He glanced at his watch. Stuart was standing by the lofty window looking out across the Embankment.

"Ten o'clock," said Dunbar. "The Commissioner will be expecting us."

"I am ready," responded Stuart.

Leaving Sergeant Sowerby seated at the table studying the note-book, Stuart and Dunbar proceeded to the smoke-laden room of the a.s.sistant Commissioner. The great man, suavely satanic, greeted Stuart with that polished courtesy for which he was notable.

"You have been of inestimable a.s.sistance to us in the past, Dr.

Stuart," he said, "and I feel happy to know that we are to enjoy the aid of your special knowledge in the present case. Will you smoke one of my cigarettes? They are some which a friend is kind enough to supply to me direct from Cairo, and are really quite good."

"Thanks," replied Stuart. "May I ask in what direction my services are likely to prove available?"

The Commissioner lighted a fresh cigarette. Then from a heap of correspondence he selected a long report typed upon blue foolscap.

"I have here," he said, "confirmation of the telegraphic report received last night. The name of M. Gaston Max will no doubt be familiar to you?"

Stuart nodded.

"Well," continued the Commissioner, "it appears that he has been engaged in England for the past month endeavouring to trace the connection which he claims to exist between the sudden deaths of various notable people, recently--a list is appended--and some person or organisation represented by, or a.s.sociated with, a scorpion. His personal theory not being available--poor fellow, you have heard of his tragic death--I have this morning consulted such particulars as I could obtain respecting these cases. If they were really cases of a.s.sa.s.sination, some obscure poison was the only mode of death that could possibly have been employed. Do you follow me?"

"Perfectly."

"Now, the death of Gaston Max under circ.u.mstances not yet explained, would seem to indicate that his theory was a sound one. In other words, I am disposed to believe that he himself represents the most recent outrage of what we will call 'The Scorpion.' Even at the time that the body of the man found by the River Police had not been identified, the presence upon his person of a fragment of gold strongly resembling the tail of a scorpion prompted me to instruct Inspector Dunbar to consult you. I had determined upon a certain course. The identification of the dead man with Gaston Max merely strengthens my determination and enhances the likelihood of my idea being a sound one."

He flicked the ash from his cigarette and resumed:

"Without mentioning names, the experts consulted in the other cases which--according to the late Gaston Max--were victims of 'The Scorpion,' do not seem to have justified their t.i.tles. I am arranging that you shall be present at the autopsy upon the body of Gaston Max.

And now, permit me to ask you a question: are you acquainted with any poison which would produce the symptoms noted in the case of Sir Frank Narcombe, for instance?"

Stuart shook his head slowly.

"All that I know of the case," he said, "is that he was taken suddenly ill in the foyer of a West-End theatre, immediately removed to his house in Half Moon Street, and died shortly afterward. Can you give me copies of the specialists' reports and other particulars? I may then be able to form an opinion."

"I will get them for you," replied the Commissioner, the exact nature of whose theory was by no means evident to Stuart. He opened a drawer.

"I have here," he continued, "the piece of cardboard and the envelope left with you by the missing cab-man. Do you think there is any possibility of invisible writing?"

"None," said Stuart confidently. "I have tested in three or four places as you will see by the spots, but my experiments will in no way interfere with those which no doubt your own people will want to make.

I have also submitted both surfaces to a microscopic examination. I am prepared to state definitely that there is no writing upon the cardboard, and except for the number, 30, none upon the envelope."

"It is only reasonable to suppose," continued the Commissioner, "that the telephone message which led Inspector Dunbar to leave your house last night was originated by that unseen intelligence against which we find ourselves pitted. In the first place, no one in London, myself and, presumably, 'The Scorpion' excepted, knew at that time that M.

Gaston Max was in England or that M. Gaston Max was dead. I say, presumably 'The Scorpion' because it is fair to a.s.sume that the person whom Max pursued was responsible for his death.

"Of course"--the Commissioner reached for the box of cigarettes--"were it not for the telephone message, we should be unjustified in a.s.suming that Mlle. Dorian and this"--he laid his finger upon the piece of cardboard--"had any connection with the case of M. Max. But the message was so obviously designed to facilitate the purloining of the sealed envelope and so obviously emanated from one already aware of the murder of M. Max, that the sender is identified at once with-- 'The Scorpion.'"

The a.s.sistant Commissioner complacently lighted a fresh cigarette.

"Finally," he said, "the mode of death in the case of M. Max may not have been the same as in the other cases. Therefore, Dr. Stuart"--he paused impressively--"if you fail to detect anything suspicious at the post mortem examination I propose to apply to the Home Secretary for power to exhume the body of the late Sir Frank Narcombe!"

Deep in reflection, Stuart walked alone along the Embankment. The full facts contained in the report from Paris the Commissioner had not divulged, but Stuart concluded that this sudden activity was directly due, not to the death of M. Max, but to the fact that he (Max) had left behind him some more or less tangible clue. Stuart fully recognized that the Commissioner had accorded him an opportunity to establish his reputation--or to wreck it.

Yet, upon closer consideration, it became apparent that it was to Fate and not to the Commissioner that he was indebted. Strictly speaking, his a.s.sociation with the matter dated from the night of his meeting with the mysterious cabman in West India Dock road. Or had the curtain first been lifted upon this occult drama that evening, five years ago, as the setting sun reddened the waters of the Imperial Ca.n.a.l and a veiled figure pa.s.sed him on the Wu-Men Bridge?

"Shut your eyes tightly, master--the Scorpion is coming!"

He seemed to hear the boy's words now, as he pa.s.sed along the Embankment; he seemed to see again the tall figure. And suddenly he stopped, stood still and stared with unseeing eyes across the muddy waters of the Thames. He was thinking of the cowled man who had stood behind the curtains in his study--of that figure so wildly bizarre that even now he could scarcely believe that he had ever actually seen it. He walked on.

Automatically his reflections led him to Mlle. Dorian, and he remembered that even as he paced along there beside the river the wonderful mechanism of New Scotland Yard was in motion, its many tentacles seeking--seeking tirelessly--for the girl, whose dark eyes haunted his sleeping and waking hours. _He_ was responsible, and if she were arrested _he_ would be called upon to identify her. He condemned himself bitterly.

After all, what crime had she committed? She had tried to purloin a letter--which did not belong to Stuart in the first place. And she had failed. Now--the police were looking for her. His reflections took a new form.

What of Gaston Max, foremost criminologist in Europe, who now lay dead and mutilated in an East-End mortuary? The telephone message which had summoned Dunbar away had been too opportune to be regarded as a mere coincidence. Mlle. Dorian was, therefore, an accomplice of a murderer.

Stuart sighed. He would have given much--more than he was prepared to admit to himself--to have known her to be guiltless.

The ident.i.ty 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 a 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 piece 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 bra.s.s 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!

Incredulous and half fearful he peered at it closely. He remembered that the impression upon the wax sealing the mysterious envelope had had a circular depression in the centre. It had been made by the head of the drawing-pin!

He found himself staring at the shelf immediately above that upon which the coin had lain. A stick of black sealing-wax used for sealing medicine was thrust in beside a bundle of long envelopes in which he was accustomed to post his Infirmary reports!