'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.'

A Loint of Paw.

There was no question that Montie Stein had, through clever fraud, stolen better than $100,000. There was also no question that he was apprehended one day after the statute of limitations had expired.

It was his manner of avoiding arrest during that interval that brought on the epoch-making case of the State of New Yorkvs. Montgomery Harlow Stein, with all its consequences introduced law to the fourth dimension.

For you see after having committed the fraud and possessed himself of the hundred grand plus, Stein had calmly entered a time machine, of which he was in illegal possession, and set the controls for seven years and one day in the future.

Stein's lawyer put it simply. Hiding in time was not fundamentally different from hiding in space. If the forces of law had not uncovered Stein in the seven-year interval that was their hard luck.

The District Attorney pointed out that the statute of limitations was not intended to be a game between the law and the criminal. It was a merciful measure designed to protect a culprit from indefinitely prolonged fear of arrest. For certain crimes, a denned period of apprehension of apprehension-so to speak-was considered punishment enough. But Stein, the D.A. insisted, had not experienced any period of apprehension at all.

Stein's lawyer remained unmoved. The law said nothing about measuring the extent of a culprit's fear and anguish. It simply set a time limit.

The D.A. said that Stein had not lived through the limit.

Defense stated that Stein was seven years older now than at the time of the crime and had therefore lived through the limit.

The D.A. challenged the statement and the defense produced Stein's birth certificate. He was born in 2973. At the time of the crime, 3004, he was thirty-one. Now, in 3011, he was thirty-eight.

The D.A. shouted that Stein was not physiologically thirty-eight, but thirty-one.

Defense pointed out freezingly that the law, once the individual was granted to be mentally competent, recognized solely chronological age, which could be obtained only by subtracting the date of birth from the date of now.

The D.A., growing impassioned, swore that if Stein were allowed to go free, half the laws on the books would be useless.

Then change the laws, said Defense, to take time travel into account; but until the laws are changed, let them be enforced as written.

Judge Neville Preston took a week to consider and then handed down his decision. It was a turning point in the history of law. It is almost a pity, then, that some people suspect Judge Preston to have been swayed in his way of thinking by the irresistible impulse to phrase his decision as he did.

For that decision, in full, was: 'A niche in time saves Stein.'