A huge fan turned slowly on one wall and the ward was pleasantly cool — it was almost like the open Air. The Hospital was close to the City’s outer wall and the ward was connected to the outside world by only a short duct and was comparatively bright; entering it, Dura had an impression of cheerfulness, of competence.

But these initial impressions were rapidly dispelled by the sight of Adda, who was suspended at the center of the room in a maze of ropes, webbings and bandages, almost all of his battered body obscured by gauzy material. A doctor — called Deni Maxx, a round, prissy-looking woman whose belt and pockets bristled with mysterious equipment — fussed around the suspended Human Being.

Adda peered at Dura and Farr from his nest of gauze. His right upper arm, which had been broken, was coated in a mound of bandages, and his lower legs were strapped together inside a cage of splints. Someone had scraped the pus from his good eye, and applied an ointment to keep out symbiotes.

Dura, oddly, felt more squeamish about Adda’s wounds now than when she had been trying to cope with them with her bare hands in the Crust-forest. She was reminded, distressingly, of the dead, displayed animals in the Museum. “You’re looking well,” she said.

“Lying sow,” Adda growled. “What by the bones of the Xeelee am I doing here? And why haven’t you got out while you can?”

The doctor clucked her tongue, tweaking a bandage. “You know why you’re here.” She spoke loudly, as if Adda were a deaf child. “You’re here to heal.”

Farr said, “Anyway, we’ll be gone soon. I’m off to work in the Harbor. And Dura is going to the ceiling-farm.”

Adda fixed Dura with a one-eyed, venomous stare. “You stupid bitch.”

“It’s done now, Adda; I won’t argue about it.”

“You should have let me die, rather than turn yourselves into slaves.” He tried to raise gauze-wrapped arms. “What kind of life do you think I’m going to have now?”

Dura found Adda’s tone repellent. It seemed wild, unconstructed, out of place in this huge, ordered environment. She found herself contrasting Adda’s violence with the quiet timidity of Ito, who was living out her life in a series of tiny movements as if barely aware of the constraints of the crush of people around her. Dura would not have exchanged places with Ito, but she felt she understood her now. Adda’s rage was crass, uncomprehending. “Adda,” she said sharply. “Leave it. It’s done. We have to make the best of it.”

“Indeed we do,” the doctor sighed philosophically. “Isn’t that always the way of things?”

Adda stared at the woman. “Why don’t you keep out of it, you hideous old hag?”

Deni Maxx shook her head with no more than mild disapproval.

Dura, angry and unsettled, asked the doctor if Adda was healing.

“He’s doing as well as we could expect.”

“What’s that supposed to mean? Why can’t you people talk straight?”

The doctor’s smile thinned. “I mean that he’s going to live. And it looks as if his broken bones are knitting — slowly, because of his age, but knitting. And I’ve sewn up the ruptured vessels; most of his capillaries are capable of sustaining pressure now…”

“But?”

“He’s never going to be strong again. And he might not be able to leave the City.”

Dura frowned; brief, selfish thoughts of extended periods of fee-paying crossed her mind. “Why not? If he’s healing up as you say…”

“Yes, but he won’t be able to generate the same level of pneumatic pressure.” Maxx frowned quizzically. “Do you understand what that means?”

Dura gritted her teeth. “No.”

“Oh dear. It’s so easy to forget you’re all upfluxers…”

Adda closed his eyes and leaned back in his gauze net.

“Look,” said Maxx, “our bodies function by exploiting the Air’s mass transport properties… No? All right.” She pointed at the fan set into the wall. “Do you know why that fan is there — why there are fans installed throughout the City? To regulate the temperature — to keep us cool, here in the heat of the South Pole. The Air we inhabit is a neutron gas, and it’s made up of two components — a superfluid and a normal fluid. The superfluid can’t sustain temperature differences — if you heat it, the heat passes straight through.

“Now — that means that if you add more superfluid to a mass of Air, its temperature will drop. And similarly if you take superfluid out the temperature rises, because normal fluid is left behind. And that’s the principle the wall fans work on.”

Farr was frowning. “What’s that got to do with Adda?”

“Adda’s body is full of Air — like yours, and mine. And it’s permeated by a network of tiny capillaries, which can draw in superfluid to regulate his temperature.” Deni Maxx winked at Farr. “We have tiny Air-pumps in our bodies… lots of them, including the heart itself. And that’s what hair-tubes are for… to let Air out of your skull, to keep your brain the right temperature. Did you know that?”

“And it’s that mechanism which may not work so well, now, for Adda.”

“Yes. We’ve repaired the major vessels, of course, but they’re never the same once they’re ruptured — and he’s simply lost too much of his capillary network. He’s been left weakened, too. Do you understand that Air also powers our muscles?… Look — suppose you were to heat up an enclosed chamber, like this room. Do you know what would happen to the superfluid? Unable to absorb heat, it would flee from the room — vigorously, and however it could. And by doing so it would raise pressure elsewhere.

“When Adda wants to raise his arm, he heats up the Air in his lungs. He’s not aware of doing that, of course; his body does it for him, burning off some of the energy he’s stored up by eating. And when his lungs are heated the Air rushes out; capillaries lead the Air to his muscles, which expand and…”

“So you’re saying that because this capillary network is damaged, Adda won’t be as strong again?”

“Yes.” She looked from Dura to Farr. “Of course you do realize that our lungs aren’t really lungs, don’t you?”

Dura shook her head, baffled by this latest leap. “What?”

“Well, we are artifacts, of course. Made things. Or at least our ancestors were. Humans — real humans, I mean — came to this world, this Star, and designed us the way we are, so that we could survive, here in the Mantle.”

“The Ur-humans.”

Maxx smiled, pleased. “You know of the Ur-humans? Good… Well, we believe that original humans had lungs — reservoirs of some gas — in their bodies. Just as we do. But perhaps their lungs’ function was quite different. You see, our lungs are simply caches of Air, of working gas for the pneumatic systems which power our muscles.”

“What were they like, the Ur-humans?”

“We can’t be sure — the Core Wars and the Reformation haven’t left us any records — but we do have some strong hypotheses, based on scaling laws and analogies with ourselves. Analogous anatomy was my principal subject as a student… Of course, that was a long time ago. They were much like us. Or rather, we were made in their image. But they were many times our size — about a hundred thousand times as tall, in fact. Because he was dominated by balances between different sets of physical forces, the average Ur-human was a meter tall, or more. And his body can’t have been based, as ours is, on the tin-nucleus bond… Do you know what I’m talking about? The tin nuclei which make up our bodies contain fifty protons and one hundred and forty-four neutrons. That’s twelve by twelve, you see. The neutrons are gathered in a spherical shape in symmetries of order three and four. Lots of symmetry, you see; lots of easy ways for nuclei to fit together by sharing neutrons, plenty of ways for chains and complex structures of nuclei to form. The tin-nucleus bond is the basis of all life here, including our own. But not the Ur-humans; the physics which dominated their structure — the densities and pressures we think they inhabited — wouldn’t have allowed any nuclear bonding at all. But they must have had some equivalent of the tin bond…”