500 Professor Sir Ernest Rutherford [June 4, 



(3) Electrons or X-rays of appropriate energy are equally able 

 to excite the characteristic radiations in an atom. 



The results which have been shown to hold for the X-rays hold 

 equally for the yS and 7 rays, which have much greater individual 

 energies, e.g.. Gray and Richardson have shown that the /? rays from 

 radio-active matter are able to excite the characteristic radiations 

 of the elements in a number of substances, while 7 rays in passing 

 through matter give rise to high-speed electrons. It was long ago 

 suggested by Bragg that /? rays and X-rays are mutually convertible 

 forms of energy, e.g., a /3 particle falling on matter may be converted 

 into an X-ray of the same energy, and the latter in passing through 

 matter may in turn be converted into an electron of identical energy. 

 This assumes that the energy of an X-ray and an electron are 

 mutually convertible, and the energy may appear under suitable 

 conditions in either of the two forms. AVhile the general evidence 

 indicates that this point of view may hold closely for the conversion 

 of the energy of a single X-ray into that of a swift electron, it is 

 very doubtful whether it holds for the converse case of the excitation 

 of an X-ray by an electron. We shall see later from experimental 

 evidence that in general the energy of the electron required to 

 excite an X-ray of definite frequency is always greater than the 

 corresponding energy carried off in the form of an X-ray. 



It was early observed that there appeared to be a close connection 

 between the emission of /? and 7 rays from radio-active matter. In 

 all cases, the two types of radiation appeared together. A closer 

 examination, however, showed that there were very marked differences 

 between the relative energies of the /? and 7 rays from different 

 radio-active elements. For example, radium C emits intense y8 rays 

 and also intense 7 rays; on the other hand, radium E emits intense 

 P rays over a wide range of velocity, but exceedingly weak 7 rays. 

 Differences of a similar kind were observed amongst a number of 

 the radio-active elements. One striking distinction, however, was 

 to be noted. All the radio-active substances which give a marked 

 line spectrum of yS rays also emitted intense 7 rays. On the other 

 hand, a substance like radium E, which gave scarcely any 7 rays at 

 all, gave a continuous spectrum of /? rays in which no lines have so 

 far been observed. It thus appeared probable that the line spectrum 

 of the y8 rays was intimately connected with the emission of 7 rays, 

 and this conclusion has been completely established by recent 

 experiments. As w^e have seen, 7 rays in passing through matter 

 give rise to high-speed yS rays. Using radium B and radium C as 

 a source of 7 rays, the y8 radiation excited in a number of metals by 

 the passage of 7 rays was analysed in a magnetic field by Messrs. 

 Eobinson and Rawlinson and the writer, and was found to consist 

 in part of definite groups of yS rays. When lead was the absorbing 

 material, the magnetic spectrum of the /? rays excited by the 7 rays 

 was found to be nearly identical with the primary yS-ray spectrum 



