"But Captain! In two years there may not be enough light alloys left on the three planets to build another ship as big as this! The service to Mars will have to be stopped long before that. The lithium mines there can't operate unless the water supply from Venus is maintained." "Well, what of it? Nobody likes to work in that Martian colony." Adam caught his breath. "But how are the atomic motors that do all the work going to operate without the lithium from Mars?" "And what of that, even? It would be temporary. Just a few months or years till another expedition could be sent out. You take things too seriously. What is there to prove that some other method of armoring space-ships won't be found? Beryllium may not be necessary after all." "Perhaps you're right." Adam was still skeptical. "But is it likely, sir? You've been on the space run so long you haven't kept up with chemistry. The armor against meteorites now is so thick that any metal but beryllium would double the weight of the vessel, and even with these seven million horsepower Buvier-Manleys we couldn't make the run from Venus to Mars. Why, sir, it would mean the end of the lithium mines, it would mean the end of atomic power, and we'd have to go back to the barbarism of the twentieth century, when they ran everything by electricity from waterfalls!" Captain McCausland raised an athletic hand. "Spare me that, Mister Mate! I have heard it at approximately a hundred banquets before starting out on this expedition. Yes, we carry the fate of the world and all that. We have to find beryllium or else the Mars mines can't be run and the atomic motors stop. I could sing it in my sleep. But suppose we do take chances and get this ship wrecked. Won't the world have to go back to 'barbarous' electric power after all? For my part, I think some of those people in the twentieth century probably had a good time." Adam was silent. There was something in the Captain's reasoning, he felt. Yet he, Adam Longworth, could not but feel that the issue was a desperately serious one for every inhabitant of the three worlds--Earth, Venus and Mars--belonging to the planetary league. The entire known supply of beryllium, the precious light, strong metal that was alone suitable for the armor of space ships, had been exhausted. All that remained was in the hulls of the few dozen ships carrying water from Venus to Mars, and from the arid deserts of Mars, bringing to Earth the equally precious lithium which was the only material with which atomic motors could be powered. Every year, in spite of the best of care, one or two space ships would be wrecked--caught in the sun's gravitational field, or lost through some small error of navigation. Soon there would be no more space ships; and no more could be built. Each of the outer planets had been explored in turn--each but the last, the outermost and most distant; Pluto. They were on their way there now; if they could not make it--