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The Wonder Book of Chemistry by  Jean Henri Fabre


 

 

FURTHER EXPERIMENTS WITH AIR

[96]

O
UR experiment with the candle burning in a bottle turned upside down in water, is one of the easiest to perform, calling for nothing that is not readily procurable. But, unfortunately, it is an incomplete experiment, for the reason that I have already indicated. It shows us that air is composed of two different gases, one that will keep a flame burning and is called oxygen, and another of a contrary nature in this respect and known as nitrogen; but it does not tell us how much there is of each, because what is left after the candle goes out, still retains a good proportion of oxygen instead of being reduced to pure nitrogen.

"A candle-flame is delicate; a moderate puff of wind will blow it out. In the bottle, I admit, it is sheltered from any current of air, but its weakness prevents its using up all the gas on which it could feed. It turns dim and then goes out altogether when this gas begins to get scarce. If the comparison is not too far-fetched, one might call the candle-flame a guest at table with a poor appetite, leaving on his plate the greater part of the food served to him. Let us, then, call in a guest with a stronger stomach and able to eat his food to the [97] last morsel, leaving nothing but the uneatable, the bare bones. I mean, let us find a kind of fuel that burns with enough energy to consume all the oxygen to the last trace and leave only the useless gas, the nitrogen.

"What shall this fuel be? Shall it be coal? No; that would not do any better than candle-grease, burning less freely, in fact, because needing the heat of a glowing furnace to keep it going, and that is out of the question in our experiment, which owes more than half of its value to its very simplicity. Shall it be sulphur? There, assuredly, we have a strong stomach needing no second invitation when oxygen is offered. Once set fire to, it burns vigorously. But it has its faults,—its suffocating fumes. Nevertheless, I should be glad to avail myself of it if there were not something better at hand. You are familiar enough with the common match, the little stick of wood tipper with sulphur, and lightly coating the sulphur we find—See who can tell me first."

"Phosphorous!" cried the two listeners at the same time.

"Yes, phosphorous, which is inflammable to a degree not attained by any other substance in common use; phosphorous, which takes fire by being merely rubbed against the sandpaper cover of the match-box or against a rough wall. Nothing equals it in the vigor and persistence with which it burns. Here, truly, we have the greedy guest who will leave nothing on his plate. But first let us get a little better acquainted with him. Phosphorous [98] is not very well known to you, as hitherto you have seen it only on the tips of matches."

"Sometimes," said Emile, "the tips are red, and sometimes blue, or yellow, or almost black. Does phosphorus have all those colors?"

"No; of itself phosphorus has only one color, which is nearly that of yellow wax. But the match manufacturer adds colored powder, sometimes of one hue, sometimes of another, according to his facy, for the sake of giving a little variety to his wares and thus pleasing the purchaser's eye. Glue is also mixed with the phosphorus to make it stick to the sulphur. So what you are used to seeing is not pure phosphorus; but I will show you some that is perfectly pure.

"A few days ago, being called to town on business, I bought a number of things that our laboratory was much in need of. A laboratory, let me explain to you , is a place devoted to scientific research; it is the scientist's workshop. Modest though our workshop is, it must have some equipment, certain implements and supplies; otherwise we should have nothing but our ten fingers for an outfit, and what could we do with them? We should simply have to content ourselves with talking and nothing else. But that I will not have, for I do not think much of chemistry carried on in words only. I wish to give you facts, things that you can see over and over again, substances that you can feel of, taste, examine, handle for yourselves, as that is the only true way to learn.

[99] "What could the blacksmith do without his anvil and hammer? Nothing. Equally helpless is the chemist without the various appliances and drugs of his laboratory. We will furnish ours, then, little by little, but in a very modest way, I assure you beforehand, as your uncle's resources do not permit luxury. We will have the indispensable, but nothing more. Nor is it altogether a misfortune to be thus forced to use one's wits a little, in devising ways and means to make what one has suffice, and in getting along without what one does not have. Our earthen dish borrowed from the kitchen, our old medicine-bottles and preserve-jars—did they not play their part well? I assure you in all sincerity, we could not have done better if we had had the outfit of a costly laboratory. Why shouldn't we continue our studies in this way, as far as may be? If you ever chance to have access to a real laboratory, and to work in it, my little lads, you will take pleasure in recalling your uncle's poor outfit and in reflecting how little it took to lay the solid foundations of useful knowledge in your minds, and how little it would need to do so for others even in our smallest villages.

"It may well happen that we shall be halted now and then by difficulties impossible for us to overcome; then, and only then, we shall be forced to appeal to the expert chemist for aid. Such is our position to-day. We had need of phosphorus, and here it is, bought recently at the druggist's in town."

[100] Uncle Paul here set before his nephews a bottle holding in water a yellow substance in the form of a stick as long and as thick as one's little finger.

"That," said he, "is pure phosphorus. It is semi-transparent, which, with its color, makes it look like a pretty piece of wax such as we have in the honeycomb. It is like wax before being bleached at the taper factory by long exposure to the sun."

"Why do you keep it in water?" asked Jules.

