All these things, he said, _radiate_ out from centres, and
act on bodies at a distance, whence comes the term "radiant energy." Now
almost all substances are opaque to some form or other of radiant energy.
Glass, for example, is transparent to light, but much less so to heat, so
that it is useful as a fire-screen; and alum is transparent to light, but
blocks heat completely. A solution of iodine in carbon bisulphide, on the
other hand, completely blocks light, but is quite transparent to heat. It
will hide a fire from you, but permit all its warmth to reach you. Metals
are not only opaque to light and heat, but also to electrical energy,
which passes through both iodine solution and glass almost as though they
were not interposed. And so on.

Now all known substances are "transparent" to gravitation. You can use
screens of various sorts to cut off the light or heat, or electrical
influence of the sun, or the warmth of the earth from anything; you can
screen things by sheets of metal from Marconi's rays, but nothing will cut
off the gravitational attraction of the sun or the gravitational
attraction of the earth. Yet why there should be nothing is hard to say.
Cavor did not see why such a substance should not exist, and certainly I
could not tell him. I had never thought of such a possibility before. He
showed me by calculations on paper, which Lord Kelvin, no doubt, or
Professor Lodge, or Professor Karl Pearson, or any of those great
scientific people might have understood, but which simply reduced me to a
hopeless muddle, that not only was such a substance possible, but that it
must satisfy certain conditions. It was an amazing piece of reasoning.
Much as it amazed and exercised me at the time, it would be impossible to
reproduce it here. "Yes," I said to it all, "yes; go on!" Suffice it for
this story that he believed he might be able to manufacture this possible
substance opaque to gravitation out of a complicated alloy of metals and
something new--a new element, I fancy--called, I believe, _helium_, which
was sent to him from London in sealed stone jars. Doubt has been thrown
upon this detail, but I am almost certain it was _helium_ he had sent him
in sealed stone jars. It was certainly something very gaseous and thin.
If only I had taken notes...

But then, how was I to foresee the necessity of taking notes?

Any one with the merest germ of an imagination will understand the
extraordinary possibilities of such a