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PROF. C. G. BARKLA ON X-RAYS AND THE THEORY OF RADIATION. 
FLUORESCENT (CHARACTERISTIC) X-RADIATION. 
Each element when traversed by X-rays emits X-radiations characteristic of the 
element ;* each characteristic radiation is unaffected by changes in the physical 
condition or state of chemical combination of the radiating element, and its quality 
is independent of that of the exciting primary radiation. But only primary radiations 
of shorter wave-length are able to excite the characteristic X-radiation (an extension 
of Stokes’s law).t 
All radiations hitherto definitely observed have fallen into two series—the K and L 
series.^ (Experiments made within the last year, and briefly described in this paper, 
have established the existence of a third, a higher frequency series, which will be 
called the J series).§ 
The absorption method of analysing a radiation showed radiation of the K series 
from a particular element to be so homogeneous that it was regarded as giving a 
spectral line—the K spectral line; but the possibility of the L radiation consisting of 
more than one line was suggested by an obvious heterogeneity in the L radiation. 
Interference experiments, || however, have shown that both the K and the L 
radiations give spectra consisting of a number of neighbouring lines. In the greater 
portion of what follows these radiations of neighbouring wave-lengih will, however, 
be classed together as K or L radiations simply. 
A characteristic radiation, unlike the scattered radiation, is uniformly distributed 
around a polarized primary beamll and also in a plane containing the direction of 
propagation of primary radiation. 
The intensity of a characteristic radiation** varies in a definite regular way with a 
variation in the wave-length of the exciting primary beam, rising rapidly and 
afterwards falling more gradually as the wave-length of the primary diminishes. 
The uniformity in the distribution of the characteristic radiation around the 
radiating substance shows that in contrast with the process of emission of scattered 
radiation, the emission of a characteristic radiation is absolutely uncontrolled by the 
primary radiation exciting it. The phenomenon of emission is not an immediate 
* Barkla, ‘Nature,’ March 9, 1905; ‘Phil. Trans. Roy. Soc.,’ January, 1905; ‘Roy. Soc. Proc.,’ 
January, 1906; ‘ Nature,’February 15, 1906; ‘Phil. Mag.,’ June, 1906;-‘Jahrb. der Radioaktivitat u. 
Elektronik,’April, 1908. Barkla and Sadler, ‘Phil. Mag.,’ September, 1907 ; ‘Nature,’February 13, 
1908. 
t Barkla, ‘ Jahr. der Rad. u. Elek.,’ April, 1908; ‘Proc. Phil. Soc. Camb.,’ May, 1909. 
| Barkla, ‘Proc. Phil. Soc. Camb.,’ May, 1909; ‘ Phil. Mag.,’ September, 1911 ; Barkla and Nicol, 
‘Nature,’ August 4, 1910; ‘Proc. Phys. Soc. Lond.,’ December, 1911. 
§ An M series has recently been observed by SlEGBAHN and others. 
|| Bragg, ‘Roy. Soc. Proc.,’ 1914; de Broglie, ‘ Comptes Rendus,’ 1914-16; Moseley, ‘Phil. Mag., 
1913-14; Siegbahn, ‘ Jahrbuch der Radioaktivitat u. Elektronik,’ 1916. 
11 Barkla, ‘Phil. Mag.,’ February, 1908; Barkla and Ayres, ‘Phil. Mag.,’ February, 1911. 
** Barkla and Sadler, ‘ Phil. Mag.,’ May, 1909. 
