Once again, there is the paradoxical fact that this outcome of
Maxwell's labours contradicts the very foundation on which he had built
his theoretical edifice. For his starting-point had been to form a
picture of the electro-magnetic field of force to which he could apply
certain well-known formulae of mechanics. This he did by comparing the
behaviour of the electrical force to the currents of an elastic fluid -
that is, of a material substance. It is true that both he and his
successors rightly emphasized that such a picture was not in any way
meant as an explanation of electricity, but merely as an auxiliary
concept in the form of a purely external analogy. Nevertheless, it was
in the guise of a material fluid that he thought of this force, and
that he could submit it to mathematical calculation. Yet the fact is
that from this starting-point the strict logic of mathematics led him
to the discovery that electricity is capable of behaviour which makes
it appear qualitatively similar to ... light!

Whilst practical men were turning the work of Faraday and Maxwell to
account by exploiting the mechanical working of electricity in
power-production, and its similarity to light in the wireless
communication of thought, a new field of research, with entirely new
practical possibilities, was suddenly opened up in the last third of
the nineteenth century through the discovery of how electricity behaves
in rarefied air. This brings us to the discovery of cathode rays and
the phenomena accompanying them, from which the latest stage in the
history of electricity originated. And here once more, as in the
history of Galvani's discoveries, we encounter certain undercurrents of
longing and expectation in the human soul which seemed to find an
answer through this sudden, great advance in the knowledge of
electricity - an advance which has again led to practical applications