But occasionally, for reasons unknown, though possibly connected with electron bombardments from space, the dust becomes electrically charged and each particle repels its neighbors. Even without wind they would tend to lift upward. Each step would raise a cloud that would refuse to settle, but would drift and wisp out through the air.

When to this a wind is added, a fully developed dust storm might be said to exist. The dust is never thick enough to obscure vision; that isn't its danger. It is rather the pervasiveness of the dust that kills.

The dust particles are extremely fine and penetrate everywhere. Clothes cannot keep them out; the shelter of a rocky ledge means nothing; even the nosepiece with its broad gasket fitting against the face is helpless to prevent the individual particle from working its way through.

At the height of a storm two minutes would suffice to arouse an unbearable itching, five minutes would virtually blind a man, and fifteen minutes would kill him. Even a mild storm, so gentle that it may not even be noticed by the people exposed, is sufficient to redden exposed skin in what are called dust burns.

David Starr knew all this and more. He knew that his own skin was reddening. He was coughing without its having any effect on clearing his caking throat. He had tried clamping his mouth shut, blowing his breath out during exhalations through the smallest opening he could manage. It didn't help. The dust crept in, working its way past his lips. The scooter was jerking irregularly now as the dust did to its motor what it was doing to David.

His eyes were swollen nearly closed now. The tears that streamed out were accumulating against the gasket at the bottom of the nosepiece and were, fogging the eyepieces, through which he could see nothing anyway.

Nothing could stop those tiny dust particles but the elaborately machined seams of a dome or a sand-car. Nothing.

Nothing?

Through the maddening itch and the racking cough he was thinking desperately of the Martians. Would they have known that a dust storm was brewing? Could they have? Would they have sent him to the surface if they had known? From his mind they must have gleaned the information that he had only a scooter to carry him back to the dome. They might have as easily transported him to the surface just outside the farm dome, or, for that matter, even inside the dome.

They must have known conditions were right for a dust storm. He remembered how the being with the deep voice had been so abrupt in his decision to return David to the surface, as though he hurried in order that time might be allowed for David to be caught in the storm.

And yet the last words of the feminine voice, the words he had not consciously heard and which, therefore, he was certain had been inserted in his mind while he was being borne through rock to the surface, were: "Have no fear, Space Ranger."

Even as he thought all this he knew the answer. One hand was fumbling in his pocket, the other at his nosepiece. As the nosepiece lifted off, the partially protected nose and eyes received a fresh surge of dust, burning and irritating

He had the irresistible desire to sneeze, but fought it back. The involuntary intake of breath would fill his lungs with quantities of the dust. That in itself might be fatal.

But he was bringing up the strip of gauze he had taken from his pocket, letting it wrap about his eyes and nose, and then over it he slapped the nosepiece again.

Only then did he sneeze. It meant he drew in vast quantities of Mars's useless atmospheric gases, but no dust was coming. He followed that by force-breathing, gasping in as much oxygen as he could and puffing it out, flinging the dust of his mouth away; alternating that with deliberate inhalations through the mouth to prevent any oncoming of oxygen drunkenness.

Gradually, as the tears washed the dust out of his eyes and no new dust entered, he found he could see again. His limbs and body were obscured by the smokiness of the force-shield that surrounded him,. and he knew the upper part of his head to be invisible in the glow of his mask.

Air molecules could penetrate the shield freely, but, small though they were, the dust particles were large enough to be stopped. David could see the process with the naked eye. As each dust particle struck the shield, it was halted and the energy of its motion converted into light, so that at its point of attempted penetration a tiny sparkle showed. David found his body an ocean of such sparkles crowding one another, all the brighter as the Martian sun, red and smokily dim through the dust, allowed the ground below to remain in semi-darkness.

David slapped and brushed at his clothing. Dust clouds arose, too fine to see even if the cloudiness of the shield had not prevented sight in any case. The dust left but could not return. Gradually he became almost clear of the particles. He looked dubiously at the scooter and attempted to start its motor. He was rewarded only by a short, grating noise and then silence. It was to be expected. Unlike the sand-cars, scooters did not, could not, have enclosed motors.

