"It is not a combination--only a concealed bolt. Our friends of the detective force might have discovered it themselves if they had taken the trouble. The first thing I noticed was that a truck had recently been wheeled through the cellar in the direction of this door, from under the grating on the street side. And this truck was not here; neither was a large case which we know was delivered here to-day. The trail extended clear up to the wall below the shelving; and yet no truck, even unloaded, could pass below that lowest shelf. The conclusion was evident. I sounded the back of the shelving and found that it covered an opening of some kind. After that, all that remained was to notice that one of the shelves was slightly soiled in just one spot, as though by the repeated contact of a hand. From this, I argued that the bolt must be attached to this board. And it was. That is all." As he spoke, the reporter entered a dark and narrow passage. "Don't shut the door," said he to his companion, who followed him. At that moment, however, the artist stumbled: and, instinctively holding out his hands to save himself from falling, he released his hold of the door, which closed with a slam. "That is unfortunate," said Sturgis; "we may have to lose some time in learning how to work the bolt from this side. Hold on; it will be prudent to keep open a line of retreat, in case of unforeseen emergencies. Hello! we are in luck. Nothing concealed on this side; the bolt in plain sight; works easily. All's well. Then let us go on; unless I am greatly mistaken, we shall find another exit on the other side." After following the underground passage for some distance, the men climbed some steps and reached a square chamber, on one side of which rose a stairway leading to a door above. The room was surmounted by a skylight, which was wide open, admitting a draught of cold air from the outside. Sturgis set down his lighted candle and proceeded to examine his surroundings. In the middle of the room stood a truck, upon which lay a long pine box. A table and a chair constituted the only furniture of the place. At one side, there was a long, low, lead-lined tank, filled to the depth of about two feet with a dark viscous liquid. Near it lay a few empty carboys. In the floor there was what seemed to be a hot-air register, of large size and of peculiar construction. The walls were bare, unbroken, save by the projection of the mouthpiece of a speaking-tube, and by a set of shelves filled with flasks, crucibles, alembics and the other paraphernalia of a chemist's laboratory. After the reporter had finished reconnoitering, he sat down upon the long box in deep thought. Sprague observed him with silent curiosity for a while; and then, with growing impatience, "I say, old man," he ventured at last to ask, "did you bring me here, armed to the teeth, to see you go off into a trance?" Sturgis started like a man suddenly awakened from a deep sleep.