THE PHENOMENA OF ELECTROSTATICS. 223 



tivity which is imparted to a gas by these various agencies may 

 be destroyed by filtering the gas through glass-wool or by plac- 

 ing the gas for a few moments between electrically charged metal 

 plates. This effect of filtration seems to show that the conduc- 

 tivity of a gas is due to something which is mixed with the gas, 

 and the effect of the electric field (between two charged plates) 

 shows that this something is charged with electricity and moves 

 under the action of the field. " We are thus led to the conclu- 

 sion that the conductivity of a gas is due to electrified particles 

 mixed up with the gas, some positive, some negative. We shall 

 call these electrified particles ions and the process by which a gas 

 is made into a conductor we shall call the process of ionization."* 



The electron"\ is a negatively charged particle of which the 

 mass is about ,^-Q of the mass of a hydrogen atom. Thus, the 

 cathode rays consist of electrons which are thrown ofif from the 

 cathode of the Crookes' tube at high velocity, the /3-rays from a 

 radio-active substance such as uranium are electrons which are 

 expelled from the atoms of the substance at high velocity. 



A simple ion is an atom of a gas from which a negatively 

 charged electron has been detached, leaving the remainder of the 

 atom positively charged. Thus, the canal rays in a Crookes' 

 tube consist of simple ions positively charged, and the a-rays 

 which are given off by a radio-active substance such as uranium 

 consist of simple ions positively charged. A compound ion con- 

 sists of a negatively charged electron or a positively charged 

 simple ion to which one or more neutral atoms cling, thus form- 

 ing a charged atomic aggregate. 



Ionization by the electric field. According to (the kinetic 

 theory of gases, a molecule of a gas travels on the\ average a 

 certain distance between successive collisions with neighboring 

 molecules. This distance is called the mean free path of the 

 molecule. The mean free path of an electron in a gas is about 

 41/2J times as great as the mean free path of a molecule of the 



* See J. J. Thomson, Conduction of Electricity Through Gases, page II. 

 f Called a corpuscle by J. J. Thomson. 

 J According to the kinetic theory. 



