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 the 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.