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The Wonder Book of Chemistry by  Jean Henri Fabre

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HE use of red-hot iron for obtaining hydrogen from water is a slow and tiresome process, requiring many repetitions of the same operation to secure even a small quantity of the gas. With live coals instead of hot iron, speedier results are obtained, but the hydrogen is not pure; it is mixed with other gases derived from the coals, and to these is due the bluish tinge of the flames, a peculiarity detected by Jules. Excellent, for practical reasons, as are these two simple and easy methods when the sole object is to show that water contains an inflammable gas, they must give place to others when it is desired to obtain a considerable quantity of hydrogen in a short time.

"Let us turn now," said Uncle Paul, "from this way of getting hydrogen from water,—I mean by the use of live coals. What we really get is a mixture of various gases that have to be examined separately if we are to avoid confusion. And let us turn, too, from the method in which red-hot iron is used; though this time the hydrogen is pure, there is very little of it. What we are looking for is some simple process that will give us all the hydrogen we desire, and that without furnace, forge, or [244] brazier, which are not always conveniently at hand. I have to tell you here that iron can decompose water without first being heated if it only has the help of a little sulphuric acid. With these two acting together, the hydrogen in water can be set free with all the ease one could ask for. I must also tell you that another common metal, zinc, is still better than iron for decomposing water,—always, of course, with the help of sulphuric acid. We can, then, use whichever of the two metals we chance to have at hand, though zinc is to be preferred. If that is lacking, we will resort to iron filings, which, as they consist of minute particles, readily yield to chemical action when brought into contact with the other substances used in this method.

"Into this tumbler I put water and some pieces of zinc from the old watering-pot that supplied me the other day with material for showing you that this metal will burn. No results are apparent as yet, all remaining quiet in the glass because cold zinc by itself has no effect on water. But I add a little sulphuric acid and stir it in well. Now things will go on unassisted. The water begins to boil violently, sending up countless bubbles of gas that burst on reaching the surface. These bubbles come from the decamped water; they are hydrogen, precisely the same inflammable gas that we obtained by using red-hot iron in the blacksmith's shop. Watch now. I hold a piece of lighted paper near the surface of the water, and each bubble, as it bursts, catches fire with a slight explosion, burning with a flame so pale as to be visible only in [245] the dark. As the bubbles follow one another thick and fast, there is an almost continuous popping."

This minature artillery popping away on the surface of the liquid, and these flames dancing on the water, certainly offered a curious spectacle. But there was something else that appeared to have even greater interest for the young spectators: the water had started to boil with no fire of any sort to heat it, and the glass had become so hot as to make one almost afraid to touch it. Uncle Paul anticipated the surprised inquiries prompted by these remarkable developments.

"Look into the glass," said he, "and you will see that the hydrogen bubbles first make their appearance on the zinc, for it is there that chemical action, resulting in the decomposition of the water, takes place. These bubbles of gas make their way up through the liquid and in doing so cause considerable commotion, just as water boiling over a fire is agitated by the bubbles of steam that are being formed. In reality the water in this glass is not in motion as a whole, but is merely stirred by the uprushing bubbles in the same way it would be stirred if you blew air into it through a straw. The boiling of the water is only apparent, only an agitation that deceives the eye."

"But the glass is awfully hot," remarked Emile; "I can't bear my hand on it."

"Very true; but the heat is still far below that of boiling water. If you should be ask me to prove it, I should only have to take the tongs and lift out the piece of zinc, whereupon the liquid would immedi- [246] ately quiet down, there being not further generation of hydrogen, which caused the commotion."

"All the same, there's lots of heat there. Where does it come from, with no fire to make it?"

"I see Emile finds it hard to get used to the idea of heat without fire. Did we need any fire to make the mixture of powdered sulphur and iron filings raise the temperature of the bottle to a burning heat? Does the mason use fire when he pours cold water on lime and makes a paste that is too hot for the hand to bear? Without fire, without live coals, without any apparent cause, great heat is produced in both cases, and chemical combination explains it. In our tumbler, here, we have an example of it. Water is being decomposed, but at the same time the opposite process, combination, is going on between the acid and the metal; and this process generates heat. We will come back to this interesting point later, and you will then see that the heating of the liquid in our glass is only what might have been expected, for zinc is really burning, or undergoing combustion.

