Electricity through Argon and Helium, 301 



investigated by Prof. J. J. Thomson (Recent Researches in 

 Electricity and Magnetism, p. 96). 



The luminosity in the bulb is confined to the plane of ihe 

 coil, and is brightest in its outer parts, nearest the wire of the 

 coil. Here the current is most intense. 



In argon the phenomena observed are as follows : — As 

 soon as the pressure is low enough to enable the discharge to 

 pass, a thin reddish ring appears in the bulb, just inside the 



Fk. 4. 



wire coil (fig. 4 a). As the pressure diminishes, this red 

 ring broadens out, and a thin ring of bright blue luminosity 

 appears in the outermost part of the bulb (fig. 4 c) . The blue 

 luminosity is represented by dark shading, the red by light. 

 On further exhaustion this blue ring expands inwards (fig. 4a 7 ) 

 until, when the pressure is so low that the discharge has a 



