Roger Kay nodded. "That much I know. I've examined one of the projectors. We've installed them at all the outposts. They're all ready, except--" "Except for the catalyst. The part of the discovery that's lost in the chemical compound that produces the catalytic gas. The ultra-sonic waves, passing through the gas, change their vibration in some way." "I see now," said Kay, "why it is directional. The ultra-sonic waves go in all directions, of course, but only those passing through the gas are disruptive. Right?" The girl nodded her beautiful blond head. "It's all very simple, and it's all in the hands of the government, except for the formula for that catalyst. Fortunately my father has a reputation as a scientist. That's why the government was willing to take a chance on having those projectors set up, even though--" Roger Kay smiled wryly. "Your father is the outstanding scientist of the System, Miss North. But even if he wasn't, we might have taken that chance. It's about the only chance. If he fails, three days from today--" "As bad as that?" "I'm afraid so. But let's not talk about it. One thing I don't know: How was the formula lost?" "Dad destroyed it. He discovered it accidentally twenty years ago, while working on something else. Never thinking that the fate of worlds might hinge upon it, he destroyed his notes almost as soon as he had made them. He's always been awfully opposed to war, you know, and he saw the terrible possibilities in the weapon if it should fall into the wrong hands." "That is still true," said a quiet voice from the doorway. Roger Kay recognized Corvo North at once from the many photographs he had seen. He rose and offered his hand. "I'm glad you're here, Mr. Kay," said the scientist. "Ann told me you were coming. Yes, it's still true that I'm opposed to war--but this isn't war. Even disregarding personal interests and patriotism, it's an attempt to save the human race. Come on into the laboratory. We've no time to waste." * * * * * Roger whistled softly under his breath as Corvo North closed the door behind them. The laboratory, spacious and well equipped, was a research worker's dream. The scientist led the way past rows of pieces of apparatus whose purpose Roger could but dimly guess, to a table at the far end of the room. Upon the table was a small box bristling with dials. The back and top were open, showing a maze of wires and coils and condensers. "Looks like a radio set with hydrophobia," Roger observed. "What connection has this with the catalyst formula?" "Nothing, directly. There's no chance, through experimentation, of my recovering that formula in time. Three years, possibly. Three days, never." "You mean that it's hopeless to try? That the System is lost?" Roger Kay was appalled.