matter fills all space, or at least all space to which gravitation
extends; for gravitation is a property of matter dependent on a
certain force, and it is this force which constitutes the matter.
In that view matter is not merely mutually penetrable;[1] but each
atom extends, so to say, throughout the whole of the solar system,
yet always retaining its own centre of force.'

It is the operation of a mind filled with thoughts of this profound,
strange, and subtle character that we have to take into account in
dealing with Faraday's later researches. A similar cast of thought
pervades a letter addressed by Faraday to Mr. Richard Phillips,
and published in the 'Philosophical Magazine' for May, 1846. It is
entitled 'Thoughts on Ray-vibrations,' and it contains one of the
most singular speculations that ever emanated from a scientific
mind. It must be remembered here, that though Faraday lived amid
such speculations he did not rate them highly, and that he was
prepared at any moment to change them or let them go. They spurred
him on, but they did not hamper him. His theoretic notions were
fluent; and when minds less plastic than his own attempted to render
those fluxional images rigid, he rebelled. He warns Phillips
moreover, that from first to last, 'he merely threw out as matter
for speculation the vague impressions of his mind; for he gave
nothing as the result of sufficient consideration, or as the settled
conviction, or even probable conclusion at which he had arrived.'

The gist of this communication is that gravitating force acts in
lines across space, and that the vibrations of light and radiant
heat consist in the tremors of these lines of force. 'This notion,'
he says, 'as far as it is admitted, will dispense with the ether,
which, in another view is supposed to be the medium in which these