There was one hitch: a stable form in normspace could become an unstable one in hyperspace. Without empiric knowledge, it was impossible for anyone going from one universe into the other to tell whether any substance he was carrying or wearing or was would remain stable. If unstable, it could turn into liquid or gas; it could turn into energy and blow up; it could cease to be a solid in any one of a number of ways.
As if that weren't bad enough, it could also happen that even a stuff previously proven to be stable in both universes could become unstable, if there was even the trace of a potentially unstable element, or if something that, stable in itself, combined with it in unstable fashion. Such an admixture could be accidental, which was what made the whole business especially tricky, and what made the reason for the inter-universe ban necessary.
The reason why that first load of the Valkyrie's had been coal was a simple one. Somewhere, Len had read that coal and diamonds were different forms of the same normspace element, and he'd thought that might carry over into the other continuum. However, even an education wouldn't have helped him know what a right first cargo to take would have been. The xhindi had told him what they did know, but their terminology was not clear. They spoke his language with outward correctness but with imperfect conceptualization; he spoke theirs not at all. Much of what they did know, they appeared to have forgotten, or only half-learned.
They managed to make him understand that certain stuffs would be definitely unsafe; they could not make it clear which stuffs would be safe, or which they would find most desirable as trade goods. He gathered that they would be satisfied with anything that came through. So he chose coal, hoping to make a splendid initial impression.
The Valkyrie reached hyperspace. It slowed down. The throbbing of its creaky engines ebbed to a hum. And it stopped and hung there in the quiet darkness of utterly alien time and place. Schiemann and Balas, expectedly, changed their appearance, but he had seen them in their monster guises before. The coal changed to something pale and glittering, but not diamonds. Everything remained quiet. The ship's instruments recorded no temperature change, but it seemed to grow colder and colder inside her.
Suddenly, Mattern knew the truth. A trap had been laid for him, and he had tumbled neatly into it. And the most shameful part was that his own desires and yearnings—deliberately fostered by the xhindi—had been the bait.
He wanted to turn to the horrible thing that Schiemann had become to scream, "Let's go back!" But he couldn't. Something held tight grip of his mind. And, looking out the portholes, he saw that the xhindi had begun to swarm.
The flickering terror of their appearance became more awesome to him than it had been at the beginning, when he'd been only a transitory shadow in hyperspace. Now, although he had no doubt that they were friendly—indeed, almost ardent in their welcoming—horror chilled him all over again. He could almost feel the molecules inside his body slow down as his viscera quivered faintly and then froze into stillness.
He looked at Schiemann and Balas. Neither of them could, he knew, see the hyperspacers. Their conditioning back on Earth's space schools had ensured this. That was the real reason for the schools; any actual training was incidental. But Schiemann knew the creatures were there, and so he could sense them. And Balas, too, certainly seemed to sense something as he stood there, tense and wary and almost understanding. It must be even worse, Len thought, to know the hyperspacers were out there and not be able to see them.