THE FIRST MEN IN THE MOON
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,
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