They stopped but ten feet short of the deadly blaze. From a narrow rift in the left wall it streamed outward, the rock at the edges of that crack turning to red at its touch. It beat upon the opposite wall, where already the stone was melting to throw over them a white glare and the glow of heat. And, like a shimmering, silken barrier, whose touch could mean only instant death, it reached across the wide tunnel at the height of a man's waist and moved slowly up and down. The heaviest armor plate ever rolled could have formed no more impenetrable a barrier. "And we almost made it," said Smithy slowly. "Look, beyond there--another hundred feet. There's the bend in the tunnel, a sharp turn--and we almost got around!" Rawson reached for Loah's light. In the wall where the flame was striking, only a dozen steps back, he had seen another dark mouth, a ragged crack in the rock. He sprang to the entrance; it might be there was another way around. His first glance told the story, for he saw the walls draw together again not a hundred feet off. "A blind alley," he groaned. * * * * * One of the two who had been their advance guard snapped his rifle to his shoulder. He was aiming at the glowing crack where the green light was issuing. "A ricochet," he growled. "It may go on in and mess 'em up." But there was no whine of a glancing bullet that followed his shot; the softened wall had cushioned the impact. Another man sprang beside him. He was shouting at the top of his voice while one hand reached into a bag that hung at his waist. "Get back, everyone," he said. "If I miss...." He did not finish the sentence, but pulled the pin from a hand grenade, then took careful aim and threw. It went high--thrown there purposely; he had not dared aim it into the flame. But it struck the crevice fairly, and they heard it rattle on inside. The next instant brought the crack and roar of its explosion. Like a winking signal light the green barrier vanished. Where it had been was only blackness and the dying glow of molten rock. Then, a hundred feet beyond, up close to the roof, the bend of the tunnel turned red; it seemed bursting into flame. Far back of them, down the long sloping way where they had come, shrill voices were screaming--and still there was no green flame to account for that tunnel end flaming red. Rawson stood motionless. Loah, and the others beside him, seemed likewise petrified, until the voice of Culver jarred them into action. "The ray!" he shouted. "It's the heat ray, damn them! Quick, jump into that cave!" * * * * * They had all retreated through fear of the grenade; they were opposite the black place into which Rawson had looked. Loah was close beside Dean; he threw her with all his strength into the black mouth of the cave, then he was one of a crowding, stumbling mass of men who followed after, and their going was lighted by a terrible torch of flame.