Like all men who worked under the great Llewes, Edmund Farley reached the point where he thought with longing of the pleasure it would give him to kill that same great Llewes.

No man who did not work for Llewes would quite understand the feeling. Llewes (men forgot his first name or grew, almost unconsciously, to think it was Great, with a capital G) was Everyman's idea of the great prober into the unknown: both relentless' and brilliant, neither giving up in the face of failure nor ever at a loss for a new and more ingenious attack.

Llewes was an organic chemist who had brought the Solar System to the service of his science. It was he who first used the Moon for large-scale reactions to be run in vacuum, at the temperature of boiling water or liquid air, depending on the time of month. Photochemistry became something new and wonderful when carefully designed apparatus was set floating freely in orbits about space stations.

But, truth to tell, Llewes was a credit stealer, a sin almost impossible to forgive. Some nameless student had first thought of setting up apparatus on the Lunar surface; a forgotten technician had designed the first self-contained space reactor. Somehow both achievements became associated with the name of Llewes.

And nothing could be done. An employee who resigned in anger would lose his recommendation and find it difficult to obtain another job. His unsupported word against that of Llewes would be worth nothing.

On the other hand, those who remained with him, endured, and finally left with good grace and a recommendation were sure of future success.

But while they stayed, they at least enjoyed the dubious pleasure of voicing their hatred among themselves.

And Edmund Farley had full reason to join them. He had come from Titan, Saturn's largest satellite, where he had singlehanded-aided by robots only-set up equipment to make full use of Titan's reducing atmosphere. The major planets have atmospherscomposed largely of hydrogen and methane but Jupiter and Saturn were too large to deal with, and Uranus and Neptune were still too expensively far. Titan, however, was Mars-size, small enough to operate upon and large enough and cold enough to retain a medium-thin hydrogen-methane atmosphere.

Large-scale reactions could proceed there easily in the hydrogen atmosphere, where on Earth those same reactions were kinetically troublesome. Farley had designed and redesigned and endured Titan for half a year and had come back with amazing data. Yet somehow, almost at once, Farley could see it fragment and begin to come together as a Llewes achievement.

The others sympathized, shrugged their shoulders, and bade him welcome to the fraternity. Farley tensed his acne-scarred face, brought his thin lips together, and listened to the others as they plotted violence.

Jim Gorham was the most outspoken. Farley rather despised him, for he was a 'vacuum man' who had never left Earth.

Gorham said, 'Llewes is an easy man to kill because of his regular habits, you see. You can rely on him.

For instance, look at the way he insists on eating by himself. He closes his office at twelve sharp and opens it at one sharp. Right? No one goes into his office in that interval, so poison has plenty of time to work.'

Belinsky said dubiously, 'Poison?'

'Easy. Plenty of poison all over the place. You name it, we got it. Okay, then. Llewes eats one Swiss cheese on rye with a special kind of relish knee-deep in onions. We all know that, right? After all, we can smell him all afternoon and we all remember the miserable howl he raised when the lunchroom ran out of the relish once last spring. No one else in the place will ever touch the relish, so poison in it will hit only Llewes and no one else...'

It was all a kind of lunchtime make-believe, but not for Farley. Grimly, and in earnest, he decided to murder Llewes.

It became an obsession with him. His blood tingled at the thought of Llewes dead, of himself able to take the credit that was rightfully his for those months of living in a small bubble of oxygen and tramping across frozen ammonia to remove products and set up new reactions in the thin, chill winds of hydrogen and methane.

But it would have to be something which couldn't possibly harm anyone but Llewes. That sharpened the matter and focused things on Llewes' atmosphere room. It was a long, low room, isolated from the rest of the laboratories by cement blocks and fireproof doors. No one but Llewes ever entered, except in Llewes' presence and with his permission. Not that the room was ever actually locked. The effective tyranny Llewes had established made the faded slip of paper on the laboratory door, reading 'Do Not Enter' and signed with his initials, more of a barrier than any lock... except where the desire for murder superseded all else.

Then what about the atmosphere room? Llewes' routine of testing, his almost infinite caution, left nothing to chance. Any tampering with the equipment itself, unless it were unusually subtle, would certainly be detected.

Fire then? The atmosphere room contained inflammable materials and to spare, but Llewes didn't smoke and was perfectly aware of the danger of fires. No one took greater precautions against one.

