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The Wonder Book of Chemistry by  Jean Henri Fabre


 

 

AIR AND COMBUSTION

[211]

T
HE next day the sparrow, quite recovered from its trying ordeal of the day before, did credit to Emile's careful ministration by its vigor and appetite. The supply of oxygen being used up, Uncle Paul had his nephews prepare, under his supervision, a little more of that gas and also some nitrogen. Burning phosphorus under the bell-glass gave nitrogen, and the decomposition of chlorate of potash gave oxygen. The boys fairly beamed with joy at being allowed to take a hand in these momentous operations. All went off according to rule and with great success. It is true that their uncle was there all the time, advising and directing; but it is also true that both Jules and Emile are unusually skilful with their hands. And so their uncle was not afraid to trust them with bell-glass, balloon, tubes, and bottle, for in hands so careful there was no danger of breakage. When the two gases had been collected the lesson began.

"Oxygen is the only breathable gas," said Uncle Paul, "the only one that will sustain animal life, and also the only one that will make fire burn. But its energies are too powerful, as was proved to you yesterday by the sparrow and the candle. These [212] energies must be toned down by the addition of an inactive gas. When a wine is too heady, we weaken it with water to make a drink that will not injure our health. In the same way oxygen, too strong for breathing or for ordinary combustion when pure, must be weakened with nitrogen, an inactive gas. This mixture gives us atmospheric air, in which nitrogen plays the part corresponding to that of water in diluted wine.

"Our burning of phosphorus under the bell-glass showed us that air is composed of two elements, oxygen and nitrogen, the quantity of the latter being four times that of the former. Now we are going to reverse the operation and make air out of the two elements here before us. Here in this bottle is oxygen, and there in that other is nitrogen. By mixing these two gases in the right proportion we ought to get air like that in which we live, air in which a candle will burn calmly and an animal breathe without danger. How are we to proceed in order to obtain this result? Our unduly strong wine,—that is, our oxygen,—must be diluted with a good deal of water, or nitrogen. In fact, we must add to oxygen four times its volume of nitrogen.

"Nothing could be simpler. I fill the bell-glass with water, and then cause a bottleful of oxygen to displace a part of the water. The bottle which is to serve as a common measure for both gases I select at random, taking care, however, that it shall be of moderate size, so that the capacity of the bell-glass may not be exceeded when the two gases are mixed. Here we have, then, our oxygen [213] in the bell-glass; and now I fill the same bottle with nitrogen, which I release inside the bell-glass, and I do this four times. This accomplished, the bell-glass contains five bottlefuls of gas,—four of nitrogen and one of oxygen, such being the proportions taught us by our experiment with phosphorus. Consequently, we have here a volume of gas in no way different from the air we breathe, as will be proved in the clearest possible manner by the two experiments we are now about to perform.

"With the gaseous mixture in the bell-glass I fill a gage or a small bottle, into which I lower a lighted candle. The candle continues to burn with its usual calmness, neither faster nor slower than in ordinary air. It behaves inside the gage just the same as it did outside. Our too strong wine is diluted just right. Diluted with nitrogen, the devouring oxygen shows by no means so keen as appetite; it consumes the candle quietly instead of making a little bonfire of it.

"Let the sparrow tell us the rest. I transfer the gas in the bell-glass to a large bottle with a wide mouth, after which I put in the bird. Does anything unusual follow? Nothing, you see. The little captive, transferred to a new prison, is rather disturbed and tries to escape, but shows no sign of painful breathing. Its breasts rises and falls as usual, its beak is not open in sign of panting, there is no agitation to indicate suffering; in short, the sparrow breathes in its glass cage exactly as it did in its wicker one, thus proving that air within is of the same kind as that outside. But to make it [214] still plainer to you that in this artificial atmosphere, the product of our skill, there is no danger of death, we will let the bird stay in the bottle a few minutes longer."

This, accordingly, was done. The boys, in some anxiety as to the result, watched the sparrow closely and were surprised to see its liveliness continue undiminished in an atmosphere made by themselves. The calmness shown by their uncle, who would have been the first to put an end to the experiment if there had been any risk to the patient, reassured them although they still had some slight misgivings, so deeply had they been impressed by the painful end of the other sparrow in the bottle of nitrogen.

"That will do," said their uncle at last. "We know all we wished to know. Set the captive free."

Jules held the bottle, open, out of the window, and the bird flew away as if nothing unusual had happened to it. With a few strokes of its wings it was on a neighboring roof, perhaps giving its comrades an account of the strange things that had taken place in the chemical laboratory.

"What is is saying to them?" Emile wondered. "Is it telling them about its glass cage and its crazy behavior and high fever in the oxygen?" Then to his uncle: "So the air the sparrow came out of is just the same as what we breathe?"

"Yes, just the same. Composed, like it, of oxygen and nitrogen in the proportions I have already named, it maintains the candle-flame and also the life of the breathing animal. With oxygen and ni- [215] trogen we made air exactly like the air that keeps us alive."

"Then the air the sparrow breathed we could breathe too?"

"We could breathe it without perceiving the slightest difference, for, as I tell you, it is the same thing."

