The old and familiar operation of charging a Leyden jar--the storing up of energy in a strained dielectric--any electrostatic charging whatever is quite analogous to the drawing aside of our flexible spring. It is making use of the elasticity of the ether to produce a tendency to recoil. Letting go the spring is analogous to permitting a discharge of the jar--permitting the strained dielectric to recover itself--the electrostatic disturbance to subside. In nearly all the experiments of electrostatics etherial elasticity is manifest. Next consider inertia. How would one illustrate the fact that water, for instance, possesses inertia--the power of persisting in motion against obstacles--the power of possessing kinetic energy? The most direct way would be, to take a stream of water and try suddenly to stop it. Open a water tap freely and then suddenly shut it. The impetus or momentum of the stopped water makes itself manifest by a violent shock to the pipe, with which everybody must be familiar. This momentum of water is utilised by engineers in the "water-ram." A precisely analogous experiment in Electricity is what Faraday called "the extra current." Send a current through a coil of wire round a piece of iron, or take any other arrangement for developing powerful magnetism, and then suddenly stop the current by breaking the circuit. A violent flash occurs, if the stoppage is sudden enough, a flash which means the bursting of the insulating air partition by the accumulated electromagnetic momentum. The scientific name for this electrical inertia is "self-induction." Briefly we may say that nearly all electromagnetic experiments illustrate the fact of etherial inertia. Now return to consider what happens when a charged conductor (say a Leyden jar) is discharged. The recoil of the strained dielectric causes a current, the inertia of this current causes it to overshoot the mark, and for an instant the charge of the jar is reversed; the current now flows backwards and charges the jar up as at first; back again flows the current; and so on, charging and reversing the charge, with rapid oscillations, until the energy is all dissipated into heat. The operation is precisely analogous to the release of a strained spring, or to the plucking of a stretched string. But the discharging body, thus thrown into strong electrical vibration, is imbedded in the all-pervading ether; and we have just seen that the ether possesses the two properties requisite for the generation and transmission of waves, viz.: elasticity, and inertia or density; hence just as a tuning fork vibrating in air excites aërial waves, or sound, so a discharging Leyden jar in ether excites etherial waves, or light.