Some hundred yards distant were clouds of green vapor that rolled into the air. At their base a fountain of mud sputtered and spouted and fell back to build up a cone. The green cloud whirled sluggishly, then was caught by the breeze and began its slow, rolling progress across the flat rock. It was coming their way, rolling down toward the ship, and Chet gripped suddenly at his companion's arm. "Come on!" he said! "I'm going away from here, and I'm going now. We'll get Diane and Kreiss: remember what a whiff of gas did to him this morning." He was drawing Harkness toward the face of the rock; he wondered at his slowness. Walt seemed fascinated by the oncoming cloud. "Wait!" Harkness paused at the top of the descending slope. Chet turned, to look where Harkness was watching. The green cloud moved slowly. As he turned to stare it touched the bow of their ship; it flowed slowly, sluggishly, along the sides, and then swept up and over the top. The lookouts of the control room were obscured, and the port from which they had come! "Cut off!" breathed Harkness, his voice heavy with hopeless conviction. "We can't get back! And now we're on our own past any doubt!" * * * * * "It may not last," Chet was urging an hour later, when, with Kreiss and Diane, they stood on high ground to look down on the ship. The sparkling sheen of the metal cylinder had changed from silver to pale green. The cloud that enveloped it was not heavy, but it was always the same. Yet still Chet insisted: "It may not last." "Sorry to disappoint you," replied Kreiss, "but there is little ground for such a belief." Again he was the professor instructing a class. "These fumeroles, in my opinion, are venting a region far below the surface. It is possible that further seismic disturbances may alter conditions; a rearrangement of the lower rock strata may close existing crevices and open others like this you have seen; but, barring that, I see no reason for thinking that this emission of what appears to be chlorine with other gases may not continue indefinitely." Chet looked at Diane. Was it a twinkle that appeared and vanished in her eyes as Herr Professor Kreiss concluded his remarks. She would laugh in the very face of death, Chet realized, but her tone was entirely serious as she offered another suggestion. "If this wind should change," she said, "and if it blew the gas in another direction, the ship could be cleared. One of us could go in long enough to switch on the air generators full." But now it was Chet who shook his head in a negative. "Remember," he told her, "when we were here before? All of the time while Walt was gone for the ship--how did the wind blow then?" "The same as now," she admitted. "And it never changed." "No,"--slowly--"it never changed." * * * * *