THE SCOP]: OF KADIOLOCV. 359 



Stances, they cause those substances to emit certain radiations. 

 Such rays are called " secondary." These rays have properties, 

 which dei)end upon the nature of the substance from which the\- 

 originate, and these substances or radiators are classified into 

 g;roups according' to their atomic weights. If the radiajtor be a 

 chemical compound, the secondary radiations depend upon the 

 chemical constitution of the radiator — e.g., the secondary radia- 

 tions from salts consist oi two parts — (i) homogeneous radia- 

 tions. ha\ing the same penetrability as those from the metal; (2) 

 scattered primary radiations, considerably more penetrating than 

 the homogeneous radiations. The latter are due to the acid 

 radical. 



}\ the metal occur in the acid radical, it merely acts in con- 

 jtmction with the rest of the radical. 



The characteristic radiation of a substance is only emitted 

 when the primary radiation has sufficient energy to bring it into- 

 play. 



{c) l'I[^■SIOLo(;ICAL Effect. 



The action of yY-rays causes nuclear changes in cells. These 

 changes are generally of a degenerati\e character. Experiments 

 on ova show that under certain conditions cell division is re- 

 tarded, and ifrequently the fcetus is a monster. 



When pregnant female rats are irradiated the young are 

 less virile than the controls, and they rarely live for one month. 

 In a])pearance. too. they are smaller than the normal young. In 

 three cases under our observation the females were sterile for a 

 period of seven months. During this period they behaved like 

 normal rats. One (female (white rat), which was treated fre- 

 quently from the first signs of pregnancy, gave birth to eleven 

 abnormally small 'black rats. They all died within five weeks 

 after birth. A similar pigmentation has been recorded in the 

 case of butterflies. 



Thus far rodent ulcers, sarcomata, carcinomata, lichen and 

 leukaemia (in which the white blood corpuscles are killed oft' by 

 irradiation) have been sticcessfully treated with X-rays. 



In this connection we may point otit that .Y-ray therapeutics 

 can only be successfully practised when the characteristic radia- 

 tions for certain treatments are definitely specified, and not 

 merely the dosa.f;e re(|tnred; hence it is essential that the medical 

 radiologist should have a thorough knowledge of the physics of 

 radiology. 



(/) Optical Properties. 



We have already seen that the intensity of the radiation 

 follows an inverse square law, the same as in ordinary optics. 

 The eft'ect of these radiations on a selenium cell (variation of 

 resistance) also follows the optical law.. Moreover, in IQ12, von 

 Laue showed that these radiations cause interference ])atterns 

 wdien they arc transmitted through crystals, consequently the 

 A" — the unknown quantity — has been eliminated, so that we may 



