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The Wonder Book of Chemistry by  Jean Henri Fabre


 

 

A DROP OF WATER

[264]

"I
promised," Uncle Paul resumed, "to show you a balloon with so strong a covering that, when inflated with hydrogen, it could rise without danger of soon breaking. The time has come to keep my promise. Emile ought to have among his old playthings just what we need for our purpose. He doubtless remembers the pretty red balloons he used to buy at two sous apiece. To hold one of those balloons by its long string was an unfailing delight to him."

"Oh, yes, I remember them," Emile was quick to rejoin. "They were more fun than my Noah's ark with all the animals arranged in pairs, or my lead soldiers that I used to set up in companies on the table. But they are all crumpled up now in my old toy-box, and they've been there a long time, ever since the day after I got them; for they would n't go up at all after the first day, but fell right down to the ground, though they were all right for a little while and went up finely."

"And have you never wondered why those little red balloons, so ready to go up the first day, were so very slow about it the next day?"

"Oh, yes, I've often wondered, but I could n't tell why it was so."

[265] "I will tell you the reason. Those balloons were filled with hydrogen, the same gas that we used yesterday for the soap-bubbles. Their covering is a fine membrane of rubber, very elastic, so that it stretches freely under the pressure of the hydrogen inside. Despite its extreme thinness such a covering is, so far as we can see, air-tight, being far superior in this respect to even the most closely woven fabric with its countless little open spaces between the meshes; and yet so subtle a gas is hydrogen that it manages to get through and make its escape. Then little by little the balloon collapses; or else, remaining full and round, it exchanges its hydrogen for air, the two gases passing through the rubber envelop in opposite directions. Consequently, whether by partial deflation or by exchange of hydrogen for air, the balloon loses much of its buoyancy before long, and after twenty-four hours cannot rise at all. To give it new life, we must again fill it with hydrogen."

"Oh, if I'd only known that before, you'd have had me teasing you to put new life into my balloons."

"A little chemistry has its advantages, has it not, my boy, were it only to put new life into toy balloons that are dead? If yours are still in good condition—I mean if there are no holes in the rubber—nothing is easier than to make them as buoyant as ever. Go and get them."

Emile ran out of the room and was back in a trice with two little red balloons, shapeless and shriveled, with not a breath of hydrogen in them; [266] nor had there been for a good while. But the child took such care of his things, was so particular about putting them away properly, that all his old toys were still in excellent condition. The rubber membranes that had once been lively little balloons, but were now all lax and lifeless, had not the slightest hole in them, as Uncle Paul soon learned by blowing into them,

"Our little balloons are all right," said he, "and now to work! I take a common bottle, the first to come to hand, holding about a liter, and into it I put some water and a good handful of zinc scraps. Then through a perforated cork stopper, fitting the neck closely, I pass a straight glass tube or, if that is lacking, a pip-stem, or, better still, a goose-quill. The free end of this I next insert in the opening of the balloon and bind it with a string to make everything tight and prevent the escape of gas. Now I pour sulphuric acid into the bottle, and when the bubbling of the mixture is well under way, and the hydrogen is being released in ample quantity, I squeeze the balloon with my hand to drive out any air through the goose-quill, and still squeezing it I push the cork firmly into place. That done, I let go of the balloon and allow things to take their course. The crumpled membrane receives the gas, swells up gradually, and is stretched taut by the pressure of the hydrogen discharged from the bottle. Now you see it all spherical, and threatening to burst if I let much more gas go in. Finally, with a strong thread, I tie tightly the mouth of the balloon a little above the goose-quill to which it is [267] attached. In that way imprisoned hydrogen is retained, for a while at least. I then free the balloon from the goose-quill, so that the hydrogen forming in the bottle may escape and not accumulate to the point of blowing out the stopper with an awkward splash of the liquid itself."

"Now let's see if it will go way up high," proposed Jules as the balloon swayed this way and that, straining to get free, but held back by his uncle's detaining hand.

"I don't want to lose it," said Emile. "Let's tie a string to it. Here's a long one that will do."

