"I can pack the explosive in the projectile, all right," Dixon answered grimly. "We will need only a lump the size of an egg, and a small container of the heavy gas that activates it. The explosive itself is a radium compound that, when allowed to come in contact with the activating gas, becomes so unstable that any sharp blow will set it off in an explosion that in a matter of seconds releases the infinite quantities of energy usually released by radium over a period of at least twelve hundred years. The cataclysmic force of that explosion should be enough to wreck a small planet." "Good!" Crawford commended weakly. "If you can only strike your blow to-night, Bruce, our world still has a chance. If only you--" The old man's voice suddenly failed. He sank back in utter collapse, his eyes closed and his last vestige of strength spent. * * * * * Knowing that the old man would probably remain in his sleep of complete exhaustion for hours, Dixon turned his attention to Ruth. To his surprise, he found her sitting up, apparently completely recovered. "I'm quite all right again," she said reassuringly. "I've been listening to what Uncle told you. Go ahead and prepare your explosive, Bruce. I'll do what I can for Uncle while you're working." Dixon donned his lead-cloth hood and tunic again and set to work. Ten minutes later he turned to Ruth with a slender foot-long cylinder of lead in his hand. "Ruth, will this fit your Uncle's projectile?" he asked. "Easily," she assured him. "But isn't it frightfully dangerous to carry in that form?" "No, it's absolutely safe now, and will be safe until this stud is turned, releasing the activating gas from one compartment to mingle with the radium compound in the other section. Then the cylinder will become a bomb that any sharp jar will detonate." "All right, let's go then," Ruth answered. "Have you any more of those lead clothes that I can wear? I could wear the globe head-piece that Uncle wore, but it would loom up in the dark like a searchlight." Dixon did not protest Ruth's going with him. There was nothing further that could be done for Emil Crawford for hours and in the hazardous sally to Crawford's laboratory he knew that Ruth's cool courage and quick wits would at least double their chances for success in their desperate mission. He provided her with a reserve hood and tunic of lead cloth, then handed her a tiny leaden pellet. "Keep this for a last resort," he told her. "It's a contact bomb that becomes ready to throw when this safety catch is snapped over. I wish we had a dozen of them, but that's the last capsule I had and there's no time to prepare more." He fished a rusty old revolver out of a drawer, and placed it in his pocket. "I'll use this gun for a last resort weapon myself," he said. "The action only works about half the time, but it's the only firearm in the place." * * * * *