and the candles of actually going out.

Of this last fact there is no doubt; for if, instead of putting a mouse into the box, you will put a lighted candle, and breathe into the tube as before, however gently, you will in a short time put the candle out.

Now, how is this?  First, what is the difference between the breath you take in and the breath you give out?  And next, why has it a similar effect on animal life and a lighted candle?

The difference is this.  The breath which you take in is, or ought to be, pure air, composed, on the whole, of oxygen and nitrogen, with a minute portion of carbonic acid.

The breath which you give out is an impure air, to which has been added, among other matters which will not support life, an excess of carbonic acid.

That this is the fact you can prove for yourselves by a simple experiment.  Get a little lime-water at the chemist’s, and breathe into it through a glass tube; your breath will at once make the lime-water milky.  The carbonic acid of your breath has laid hold of the lime, and made it visible as white carbonate of lime—in plain English, as common chalk.

Now I do not wish, as I said, to load your memories with scientific terms: but I beseech you to remember at least these two, oxygen gas and carbonic acid gas; and to remember that, as surely as oxygen feeds the fire of life, so surely does carbonic acid put it out.

I say, “the fire of life.”  In that expression lies the answer to our second question: Why does our breath produce a similar effect upon the mouse and the lighted candle?  Every one of us is, as it were, a living fire.  Were we not, how could we be always warmer than the air outside us?  There is a process; going on perpetually in each of us, similar to that by which coals are burnt in the fire, oil in a lamp, wax in a candle, and the earth itself in a volcano.  To keep each of those fires alight, oxygen is needed; and the products of combustion, as they are called, are more or less the same in each case—carbonic acid and steam.

These facts justify the expression I just made use of—which may have seemed to some of you fantastical—that the fire and the candles in the crowded room were breathing the same breath as you were.  It is but too true.  An average fire in the grate requires, to keep it burning, as much oxygen as several human beings do; each candle or lamp must have its share of oxygen likewise, and that a very considerable one, and an average gas-burner—pray attend to this, you who live in rooms lighted with gas—consumes as much oxygen as several candles.  All alike are making carbonic acid.  The carbonic acid of the fire happily escapes up the chimney in the smoke: but the carbonic acid from the human beings and the candles remains to poison the room, unless it be ventilated.

Now, I think you may understand one of the simplest, and yet most terrible, cases of want of ventilation—death by the fumes of charcoal.  A human being shut up in a room, of which every crack is closed, with a pan of burning charcoal, falls asleep, never to wake again.  His inward fire is competing with the fire of charcoal for the oxygen of the room; both are making carbonic acid out of it: but the charcoal, being the stronger of the two, gets all the oxygen to itself, and leaves the human being nothing to 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 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