106 MODERN THEORIES OF ELECTRICITY AND MATTER. 



rent of displacement in the dielectric, so that there exist only closed 

 circuits. 



The system of the §ix differential equations, called Maxwell's 

 equations, brings out in mathematical form the relation which exists 

 at each point of an electro-magnetic field between the current of 

 displacement and the magnetic field, as well as between the rate of 

 change of the magnetic induction and the resulting electric field. 

 These perfectly symmetrical relations show tliat all variations of 

 an electric field cause a magnetic field, and vice versa. Starting 

 from these equations, jMaxwell proved that every perturbation of an 

 electro-magnetic field should be propagated in a vacuinn, with a 

 velocity equal to that of light in a vacuum, and he draws the conclu- 

 sion that the medium which transmits electro-magnetic actions in 

 the vacuum is the same as that which transmits light, and that light is 

 very likely an electro-magnetic phenomenon. This conception has 

 served as the basis of the electro-magnetic theory of light, now uni- 

 versally adopted as the result of the experiments of Hertz and 

 numerous physicists upon electro-magnetic waves. In the develop- 

 ment of the ideas of Faraday and Maxwell, a preponderating 

 influence was attributed to the role of the dielectric medium, so that 

 little attention was paid for some time to the nature of electricity; 

 and this question was relegated to a subordinate place, and received 

 onl}^ an indirect interpretation. There was no longer the conception 

 of charges of electricity localized in a determined region, nor of a 

 fluid flowing through a conductor. The main conceptions were of 

 energy localized in the dielectric medium and the differential equa- 

 tions which determine the field in the medium. Recent progress in 

 research has brought us bade to a more concrete conception of tlie 

 nature of electricity^ 



The first impulse in this direction was the result of investigations 

 of electrolysis and modern theories of this phenomenon. It was 

 established with certainty that the passage of electricity in the elec- 

 trolyte is always accompanied by the transportation of matter. Elec- 

 trolytes are aqueous solutions of acids, bases, and mineral salts, or 

 these bodies in a fused condition. It is now admitted that the mole- 

 cules of a dissolved substance are totall}^ or partially dissociated 

 in two ions — one ion formed by the metal, or hydrogen, and charged 

 positively; another formed by the acid radical, and charged nega- 

 tively. When there is set up in the solution an electric field the ions 

 move toward the electrodes of contrary sign, transporting across the 

 liquid their charges, which they give up to the electrodes, and 

 themselves become free in a neutral state. Ions are, then, the actual 

 carriers of electricity in electrolytes, and the current is a current of 

 convection. It follows from Faraday's laws that all monovalent 

 ions carry the same amount of charge, q, corresponding to 9G,600 



