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.

"And that is what killed poor Boris," said the Major, as he watched the
eddying whirls of smoke which settled and rested upon the surface of the
gas. "Exactly," I said, "and probably Ashton as well. His skull was
fractured, it is true, but the divisional surgeon at the inquest
reported, you may remember, that the fracture was not sufficient of
itself to have caused instant death. It was ten minutes or more, I
should say, from the time I was first awakened by Ashton's cry, until we
finally broke in the door and reached his side. By that time he had
suffocated. The gas, as no doubt you know, is not a poisonous one, but
containing no oxygen which the lungs can take up, acts very much the
same as water would if breathed into the lungs."

Muriel looked at me with admiring eyes. I did not tell her that my
father had inte