Now in dealing with electricity as a mode of motion, it will be necessary to show that electricity is also a form of energy in the same way that heat and light are forms of energy. If it can be demonstrated that electricity is a form of energy, then it can easily be demonstrated that work can be done by it, and that that work may take a mechanical form in the same way that the energy of heat and light may produce mechanical results. Clerk Maxwell has given us, in his greatest work, his conception of the two kinds of energy due to electricity and magnetism. On the subject he writes: "In the theory of Electricity and Magnetism accepted in this treatise, two forms of energy are recognized, the Electro-Static and Electro-Kinetic (paragraphs 630 and 636), and these are supposed to have their seat not merely in electrified or magnetized bodies, but in every part of surrounding space, wherever electric or magnetic force is observed to act. Hence our theory agrees with the undulatory theory of light in assuming the existence of a medium which is capable of becoming susceptible to two forms of energy."[24] The question has arisen many times as to what is meant by the terms Electro-Static and Electro-Kinetic energy used by Maxwell, and various hypotheses have been advanced to explain the same. Electro-static energy is said to be that phase of electricity in which we deal with stresses set up in the Aether by an electrified body at rest, whether that body be small or large. It further deals with the process of induction, that is, the action of an electrified body upon another body, such action taking place through the medium between the two bodies. Electro-kinetic energy is the energy due to electricity in motion. On this point Maxwell says: "A conducting circuit in which the current has been set up has the power of doing work in virtue of the current, for it is really and truly energy. It appears, therefore, that a system containing an electric current is a seat of energy of some kind; and, since we can form no conception of electric current except as a kinetic phenomenon, its energy must be kinetic energy, that is to say, the energy which a moving body has in virtue of its motion." (Arts. 551 and 552.) It is not our purpose to deal with the electro-kinetic form of energy referred to by Maxwell in this chapter. We shall deal with that form of energy due to electricity in the succeeding chapter. We will consider first the effect of the electro-static energy in relation to electrified bodies, but I wish it to be distinctly understood, that in all the different kinds of electric phenomena manifested, the Aether plays the chief part, and without it, none of the phenomena observed could be produced; because, what Aether is to light and heat, so it is to electricity, being that medium which by its motions propagates and gives rise to all electrical phenomena.