THE MODERN THEORY OF LIGHT, 443 



Wliat properties are essential to a uiediuiii capable ol' traiisiititting 

 wave motion ? ]ion<ilily we may say two: elasticity und inertia. Elas- 

 ticity in some form, or some e(piivalent otit, iu order to be able to store 

 up energy and etfect recoil; inertia, in order to enable the disturbed 

 substance to over-shoot the mark and oscillatel)eyond its place of equi- 

 librium to and fro. Any medium possessing these two properties can 

 transmit waves, and unless a medium possesses these properties in some 

 form or other, or some equivalent for them, it may be said with moder- 

 ate security to be incompetent to transmit waves. But if we make this 

 latter statement one must be prepared to extend to the terms elasticity 

 and inertia their very largest and broadest signification, so as to include 

 any possible kind of restoring force, and any possible kind ol' persist- 

 ence of motion respectively. 



These matters may be illustrated in manj" ways, but perhaps a sim- 

 ple loaded lath or spring in a vise will serve well enough. Pull aside 

 one end, and its elasticity tends to make it recoil 5 let it go and its in- 

 ertia causes it to over-shoot its normal position ; both causes together 

 cause it to swing to and fro till its energy is exhausted. A regular 

 series of such springs at ecpial intervals in space, set going at regular 

 intervals of time one after the other, gives you at once a wave motion 

 and appearance which the most casual observer must recognize as such. 

 A series of pendulums will do just as well. Any Mave-transmitting 

 medium must similarly possess some form of elasticity and of inertia. 



But now proceed to ask what is this a4her which iu the case of light 

 is thus vibrating °? What corresponds to the elastic displacment and re- 

 coil of the spring or pendulum ? What corresponds to the inertia 

 whereby it over-shoots its mark ? Do we know these properties in the 

 iether in any other way '? 



The answer, given first by Clerk Maxwell, and now reiterated and 

 insisted on by experiments performed in every important laboratory in 

 the world, is: 



The elastic displacement corresponds to electrostatic charge 



(roughly speaking, to electricity). 

 The inertia corresponds to magnetism. 

 This is the basis of the modern electro-magnetic theory of light. Now 

 let me illustrate electrically how this can be. 



The old and familiar operation of charging aLeyden jar — the storing 

 up of energy in a strained dielectric — any electro-static charging what- 

 ever — is quite analogous to the drawing aside of our flexible spring. 

 It is making use of the elasticity of the tether to produce a tendency to 

 recoil. Letting go the si)ring is analogous to permitting a discharge of 

 the jar — permitting the strained dielectric to recover itself— the elec- 

 tro-static disturbance to subside. 



In nearly all the experiments of electro-statics setherial elasticity is 

 manifest. 

 Next consider inertia. How would one illustrate the fact that water. 



