CONDUCTION THROUGH GASES 159 



which we have described in Chapter V., indicates 

 that an electric transfer through such liquids 

 involves a movement of the chemical constituents 

 of the substance decomposed. In fact, as we have 

 seen, that movement has been experimentally 

 demonstrated, and the passage of the ions rendered 

 visible. We must suppose, then, that the electron 

 forming the effective negative essence of the anion, 

 is, in liquid electrolytes, attached to an atom of 

 matter. This atom may possibly be associated 

 with other atoms or molecules forming a complex 

 ion, but the point is that the isolated electron 

 cannot slip from one atom to another, and thus 

 carry an electric current through the liquid ; the 

 electron cannot move without a corresponding 

 movement of matter — of matter, that is, in its 

 atomic or molecular sense. 



Here again the motion of the positive ion 

 involves the simultaneous passage of a particle 

 of matter of at least atomic dimensions. The 

 positive ion consists of an atom of the electrolyte 

 with one of its electrons missing. In this way, 

 a unit of negative electricity is removed from it, 

 that is, it is left with a positive charge. 



In metals an electric current flows without 

 chemical change in the substance of the con- 

 ductor, so that, in this case, we must imagine 

 the electrons to be freely mobile. They pass 

 from atom to atom, and thus carry the current 

 when an electromotive force acts. In the presence 

 or absence of such a force, they may be regarded 

 as existing within the metal in a state resembling 

 in many ways the state of a gas in a closed 

 vessel. Estimates have been made of the 

 number of electrons present in a given volume ; 



