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 understand--has vastly diminished among them. The first question in ventilation, therefore, is to get this carbonic acid safe out of the room, while it is warm and light and close to the ceiling; for if you do not, this happens--The carbonic acid gas cools and becomes heavier; for carbonic acid, at the same temperature as common air, is so much heavier than common air, that you may actually--if you are handy enough--turn it from one vessel to another, and pour out for your enemy a glass of invisible poison. So down to the floor this heavy carbonic acid comes, and lies along it, just as it lies often in the bottom of old wells, or old brewers' vats, as a stratum of poison, killing occasionally the men who descend into it. Hence, as foolish a practice as I know is that of sleeping on the floor; for towards the small hours, when the room gets cold, the sleeper on the floor is breathing carbonic acid. And here one word to those ladies who interest themselves with the poor. The poor are too apt in times of distress to pawn their bedsteads and keep their beds. Never, if you have influence, let that happen. Keep the bedstead, whatever else may go, to save the sleeper from the carbonic acid on the floor.