The trouble was that he wasn't working with any natural phenomenon, but with life. He grimaced at the twist of his logic, but the sense remained. Something came into the back of his mind from a phrase in Hardwick's notes. It teased him, until his mind almost had it, and then another taboo clamped down on his thoughts. He fought it out, standing still while Pat stared at him doubtfully. Twice he could feel himself almost black out, but he tracked the taboo down in his mind, pursued it into its lair, and strangled it. It died hard, but left his answer available. "K-40," he said. His voice was steady, and Pat relaxed, unable to see the complete fatigue inside him. Disciplining himself seemed to be the hardest possible task. "Radio-active potassium isotope. It's supposed to be mixed up somehow with the life processes. Some scientists claim it's essential to life." She reached for the phone, and spoke into it briefly. Then there was a wait, before she handed it over, and a voice came on. "This is General Dawes at Oak Ridge. Who's calling?" "William Norden, Project A-sub-zero, Moon Base. I want five pounds of K-40 up here in four hours. Use my top priority and mark the shipment for delivery to me only." There was the usual few seconds of waiting while the message traveled to Earth, and back. "Five _pounds_?" the voice asked, incredulously. "Five pounds! And I may want more as fast as it can be gathered." Pat was on another phone. Before Earth could answer again, there was a click, and General Miles' voice broke in sharply. "This is Miles, Dawes. _Give Norden what he wants._" A sputter of protest began, but it ended abruptly as Miles' voice reached Earth. The silence was broken by a sigh. "Okay, Norden, we'll get it to you in four hours somehow." It arrived in less time, and Norden and Pat began the tricky job of getting the highly active elements into a container which was both chemically and radio-actively safe. They clamped it over the cage at last, and watched the pressure on the female cages. The results weren't spectacular, but they were unquestionable. And later, when they had reduced the amount of K-40 to a thin coating, it still worked. The quantity of potassium made very little difference beyond a certain minimum. The effects still weren't good enough. They tried painting various substances with the chloride of the potassium, with equally good results and much greater ease of handling. The nitrate was even better to work with. But it took them until late that night before they learned that coating the nitrate over cleaned iron was a major step forward. Until then it had been all hit and miss, except for vague directional hunches. Norden looked at Pat, who seemed ready to drop. "Better go back for some rest," he suggested. She shook her head, but did agree to lie down while he began re-checking their results to date. Their best efforts had quieted the excitement of the females by no more than ten percent.