180 THE REALITIES OF MODERN SCIENCE 



free one of the component electrons of the molecule. 

 If this is to be the effect, the electron, before its col- 

 lision with the molecule, must have fallen through a 

 sufficient difference of potential to acquire the amount 

 of kinetic energy necessary for disrupting the system 

 of nucleus and electrons which constitutes the molecule. 

 When an electron has been shaken from a gas molecule, 

 so that it is free to pursue an independent path and to 

 have an individual existence, the molecule is said to be 

 ionized. The two parts which are thus formed are 

 called "ions," meaning "goers," but it is preferable to 

 call the portion which is still of molecular size the ion 

 and to speak of the other moving part as an electron, 

 since that is what it really is. 



As we shall see, the electron may later join company 

 with a neutral molecule, that is, one which has not been 

 ionized. In this case also we would speak of the new 

 combination as an ion. We may thus have ions, pos- 

 itive or negative, depending upon whether they are 

 formed from a neutral molecule by knocking off an 

 electron or by combination with an electron. The 

 electrons and the ions are "goers" whose motion is 

 conditioned by the systems of potential energy which 

 they form with the positive and negative plates. 



Now in air at ordinary pressures and temperatures 

 the mean free path 1 is comparatively short. A moving 



1 The distance which a molecule in its haphazard motion would 

 travel on the average between two successive impacts with its fel- 

 lows is called its mean free path. This will be smaller the larger 

 the number of molecules per unit volume, that is, the greater the 

 density of the gas. The average distance through which an electron 

 may move between successive impacts with the molecules of its gase- 

 ous path will depend upon the mean free path of the gas molecules. 



