"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."