On Earth all atoms are composed of small heavy protons that are always positive in charge, and larger lighter electrons that are always negative. In Arret the protons were negative, and the electrons positive. The result was two worlds occupying the same space at the same time, yet with matter so essentially and completely different that each world was intangible to the other. They had named the unseen world Arret, the reverse of Terra. Finding it impossible to work directly upon most forms of matter, the experimenters had finally evolved a silver alloy that served as a medium both for sending objects into Arret and then bringing them back to Earth. By focussing the flame of the projection apparatus upon a Silver Belt of this alloy, the electrical charges of the Belt's atoms were reversed, automatically causing the Belt to vanish from Earth and materialize in Arret. At the same time the atoms of any object within the Belt's immediate radius were similarly transformed, and that object was taken into Arret with the Belt. The recall mechanism functioned by broadcasting a power wave that again reversed the atomic charge of the Belt and its contained object back to that of Earth. At the same time the recall wave exerted an attractive force that drew the atoms back to a central point in the laboratory, where they were re-materialized upon the same floor-plate from which they had originally been sent. * * * * * The twelve-hour time limit was half up when Benjamin Marlowe and Larry Powell finally straightened up wearily from their work over the recall mechanism, their repairs completed. It had been one o'clock in the afternoon when Joan Marlowe vanished from Earth in the roseate flare of the projector. It was now nearly seven o'clock. With nerves tense from anxiety, the two men crossed over to the control-panel of the recall apparatus. This time they donned goggles of dark glass to shield their eyes from the blinding green glare. Marlowe threw the main switch, and the banked tubes came to life in a flood of vivid emerald light. Marlowe began stepping the rheostat up gradually to more power, advancing it with cautious slowness to avoid any chance of a repetition of the previous accident. The green radiance streaming from the tubes in every direction began to throb with an electric force that the two men could feel pulsing through their own bodies. There was a click as the rheostat struck the last notch. The green radiance was now a searing flame that half-blinded them even through the thick dark glass of their protective goggles, while the vibrant force of the green rays was sweeping through their bodies with a tingling shock that nearly took their breath away.