"I asked because I thought it so strange we could live in air made by our own hands, with our drugs and our outfit of bottles and tubes. And there's something else, stranger still, that came into my head. Let me tell you what it was. Our oxygen here was furnished by a salt, chlorate of potash, in which chemical combination had stored it up. You have told us that there are many other salts, all rich in oxygen, from which this gas could be obtained if it were n't so hard to decompose them. One of these seems to me particularly interesting, the one used for building houses."

"You mean limestone, carbonate of lime?"

"Yes, carbonate of lime. That salt has oxygen just like the others, has n't it?"

"Without a doubt. What of it?"

"Well, if limestone has oxygen, the oxygen could be taken from it?"

"If absolutely necessary the thing could be done, but I warn you it would be enormously difficult in practice."

"No matter; it could still be done. Then we may think of chemistry as telling us that limestone can be breathed, and the idea of limestone as just so [216] much air that we could breathe strikes me as rather funny."

"You go too far in imagining limestone could be made to furnish oxygen. There is no impossibility in that."

"Could we really breathe air that is made partly out of limestone?" asked Jules, as surprised at his uncle's answer as at Emile's queer ideal

"Why not? The sparrow, with more delicate ogans of breathing than we, breathed air containing oxygen from chlorate of potash, another mineral substance,—in fact, another kind of stone. To accustom you a little to these curious changes and shiftings in which certain elements are used for one thing to-day, for another to-morrow, and for still a third the next day, with no ultimate loss or gain of a single particle of matter, listen to what I am going to tell you now that the opportunity occurs.

"When the lime-burner fires the limestone in his furnace, the carbonic acid which it contains, and which is an invisible gas, escapes and is scattered far and wide in the atmosphere. Vegetables and plants and trees feed on carbonic acid through their leaves. I simply state the fact here, postponing the demonstration until later. They take in from the air the carbonic acid coming from a thousand sources, the least of which, hardly counting at all, is the lime-kiln. They break it up, keep the carbon, and reject the oxygen in a pure state. This oxygen spreads through the atmosphere, becoming a part of our breathable air. Who, then, would venture to [217] deny that in a whiff of air breathed by us there may sometimes be a little oxygen from limestone,—from building-stone, in fact? The gas from stone such as is used in building may, indeed, sometimes help to keep us alive. The elements come and go between one compound and another; substances cease to be, and in doing so give up their material to new substances; the indestructible elements, released form one combination, reappear with properties unchanged in another. Whether it comes from air, chlorate of potash, plaster of Paris, iron-rust, marble, or limestone, oxygen is always oxygen, nothing more and nothing less, provided it be disengaged from all chemical union. Thus the same gas can in turn rust a piece of iron, reduce a stick of wood to ashes, feed a flame, incorporate itself in a wayside pebble and there lose its activity, or send the blood coursing through an animal's veins. Who knows whence comes the carbon in a mouthful of bread? What part may it not have played before entering into the wheat, and what part may it not play afterward? We get lost when we try to follow in imagination the travels of a bubble of oxygen or lump of coal through all the things that are being continually made and unmade.

"We will not dwell further at present on these wonders, but will return to our subject of air artificially obtained. When, just now, I put oxygen and nitrogen into the same bell-glass, did anything remarkable take place? No; there was not the least rise in temperature, no light, no tumultuous conflict between the two elements,—nothing, in short, [218] that ordinarily accompanies chemical union. Brought together, oxygen and nitrogen did not act on each other; hence, they did not combine chemically in the resulting atmospheric air, but merely mixed. Now, I assure you there is an immense difference between the chemical combination of two gases and their simple mixture. There is an extremely powerful liquid, an acid, which eats into and dissolves most metals, even the hardest, with as much ease as water dissolves sugar; and it is appropriately called aqua fortis>  (the Latin for strong water). Its chemical name is nitric acid. The Latin term is very expressive, for very few substances can resist the furious strength of this liquid. Our skin, touched in any spot by a drop of it, quickly turns yellow, dies, and peels off in shreds. That is what results from the chemical combination of oxygen and nitrogen. By merely mixing the two gases we get atmospheric air, on the uninterrupted supply of which to the lungs depends our very life; but the chemical union of these same gases produces something that kills. I ask you to note especially here the utter difference between two substances that are nevertheless both made of the same elements. This difference is not unlike that observed by you between the simple mixture of sulphur and iron filings and the chemical combination of these two substances, the combination having none of the properties of either sulphur or iron.

"Thus, air is a simple mixture of oxygen and nitrogen in the proportion of four liters of the latter to one of the former. Oxygen maintains combustion [219] and respiration; or, in simpler language, it makes things burn and animals breathe; but nitrogen merely moderates the powerful energies of the oxygen mixed with it in the air around us. What takes place in the act of breathing deserves our serious study, but the proper time has not yet come for that. Some day, after we have learned certain things that will prepare the way, we will take it up again, in detail. At present let us confine our attention to the subject of combustion, or burning, and especially to that seen every day in our own fireplaces. A substance burns when it combines with oxygen, and so in every act of burning there must be both something to burn and oxygen to make it burn. Let us look into this a little more closely.

