e motion of light, in that it is periodic both
in time and space, and in that aetherial wave motion Maxwell states that
two forms of energy are called into play, which he calls Electro-Static
and Electro-Kinetic. These correspond respectively to the elasticity and
inertia in the older theory of the wave motion of light. It was upon
this basis that Maxwell built up his electro-magnetic theory. Even this
theory does not tell us what is the exact character or nature of the
periodic wave motion of the Aether. All it tells us is, that the
electro-magnetic wave motion of the Aether is the same in nature and
character as the wave motion which produces light and heat. Thus it
shows that light and electricity have a common origin, and proves that
light is nothing more or less than an electro-magnetic phenomenon.
Maxwell gave a number of proofs in support of his theory. He showed that
the velocity of the electro-magnetic waves was almost identical with the
velocity of light waves, his results being as follows--

VELOCITY OF ELECTRO-MAGNETIC VELOCITY OF LIGHT IN
WAVES. METRES PER SEC.

Weber 310,740,000 metres per sec. Fizeau 314,000,000
Maxwell 288,000,000 " " Foucault 298,360,000
Thompson 282,000,000 " "

From these figures it can readily be seen that the velocity of an
aetherial wave, generated by electric means, is approximately the same
as an aetherial wave generated by a luminous body. Thus one of the most
important results of Maxwell's theory was to show that electro-magnetic
disturbances produced in the Aether by an electrified or magnetic body
might be propagated through space with a velocity equal to that of
light.

It was left, however, for Professor Hertz to place the electro-magnetic
theory of light upon a sure and certain foundation. The results are to
be found in his work on _Electric Waves_, translated by Professor Jones,
1893.

In his paper on "The Velocity of Propagation of Electro-dynamic Action,"
he gave definite and experimental proof of the hitherto theoretical fact
that the velocity of the electric waves in air was exactly the same as
that of light, whereas he found that in wires the ratio was not the
same, being 4 to 7. This was afterwards found out to be an error by some
experiments made at Geneva, when it was discovered that the propagation
in wires was the same as in the air. Among his experiments Hertz