That was an eye-opening paragraph. Suddenly I saw just why Bert had been trifling with the truth, why he had concealed Joey’s presence from Marie, why he had decided to go back to the surface on such short notice, why he had been so far from completely frank with me — and even why the local Council had been so reluctant to let us both leave.

I also saw that I was in no position to criticize him for any of it. There was not a word to be said against him which didn’t apply with equal force to me. The only reason I hadn’t done as much, under exactly the same motivation, was that I’d been in no position to.

I couldn’t blame him, or even criticize him. I have failings, but I’m not that much of a hypocrite. I could be sorry for him; as he’d said, his chances were gone.

Marie might conceivably come to realize that Joey was a hopeless case as far as she was concerned, even after this discovery that he was alive after all. She might possibly settle for me if that happened. But after the last few weeks and the discoveries of the last few minutes she’d never, never have any use for Bert.

I gave him as sympathetic a look as I could as all this dawned on me, but I could think of nothing to write. He answered with a bitter grin and waved me toward the door. I went. The others, except the doctor, followed me.

I wasn’t through learning for the day, though. As I went through the huge valve and became visible from the tunnel outside, Marie’s voice met me. It had sharp edges, but otherwise it resembled a heavy club.

“Just where did you come up with the idea that these people weren’t getting oxygen through their lungs? If I killed Bert I’m not too sorry, but it’s your fault.”

Even I had had time to see that this question would be coming, but I’d had no chance to work out a very good answer. While the doctor had been working on Bert I’d been doing the same with my memory. It was evident enough that my theory of oxygen-food was out the window, but I still wasn’t able to find a better one.

All I could do was repeat the theory and my reasons for it. I also assured Marie that she hadn’t actually killed Bert. Somehow my reasoning didn’t look as airtight written out as it had felt when I was thinking it through in the first place — quite aside from the fact that it was now obviously wrong. In spite of this, Marie seemed to calm down as I wrote page after page, let her read each, and cleared it and went on to the next. The forced pauses may have helped.

I admit you convinced me before,” she said when I was done, ‘and I don’t see what the hole is myself. Joey, in the time you’ve been here have you found out enough to let you tell us what’s wrong with this notion?”

“I think so,” he wrote. He paused, and positioned himself outside the port so that Marie could read as he wrote. I swam to a spot a little further above and behind him, so I could do the same.

“Your big mistake was natural. You were quite right in

observing that we aren’t breathing, as far as chest motions go. But in spite of that we are getting oxygen from this liquid. It’s wonderful stuff. You might regard its molecular structure as vaguely comparable to hemoglobin in that it binds oxygen molecules loosely to its surface. I don’t know just how many, but the number is large. It doesn’t have the porphyrin groups of hemoglobin; they went to great lengths to make it transparent to visible light. I couldn’t draw you its structural formula from memory. But I’ve seen it. It’s perfectly understandable.

“Now, think a minute. Liquid oxygen has a molecular concentration about four thousand times that of the gas we normally breathe. The reason we have to breathe is that diffusion, at sea-level concentrations, won’t get enough oxygen through your windpipe to keep an animal as large as a human being going. You can’t live in liquid oxygen, of course, because of temperature problems. However, in this liquid the concentration of almost-free oxygen is far, far higher than in the atmosphere — a long way short of what it is in LOX, but very high. That was another problem; while they were at it, they made the kernel of this molecule with a structure which would break down endothermically at temperatures above a few hundred degrees. A fire will tend to damp itself out, therefore. But that’s a side issue, as far as breathing is concerned.

“When molecules of the stuff give up their oxygen in your lungs, nearby molecules pass on more O2 to the ones which have lost it; others replenish those, and so on. It’s a bucket-brigade situation, but it’s described by just the same equations that you’d use for a diffusion problem. The rate of oxygen transport depends on the concentration difference between the inside of your lungs and outside, and on the area of the barrier through which the diffusion is taking place — in this case, the smallest cross-section area of your windpipe. In this case, the oxygen concentration around us is enough to keep us going by diffusion down our windpipes. I’m not sure about carbon-dioxide elimination, but I believe your theory is more nearly right there; it’s taken care of by binding into insoluble carbonates in the intestines and gotten rid of as solid waste. As I say, that seems a little funny to me, and I may have misunderstood what I read about it. I’m going to dig into the matter more when I have time. I’m no physiologist, but it’s fascinating reading, especially the history of its development.”

“But why such a fancy arrangement? A less efficient oxygen carrier would still work as long as you pumped fresh supplies into your lungs! That’s why we breathe, anyway!” Marie couldn’t have been thinking at the top of her form just then; even I could see the answer. I took the pad from Joey — in fact, he held it out to me, with a suspicion of a grin on his face — and started my own exposition.

“Pumping a liquid even denser than water through your windpipe would call for tremendous effort and probably dangerously high lung pressures. I tried it just after I made the change, and I know it hurts. I wouldn’t be surprised if you could rupture lung tissues that way. It’s a logical chain: fill body cavities with liquid so that outside pressure can be matched without serious volume change; then you can’t pump the liquid with your normal breathing equipment; so you have to give it a high enough free-oxygen concentration to diffuse an adequate supply down your throat. Simple once you see it. What’s the primary source of oxygen, though, Joey?”

“Just what you’d expect. Photosynthesis. That’s where most of the power produced here goes. About three-quarters of the oxygen comes from gene-tailored algae living at the interface between the ocean and the breathing liquid. The rest comes from the farm plants. Loss to the ocean is low because of the favorable partition ratio.”

I took the pad again.

“Well, at least I was right in guessing why laughing is dangerous, and why they do away with the coughing reflex; either action could rupture your lungs.”

“Of course,” agreed Joey. ‘I don’t claim to know the whole story yet — even Bert, who’s been here much longer, probably doesn’t. Remember, all we could learn about it was what we read, and that was only what happened to be lying around written in languages we knew. We weren’t told any of it by these people. Not only is it impossible to talk to them on such a level; I’m pretty sure most of them don’t know it either.

How many people at the surface, out of any given fifteen thousand, would be doctors or physiologists or even engineers?”

“That’s why they need us so badly,” I interjected. ‘Bert must have told you about that.”

“Who’d believe Bert?” snapped Marie — we’d been holding all our writings so she could read them, of course, even when they weren’t specifically meant for her. Joey took over the pad.