"It is not enough to know how to get hydrogen with zinc and sulphuric acid; you must also provide something to receive and hold the gas. A little difficulty presents itself at the start. We have to do with three substances,—water to furnish the hydrogen, and sulphuric acid and zinc for decomposing the water and so releasing the hydrogen. All the water and all the zinc to be used in the operation may be put into the glass at once, but the sulphuric acid should be added little by little as it is needed. [247] If poured in copiously and all at once, it would raise an unmanageable commotion. Under these conditions the operation would be too rapid, and the operator would run the risk of being splashed with the boiling liquid. Consequently, the sulphuric acid should be poured in gradually, a fresh supply being added whenever the release of gas slows up. Moreover, these successive additions of acid should be made without opening or in any way disarranging the vessel in which the hydrogen is generated. In this manner we prevent any hydrogen and form a dangerous mixture.

"The vessel commonly used in this operation is a sort of bottle with two necks, one in the usual central position, the other at one side. Into this bottle is put a handful of zinc cut into small pieces, or, better, a small sheet of zinc rolled up so as to pass through the neck. Enough water is then poured in to cover the metal completely. Through one of the necks, no matter which, is passed a glass tube, which is held in place by a tightly fitting cork stopper with a hole in it to receive the tube, and which is bent over and downward on the outside like the one we used in producing oxygen. Finally, through the other neck and into the liquid is passed a straight glass tube, which is held in place in the same manner as its companion. The apparatus is now ready for use, only sulphuric acid having to be added. For this purposed the straight tube is equipped at the top with a small glass funnel, through which the acid required is gradually introduced. As long as [248] the release of gas proceeds satisfactorily, no further attention is necessary; but if it slackens, a fresh dose of acid is poured in. This arrangement is very simple and very ingenious. The straight tube, extending into the water as it does, admits no air to mingle with the hydrogen, a thing to be carefully avoided, as will be shown; but it does allow the introduction of sulphuric acid whenever needed. Furthermore, the hydrogen that is being released cannot get out this way, as the water keeps it back; hence its only issue is through the bent tube, the nearer end of which is in the second neck of the bottle and well above the water. In other words, the gas-factory has two doors and only two,—the straight tube, which allows entrance but not exit, and the bent tube, which offers a way out, but no passage inward, when the apparatus is in operation and discharging its hydrogen.


"One thing more. Suppose the bent tube gets stopped up in some way, or that it is too small for a sufficiently rapid discharge of the gas set free by the decomposition of the water; what will happen? The gas collected in the bottle, and unable to get [249] out, will press downward on the liquid and drive it up through the straight tube until it overflows the funnel at the top. This rising of the water in the straight tube warns us that something is wrong with our apparatus, blocking the issue of the gas. But, unless we pour in too much sulphuric acid at a time, we need not trouble ourselves to watch the straight tube for signs of danger.

"Such is the hydrogen apparatus used in laboratories, and I regret that I cannot show you one in operation to supplement my description; but a two-necked bottle is not an easy thing to procure in our village."

"That's so," chimed in Emile. "I've never seen anything of the sort around here. A bottle with two mouths, one for pouring in and another for pouring out, is n't a thing you 'd be likely to find In any rubbish heap. And so we can't have our hydrogen factory, after all," he concluded in a plaintive tone.

"But should I have aroused your expectations at the blacksmith's if I had n't known beforehand that I could gratify them? With an old pickle-jar and a little ingenuity, need we despair of success? Your uncle thinks not. What does our modest bottle lack to make it a serviceable piece of apparatus? Two mouths; and we will supply them without further delay."

So saying, he took a good-sized cork stopper that had belonged to some demijohn, and shaped it carefully with a file to make it fit the large neck of the pickle-jar. Then he punched two holes in it, and in [250] one of these he fixed the bent glass tube, pushing it only just through the cork, while through the other hole he passed the straight tube, forcing it down much farther. Bits of zinc and enough water to cover them well were next put into the jar, after which the stopper was inserted, with a little moist clay around the edge to prevent any escape of gas. Emile was delighted with the turn things were taking: he was going to see hydrogen made in as large quantities as any one could wish. Everything was arranged in accordance with his uncle's previous description.