"Before talking about strings," was his uncle's advice, "let us consider the matter a little. How much gas does our little balloon hold? A liter at the most. Conseqently, the weight of the hydrogen that fills it is about a decigram. The same volume of air weights fourteen times as much, making the difference between the two about thirteen decigrams. We will say the rubber covering weighs a grain, or ten decigrams. Three decigrams, then, are left as the measure of our little balloon's upward tug on its string. And with such feeble power at its disposal you would have our balloon drag the weight of a long string. Why not a rope or a ship's cable?"

"You are right. The balloon is n't strong enough to lift a long string up into the air. Then suppose we take a fine thread."

Attached to the end of a thread of this sort, the balloon rose, but, disappointing the children's expectations, it refused to go very high.

"It has stopped going up," said they. "Why?"

[268] "Because the higher it rises the more thread it has to pull up after it, and the weight of his is added to that of the balloon itself. So the time soon comes when the weight of the rubber covering, the hydrogen, and the lifted thread all together equal the weight of the air displaced by the hydrogen; and from that moment further ascent becomes impossible. As Emile wishes to keep this balloon we will inflate another and let it go without a thread to weigh it down. Free from this, it will go up as high as you wish."

And in fact, set free with no thread to shorten its flight by increased weight, the little balloon rises quickly and is very soon out of sight. How high will it go? Who knows? But whatever height it attains, sooner or later it will come down, because through the fine rubber covering there is taking place a constant exchange of gases, hydrogen going out and air coming in. Thus becoming always heavier, the balloon gradually falls back to the ground. But meanwhile who can say whither the winds may have wafted it?

"If Emile had n't kept the red balloons that we used to play with, could n't we have used a pig's bladder instead?" asked Jules. "That would give us a balloon as ready made, easy to get, and of a good size."

"I should have tried the pig's bladder only in case I had been unable to get anything better. It would have given us a large balloon, it is true, and made of a strong membrane; but it is loaded here and there with a layer of fat, which would be ob- [269] jectionable. You must not forget that the covering of our balloon should be as light as possible, so that the hydrogen may not lose its power of flight by having too great a load to cary. A liter of his gas cannot raise much more than a gram. Let us say the bladder will hold four liters, making about four grams that the contained hydrogen can raise. If, then, the bladder exceeds this weight, the balloon cannot rise. Consequently, if we propose to send up a balloon of this sort, the membrane must first be rid of its layer of fat, scraped down, its thickness reduced here and there, and all superfluous material remove, so as to lessen the weight as much as possible, while at the same time care must be taken not to make any holes in the membrane. With theses precautions, which call for the utmost patience and skill, success is possible.

"When our village festival is held, one of its features is the ascent of a balloon, filled, not with hydrogen, but with hot air. It goes up amid cheers from the crowd and salvos from the village artillery, consisting of mortars loaded with charges of powder well rammed home. Why should not we, too, in our chemical pastime, have a discharge of artillery to accompany the ascent of our balloons? A bottle shall be the mortar, hydrogen the powder. Every time we set fire to hydrogen, even to a tiny bubble of it rising to the surface of the water, you heard a slight explosion. It is only necessary to find out what causes this explosion, and then to produce a louder one. It is presumable that air is necessary here, or more accurately the oxygen contained in [270] air. Let us see, then, how hydrogen and air behave when mixed together.

"Into this narrow-necked bottle holding about a quarter of a liter, I pour enough water to fill it one third full, and then turn it upside down in the bowl so that it may receive a supply of hydrogen, using the bent tube once more and starting the release of the gas with sulphuric acid as before. Air fills two thirds of the bottle, water the other third. Hydrogen from our apparatus expels this water and takes its place. Thus the bottle contains a mixture of air and hydrogen, two parts of the former to one of the latter. I cork it tightly and wrap it up in several folds of a towel, leaving the neck free. Our gun is loaded. Now to fire it off."

With these words Uncle Paul grasped the towel-wrapped bottle by its middle with one hand, uncorked it, and held its mouth to the flame of a candle burning on the table. An explosion followed, and it was loud enough to make both the children start.

"Hurrah for the little hydrogen pistol!" cried Emile. "Famous gunpowder that makes. Once more, Uncle, please!"

