338 
PEOF. C. G. BARKLA ON X-RAYS AND THE THEORY OF RADIATION. 
intensity or character of the fluorescent radiation emitted, show the same thing. 
When the greater portion of the energy of the expelled electrons, even in excess of 
that essential to the production of the fluorescent radiation, was absorbed in a second 
substance, the intensity of the fluorescent X-radiation was the same as when the 
electrons were completely stopped by the substance originally emitting them. 
There appears to be no escape from the conclusion that the emission of fluorescent 
X-radiation is completely independent of the career of electrons subsequent to their 
expulsion from the atoms. 
As ionization is produced by expelled electrons constituting the corpuscular 
radiation, it follows that the simple process of ionization is not accompanied by X-ray 
fluorescence. 
Independence of Electron During or Preceding Expulsion. 
It does not appear possible that the expelled electron is itself the source of this 
radiation during the final simple process of ejection, that is, during the final unidirec¬ 
tional flight which carries it beyond the bounds of the atom, for the homogeneous 
fluorescent X-radiation is both independent of the velocity of expulsion of these 
electrons, and thus of the period of actual escape, and according to our conception of 
homogeneous radiation implies a regular vibration. 
Again, the vibration of the electron (which is finally expelled from an atom) or of 
other components of a normal atom when under the influence of a primary Rontgen 
radiation must of course depend in character upon the primary radiation and be 
ultimately of the same frequency. This forced vibration would produce a radiation 
with properties dependent upon and possibly ultimately identical with those of the 
primary radiation. Such a radiation, due to the forced vibration of electrons within 
the atom, is actually emitted ; it however constitutes the scattered X-radiation. 
On the other hand, the fluorescent X-radiation is characteristic not of the exciting 
primary radiation, hut of the excited substance; it cannot be produced by such a 
controlled motion but only by a free vibration. The emission of a fluorescent radia¬ 
tion, too, is not general but conditional, that is to say, some special atomic condition 
appears to be essential to the production of the fluorescent radiation. We know that 
vibrations are produced within the atom both by Rontgen radiations of longer wave¬ 
length, and by the impact of electrons moving more slowly than the velocity critical 
to the production of a particular characteristic X-radiation ; yet the characteristic 
radiation has never been observed in these cases. 
We thus arrive at the conclusion that the fluorescent X-radiation is due to a free 
vibration occurring within the atom when in an abnormal state—probably after the 
ejection of a high speed electron.* 
* See Barkla, “ Der Stand der Forschung iiber die sekundare Rontgenstralilung ” (‘ Jakrb. d. Radio- 
aktivitat und Elektronik,’ April, 1908). 
