RONTGEN RAYS 



BY G. W. C. KAYE, B.A., B.Sc. 



Trinity College, Cambridge 



In 1895 Prof. W. C. Rontgen of Warburg discovered, it may be 

 said almost accidentally, the rays which now bear his name. 

 During some experiments on the passage of an electric discharge 

 through an exhausted tube, in which the gas pressure was so 

 low that the glass walls glowed with vivid phosphorescence, he 

 noticed that a screen of barium platino-cyanide in the neighbour- 

 hood of the tube shone out brightly. By the interposition of 

 obstacles he was at once able to attribute the sharp shadows 

 produced on the screen to the interception of some type of 

 11 ray " which proceeded in straight lines from the walls of the 

 tube. Further investigation revealed the fact that these X rays 

 —as Rontgen called them— invariably attend the stoppage of 

 cathode rays by solid bodies. 



But the distinctive feature of the Rontgen rays— as they are 

 now generally known — is (as Rontgen himself noticed) their 

 ability to penetrate in different degrees many substances quite 

 opaque to ordinary light. The extent to which suffering humanity 

 has benefited by the application of this property in therapeutics 

 and medical problems can scarcely be estimated. As a measure 

 of the penetrating effect, Rontgen in one of his Memoirs pro- 

 pounded his "density law" — that the absorbing powers of 

 different materials are roughly proportional to their thickness 

 and density. 



In addition to this property of exciting fluorescence in 



suitable materials, Rontgen ascertained that the X rays affect 



a photographic plate in the same manner as light. J. J. Thomson 



and others found in 1896 that they ionise or impart temporary 



electrical conductivity to a gas which, in the absence of the 



rays, is a nearly perfect insulator. While most of the earlier 



work on the Rontgen rays depended on their photographic 



or fluoroscopic actions, practically all recent investigators have 



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