The bottle-pistol was fired off again charged in the same manner as before,—that is to say, filled with the invisible powder of mixed hydrogen and air. Uncle Paul repeated the process as many times as was desired, and the explosions were more or less violent according to the proportions of hydrogen and air used. Some were full-toned and short, like the discharge of a firearm, others were only a noisy blowing, and others still, to Emile's unfailing [271] amusement, sounded like the yelping of a dog that has had its tail stepped on.

"My artillery," resumed Uncle Paul, "shows you that hydrogen and air form an explosive mixture that will take fire immediately at the touch of a flame and will explode violently. This mixture, invisible though it is, has no lack of power, and could blow its container to pieces if denied a sufficient outlet. That is why I wrap the bottle in a towel to arrest the fragments if the rupture should occur; and for a like reason it is well in theses experiments to select a bottle of rather small size, a quarter of a liter at most. With a larger one there would be serious risk of injury: the gun might burst and wound the gunner.

"Air, as you know, is a mixture of an active gas, oxygen, and an inactive one, nitrogen. In exploding hydrogen it is clear that nitrogen plays no part; or, rather, by its inertia and considerable volume it impedes chemical action and muffles the detonation. The oxygen alone is active. Let us, then, do away with the nitrogen and use pure oxygen, and the detonation will be much louder. What we need for this purpose is all ready here, as I took care this morning to prepare a bottle of oxygen beforehand. It stands there in the corner, upside down, with its mouth in a glass of water. Before going farther I must tell you that to get the loudest explosion, hydrogen and oxygen should be mixed in the proportion of two parts of the former to one of the latter.

"I fill with water a wide-mouthed glass jar to [272] serve as receiver for holding our explosive mixture. Turning it upside down with its mouth in the water, I discharge into it one part of oxygen, using the first bottle at hand as a measure. Then I add two parts or bottlefuls of hydrogen. That done, our gunpowder is ready. Look into the jar. What do you see there? Nothing. Nevertheless it holds a dangerous explosive which it would be unwise to set fire to without proper precautions. The glass jar would fly into fragments that might seriously disfigure us. But there is nothing really to be feared as long as no match is applied. If you ever undertake experiments of this sort by yourselves, bear in mind that the mere fact of having water at hand does not insure you against the serious results of carelessness. This explosive does not in the least mind getting wet; it could stay next to water indefinitely without losing any of its terrible explosive power. Dry and wet are all the same to it.

"With the aid of a funnel, I fill with the gaseous mixture the bottle we have just been using, corking it and wrapping it carefully in a cloth to guard against a rupture, a thing more to be feared now than in our previous experiment. Now I have only to uncork my piece of artillery and hold its mouth near the candle-flame. Attention! Say 'Fire!' "

"Fire!" cried the boys in unison.

Bang!>  went the gun, and the room rang as if a rifle had been discharged. Emile gave a jump; indeed, he was almost frightened.

"To think that something you can't even see [273] should make such a noise!" he exclaimed. "I can hardly believe it. If I'd known beforehand what was coming I should have stopped up my ears."

"Ah, indeed! That would look well, would n't it, Emile stopping up his ears so as not to hear the chemical pistol go off! No flinching, my boy, or I stop firing."

The candle having been relighted—for it was blown out with each burst of ignited gas from the bottle—Uncle Paul repeated the experiment. The explosion made the window-panes rattle; but this time, to make amends for his former show of fear, Emile stood as firm as a rock. He even forced himself to watch the proceedings with unwavering attention, and he saw a tongue of flame almost a meter long dart out with furious impetuosity from the mouth of the bottle. After a few more repetitions of this performance he became so hardened that he asked the great favor of being allowed to hold the gun in his own hand and fire it off as his uncle had done.

"I grant your request most willingly," was the reply. "There is nothing further to fear now. The bottle has been well tested, it has withstood the strain put upon it, and it will continue to do so. However, as an extra precaution keep it securely wrapped in the cloth, so that if by any chance it should burst the pieces would still stay in the wrapping. Grasp it boldly in your hand; there is nothing to be afraid of."

