Dick Rennell was to experience many thrilling moments afterward, but there was never a period, measurable by seconds, yet seeming to extend through all eternity--never a period quite so fraught with suspense as, hovering there, he watched the flight of that silvery plane speeding straight toward the executive mansion while all around it the shells bloomed and spread. It was over the White House grounds. The archies had failed; they were being outmaneuvered, they could not be swung in time to follow the trajectory of the plane. Dick held his breath. Then suddenly the silvery ship dissolved in a blaze of fire, a shower of golden sparks such as fly from a rocket, and simultaneously the last bomb that she was to drop broke upon the ground below. Down she plunged, instantly invisible as she escaped the finger of the black beam; but she dropped into the vortex of ruin that she herself had created. Into a pit of blazing fire, criss-crossed by falling trees, that had engulfed the battery and a score of men. Then suddenly Dick understood. He flung home the soaring lever, banked, and headed, not for the White House, but for the flat roof of the hotel from which the black searchlight was still projecting itself through the skies. He hovered above, and dropped, light as a feather, upon the rooftop. * * * * * There was only one person there--an old man dressed in a shabby suit, kneeling before a great block of stone that had been dislodged upward from the parapet and formed a sort of table. Upon this table the old man had placed a large, square box, resembling an exaggerated kodak, and it was from the lens of this box that the black beam was projecting. Dick sprang from his cockpit as the old man rose in alarm. He ran to him and caught him by the arm. "Luke Evans!" he cried. "Thank God you've come back in time to save America!" CHAPTER VI _The Gas_ In the Blue Room of the White House the Council listened to old Luke Evans's exposition of his invention with feelings ranging from incredulity to hope. "I've been at work all the time," said the old man, "not far from here. I knew the day would come when you'd need me. I put my pride aside for the sake of my country." "Tell us in a few words about this discovery of yours, Mr. Evans," said Colonel Stopford. Luke Evans placed the square black case upon the table. "It's simple, like all big things, sir," he answered. "The original shadow-breaking device that I invented was a heavy, inert gas, invisible, but almost as viscous as paint. Applied to textiles, to inorganic matter, to animal bodies, it adheres for hours. Its property is to render such substances invisible by absorbing all the visible light rays that fall upon it, from red to violet. Light passes through all substances that are coated with this paint as if they did not exist." "And this antidote of yours?" asked Colonel Stopford.