Farley thought impatiently of the man on whom it seemed so difficult to wreak a just vengeance; the thief playing with his little tanks of methane and hydrogen where Farley had used it by the cubic mile. Llewes for the little tanks and fame; Farley for the cubic miles and oblivion.

All those little tanks of gas; each its own color; each a synthetic atmosphere. Hydrogen gas in red cylinders and methane in striped red and white, a mixture of the two representing the atmosphere of the outer planets. Nitrogen in brown cylinders and carbon dioxide in silver for the atmosphere of Venus. The yellow cylinders of compressed air and the green cylinders of oxygen, where Earthly chemistry was good enough. A parade of the rainbow, each color dating back through centuries of convention.

Then he had the thought. It was not born painfully, but came all at once. In one moment it had all crystallized in Farley's mind and he knew what he had to do.

Farley waited a painful month for September i8th, which was Space Day. It was the anniversary of man's first successful space flight and no one would be working that night. Space Day was, of all holidays, the

one most meaningful to the scientist in particular and even the dedicated Llewes would be making merry then.

Farley entered Central Organic Laboratories-to use its official title-that night, certain he was unobserved. The labs weren't banks or museums. They were not subject to thievery and such night watch men as there were had a generally easy-going attitude toward their jobs.

Farley closed the main door carefully behind him and moved slowly down the darkened corridors toward the atmosphere room. His equipment consisted of a flashlight, a small vial of black powder, and a thin brush he had bought in an art-supply store at the other end of town three weeks before. He wore gloves.

His greatest difficulty came in actually entering the atmosphere room. It's 'forbiddenness' hampered him more than the general forbiddenness of murder. Once in, however, once past the mental hazard, the rest was easy.

He cupped the flashlight and found the cylinder without hesitation. His heart was beating so as almost to deafen him, while his breath came quickly and his hand trembled.

He tucked the flash under his arm, then dipped the tip of the artist's brush into the black dust. Grains of it adhered to the brush and Farley pointed it into the nozzle of the gauge attached to the cylinder. It took eons-long seconds for that trembling tip to enter the nozzle.

Farley moved it about delicately, dipped it into the black dust again, and inserted it once more in the nozzle. He repeated it over and over, almost hypnotized by the intensity of his own concentration. Finally, using a bit of facial tissue dampened with saliva, he began to wipe off the outer rim of the nozzle, enormously relieved that the job was done and he'd soon be out.

It was then his hand froze, and the sick uncertainty of fear surged through him. The flashlight dropped clattering to the floor.

Fool! Incredible and miserable fool! He hadn't been thinking I Under the stress of his emotion and anxiety, he had ended at the wrong cylinder!

He snatched up the flash, put it out, and, his heart thumping alarmingly, listened for any noise.

In the continuing dead silence, he regained a portion of his self-control, and screwed himself to the realization that what could be done once could be done again. If the wrong cylinder had been tampered with, then the right one would take two minutes more.

Once again, the brush and the black dust came into play. At least, he had not dropped the vial of dust; the deadly, burning dust. This time, the cylinder was the right one.

He finished, wiping the nozzle again, with a badly trembling hand. His flash then played about quickly and rested upon a reagent bottle of toluene. That would do. He unscrewed the plastic cap, splashed some of the toluene on the floor, and left the bottle open.

He then stumbled out of the building as in a dream, made his way to his rooming house and the safety of his own room. As nearly as he could tell, he was unobserved throughout.

He disposed of the facial tissue he had used to wipe the nozzles of the gas cylinders by cramming it into the flash-disposal unit. It vanished into molecular dispersion. So did the artist's brush that followed.

The vial of dust could not be so gotten rid of without adjustments to the disposal unit he did not think it safe to make. He would walk to work, as he often did, and toss it off the Grand Street bridge...

Farley blinked at himself in the mirror the next morning and wondered if he dared go to work. It was an idle thought; he didn't dare not go to work. He must do nothing that would attract attention to him on this day of all days.

With grayish desperation, he worked to reproduce normal acts of nothingness that made up so much of the day. It was a fine, warm morning and he walked to work. It was only a flicking motion of the wrist that was necessary to get rid of the vial. It made a tiny splash in the river, filled with water, and sank.

