

Chapter I 



BIOPHYSICALLY ACTIVE X-RAYS 



It was at the University of Wurzburg, Bavaria, in November, 1895, 

 that Wilhelm C. Rontgen* discovered x-rays. While experimenting 

 and presumably repeating some of Lenard's contemporary experiments 

 on cathode rays in extreme vacuum he is supposed to have observed 

 that a sheet of paper covered with barium platinocyanide placed near 

 a Hittorf tube (according to Zehnder [1933] )f or near a Lenard tube 

 (according to Stark [1935]) became fluorescent although the tube had 

 been enclosed in an opaque cardboard cover. 



In accordance with Rontgen's will, all papers and letters bearing 

 dates 1895 to 1900 were destroyed so that no written record of the de- 

 tails of his first experiments is available. 



His first publication, "Ona New Kind of Ray," which appeared in 

 1895, contained a description of the three chief properties of the new 

 ray, i.e., its effect on a photographic plate, its ability to produce fluores- 

 cence in many substances, and its ability to make the air through which 

 it had passed electrically conductive. He discovered that substances 

 varied in opacity and that flesh was more transparent than bone. He 

 further recognized that x-rays which originated from a tube that pos- 

 sessed a comparatively low air pressure were more easily absorbed by 

 substances than x-rays orginating from a tube exhausted to a very high 

 vacuum. He called the latter " hard " and the former " soft " x-rays; 

 i.e., it was harder to pass a current through the tubes when they were 

 very highly evacuated. 



Gas Tubes 



The general method of producing roentgen rays is always the same, 

 namely: accelerating electrons to a high velocity in an electrostatic 

 field and then suddenly stopping them by collision with a solid body, 

 the so-called target. The electrons are liberated by the positive-ion 

 bombardment from the surface of a concave aluminum cathode immersed 



* His signature is written Rontgen; its modern version is Roentgen. 

 t Brackets enclosing dates [1933] refer to a citation in the bibliography at the end 

 of each chapter. 



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