at light
is an electro-magnetic phenomena." In the wave theory of light we have
seen (Art. 70), that two properties are necessary to any medium before
it is capable of transmitting wave motion of any kind. Those two
properties are elasticity and inertia. Water possesses these properties,
and so can transmit ocean waves; air also possesses these properties,
and so can transmit sound waves; and Aether, being matter, also
possesses these properties (Arts. 47 and 48), and is therefore capable
of transmitting light waves. The elasticity is essential in order for
the medium to store up energy, and also to enable it to resume its
original shape after deformation, while the inertia is necessary in
order that the medium may transmit the impulse, and oscillate to and fro
until the impulse received has been passed on. This elasticity and
inertia may be well illustrated by the bending of a lath or cane. If we
pull one end down, holding the other end quite still, we shall see that
the lath oscillates to and fro until gradually it comes to rest. The
elasticity of the lath allows it to be pulled out of its original
position, and also enables it to rebound, while its inertia causes it to
swing back again past its original position. Both combined together
cause it to swing backwards and forwards till its energy is used up. If
such a series of springs could be set in motion at equal intervals of
space and time, we should then have a good illustration of a wave
motion.

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