310 Sir E. Rutherford on the 



through one or more distinct regions which give rise to the- 

 corresponding characteristic radiations, while the ft particle 

 from radium C escapes in such a direction that it does not 

 pass through the corresponding regions but does pass through 

 a new region not involved in the case of the expulsion of the 

 ft particle from radium B. 



The general evidence available indicates that radium B and 

 radium D have identical general physical and chemical pro- 

 perties with those of lead, although differing from the latter 

 in atomic weight. If this be correct, we should anticipate 

 that these elements should give identical X ray spectra when 

 bombarded by cathode rays. On the other hand, from the- 

 work of Rutherford and Richardson, radium B and radium D 

 are known to emit types of 7 rays which are widely different 

 in relative amount and penetrating power. Such results are, 

 however, at once intelligible if it be supposed that the 

 ft particles from these two elements are expelled in different 

 directions with regard to the atomic structure. 



It is to be anticipated that some of the lines of the 7-ray 

 spectra of these two radioactive elements should be coincident, 

 but some may be absent or very faint in one spectrum and 

 strong in another. In fact, the relative intensities of the 

 spectral lines of the 7 rays from radioactive substances may in 

 all cases be very different from those which would be observed 

 when the element is bombarded by cathode rays. In the- 

 latter case, all types of characteristic radiation have a chance 

 of excitation — supposing of course account is taken of the 

 speed of the incident cathode rays — since the ft particles 

 enter the atom on an average equally in all directions. 



Theory of the origin of the ft and 7 rays. 



One fundamental fact that has to be taken into account in 

 considering the origin of the ft and 7 rays is the conversion 

 of the energy of a 7 ray into the form of a high speed electron 

 or ft ray, and vice versa. This point has been emphasized by 

 Bragg in his papers on the nature of X rays. He supposed 

 that the energy of a single X ray could be converted by its 

 passage through matter into the energy of a single ft ray of 

 appropriate speed, and that no loss of energy occurred in the 

 process. It would appear, however, necessary to generalize 

 this conception, for it will be seen later that there is consider- 

 able evidence from a study of the ft and 7 rays from radio- 

 active matter that a train of X rays of the same frequency 

 may be given out each of definite energy, and that the whole 

 energy of this train of waves may, under suitable conditions, 