"I keep it in water because if exposed to the air it would soon catch fire. So inflammable is it that the slightest heat is enough to ignite it."

"But the phosphorus on matches doesn't catch fire like that; you have to scratch it."

"I told you that in matches it is not used in a pure state. It is mixed with certain other substances with glue and colored powder, which lessen its inflammability. But even so it sometimes occurs in summer that matches take fire of their own accord. This is a serious fault which, added to others, will perhaps some day induce us to give up the use of phosphorus, when science has discovered something better to take its place."

"And why," asked Emile, "doesn't it catch fire in the water, if it is so eager to burn?"

"Has Emile forgotten what I told him yesterday? In order to have fire, two things are necessary, each as indispensable as the other,—the thing that burns and the thing that makes it burn, this latter being a gas called oxygen, contained in the air. Combustion is brought about when the two combine. Where there is no air—or, rather, where [101] there is no oxygen—combustion is impossible, however inflammable the fuel may be. So I guard the phosphorus from risk of taking fire by keeping it in water, which protects it from the air. This being excluded, we are sure the phosphorus will not catch fire.

"Still another precaution is not out of place with this dangerous substance. The bottle holding the phosphorous in water might get broken, which would expose the phosphorous to the air. Therefore we must guard against any chance shock or fall, and to this end the bottle is enclosed in a tin box which serves as a shock-absorber. In this double enclosure and immersed in water, phosphorus is kept indefinitely on the druggist's shelf without any danger.

"It remains for me to add that a burn from phosphorus is a serious accident, a most painful accident. There is nothing that smarts more than a wound made by this terrible stuff. Neither live coals nor red-hot iron can cause so acute and lasting a pain. I will leave you to imagine the fate of the thoughtless person who, wrapping a piece of phosphorus in paper, should put it into his pocket, proposing to amuse himself with it later and make it shine in the dark. The heat of his body would set fire to his perilous possession, and the imprudent one would be burned to his very entrails, while he filled the air with his shrieks of agony. Be careful then, my boy, not to play with this terrible substance. If the thirst for knowledge should tempt you to handle it at all, do so with the utmost caution. [102] I appeal here to Emile's obedience and Jules's prudence. Had I not perfect confidence in you, did I not know you to be incapable of reckless folly, I should double-lock and triple-lock my arsenal of drugs and banish phosphorus from our lessons forever.

"I should be all the more impelled to this course because the risk of fire and serious burns is not the only danger to be considered; there is another peril, which you must be still more careful to guard against. Phosphorous is a deadly poison, a few particles of it being enough to cause death with frightful sufferings. I will say no more; you are warned. Look on phosphorous as one of the most formidable of foes, and let no carelessness on your part lay you open to its assaults.

"After these warnings, dictated by prudence, I will now explain how phosphorous may be used to show what air is made of. We must burn a little in a certain volume of air properly separated from the rest of the atmosphere. Our container in this operation should be of considerable size, so that the glass may be far enough from the flame to escape the risk of being cracked or broken by the heat. A common glass preserve-jar holding two liters or more, and as large at the top as at the bottom, would do very well if I had not something better. This something better is a chemist's bell-glass, a recent purchase of mine, which we shall find to be one of the most useful appliances in our laboratory. I will ask you to be especially careful of it. It is, as you see, a simple container of color- [103] less glass, cylindrical in form, with a dome-shaped top surmounted by a little knob for taking hold of. Some are made with flaring mouth, reminding one of the shape of our bronze church bells, whence comes the name of bell-glass. For certain delicate plants requiring shelter and warmth gardeners use similar glass covers, but they are commonly too large and cumbrous for laboratory use. If one could find a medium-sized one, it would do perfectly.

"A large bottle, a gardener's glass plant-protector of suitable size, or the ordinary chemist's bell-glass—any one of these that should chance to fall into our hands would serve as a contain for burning phosphorous; but Uncle Paul's purse has procured for us a real laboratory bell-glass, so let us be grateful for it and proceed with our experiment.


[Illustration]

"The combustion, the burning of our bit of phosphorous, must take place on water, to prevent any communication between the air in the glass and that outside. Consequently, the phosphorus has to be placed on a tiny raft that will keep it dry, and for this purpose we can use any small object that will float, as a piece of cork or a bit of wood. But this float of ours would catch fire if unprotected from the burning phosphorous. Accordingly, we will put the latter in a tiny earthenware cut resting on the float; and for our cup we will simply take a concave fragment of some old broken pot. Now all is ready and we will proceed to business.

[104] "First we must cut off a piece of our stick of phosphorus. It is soft enough to cut with a knife, being of about the hardness of rather firm wax; but it is not a thing to be cut as carelessly as one might whittle a stick of pine wood, the mere friction of the knife being likely to prove sufficient to set fire to the phosphorus if it were exposed to the air, with consequent serious injury to the clumsy operator. The inflammable stuff should not be held in the air except by the finger-tips and for as short a time as possible, and the cutting should take place under water. Watch me."

