"Thorium," he whispered. "Yes, that was it--thorium. The greatest of their improvisations! For when they devised the techniques for the fission of thorium, so infinitely more plentiful in supply than uranium, no combination of powers that lacked the formulae could longer challenge them. They wrote the peace that they wanted with those formulae. And that peace forbade atomic fission for the rest of the world. No nation dared touch it, no scientist anywhere, except in America, dared to experiment with it, knowing that the slightest radiation detectable by Washington's scintillometers might bring within the next half hour obliteration, complete obliteration. "And actually, that was the conquest of the world. Yet they chose instead of ruling their conquered world to erect around and above the Western Hemisphere that vast umbrella of radioactivity, the Atomic Curtain. And the two continents of North and South America were lost behind it as utterly as if they had been swallowed in the seas that girt their shores." "Even with their thorium, they must have been afraid," I said. O'Hara scoffed at me. "Of what? Maybe of themselves, maybe of their own power, but certainly of nothing else. They could have destroyed the other nations of the world, and the others were so well aware of it that they have not since risked the science of fission, still dreading the rain of warheads that might come screeching through the Atomic Curtain. Even today, although a century and a half has passed, we're still so obsessed with that dread that we maintain the International Patrol, ceaselessly flying the outer fringe of radiation in the stupid hope that if that rain of warheads comes, we shall have a warning of it. But what good would a warning do? A few minutes to pray, perhaps. For there's no defense and so it is a stupid, useless dread, made terrible because beyond the Curtain all is mystery to us, something that we no longer understand--" "And cannot help," I pointed out. But O'Hara did not hear me. "That's it," he cried. "The Mystery! The greatest of all powers, unknown to us, a terra incognita. I'm surprised that we, dreading them so much and knowing nothing of their mind, do not worship them, for dread and ignorance and mystery are the requisites for a god. A race of gods!" "They may be all of that," I said. "I rather like the notion that I am descended from a race of gods. Inflates my ego, which can bear a little of it. Can't I go to sleep now?" "Not even a race of gods could possibly keep you from that," O'Hara said, and he put out the light.