"After the publication of 'On physical lines of force,' Maxwell's agenda included the experimental verification of three predictions of his theory. He planned to renew his attempts at detecting gyromagnetic effects. He envisioned precise measurements of the inductive capacity ε of various transparent substances in order to verify the theoretical relation with the optical index (ε = n^2). Most importantly, he intended to verify the identity of the velocity of light with the ratio of absolute electromagnetic and electrostatic charge units by improving on Weber and Kohlrausch's measurement. His enrollment in the British project for electric standards eased this task. In 1864 he imagined an arrangement based on the direct comparison between an electrodynamic and an electrostatic force. Four year laters [sic] he published the results of a more sophisticated experiment based on the same principle.

Considering that the electromagnetic derivation of the velocity of light was his most important result, Maxwell tried to 'clear the electromagnetic theory of light of all unwarranted assumptions.' The velocity of light could not possibly depend on the shape of vortices or on their kind of elasticity. In 1864 Maxwell managed to reformulate his theory without any specific mechanism and to describe wave propagation in purely electromagnetic terms."

Considering that the electromagnetic derivation of the velocity of light was his most important result, Maxwell tried to 'clear the electromagnetic theory of light of all unwarranted assumptions.' The velocity of light could not possibly depend on the shape of vortices or on their kind of elasticity. In 1864 Maxwell managed to reformulate his theory without any specific mechanism and to describe wave propagation in purely electromagnetic terms."

In Darrigol, Electrodynamics from Ampère to Einstein, pp. 154-155.

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