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 burned in a 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 combustion1, as they are called, are more or less the same in each case-carbonic acid gas and steam.
These facts justify the expression I just made use of, 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 gas. The carbonic acid gas of the fire happily escapes up the chimney in the smoke; but the carbonic acid gas from the human beings and the candles remains to poison the room, unless it be ventilated.
Now, I think, we may see what ventilation means, and why it is needed.
1 Combustion: state of burning.
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 gas 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 theaters, where the air is palpably1 more foul, and therefore more injurious, than down below.
The first question in ventilation, therefore, is to get this carbonic acid gas 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 gas 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 gas comes, and lies along it, just as it lies often in the bottom of old wells, as a stratum2 of poison,1 Palpably: plainly; evidently. 2 Stratum: layer.
killing occasionally the men who descend into it.
And now, what becomes of this breath which passes from your lips? Is it merely harmful, merely waste? God forbid! God has forbidden that anything should be merely harmful or merely waste in this so wise and well-made world. The carbonic acid gas which passes from your lips at every breath is a precious boon to thousands of things of which you have daily need. Indeed, there is a sort of hint at physical truth in the old fairy tale of the girl from whose lips, as she spoke, fell pearls and diamonds; for the carbonic acid gas of your breath may help hereafter to make the pure carbonate of lime of a pearl or the still purer carbon of a diamond.
Nay, it may go- in such a world of transformations1 dowe live-to make atoms of coal strata which shall lie buried for ages beneath deep seas, shall be upheaved in continents which are yet unborn, and there be burned for the use of a future race of men, and resolved into their original elements. Coal, wise men tell us, is, on the whole, breath and sunlight; the breath of living creatures who have lived in the vast swamps and forests of some primeval world, and the sunlight which transmuted2 that breath into the leaves and stems of trees, magically locked up for ages in that black stone, to become, when it is burned at last, light and carbonic acid gas, as it was at first.
For though you must not breathe your breath again, you may1Transformations: changes.
2Transmuted: changed from one form or nature into another.
at least eat your breath, if you will allow the sun to transmute it for you into vegetables; or you may enjoy its fragrance and its color in the shape of a lily or a rose. When you walk in a sunlit garden, every word you speak, every breath you breathe, is feeding the plants and flowers around. The delicate surface of the green leaves absorbs the carbonic acid gas and parts it into its elements, retaining the carbon to make woody fiber, and courteously returning you the oxygen to mingle with the fresh air and be inhaled by your lungs once more. Thus do you feed the plants, just as the plants feed you; while the great life- giving sun feeds both; and the geranium standing in the sick child"s window does not merely rejoice his eye and mind by its beauty and freshness but repays honestly the trouble spent on it: absorbing the breath which the child needs not, and giving to him the breath which he needs.