A mental picture of the envelope arose before him, the number, 30, written upon it and the two black seals securing the lapels. He paused again in his walk. His reflections had led him to a second definite point and he fumbled in his waistcoat pocket for a time, seeking a certain brass coin about the size of a halfpenny, having a square hole in the middle and peculiar characters engraved around the square, one on each of the four sides. He failed to find the coin in his pocket, however, but he walked briskly up a side street until he came to the entrance to a tube station. Entering a public telephone call-box, he asked for the number, City 400. Being put through and having deposited the necessary fee in the box: "Is that the Commissioner's Office, New Scotland Yard?" he asked. "Yes! My name is Dr. Keppel Stuart. If Inspector Dunbar is there, would you kindly allow me to speak to him." There was a short interval, then: "Hullo!" came--"is that Dr. Stuart?" "Yes. That you, Inspector? I have just remembered something which I should have observed in the first place if I had been really wide-awake. The envelope--you know the one I mean?--the one bearing the number, 30, has been sealed with a Chinese coin, known as _cash_. I have just recognized the fact and thought it wise to let you know at once." "Are you sure?" asked Dunbar. "Certain. If you care to call at my place later to-day I can show you some _cash_. Bring the envelope with you and you will see that the coins correspond to the impression in the wax. The inscriptions vary in different provinces, but the form of all _cash_ is the same." "Very good. Thanks for letting me know at once. It seems to establish a link with China, don't you think?" "It does, but it merely adds to the mystery." Coming out of the call-box, Stuart proceeded home, but made one or two professional visits before he actually returned to the house. He now remembered having left his particular _cash_ piece (which he usually carried) in his dispensary, which satisfactorily accounted for his failure to find the coin in his waistcoat pocket. He had broken the cork of a flask, and in the absence of another of correct size had manufactured a temporary stopper with a small cork to the top of which he had fixed the Chinese coin with a drawing-pin. His purpose served he had left the extemporised stopper lying somewhere in the dispensary. Stuart's dispensary was merely a curtained recess at one end of the waiting-room and shortly after entering the house he had occasion to visit it. Lying upon a shelf among flasks and bottles was the Chinese coin with the cork still attached. He took it up in order to study the inscription. Then: "Have I cultivated somnambulism!" he muttered. Fragments of black sealing-wax adhered to the coin!