"There is nobody living who really understands this matter very well, for it is an extremely obscure, though very important, subject," replied Monsieur Roger. "But, to resume our explanation. Besides oxygen and nitrogen, there is also in the air a little carbonic acid and vapor. The carbonic acid will bring us back to the point from which we started,—the phenomenon of breathing. Carbonic acid is a gas formed by oxygen and carbon. The carbon is a body which is found under a large variety of forms. It has two or more varieties,—it is either pure or mixed with impurities. Its varieties can be united in two groups. The first group comprises the diamond and graphite, or plumbago, which are natural carbon. The second group comprises coal, charcoal, and the soot of a chimney, which we may call, for convenience, artificial carbon. When oxygen finds itself in contact with carbonaceous matter,—that is to say, with matter that contains carbon,—and when the surrounding temperature has reached the proper degree of heat, carbonic acid begins to be formed. In the oven and the furnace, coal and charcoal mingle with the oxygen of the air and give the necessary heat; but it is first necessary that by the aid of a match, paper, and kindling-wood you should have furnished the temperature at which oxygen can join with the carbon in order to burn it. That is what we may call an active or a live combustion; but there can also be a slow combustion of carbon,—a combustion without flame, and still giving out heat. It is this combustion which goes on in our body by means of respiration."
"Ah, now we have come around to it!" cried Miette. "That is the very thing I was inquiring about."
"Well, now that we have come around to it," answered Monsieur Roger, "tell me what I began to say to you on the subject of respiration."
"That is not very difficult," answered Miette, in her quiet manner. "You told us that we swallowed oxygen and gave out carbonic acid; and you also said, 'Whence comes this carbonic acid? From combustion.' That is why I said, just now, 'We have come around to it.'"
"Very good,—very good, indeed, only we do not swallow oxygen, but we inhale it," said Monsieur Dalize, charmed with the cleverness of his little girl.
"What, then, is the cause of this production of carbonic acid?" continued Monsieur Roger. "You don't know? Well, I am going to tell you. The oxygen of the air which we breathe arrives into our lungs and finds itself in contact with the carbon in the black or venous blood. The carbon contained here joins with the oxygen, and forms the carbonic acid which we breathe out. This is a real, a slow combustion which takes place not only in our lungs,—as I said at first, in order not to make the explanation too difficult,—but also in all the different portions of our body. The air composed of oxygen and nitrogen—for the nitrogen enters naturally with the oxygen—penetrates into the pulmonary cells, spreads itself through the blood, and is borne through the numberless little capillary vessels. It is in these little vessels that combustion takes place,—that is to say, that the oxygen unites with the carbon and that carbonic acid is formed. This carbonic acid circulates, dissolved in the blood, until it can escape out of it. It is in the lungs that it finds liberty. When it arrives there it escapes from the blood, is exhaled, and is at once replaced by the new oxygen and the new nitrogen which arrive from outside. The nitrogen absorbed in aspiration at the same time as the oxygen is found to be of very much the same quantity when it goes out. There has therefore been no appreciable absorption of nitrogen. Now, this slow combustion causes the heat of our body; in fact, what is called the animal-heat is due to the caloric set free at the moment when the oxygen is converted into carbonic acid, in the same way as in all combustion of carbon. In conclusion, I will remind you that our digestion is exercised on two sorts of food,—nitrogenous food and carbonaceous food. Nitrogenous food—like fibrin, which is the chief substance in flesh; albumen, which is the principal substance of the egg; caseine, the principal substance of milk; legumine, of peas and beans—is assimilated in our organs, which they regenerate, which they rebuild continually. Carbonaceous foods—like the starch of the potato, of sugar, alcohol, oils, and the fat of animals—do not assimilate; they do not increase at all the substance of our muscles or the solidity of our bones. It is they which are burned and which aid in burning those waste materials of the venous blood of which I have already spoken. Still, many starchy foods do contain some nutritive principles, but in very small quantity. You will understand how little when you know that you would have to eat about fifteen pounds of potatoes to give your body the force that would be given it by a single pound of beef."
"Oh," said Miette, "I don't like beef; but fifteen pounds of potatoes,—I would care still less to eat so much at once."
"All the less that they would fatten you perceptibly," replied Monsieur Roger; "in fact, it is the carbonaceous foods which fatten. If they are introduced into the body in too great a quantity, they do not find enough oxygen to burn them, and they are deposited in the adipose or fatty tissue, where they will be useless and often harmful. You see how indispensable oxygen is to human life, and you now understand that if respiration does not go on with regularity, if the oxygen of your room should become exhausted, if the lungs were filled with carbonic acid produced by the combustion of fuel outside the body, there would follow at first a great deal of difficulty in breathing, then fainting, torpor, and, finally, asphyxia."
These last words, pronounced by Monsieur Roger with much emotion, brought before them a remembrance so recent and so terrible that all remained silent and thoughtful. It was Miss Miette who first broke the spell by asking a new question of her friend Roger. Asphyxia had recalled to her the fire. Then she had thought of the manner of extinguishing fire, and she said, all of a sudden, her idea translating itself upon her lips almost without consciousness,—
"Why does water extinguish fire?"