Powell’s visiplate had turned toward him in a silent question.

“You see,” came the cautious explanation, “all we need to do to drive him out of his rut is to increase the concentration of carbon monoxide in his vicinity. Well, back at the Station there’s a complete analytical laboratory.”

“Naturally,” assented Powell. “It’s a Mining Station.”

“All right. There must be pounds of oxalic acid for calcium precipitations.”

“Holy space! Mike, you’re a genius.”

“So-so,” admitted Donovan, modestly. “It’s just a case of remembering that oxalic acid on heating decomposes into carbon dioxide, water, and good old carbon monoxide. College chem, you know.”

Powell was on his feet and had attracted the attention of one of the monster robots by the simple expedient of pounding the machine’s thigh.

“Hey,” he shouted, “can you throw?”

“Master?”

“Never mind.” Powell damned the robot’s molasses-slow brain. He scrabbled up a jagged brick-size rock. “Take this,” he said, “and hit the patch of bluish crystals just across the crooked fissure. You see it?”

Donovan pulled at his shoulder. “Too far, Greg. It’s almost half a mile off.”

“Quiet,” replied Powell. “It’s a case of Mercurian gravity and a steel throwing arm. Watch, will you?”

The robot’s eyes were measuring the distance with machinely accurate stereoscopy. His arm adjusted itself to the weight of the missile and drew back. In the darkness, the robot’s motions went unseen, but there was a sudden thumping sound as he shifted his weight, and seconds later the rock flew blackly into the sunlight. There was no air resistance to slow it down, nor wind to turn it aside—and when it hit the ground it threw up crystals precisely in the center of the “blue patch.”

Powell yelled happily and shouted, “Let’s go back after the oxalic acid, Mike.”

And as they plunged into the ruined substation on the way back to the tunnels, Donovan said grimly: “Speedy’s been hanging about on this side of the selenium pool, ever since we chased after him. Did you see him?”

“Yes.”

“I guess he wants to play games. Well, we’ll play him games!”

They were back hours later, with three-liter jars of the white chemical and a pair of long faces. The photo-cell banks were deteriorating more rapidly than had seemed likely. The two steered their robots into the sunlight and toward the waiting Speedy in silence and with grim purpose.

Speedy galloped slowly toward them. “Here we are again. Whee! I’ve made a little list, the piano organist; all people who eat peppermint and puff it in your face.”

“We’ll puff something in your face,” muttered Donovan. “He’s limping, Greg.”

“I noticed that,” came the low, worried response. “The monoxide’ll get him yet, if we don’t hurry.”

They were approaching cautiously now, almost sidling, to refrain from setting off the thoroughly irrational robot. Powell was too far off to tell, of course, but already even he could have sworn the crack-brained Speedy was setting himself for a spring.

“Let her go,” he gasped. “Count three! One—two—”