in the propagation of light might also be the agent in electromagnetic phenomena. Faraday says:
For my own part, considering the relation of a vacuum to the magnetic force, and the general character of magnetic phenomena external to the magnet, I am much more inclined to the notion that in the transmission of the force there is such an action, external to the magnet, than that the effects are merely attraction and repulsion at a distance. Such an action may be a function of the ether, for if there be an ether, it should have other uses than simply the conveyance of radiation.
This expression of Faraday is the key-note of Maxwell's theory. In examining the properties of the medium necessary to transmit electric and magnetic forces, he concentrates his attention on two quantities having direction, namely, the magnetic and electric polarization of the medium at every point. He shows that these states of polarization are propagated in waves, and that these waves have all the properties of light-waves. They are transverse, no longitudinal wave occurs, and moreover for the first time the conditions at the surface of separation of two media are exactly sufficient to give the proper explanation of reflection and refraction. Everything accomplished by any undulatory theory was accomplished by the electromagnetic theory, with this in addition, so that it is perhaps surprising that it remained for the experimental production in 1888 by Hertz of undoubtedly electromagnetic waves having all the properties predicted by Maxwell to give this theory the overwhelming preponderance that it has since maintained.
We may now touch upon the question, what is a mechanical theory. A mechanical theory is one that can be stated in terms of the principles of mechanics. The laws of mechanics, as they have been held since their exact statement by Newton, are all embraced in the single mathematical principle of least action, best comprised in the enunciation of Hamilton. In this enunciation occur two functions representing the two forms of energy, kinetic and potential. If these depend in a certain simple manner on two quantities having direction, or vectors, irrespective of their physical nature, the differential equations follow, which lead to wave propagation. Maxwell's field vectors have this property, and consequently Maxwell's theory is a mechanical theory. I will now define the properties of the ether, as they seem to me to be required by our present-day notions. The ether connotes those properties of space in virtue of which a change in either of two field vectors at any point gives rise to a field of the other sort, the lines of which tend to symmetrically surround the lines of the original and varying vector in circles. In addition the direction of these surrounding lines is contrary according to the field that we begin with. This is a qualitative statement in plain English of what is quantitatively stated in the six differential equations of Maxwell's theory, and it avoids the use of the
