CHAPTER XXVI.
WHY WATER PUTS OUT FIRE.
"You have never seen oxygen any more than you have seen air," continuedMonsieur Roger. "You have never seen it, and you never will see it withyour eyes,--for those organs are very imperfect. I need not thereforesay oxygen is a colorless gas; and yet I will say it to you by force ofhabit. All books of chemistry begin in this way. Besides this, it iswithout smell and without taste. Oxygen is extremely well fitted forcombustion. A half-extinguished candle--that is, one whose wick is stillburning but without flame--will relight instantly if placed in a globefull of oxygen. Almost all the metals, except the precious metals, suchas gold, silver, and platinum, burn, or oxydize more or less rapidly,when they are put in contact with oxygen; for, besides those livelycombustions, in which metals, or other materials, become hot and aremaintained in a state of incandescence, there are other kinds of burningwhich may be called slow combustions. You have often had under youreyes, without knowing it, examples of these slow combustions. Forexample, 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 thanthe product of the combustion of the iron. The oxygen which is found inthe air, or the water, has come in contact with the bit of iron and hasmade it burn. It is a slow combustion, without flames, but itnevertheless releases some heat. Verdigris, in some of its forms, isnothing less than the product of the combustion----"
"Of copper," interrupted Miette again.
"Miette has said it. These metals burn when they come in contact withthe oxygen of the air,--or, in the language of science, they areoxydized; and this oxydation is simple combustion. Therefore, oxygen isthe principal agent in combustion. The process which we call burning isdue to the oxygen uniting itself to some combustible body. There is nodoubt on that subject, for it has been found that the weight of theproducts of combustion is equal to the sum of the weight of the bodywhich burns and that of the oxygen which combines with it. In theexperiment which we have made, if the oxygen has diminished in theglobe, if it seems to have disappeared, it is because it has uniteditself and combined with the carbon of the candle to form the flame. Inthe same way in Lavoisier's experiment it had combined itself with themercury to form the red pellicles. The candle had gone out when all theoxygen in the globe had been absorbed; the red pellicles had ceased toform when they found no more oxygen. In this way Lavoisier discoveredthat 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 somequalities that are precisely contrary to those of oxygen. Oxygen is theagent of combustion. Nitrogen extinguishes bodies in combustion. Oxygenis a gas indispensable to our existence, with which our lungs breathe,and which revives our being. The nitrogen, on the contrary, contains noproperties that are directly useful to the body. Animals placed in aglobe full of nitrogen perish of asphyxia. In other words, they drown inthe gas, or are smothered by it. I suppose you will ask me what is theuse of this gas, and why it enters into the composition of the air? Youwill ask it with all the more curiosity when you know that the aircontains four times as much nitrogen as oxygen; to be exact, a hundredcubic feet of air contains seventy-nine cubic feet of nitrogen andtwenty-one cubic feet of oxygen. Now, the important part that nitrogenplays is to moderate the action of the oxygen in respiration. You maycompare this nitrogen mixed with oxygen to the water which you put in aglass of wine to temper it. Nitrogen possesses also another propertywhich is more general: it is one of the essential elements in a certainnumber of mineral and vegetable substances and the larger portion ofanimal substances. There are certain compounds containing nitrogen whichare indispensable to our food. An animal nourished entirely on foodwhich is destitute of nitrogen would become weak and would soon die."
"Excuse me, Monsieur Roger," said Albert Dalize: "how can nitrogen enterinto our food?"
