"I don't know if the hydrogen and the oxygen were aware of the honor which was being done them; but certainly they combined with no more manners than if their spectator was an ordinary boy. Now, I may add, you must not confound combinations with mixtures; thus, air is a mixture of oxygen and nitrogen, while water is a combination of hydrogen and oxygen. This combination is a union of the molecules of the two gases which produces a composite body formed of new molecules. These new molecules are water. Now, this last word recalls to me Miette's question."
"Yes," said the latter: "why does water put out fire?"
"There are two reasons for this phenomenon," said Monsieur Roger: "the first is that water thrown upon the fire forms around the matter in combustion a thick cloud, or vapor, which prevents the air from reaching it. The wood, which was burning—that is to say, which was mingling with the oxygen of the air—finds its communication intercepted. The humid vapor has interposed between the carbon of the wood and the oxygen of the air; therefore, the combustion is forced to stop. Further, water falling upon the fire is transformed, as you very well know, into vapor, or steam. Now, this conversion into vapor necessitates the taking up of a certain quantity of heat. This heat is taken away from the body which is being burned, and that body is thus made much cooler; the combustion therefore becomes less active, and the fire is at last extinguished."
"Very good," said Miette; "but still another question, and I will let you alone."
"You promise?"
"Yes."
"Well, then, what is your last question?"
"Why is a candle put out by blowing on it, and why do they light a fire by doing the same thing?"
"In these two cases there are two very different actions," replied Monsieur Roger: "in the first there is a mechanical action, and in the second a chemical action. In blowing upon a candle the violence of the air which you send out of your mouth detaches a flame which holds on only to the wick. The burning particles of this wick are blown away, and consequently the combustion is stopped. But the case is very different when you blow with a bellows or with your mouth upon the fire in the stove. There the substance in combustion, whether wood or coal, is a mass large enough to resist the violence of the current of air you throw in, and it profits from the air which you send to it so abundantly, by taking the oxygen which it contains and burning up still more briskly.
"Now, that is the answer to your last question; and I must beg you to remember your promise, and ask me no more hard questions to-night."