So they took the _Valkyrie_, the two of them--and Balas, of course, but naturally nobody would consult a madman--and headed for hyperspace. Len knew exactly where to go, even though he had no charts. The breakthrough he wanted was in their own sector and it had been carefully marked for him in his mind. Schiemann left all the details to him, even the selection of cargo. Len chose coal. He knew that what the xhindi wanted was normspace materials, but not precisely what materials. Their normspace value did not matter, because normspace matter changed to another form of itself when it got to hyperspace, and that was where the possibility of enormous profit came in. Something cheap in normspace could become something quite rare and expensive in hyperspace, and vice versa. The distribution of elements was different between the two universes; each one essentially complemented the other. 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. * * * * *