He sat at his desk, later that morning, staring at his hand computer. Now that it had all been done, would it work? Llewes might ignore the smell of toluene. Why not? The odor was unpleasant, but not disgusting. Organic chemists were used to it.

Then, if Llewes were still hot on the trail of the hydro-genation procedures Farley had brought back from Titan, the gas cylinder would be put into use at once. It would have to be. With a day of holiday behind him, Llewes would be more than usually anxious to get back to work.

Then, as soon as the gauge cock was turned, a bit of gas would spurt out and turn into a sheet of flame.

If there were the proper quantity of toluene in the air, it would turn as quickly into an explosion-- So intent was Farley in his reverie that he accepted the dull boom in the distance as the creation of his own mind, a counter-point to his own thoughts, until footsteps thudded by.

Farley looked up, and out of a dry throat, cried, 'What- what-'

'Dunno,' yelled back the other. 'Something wrong in the atmosphere room. Explosion. Hell of a mess.'

The extinguishers were on and men beat out the flames and snatched the burned and battered Llewes out of the wreckage. He had the barest flicker of life left in him and died before a doctor had time to predict that he would.

On the outskirts of the group that hovered about the scene in grim and grisly curiosity stood Edmund Farley. His pallor and the glisten of perspiration on his face did not, at that moment, mark him as different from the rest. He tottered back to his desk. He could be sick now. No one would remark on it.

But somehow he wasn't. He finished out the day and in the evening the load began to lighten. Accident was accident, wasn't it? There were occupational risks all chemists ran, especially those working with inflammable compounds. No one would question the matter.

And if anyone did, how could they possibly trace anything back to Edmund Farley? He had only to go about his life as though nothing had happened.

Nothing? Good Lord, the credit for Titan would now be his. He would be a great man. The load lightened indeed and that night he slept.

* * *

Jim Gorham had faded a bit in twenty-four hours. His yellow hair was stringy and only the light color of his stubble masked the fact that he needed a shave badly.

'We all talked murder,' he said.

H. Seton Davenport of the Terrestrial Bureau of Investigation tapped one finger against the desktop methodically, and so lightly that it could not be heard. He was a stocky man with a firm face and black hair, a thin, prominent nose made for utility rather than beauty, and a star-shaped scar on one cheek.

'Seriously?' he asked.

'No,' said Gorham, shaking his head violently. 'At least, I didn't think it was serious. The schemes were wild: poisoned sandwich spreads and acid on the helicopter, you know. Still, someone must have taken the matter seriously after all... The madman! For what reason?'

Davenport said, 'From what you've said, I judge because the dead man appropriated other peoples' work.'

'So what,' cried Gorham. 'It was the price he charged for what he did. He held the entire team together.

He was its muscles and guts. Llewes was the one who dealt with Congress and got the grants. He.was the one who got permission to set up projects in space and send men to the Moon or wherever. He talked spaceship lines and industrialists into doing millions of dollars of work for us. He organized Central Organic.'

'Have you realized all this overnight?'

'Not really. I've always known this, but what could I do? I've chickened out of space travel, found excuses to avoid it.

I was a vacuum man, who never even visited the Moon. The truth was, I was afraid, and even more afraid to have the others think I was afraid.' He virtually spat self-contempt.

'And now you want to find someone to punish?' said Davenport. 'You want to make up to the dead

Llewes your crime against the live one?'

'No! Leave psychiatry out of this. I tell you it is murder. It's got to be. You didn't know Llewes. The man was a monomaniac on safety. No explosion could possibly have happened anywhere near him unless it were carefully arranged.'

Davenport shrugged. 'What exploded, Dr. Gorham?' It could have been almost anything. He handled organic compounds of all sorts-benzene, ether, pyridine-all of them inflammable.'

'I studied chemistry once, Dr. Gorham, and none of those liquids is explosive at room temperature as I remember. There has to be some sort of heat, a spark, a flame.' There was fire all right.'

'How did that happen?'

'I can't imagine. There were no burners in the place and no matches. Electrical equipment of all sorts was heavily shielded. Even little ordinary things like clamps were specially manufactured out of beryllium copper or other non-sparking alloys. Llewes didn't smoke and would have fired on the spot anyone who approached within a hundred feet of the room with a lighted cigarette.'

'What was the last thing he handled, then?'

