The Nevian physician directed his key tube upon the transparent wall of the chamber and an opening appeared, an opening which vanished as soon as he had stepped through it; Costigan kicked a valve open; and from various innocent tubes there belched forth into the water of the central lagoon and into the air over it a flood of deadly vapor. As the Nevian turned toward the prisoner there was an almost inaudible hiss and a tiny jet of the frightful, outlawed stuff struck his open gills, just below his huge, conical head. He tensed momentarily, twitched convulsively just once, and fell motionless to the floor. And outside, the streams of avidly soluble liquefied gas rushed out into air and into water. It spread, dissolved, and diffused with the extreme mobility which is one of its characteristics; and as it diffused and was borne outward the Nevians in their massed hundreds died. Died not knowing what killed them, not knowing even that they died. Costigan, bitterly resentful of the inhuman treatment accorded the three and fiercely anxious for the success of his plan of escape, held his breath and, grimly alert, watched the amphibians die. When he could see no more motion anywhere he donned his gas-mask, strapped upon his back a large canister of the poison--his capacious pockets were already full of smaller containers--and two savagely exultant sentences escaped him. "I am a poor, ignorant specimen of ape that can be let play with apparatus, am I?" he rasped, as he picked up the key tube of the specialist and opened the door of his prison. "They'll learn now that it ain't safe to judge by the looks of a flea how far he can jump!" He stepped out through the opening into the water, and, burdened as he was, made shift to swim to the nearest ramp. Up it he ran, toward a main corridor. But ahead of him there was wafted a breath of dread Vee-Two, and where that breath went, went also unconsciousness--an unconsciousness which would deepen gradually into permanent oblivion save for the prompt intervention of one who possessed, not only the necessary antidote, but the equally important knowledge of exactly how to use it. Upon the floor of that corridor were strewn Nevians, who had dropped in their tracks. Past or over their bodies Costigan strode, pausing only to direct a jet of lethal vapor into whatever branching corridor or open door caught his eye. He was going to the intake of the city's ventilation plant, and no unmasked creature dependent for life upon oxygen could bar his path. He reached the intake, tore the canister from his back, and released its full, vast volume of horrid contents into the primary air stream of the entire city.