Without flinching, Emile managed the battle when it had been charged by his uncle. With sober face [274] and standing rigidly erect, like an artilleryman discharging his cannon, he fired off the glass gun. Jules then took his turn, his delicate hand trembling a little with excitement. And so they alternated until the explosive gas was all used up. These repeated discharges made the rest of the family wonder what in the world Uncle Paul and his nephews could be up to with their chemicals and their bottles and their other pieces of apparatus.

"Now that our gun is silenced for want of ammunition," said Uncle Paul, "let us see what we have left from all this burning of hydrogen in oxygen. The two gases combine when the explosion takes place, and this combination is attended by a streak of flame, not very bright, which you saw shoot from the bottle. A new substance, a compound of hydrogen and oxygen, is formed as the explosion takes place, but it cannot be seen, as it is an invisible vapor. This must be condensed if we wish to examine it. But instead of igniting a great volume of gas all at once, which would be somewhat dangerous and, besides, would not serve the purpose of careful study, we will arrange matters so that our hydrogen and oxygen shall combine a little at a time; that is, we will light a jet of hydrogen and let it burn in the air so that we can watch it closely.

"Let us begin with the preparation of our apparatus. It will be the same as we used for blowing our soap-bubbles, except that the straight tube having a straw in its upper end will be replaced by another with a tapering tip. The channel as it is now would not do, being too large; it must be made [275] smaller until the outlet does not much exceed the size of a pin. This is how it is done. A piece of glass tubing, as easy to melt as possible, is held in the flame of an alcohol lamp and twirled between the fingers so as to be heated equally all around. When it seems soft enough it is pulled so that the dough-like heated part stretches into a thread and two sections are obtained with a mere string of glass between them. With one sharp tap in the middle of this connection we get two tapering tubes, either one of which will serve our purpose. Accordingly, we select one and thrust its blunt end into one of the two holes in the jar's stopper. The second hole receives, as before, the straight tube for the introduction of the acid. I will add that a clay pipe-stem could be used if necessary instead of the tapering glass tube. Of course, if you have a two-necked jar, one neck would receive the tapering tube and the other the tube for the acid.

"Water, zinc, and sulphuric acid having been put into the jar, hydrogen is released and flows in a jet through the tiny outlet of the tapering tube. I propose to light this jet, but here we must exercise a little prudence. We have just seen that hydrogen and air make an explosive mixture. Now, when the release of hydrogen began, the jar contained air, and hence an explosive mixture is inevitable at the start. If we were so careless as to hold a lighted match to the gas jet at this stage of operations, the dangerous mixture would explode in the jar, which might not withstand the shock; it might be blown to bits, or at the very least the cork [276] would pop out and with it would come splashes of the acid, attaining our clothes with red spots and, much worse, perhaps getting into our eyes. I warn you to beware of this explosive mixture and to be constantly on your guard if you are preparing hydrogen by yourselves. You must watch every moment to see that no air mixes with the gas you are intending to light.

"In the experiment before us the presence of air is inevitable at the start. What, then, is to be done? The release of gas must be allowed to go on for a while. As it comes out of the jar, the hydrogen brings air along with it; and, as this is not renewed, a time comes when there is none or next to none left. But as there is nothing to indicate when this moment has arrived, we must simply wait a while, and our waiting should be rather too long than too short, so as to assure us of safety. Here patience is prudence."

They waited while the release of hydrogen went on and the liberated gas came whistling through the tiny outlet of the tube. After a few minutes it was thought safe to proceed.

"That ought to do," declared Uncle Paul. "There can hardly be any more air left in the jar by thins time. But we may be mistaken, and so, to prevent accident, I wrap the jar in a towel to arrest, if necessary, any flying fragments or splashes. With this final precaution, I hold a lighted paper to the hydrogen jet. Instantly the gas takes fire and burns quietly with a very pale yellow flame. All danger is past. As there was no explosion when I lighted [277] the gas, there will be none. All the air is driven out of the jar, only hydrogen issues from the tapering tube. Our towel was not needed, but it should nevertheless be used whenever we suspect there may be an explosive mixture in the jar. Not to hide from view what is going on there, I now remove the protecting cloth, which is of no further use. Instead of worrying about an explosion that cannot occur, let us examine our hydrogen flame closely.

