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PROF. C. G. BARKLA ON X-RAYS AND THE THEORY OF RADIATION. 
absorption does not differ greatly from that for the L electrons, we may class them all 
as L electrons. As all electrons leave the atom with the same velocity, and as the 
emission of the fluorescent radiation is subsequent to the absorption of primary 
radiation, we may express the fact in the following way :—The energy of a quantum 
of K radiation is the energy required to move an electron from the position and 
state of a K electron to the position and state of a lower frequency electron, 
an L electron. (We are led to regard a K electron as being situated nearer to the 
centre of the atom than the lower frequency electrons, and, in general, the frequency 
of vibration of electrons to be in the order of their distance from the surface. Thus 
the J, K, L, M electrons are arranged in this order from the centre outwards.) 
But when a K electron is hurled out of the atom, it is possible that, for 
stability, another electron will fall into the position and state of the K electron. 
If it falls from the position and state of an L electron, the energy of a quantum 
of K radiation must be re-emitted. The emission of a quantum of K radiation for 
each electron expelled we have found to be an experimental fact. 
Reasoning by analogy, we conclude that to displace an L electron necessitates the 
absorption of the energy of one quantum of L radiation more than that required to 
eject a still lower frequency electron. This energy is re-emitted as a quantum of 
L radiation when an electron falls from the position and state of one of these lower 
frequency electrons to the position and state of the displaced L electron. 
Thus, on this hypothesis, the energy of primary radiation absorbed per K electron 
emitted is h(n + n K + 7i L + n yi +...), he., the sum of the kinetic energy of the ejected 
electron, which is hn, and the energy of one quantum of each of the characteristic 
X-radiations of series K, L, M,_ Similarly, the energy absorbed per L electron 
emitted is h (n + n L + n M ...), and so on. 
If, after a K electron is ejected, its place is taken by an L electron, and the place 
of that by an M electron, and so on, then for each K electron ejected there is 
emitted one quantum of each of the K, L, M, ... X-radiations. 
T ,i • ,, r» , • energy of K characteristic X-radiation • F K • i , 
In this case the traction -^----— : -, i.e. is equal tc 
energy ot K corpuscular radiation 
— (or —) as before. But what has previously been regarded as purely K corpuscular 
n \ fj. K J 
radiation is K corpuscular radiation together with a weak corpuscular radiation 
produced by the re-absorption of the L, M, ... characteristic X-radiations emitted in 
association with the K corpuscles. That is, if there be additional L characteristic 
radiation associated with the emission of K corpuscles, the ionization produced by its 
absorption must be added to that produced more directly by the Iv corpuscular 
radiation. Thus more accurately our comparison of the energies of fluorescent and 
corpuscular radiations has given 
_ Iv characteristic X-radiation (energy)_ 
K corpuscular radiation+ L characteristic radiation accompanying K electrons (energy) ’ 
