And now, as to the nature of these inquiries. Here, unhappily, comes a
grave difficulty. I am no scientific expert, and if I were to attempt to
set forth in the highly scientific language of Mr. Cavor the aim to which
his experiments tended, I am afraid I should confuse not only the reader
but myself, and almost certainly I should make some blunder that would
bring upon me the mockery of every up-to-date student of mathematical
physics in the country. The best thing I can do therefore is, I think to
give my impressions in my own inexact language, without any attempt to
wear a garment of knowledge to which I have no claim.

The object of Mr. Cavor's search was a substance that should be
"opaque"--he used some other word I have forgotten, but "opaque" conveys
the idea--to "all forms of radiant energy." "Radiant energy," he made me
understand, was anything like light or heat, or those Rontgen Rays there
was so much talk about a year or so ago, or the electric waves of Marconi,
or gravitation. 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