PROF. C. G. BARKLA ON X-RAYS AND THE THEORY OF RADIATION. 
317 
Difficult as the first two results would be to explain on any entity or quantum 
theory—that is, any theory assuming radiation itself to exist in definite indivisible 
bundles or quanta—perhaps the strongest evidence against this is provided by the 
third fact stated above. The distribution calculated and given by the expression 
D = D /2 (1 + cos 2 0) where I 9 is the intensity of radiation scattered in a direction 
making an angle 9 with that of propagation of the primary beam depends essentially 
on a steady variation with direction, not of a number of indivisible entities, but of the 
energy density around a single radiating charge—-an electron in this case. 
4. Further, measurements of the energy of radiation scattered* have furnished one 
of the most searching and critical of tests that could be applied to any theory. 
In the early experiments it was found that the intensity of the radiation scattered 
from light elements was simply proportional to the mass of scattering substance 
traversed.! That is, the intensity of scattering per atom or molecule of scattering 
substance was proportional to its atomic or molecular weight—independent of 
density, chemical combination, or state of ionization of the substance. It thus became 
evident that neither atoms, molecules, nor gaseous ions were the scattering units, but 
that the scattering particles were the constituent electrons, and that in general, for 
light atoms, the number per atom was proportional to the atomic weight. 
Using the result of a calculation given by Sir J. J. Thomson, for the fraction of a 
beam lost by scattering per centimetre of its path, 
/ = 
8 
— N — u 2 
3 m 2 ^ ’ 
where N is the number per cubic centimetre of particles of charge e and mass m, it 
was calculated that the number of electrons per atom was several times the atomic 
weight. With the much more accurate data available for N, —, e, a few years ago 
I showed that the intensity is such as on the old spreading wave theory would be given 
by 1 electron per atom of H, 6 per atom of C, 7 per atom of N, 8 for 0, 15 or 16 for 
S, &c. 
These results concerning the constitution of the atom have been confirmed by 
independent evidence and are becoming generally accepted. Thus Rutherford,J 
from experiments on the scattering of a particles, arrived approximately at the 
complementary conclusion—that the central nucleus charge of an atom is ~ where w 
* Barkla, ‘Phil. Mag.,’ May, 1904; ‘Jahrb. d. Radioaktivitat u. Elektronik,’ 1908 ; ‘Phil. Mag.,’ 
May, 1911. 
t Hydrogen scattered about twice as much as the other elements, mass for mass. This at the time was 
thought to be due to impurity. Crowther, however, later obtained a value only slightly lower 
than this. 
1 ‘Phil. Mag.,’ May, 1911. 
2 Y 2 