He would have to walk. The thought was not a particularly frightening one. The farm dome was little more than two miles away and he had plenty of oxygen. His cylinders were full. The Martians had seen to that before sending him back.

He thought he understood them now. They did know the dust storm was coming. They might even have helped it along. It would be strange if, with their long experience with Martian weather and their advanced science, they had not learned the fundamental causes and mechanisms of dust storms. But in sending him out to face the storm, they knew he had the perfect defense in his pocket. They had not warned him of either the ordeal that awaited him or of the defense he carried. It made sense. If he were the man who deserved the gift of the force-shield, he would, or should, think of it himself. If he did not, he was the wrong man for the job.

David smiled grimly even as he winced at the touch of his clothing against inflamed skin as he stretched his legs across the Martian terrain. The Martians were coldly unemotional in risking his life, but he could almost sympathize with them. He had thought quickly enough to save himself, but he denied himself any pride in that. He should have thought of the mask much sooner.

The force-shield that surrounded him was making it easier to travel. He noted that the shield covered the soles of his boots so that they never made contact with the Martian surface but came to rest some quarter inch above it. The repulsion between himself and the planet was an elastic one, as though he were on many steel springs. That, combined with the low gravity, enabled him to devour the distance between himself and the dome in swinging giant strides.

He was in a hurry. More than anything else at the moment he felt the need of a hot bath.

By the time David reached one of the outer locks of the farm dome the worst of the storm was over and the light flashes on his force-shield had thinned to occasional sparks. It was safe to remove the mask from his eyes.

When the locks had opened for him, there were first of all stares, and then cries, as the farmboys on duty swarmed about him.

"Jumping Jupiter, it's Williams!"

"Where've you been, boy?"

"What happened?"

And above the confused cries and simultaneous questioning there came the shrill cry, "How did you get through the dust storm?"

The question penetrated, and there was a short silence.

Someone said, "Look at his face. It's like a peeled tomato."

That was an exaggeration, but there was enough truth to it to impress all who were there. Hands were yanking at his collar which had been tightly bound about his neck in the fight against the Martian cold. They shuffled him into a seat and put in a call for Hennes.

Hennes arrived in ten minutes, hopping off a scooter and approaching with a look that was compounded of annoyance and anger. There were no visible signs of any relief at the safe return of a man in his employ.

He barked, "What's this all about, Williams?"

David lifted his eyes and said coolly, "I was lost."

"Oh, is that what you call it? Gone for two days and you were just lost. How did you manage it?"

"I thought I'd take a walk and I walked too far."

"You thought you needed a breath of air, so you've been walking through two Martian nights? Do you expect me to believe that?"

"Are any sand-cars missing?"

One of the farmboys interposed hastily as Hennes reddened further. "He's knocked out, Mr. Hennes. He was out in the dust storm."

Hennes said, "Don't be a fool. If he were out in the dust storm, he wouldn't be sitting here alive."

"Well, I know," the farmboy said, "but look at Mm."

Hennes looked at him. The redness of his exposed neck and shoulders was a fact that could not be easily argued away.

He said, "Were you in the storm?"

"I'm afraid so," said David.

"How did you get through?"

"There was a man," said David. "A man in smoke and light. The dust didn't bother him. He called himself the Space Ranger."

The men were gathering close. Hennes turned on them furiously, his plump face working.

"Get the Space out of here!" he yelled. "Back to your work. And you, Jonnitel, get a sand-car out here."

It was nearly an hour before the hot bath he craved was allowed David. Hennes permitted no one else to approach him. Over and over again, as he paced the floor of his private office, he would stop in midstride, whirl in sudden fury, and demand of David, "What about this Space Ranger? Where did you meet him? What did he say? What did he do? What's this smoke and light you speak of?"

To all of wliich David would only shake his head slightly and say, "I took a walk. I got lost. A man calling himself the Space Ranger brought me back."

