"I imagined that there must be," said Marsden, "and I haven't the least doubt that we shall find it unlocked." He got up from his chair and glanced at the police surgeon, who was standing by himself in front of the fire. "I don't think we need keep you any longer, Doctor Sinclair," he added. "I'll let you know what time we fix for the P.M. as soon as I've seen Ashford." The surgeon, who seemed ready enough to depart, picked up his hat and coat. "You can tell him to 'phone me at my house," he replied. "I shall be there till midday for certain." He nodded a general good-night, and, accepting Colin's offer to escort him to the front door, accompanied the latter through the hall and down the outer corridor. "We shall be bound to come across each other again during the next few days," he said as they shook hands. "I only hope that when all this is over we shall have the pleasure of meeting under less distressing circumstances." Colin returned some more or less suitably polite rejoinder, and, shutting the door, made his way back to the study. He found the two detectives standing in front of the safe, the sergeant stooping down and apparently engaged in some experiment with the lock. Marsden looked round at his entrance. "We'll leave Humphries to finish up here," he said. "I want you to take me to the laboratory; and afterward, if you'll call down the servants, I'd like to have a few minutes' conversation with both of them." "I don't suppose you'll get much out of the cook," said Colin doubtfully. "According to Mrs. Ramsay, she's collapsed for the night." "She'll talk all right," was the somewhat cynical answer. "Women can always pull themselves together if there's a chance of using their tongues." He stepped forward briskly, and, following Colin to the back of the house, turned down the side passage which led to the laboratory. The door of the latter apartment was still open, and at the sight of the big, brilliantly lit interior he pulled up with an exclamation of surprise. "Hullo!" he remarked. "I'd no idea it was such a size. The old man must have been pretty well off if he could afford to run up places like this." He glanced round the room as though in search of the additional exit, and, without waiting for his question, Colin pointed toward a high screen which jutted out at right angles from the wall. "It's behind there," he explained. "I never thought of looking to see if it was open. The Professor told me that he only used it in summer time." "He used it to-night," was the detective's reply. "If he hadn't he would probably be alive now."