She did not, however, look quite good enough to take my mind off my need for the money to buy a new liver, so I got down to business. 'The little red marbles over there are blood-diamonds,' I told her. 'They're radioactive. Not so much that they'll hurt you, but they stay warm. Which is one way you can tell the real one from a fake: Anything over about three centimeters is a fake. A real one that big generates too much heat-the square-cube law, you know. So it melts.'
'So the ones your friend was trying to sell me-'
'Were fakes. Right.'
She nodded, still smiling. 'What about what you were trying to sell us, Audee? Real or fake?'
The Third of Vastra's House had discreetly vanished by then, so I took a deep breath and told Dorotha the truth. Not the whole truth, maybe, but nothing but the truth.
'All this stuff here,' I said, 'is what thousands of people have found after a hell of a lot of digging. It's not much. The punch, the piezophone, and two or three other gadgets that we can make work; a few busted pieces of things that they're still studying; and some trinkets. That's it.'
'That's the way I heard it,' she said. 'And one more thing. None of the discovery dates on these things is less than twenty years old.' She was smarter and better informed than I had expected.
'And the conclusion you can draw from that,' I agreed, 'is that as nothing new has been found lately, the planet has probably been mined dry. You're right. That's what the evidence seems to show. The first diggers found everything useful that there was to be found .. . so far.'
'But you think there's more.'
'I hope there's more. Look. Item. The tunnel walls. You see
they're all alike-the blue walls, perfectly smooth; the light coming from them that never varies; the hardness. How do you suppose the Heechee made them?'
'Why, I don't know.'
'Neither do I. Or anybody else. But every Heechee tunnel is the same, and if you dig into them from the outside you find the same basic substrate rock, then a boundary layer that's sort of half wall-metal and half substrate, then the wall itself. Conclusion: The Heechee didn't dig the tunnels and then line them, they had something that crawled around underground like an earthworm, leaving these tunnels behind. And one other thing: they overdug. That's to say they dug lots of tunnels they didn't need, going nowhere, never used for anything. Does that suggest anything to you?'
'It must have been cheap and easy?' she guessed.
I nodded. 'So it was probably an automatic machine, and there really ought to be at least one of them, somewhere on this planet, to find. Next item. The air. They breathed oxygen like we do, and they must have got it from somewhere. Where?'
'Why, there's oxygen in the atmosphere, isn't there?'
'Hardly any. Less than a half of a percent. And most of what there is isn't free oxygen; it's compounded into carbon dioxide and other garbage. There's no water vapor to speak of, either. Oh, a little-not as much as, for instance, sulfur dioxide. When water seeps out of the rock it doesn't come out as a fresh, clear spring. It goes into the air as vapor pretty fast. It rises-the water molecule being lighter than the carbon dioxide molecule. When it reaches a point where the sun can get at it it splits into hydrogen and oxygen. The oxygen and half the hydrogen mostly go into turning the sulfur dioxide into sulfuric acid. The rest of the hydrogen just escapes into space.'
She was looking at me quizzically. 'Audee,' she said gently, 'I already believe you're an expert on Venus.'
I grinned. 'But do you get the picture?'
'I think so. It looks pretty bad.'
'It is pretty bad, but all the same the Heechee managed to get that little bit of oxygen Out of the mixture, cheaply and easily-remember those extra tunnels they filled-along with inert gases like nitrogen-and they're present only in trace amounts-enough to make a breathing mixture. How? I don't know, but if there's a machine that did it I'd like to find that machine. Next item: aircraft. The Heechee flew around the surface of Venus a lot.'
'So do you, Audee! Aren't you a pilot?'
'Airbody pilot, yes. But look what it takes to make an airbody go. There's a surface temperature of seven- thirty-five K, and not enough oxygen to keep a cigarette lit. So my airbody has to have two fuel tanks, one for the fuel, one for the thing to burn it with. That's not just oil and air, you know.'
'' . ,
It 15fl t.
'Not here, Dorotha. Not at the kind of ambient temperatures we've got. It takes exotic fuels to get that hot. Did you ever hear of a fellow named Carnot?'
'Old-time scientist, was he? The Carnot cycle fellow?'
'Right again.' That was the third time she'd surprised me, I noted cautiously. 'The Carnot efficiency of an engine is expressed by its maximum temperature-the heat of combustion, let's say-divided by the temperature of its exhaust. Well, but the temperature
of the exhaust can't be lower than the temperature of whatever it's exhausted into-otherwise you're not running an engine, you're running a refrigerator. And you've got that seven-thirty-five air temperature to fight, so even with special fuels you have basically a lousy engine. Any heat engine on Venus is lousy. Did you ever wonder why there are so few airbodies around? I don't mind; it helps to have something close to a monopoly. But the reason is that they're so damn expensive to run.'
'And the Heechee did it better?'
'I think they did.'
She laughed again, unexpectedly and once more very attractively. 'Why, you poor fellow,' she said in good humor, 'you're hooked on the stuff you sell, aren't you? You think that one of these days you're going to find the mother tunnel and pick up a few billion dollars' worth of Heechee stuff!'
I wasn't pleased with the way she put that. I wasn't all that happy with the meeting I had set up with Vastra's Third, for that matter; I'd figured that, away from her boyfriend, I could pick this Dorotha Keefer's brains about him pretty easily. It wasn't working out that way. She was making me aware of her as a person, which was an undesirable development in itself-you can't treat a mark as a mark if you think of him, or her, as a fellow human being.
