Quotes by James Clerk Maxwell

Energy in electromagnetic phenomena is mechanical energy

In speaking of the Energy of the field, however, I wish to be understood literally. All energy is the same as mechanical energy, whether it exists in the form of motion or in that of elasticity, or in any other form. The energy in electromagnetic phenomena is mechanical energy.

Maxwell quote from Joseph Newman's website..

The Theory I propose may ... be called a Theory of the Electromagnetic Field because it has to do with the space in the neighborhood of the electric or magnetic bodies, AND IT MAY BE CALLED A DYNAMICAL THEORY, BECAUSE IT ASSUMES THAT IN THAT SPACE THERE IS MATTER IN MOTION, BY WHICH THE OBSERVED ELECTROMAGNETIC PHENOMENA ARE PRODUCED.