Tom nodded, and we went on. There were three doors on the opposite side of the wall. Swenton passed to the first and opened it. “The storeroom,” he explained. Within were wooden cases of glassware, large carboys of acid, glass tubing on racks and wire on spools. In one corner was apparently a hospital for broken or disused pieces of apparatus. We turned from this to the second door. “The balance room,” said Swenton, as he threw open the portal. Three balances in polished wood and shining glass met our eyes. There was nothing else in the room. Swenton opened a third door. “The spectroscope room,” he said. “Beyond is the doctor’s private laboratory.” A big piece of apparatus on the table was covered with a green cloth. Beyond was a wooden door. Despite myself, I felt a queer, nervous tremor pass over my frame, as I looked at the commonplace wooden panels, behind which Dr. Heidenmuller had sat dead, killed by the same mysterious power which had slain the men I had seen lying quietly at the bottom of Portsmouth Harbor. Tom and Hamerly were as keen as hounds on a scent, Swenton interested but more indifferent, Dorothy pale, her eyes glittering with excitement. Hamerly reached the door first, tried it and it swung back. The incandescent had not been turned on in the spectroscope room, and the only light which entered was the golden lane, which came through from the main laboratory. It seemed like a stage setting. The light fell on a heavy wooden table and a couple of Windsor chairs. The rest was but faintly outlined. A moment’s pause on the threshold, as if we expected to meet some horror, we knew not what, and then we rushed in together. There was nothing to be seen. Wood panelled walls; windows sealed by wooden shutters; the wooden table and the two wooden chairs; that was all. We stood there silent, until Tom broke the quiet. “Nothing to do but to Sherlock Holmes it,” he said. “We have all day to run this thing down. Swenton, there’s a piece of apparatus here that I need. The doctor may never have had it outside his room as a whole, yet we may find traces of it in the laboratory or the storeroom. Are you willing to help us hunt?” “I should be the most ungrateful man living, sir, if I were not,” said Swenton earnestly. “I owe my wife’s life to you and Miss Haldane.” He glanced at Dorothy. “So that’s where you have been the last two mornings,” I whispered to her, as Tom went on. “I found them just coming out of great distress,” she answered simply; “I am so glad I was able to help.” “Now,” cried Tom, “let’s sit down to another counsel of war. Come out into the outer laboratory and we’ll talk it all over.” The drawn shades, the bright gleam of the laboratory lamps reflected back from polished tile and cabinet door, gave a distinctly cheerful aspect to the scene as we settled down.