330 
PROF. C. G. BARKLA ON X-RAYS AND THE THEORY OF RADIATION. 
(4) Support for the general conclusion given above can also be obtained from some 
results incidentally got by Bragg and Porter. # I have used these experimental 
results to determine the fractions as given in Table II., with the results given 
below:— 
Primary radiation. 
f k /e k . 
Fe. 
Ni. 
Cu. 
Zn. 
Zn X-rays. 
0-236 
0-322 
As X-rays. 
0-227 
0-336 
0-394 
0-432 
Sn X-rays. 
0-17 
0-29 
0-34 
0-39 
Ail these values are a little higher than Sadler’s and indicate a smaller change 
with the wave-length of the primary radiation, that is, a smaller slope in the curves 
of fig. 4 ;f but the important features to be observed in the results of Table II. are in 
evidence here. They are so consistent, especially in the case of the radiators of higher 
atomic weight, that little error in these seems possible. 
Briefly and approximately the results of experiments on the heavier elements are 
these :—When the wave-length (/a) of the primary radiation is just less than that 
(/u K ) of the fluorescent (characteristic) radiation, of the energy specially absorbed in 
association with the emission of the fluorescent radiation, nearly 0'5 is re-emitted as 
fluorescent radiation. As /u. becomes less, the fraction diminishes almost propor¬ 
tionately, until for a primary radiation of very small wave-length, the energy of the 
fluorescent radiation becomes a very small fraction of the energy of the primary 
radiation (E K ) specially absorbed. 
Energy of Corpuscular ( Electronic) Radiation. 
Though measurements of the energy of the corpuscular radiation emitted by 
substances exposed to X-rays have been made by a number of investigators, the 
changes taking place as the wave-length of the exciting radiation passes from one 
side of a spectral line to the other have not been studied. In addition the results 
obtained by different investigators have been by no means consistent. It seemed 
probable that much more reliable results could be obtained by experimenting on 
substances in the gaseous state, and estimating the energy of corpuscular radiation 
by the ionization it produced on complete absorption in the gas itself. The method 
* ‘Roy. Soc. Proc.,’ May, 1911. 
f These results from experiments with lighter elements agree more closely with theory than do those 
of Sadler. 