'Hard to tell. The place was a shambles.'

'I suppose it has been straightened out by now, though.'

The chemist said with instant eagerness, 'No, it hasn't. I took care of that. I said we had to investigate the cause of the accident to prove it wasn't neglect. You know, to avoid bad publicity. So the room hasn't been touched.'

Davenport nodded. 'All right. Let's take a look at it.'

In the blackened, disheveled room, Davenport said, 'What's the most dangerous piece of equipment in the place?'

Gorham looked about. The compressed oxygen tanks,' he said, pointing.

Davenport looked at the variously colored cylinders standing against the wall cradled in a binding chain. Some leaned heavily against the chain, tipped by the force of the explosion.

Davenport said, 'How about this one?' He toed a red cylinder which lay flat on the ground in the middle of the room. It was heavy and didn't budge. That one's hydrogen,' said Gorham.

'Hydrogen is explosive, isn't it?' That's right-when heated.'

Davenport said, Then why do you say the compressed oxygen is the most dangerous. Oxygen doesn't explode, does it?'

'No. It doesn't even burn, but it supports combustion, see. Things burn in it.'

'So?'

'Well, look here.' A certain vivacity entered Gorham's voice; he was the scientist explaining something simple to the intelligent layman. 'Sometimes a person might accidentally put some lubricant on the valve before tightening it onto the cylinder, to make a tighter seal, you know. Or he might get something inflammable smeared on it by mistake. When he opens the valve then, the oxygen rushes out, and whatever goo is on the valve explodes, wrenching off the valve. Then the rest of the oxygen blows out of the cylinder, which would then take off like a miniature jet and go through a wall; the heat of the explosion would fire other inflammable liquids nearby.'

'Are the oxygen tanks in this place intact?'

'Yes, they are.'

Davenport kicked the hydrogen cylinder at his feet. The gauge on this cylinder reads zero. I suppose that means it was in use at the time of explosion and has emptied itself since then.'

Gorham nodded. 'I suppose so.'

'Could you explode hydrogen by smearing oil on the gauge?'

'Definitely not.'

Davenport rubbed his chin. 'Is there anything that would make hydrogen burst into flame outside of a spark of some sort?'

Gorham muttered, 'A catalyst, I suppose. Platinum black is the best. That's powdered platinum.' Davenport looked astonished. 'Do you have such a thing?'

'Of course. It's expensive, but there's nothing better for catalyzing hydrogenations.' He fell silent and stared down at the hydrogen cylinder for a long moment. 'Platinum black,' he finally whispered. 'I wonder-'

Davenport said, 'Platinum black would make hydrogen burn, then?'

'Oh, yes. It brings about the combination of hydrogen and oxygen at room temperature. No heat necessary. The explosion would be just as though it were caused by heat, just the same.'

Excitement was building upin Gorham's voice and he fell to his knees beside the hydrogen cylinder. He passed his finger over the blackened tip. It might be just soot and it might be-'

He got to his feet, 'Sir, that must be the way it was done. I'm going to get every speck of foreign material off that nozzle and run a spectrographic analysis.'

'How long will it take?'

'Give me fifteen minutes.'

Gorham came back in twenty. Davenport had made a meticulous round of the burned-out laboratory. He looked up. 'Well?'

Gorham said triumphantly, 'It's there. Not much, but there.'

He held up a strip of photographic negative against which there were short white parallel lines, irregularly spaced and of different degrees of brightness. 'Mostly extraneous material, but you see those lines...'

Davenport peered closely. 'Very faint. Would you swear in court that platinum was present?'

'Yes,' said Gorham at once.

'Would any other chemist? If this photo were shown a chemist hired by the defense, could he claim the lines were too faint to be certain evidence?'

Gorham was silent.

Davenport shrugged.

The chemist cried, 'But it is there. The stream of gas and the explosion would have blown most of it out. You wouldn't expect much to be left. You see, that, don't you?'

Davenport looked about thoughtfully. 'I do. I admit there's a reasonable chance this is murder. So now we look for more and better evidence. Do you suppose this is the only cylinder that might have been tampered with?'

'I don't know.'

Then the first thing we do is check every other cylinder in the place. Everything else, too. If there is a murderer, he might conceivably have set other booby traps in the place. It's got to be checked.'

