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The Wonder Book of Chemistry by  Jean Henri Fabre


 

 

BURNING PHOSPHORUS

[124]

P
REPARATIONS had been made for some new experiments, which pupils always like. On the table stood the tin box containing the bottle with the phosphorous; also there was the famous bell-glass resting on a plate, in the middle of which was a saucer full of lime.

"What is Uncle going to show us with all these fixings?" queried the boys.

"The air we breathe," he began, "is still very imperfectly known to you. Of the two elements composing it, only one, nitrogen, has been shown you; the other, oxygen, less abundant but much more important, is hardly known to you except by name. You recall what the experiment with burning phosphorous showed us,—that oxygen forms the fifth part of our atmosphere,—and you also know, rather from my telling you than from the evidence of actual facts, that is the gas needed when anything is to be burned. Without oxygen the flame goes out, and without it the life of an animal also comes to an end. But what is this gas? What will it do alone, by itself, and not mixed with nitrogen as it is in the atmosphere? That, my little friends, [125] is the important question. I am going to try to answer it for you.

"In five liters of atmospheric air there are four liters of nitrogen and one of oxygen; so that is the source we must go to when we wish to obtain in a pure state either one of these two elements. Now, in the atmosphere the two gases are not chemically combined, but simply mixed, as I shall have occasion to prove to you later. As they are only mixed, a simple separation of the two is all that is needed, though even this is a difficult matter; for how can we separate two substances that cannot be handled or even seen? A little while ago when we mixed powdered sulphur and iron filings, Emile thought it not impossible to separate the two, grain by grain, at a great cost of time and patience. And he was right; the task is not too much for nimble fingers and sharp eyes. With this mixture called air, however, it is a very different thing. The two substances forming the mixture can be neither seen nor felt, and if they could be seen it would still be hardly any easier to separate them, so subtle is their nature. What, then, are we to do?"

"It was easy enough o separate iron and sulphur," said Jules, after a moment's reflection, "by using a magnet, even though the two substances were both of them powdered very fine. Couldn't we use some means for sorting the two gases that air is made of?"

"Yes," chimed in Emile, "I'd like to find something that we could hold in the air and make it atract one of the gases and leave the other behind, [126] just as the magnet attracted the iron filings and left the sulphur."

"Do you know, my lads, that what you say shows more understanding of the matter than I had expected?" rejoined Uncle Paul. "Your answers delight me, anticipating as they do what I was about to propose as the only practicable means to be employed. What Emile says he would like to find is already known to you; you have seen it in operation, and no longer ago than day before yesterday."

"Phosphorous?" queried the boys.

"Yes, phosphorous. When it was burned under the bell-glass, did it not take to itself the oxygen and leave its companion, nitrogen, in the glass?"

"Yes, that's just what it did."

"Didn't it behave very much like the magnet you thrust into the mixture of iron filings and sulphur so that it drew the iron filings to itself and left the sulphur on the paper?"

"To be sure it did!"

"The magnet attracts iron, but has no effect on sulphur, which is thus by itself. In the same way burning phosphorous attracts and retains the oxygen in the air, but leaves the nitrogen, for which it has no liking."

"Now we have it, I think," said Jules. "The magnet, covered with iron filings, was drawn out of the mixture, and then we rubbed off the filings on to another piece of paper away from the sulphur. Let's make the phosphorous take all the oxygen it wants, and then we'll take it away again."

"A capital suggestion," applauded Uncle Paul; [127] "but, unfortunately, it won't quite work. The magnet readily gives up its load of filings, but not so with phosphorus and its load of oxygen. I have told you of its voracious appetite. Once having got its fill oxygen, it is impossible to make it disgorge except by forcible means not at our command in our humble laboratory. What it gets it keeps a good hold of, so that with our modest resources we should never succeed in making it let go."

