The identity of the missing cabman now engaged his mind. It was quite possible, of course, that the man had actually found the envelope in his cab and was in no other way concerned in the matter. But how had Mlle. Dorian, or the person instructing her, traced the envelope to his study? And why, if they could establish a claim to it, had they preferred to attempt to steal it? Finally, why all this disturbance about a blank pieced of cardboard?
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?"