deep in reflection, stuart walked alone along the embankment. the full facts contained in the report form 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 association 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 canal and a veiled figure passed 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 passed 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 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?"

"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 this particular cash piece (which he usually carried) in his dispensary, which satisfactorily accounted fro 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 extemporized stopper 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 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!

one hand raised to his head, stuart stood endeavouring to marshal his ideas into some sane order. then, knowing what he should find, he raised the green baize curtain hanging from the lower shelf, which concealed a sort of cupboard containing miscellaneous stores and not a little rubbish, including a number of empty cardboard boxes.

a rectangular strip had been roughly cut from the lid of the topmost box!

the mysterious envelope and its contents, the wax and the seal—all had come from his own dispensary!