Properties of the Electric Field. 151 



energy due to the alteration in the motion of the atoms : this 

 may be represented by the addition to the ordinary expres- 

 sion for the energy of a term w for each atom on which a 

 unit tube falls, — w for each atom which a tube leaves ; we shall 

 suppose that w depends on the nature of the atom, that it is 

 not the same for zinc as for copper and so on. The existence 

 of this energy will produce the same effect as if the atoms of 

 different substances attracted electricity with different degrees 

 of intensity : this is the assumption made by von Helrnholtz, 

 and it has been shown by him to be sufficient to account for 

 contact electricity. 



The ends of an unclosed tube of induction are places where 

 electrification exists, and therefore are always situated on 

 matter. According to our view, the ends of a tube of finite 

 length are on free atoms as distinct from molecules, the 

 atoms in the molecule being connected by a short tube whose 

 length is of the order of the molecular distance. On this 

 view, therefore, the existence of free electricity, whether on 

 a metal, an electrolyte, or a gas, always denotes the existence 

 of free atoms. The production of electrification must be 

 accompanied by chemical dissociation, the disappearance of it 

 by chemical combination ; changes in electrification are on 

 this view always accompanied by chemical changes. This was 

 long thought to be a peculiarity of the passage of electricity 

 through electrolytes, but recent experiments seem to show that 

 it is also the case when electricity passes through gases. 

 Thus, for example, those gases which conduct readily when 

 hot are those which dissociate when heated, and are thus 

 undergoing chemical changes when the electricity passes 

 through them. Again, it is known that the passage of elec- 

 tricity through many gases causes chemical changes to take 

 place — the production of ozone is the most familiar instance 

 of this, but there are a multitude of others. Lastly, R. v. 

 Helmholtz and Richarz have found that when electricity 

 passes through a gas, a steam-jet in the neighbourhood is 

 influenced in the same way as it, is when free atoms are pro- 

 duced by chemical changes. All these results seem to point 

 to the conclusion that the passage of electricity through gases 

 is accompanied by changes in the pairing of the atoms of the 

 gas. Although we have no such direct evidence of the same 

 effect when electricity passes through metals, it must be 

 borne in mind that direct evidence in this case is very much 

 more difficult to obtain, and there are many reasons for taking 

 the view that the passage of electricity through metals is 

 performed in much the same way as it is through electrolytes 

 and gases. 



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