MOTIONS OF ELECTRONS 183 



in a gas, but the combinations are prevented by the 

 other molecular masses from moving toward the elec- 

 trodes. In the case of metals, which are the best con- 

 ductors, there are always electrons moving about through 

 the substance. Although the number per cubic cen- 

 timeter is very large, it is small compared to the total 

 number of electrons in this volume. A metal then con- 

 sists of a large number of neutral molecules, a few 

 positive molecules (they can hardly be called "ions" 

 since they cannot "go"), a smaller number, perhaps, 

 of negative molecules and a number of free electrons 

 just equal at any instant to the difference between the 

 number of positive and negative molecules. 



In the case of those liquids which conduct electricity 

 (pure water, for example, does not) we shall see later 

 that there are positive and negative ions but no free 

 electrons. The ionization of liquids is not the result of 

 collisions with free electrons, since there are none pres- 

 ent. It is in the nature of a spontaneous dissociation 

 and depends only upon the chemical composition of the 

 liquid. Of this phenomenon of "electrolytic dissocia- 

 tion" we shall have more to say later. 



For the moment, the important matter is to obtain a 

 general picture of the conduction of electricity. Con- 

 duction occurs as the result of the motion of electrons. 

 This may be an actual motion or it may be a motion of 

 what we might call "certificates of electronic indebted- 

 ness." A positive ion is essentially a certificate of 

 electronic indebtedness which may be transferred at 

 will and may be satisfied at any point in the universe 

 where there is an excess electron. (Sometimes the 

 certificate will call for more than one electron, as is true, 