"That is a very good question," added Miette, laughing; "surely youcannot 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, arewe not? we eat meat and flesh of animals. And what flesh do we chieflyeat? 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 theatmosphere and have fixed it in their tissues. Herbivorous animals, ineating vegetables, eat nitrogen, and we, who are carnivorous, we alsoeat nitrogen, since we eat the herbivorous animals. We also eatvegetable food, many kinds of which contain more or less nitrogen. Doyou 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," repliedMonsieur Roger. "But, to resume our explanation. Besides oxygen andnitrogen, there is also in the air a little carbonic acid and vapor. Thecarbonic acid will bring us back to the point from which westarted,--the phenomenon of breathing. Carbonic acid is a gas formed byoxygen and carbon. The carbon is a body which is found under a largevariety of forms. It has two or more varieties,--it is either pure ormixed with impurities. Its varieties can be united in two groups. Thefirst group comprises the diamond and graphite, or plumbago, which arenatural carbon. The second group comprises coal, charcoal, and the sootof a chimney, which we may call, for convenience, artificial carbon.When oxygen finds itself in contact with carbonaceous matter,--that isto say, with matter that contains carbon,--and when the surroundingtemperature has reached the proper degree of heat, carbonic acid beginsto be formed. In the oven and the furnace, coal and charcoal mingle withthe oxygen of the air and give the necessary heat; but it is firstnecessary that by the aid of a match, paper, and kindling-wood youshould have furnished the temperature at which oxygen can join with thecarbon in order to burn it. That is what we may call an active or alive combustion; but there can also be a slow combustion of carbon,--acombustion without flame, and still giving out heat. It is thiscombustion which goes on in our body by means of respiration."
"Ah, now we have come around to it!" cried Miette. "That is the verything 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. "Youtold us that we swallowed oxygen and gave out carbonic acid; and youalso said, 'Whence comes this carbonic acid? From combustion.' That iswhy 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 hislittle 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 findsitself in contact with the carbon in the black or venous blood. Thecarbon contained here joins with the oxygen, and forms the carbonic acidwhich we breathe out. This is a real, a slow combustion which takesplace not only in our lungs,--as I said at first, in order not to makethe explanation too difficult,--but also in all the different portionsof our body. The air composed of oxygen and nitrogen--for the nitrogenenters naturally with the oxygen--penetrates into the pulmonary cells,spreads itself through the blood, and is borne through the numberlesslittle capillary vessels. It is in these little vessels that combustiontakes place,--that is to say, that the oxygen unites with the carbon andthat carbonic acid is formed. This carbonic acid circulates, dissolvedin the blood, until it can escape out of it. It is in the lungs that itfinds liberty. When it arrives there it escapes from the blood, isexhaled, and is at once replaced by the new oxygen and the new nitrogenwhich arrive from outside. The nitrogen a
bsorbed in aspiration at thesame time as the oxygen is found to be of very much the same quantitywhen it goes out. There has therefore been no appreciable absorption ofnitrogen. Now, this slow combustion causes the heat of our body; infact, what is called the animal-heat is due to the caloric set free atthe moment when the oxygen is converted into carbonic acid, in the sameway as in all combustion of carbon. In conclusion, I will remind youthat our digestion is exercised on two sorts of food,--nitrogenous foodand carbonaceous food. Nitrogenous food--like fibrin, which is the chiefsubstance in flesh; albumen, which is the principal substance of theegg; caseine, the principal substance of milk; legumine, of peas andbeans--is assimilated in our organs, which they regenerate, which theyrebuild continually. Carbonaceous foods--like the starch of the potato,of sugar, alcohol, oils, and the fat of animals--do not assimilate; theydo not increase at all the substance of our muscles or the solidity ofour bones. It is they which are burned and which aid in burning thosewaste materials of the venous blood of which I have already spoken.Still, many starchy foods do contain some nutritive principles, but invery small quantity. You will understand how little when you know thatyou would have to eat about fifteen pounds of potatoes to give your bodythe force that would be given it by a single pound of beef."
"Oh," said Miette, "I don't like beef; but fifteen pounds ofpotatoes,--I would care still less to eat so much at once."
