Take another instance, which is only too common: If you are in a crowded
room, with plenty of fire and lights and company, doors and windows all
shut tight, how often you feel faint--so faint, that you may require
smelling-salts or some other stimulant. The cause of your faintness is
just the same as that of the mouse's fainting in the box: you and your
friends, and, as I shall show you presently, the fire and the candles
likewise, having been all breathing each other's breaths, over and over
again, till the air has become unfit to support life. You are doing your
best to enact over again the Highland tragedy, of which Sir James Simpson
tells in his lectures to the working-classes of Edinburgh, when at a
Christmas meeting thirty-six persons danced all night in a small room
with a low ceiling, keeping the doors and windows shut. The atmosphere
of the room was noxious beyond description; and the effect was, that
seven of the party were soon after seized with typhus fever, of which two
died. You are inflicting on yourselves the torments of the poor dog, who
is kept at the Grotto del Cane, near Naples, to be stupified, for the
amusement of visitors, by the carbonic acid gas of the Grotto, and
brought to life again by being dragged into the fresh air; nay, you are
inflicting upon yourselves the torments of the famous Black Hole of
Calcutta; and, if there was no chimney in the room, by which some fresh
air could enter, the candles would soon burn blue--as they do, you know,
when ghosts appear; your brains become disturbed; and you yourselves run
the risk of becoming ghosts, and the candles of actually going out.

Of this last fact there is no doubt; for if, instead of putting a mouse
into the box, you will put a lighted candle, and breathe into the tube,
as before, however gently, you will in a short time put the candle out.

Now, how is this? First, what is the difference between the breath you
take in and the breath you give out? And next, why has it a similar