122 ESSAYS BIOGRAPHICAL AND CHEMICAL 



hydrogen or neon; sometimes bluish-white, as with 

 carbonic acid or krypton ; sometimes purple-red, as with 

 argon or nitrogen. When examined through a prism or 

 a spectroscope, this light is seen to consist of a number of 

 colours, which blend to give the colour seen with the 

 naked eye. 



Thus the brilliantly red spectrum of hydrogen is easily 

 shown to be a compound impression ; the red light, which 

 is the brightest, is mixed with and slightly modified by 

 a blue- green and a violet light. Tubes which are well 

 adapted to show this light were invented by a German 

 physicist named Plticker in the 'fifties. Twenty-five 

 years later, Sir William Crookes, with the aid of his 

 skilful assistant, Mr. Gimingham, improved the then 

 existing form of air-pump, invented by Dr. Hermann 

 Sprengel, so that it became capable of exhausting the 

 air much more completely than was previously pos- 

 sible. 



He found that, at a much greater exhaustion than that 

 which causes gases to glow and give out their spectrum, a 

 current of high-tension electricity produced in the tube a 

 violet or a green phosphorescence, according as the glass 

 of which it was made contained lead and potash, or lime 

 and soda, combined with the silica, or sand. 



Moreover, the position of this curious phosphorescent 

 glow depended on the shape and direction of the wire or 

 plate from which the negative electricity passed into the 

 tube. From a wire the glow proceeded in all directions 

 perpendicular with its length, so as to colour the tubes 

 immediately surrounding the wire with phosphorescent 

 light. If the wire, however, were terminated with a plate, 

 then the phosphorescent light appeared mostly between 

 the front of the plate and the positive wire of the vacuum- 

 tube. Supposing the plate were curved, so as to form a 

 concave metallic reflector, the light of what was evidently 



