456 Prof. E. Rutherford on the Origin of 



straight line he produced, it can he shown by extrapolation that 

 an element of atomic weight 214 should emit a characteristic 

 radiation whose value of yu,/D = '04 about for aluminium. 

 This is in good accord with the value /x/D = '0406 for 

 aluminium found by Soddy and Russell for the penetrating 

 7 rays from radium 0. It would thus appear probable that 

 the penetrating y radiation from radium C is to be regarded 

 as the characteristic radiation of that element excited by the 

 escape of (3 particles from the atomic system. 



Whiddington * has shown that the ft or cathode particle 

 incident on matter must have a definite minimum speed for 

 each element before the characteristic radiation of the latter 

 is excited. Over the range examined, this velocity is directly 

 equal to AxlO 8 cm. per second, where A is the atomic 

 weight of the element. If it be supposed that this law holds 

 generally for all the elements, the velocity of the ft particle 

 required to excite the characteristic radiation of radium 

 of atomic weight 214 should be 0*71 of the velocity of light. 

 It is not improbable, however, that the energy \mu 2 rather 

 than the velocity u is the determining factor for high-speed 

 ft particles. Taking this into consideration, the velocity 

 required to excite the characteristic radiation is given by 



w = 10 8 A^/m /m, where m is the mass of the ft particle 

 for slow speeds. On this hypothesis, ^ = *63 of the velocity 

 of light. The mean value for the two methods of calculation 

 gives u = -Q7. The corresponding energy of the ft particle 

 for the mean value of u is 1*5 x 10 13 e ergs, where e is the 

 charge carried by the ft particle. If it be supposed that 

 the whole energy of the ft particle is converted into y radia- 

 tion, the energy absorbed in exciting the characteristic y ray 

 should be 1*5 x 10 13 e ergs. 



If the complexity of the ft radiation is connected with the 

 emission of y rays, it is to be expected that some definite 

 relation should exist between the energies of the ft particles 

 in each of the groups emitted. From this point of view, it 

 is of interest to examine whether there is any evidence of 

 such a relation for the ft rays emitted by radium B and 0. 

 The results given by Danysz are included in the table on 

 the opposite page. 



The various groups differ widely in photographic intensity. 

 This is designated in column 1 by the symbols s = strong, 

 m — mean, £= feeble, vf = very feeble. Each group is defined 

 by a number given in the second column ; for the sake of 

 completeness, two groups of rays of low velocity A and B 



* WlMddinptoti, Proc. Rot. Soc. A. lxxxv. p. 3.23 (1911). 



