724 Professor C. G. Barkia [Maj 26, 



tion, lunsfc be re-emitted. The emission of a quantum of K radiation^ 

 for each electron expelled we have found to be an experimental fact. 

 V)j analogy the energy of a quantum of L radiation will presumably be 

 emitted when an electron falls from the position and state of a lower 

 frequency electron to the position and state of the displaced L 

 electron. 



It is possible that there are not electrons of every series in every 

 atom of the same substance, or, if there are, they may not be in the- 

 best position to enable them to replace an electron of an adjacent 

 higher series. If, for instance, a K electron were ejected from an 

 atom which did not contain an L electron, or in which the L electron 

 was situated on the other side of the centre, its place might be taken 

 by an M electron, with the result that the energy emitted as character- 

 istic radiation would be equal to that of one quantum of K radiation 

 + one quantum of L radiation. This would be equal to one quantum 

 of radiation of slightly greater frequency than what we have pre- 

 viously regarded as the principal component of the K radiation, and 

 might account for a neighbouring higher frequency spectral line in 

 the K radiation, as was suggested by Kossel. 



Whatever the exact process of emission of the fluorescent character- 

 istic radiations, the experiments described above lead to the conclusion 

 that a characteristic radiation (of series K, say) is probably emitted 

 when an outer electron falls into the position of a displaced K electron, 

 the energy previously absorbed in displacing the K electron in excess- 

 of that carried away as kinetic energy being now re-emitted as a 

 quantum of K radiation, or, what seems more probable, as a quantum 

 of K radiation accompanied by quanta of lower frequency radiations, 

 due to successive falls of electrons between the outer rings. 



In conchision, it seems absolutely certain that electric radiation 

 can be and is both emitted and absorbed in quantities which are not 

 multiples of any particular unit ; that radiation even by individual 

 electrons is in many cases a continuous phenomenon. But that in 

 certain processes within the atom — probably when an electron falls 

 from one ring to another — the phenomenon takes place in a very 

 definite manner, involving the radiation of a definite amount of 

 energy which is proportional to the frequency of vibration. 



Though there are many problems of absorption and radiation 

 still awaiting solution, as indeed there always must be, the difficulties 

 seem to be definitely shifting from the radiation itself to the structure 

 of the atom and to processes taking place within the atom. The evi- 

 dence obtained from experiments on X-rays indicates that there is no 

 such thing as a quantum of radiation in the sense in which it has 

 been frequently used. There is no entity apart from the atom. The 

 structure of the atom only permits of the true absorption of a quan- 

 tum of energy, and when the original configuration is reached it 

 re- emits that quantum. 



It may be briefly added that the evidence suggests that mere 