Uncle Paul put his fingers into the bottle and drew out the phosphorous stick, which gave out a rather strong smell of garlic, with some slight wisps of white smoke. The children were told that this smell of garlic is the natural odor of phosphorus, and that the white smoke would be found to give out light if looked at in the dark. Matches emit, to a lesser extent, these same odorous fumes. The phosphorus was immediately plunged into the bowl of water, and there, both hands under water, Uncle Paul with a knife cut off a piece about as large as two peas. This fragment was placed on a bit of broken crocker, and that on a little wooden raft of sufficient buoyancy to float its load; then the whole thing was set on the surface of the water in the middle of the bowl. A lighted match quickly started the phosphorus to burning, and Uncle Paul hastened to cover it with the bell-glass, which was, of course, full of air.

Behold, then, the phosphorous blazing away with [105] a violence quite new to the boys, who up to this time had seen no more of the inflammable stuff than the minute quantity at the tip of a match. The flame crackled, the light was brilliant, almost blinding. A dense cloud of white smoke formed, giving the appearance of milk to the contents of the bell-glass. At the same time the water in the bowl rose so rapidly in the glass that Uncle Paul was obliged to add more, in order not to leave the bottom of the bowl dry, for that would have let air into the bell-glass. So thick was the milky-looking cloud that the phosphorus flame could no longer be seen; or, if seen, it was only at intervals, like lightning in a mass of clouds. But the jets of light became more and more infrequent and feeble, and finally ceased altogether.


[Illustration]

"It is over," Uncle Paul announced. "The phosphorus has used up all the oxygen in the air contained in the bell-glass, and there is nothing left but nitrogen, which will not support combustion, although there is still some combustible matter left on the bit of crockery. We shall see it when the white smoke has cleared away. Meanwhile let us talk a little about this smoke, which seems to attract your attention by reason of its beautiful milky appearance. It comes from burned phosphorus—that is to say, from the phosphorous combined with the oxygen of the atmosphere. A brilliant light, of such intensity as to try our eyes, [106] accompanied this act of combining, as it always does. I say nothing of the heat, to which the bit of broken crockery could testify if it could speak. These fumes are easily dissolved in water, and thus there is left a vacant space which the water from the dish rises to fill, little by little, in this way showing how much oxygen has disappeared. We should have to wait only about twenty minutes, more of less, for the contents of the glass to become as clear and transparent as at the start. But to hasten the process and not to put your patience to such a test, let us see what this will do: we shake the bell-glass gently so that the moving water washes the interior and takes up the smoke. By this operation the contents will soon be made clear."

With a little careful management in shaking the glass, the gas within was soon made to resume its original transparency, and then there was revealed, on the bit of broken earthenware, the residue of that had been placed there, but now of a reddish color and, indeed, so changed in appearance that the boys would not have recognized it as phosphorous. Melted by the heat so that it was spread out on the piece of earthenware, it had quite an altered look. But to convince his hearers that it was still phosphorous, their uncle tilted the bell-glass slightly, so as to bring the little raft near its edge, when it became an easy matter to withdraw the raft and its load.

"What we have here," said he, "is really phosphorous despite the reddish tinge that heat and melting have given it. There is even more left over [107] than there was burned. You shall judge for yourselves."

The potsherd was taken out into the garden so as not to mingle the disagreeable phosphorous fumes with the air of the workroom, then with a match the reddish substance was set fire to, and it burned with the bright light and dense white smoke attending the combustion under glass. Thus it was proved that there remained some phosphorous, even a good deal, as it burned for a considerable time; and in this instance every particle of it was consumed, the last trace being dissipated in the air as white smoke.

"If combustion stopped under the bell-glass," Uncle Paul continued, "it was not for lack of something to burn, for there was a good deal of it left at the end, but for lack of the gas necessary to support combustion—oxygen, in a word. It stopped when the last trace of this gas was used up, phosphorous being able to burn as long as there is any oxygen left, however little it may be. Consequently, the bell-glass now contains nothing but pure nitrogen, a gas in which no substance whatever can burn.

"The phosphorous experiment tells us once again, but more positively and more distinctly, what the candle experiment; the atmosphere contains two gases, oxygen, which supports combustion, and nitrogen, in which neither candle nor phosphorous nor anything else can burn. It tells us, also, in what proportions the two gases, both simple substances, both metalloids, are combined in the atmosphere. Our bell-glass is cylindrical. If we divide [108] its height into five equal parts, these will represent equal capacities, equal volumes. Now we see that the water that rose in the glass and took the place of the departed oxygen has mounted to a fifth of the total height, nitrogen occupying the other four-fifths. Thus the air about us has four times as nitrogen as oxygen; or, to express it differently, in five liters of air there are four of nitrogen and one of oxygen.

"We will stop here for to-day. To-morrow, let me notify you in advance, our chemical experiments will call for two live and uninjured sparrows. Set your snares and catch them. I must ask you also to be careful not to molest any of the various species of garden birds, industrious hunters of insects and worms that are the scourge of agriculture; but I gladly give you free rein in regard to those pillaging sparrows that, eager to find some tender foliage to tickle their palates in the spring, fly down from the neighboring roofs and nip my peas as fast as they sprout. I must have two of these birds for our instruction and to serve as a lesson to their brother marauders."


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