PROF. C. G. BARKLA ON X-RAYS AND THE THEORY OF RADIATION. 
31.9 
f is simply — where X is the electric intensity in the primary wave producing the 
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disturbance. (See Sir J. J. Thomson’s ‘ Conduction through Gases.’) 
Thus the theory assumes that radiation can take place from these electrons in any 
quantity whatever, and is not confined to units or quanta; that radiation is a 
continuous process not depending on any limiting or critical condition. 
5. Again, in perfect agreement with this theory, I have found that the intensity of 
the radiation scattered from light elements varies little with the wave-length of the 
primary radiation. This indeed is also indicated by the above equation, the derivation 
of which was based on the assumption of independent action of the electrons. 
6. And more recently* it has been found that the intensity from the heavier atoms 
in which the constituent electrons are more closely packed, increases rapidly and 
continuously with the wave-length of the radiation unless this is very small. Such 
a result is to be expected on the wave theory when the wave-length becomes 
comparable with the size of the atom; ultimately a group of electrons and not an 
individual electron moves as a whole and becomes the scattering unit.t Further 
experimental results support this conclusion, and indicate that the radiation is 
emitted not in quanta but in any quantity, and that the process is a continuous 
process. 
There is thus in the phenomena of scattering not only no suggestion of a quantum 
or entity in radiation, or of any discontinuity in the process of radiation involved, 
but there is some of the strongest positive evidence against any such theory. The 
tests which have been applied are the most searching and sensitive ; the results 
appear conclusive. The phenomena observed become meaningless on any quantum or 
entity theory. 
Absorption. 
As in the transmission of X-rays (particularly of short wave-length) through 
matter consisting of light elements only, the energy absorbed is practically all 
re-emitted as scattered X-radiation, the quantities radiated by each electron are 
identical with those absorbed. When there is little other absorption, viz., that 
associated with the emission of corpuscular and fluorescent X-radiation, the total 
absorption may be approximately calculated from the number of electrons in the 
matter traversed. It follows that this process of absorption is also a process which 
takes place in any quantity whatever, and is unlimited by any critical condition. 
* Barkla and J. Dunlop, ‘ Phil. Mag.,’ March, 1916. 
t When the group comprises all the electrons in the atom, the scattering per atom becomes on this 
theory proportional to the (atomic number) 2 , instead of to the atomic number, when the electrons scatter 
independently. 