"All the less that they would fatten you perceptibly," replied MonsieurRoger; "in fact, it is the carbonaceous foods which fatten. If they areintroduced into the body in too great a quantity, they do not findenough oxygen to burn them, and they are deposited in the adipose orfatty tissue, where they will be useless and often harmful. You see howindispensable oxygen is to human life, and you now understand that ifrespiration does not go on with regularity, if the oxygen of your roomshould become exhausted, if the lungs were filled with carbonic acidproduced by the combustion of fuel outside the body, there would followat 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 allremained silent and thoughtful. It was Miss Miette who first broke thespell by asking a new question of her friend Roger. Asphyxia hadrecalled to her the fire. Then she had thought of the manner ofextinguishing fire, and she said, all of a sudden, her idea translatingitself upon her lips almost without consciousness,--
"Why does water extinguish fire?"
Monsieur Roger, drawn out of his thoughts by this question, raised hishead, 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 nota simple body, but that it is composed of two gases, next turned hisattention 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 soskilfully that he succeeded in showing that water was formed by thecombination of two gases."
"Of two gases!--water?" cried Miette.
"Certainly, of two gases. One of these gases is oxygen, which we havealready 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 notLavoisier, however, who first discovered hydrogen. This gas had beendiscovered before his time by the chemists Paracelsus and Boyle, who hadfound out that in placing iron or zinc in contact with an acid calledsulphuric acid, there was disengaged an air "like a breath." This air"like a breath" is what we now call hydrogen. Lavoisier, with theassistance of the chemist Meusnier, proved that it was this gas which incombining with oxygen formed water. In order to do this he blew acurrent of hydrogen into a retort filled with oxygen. As this hydrogenpenetrated into the retort, he set fire to it by means of electricsparks. Two stop-cocks regulated the proper proportions of the oxygenand the hydrogen in the retort. When the combustion took place, they sawwater form in drops upon the sides of the retort and unite at thebottom. Water was therefore the product of the combination of hydrogenwith oxygen. The following anecdote is told in regard to thiscombination. 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 hisscholar the necessary apparatus for making the combination of hydrogenand oxygen, and, at the moment when he was about to send the electricspark into the retort, he said, bowing his head,--
"'If it please your Royal Highness, this hydrogen and oxygen are aboutto have the honor of combining before you.'
"I don't know if the hydrogen and the oxygen were aware of the honorwhich was being done them; but certainly they combined with no moremanners than if their spectator was an ordinary boy. Now, I may add, youmust not confound combinations with mixtures; thus, air is a mixture ofoxygen and nitrogen, while water is a combination of hydrogen andoxygen. This combination is a union of the molecules of the two gaseswhich produces a composite body formed of new molecules. These newmolecules are water. Now, this last word recalls to me Miette'squestion."
"Yes," said the latter: "why does water put out fire?"
"There are two reasons for this phenomenon," said Monsieur Roger: "thefirst is that water thrown upon the fire forms around the matter incombustion a thick cloud, or vapor, which prevents the air from reachingit. The wood, which was burning--that is to say, which was mingling withthe oxygen of the air--finds its communication intercepted. The humidvapor has interposed between the carbon of the wood and the oxygen ofthe air; therefore, the combustion is forced to stop. Further, waterfalling upon the fire is transformed, as you very well know, into vapor,or steam. Now, this conversion into vapor necessitates the taking up ofa certain quantity of heat. This heat is taken away from the body whichis being burned, and that body is thus made much cooler; the combustiontherefore becomes less active, and the fire is at last extinguished."
"Very good," said Miette; "but still another question, and I will letyou 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 fireby doing the same thing?"
"In these two cases there are two very different actions," repliedMonsieur Roger: "in the first there is a mechanical action, and in thesecond a chemical action. In blowing upon a candle the violence of theair which you send out of your mouth detaches a flame which holds ononly to the wick. The burning particles of this wick are blown away, andconsequently the combustion is stopped. But the case is very differentwhen you blow with a bellows or with your mouth upon the fire in thestove. There the substance in combustion, whether wood or coal, is amass large enough to resist the violence of the current of air you throwin, and it profits from the air which you send to it so abundantly, bytaking 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 toremember your promise, and ask me no more hard questions to-night."
"Yes, friend Roger," said Miette, "I will leave you alone; you may go tosleep."
"And it will be a well-earned sleep," added Madame Dalize, with theassent of every one.
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