Far overhead, through the great round orifice, darted a tremendous shape, pointed, glittering. "Why, that's the _Althea_," Nona exclaimed. "Yes. Now watch. Damn--all we can do is watch," Grant gritted between his teeth. * * * * * Down sped the gleaming liner, pride of the fleet. The men at the mirror were swerving it on gimbals until a ray from it flashed on the burnished nose. As though it were a physical impact, the vessel slackened its tremendous speed and hung suspended midway between the cloud concavity and the island. The men with the nozzle spurred into activity. A thin stream of fluid shot out of the orifice straight up for the captive liner. The tip of the expanding spray impinged on the hull--and Nona gasped her astonishment. For the liquid passed clean through the hull as though it were a porous network instead of four-inch thick beryllium-steel. "Just as I thought," Grant groaned. "Lethal gas that penetrates everything. Those poor people on board--for their own sakes I hope none remained alive to hit this." "Can't we do anything?" Nona asked desperately. "Nothing for the _Althea_. But plenty to prevent any more disasters like it." There was a hard ring to his voice. "Come on." He stepped off the transparent slab onto the stone floor of the island. "Where to?" asked Nona, following. "We're going to locate that orange oval we saw from the _Althea_. That's the secret of all this. The pool of liquid fire here is unimportant, secondary." They were at one edge of the floating island. The other side was hidden from them by the solid wall that stretched across its full diameter. "We'll scout beyond there," Grant pointed out. "I'll miss my guess if what we're looking for is not on the other side." As they started for the wall, they saw the Althea brought slowly down to the rock, another captive to swell the motionless fleet. It did not take them long to reach the barrier. Some fifty feet high it was, of smooth polished Ganymedan stone, and no door or opening in its straight unbroken surface. "How shall we get through?" Nona asked. Grant surveyed it thoughtfully. "There must be a hidden spring somewhere," he said. He walked carelessly along the wall, tapping it idly here and there. His quick probing fingers were searching. With a sharp "Ah!" he stopped short. He bent over a moment; his fingers moved deftly. Then he straightened with a grunt of satisfaction. A section of the seemingly solid, immovable stone was sliding silently open. He looked through. * * * * *