"Here is the straight tube," the boy pointed out in eager interest, "that you'll pour the acid into when the time comes, and there is bent one for letting the hydrogen out. The old pickle-jar is going to do very well, now that it has two mounts made in the big stopper. But there's one thing we have n't got yet,—the little funnel for pouring in the acid."

"I have none," replied his uncle.

"What shall we do, then? The tube is so small we can't pour anything into it without a funnel."

"Let us ask Jules and see whether he would allow such a trifle to defeat our purpose."

"You will laugh at my idea," said Jules, on being thus turned to for advice, "but why could n't we use a little piece of paper rolled up into a cone open at the point?"

"Your suggestion is unanimously adopted. Lacking a regular chemist's funnel, we could hardly do better. Your little paper cone shall take the place [251] of the small glass funnel; but it will soon go to pieces, I warn you, for sulphuric acid is exceedingly destructive. However, that does n't matter in this case, as we can renew our paper funnel as often as necessary. Economy in this particular is not required."

So said, so done. The paper funnel inserted in the upper end of the straight tube made it possible without the slightest difficulty to pour in the sulphuric acid, whereupon the water in the jar immediately began to boil, as it appeared to the eye, and hydrogen came out through the bent tube, the further end of which went down into the water in the bowl. The boys hastened to touch a lighted paper to the gas bubbles thus created. Quick flashes of flame, a crackling sound, a pale white light—all these duly followed as the gas came rushing out of the pickle-jar and was ignited. It was really and truly hydrogen; a regular laboratory outfit could not have given better results.


"You are pretty well acquainted with this water artillery now," said Uncle Paul. "Let us pass on to something else and set fire to a large volume of hydrogen. I dissolve a little soap in water, and into this soapy water I lower the end of the tube through which the gas is discharged. If we took a straw and blew through it, we should get plenty of foam. The bottle blows in its own peculiar way: [252] it sends a jet of gas into the midst of the soap-suds and makes a mass of tiny bubbles, all filled with hydrogen. In this manner we obtain a certain amount of the inflammable gas stored up in little thin-walled cells. I apply a piece of lighted paper, and the gas catches fire. The explosion is louder, the flame larger than before, though the light produced is still very pale."

At the request of the young pupils, who were quite fascinated with this exhibition, the experiment was repeated and a still greater volume of gas was produced, which was then exploded with fine effect.

"We have nothing more to learn from this play-thing," concluded Uncle Paul. "It has shown us how readily hydrogen catches fire: hardly do we touch the lighted paper to the bubbles, when the imprisoned gas explodes. Let us now proceed to another experiment, which will show us that hydrogen, so highly inflammable in itself, can yet be used for putting out fire. It burns as nothing else will, and yet it stops the burning of anything plunged, all afire, into the midst of this gas. It will put out a lighted candle as quickly as will nitrogen. Let us prove it. I will plunge the end of the bent tube of our apparatus into the bowl of water and fill with hydrogen either the gage or a tall bottle with a wide neck, proceeding exactly as I did with oxygen."

Accordingly the gage was filled, after which Uncle Paul continued:

"Here is our gage, now, full of hydrogen. I lift it out of the water."

With these words he took the gage by its foot and [253] withdrew it from the bowl, holding it upside down as one would to empty it of a liquid. This procedure seemed to the boys to betray absent-mindedness on their uncle's part.

"If you hold it that way," they exclaimed, "the gas will all get out. The mount is pointing down, and it is n't corked."

"No, my lads, the hydrogen will not get out. It is much lighter than air, and so tends to rise and not to fall. To keep it from escaping, we must block its way above, not below, and this I do by holding the gage upside down. There being no outlet upward, the gas is held captive. As to the open mouth below, we need not give it a thought; our hydrogen cannot go down and get out that way. I put a lighted candle into the gage and push it up almost to the inverted bottom. See what happens. The lowest layer of hydrogen, being next to the outside air, immediately catches fire with a slight explosion, and the flame gradually works its way up to the top of the column of gas. But as for the candle-flame, it went out at the very first, being smothered by the hydrogen as quickly and completely as it would have been by nitrogen."

