"You know everything," gasped Dr. Cooper, falling back in his chair in consternation. "You are not far out. What were you doing? What was in the vial?" "A deadly poison. The drops I poured into the glass would put an end to a man's life in a few seconds, and it would be next to impossible to discover the cause of death." "An interesting experiment. If it would put an end to a man's life it would put an end to a woman's. Are you a double-dyed knave, or an egregious fool? Do you not see the crime your accomplice was meditating?" "I am not his accomplice," cried Dr. Cooper in a violent tone. "He told me he wanted to try it upon some animals." "A likely story. This deadly poison was to be administered to his stepdaughter. He paved the way by informing the old woman in this house that the young lady is sinking fast. He is caught in his own trap. Where is the vial?" "Mr. Nisbet has it." At this moment I saw confirmed the fancy I had entertained of a movement in the wall between the bedrooms. A panel was softly and noiselessly pushed, and Mr. Nisbet's face appeared. It was of an ashen whiteness; he must have overheard every word of the conversation. As his eyes met mine he swiftly retreated; the panel closed, and then came the sound of the snap of a lock. "What was that?" cried Rivers, starting up. I told him hurriedly what I had seen, and he went to the wall and examined it. "It is a cunning contrivance," he said, "and is hidden somewhere in these wide headings." He pushed against the wall without effect. "You, too," he added grimly to Dr. Cooper, "might never have left the house alive. Let us finish the night's work. You will come out with us. Leave the door open, and set that chair against it, in case he slips in here, and tries to make his escape. We will take the law into our own hands. I never travel without the darbies." He took a pair of handcuffs from his pocket, and put them back with a satisfied smile. We joined Ronald and Bob in the passage, and questioned them. Mr. Nisbet had made no attempt to open his door, but Bob had peeped through the keyhole a few minutes after he had taken up his station, being attracted by the glimmering of a light in the room, which he accepted as a proof that Mr. Nisbet was awake. By means of this light he had obtained a partial view of the room, but before he could catch sight of Mr. Nisbet the keyhole was masked from within, and he could see nothing more. "Mr. Nisbet!" Rivers called out as he rapped smartly at the door. We listened for an answer, but received none, and Rivers repeated his summons several times in vain. No movement within the room reached our ears. We did not make more noise than was absolutely necessary, but it brought Mme. Bernstein out, to whom Ronald explained what we were doing, and hoped we were not alarming Beatrice. "Oh, no," said Mme. Bernstein, "she is sleeping like an angel."