MOTIONS OF ELECTRONS 189 



is merely typical of those which may be observed. 

 Near the negative electrode, or cathode, 1 the positive 

 ions are ionizing the gas and combining with the excess 

 electrons of the cathode, and a violet glow results. 

 In the region of the so-called " positive column" there 

 are striae, indicating the successive layers of the gas 

 where ionization and recombination occur. When the 

 vacuum of the tube is carried further a condition is 

 reached where, because of the small number of mole- 

 cules available, but little ionization occurs and the 

 effects are due largely to the electrons. These fly 

 away from the cathode in radial lines. Their existence 

 was first noted by Crookes in 1876, who spoke of them 

 as " radiant matter." A brilliant phosphorescence in- 

 dicates their impacts with the walls of the glass tube. 

 The fact that they proceed 

 radially is usually illustrated 

 by interposing a piece of 

 mica, as the Maltese cross of 

 Fig. 18, and observing that it 

 casts a well-defined shadow 

 within which there is no phos- 

 phorescence. Some of the ex- FlG - 18 - 

 periments by which it was proved that the phenom- 

 enon was one of corpuscles rather than of light rays, 

 that is, the experiments which led to the identification 

 of the electron, will be described in Chapter XXII. 



1 Current was considered to flow from a positive to a negative 

 electrode before the electron was discovered. It was therefore 

 spoken of as flowing from the ''anode " to the "cathode." The tube 

 which we are considering has had associated with it the names 

 of various scientists. It is preferably described by the name of 

 either Crookes or Geissler. 



