* * * * *

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
constitutional complaint, consumption, simply from breathing the warm
breath of their friends below. But since the cages have been altered,
and made to range side by side from top to bottom, consumption--I