What analogy, may be asked, is there in electro-magnetic phenomena to correspond to this elasticity and inertia of the Aether, so essential to the propagation of light? Let us look at the familiar illustration of charging a Leyden jar. In charging a Leyden jar with electricity a certain amount of energy is spent, work is done, and the result is found in the electrified state of the jar. That which has actually been accomplished is the storing up of energy in the Aether around the jar. This storing up of energy is analogous to pulling aside the lath, and is the making use of the elasticity of the Aether, in order to produce a tendency to recoil. When the jar is discharged, which is analogous to letting go the lath, the Aether seeks to recover its former condition by discharging the energy it received. In these operations the elasticity of the Aether is called into play. After the jar is discharged, however, the recoil of the Aether produces a current, and the inertia of the current causes it to overshoot its original position, and for an instant the charge of the jar is reversed. The current now flows backwards in the same way that the lath returned back, and charges the jar as at first. This discharging and recharging continue backward and forward, so to speak, until all the energy which was originally given to the jar has been expended, and it resumes its normal condition. In this experiment the elasticity and inertia of the Aether have both been called into play, so that we see in this electrical experiment a similar illustration of the elasticity and inertia of the Aether, as manifested in the undulatory or wave theory of light. The question now arises, what are the corresponding properties as given by Maxwell in his electro-magnetic theory? In Art. 782 he writes: "In the theory of electricity and magnetism adopted in this treatise two forms of energy are recognized--the electro-static and the electro-kinetic--and these are supposed to have their seat, not merely in the electrified or magnetized bodies, but in every part of the surrounding space where electric or magnetic force is observed to act. Hence our theory agrees with the undulatory theory in assuming the existence of a medium which is capable of becoming a receptacle of two forms of energy." Faraday, in his _Experimental Researches_, paragraph 3075, in referring to the character of magnetic phenomena external to the magnet, writes: "I am more inclined to the notion that in the transmission of force there is such an action external to the magnet, than that the effects are merely attraction and repulsion at a distance. Such a function may be a function of the Aether if it should have other uses than simply the conveyance of radiations" (light and heat). From this extract we learn that Faraday was also of the opinion that the Aether around a magnet or any electrified body was directly concerned in the propagation of the