This seemed very strange to the boys; they wondered how a gas that burns so well itself could put out a fire already burning. But the explanation soon given by their uncle was found to be simple enough.

"All burning," said he, "let us repeat and continue to repeat until the mind is quite familiar with this first principle—all burning, I say, is nothing [254] but the chemical combination of some substance with oxygen, which is always present in the air. Where there is no oxygen, there nothing will burn. Well, then, the candle, on being thrust into the gage of hydrogen, went out because it did not find there the gas necessary for feeding its flame; if found no oxygen, and the other gas was unable to take its place, although very inflammable in itself. This gas, the hydrogen, took fire, but at first only in the bottom layer, because there and only there, next to the outlet, was there any air to feed the flame. Then this flame worked slowly upward from bottom to top, as the consumed hydrogen gave place to the air crowding in from below.

"Hydrogen is about fourteen times as light as air. This has been ascertained by means of chemists' scales, which are so delicately poised as to tip under the weight of a hair. Although an extremely light gas, hydrogen still weighs something, about one decigram to the liter. No other substance even among the most subtle, the gases, weighs so little. A liter of water weighs a kilogram, or ten thousand times as much as hydrogen. The heaviest of known substances is a metal called platinum, which weighs twenty-seven times as much as hydrogen. Between these two extremes range all the other substances known to us, some being heavy and others light according to their [255] position in this scale. Accepting these statements as verified, we will confine ourselves to showing by experiment that hydrogen is indeed much lighter than air.

"You have just seen how the gage must be held,—that is, with its mouth downward, to keep the hydrogen in. On account of its extreme lightness this gas escapes upward. Hence we may keep it confined by interposing some obstacle to this upward flight. Now let us prove that under the opposite conditions it will escape. We hold the gage upright, its mouth at the top, as if we had to do with nitrogen, oxygen, or atmospheric air, all three of about the same weight. Nothing standing in its way above, the hydrogen will quickly escape, you may be sure."

Refilled with hydrogen, the gage was set upright on the table, and they waited a few minutes. Nothing could be seen to go our or to come in. The sharpest eye could not have detected the departure of one gas and its immediate replacement by another.

"We have waited long enough," announced Uncle Paul. "There cannot be any hydrogen left there now. It is gone, and air has taken its place."

"How do you know?" asked Emile. "For my part, I can't see that anything has happened."

"Nor I, either; and if we had only our three pairs of eyes to decide the matter, the gage would keep its secret and never tell us what has taken place. But a lighted candle will tell us what our eyes cannot. If it keeps on burning in the gage, it will show that the latter contains air; if, on the con- [256] trary, it goes out after setting fire to the contents, it will mean that hydrogen is present."

A lighted candle was lowered into the gage and continued to burn there the same as before, proving that the hydrogen was gone and air, a heavier gas, had taken its place.

"If we lowered an open can of oil into a barrel of water," Uncle Paul went on, "what would happen? The water, being heavier than the oil, would force the latter out of the can and take its place, while the oil, being lighter than water, would rise and float on the surface. That is the way air and hydrogen act when the gage is set upright. But I have a still better experiment to show you in proof that hydrogen is lighter than air. With a few straws and a little soap-suds we can give a fine demonstration of the lightness of hydrogen. This is the way of it. You know better than I what will happen if we wet the end of a straw in soapy water and then gently blow through the straw. Emile used to play at the game not so very long ago, and he found it great fun."

"You mean blowing soap-bubbles?" Emile was quick to rejoin. "Oh, that's fine, Uncle! On the end of the straw there comes a bubble, and it swells out bigger and bigger, to the size of an apple or an orange if you blow it the right way. And you can see all the colors of the rainbow on it,—blue and green and red, and so on,—more beautiful than the finest flowers in our garden. You don't dare move the straw for fear the magnificent bubble will burst. But before long it does burst, [257] anyway, all of a sudden, and you don't know where it's gone to. How sorry I've felt, many a time, because my soap-bubbles would n't fly up into the air and soar about with all their splendid colors!"

