Ionization of Gases and the Ahsorjption of Rbntgen Rays. 319 
for the absorbing elements between the points A and B in fig. 1. The 
second column of values of Kcl>/j\^4 gives the average of these quantities 
between the points B and C. Whilst Q/Qi shows how the constant falls 
as the point B is passed through. The last column locates approximately 
the element at the point B. The absorbing elements were Al (13), Fe (26), 
m (28), Cu (29), Zn (30), Vd (46), Ag (47), (50), (79), Au (80). 
Suppose at the point B we could imagine a critical element so constructed 
that for a very small diminution in the wave length of the incident radiation 
its K ring would begin to radiate, i. e. the constant of proportion between 
K(ti and suddenly changes from to Q, or suppose we had two elements, 
one whose atomic number were just above that of B, and the other just 
below that of B, then for a given wave length of incident radiation in 
passing from the former to the latter the K ring would radiate. It will be 
seen that in (ii) x = for these are analogous to Jc in Table I, the corpus- 
cular factor. Barkla and Shearer* have shown that Vk = Vi^ii X is constant. 
Also A = C, for we believe that the seat of absorption is within the atom. 
At the transition point B the increase in absorption is not due to a great 
increase in the number of atoms originally ionized. Then the increase in 
the absorption is 
- at B . . . . (iii.) 
and similarlv 
n^ + n^i -f 
+ n^i + 
at C . . . . (iv.) 
n^i 4- nn . . . 
The theory outlined above can be put to a direct test, for Moseley has 
actually obtained both the Ka and lines for some of the elements about the 
region of atomic number 40. 
The general formula he gives! for the frequency of the Ka line is 
and for the Lo line 
»i. = (p - y - ^•^)'"» 
in which Uq is the fundamental frequency of line spectra and =: 109720 x 
(velocity of light). 
We therefore obtain 
!^«=5-4(^-i .... (V.) 
^La - 7-4/ 
for an element whose atomic number is Then if the transition in the 
constant of proportion for Ag X-rays takes place at Palladium (46) 
^ =7-35. 
^ Phil. Mag. (6) xxx, p. 753. 
t Phil. Mag. (6), xxvii, p. 712. 
