase in amount and in penetrating power with the 

 atomic weight of the material on which the ft rays impinge. 



* Roy. Soc. Proc. A. vol. lxxxv. p. 323 (1911) ; vol. lxxxvi. pp. 360, 

 370(1912). 



t Communicated by Prof. E. Rutherford, F.R.S. 



j J. A. Gray, Proc. Roy. Soc. A. vol. lxxxv. p. 131. and vol. lxxxvi. 

 p. 613. 