"You won't have any reason to be sorry this time, my child," his uncle assured him, "for you are going to see you bubbles soar upward in fine style, and that without any coaxing."

"My beautiful soap-bubbles?"

"Your beautiful soap-bubbles."

"Then I shall like them better than ever."

"Show us first some of the kind you know so well how to make."

Emile took a straw, thrust one end into the soapy water that had been prepared, withdrew it, and blew gently into the dry end, making a series of bubbles, the largest of about the size of one's fist. All, as the filmy envelop became thinner with continued blowing, showed the brilliant hues of the rainbow; but they also, as soon as detached from the straw, all few softly to the floor. Not one would float aloft in the air.

"And why should they rise?" asked Uncle Paul. "They are filled with air that is no different from the air all about them, and consequently receive no impulse from this gas either to rise or to fall. But their covering, which is made of a thin film of soapy water, is heavier than air, and so causes them to fall. Hence, if we wish our bubbles to rise, we must fill them with a gas lighter than air, a gas that will by its lightness not only make up for the weight of [258] the envelop, but will also lift it up and bear it skyward. That gas is hydrogen."

"But how can I fill my bubbles with that?" asked Emile. "I can't blow hydrogen into them with my mouth."

"We will make the bottle do the blowing, and it will blow as well as any of us. First I take out the bent tube and put in a straight one, running side by side with that used for pouring in the acid and extending upward a little farther. But, as the tube is rather large, I contrive a smaller outlet at the top by inserting a straw wrapped at the lower end in a bit of wet paper. It is at the upper end of this straw, whence issues a jet of hydrogen—or a breath of hydrogen, if you prefer to call it so—that the soap-bubbles will form. I place the bottle upright so that the little balloons, borne upward by the lightness of the gas, will find nothing in their way. Now all we need do is to take a wisp of paper or something else and from time to time put a drop of the soap-suds on the end of the straw, whereupon we shall see bubbles form, filled with hydrogen."

No sooner said than done. At the end of the straw, which was kept supplied with soapy water, there appeared a succession of transparent globes, sometimes larger, sometimes smaller, but always in an upright position on the tip of the straw and straining to get away from it. Many succeeded as soon as they were big enough, and then away [259] they soared, rising rapidly and soon reaching the ceiling of the room, where they burst on touching it. Others burst before they could get clear of their moorings. Not for a good deal would the boys have missed seeing this ravishing spectacle. With wondering gaze they followed each balloon every instant, from start to finish. First they beheld it as a tiny bubble, then steadily swelling and all aglow with brilliant colors. It would sway a little on the end of the straw, then tear itself away, and off it would go in its flight to the ceiling. Oh, how gracefully it rose! But all too soon the ceiling was reached and the magnificent sphere shattered. Another soon followed it, however, and then another, and still another, as many as one chose. Jules was thoughtful, Emile jubilant.


"I am going to make this chemical diversion of yours still more entertaining," said Uncle Paul. "Ties a bit of candle to a long stick, light the candle, and then hold it up to a bubble while it is in the air."

Emile was not slow to carry out these instructions. Fastening a candle to a reed, he gave chase after one of the bubbles as it rose. Flack!>  went the little balloon, there was a sudden burst of flame in mid-air, and then nothing more; the whole thing had vanished. Emile gave a start; he had not expected so sudden a flash or one of so short duration.

"Does that startle you?" asked his uncle. "Did n't you know that hydrogen is exceedingly inflammable? Touch a lighted candle to a bubble filled with this gas, and there can't fail to be an in- [260] stantaneous outburst of flame. That is the whole secret of these little a๋rial fireworks so astonishing to you."

"Yes, it's simple enough, but I was n't expecting it."

"Now that you know what to expect, let us try it again."

