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The excerpt represents the core issue or deciding factor on which you must meditate, and is drawn from Sanitary and Social Lectures by Charles Kingsley: 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
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