“I came to suggest that,” said the other. “In fact, I kept a cab waiting at the door with that idea.” Derrick Yale did not speak during the journey to Bayswater, and it was not until he stood in the hall of the house in Marisburg Place that he broke the silence. “We ought to find a small steel cylinder somewhere,” he said slowly. The policeman standing on duty in the hall came forward and saluted the inspector. “We found an iron bottle in the garage, sir?” he said. “Ah!” cried Derrick Yale triumphantly. “I thought so!” He almost ran up the stairs ahead of the detective and paused in the passage, which was now lighted. The little oak table stood against the ventilator and toward that he moved. Then he went down on his hands and knees and sniffed the carpet. Presently he choked and coughed and got up, red in the face. “Let me see that cylinder,” he said. They brought it to him. The policeman’s description of it as a bottle was nearer the truth. It was an iron bottle, at the end of which was a small pipe to which was attached a tiny turn-key. “And now there ought to be a cup somewhere,” he said, looking round, “unless he brought it in a bottle.” “There was a small glass bottle in the garage near this, sir,” said the policeman who had found it, “it is broken, though.” “Bring it to me quickly,” said Yale. “And I can only hope that it isn’t so completely smashed that none of its contents are left.” The stout Mr. Parr was regarding him sombrely. “What is all this about?” he asked, and Derrick Yale chuckled. “A new way of committing a murder, my dear Mr. Parr,” he said airily, “now let us go into the room.” The body of Marl lay on the bed covered by a sheet and the circular patch of wet on the pillow had not dried. The windows were open and a fitful wind kept the curtains fluttering. “Of course you can’t smell it here,” said Yale speaking to himself, and again went on his knees and nosed the carpet. And again he coughed and rose hurriedly. By this time they had returned with the lower half of a glass bottle. It contained a few drops of liquid, and this Yale poured into his hand. “Soap and water,” he said; “I thought it would be. And now I’ll explain how Marl was killed. Your thief, ‘Flush’ Barnet, heard a hissing sound. It was the sound of a heavy gas escaping from this cylinder. I may be wrong, but I should imagine there is enough poison gas in that little iron bottle to settle your account and mine. It is still lying on the floor, by the way. It is one of those heavy gases which descend.” “But how did it kill Marl? Did they pump it through the grating on to his head?” Derrick Yale shook his head. “It is a much simpler and a much more deadly method which the Crimson Circle employed,” he said quietly. “They blew bubbles.” “Bubbles!” Derrick Yale nodded.