The experiment was repeated several times, Emile allowing the bubbles to rise half-way to the ceiling and then touching them off with the candle. Not one of them, however quickly it rose, escaped the alert incendiary's pursuit. Thus it was shown in a highly diverting manner how readily hydrogen catches fire. Jules, who never asked idle questions, finally broke his silence,

"Our soap-bubbles," said he, "hit against the ceiling and that's the end of them. Would they go up very high if they had plenty of room? Where would they go to?"

"In the open air and with nothing in the way they might rise to a great height if they did n't burst too soon; but the least agitation, the slightest breath or air, is sufficient to destroy them, so delicate and fragile a thing is a soap-bubble. Nevertheless if the atmosphere is very calm, the bubbles may last long enough to soar out of sight. We can try the thing out of doors this minute, for luckily the air is perfectly calm just now. Not a leaf is stirring on the tree in the garden."

The apparatus was carried outside and the bubble-blowing resumed. Many of the bubbles burst when no higher than the roof of the house, while others, [261] though very few, rose out of sight. In a short time even Emile's sharp eyes could no longer distinguish them against the blue sky.

"Do they go very high?" asked Jules.

"I think not. A hundred meters, more or less, but at that height, being so small and so transparent, they become invisible. Their extremely thin and delicate covering, too, bursts before long. The one you are looking for now up there, hoping to catch another glimpse of it, is probably no longer in existence."

"But if the covering never broke, how high would the bubbles go?"

"On that point I can speak rather more definitely. Learned men who wish to explore the upper regions of the atmosphere and find out what is going on there, make enormous balloons of some strong fabric, and then varnish them outside and fill them with hydrogen, just as we are now filling our soap-bubbles. With these durable balloons they can go up to any height they please. The most daring have gone up ten thousand meters."

"Why not higher?" asked Jules. "I'd have gone a good deal higher if I'd been in their place. I should have wanted to see what there is at the very top of the blue sky. How beautiful it must be up there above the clouds!"

"In their place you would have done as they did, my dear boy, or probably not so much, for it takes almost superhuman courage to dare to visit those high regions. When you get to where there is not air enough, breathing becomes impossible and you [262] have to come down in a hurry, or you are dead in a few minutes. That is why, up to the present time, the greatest height attained by man is about ten thousand meters."

"But the hydrogen balloon could go still higher if there was no danger for the balloonist?"

"Without a doubt, much higher."

"How high?"

"I can't tell you exactly, but let us say twice as high. All that I can be sure of is that balloon-ascent has some fixed limit, no matter how light and how skilfully constructed the balloon may be. The layer of atmosphere is thought to be only about fifteen leagues in thinkness. Nothing that rises from the earth by reason of its superior lightness to air can pass that limit, since it no longer has air to buoy it up. At about fifteen leagues above the earth's surface, therefore, the ascent of any substance, even though it be the lightest gas, ceases."

"I'd be satisfied with ten thousand meters, or even much less, if I only had a strong enough covering to my balloon not to break at a mere nothing, as our soap-bubbles do."

"You shall have your strongly covered balloon no later than to-morrow."

"And can I send it up as high as I want to?"

"Yes, as high as you want to."

At this prospect Emile clapped his hands with delight, while a gentle smile of satisfaction showed that Jules, too, was not indifferent to their uncle's promise. If he could not, himself, explore that beautiful blue void whose mystery so fascinated him, [263] he could at least send a hydrogen balloon up there.

"One more question," said he "before we stop blowing soap-bubbles. When they are filled with our breath they have all sorts of bright colors, and when they are filled with hydrogen they look just the same, so it is n't what's in the bubbles that makes those beautiful colors?"

"No, my boy; those colors, which are the same as you see in the rainbow, do not come from air or hydrogen, nor do they come from the soapy water that makes the outside of the bubble. They are simply the play of light on the extremely thin covering. Whenever any transparent substance, whatever its nature, is in the form of an exceedingly thin film, the light striking on it causes this splendid coloring. Put a drop of oil, for instance, on still water, and the drop will spread out in the thinnest layer imaginable, whereupon the rich colors you speak of will appear. A soap-bubble or a thin layer of oil or a film of any transparent substance is called iridescent because it shows the colors of the rainbow, which the ancients called iris."

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