energy is
supposed to be partly potential and partly kinetic, and our theory
agrees with the undulatory theory in assuming the existence of a medium
capable of becoming a receptacle for two forms of energy.

Now, the properties of bodies are capable of quantitative measurement.
We therefore obtain the numerical value of some property of the
medium--such as the velocity with which a disturbance is propagated in
it, which can be calculated from experiments, and also observed directly
in the case of light. If it be found that the velocity of propagation of
electro-magnetic disturbance is the same as the velocity of light, we
have strong reasons for believing that light is an electro-magnetic
phenomenon.

It is, in fact, found that the velocity of light and the velocity of
propagation of electro-magnetic disturbance are quantities of the same
order of magnitude. Neither of them can be said to have been determined
accurately enough to say that one is greater than the other. In the
meantime, our theory asserts that the quantities are equal, and assigns
a physical reason for this equality, and it is not contradicted by the
comparison of the results, such as they are.

Lorenz has deduced from Kirchoff's equations of electric currents a new
set of equations, indicating that the distribution of force in the
electro-magnetic field may be considered as arising from the mutual
action of contiguous elements, and that waves, consisting of transverse
electric currents, may be propagated, with a velocity comparable with
that of light, in non-conducting media. These conclusions are similar to
my own, though obtained by an entirely different method.

The most important step in establishing a relation between electric and
magnetic phenomena and those of light must be the discovery of some
instance in which one set of phenomena is affected by the other. Faraday
succeeded in establishing such a relation, and the experiments by which
he did so are described in the nineteen series of his "Experimental
Researches." Suffice it to state here that he showed that in the case of
aray of plane-polarised light the effect of the magnetic force is to
turn the plane of polarisation round the direction of the ray as an
axis, through a certain angle.

The action of magnetism on polarised light leads to the conclusion that
in a medium under the action of a magnetic force, something belonging to
the same mathematical class as an angular velocity, whose