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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 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.

How, then, shall we get rid of the foul air at the top of the room? After all that has been written and tried on ventilation, I know no simpler method than putting into the chimney one of Arnott’s ventilators, which may be bought and fixed for a few shillings; always remembering that it must be fixed into the chimney as near the ceiling as possible. I can speak of these ventilators from twenty-five years’ experience. Living in a house with low ceilings, liable to become overcharged with carbonic acid, which produces sleepiness in the evening, I have found that these ventilators keep the air fresh and pure; and I consider the presence of one of these ventilators in a room more valuable than three or four feet additional height of ceiling. I have found, too, that their working proves how necessary they are, from this simple fact:–You would suppose that, as the ventilator opens freely into the chimney, the smoke would be blown down through it in high winds, and blacken the ceiling: but this is just what does not happen. If the ventilator be at all properly poised, so as to shut with a violent gust of wind, it will at all other moments keep itself permanently open; proving thereby that there is an up-draught of heated air continually escaping from the ceiling up the chimney. Another very simple method of ventilation is employed in those excellent cottages which Her Majesty has built for her labourers round Windsor. Over each door a sheet of perforated zinc, some 18 inches square, is fixed; allowing the foul air to escape into the passage; and in the ceiling of the passage a similar sheet of zinc, allowing it to escape into the roof. Fresh air, meanwhile, should be obtained from outside, by piercing the windows, or otherwise. And here let me give one hint to all builders of houses. If possible, let bedroom windows open at the top as well as at the bottom.