PROF. C. Ot. BARKLA ON X-RAYS AND THE THEORY OF RADIATION. 
335 
But as fi becomes smaller, an increasing fraction of the energy absorbed (K absorption) 
goes into the K corpuscular radiation. And the rate of change indicates that with 
a primary radiation of very small wave-length, practically the whole of the K 
absorption would be accounted for by the K corpuscular radiation alone. There is, 
in addition, very strong indirect evidence that when /a is very small, nearly all the 
energy absorbed is re-emitted as corpuscular radiation, for, as already pointed out, 
whatever the absorbing substance under these conditions, the energy of this 
corpuscular radiation is approximately a constant. 
Similar measurements have been made of the energy of corpuscular radiation 
emitted from iodine from observations of the ionization in methyl iodide. Owing to 
complications—the causes of which need not be discussed—difficulty was found in 
determining accurate values by extrapolation, and such as were obtained only indicated 
the energy of corpuscular radiation when the wave-length of the primary radiation 
was slightly less than that of the K fluorescent radiation. They show that 
when the K radiation from barium (fx Ka = 0'39 x 10~ 8 cm.) is incident on iodine 
(mk« = 0'44xl0 -8 cm.) nearly 0'5 of the K absorption is accounted for by the K 
corpuscular radiation from iodine. This is in as close agreement with the 
corresponding value for bromine given above as could be expected. 
scattering, showing there is little energy truly absorbed except as the accompaniment of the emission of 
corpuscular radiation. 
When absorption is accompanied by the emission of only very little fluorescent (characteristic) 
X-radiation and a negligible amount of scattered radiation, the number of high speed electrons emitted by 
the absorbing substance is approximately independent of the substance; so for a radiation of a particular 
frequency absorbed, the number of high speed electrons emitted is proportional to the energy absorbed. 
Even when a considerable amount of fluorescent radiation is emitted—about half of the total energy 
absorbed—the number of high speed electrons emitted by certain substances is still approximately propor¬ 
tional to the energy absorbed minus that re-emitted as fluorescent radiation. Thus, in many cases, 
energy of high speed electrons emitted by an absorbing substance = k (E - S - F) where k is a constant, 
E is energy of primary radiation absorbed, S the amount of energy scattered, and F the amount re-emitted 
as fluorescent radiation. 
This has been shown to be true (approximately) in C 2 H 5 B 1 ’ when S is small and F = JE. And if we 
assume that the energy of X-radiations differing in wave-length may be measured by their total ionizing 
power in a given gas, it is true in CgHsBr as the wave-length of the primary diminishes and the 
proportions of fluorescent and corpuscular radiations change. 
This is strong evidence both that the equation holds in ethyl bromide while the wave-length of the 
primary radiation changes considerably, and at the same time that the energy of X-radiation of various 
wave-lengths may be compared at least approximately by their ionizing powers. 
As the equation holds under such a variety of conditions, and through various transformations, we 
conclude that under these conditions the energy of the corpuscular radiation is not only proportional to, 
but equals that of the energy of X-rays disappearing, or that k = 1 . (Under other conditions, probably 
in lighter elements, there appears to be appreciably less corpuscular radiation than would be expected from 
this relation. The energy disappearing may not be re-emitted in radiation of any recognisable form.) 
As the energy of a corpuscle, too, has been found to be approximately that of a quantum of radiation 
measured by other methods, we conclude that in the cases given above the number of quanta of X-radiation 
absorbed, when not re-emitted as X-radiation, is measured by the number of high speed corpuscles emitted. 
