Lavenne drew the blade from the sheath, noticed that the poison, which doubtless was only soluble in an acid solution, like, for instance, the juice of a fruit, showed no sign of its presence on the silver, inserted the blade again in the sheath the other way about, and returned the dagger to the _cache_, which he then closed. His work was now done, there was nothing left but to extinguish the lamp and leave the room. He looked about to see that everything was in perfect order, and then, taking the _crochet_ from his pocket, he approached the door. The lock turned quite easily to the instrument, but the door did not open. He withdrew the _crochet_, reinserted it, and made the turn with his wrist, and again with the same result. The door was bolted now as well as locked. Lavenne drew the back of his hand across his forehead, which was covered with sweat. It was quite useless to try again. It was not the fault of the lock. He remembered now that Brujon, before he opened the door to show him the room, had placed one hand on the wall beside the door. Brujon was stout, and Lavenne had fancied that he leaned his hand on the wall to rest himself. He knew now that Brujon must have touched a spring withdrawing a secret bolt, without the release of which the door would not open. When Brujon had closed the door, he must have forgotten to touch another spring which would have re-shot the bolt. Owing to this forgetfulness, Lavenne had been able to enter the room simply by picking the lock. But Camus, who seemed never to forget precaution, had not forgotten to touch the bolting spring, with the result that Lavenne was now a prisoner in a prison that threatened to be his tomb. He knew that it was quite futile, with the means at his disposal, to make any further attempt upon the lock; even had he possessed a crow-bar and all the tools necessary, the noise of the breaking open of the door would arouse the house. The doorway being impossible, he turned to the window, which he had not yet examined. The lamp held close to the window showed nothing of the dark world outside, but it showed very definitely strong iron bars almost touching the glass. The window being impossible, he turned to the floor. There was just a chance that the hollow-sounding portion of the parquet between the table and the window curtains might disclose a means of exit. There was, in fact, more than a chance, for a man like Camus, who forgot nothing, would be the least likely man to forget to provide a secret way of escape from this chamber of secrecy. Lavenne was not wrong; the parquet on close examination showed the outline of a trap-door so well constructed as to be perfectly indistinguishable to the gaze of a person who was not searching for it. In five minutes, or less, he had discovered the button of the opening spring. He pressed on it, and the flap, instead of rising, as in the ordinary trap-door, sank, disclosing a perpendicular ladder leading down into absolute darkness.