“Ha!” he exclaimed.  “A gas cylinder!”

“What!”

He fingered the green cable.

“This is not cable at all,” he cried; “it’s covered tubing!  Do you see?”

He descended and rejoined me.

“You see?” he continued.  “A call from the exchange would ring the bell in the ante-room here.  This devilish contrivance”—­he pointed to the false telephone—­“is really hollow.  The weight of the receiver hermetically closes the end of the tube, no doubt.  But any one answering the call and taking up the duplicate instrument would receive the full benefit of the contents of the cylinder which lies up there on the roof!”

“My God, Gatton!” I muttered.  “The fiends!  But why was the contrivance not removed?”

“They hadn’t time,” he said grimly.  “They had not counted on the death-grip of the victim!”

I heard a car come racing up to the gate, followed by the sound of many excited voices.

“At last we know where the gray mist came from,” I said, as Gatton and I walked through the cottage to meet the new arrivals.

“We know more than that,” he retorted.  “We know how Sir Marcus died!”

“Gatton!” I cried excitedly, as we approached a group waiting in the porch—­“do you mean—­”

He looked at me grimly.

“I mean,” he said slowly, “that I have not forgotten the gas-plug in the wall of that recess in the supper-room at the Red House!  The only thing I was doubtful about (the means by which the victim was induced to admit the gas into the room) is now as clear as daylight.”

“You are right, Gatton,” I agreed.  “The same trick has succeeded twice.”

“The same trick, as you say, Mr. Addison; with one trifling variation, a device which would only suggest itself to such a brain as that of—­”

“Dr. Damar Greefe!” I cried.

“I believe you are right.”

And now fell an awesome silence; for whilst Gatton and I stood bare-headed, the unfortunate Eric Coverly was being carried out to the waiting car; and even as I turned my eyes away in horror from that spectacle, I was endeavoring to frame the words in which I should acquaint Isobel with this second ghastly tragedy.

Here, indeed, was a new development of “the Oritoga mystery”; and so queerly does the mind depart from the actualities at such a moment that I found myself thinking, even whilst Gatton was talking to me, of the bold head-lines which would greet readers of the press in the morning—­and of the renewed excitement which would sweep throughout the length and breadth of the land when this dreadful alibi was proven.

Over the details of that gruesome tragedy I feel myself compelled to pass lightly, for even now the horror of it remains with me.  The fumes of the poisonous gray mist lingered for hours in the house; and there were official visitations, testimonies and attestations, and the hundred and one formalities which invariably accompany such a tragedy but which I need not deal with in detail here.