THE ELECTRON THEORY. 



28l 



air. When the pressure of the gas in the bulb has been reduced 

 to a few centimeters of mercury, the spark begins to be nebulous, 

 and a continued reduction of pressure causes the luminosity 

 ultimately to fill the entire tube. When the pressure has been 

 reduced to a few millimeters of mercury the discharge presents 

 the following features, as shown in Fig. 206. There is a thin 

 layer of luminosity spread over the surface of the cathode C, 

 and beyond this there is a comparatively dark space D called 

 the Crookes dark space, the width of which depends upon the 

 pressure of the gas, increasing as the pressure of the gas di- 

 minishes. This Crookes dark space extends to a 

 boundary which is approximately a surface traced 

 out by lines of constant length drawn normally to 

 the surface of the cathode. Beyond the Crookes 

 dark space is a luminous region N called the nega- 

 tive glow, and beyond the negative glow is another 

 comparatively dark region F which is called the 

 Faraday dark space. Beyond the Faraday dark 

 space is a luminous column P extending to the 

 anode A and called the positive column. This 

 positive column usually exhibits alternations of 

 bright and dark spaces which are called striations. 

 The effects here descibed are exhibited at their best 

 in a vacuum tube in which the pressure has been 

 reduced to a few millimeters of mercury. Such a 

 vacuum tube is called a Geissler tube. When the 

 exhaustion of the vacuum tube is carried further, 

 the dark space which surrounds the cathode (the 

 Crookes dark space) expands until it fills the entire tube. The 

 glass walls of the tube then show a yellowish-green or blue 

 luminescence (according as the tube is made of soda glass 

 or lead glass) and a slight negative glow may remain in 

 portions of the tube remote from the cathode. These effects, 

 which were first studied by Crookes in England and by Plucker 

 and Hittorf in Germany, are exhibited at their best in a vacuum 



Fig. 206. 