Hennes gave up eventually. The dome doctor took charge. David got his hot bath. His body was anointed with creams and injected with the proper hormones. He could not avoid the injection of Soporite as well. He was asleep almost before the needle was withdrawn.

He woke to find himself between clean, cool sheets in the sick bay. The reddening of the skin had subsided considerably. They would be at him again, he knew, but he would have to fight them off but a little while longer.

He was sure he had the answer to the food-poisoning mystery now; almost the whole answer. He needed only a missing piece or two, and, of course, legal proof.

He heard the light footstep beyond the head of his bed and stiffened slightly. Was it going to begin again so soon? But it was only Benson who moved into his line of vision. Benson, with his plump lips pursed, his thin hair in disarray, his whole face a picture of worry. He carried something that looked like an old-fashioned clumsy gun.

He said, "Williams, are you awake?"

David said, "You see I am."

Benson passed the back of his hand across a perspiring forehead. "They don't know I'm here. I shouldn't be, I suppose."

"Why not?"

"Hennes is convinced you're involved with this food poisoning. He's been raving to Makian and my- self about It. He claims you've been out somewhere and have nothing to say about it now other than ridiculous stories. Despite anything I can do, I'm afraid you're in terrible trouble."

"Despite anything you can do? You don't believe Hennes's theory about my complicity in all this?"

Benson leaned forward, and David could feel his breath warm on his face as he whispered, "No, I don't. I don't because I think your story is true. That's why I've come here. I must ask you about this creature you speak of, the one you claim was covered with smoke and light. Are you sure it wasn't a hallucination, Williams?"

"I saw him," said David.

"How do you know he was human? Did he speak English?"

"He didn't speak, but he was shaped like a human." David's eyes fastened upon Benson. "Do you think it was a Martian?"

"Ah"-Benson's lips drew back in a spasmodic smile-"you remember my theory. Yes, I think it was a Martian. Think, man, think! They're coming out in the open now and every piece of information may be vital. We have so little time."

"Why so little time?" David raised himself to one elbow.

"Of course you don't know what's happened since you've been gone, but frankly, Williams, we are all of us in despair now." He held up the gun-like affair in his hand and said bitterly, "Do you know what this is?"

"I've seen you with it before."

"It's my sampling harpoon; it's my own invention. I take it with me when I'm at the storage bins in the city. It shoots a little hollow pellet attached to it by a metal-mesh cord into a bin of, let us say, grain. At a certain time after shooting an opening appears in the front of the pellet long enough to allow the hollow within to become packed with grain. After that the pellet closes again. I drag it back and empty out the random sample it has accumulated. By varying the time after shooting in which the pellet opens, samples can be taken at various depths in the bin."

David said, "That's ingenious, but why are you carrying it now?"

"Because I'm wondering if I oughtn't to throw it into the disposal unit after I leave you. It was my only weapon for fighting the poisoners. It has done me no good so far, and can certainly do me no good In the future."

"What has happened?" David seized the other's shoulder and gripped it hard. "Tell me."

Benson winced at the pain. He said, "Every member of the farming syndicates has received a new letter from whoever is behind the poisoning. There's no doubt that the letters and the poisonings are caused by the same men, or rather, entities. The letters admit it now."

"What do they say?"

Benson shrugged. "What difference do the details make? What it amounts to is a demand for complete surrender on our part or the food-poisoning attacks will be multiplied a thousandfold. I believe it can and will be done, and if that happens, Earth and Mars, the whole system, in fact, will panic."

He rose to Ms feet. ''I've told Makian and Hennes that I believe you, that your Space Ranger is the clue to the whole thing, but they won't believe me. Hennes, 1 think, even suspects that I'm in it with you."

He seemed absorbed in his own wrongs. David said, "How long do we have Benson?"

"Two days. No, that was yesterday. We have thirty-six hours now."

Thirty-six hours!

David would have to work quickly. Very quickly. But maybe there would yet be time. Without knowing it Benson had given him the missing piece to the mystery.