106 THE PATH OF SCIENCE 



wheel of light metal built inside the tube, which led to his 

 conclusion that the cathode rays consisted of small particles. 

 Like Hittorf, he found that the rays could be deviated by a 

 magnet, and as a consequence they must consist of charged 

 particles. When they fell on an extra electrode inserted in 

 the tube, they charged it negatively. Moreover, their velocity 

 could be determined and was found to be very high, in some 

 cases approaching the velocity of light. Crookes suggested 

 that the cathode rays consisted of a new form of matter. 



In 1892 H. Hertz, who had discovered the long wave elec- 

 tromagnetic waves, which are used in radio work, published a 

 paper in which he showed that cathode rays can pass through 

 thin metal foils. Two years later, P. Lenard made a tube 

 containing a thin aluminum window, through which the 

 cathode rays could escape into the air. At that time Wilhelm 

 Roentgen was professor of physics at the University of Wurz- 

 burg. He covered an ordinary Hittorf vacuum tube with 

 black paper, probably to see whether Lenard's rays could 

 escape into the open from an ordinary glass tube. He noticed 

 that barium platinocyanide crystals glowed by fluorescence in 

 the dark room, although there was black paper between the 

 tube and the crystals, and realized that some rays from the 

 tube must penetrate the black paper. Then he found that 

 these rays would affect a photographic plate, would pass 

 through matter generally, and so enable the structure of 

 things to be photographed as shadows. This work was the 

 very important discovery of the x-rays. It had been missed 

 by many earlier experimenters. Perhaps Morgan had ac- 

 tually produced x-rays in his experiments a hundred years 

 before Roentgen recognized them. Hittorf and Crookes cer- 

 tainly must have produced x-rays hundreds of times, and 

 Crookes actually fogged a box of photographic plates in his 

 laboratory. It was only when he heard of Roentgen's dis- 

 covery many years later that he understood that his plates 

 had been fogged by the x-rays produced from his own vacuum 

 tubes. 



