JoLY — Radio- Therapy : its Scientific Basis and its Teaching. 495 



are generated and fresh j3 rays or electrons are liberated. Eaeli of these 

 X rays carries its quantum of energy depending on its wave-lengtli or 

 frequency, aod is in kind identical with the -y ray emitted by radioactive 

 elements ; but the quantum is less tlian is generally associated witji the 

 y ray. It is now known, also, tliat the X rays from the anti-cathode are 

 evolved in spectra cliaracterislie of tlie nature of the element forming the 

 material of the anti-cathode. 



The increasing gradation of energy from the wave of light to the y ray 

 introduces certain differences into the phenomena they give rise to. Tliis 

 must be held in mind, for these differences may be important therapeutically. 

 I shall refer here to two important instances. 



(1) Photo-electric effects appear in certain cases to be limited by chemical 

 structure. Thus the sulphides of the metals yield free electrons under light 

 stimulus ; the corresponding sulphates do not, Tiiere is, however, no such 

 limitation when the electrons are evoked by X or y rays. Tlie limitation 

 imposed in this ease by cliemical structure almost certainly arises from the 

 small velocity with which the photo-electrons are emitted. There is no 

 reason to believe that liglit fails to evoke the electron, but tlie molecular 

 structure is such as to secure a large absorption. 



(2) Outside the region of the ultra-violet, but long before the higher 

 frequencies of X rays are reached, there is a spectral region which has been 

 named the Schumann region. This is concerned with wave-lengths of from 

 2000 to 1500 Angstrom units (10"' cms). The absorption of these waves in 

 air, water, etc., is very rapid — far more so than that of visible light. To 

 these waves we will again refer. The point in the present connexion is 

 that the greater frequency has not been attended with increased penetration. 

 This may be due to resonance with some harmonic of the atom. Selective 

 absorption by matter is a conspicuous piienomenon in the case of waves of 

 low frequency. Dealing with such waves, it must not be forgotten that media 

 may be sensitized for particular rays by adding photo-electric substances 

 which absorb those rays. Thus photo-electric effects on bacterial culture can 

 be greatly increased by the addition of eosine or similar dyes. 



The /3 and y radiations given out by radioactive bodies are those which 

 most concern us. I cannot stop to discuss the views which have been offered 

 as to the source of these radiations in the atom and the kind of properties 

 which must be ascribed to the atom in order to account for the origin and 

 interchange of quanta within it. Suffice it to say that Rutherford has 

 defined in general terms the necessary conditions.' Nor is it possible here 



1 Rutherford, I'hil. Mag., vol. xxviii, Sept., 19U. 



