A.—MATHEMATICS AND PHYSICS. 35 
E 
“still to be explored. In approaching this unknown region from the 
violet end the most characteristic property of the radiations appears to 
be their intense absorption by practically every kind of matter. This 
result is not very surprising from the quantum standpoint. The 
“quantum of these radiations is in excess of that which corresponds to 
the ionising potential of every known molecule, but it is of the same 
order of magnitude. Furthermore, it is large enough to reach not 
only the most superficial, but also a number of the deeper-seated 
electrons of the atoms. There is evidence, both theoretical and experi- 
‘mental, that the photoelectric absorption of radiation is most intense 
when its quantum exceeds the minimum quantum necessary to eject 
the absorbing electron but does not exceed it too much. In the simplest 
theoretical case the absorption is zero for radiations whose frequencies 
lie below the minimum quantum, rises to a maximum for a frequency 
comparable with the minimum, and falls off to zero again at infinite 
frequency. This case has not been realised in practice, but, broadly 
judged, the experimental data are in harmony with it. On these general 
grounds we should expect intense absorption by all kinds of matter for 
the radiation between the ultra-violet and the X-ray region. 
: The closeness of the similarity in the properties of X-rays and light 
is, I think, even yet inadequately realised. It is not merely a similarity 
_along broad lines, but it extends to a remarkable degree of detail. It 
is perhaps most conspicuous in the domains of photoelectric action and 
of the inverse phenomenon of the excitation of radiation or spectral lines 
by electron impacts. Whilst there may still be room for doubt as to 
the precise interpretation of some of the experimental data, the impres- 
sion I have formed is that each important advance tends to unify rather 
than to disintegrate these two important groups of phenomena. 
OO 
