CATHODE RAYS AND RONTGEN RAYS. 273 



uppermost. J. J. Thomson in 1897 returned to the emission of par- 

 ticles, but these projectiles were no longer molecules, atoms or ions — 

 the smallest division of matter recognized, but the fragments of atoms, 

 atomic corpuscles. Finally, M. Villard in 1899 determined the nature 

 of these bodies, and showed that they were formed of hydrogen, in 

 short corpuscles or fragments of atomic hydrogen. It was shown 

 that the cathode rays exhibit the spectrum of hydrogen, and if every 

 trace of this gas is successfully removed the cathode emission is 

 suddenly suppressed. 



II. 



After this presentation of the theoretical interest of these new rays 

 it will be well to give a short description of them. Their appearance 

 is dependent upon conditions of the electric discharge in rarefied 

 gases. Phenomena of this character are frequently seen, as for exam- 

 ple, the illumination of Geissler tubes, or of the electric bulb. As 

 these experiments are among the most brilliant and most attractive 

 that can be performed with electricity they are shown on every occa- 

 sion, as much for the beauty of the spectacle as for the instruction of 

 the spectator. 



Let us imagine, then, an electric bulb, an oval vessel of glass in 

 which are placed two metallic poles, two bulbs or, in short, two 

 electrodes of some shape or other, separated by smaller or greater 

 intervals, and charged with electricity. Their electrification will be 

 maintained, for example, by placing them in connection with the 

 induction poles of a Ruhmkorff coil. An electrostatic machine can 

 also be used, if furnished with a condenser whose collector is con- 

 nected with one of the electrodes. A short tube provided with a stop- 

 cock allows the ovoid bulb to be exhausted of air. When the electric 

 tension passes a certain limit a current is established. A flash of 

 flame passes from the positive electrode (the anode) to the negative 

 electrode (the cathode). Under these conditions, having a rarefied 

 gas and suitable charge of electricitjr, this luminous trajectory, instead 

 of being blinding white, sharp, rectilinear or zig zag as the ordinary 

 spark is constituted, appears as a diffuse glow, varied in color accord- 

 ing to the nature of the gas. 



If the bulb or flask which contains the electrodes permits changing 

 the place of the positive pole and approaching it to different points of 

 the surface of the glass, the luminous trail is seen always to leave the 

 wandering point of attachment in order to pass to the fixed negative 

 pole. The passage will be more or less direct or rectilinear, it will 

 approach more or less the axis of the bulb, and will vary in conse- 

 quence with the shape of the same. And by displacing the positive 

 pole, the current, this trajectory of discharge, can be directed at will. 

 In ordinary cases this is what usually occurs, especially when the rare- 

 sm 1901 18 



