348 
PROF. C. G. BARKLA ON X-RAYS AND THE THEORY OF RADIATION. 
Quantum Theory. 
In the above investigation of the energy transformations accompanying the 
phenomenon of X-ray fluorescence the results of greatest significance in their bearing 
on the quantum theory are probably the following :—■ 
(1) The energy absorbed per K electron expelled is greater than that per L electron 
expelled by the energy of one quantum of K radiation approximately. This leads to 
the statement that the energy of a quantum of K radiation is that which must be 
absorbed to displace a K electron from its normal position and state to the position 
and state of an L electron. 
(2) A quantum of K radiation is emitted for each K electron ejected. 
(3) The number of electrons in the secondary corpuscular radiation plus the 
number of quanta of fluorescent X-radiation, that is, the total number of electrons 
emitted by the direct action of the primary X-radiation together with those emitted 
indirectly through the agency of the associated fluorescent X-radiation is not equal to 
the number of quanta of primary radiation absorbed, but varies from equality with 
to double that number. Thus if, for the sake of clearness, we regard the first 
approximation as being accurately true, N quanta of primary radiation cause the 
emission of 
■J-N electrons (directly)+ ^N (indirectly through the fluorescent radiation) 
when y. is just less than /u. K> 
§N (directly) + §N (indirectly) when /x is |-/x K , 
N (directly) + N (indirectly) when u is very small, 
or, in general, 
N ,J ' K (directly)-l—N (indirectly) for any value of u* 
M + Mk M + Mk 
Thus, contrary to what has sometimes been supposed, the energy of a quantum does 
not persist through these energy transformations as an indivisible entity. 
<4) if we consider the energy absorbed by each atom from which a K electron is 
expelled, we see that when /u is just less than ,u K , approximately two quanta of 
primary radiation are absorbed per atom, 
when fx = \-/x K , l|- quanta of primary radiation are absorbed per atom, 
•>} U 4Mk> f4 )> >) )> i> 
or, in general, 
when ft =/uk, .(!+/) 
* If there are L, M, ... characteristic X-radiations associated with the emission of K electrons—indicated 
above as a possibility—the number of electrons emitted will be greater than is given by this expression; 
in this case it will depend on the number of series (K, L, M, ...) of characteristic radiations. 