"At the tip of the tapering tube we see a steadily burning flame, fed by the gas constantly coming out; and this flame is of a very pale yellow and gives scarcely any light. Such is the appearance of burning hydrogen, of which until now you have caught only fleeting glimpses. It is not a bright flame, but an exceedingly hot one. Try it"


[Illustration]

The boys held each a finger-tip near the paltry little flame, and each drew it back quickly. The heat was unbearable.

"Oh, oh!" Emile cried out with pain. "That flame does n't look like much, but it 's a good heater, all the same."

"It ought to be, coming as it does from burning hydrogen, the best of fuels. Do you remember what the blacksmith showed us?"

"Do you mean his wetting the coal in his forge so as to heat his iron white-hot?"

"Yes. Water, decomposed by burning coal, gives hydrogen; and this hydrogen burns and thus adds considerably to the heat."

[278] "Then we could make iron red-hot in this flame, though it is so small and pale?"

"You could make it not only red-hot, but white-hot. Look here. I hold the end of an iron wire in the flame, and immediately it becomes dazzling to the eye. It is thus that the blacksmith's iron bar is heated in the forge when he wets his coal.

"Another peculiarity, less important but more curious, is that the hydrogen flame sings. Yes, my little friends, it sings, and you shall hear its song as soon as I have provided it with a suitable musical instrument. This instrument is a glass tube about as long and as slender as a walking-stick. But the tube may be shorter and wider, with corresponding changes in the tone, deeper if the tube is wide, on a higher note if it is narrow. For want of such an instrument a lamp chimney will serve, or a paste-board or even paper cylinder. They are made in several lengths and calibres. I have prepared a number of these tubes, one of which is of glass. I begin with that."

Uncle Paul, holding the glass tube upright, lowered it so as to enclose the flame, whereupon there was heard a musical sound, continuous and full, not unlike that of an organ-pipe. According as the tube was lowered or raised, causing a greater or less extent of flame to be within it, the note was on one pitch or another, changing abruptly from octave to octave in a manner to jar upon the least sensitive ear. Sometimes the note was tremulous, then uniform, and then tremulous again. Occasionally the effect was that of a solemn prayer muttered in the [279] hushed seclusion of some chapel, after which would come a response in high falsetto. In short, whatever the note, it always had a somewhat strident tone, affecting the ear as a disagreeable buzzing. By repeatedly changing the tube, selecting now a long one and now a short, now of greater calibre and now one of less, sometimes one of paper, then of glass or metal or pasteboard, Uncle Paul ran through the entire gamut in a riot of ear-piercing notes.

"Oh, what a crazy medley!" exclaimed the young hearers, overcome with laughter and unable to stand the discord any longer. They sought relief by stopping up their ears. "Oh, if Bull were only here! He barks when any one plays the flute or violin, and what fun it would be to hear him join in with this hydrogen orchestra! Let 's go and find him."

The dog was soon found, and he came readily enough, perhaps thinking he was going to get a bone. At the first note of the mad music the animal became excited and began to howl and moan, voicing his bewildered surprise in the strangest kind of sounds, all to the fit accompaniment of the hydrogen refrain. Emile and Jules burst out laughing at this instrumental and vocal concert, and even their sedate uncle failed to preserve his usual gravity.

"Put that disorderly pupil out of the class room immediately," he commanded, "or I shall be joining in your foolish merriment and the lesson on hydrogen will go unfinished."

When the dog had been banished and the hilarity calmed, Uncle Paul continued:

"You will of course understand that it was not [280] for the sole purpose of treating you to such a charivari that I took it into my head to enclose the hydrogen flame in a tube; there was back of this mad concert a serious motive, which I will explain after answering a question that both of you must surely have on your tongue's end. What make the hydrogen flame sing? Enclosed in the tube, the gas jet meets with air, and so there is continually being formed an explosive mixture that causes a succession of tiny detonations, one immediately after another, making the column of air in the tube vibrate. The sound we hear is due to this vibration.

