CH.IV.] VACUUM TUBE 25 



forming a band or thread of light, while the original 

 spark-gap may be shortened down gradually to one- 

 eighth of an inch, or even less, without any spark 

 taking place across it, showing that rarified air is 

 a very good conductor. When the best conducting 

 stage is reached the tube is filled with a glow, called 

 the positive column ; and both ends of the tube are 

 apt to look alike. If we exhaust still further and 

 to exhaust even as far as this something better than 

 an ordinary air-pump is necessary, an oil or mercury 

 pump being the most suitable the column of light 

 is seen to fill the whole tube, to gradually lose its 

 bright red or crimson tint, and to break up into a 

 number of very narrow discs, like pennies seen edge- 

 ways. At the same time the spark-gap must be 

 widened to something more like a quarter or half an 

 inch, to prevent the discharge from taking that path, 

 and a dark space near the cathode now begins to be 

 visible, the cathode itself being covered all over with 

 a glow, while the anode is usually only illuminated 

 at a point or two. The striae, into which the positive 

 column has been broken up, thicken and separate as 

 exhaustion proceeds. The dark space near the 

 cathode also enlarges, driving as it were the positive 

 column before it into the anode, and looking as if it 

 would presently fill the tube ; but before it can do 

 this it is noticed that the glow on the cathode itself 

 is coming off as a kind of shell, leaving another dark 

 space, a narrower and much darker space, inside it. 

 The first dark space has been called Faraday's dark 

 space ; the second is generally known by the name of 

 Crookes'. This second dark space now increases in 

 thickness, pushing the glow before it as the vacuum 

 gets better and better ; but the terminals of the 

 spark-gap must now be pulled still further apart, else 



