"You have never seen oxygen any more than you have seen air," continued Monsieur Roger. "You have never seen it, and you never will see it with your eyes,—for those organs are very imperfect. I need not therefore say oxygen is a colorless gas; and yet I will say it to you by force of habit. All books of chemistry begin in this way. Besides this, it is without smell and without taste. Oxygen is extremely well fitted for combustion. A half-extinguished candle—that is, one whose wick is still burning but without flame—will relight instantly if placed in a globe full of oxygen. Almost all the metals, except the precious metals, such as gold, silver, and platinum, burn, or oxydize more or less rapidly, when they are put in contact with oxygen; for, besides those lively combustions, in which metals, or other materials, become hot and are maintained in a state of incandescence, there are other kinds of burning which may be called slow combustions. You have often had under your eyes, without knowing it, examples of these slow combustions. For example, you have seen bits of iron left in the air, or in the water, and covered with a dark-red or light-red matter."

"That is rust," said Miette.

"Yes, that is what they call rust; and this rust is nothing less than the product of the combustion of the iron. The oxygen which is found in the air, or the water, has come in contact with the bit of iron and has made it burn. It is a slow combustion, without flames, but it nevertheless releases some heat. Verdigris, in some of its forms, is nothing less than the product of the combustion——"

"Of copper," interrupted Miette again.

"Miette has said it. These metals burn when they come in contact with the oxygen of the air,—or, in the language of science, they are oxydized; and this oxydation is simple combustion. Therefore, oxygen is the principal agent in combustion. The process which we call burning is due to the oxygen uniting itself to some combustible body. There is no doubt on that subject, for it has been found that the weight of the products of combustion is equal to the sum of the weight of the body which burns and that of the oxygen which combines with it. In the experiment which we have made, if the oxygen has diminished in the globe, if it seems to have disappeared, it is because it has united itself and combined with the carbon of the candle to form the flame. In the same way in Lavoisier's experiment it had combined itself with the mercury to form the red pellicles. The candle had gone out when all the oxygen in the globe had been absorbed; the red pellicles had ceased to form when they found no more oxygen. In this way Lavoisier discovered that the air was formed of a mixture of two gases: the first was oxygen, of which we have just spoken; the second was nitrogen. The nitrogen, which is also a colorless, odorless, and tasteless gas, possesses some qualities that are precisely contrary to those of oxygen. Oxygen is the agent of combustion. Nitrogen extinguishes bodies in combustion. Oxygen is a gas indispensable to our existence, with which our lungs breathe, and which revives our being. The nitrogen, on the contrary, contains no properties that are directly useful to the body. Animals placed in a globe full of nitrogen perish of asphyxia. In other words, they drown in the gas, or are smothered by it. I suppose you will ask me what is the use of this gas, and why it enters into the composition of the air? You will ask it with all the more curiosity when you know that the air contains four times as much nitrogen as oxygen; to be exact, a hundred cubic feet of air contains seventy-nine cubic feet of nitrogen and twenty-one cubic feet of oxygen. Now, the important part that nitrogen plays is to moderate the action of the oxygen in respiration. You may compare this nitrogen mixed with oxygen to the water which you put in a glass of wine to temper it. Nitrogen possesses also another property which is more general: it is one of the essential elements in a certain number of mineral and vegetable substances and the larger portion of animal substances. There are certain compounds containing nitrogen which are indispensable to our food. An animal nourished entirely on food which is destitute of nitrogen would become weak and would soon die."

"Excuse me, Monsieur Roger," said Albert Dalize: "how can nitrogen enter into our food?"

"That is a very good question," added Miette, laughing; "surely you cannot eat nitrogen and you cannot eat gas."

"The question is indeed a very sensible one," answered Monsieur Roger; "but this is how nitrogen enters into our food. We are carnivorous, are we not? we eat meat and flesh of animals. And what flesh do we chiefly eat? The flesh of sheep and of cattle. Sheep and cattle are herbivorous: they feed on herbs, on vegetables. Now, vegetables contain nitrogen. They have taken this nitrogen, either directly or indirectly, from the atmosphere and have fixed it in their tissues. Herbivorous animals, in eating vegetables, eat nitrogen, and we, who are carnivorous, we also eat nitrogen, since we eat the herbivorous animals. We also eat vegetable food, many kinds of which contain more or less nitrogen. Do you understand?"

"Yes, I understand," said Miette.

"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?"

Monsieur Roger, drawn out of his thoughts by this question, raised his head, looked at Miette, and said to her,—

"In the first place, do you know what water is?"

"No; but you were going to tell me."

"All right. The celebrated Lavoisier, after having shown that air is not a simple body, but that it is composed of two gases, next turned his attention to the study of water, which was also, up to that time, considered to be an element; that is, a simple body. He studied it so skilfully that he succeeded in showing that water was formed by the combination of two gases."

"Of two gases!—water?" cried Miette.

"Certainly, of two gases. One of these gases is oxygen, which we have already spoken of, and the other is hydrogen."

"Which we are going to speak of," added Miette.

"Of course," answered Monsieur Roger, "since you wish it. But it was not Lavoisier, however, who first discovered hydrogen. This gas had been discovered before his time by the chemists Paracelsus and Boyle, who had found out that in placing iron or zinc in contact with an acid called sulphuric acid, there was disengaged an air "like a breath." This air "like a breath" is what we now call hydrogen. Lavoisier, with the assistance of the chemist Meusnier, proved that it was this gas which in combining with oxygen formed water. In order to do this he blew a current of hydrogen into a retort filled with oxygen. As this hydrogen penetrated into the retort, he set fire to it by means of electric sparks. Two stop-cocks regulated the proper proportions of the oxygen and the hydrogen in the retort. When the combustion took place, they saw water form in drops upon the sides of the retort and unite at the bottom. Water was therefore the product of the combination of hydrogen with oxygen. The following anecdote is told in regard to this combination. A chemist of the last century, who was fond of flattery, was engaged to give some lessons to a young prince of the blood royal. When he came to explain the composition of water, he prepared before his scholar the necessary apparatus for making the combination of hydrogen and oxygen, and, at the moment when he was about to send the electric spark into the retort, he said, bowing his head,—