He discarded all but attentive listening, making his mind focus on some plan to trap his adversary.
What his mind had, with seeming whimsicality, obtruded during his moment of terror, came back to Roger. “Law of Nature.” seemed to prod at his thoughts. What law of Nature? How would it help?
Almost as though some inner monitor was going to save him, a mental visualization of the laboratory seemed to become clear to his mind. He saw the ceilings, with the slim pipes that ran here and there to openings; and he connected the vision with the fact that their fire-protective apparatus had not functioned, when the alarm had been set off. The tanks of heavy gas, under pressure, were still charged.
“Gravity!” Roger’s mind grasped at an idea, “that’s the Law of Nature I am trying to think up.”
As if he had received a key to a tantalizing problem, Roger solved his course of procedure in a flash. In his mind he ran over their stock of chemicals. Hydrocyanic acid, a stinging, powerful combination of cyanogen and hydrogen; and hydrochloric acid—and many more.
One of these, akin to a tear gas, would do. But he was cautious, and in spite of the pressing uncertainty he paused to be sure he would not take for his plan anything that could, in combination with the fire-smothering gas, cause an explosion.
Almost at once he had the solution. Sulphuretted hydrogen—the common, refined gas that comes in the city mains from gas plants to stoves and gas jets—that would not explode in combination with the heavy gas in the compression-tank system!
He wanted a gas that would stupefy: but he needed to be sure that it would lie, close to the floor.
The gas in the fire-prevention apparatus was such a heavy gas that on being liberated, under pressure, it would settle rapidly, diffusing and spreading, as if it could be likened to a cloud, surcharged with moisture, settled on the earth, enfolding it like a blanket.
There, in the upper room, was the means of releasing the city gas, which, Roger knew, would stupefy of its own constituents—even kill, in time. He did not intend to give it that much time! He merely had the desire to put his assailant into a state where he could not leave.