"When we wish to make a fire burn more briskly, what do we do? We take the bellows and blow air on the fuel,—the wood, coal, or charcoal. At each blast from the bellows the fire revives and its strength increases. The live coals, at first of a dull red, grow bright red and then glowing white. Air brings new life to the fire by giving it oxygen. But if we wish to keep the fuel from burning up too fast, what do we do? We cover the fire with ashes and thus guard if from too free contact with the air. Under this covering the coals remain alive for a long time, being only very gradually consumed. Thus fire is kept up in a fireplace only by continual supply of air, the oxygen of which combines with the fuel as combustion goes on.

"If a fire is to be brisk and give out good heat, air must be supplied in a rapid current proportional to [220] the amount of the fuel. In the economical foot-warmer, heated by a few live coals, there is but little air admitted, and it gets at the coals only through a covering of ashes. Combustion is proportionately slow, and the heat given out is slight, but it is lasting. On the other hand, in the great blast furnaces of our iron-works, consuming their fuel by the cartload, air is supplied in powerful gusts by blowing-machines that raise a veritable tornado. This hurricane of air fans something more that a brazier of live coals; it creates a sort of roaring inferno. Call to mind our sitting-room stove, when, having been first cleaned out and then well filled with fuel, it burns with a kind of subdued rumbling."

"I know what you mean," Emile broke in; "we say then that the stove is snoring."

"Yes, and it is the cause of this snoring that I now wish to explain to you. If the door of the ash-pit is open, or at least partly open, the stove snores; but f it is closed the stove is quiet. Why is this? Evidently because something rushes noisily into the stove when an entrance is left for it. What this something is will not be hard to find out. Hold your hand near the door of the ash-pit and you will feel a lively current of air. So it must be air that goes with a snoring sound through the bed of burning coal. That is what we call a draft. A stove that snores has a good draft; that is, plenty of air passes through its burning fuel, and so the fire is vigorous and gives out a good hear. A silent stove has a poor draft; air comes in but slowly, and the fire is low. According to the freedom or hindrance [221] with which air is admitted,—that is, according to the strength or weakness of the draft,—the fire burns rapidly or slowly.

"Now let us seek the cause of this draft. Over a hot stove, wave a piece of burning paper, and you will see the burnt particles rise in an eddy, going to a greater or less height, sometimes even up to the ceiling. Those bits of burnt paper, light though they are, do not go up like that of their own accord; an upward draft must carry them. This draft is produced by the ascending flow of air that has been warmed by contact with the stove and thus made lighter; it rises and is immediately replaced by cold air, which in its turn becomes warm and rises. Although air is invisible, its ascent can be inferred from the rising particles of burnt paper that are carried up with it, very much as the imperceptible movement of nearly quiet water is shown by the drifting of objects floating on its surface.

"There is another experiment I advise you to perform next winter, when the stove is going. Take a sheet of paper and cut out a circle as large as your hand; then with a pair of scissors cut this circle into a spiral ribbon, following a line that starts from the edge and gradually approaches the center. Attach the middle of this spiral to the lower end of a wire hung vertically over the stove, and then let go of your paper ribbon. It will stretch out, by its own weight, into a sort of cork-screw, big at the bottom and small at the top where the wire supports it. If the stove is hot you will [222] see the corkscrew whirl round and round like some ingenious piece of mechanism. The cause of the whirling is this: The paper ribbon presents its surface diagonally to the current of hot air that is continually rising, and from the push it thus receives all along it length comes the operation of the little mechanism. It is this same push of moving air against the diagonal sails of a windmill that causes them to turn.

"Thus it is proved that on being heated, air becomes lighter and consequenty rises, while cold air rushes in to take its place. The push of this rising air turns out paper corkscrew, and it is this same current that carries upward the bits of burnt paper. You will now be able to understand what takes place in our stoves and fireplaces when we say that they are drawing well. If the air were all of the same temperature in the chimney, in the room, and out of doors, there would be no draft. But as soon as the fire is lighted, a change of conditions is brought about: the column of air in the stovepipe or in the flue of the chimney gets warm, becomes lighter, and rises. The hotter the air and the taller the column, the faster the ascent. As the hot air rises, cold air, which is heavier, rushes toward the fire, makes it burn, becomes heated in doing so, and passes up in the ascending column. In this way a continual current of air is established from the bottom of the chimney to its top. In passing over and through the burning fuel, this constantly renewed air-current feeds the fire with its oxygen, and as soon as it is heated and has taken on its load of [223] carbon continues its journey up the chimney, taking the smoke along with it and finally escaping into the open air. That is how a chimney draws, how a stove snores. The draft acts like a pair of bellows working of its own accord, renewing the air as fast as the oxygen is used up, and so keeping the fire going. To maintain a good fire, follow this simple rule: allow free entrance of air with its fresh supply of oxygen, and free exit of air that has been used and no longer has any oxygen, or at any rate has not enough of it. Let plenty of air come in below, let it circulate without hindrance through the burning fuel, and then let it pass upward and give place to a fresh supply. By so doing you will have a fire that, with sufficient fuel, will do you credit."


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