(after referring to 'the velocity which is the conductance in electrostatic measure, and the resistance in electromagnetic measure of one and the same conductor', and stating that this velocity is 'not very different from that of light'): "But its relationship to the velocity of light was brought out in a manner by James Clerk Maxwell to make it really a part of theory which it never was before. James Clerk Maxwell pointed out its application to the possible or probable explanation of electric effects by the influence of a medium, and showed that that medium - the medium whose motions constitute light - must be ether. Maxwell's 'electromagnetic theory of light' marks a stage of enormous importance in electromagnetic doctrine, and I cannot doubt but that in electromagnetic practice we shall derive great benefit from a pursuing of the theoretical ideas suggested by such considerations." Kelvin (Thomson)

"We thus have simply the undulatory theory of light, as an inevitable consequence of believing that the displacement of an elastic solid by which, in my old paper (of 1847), I gave merely a 'representation' of the electric currents and the corresponding magnetic forces, is a reality. But to give anything like a satisfactory material realization of Maxwell's electromagnetic theory of light, it is necessary to show electrostatic force in relation to the force (X, Y, Z) of my formulas; to explain the generation of heat according to Ohm's Law in virtue of the action of this force when it causes an electric current to flow through a conductor; and to show how it is that the velocity of light in ether is equal to, or perhaps we should rather say, is, the number of electrostatic units in the electromagnetic unit of electric quantity. All this essentially involves the consideration of ponderable matter permeated by, or imbedded in ether, and a tertium quid which we may call Electricity, a fluid go-between, serving to transmit force between ponderable matter and ether ... I see no way of suggesting properties of matter, of Electricity, or of ether, by which all this, or any more than a very slight approach to it, can be done, and I think we must feel at present that the triple alliance, ether, Electricity, and ponderable matter is rather a result of our want of knowledge, and of capacity to imagine beyond the limited present horizon of physical science, than a reality of nature." Lord Kelvin (Sir William Thomson)