454 Mr. J. H. Jeans on the 



positive. Before the ions have taken up their equilibrium 

 positions there will be a flow of ions through the body, and 

 this is a " current of electricity." 



A third class of body can be imagined in which the ions 

 are closely bound into atoms, but the bond between the 

 various atoms of a molecule is very slight : this is the class of 

 electrolytes. A current in this case involves an actual transfer 

 of atoms through the electrolyte. 



Consider, for instance, the electrolysis of hydrochloric acid. 

 The hydrogen atom has a convergence of force equal and 

 opposite to that of a negative ion. Every pair of hydrogen 

 atoms as they arrive at the cathode will combine with two of 

 the negative ions which have been conveying the current 

 through the metal cathode, and will together form a neutral 

 hydrogen molecule. So also at the anode, the pairs of 

 chlorine atoms combine into molecules, and in doing so 

 liberate the ions which are to carry the current away from 

 the anode. Faraday's Laws of electricity follow at once. 



Conclusion. 



§ 42. This concludes the comparison between the phe- 

 nomena which are to be expected from our hypothetical 

 matter and those which are observed to occur in nature. It 

 must be left for individual judgments to decide whether the 

 test afforded in this way is sufficiently strict to be worth 

 anything ; and if so, to decide what is the measure of pro- 

 bability that our hypothetical matter gives a clue to the 

 structure of actual matter. 



§ 43. In conclusion, reference may be made to a question 

 which demands an answer in the case of this, as in the case 

 of every other hypothesis which attempts to place the structure 

 of matter on a purely electrical or aethereal basis. The only 

 difference which the aether-equations of electricity can re- 

 cognize between a negative and a positive charge of electricity 

 is a mere difference of sign ; so that if we regard a negative 

 ion as an aether-structure, we are inevitably led to regard the 

 possibility of positive ions differing from negative ions only 

 by a difference of sign. On the other hand, the predominance 

 of the negative ion in most material phenomena, and in the 

 emission of light (as evidenced by the Zeeman Effect) seems 

 to suggest the view that positive and negative ions differ in 

 something more than mere sign. An}^ attempt to explain 

 matter in terms of aether must therefore face the problem of 

 reducing what appears to be a difference in quality to a 

 difference in sign only. 



We can explain these facts, in terms of our present theory, 

 by supposing (§ 26) that the outermost shell of ions in any 