"But now let us drop this subject and see what is produced when hydrogen is burned. What becomes of the burned hydrogen? I take the glass tube again, and wipe it thoroughly inside with a wad of blotting-paper on a stick. So now we have it in a perfectly clean condition, without a trace of moisture on the inside. Again I pass it over the hydrogen flame. But don't listen now to the singing of the gas; watch what is going on in the tube. A fine dew soon forms on the inner surface of the glass, increasing gradually until it trickles down in colorless drops. This liquid is the burned hydrogen, the compound resulting from the combining of hydrogen with the oxygen of the atmosphere. From its appearance one would call it water, but before we can be sure we must taste it.

"With the tube I am using it is hardly possible to wet the finger-tip in the drops running down the inside; so let us make a slight change, substituting for the tube a large jar with a wide mouth. I wipe [281] the inside carefully and then introduce the hydrogen flame. Dew again appears, and drops collect and run down. If we wait long enough, some of them will reach the mouth and you can wet the tip of a finger in them."

After the flame had burned some time in the jar, which was held upside down and, to enable the condensed liquid to gather in one place, a little slanting, some drops did in fact collect>  at one point on the mouth of the jar. At their uncle's bidding the boys hastened to wet a finger-tip in the liquid and taste it.

"It has n't any taste," declared Jules, "or any smell or color. I should almost say it was water."

"You may say so without the 'almost,' for that is just what it is. This is the wonderful thing I wished to teach you when I made the flame sing. Water is burnt hydrogen; it is composed of hydrogen and oxygen. Commonly looked upon as the direct opposite of fire, water in reality combines the elements essential to the hottest kind of a fire, namely, hydrogen, the best of fuels, and oxygen, in which metals, even including iron, will burn. The two gases are present in unequal parts, one of oxygen to two of hydrogen. That is why, when I prepared the mixture that exploded so loudly, I put into the bottle two measures of hydrogen and one of oxygen. The explosion of this mixture produced a little water, which, vaporized by the high temperature, rushed with much violence and noise out of the bottle. From the loudness of the explosion, causing the window-panes to rattle at each occurrence, you [282] might think a good deal of water had been produced. Undeceive yourselves; great as was the noise, the quantity of water produced was small, very small, perhaps a single drop. You shall judge for yourselves from the figures given by chemistry. It tells us that to make one liter of water we must have one thousand, eight hundred, and sixty liters of our explosive mixture, of which six hundred and twenty are oxygen and one thousand, two hundred, and forty, or twice that amount, hydrogen. How much water, then, would come from out little bottle holding a quarter of a liter? Hardly any at all. What a noisy celebration over a chemical marriage from which is to be born a single drop of water!

"Now we can see the reason for using sulphuric acid with our metal, be it zinc or iron, in decomposing water. We know that an acid unites with metal if the latter is first converted into an oxid. A salt is formed by this union of acid and oxid. Zinc, sulphuric acid, and water are put into a jar together. The acid has a natural liking for the metal if the metal first becomes an oxid; and the metal, on its part, has a liking for the oxygen. The combined effect of these two likings is the decomposition of the water, which lets its hydrogen escape and gives its oxygen to the zinc. Thus the metal burns at the expense of the water, which furnishes the element without which nothing can burn. It burns, I say, or in other words it becomes an oxid, which immediately combines with the sulphuric acid to make a salt called sulphate of zinc. In short, what zinc undergoes with the help of water is exactly what it [283] undergoes in a brazier when it burns with the help of air. The metal burns there, or is turned into an oxid. It is this burning of the metal that produces the heat generated by our hydrogen apparatus when it is in operation. You were surprised that a liquid should get hot without fire. Now the mystery is explained.

"I said the zinc was converted into a salt. This salt, this sulphate of zinc, readily dissolves in the water used in the experiment and only slightly diminished in volume by the decomposition of a very small part of it. The metal, then, disappears from sight just as sugar in melting disappears. Let us now take a look at out hydrogen apparatus. For some time it has been producing no gas. The zinc is all gone except a few blackish flakes resulting from impurities in the metal. It is dissolved in the liquid, having first been turned into salt, and the liquid itself remains as colorless as at first. We will let the jar stand in a corner, and little by little the dissolved substance will crystallize out, and we shall have a white deposit of an unbearably sharp taste. This white substance will be sulphate of zinc."


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