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?'