"Let it keep its oxygen, then!" cried Jules in vexation at seeing his project fail just as he thought it about to succeed. "I'll try another way. Isn't there something that will work just the opposite of phosphorous,—something that will take the nitrogen from the air and leave the oxygen alone by itself? That would be much simpler."

"No doubt that would be much simpler, but—"

"Is there a but?"

"Alas, yes, and a most serious one! You must know that nitrogen is a most unsociable element, decidedly hostile to the notion of alliances. No element will have anything to do with it, as a rule, and it has no use for a partner of any sort. Chemical combination it abhors, and only when coaxed by the most skilful and delicate devices will it consent to any such union. Let us not, then, for a moment think of withdrawing nitrogen from the air by combining it with another substance; all attempts in that direction would be sure to result in failure.

"Must we, then, give up in despair? Not at all. The first method is excellent if we only use it with discretion. Phosphorous, it is true, keeps a most [128] obstinate hold on the gas it has united with in the act of burning, and it is useless to expect it to let go of the oxygen it has taken from the air. But, fortunately, not all simple substances are like it. We shall find some more accommodating, willing surrender their plunder without too much coaxing. For to-day we will content ourselves with learning how this gas is accumulated and, as we might say, stored up in a burnt substance; and for the purposes of this demonstration phosphorous will serve.

"You have not forgotten how smoke formed in the bell-glass when phosphorous was burned there, in our experiment of day before yesterday. That thick cloud of milk-white appearance made too deep an impression on you to be soon forgotten. And you remember how, little by little, it dispersed, being taken up the water in the bowl. If I had not called your attention to this point, perhaps that disappearance would have seemed to you a real instance of annihilation, and you would have retained the notion, so generally held, that fire reduces to nothing the material that it burns. I instructed you to the contrary, but that is not enough; I wish to add to my mere assertion the more convincing testimony of fact. Accordingly, I propose to show you that fire does not annihilate, but only transforms; that it changes the appearance and the properties of the matter without affecting its existence. Phosphorous will furnish us a fine example of this, and at the same time give us some knowledge of the chief topic of our to-day's lesson. The experiment I am proposing will show us, on the other hand, the indestructi- [129] bility of matter by fire, and, on the other, the storing up of oxygen by combustion.

"The white fumes given out by burning phosphorous are very easily dissolved in water, which accounts for their prompt disappearance in our recent experiment. To preserve them, to let them take on the state natural to them when cold, and then to examine them at leisure, it is absolutely necessary to examine them at leisure, it is absolutely necessary to do the burning where there is no water. Nor is even this precaution sufficient, so great is the liking for water of this compound formed by burning phosphorous. The atmosphere is always moist, whence come rain and dew. However dry it may seem to us, it is sure to contain more or less of the invisible vapor of water, which the burned phosphorous would greedily pounce upon, dissolving itself therein just as sugar dissolves in water. Consequently, we must have perfectly dry air in the bell-glass where the burning is to take place.

"This dry air I obtain by means of quicklime,—that is to say, lime before it is slaked by the mason, or, in other words, lime just as it comes from the lime-kiln where it has been prepared. You don't need to be told what happens to a piece of lime left in the air for some time."

"I know what you mean," said Jules. "The piece of lime gradually cracks open, and then crumbles to dust, just as it does when you sprinkle it with water; only then it crumbles a good deal faster."

"That is it. Sprinkled with water, a piece of lime cracks, splits, crumbles to dust. Exposed to the air for some time, it acts in the same way, but [130] more slowly. Why? Because it absorbs the moisture in the surrounding air, until, little by little, this moisture has had the same effect on it as a fine spray of water would have had. Thus it is that lime has the habit of attracting moisture, however little there may be within its reach. It wrenches it by force, we might say, from the surrounding atmosphere, and takes every bit of it. Here we have, then, a very easy and convenient way to obtain perfectly dry air.

