324 
PROF. C. G. BARKLA ON X-RAYS AND THE THEORY OF RADIATION. 
but higher value until another series of spectral lines in one of the substances is 
passed when a further change occurs. This is shown in fig. 3, in which the upper 
portion shows the ratio of ionizations plotted against wave-length of the X-rays used. 
In the figure the horizontal broken line indicates the relation that would have existed 
if there had been no radiation characteristic of R within the range of wave-lengths 
used, and no corresponding increase in the corpuscular radiation from the substance 
R. The rise of the curve above the broken line gives the increase in ionization 
associated with the emission of the fluorescent X-radiation of series K, say. As 
ionization is due to the emission of high speed electrons by the substance, the differ¬ 
ence between the two curves is due to additional electrons emitted in association 
with the fluorescent characteristic X-radiation of series K. We thus see and are 
able to measure accurately the effect of what will be called the K corpuscular radiation, 
to distinguish it from the L, M, N corpuscular radiations which are emitted in 
association with the similarly named fluorescent (characteristic) X-radiations. 
As it has recently been shown* that there is no appreciable difference between the 
velocities of these electrons of different groups K, L, &c., it follows that ionization 
experiments give us accurate data for determining the relative numbers of electrons 
in the various groups emitted by a substance when exposed to X-rays. This method 
has been used in some preliminary determinations which are described later. 
Energy Relations between the Corpuscular and the Fluorescent 
Radiations, and the Exciting Primary Radiations. 
In investigating the process of emission of the characteristic X-radiation when this 
is excited by another shorter wave-length radiation, a study of the relation between 
the energies of primary radiation absorbed and the associated corpuscular and fluores¬ 
cent radiations emitted, is of fundamental importance. It is, however, necessary first 
* Barkla and Shearer, ‘Phil. Mag.,’ December, 1915. 
