From the waiting-room they went up a short flight of stairs into the small apartment in which Stuart saw his patients. Dunbar looked slowly about him, standing in the middle of the room, then crossed and stared out of the window into the narrow lane below. "Where were you when he gave you the envelope?" he snapped suddenly. "At the table," replied Stuart with surprise. "Was the table-lamp alight?" "Yes. I always light it when seeing patients." "Did you take the letter into the study to seal it in the other envelope?" "I did, and he came along and witnessed me do it." "Ah," said Dunbar, and scribbled busily in his note-book. "We are badly tied at Scotland Yard, doctor, and this case looks like being another for which somebody else will reap the credit. I am going to make a request that will surprise you." He tore a leaf out of the book and folded it carefully. "I am going to ask you to seal up something and lock it away! But I don't think you'll be troubled by cowled burglars or beautiful women because of it. On this piece of paper I have written--_a"_--he ticked off the points on his fingers: "what I believe to be the name of the man who cut out the cardboard and sealed it in an envelope; _b_: the name of the cabman; and, _c_: the name of the man who rang me up here last night and gave me information which had only just reached the Commissioner. I'll ask you to lock it away until it's wanted, doctor." "Certainly, if you wish it," replied Stuart. "Come into the study and you shall see me do as you direct. I may add that the object to be served is not apparent to me." Entering the study, he took an envelope, enclosed the piece of paper, sealed the lapel and locked the envelope in the same drawer of the bureau which once had contained that marked 30. "Mlle. Dorian has a duplicate key to this drawer." he said. "Are you prepared to take the chance?" "Quite," replied Dunbar, smiling; "although my information is worth more than that which she risked so much to steal." "It's most astounding. At every step the darkness increases. Why should _anyone_ have asked me to lock up a blank piece of cardboard?" "Why, indeed," murmured Dunbar. "Well, I may as well get back. I am expecting a report from Sowerby. Look after yourself, sir. I'm inclined to think your pretty patient was talking square when she told you there might be danger." Stuart met the glance of the tawny eyes. "What d'you mean, Inspector? Why should _I_ be in danger?" "Because," replied Inspector Dunbar, "if 'The Scorpion' is a poisoner, as the chief seems to think, there's really only one man in England he has to fear, and that man is Dr. Keppel Stuart."