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?"