278 CATHODE RAYS AND RONTGEN RAYS. 



little ways back in saving that it was similar to a row of electrified 



particles, that is to say. to a current. It is necessary, therefore, that 

 the charge which it transports should be made manifest. Crookes 

 believed that he had succeeded in doing this. Ebert and Wiedemann 

 showed the fallacy of his demonstration, hut it was a young French 

 physicist, M. Jean Perrin, who. by a ven neat experiment, made plain 

 the essential character of cathode rays, which is that they must be 

 charged with negative electricity. 



The cathode phenomena, such as we have described them, tills the 

 whole of the interior of the bulb; within it. it begins and ends. l T p 

 to 1894 it had been impossible to study these rays under the experi- 

 mental conditions in which they occur. The rays remain shut up in 

 their birthplace as in a prison. Lenard succeeded in liberating them, 

 and his beautiful experiments of 1894, which drew these captive rays 

 from their prison of glass, created a great enthusiasm among physicists. 



The cathode rays are stopped by glass: this is well known. Most 

 other substances act the same way. However, Hertz in 1883 had 

 announced that metallic plates would permit the passage of these rays 

 provided they were sufficiently thin; their thickness should not be 

 greater than a few thousandths of a millimeter (micron). Lenard sug- 

 gested replacing the fluorescent portion of the glass tube on which the 

 cathode pencil strikes by a piece of metal, and it was necessary that 

 this plate should be stout enough not to yield to the pressure of the 

 air. Herein lay the difficulty, which Lenard .succeeded in overcoming. 

 He arranged in his Crookes tube a small window, in which he inserted 

 a plate of aluminium three-thousandths of a millimeter in thickness. 

 This leaf proved to be capable of resisting atmospheric pressure and 

 of sustaining the vacuum within. The cathode rays, more subtle than 

 ■ aseous molecules, passed through, permitting them to be studied 



ithout. 

 "hey behaved without exactly as within the tube; they proved to be 



ctilinear. deflected by a magnet and capable of producing fluores- 

 cence; also equally capable of making an impression on a photographic 

 plate. Most extraordinarily they had preserved their negative elec- 

 trification in spite of the thickness of the metal which they had tra- 

 versed. This fact was unexpected and unexampled. It indicates that 

 the negative electrical charge is an essential and indelible character of 

 the cathode ray, and that it can not lose it without ceasing to exist. 



These experiments taught at the same time that the cathode rays 

 possess a very limited power of penetration, even through gases. 

 Unless these gases are extremely rarefied the rays are quickly stopped 

 and scattered by molecular obstacles. On the contrary, when the 

 vacuum is pushed very far they remain unchanged; it has been possi 

 hie to follow them the length of a meter and a half without noticing 

 any diminution of power. 





