“When again,” he continued, “the slipper shall be in the receptacle of which you hold the key, that key must be given to me!”

I thought I saw the drift of his words now; I thought I perceived with what object I had been trapped and borne to this mysterious abode for whose whereabouts the police vainly were seeking.  By the exercise of the gift of divination it would seem that Hassan of Aleppo had forecast the future history of the accursed slipper or believed that he had done so.  According to his own words I was doomed once more to become trustee of the relic.  The key of the case at the Antiquarian Museum, to which he had prophesied the slipper’s return, would be the price of my life!  But—­

“In order that these things may be fulfilled,” he continued, “I must permit you to return to your house.  So it is written, so it shall be.  Your life is in my hands; beware when it is demanded of you that you hesitate not in yielding up the key!”

He raised his hands before him, making a sort of obeisance, I doubt not in the direction of Mecca, drew aside one of the yellow hangings behind him and disappeared, leaving me alone again in that nightmare apartment of yellow and green and gold.  A moment I stood watching the swaying curtain.  Utter silence reigned, and a sort of panic seized me infinitely greater than that occasioned by the presence of the weird Sheikh.  I felt that I must escape from the place or that I should become raving mad.

I leapt forward to the curtain which Hassan had raised and jerked it aside; it had concealed a door.  In this door and about level with my eyes was a kind of little barred window through which shone a dim green light.  I bent forward, peering into the place beyond, but was unable to perceive anything save a vague greenness.

And as I peered, half believing that the whole episode was a dreadful, fevered dream, the abominable fumes of hashish grew, or seemed to grow, quite suddenly insupportable.  Through the square opening, from the green void beyond, a cloud of oily vapour, pungent, stifling, resembling that of burning Indian hemp, poured out and enveloped me!

With a gasping cry I fell back, fighting for breath, for a breath of clean air unpolluted with hashish.  But every inhalation drew down into my lungs the fumes that I sought to escape from.  I experienced a deathly sickness; I seemed to be sinking into a sea of hashish, amid bubbles of yellow and green and gold, and I knew no more until, struggling again to my feet, surrounded by utter darkness—­I struck my head on the corner of my writing-table . . . for I lay in my own study!

My revolver, unloaded, was upon the table beside me.  The night was very still.  I think it must have been near to dawn.

“My God!” I whispered, “did I dream it all?  Did I dream it all?”

CHAPTER XXI

THE BLACK TUBE