'I'll get started-' began Gorham eagerly.

'Uh-not you,' said Davenport. 'I'll have a man from our labs do it.'

The next morning, Gorham was in Davenport's office again. This time he had been summoned. Davenport said, 'It's murder, all right. A second cylinder had been tampered with.'

'You see!'

'An oxygen cylinder. There was platinum black inside the tip of the nozzle. Quite a bit of it.'

'Platinum black? On the oxygen cylinder?'

Davenport nodded. 'Right. Now why do you suppose that would be?'

Gorham shook his head. 'Oxygen won't burn and nothing will make it burn. Not even platinum black.'

'So the murderer must have put it on the oxygen cylinder by mistake in the tension of the moment. Presumably he corrected himself and tampered with the right cylinder, but meanwhile he left final evidence that this is murder and not accident.'

'Yes, Now it's only a matter of finding the person.'

The scar on Davenport's cheek crinkled alarmingly as he smiled. 'Only, Dr. Gorham? How do we do that? Our quarry left no calling card. There are a number of people in the laboratories with motive; a greater number with the chemical knowledge required to commit the crime and with the opportunity to do so. Is there any way we can trace the platinum black?'

'No,' said Gorham hesitantly. 'Any of twenty people could have gotten into the special supply room without trouble. What about alibis?'

'For what time?'

'For the night before.'

Davenport leaned across his desk. 'When was the last time, previous to the fatal moment, that Dr. Llewes used that hydrogen cylinder?'

'I-I don't know. He worked alone. Very secretly It was part of his way of making sure he had sole credit.'

'Yes, I know. We've been making our own inquiries. So the platinum black might have been put on the cylinder a week before for all we know.'

Gorham whispered disconsolately, Then what do we do?'

Davenport said, The only point of attack, it seems to me, is the platinum black on the oxygen cylinder.

It's an irrational point and the explanation may hold the solution. But I'm no chemist and you are, so if the answer is anywhere it's inside you. Could it have been a mistake-could the murderer have confused the oxygen with the hydrogen?'

Gorham shook his head at once. 'No. You know about the colors. A green tank is oxygen; a red tank is hydrogen.'

'What if he were color blind?' asked Davenport.

This time Gorham took more time. Finally he said, 'No. Color-blind people don't generally go in for chemistry. Detection of color in chemical reactions is too important. And if anybody in this organization were color blind, he'd have enough trouble with one thing or another so that the rest of us would know about it.'

Davenport nodded. He fingered the scar on his cheek absently, 'All right. If the oxygen cylinder wasn't smeared by ignorance or accident, could it have been done on purpose? Deliberately?'

'I don't understand you.'

'Perhaps the murderer had a logical plan in mind when he smeared the oxygen cylinder, then changed his mind. Are there any conditions where platinum black would be dangerous in the presence of oxygen?

Any conditions at all? You're the chemist, Dr. Gorham.'

There was a puzzled frown on the chemist's face. He shook his head. 'No, none. There can't be. Unless-'

'Unless?'

'Well, this is ridiculous, but if you stuck the oxygen jet into a container of hydrogen gas, platinum black on the gas cylinder could be dangerous. Naturally you'd need a big container to make a satisfactory explosion.'

'Suppose,' said Davenport, 'our murderer had counted on filling the room with hydrogen and then having the oxygen tank turned on.'

Gorham said, with a half-smile, 'But why bother with the hydrogen atmosphere when-' The half-smile vanished completely while a complete pallor took its place. He cried, 'Farley! Edmund Farley!'

'What's that?'

'Farley just returned from six months on Titan,' said Gorham in gathering excitement. Titan has a hydrogen-methane atmosphere. He is the only man here to have had experience in such an atmosphere, and it all makes sense now. On Titan a jet of oxygen will combine with the surrounding hydrogen if heated, or treated with platinum black. A jet of hydrogen won't. The situation is exactly the reverse of what it is here on Earth. It must have been Farley. When he entered Llewes' lab to arrange an explosion, he put the platinum black on the oxygen, out of recent habit. By the time he recalled that the situation was the other way round on Earth, the damage was done.'

Davenport nodded in grim satisfaction. That does it, I think.' His hand reached out to an intercom and he said to the unseen recipient at the other end, 'Send out a man to pick up Dr. Edmund Farley at Central Organic.'