THE NEW KNOWLEDGE. 





bulb is illustrated in Fig. 14. Near the positive pole, a, is 



situated a cloud of light which, when the pressure of the 

 air and the current of electricity lie within cer- 

 tain definite limits, shows beautiful fluctuating 

 stria tions. 



The negative pole, or cathode, k, is sur- 

 rounded by a faint glow, and beyond the glow 

 lies a dark space, k to 6. If the exhaustion of 

 the bulb be now increased, the dark space 

 extends farther and farther through the bulb, 

 until, finally, it strikes the other side of the 

 bulb and impinges on the glass. When it strikes 

 the glass a brilliant phosphorescent light ap- 

 pears, green if the glass contain soda, blue if 

 it contain lead, and the glass itself, or the metal 

 within the bulb, becomes a source of X-rays. 

 Fig. 16 is a photographic representation of such 

 a bulb in action. The positive and negative 

 terminals (the anode and cathode) are marked -f- 

 and respectively. Looking only at the wire 

 ^7 which the current leaves the bulb (this wire 



ena in vac- is called the cathode; it is the one marked 



uum tube. m the figure), you will see a velvety glow about 



the terminal of the wire within 



the bulb. In front of this is the 



space called the dark space; in 



front of this, again, the bulb is 



luminous for some distance, this 



luminous portion being called the 



negative glow; and, finally, the 



brilliant phosphorescence of the F - 15 



pear-shaped end of the tube will Showing that the phosphores- 



be noticed as a mass of white. cence arises from the cathode. 



Fig. 14. 





