“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 photocell 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 even already he could have sworn the crack-brained Speedy was setting himself for a spring.

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

Two steel arms drew back and snapped forward simultaneously and two glass jars whirled forward in towering parallel arcs, gleaming like diamonds in the impossible sun. And in a pair of soundless puffs, they hit the ground behind Speedy in crashes that sent the oxalic acid flying like dust.

In the full heat of Mercury’s sun, Powell knew it was fizzing like soda water.

Speedy turned to stare, then backed away from it slowly – and as slowly gathered speed. In fifteen seconds, he was leaping directly toward the two humans in an unsteady canter.

Powell did not get Speedy’s words just then, though he heard something that resembled, “Lover’s professions when uttered in Hessians.”

He turned away. “Back to the cliff, Mike. He’s out of the rut and he’ll be taking orders now. I’m getting hot.”

They jogged toward the shadow at the slow monotonous pace of their mounts, and it was not until they had entered it and felt the sudden coolness settle softly about them that Donovan looked back. “Greg!”

Powell looked and almost shrieked. Speedy was moving slowly now – so slowly – and in the wrong direction. He was drifting; drifting back into his rut; and he was picking up speed. He looked dreadfully close, and dreadfully unreachable, in the binoculars.

Donovan shouted wildly, “After him!” and thumped his robot into its pace, but Powell called him back.

“You won’t catch him, Mike – it’s no use.” He fidgeted on his robot’s shoulders and clenched his fist in tight impotence. “Why the devil do I see these things five seconds after it’s all over? Mike, we’ve wasted hours.”

“We need more oxalic acid,” declared Donovan, stolidly. “The concentration wasn’t high enough.”

“Seven tons of it wouldn’t have been enough – and we haven’t the hours to spare to get it, even if it were, with the monoxide chewing him away. Don’t you see what it is, Mike?”

And Donovan said flatly, “No.”

“We were only establishing new equilibriums. When we create new monoxide and increase Rule 3 potential, he moves backward till he’s in balance again – and when the monoxide drifted away, he moved forward, and again there was balance.”

Powell’s voice sounded thoroughly wretched. “It’s the same old runaround. We can push at Rule 2 and pull at Rule 3 and we can’t get anywhere – we can only change the position of balance. We’ve got to get outside both rules.” And then he pushed his robot closer to Donovan’s so that they were sitting face-to-face, dim shadows in the darkness, and he whispered, “Mike!”

“Is it the finish?” – dully. “I suppose we go back to the Station, wait for the banks to fold, shake hands, take cyanide, and go out like gentlemen.” He laughed shortly.

“Mike,” repeated Powell earnestly, “we’ve got to get Speedy.”

“I know.”

“Mike,” once more, and Powell hesitated before continuing. “There’s always Rule 1. I thought of it – earlier – but it’s desperate.”

Donovan looked up and his voice livened. “We’re desperate.”

“All right. According to Rule 1, a robot can’t see a human come to harm because of his own inaction. Two and 3 can’t stand against it. They can’t, Mike.”

“Even when the robot is half cra- Well, he’s drunk. You know he is.”

“It’s the chances you take.”

“Cut it. What are you going to do?”

“I’m going out there now and see what Rule 1 will do. If it won’t break the balance, then what the devil – it’s either now or three-four days from now.”

“Hold on, Greg. There are human rules of behavior, too. You don’t go out there just like that. Figure out a lottery, and give me my chance.”

“All right. First to get the cube of fourteen goes.” And almost immediately, “Twenty-seven forty-four!”

Donovan felt his robot stagger at a sudden push by Powell’s mount and then Powell was off into the sunlight. Donovan opened his mouth to shout, and then clicked it shut. Of course, the damn fool had worked out the cube of fourteen in advance, and on purpose. Just like him.

The sun was hotter than ever and Powell felt a maddening itch in the small of his back. Imagination, probably, or perhaps hard radiation beginning to tell even through the insosuit.

Speedy was watching him, without a word of Gilbert and Sullivan gibberish as greeting. Thank God for that! But he daren’t get too close.

He was three hundred yards away when Speedy began backing, a step at a time, cautiously – and Powell stopped. He jumped from his robot’s shoulders and landed on the crystalline ground with a light thump and a flying of jagged fragments.

He proceeded on foot, the ground gritty and slippery to his steps, the low gravity causing him difficulty. The soles of his feet tickled with warmth. He cast one glance over his shoulder at the blackness of the cliff’s shadow and realized that he had come too far to return – either by himself or by the help of his antique robot. It was Speedy or nothing now, and the knowledge of that constricted his chest.

Far enough! He stopped.

“Speedy,” he called. “Speedy!”

The sleek, modern robot ahead of him hesitated and halted his backward steps, then resumed them.

Powell tried to put a note of pleading into his voice, and found it didn’t take much acting. “Speedy, I’ve got to get back to the shadow or the sun’ll get me. It’s life or death, Speedy. I need you.”