182 THE REALITIES OF MODERN SCIENCE 



be struck by another electron traveling at such a speed 

 as to combine with it instead of again disrupting it. 

 As a negative ion it would then take up a directed 

 motion toward the positive electrode. The molecule 

 has now in its group an extra electron. None of the 

 electrons are therefore as firmly held as if the normal 

 number had not been exceeded. An impact even with 

 a neutral molecule may sometimes be sufficient to jar 

 off an electron. Whether the electron jarred loose in 

 this way is the latest addition or one of the original ones 

 makes no difference and can probably never be de- 

 termined, since they are all alike anyway. 



Before discussing further the picture, which we have 

 just obtained as to the mechanism for the conduction 

 of electricity through gases, it is well to note that it is 

 that of the general case. If all the molecules of the 

 gas are removed, leaving a vacuum, the current can 

 be carried only by electrons, since there are no mole- 

 cules to form ions, and hence all the electrons must be 

 supplied and released at the negative electrode. If 

 electrons are not so released there can be no current. 

 In the case of a gas, if there happen originally to be 

 no free electrons between the electrodes, there can be 

 no current, unless some electrons are released at the 

 negative electrode. If, however, the potential differ- 

 ence is sufficiently high the presence or release of a few 

 electrons will result in ionization, and hence in the self- 

 perpetuation of the supply of carriers. 



In conduction through solids, molecular motion is 

 restricted and the entire transfer of electricity is due 

 to the motion of the electrons. In this case com- 

 binations probably occur in much the same manner as 



