"Who are you?" asked the detective brusquely. The man looked in the direction of the voice; and, as his eyes became accustomed to the light, returned the detective's surprised stare with a calm and searching look. "Checkmate!" he murmured quietly to himself at last. Then, without seeming haste, he passed back into the private office, before the astonished detective could make any attempt to stop him. Recovering himself quickly, the detective followed the sounds of the retreating footsteps to the cellar stairs. Then, fearful of an ambush, he fired his revolver as a signal to his companion on the roof; and, after striking a match, he cautiously descended, reaching the cellar just in time to see Murdock disappear into the underground passage. He rushed to the spot; and, unable to find the door, he pounded with all his might upon the shelves, causing the bottles to dance and rattle. "Come, now," he shouted, "the game's up! You may as well be reasonable. You can't possibly escape, for you're surrounded." No answer came from within. The man tried his powerful strength upon the door without any perceptible effect. When the second detective arrived upon the scene, he found the first one removing the bottles from the shelves by the light of a match held in his left hand. "Get a light and an axe, Jim. There's a secret door here which we'll have to break in; I can't find any way of opening it." A few minutes later, the detectives, after dealing upon the shelves some telling blows with an axe, again called upon Murdock to surrender. Receiving no answer to their summons, the men stood irresolute for a few seconds. Then, with grim determination, they attacked the door; raining the blows upon it fast and furiously, and filling the air with a shower of splinters. At length a final stroke sent the weakened hinges from their fastenings, and the men rushed through the underground passage into the murderer's laboratory. A hasty, startled glance told them that Murdock was not there. They started for the stairs and were met by a policeman who was just entering from Murdock's office. "Have you got him?" asked the detectives in chorus. "No," replied the policeman surprised; "Mr. Sturgis says he went down here about twenty minutes ago." "We chased him in from the other end not ten minutes ago." The policeman hurried down the stairs. Murdock's valise and package stood conspicuous upon the long pine box. But of Murdock there was no sign. "Gone!" exclaimed one of the detectives deeply mortified at the thought that his quarry had slipped through his fingers. "Gone! How? Where? He cannot have escaped. He cannot----What is it, Mr. Sturgis?" He had suddenly caught sight of the reporter, half way up the stairs. Weak and ill, Sturgis, with blanched face, clung unsteadily, with one hand, to the railing; while, with the other, he pointed toward the lead-lined vat, whose dark viscous contents were bubbling like boiling oil.