PROF. C. G. BARKLA ON X-RAYS AND THE THEORY OF RADIATION. 
359 
limit to the frequency or wave-length, for the absorption of penetrating radiations is 
principally due to the process called scattering, the laws governing which are entirely 
different from the laws of absorption connected with the emission of electrons. The 
process of scattering imposes the limit on penetrating power, and where scattering 
varies little with wave-length, gives the appearance of an approximate limit to that 
wave-length. Such a small variation of absorbability with a variation of wave-length 
(as measured by absorption in heavy substances) has been observed in the region for 
which results have been given. 
Again, an empirical formula which connects wave-length and penetrating power, 
when absorption is due to one process, can scarcely be expected to hold when 
absorption is due to an entirely different process and is governed by entirely different 
laws. 
In addition, since the discovery of the J series of radiations of higher frequency 
than those of the K series (see p. 349), it seems highly probable that radiations of 
higher frequency than that of the radiation supposed to have been observed can be 
generated, for this has been indicated as belonging to the K series. 
For these reasons we think that further evidence will be needed in support of 
such conclusions, though results of the same kind may perhaps be expected 
elsewhere.] 
GENERAL CONCLUSIONS UPON X-RADIATION AND THE QUANTUM THEORY. 
Thus all the available evidence shows that X-radiation may be and is emitted by 
electrons—probably in certain cases by groups of electrons, or even atoms—as a 
continuous process and in any quantity whatever. It is frequently emitted in 
quantities almost infinitesimal in comparison with a quantum. It is, however, 
emitted in quanta from atoms when certain critical conditions resulting in the 
ejection of certain electrons are reached, the process of radiation then taking place in 
a perfectly definite manner involving the radiation of a definite amount of energy 
which is proportional to the frequency of vibration. 
Absorption, too, normally takes place in very minute quantities—very small in 
comparison with a quantum. But in certain processes, which usually account for 
nearly the whole absorption, the radiation is absorbed in quantities greater than 
a quantum of the primary radiation, quantities varying with the conditions from one 
to two quanta approximately. There is no evidence of absorption of X-radiation 
in whole quanta, though the conditions are frequently such as to give an approxi¬ 
mation to this. 
-All this evidence seems to indicate that a quantum of radiation in the sense in 
which it has frequently been used, i.e., as an indivisible bundle of radiant energy, 
does not exist. The process of radiation may be, and is, continuous—at any rate, 
within limits extending to far smaller quantities than the quantum. The quantum 
is a unit of atomic energy which must be absorbed to change the configuration of 
