"Brandon's ultra-radio comes next, but it's got more angles to it than a cubist's picture of a set of prisms; so many that I don't know where to begin. There, that job's done—let's sit down and I'll talk at you awhile. Maybe between us we can figure out where to start. I've got everything to build it lined up except for the tube, but that's got me stopped cold. You see, fields of force are all right in most places, but I've got to have one tube, and it's got to have the hardest possible vacuum. That means a mercury-vapor super pump. Mercury is absolutely the only thing that will do the trick and the mercury is one thing that is conspicuous by its absence in these parts. So are tungsten for filaments, tantalum for plates, and platinum for leads; and I haven't found anything that I can use as a getter, either—a metal, you know, to flash inside the tube to clean up the last traces of atmosphere in it."
"I didn't suppose that such a simple thing as a radio tube could hold you up, after the perfectly unbelievable things that you have done already—but I see now how it could. Of course, the tubes in our receiver over there are too small?"
"Yes, they are only receiver and communicator tubes, and I need a high-power transmitting tube—a fifty-kilowatter, at least. I'd give my left leg to the knee joint for one of those big water-cooled, sixty-kilowatt ten-nineteens right now—it would save us a lot of grief."
"Maybe you could break up those tubes and use the plates and so on?"
"I thought of that, but it won't work—there isn't half enough metal in the lot, and the filaments in particular are so tiny that I couldn't possibly work them over into a big one. Then, too, we haven't got many spare tubes, and if I smash the ones we're using, I put our communicators out of business for good, so that we can't yell for help if we have to drift home—and I still don't get any mercury."
"Do you mean to tell me there's no mercury on this whole planet?"
"Not exactly; but I do mean that I haven't been able to find any, and that it's probably darned scarce. And since all the other metals I want worst are also very dense and of high atomic weight, they're probably mighty scarce here, too. Why? Because we're on a satellite, and no matter what hypothesis you accept for the origin of satellites, you come to the same conclusion—that heavy metals are either absent or most awfully scarce and buried deep down toward the center. There are lots of heavy metals in Jupiter somewhere, but we probably couldn't find them. Jupiter's atmosphere is one mass of fog, and we couldn't see, since we haven't got an infra-red transformer. I could build one, in time, but it would take quite a while—and we couldn't work on Jupiter, anyway, because of its gravity and probably because of its atmosphere. And even if we could work there, we don't want to spend the rest of our lives prospecting for mercury." Stevens fell silent, brow wrinkled in thought.
"You mean, dear, that we're..." Nadia broke off, the sentence unfinished.
"Gosh, no! There's lots of things not tried yet, and we can always set out to drift it. I was thinking only of building the tube. And I'm trying to think ... say, Nadia, what do you know about Cantrell's Comet?"
"Not a thing, except that I remember reading in the newspapers that it was peculiar for something or other. But what has Cantrell's Comet got to do with the high cost of living—or with radio tubes? Have you gone cuckoo all of a sudden?"