PROF. C. G. BARKLA ON X-RAYS AND THE THEORY OF RADIATION. 
337 
Theory oe Fluorescent X-radiation. 
The conditions essential to the emission of a fluorescent X-radiation show that it 
originates in some exceptional disturbance of an atomic system. In the phenomena 
considered, the most violent disturbance of the atom of which we have any knowledge 
is that accompanying the ejection of a high speed electron. It is natural to regard 
this as the probable source, direct or indirect, of the characteristic X-radiation. 
Investigation completely confirms this view. 
Independence of Electron after Expulsion. 
There is considerable and apparently conclusive evidence that the electrons after 
expulsion take no part in the process of radiation*—that is, the process of emission 
of the characteristic radiation as ordinarily detected. They must, of course, on 
encountering other atoms, give rise to some radiation—this, however, is not the 
fluorescent radiation which is emitted in such intensity as that observed. That this 
is so follows from the following considerations :—- 
As the exciting primary radiation becomes more penetrating (of shorter wave¬ 
length), the speed with which electrons are emitted from a substance exposed 
to this primary radiation becomes greater, whereas the fluorescent radiation remains 
homogeneous and unchanged in character. Now all experiments on the production of 
X-rays show that as the speed of the generating cathode rays increases, although an 
intense homogeneous X-radiation may be emitted, there is emitted also a heterogeneous 
radiation of comparable intensity, which becomes more penetrating. No such hetero¬ 
geneity or change in penetrating power is observable in the fluorescent radiation. 
Again, the ionizing power of the fluorescent radiation is comparable with—in 
limiting cases it is equal to—that of the secondary corpuscular radiation. This 
corpuscular radiation cannot then produce the fluorescent X-radiation by any process 
subsequent to expulsion, for all experiments on the relation between the ionizing 
power of cathode rays and of the X-rays they produce show that only a small 
fraction of the ionizing power goes into the resulting X-radiation. 
Again, if the fluorescent X-radiation was produced by electrons subsequent to their 
expulsion from the parent atoms, there could be no distinction between the effectiveness 
of various groups of electrons provided they had the same velocity. Yet the intensity 
of the fluorescent X-radiation is most distinctly related not to the whole number 
but only to a particular group of electrons in the corpuscular radiation. 
Direct experiments made by Chapman! in order to determine whether the nature 
of the substance subsequently bombarded by the electrons had any influence upon the 
* Barkla, ‘ Proc. Camb. Phil. Soc.,’ May, 1909. 
t Chapman and Piper, ‘Phil. Mag.,’ June, 1910; Chapman, ‘Phil. Mag.,’April, 1911; ‘Phil. Mag.,’ 
March, 1913. 
