616 Dr. J. Kunz : Abrupt Limit of Distance in the 



to the positive rays suddenly appears and increases, when the 

 distance between the Rontgen tube and the screen is further 

 decreased. If the interior tube is moved in the opposite 

 direction, the luminosity of the screen gradually diminishes 

 and eventually disappears suddenly at the point where it 

 appeared in the original movement. The points of appearance 

 and disappearance found in very numerous successive ex- 

 periments lay within a range of about 4 mm. The distance 

 between the cathode and the screen was then about 41 cm. 

 in air under a suitable low pressure. 



One might object that the abrupt appearing and disap- 

 pearing of: the phosphorescent light depends on a physiolo- 

 gical property of the eye. The light of the appearing stars 

 flashes up at once in the evening, though we should expect a 

 quite continuous transition. That the effect is really produced 

 by the positive rays was proved by introducing a revolving 

 disk with sectorial openings between the screen and the eye. 

 When the distance between the cathode and the screen was 

 chosen so that the phosphorescent light just appeared, and if 

 the disk was then set in revolution, the light, though dimmer, 

 did not disappear. Moreover, the critical distance between 

 the screen and the cathode was found to be the same by 

 different observers. 



The same experiment was made with air, carbonic acid, and 

 hydrogen at various low pressures suitably chosen. The 

 result was the same in all these gases. There is an abrupt 

 limit of distance in the power of the positive rays to produce 

 phosphorescence. 



Moving positive particles passing through a gas and im- 

 pinging against a metal plate produce corpuscles, and the 

 gas in the tube becomes a comparatively good conductor. 

 Professor J. J. Thomson * explains the existence of an abrupt 

 limit of the ionizing power of the ol particles of the radio- 

 active elements in the following way : — When the energy of 

 an a. particle falls below a certain value, a negatively elec- 

 trified corpuscle gets attached to the particle, so that its 

 positive charge is neutralized and it becomes an uncharged 

 atom with properties quite different from those of the original 

 ol particle. 



In order that an a particle and a corpuscle should separate 

 after a collision between the two, it is necessary that the 

 relative velocity of the particle and the corpuscle before 

 impact must exceed a definite value v, which depends on the 

 charges E and e on the particle and the corpuscle respectively, 



* J. J. Thomson, ' Conduction of Electricity through Gases/ 2nd ed., 

 p. 360. 



