"Ah! That, my boy, is where our scientists have triumphed. True, his rays were terrible. They could not be combatted when he first returned. The strange chemicals and gases of the Moon men defied analysis or duplication. His citadel atop the city of Dorn is proof against them all; proof against explosives and rays of all kinds known to him. The disintegration and decomposition rays have no effect on the crystal of its walls. It is hermetically sealed from the outer air so can not be gassed. The vibration impulses have no effect upon its reinforced structure. But there is a ray, a powerful destructive agent, against which it is not proof. And our scientists have developed this agency. You shall have the privilege of pressing the release of the energy that destroys the arch-fiend in his lair. His dominance over, the empire will fall. We shall take it--for you." A strange exaltation shone from the faces of those in the room, and Karl found that it was contagious. His bosom swelled and he itched to handle the controls of this wonderful ray. "This ray," continued the brother of old Rudolph, "carries the longest vibrations ever measured, the vibrations of infra-red, the heat-ray. We have succeeded in concentrating a terrific amount of power in its production, and with it are able to produce temperatures in excess of that of the interior of the earth, where all substances are molten or gaseous. The Zar's crystal palace cannot withstand it for a second. He cannot escape!" "How'll you know he's there at the time?" Karl was greatly excited, but he was curious too. "Come with me, my boy. I'll show you." The old man led him from the room and the others followed respectfully. * * * * * They stopped at a circular port and Karl saw that they were high above the earth in a vessel that hovered motionless, quivering with what seemed like human eagerness to be off. "This vessel?" he asked. "It's a huge sphere; the base of our operations. To it we drew the aero on which you were fighting. A magnetic force discovered by our scientists and differing only slightly from that used in counteracting gravity. We let the rest of them go; foolishly I think. But it's done now and we have no fear. From this larger vessel we shall send forth smaller ones, armed with the heat-ray. The flagship of the fleet is to be yours and you'll lead the attack on Dorn. Here--I'll show you the Zar." They had reached the room of the detectoscopes--a mass of mechanisms that reminded Karl of nothing so much as the vitals of the intermediate levels which he had visited with Leon--and Rhoda. He knew that he flushed when he thought of her. What a fool he had been!