Increasing difficulty came with every step. Now they were sliding and rolling into a deep crater, now scrambling up its steep sides with hands torn and bodies bruised by the jagged boulders. A yawning crevasse opened before them and they were forced to skirt its edge for fully a half mile in the wrong direction before they found a crossing. And the cold was unbelievably intense. Numbed and silent, with their eyes half blinded and lungs seared by the frosty air, they struggled on toward the three pillars of flame. And still Tom Fuller carried on, though Luke was now in the lead. They had covered probably half the distance to the flaming columns when shouts arose behind them. The guards were on their trail. "Can't--find us," Fuller panted. "The mists----" "Hell, the mists are clearing," Luke snarled. "You ain't so damn smart as you think." What he said was true. Though there was less light on account of the new angle with the sun farther below the horizon, the red mist was definitely lighter in color, noticeably less dense. Visibility was good to several hundred yards. Luke turned his head, but could see nothing of their pursuers. "They can't," Fuller insisted weakly. Luke, pushed on with renewed vigor, ignoring him, cursing. And then there came faintly to his ears the twang of a dart gun; the shrill scream of its deadly vibrating missile; a violent blow that flung him headlong. * * * * * Like a cat, he bounced to his feet, crouching with Chan Dai's dart gun at his shoulder. A strangely grotesque heap was at his feet--Tom Fuller. Off there in the thinning mist he saw a shadowy figure and he fired at it twice. Whether his darts found their mark he was never to know, for a wall of white swept down suddenly to obscure his vision. Snow! Great massed flakes falling endlessly--the moisture of the mist crystallized and closing in on him to hide him even more safely, than had the mists themselves. He was on his knees then at Fuller's side. A brilliant flash and a screaming roar over amongst the rocks apprised him of the fact that the guard's dart had gone wide. And yet Fuller was down, moaning with pain. Luke tried to turn him over and found that his body had taken on tremendous weight. He was flattened, crushed to the rocky surface of Vulcan by the full force of its gravity! "What the devil!" he grunted as he heaved and strained. "What'd they do to you, old man?"