444 THE MODERN THEORY OF LIGHT. 



for instance, possesses inertia — the power of persisting in motion against 

 obstacles — the power of i^ossessiug kinetic energy? The most direct 

 way would be, to take a stream of water aud 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 utilized b^^ engineers in the " water-ram." 



A i)recisely analogous experiment in electricity is what FarSday 

 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, aud then suddenly stop the current by breaking the circuit. 

 A violent Hash occurs if the stoppage is sudden enough, a Hash which 

 means the bursting of the insulating air partition by the accumulated 

 electro magnetic momentum. 



Briefly we may say that nearly all electro-magnetic experiments illus- 

 trate the fact of a^therial inertia. 



Now return to consider what happens when a charged conductor (say 

 a Leyden jar) is discharged. The recoil of the strained di-electric 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 uatil the energy is all dissipated into heat. The operation 

 is precisely analogous to the release of a strained spring, or to the pluck- 

 ing of a stretched string. 



But the discharging body thus thrown into strong electrical vibration 

 is embedded in the all-pervading ;TBther, and we have just seen that the 

 nether possesses the two properties requisite for the generation and trans- 

 mission of waves, viz, elasticity, and inertia or density ; hence just as 

 a tuning-fork vibrating in air excites aerial waves or sound, so a dis- 

 charging Leyden jar in tether excites {etherial waves or light. 



^Etherial waves can therefore be actually produced by direct electri- 

 cal means. I discharge here ajar, and the room is for an instant filled 

 with light. With light, I say, though you can see nothing. Youcan 

 see and hear the spark indeed (but that is a mere secondary disturb- 

 ance we can for the present ignore), I do not mean any secondary dis- 

 turbance. I mean the true tetherial waves emitted by the electric oscil- 

 lation going on in the neighborhood of this recoiling di-electric. You 

 X)ull aside the prong of a tuning-fork aud let it go : vibration follows 

 and sound is produced. Y^ou charge a Leyden jar and let it discharge: 

 vibration follows and light is excited. 



It is light, just as good as any other light. It travels at the same 

 pace, it is reflected aud refracted according to the same laws ; every 

 experiment known to optics can be performed with this fetherial radia- 

 tion electrically produced, and yet you cannot see it. Why not "? For 

 no fault of the light, the fault (if there be a fault), is in the eye. The 



