"Why?" asked McQuade, who, like the others, followed my every word with intense interest.
"Because, had the time been very long, I, like Mr. Ashton, should never have risen at all. You would have found me here this morning, as he was found."
For answer I took a box of wax tapers from my pocket and lighted one. "Have you ever heard of the Cave of Dogs, near Naples?" I inquired.
"Carbon dioxide," gasped the Major with a look of comprehension.
Sergeant McQuade looked blank, and I saw that to him neither my question nor the Major's answer had conveyed any definite meaning. "Look," I cried, as I held the match out before me, where it burned with a bright, clear flame.
McQuade's mystification increased. I think he wondered if I were trying to play some practical joke upon him. But, when I slowly lowered the taper until it reached a point a few inches above my knee, and its flame faded away and then suddenly went out, as though the match had been plunged into a basin of water, his expression slowly cleared, and he gave a significant grunt. "Carbonic-acid gas," he said. "I understand. But where does it come from?"
"That I do not know, at the moment," I said, "but I think there should be no great difficulty in finding out. This room has been closed for a long time. Even when Mr. Ashton came here, it was opened for only a few moments. Neither he nor I opened the windows, because of the rain, as you know. Somehow, just how I cannot say, a slow stream of carbonic-acid gas finds its way into this room. It is the product of combustion, as you of course know, and is produced in large quantities by burning coal. It may come through the register from the furnace, or from some peculiar action of partially slacked lime in the plaster of the walls. Wherever it comes from, being heavier than air, it slowly settles to the floor, where it collects, becoming deeper and deeper, just as water collects and rises in a tank. Look." I tore a few sheets from the magazine I had been reading the night before, which still lay upon the bed, and lighting them with another match, extinguished the flame, but allowed the smoke from the smoldering paper to spread about the room. It slowly sank until it rested upon the surface of the heavy gas, like a layer of ice upon the surface of a body of water. It showed the carbon dioxide to be considerably over two feet deep, and some six or eight inches below the level of the top of the bed. I knew it must have risen higher during the night, as it was its deadly fumes, closing about my pillow and beginning to enter my lungs, that caused my troubled dreams, as well as, ultimately, the feeling of suffocation which had caused me to awake so suddenly. A considerable portion of the gas had evidently flowed out through the open door, as I lay across the threshold, after my escape from the room.