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