306 DISCOVERY CH. 



digestive system were largely a matter of conjecture ; 

 and an operation was necessary to discover the exact 

 origin of the disorder. Rontgen rays enable almost the 

 whole of the internal anatomy to be scrutinised and 

 photographed, so that no operation need be undertaken 

 until its necessity has been demonstrated by ocular 

 evidence. 



The beneficial uses of X-rays are alone sufficient to 

 make the discovery of the highest significance in the 

 history of humanity ; but advance has not been limited 

 to them. The discovery directed renewed attention to 

 the subject of phosphorescence and fluorescence, and the 

 field thus opened up has proved to be the richest ever 

 explored. 



Certain substances, such as salts of uranium, phos- 

 phoresce, or continue to shine, after Rontgen rays have 

 been permitted to fall upon them. In 1896 Henri 

 Becquerel found that uranium salts which had been 

 made phosphorescent by exposure to sunlight gave out 

 rays having similar properties to those discovered by 

 Rontgen ; that is, they could pass through materials 

 opaque to ordinary light, would affect a photographic 

 plate, and so on. Further investigations showed that 

 these functions are inherent in uranium and its com- 

 pounds, which are thus independent of the exciting 

 influence of Rontgen rays or sunlight. Similar pro- 

 perties were shortly afterwards found by Schmidt to be 

 possessed by thorium ; and then Prof, and Madame 

 Curie, by infinite labour, succeeded in isolating from 

 pitchblende two new substances, radium and polonium, 

 which were far more active in their radiations than any 

 others. The discovery of actinium by Debierne quickly 

 followed, and, said Sir J. J. Thomson, in 1909 : " Now 

 the researches of Rutherford and others have led to the 



