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 velocity of propagation of disturbance may be shown, by the same analysis, to be cε-$1⁄2$μ-$1⁄2$; so that it is diminished when μ 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 contemporaries 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, lie 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 Electromagnetic Field," which was read to the Royal Society in 1864;