188 THE REALITIES OF MODERN SCIENCE 



These ultra-violet radiations may also serve to shake 

 electrons loose from the metals on which they fall. In 

 fact, the electrons of the surface of the metal are forced 

 to vibrate with the same high frequency as do the 

 electrons of the source of the ultra-violet light. The 

 phenomenon of this radiation of energy is similar to 

 that of sound, as discussed in Chapter VI, in the one 

 respect that energy is transferred by a wave motion 

 from a vibrating source and results in a similar vibra- 

 tion of a distant particle. 



It is immaterial, so far as concerns merely the fact 

 of shaking loose an electron, whether the electron ac- 

 quires the necessary energy indirectly as its share of 

 the total increase of energy received by the entire body 

 in being heated, or directly by transmission from a 

 distant electron. Of course the electron may also 

 receive the necessary increase of energy by the direct 

 impact of some molecular mass, as in the case of the 

 conduction of electricity through gases, when the pos- 

 itive ions of the gas collide violently with the negative 

 electrode toward which they naturally move. 



The emission of light which accompanies ionization 

 is well illustrated in the discharge which takes place 







FIG. 17. 



in a partially evacuated tube like that of Fig. 17. The 

 appearance of the discharge depends upon the mean 

 free path of the gas molecules in the tube and hence 

 upon the evacuation. The form shown in the figure 



