356 
PROF. C. G. BARKLA ON X-RAYS AND THE THEORY OF RADIATION. 
corresponding features all occurred with radiation of shorter wave-length than in 
paper and paraffin wax which consisted of lighter elements. 
In these experiments comparatively heterogeneous primary beams were used, so 
that the particular absorbability or wave-length at which the characteristic radia¬ 
tions appeared could not be determined with any accuracy. The absorbability in 
aluminium, however, was of the order of magnitude of that obtained more exactly by 
other methods. 
Absorption Method. 
It has been shown that over the range of wave-lengths between those of the 
spectral lines of two consecutive series and not near to the longer wave-length series, 
the absorptions in any pair of substances are in an approximately constant ratio for 
any wave-length. Any marked deviation from proportionality in the absorptions in 
two substances as the wave-length of the radiation changes, shows the existence 
of a characteristic radiation. Thus as the wave-length of a radiation diminishes, the 
n , • absorption in substance A • , ,, j. 
traction —-;-:- x - =■ remains approximately constant until the radiation 
absorption m substance B 
becomes of shorter wave-length than that of a radiation characteristic of A or B. 
In this case the corresponding absorption increases rapidly and the ratio of absorptions 
is changed. The absorptions of homogeneous radiations were therefore determined 
in iron and aluminium, so as to provide a further test of the existence of a radiation 
characteristic of aluminium. 
t-, • f. i , i , ,i ,• absorption in Fe j i ■ , 
Passing from long waves to shorter, the ratio —:- t—- r suddenly rises at 
& & absorption m A1 
wave-length approximately l'9xl0 -8 cm., which agrees with the determined wave¬ 
length of the K radiation of iron (fig. 9). There is, further, a sudden drop in the ratio, 
Fig. 9. Showing approximate wave-lengths of the K radiation from Fe and the J radiation from A1 
from relative absorption curve. 
