to poison the room, unless it be ventilated.

Now, I think you may understand one of the simplest, and yet most
terrible, cases of want of ventilation--death by the fumes of charcoal. A
human being shut up in a room, of which every crack is closed, with a pan
of burning charcoal, falls asleep, never to wake again. His inward fire
is competing with the fire of the charcoal for the oxygen of the room;
both are making carbonic acid out of it: but the charcoal, being the
stronger of the two, gets all the oxygen to itself, and leaves the human
being nothing to inhale but the carbonic acid which it has made. The
human being, being the weaker, dies first: but the charcoal dies also.
When it has exhausted all the oxygen of the room, it cools, goes out, and
is found in the morning half-consumed beside its victim. If you put a
giant or an elephant, I should conceive, into that room, instead of a
human being, the case would be reversed for a time: the elephant would
put out the burning charcoal by the carbonic acid from his mighty lungs;
and then, when he had exhausted all the air in the room, die likewise of
his own carbonic acid.

* * * * *

Now, I think, we may see what ventilation means, and why it is needed.

Ventilation means simply letting out the foul air, and letting in the
fresh air; letting out the air which has been breathed by men or by
candles, and letting in the air which has not. To understand how to do
that, we must remember a most simple chemical law, that a gas as it is
warmed expands, and therefore becomes lighter; as it cools, it contracts,
and becomes heavier.

Now the carbonic acid in the breath which comes out of our mouth is warm,
lighter than the air, and rises to the ceiling; and therefore in any
unventilated room full of people, there is a layer of foul air along the
ceiling. You might soon test that for yourselves, if you could mount a
ladder and put your heads there aloft. You do test it for yourselves
when you sit in the galleries of churches and theatres, where the air is
palpably more foul, and therefore more injurious, than down below.

Where, again, work-people are employed in a crowded house of many
storeys, the health of those who work on the upper floors always suffers
most.

In the old monkey-house of the Zoological Gardens, when the cages were on
the old plan, tier upon tier, the poor little fellows in the uppermost
tier--so I have been told--always died first of the monkey's