"In order to live," continued Monsieur Roger, "you must breathe. You don't doubt that?"

"No," said Miss Miette, seriously.

"Now, respiration consists in the absorption by the blood of some of the oxygen of the air and in breathing out carbonic acid. The oxygen, in combining with the carbon and hydrogen of the blood, excites a real combustion in the lungs, which results in the production of heat and in the exhalation of vapor and carbonic acid."

Monsieur Roger was going to continue in the same scientific tone, when Monsieur Dalize remarked to him that his explanation did not seem to be at all understood by the children.

The latter, a little embarrassed, held their tongues.

"You are right," replied Monsieur Roger, addressing Monsieur Dalize; "that is a silence which is certainly not very flattering. I intend to profit by this lesson by beginning once more at the beginning."

"You are right," said Miette.

"Well, then, respiration is the very important function whose object is to introduce air into our lungs.

"What are the lungs, and why is it necessary to introduce air into them? And, in the first place, how is this air introduced? Through the mouth and through the nose. Then it passes through the larynx and arrives at a large tube, which is called the trachea, or wind-pipe. It is this tube which, as I shall show you, forms the two lungs. As it enters the chest, this tube branches out into two smaller tubes, which are called the primary bronches. One of these bronches goes to the right, to make the right lung; the other to the left, to make the left lung. Each primary bronche is soon divided into a number of little tubes, called secondary bronches. The secondary bronches divide up into a number of other tubes, which are still smaller; and so on, and so on. Imagine a tree with two branches, one spreading towards the right, and the other towards the left. Upon these two branches grow other branches; upon these other branches still others, and so on. The branches become smaller and smaller until they become mere twigs. Now, imagine these twigs ending in leaves, and you will have some idea of that which is sometimes called the pulmonary tree, with its thousand branches."

"No," said Miette: "bronches."

"Bronches,—you are right," said Monsieur Roger, who could not help smiling at Miss Miette. "The tree which I have taken as a comparison finishes by dividing itself into twigs, which, as I have said, end in leaves. But you know, of course, that the twigs of the pulmonary tree in our breast do not end in leaves. They end in a sort of very small cells, surrounded by very thin walls. These cells are so small that they need a microscope to detect them, and their walls are very, very thin; the cells are all stuck fast together, and together they constitute a spongy mass, which is the lung. Now let us pass to the second question: Why is it necessary to introduce air into the lungs?"

"Yes," said Miette; "let us pass to that."

"The blood, in going out of the heart, circulates to all the parts of the body in order to make necessary repairs; at the same time it charges itself with all the old matter which has been used up and is no longer any good and carries it along. Now, what is it going to do with this old matter? It will burn it. Where will it burn it? In the lungs. Now, there can be no combustion when there is no air. The blood, wishing to burn its waste matter, and wishing also to purify the alimentary principles which the veins have drawn from the stomach, has need of air. Where will it find it? In the lungs. And that is why it is necessary to introduce air into our lungs, or, in other words, that is why we breathe. The lungs are a simple intermediary between the air and the blood. Among the cells of the lungs veins finer than hair wind and turn. These veins gather up the blood filled with waste matter. It is blood of a black color, which is called venous blood. The walls of the veins which transport the blood are so thin that air, under the atmospheric pressure,—this pressure which I have told you all about,—passes through them and into the blood. Then the venous blood charges itself with the oxygen contained in the air, and frees itself from what I have called its waste material, and which is nothing less than carbon. Immediately its aspect changes. This venous blood becomes what is called arterial blood; this black blood becomes rich vermilion,—it is regenerated. It goes out again to carry life to all our organs. Now, this time," asked Monsieur Roger, pausing, "have I made myself understood?"

"Yes," said Miette, speaking both for Paul and for herself; "yes, we have understood,—except when you speak of oxygen, of carbon, and of combustion."

"Oh, I was wrong to speak of them," answered Monsieur Roger, pretending to be vexed.

"That may be," answered Miss Miette, very calmly; "but as you did speak of them, you must tell us what they are."

"Yes, you must, my friend," remarked Monsieur Dalize, taking sides with his little girl.

"Mustn't he, papa? mustn't Monsieur Roger explain?" asked Miette.

"Come, now," said Monsieur Roger, in a resigned tone. "You must know, then, that air is composed of two gases,—oxygen and nitrogen; therefore, when we breathe, we send into our lungs oxygen and nitrogen. You might think, when we throw out this air, when we exhale,—you might think, I say, that this air coming out of our lungs is still composed of oxygen and of nitrogen in the same proportions. Now, it is not so at all. The quantity of nitrogen has not varied, but, in the first place, there is less oxygen, and there is another gas,—carbonic acid gas; where, then, is the oxygen which we have not exhaled, and whence comes this carbonic acid which we did not inhale? Then, besides, in the air exhaled there is vapor. Where does that come from? These phenomena result from the combustion of which I speak; but, in order that you should understand how this combustion occurs, I must explain to you what is oxygen and what is nitrogen. And as it is a long story, you must let me put it off till this evening; then I will talk until you are weary, my dear little Miette."

Miette looked at Albert and Paul, and answered for them with remarkable frankness:

"It will be only right if you do weary us. It is we who asked you, and, besides, we have so often wearied you that it is only right you should have your revenge on us. Still——"

"Still, what?"

"Still, we can trust you," added Miette, laughing, and throwing her arms around Roger's neck.