The chief nodded and bounded off with his subordinates. In this gravity, one man could carry a good-sized tank.

Vesey beat a fist into his palm. There was agony on his face.

Catherine laid a hand on his arm. "You've no choice," she said gently. "Ammonia is rough stuff, but it would be worse if children started getting trampled."

Gilchrist, leaning against the wall, straightened. It was as if a bolt had snapped home within him. His shout hurt their eardrums.

"Ammonia!"

"Yes," said Vesey dully. "What about it?" Breath smoked from his mouth, and his skin was rough with gooseflesh.

"I—I—I—It's your ... y-y-your answer!"

They had set up a heater in his laboratory so he could work, but the test was quickly made. Gilchrist turned from his apparatus and nodded, grinning with victory. "That settles the matter. This sample from the pile room proves it. The air down there is about half ammonia."

Vesey looked red-eyed at him. There hadn't been much harm done in the riot, but there had been a bad few minutes. "How's it work?" he asked. "I'm no chemist."

Alemán opened his mouth, then bowed grandly. "You tell him, Thomas. It is your moment."

Gilchrist took out a cigaret. He would have liked to make a cavalier performance of it, with Catherine watching, but his chilled fingers were clumsy and he dropped the little cylinder. She laughed and picked it up for him.

"Simple," he said. With technicalities to discuss, he could speak well enough, even when his eyes kept straying to the girl. "What we have down there is a Haber process chamber. It's a method for manufacturing ammonia out of nitrogen and hydrogen—obsolete now, but still of interest to physical chemists like myself.

"I haven't tested this sample for nitrogen yet, but there's got to be some, because ammonia is NH₃. Obviously, there's a vein of solid nitrogen down under the Hill. As the heat from the pile room penetrated downward, this slowly warmed up. Some of it turned gaseous, generating terrific pressure; and finally that pressure forced the gas up into the pile room.

"Now, when you have a nitrogen-hydrogen mixture at 500 degrees and 600 atmospheres, in the presence of a suitable catalyst, you get about a 45 percent yield of ammonia—"

"You looked that up," said Catherine accusingly.

He chuckled. "My dear girl," he said, "there are two ways to know a thing: you can know it, or you can know where to look it up. I prefer the latter." After a moment: "Naturally, this combination decreases the total volume of gas; so the pump has to pull in more hydrogen from outside to satisfy its barystat, and more nitrogen is welling from below all the time. We've been operating quite an efficient little ammonia factory down there, though it should reach equilibrium as to pressure and yield pretty soon.

"The Haber process catalyst, incidentally, is spongy iron with certain promoters—potassium and aluminum oxides are excellent ones. In other words, it so happened that the Hill is a natural Haber catalyst, which is why we've had this trouble."

"And I suppose the reaction is endothermic and absorbs heat?" asked Catherine.

"No ... as a matter of fact, it's exothermic, which is why the pile is actually a little hotter than usual, and that in spite of having to warm up all that outside air. But ammonia does have a considerably higher specific heat than hydrogen. So, while the gas in our pipes has the same caloric content, it has a lower temperature."

"Ummm—" Vesey rubbed his chin. "And the radiation?"

"Nitrogen plus neutrons gives carbon-14, a beta emitter."

"All right," said Catherine. "Now tell us how to repair the situation."

Her tone was light—after all, the answer was obvious—but it didn't escape Gilchrist that she had asked him to speak. Or was he thinking wishfully?

"We turn off the pile, empty the pipes, and go into the room in spacesuits," he said. "Probably the simplest thing would be to drill an outlet for the nitrogen vein and drop a thermite bomb down there ... that should flush it out in a hurry. Or maybe we can lay an impermeable floor. In any event, it shouldn't take more than a few days, which the batteries will see us through. Then we can go back to operation as usual."

Vesey nodded. "I'll put Jahangir on it right away." He stood up and extended his hand. "As for you, Dr. Gilchrist, you've saved all our lives and—"

"Shucks." His cheeks felt hot. "It was my own neck too."

Before his self-confidence could evaporate, he turned to Catherine. "Since we can't get back to work for a few days, how about going down to the bar for a drink? I believe it'll soon be functioning again. And, uh, there'll doubtless be a dance to celebrate later—"

"I didn't know you could dance," she said.

"I can't," he blurted.

They went out together. It is not merely inorganic reactions which require a catalyst.