284 Maxwell. 



velocity of propagation of disturbance may be shown, by the 

 same analysis, to be ct~i^~i ; so that it is diminished when /u is 

 greater than unity, i.e., in paramagnetic bodies. This inference 

 had been anticipated by Faraday : " Nor is it likely," he wrote,* 

 " that the paramagnetic body oxygen can exist in the air and 

 not retard the transmission of the magnetism." 



It was inevitable that a theory so novel and so capacious as 

 that of Maxwell should involve conceptions which his contempo- 

 raries understood with difficulty and accepted with reluctance. 

 Of these the most difficult and unacceptable was the principle 

 that the total current is always a circuital vector ; or, as it is 

 generally expressed, that " all currents are closed." According 

 to the older electricians, a current which is employed in charging 

 a condenser is not closed, but terminates at the coatings of the 

 condenser, where charges are accumulating. Maxwell, on the 

 other hand, taught that the dielectric between the coatings 

 is the seat of a process the displacement-current which is 

 proportional to the rate of increase of the electric force in the 

 dielectric ; and that this process produces the same magnetic 

 effects as a true current, and forms, so to speak, a continuation, 

 through the dielectric, of the charging current, so that the 

 latter may be regarded as flowing in a closed circuit. 



Another characteristic feature of Maxwell's theory is the 

 conception for which, as we have seen, he was largely indebted 

 to Faraday and Thomson that magnetic energy is the kinetic 

 energy of a medium occupying the whole of space, and that 

 electric energy is the energy of strain of the same medium. 

 By this conception electromagnetic theory was brought into 

 such close parallelism with the elastic- solid theories of the 

 aether, that it was bound to issue in an electromagnetic theory 

 of light. 



Maxwell's views were presented in a more developed form 

 in a memoir entitled "A Dynamical Theory of the Electro- 

 magnetic Field," which was read to the Koyal Society in 1864 ;f 



* Faraday's laboratory note-book for 1857 : of. Bence Jones's Life of Faraday, 

 ii, p. 380. 



t Phil. Trans, civ (1865), p. 459 : Maxwell's Scient. Papers, i, p. 526 