"A few hours ago I took care to place a saucer of quicklime in the middle of a large plate, and covered it with the bell-glass, the latter resting on the plate and being, of course, full of air. With this precaution the fumes of the burning phosphorous cannot escape, and not only the imprisoned air, but also the surface of the plate under the glass and the inside of the glass itself, will be rendered very dry. Now for the burning of our phosphorous."

Uncle Paul cut off a piece of this substance under water and dried it carefully with blotting-paper. Then, placing it on a bit of broken crockery, he withdrew the saucer full of lime and substituted the phosphorous, which he set fire to, whereupon the bell-glass with its contents of dry air was immediately replaced on the plate. The burning did not at first differ in any way from that already seen by the boys; there was the same bright light, with the same eddies of dense white smoke. But at this point there occurred something new: on the inside of the cold glass the smoke condensed and became a beautiful, white, flaky substance that detached itself and [131] fell here and there with all the appearance of falling snow. Soon the plate was covered with a layer of this strange snow-like substance that had come from the very heart of the flames.

"Well, Emile," said his uncle, "what do you think of this snow?"

"I think it is very wonderful. Who would ever have expected fire to make a snow-storm? But I know well enough it isn't real snow, though one might be fooled by its looks. Those flakes, all so white and beautiful, must come from the burning phosphorous, for they couldn't come from anything else."

"Yes, that is quite clear. The substance we see forming here has nothing of snow about it except its appearance. In reality it is quite another thing, as we shall soon see. But first let us make it snow a little more. The fire is dying down; we will feed it."

Uncle Paul raised the bell-glass a little, and the burning, which had begun to languish, started up again with all its former vigor.

"Air was beginning to fail," said he; "the phosphorous had nearly exhausted its supply of gas, and was about to die down, but, by raising the bell-glass a little, I have let in more air, and the fire revives. Let us give it a little more air still, so that we may be sure to have enough of this strange snow."

When, after three of four renewals of air, the layer of snow on the plate was deemed thick enough, Uncle Paul took a pair of pincers and drew out the piece of crockery with the phosphorous on it and [132] carried it into the garden, that the still unconsumed material might there burn itself out with no inconvenience to lungs or sense of smell.

"Now, my young friends," he resumed, "I invite you to examine what there is in the plate. It is, as you see, a white, flaky substance looking much like snow. That is what phosphorous turns to when burned. Fire has not destroyed it, but has changed it into something else, and the change is so complete that if you did not know where this false snow came from you could never guess its nature. I repeat, fire does not destroy anything; what it devours, what it consumes, is not reduced to nothing, but changed into something else, which sometimes vanishes from before our eyes as an invisible gas, and at other times arrests even the least heedful attention as a much grosser substance. What you see here in the plate—this stuff that we can feel, smell, and taste—is phosphorous consumed by fire, phosphorous still in existence though it has been burnt up. Thus is illustrated before your eyes the first point I had in view in this experiment,—namely, that nothing is ever annihilated even by the action of fire.

"Suppose we had here a fine pair of scales for weighing, exactly balanced as are scales used by chemists, and capable of telling us with precision the weight of a fly's wing. The weighing of even so delicate operations of chemistry. With a pair of scales of such sensitiveness, we could have ascertained the weight of the piece of phosphorous in milligrams. Nothing then would have stood in the way [133] of our burning the whole of that piece under the bell-glass by renewing the air as often as was necessary; and at the end we could have taken a feather and swept up the snowy deposit even to the last flake, after which we could have weighed it on our scales. Let us suppose these two weighings to have been carried out, one before the phosphorous was burned and the other after. Now, which will weigh the more, the unburnt substance or the burnt?

"Misled by that false notion of fire as a destroyer, the novice would answer at once that the burnt substance would weigh less than the other, arguing that if fire does not entirely destroy, it at leasts destroys in part. Buy you, my boys, forewarned against this error in our earlier talks, and having had your eyes opened by a number of experiments, will not, I believe, make this foolish answer."

"I should think not," was the confident reply of Jules. "I should say the burnt phosphorous would be heavier than the unburnt."

"Your reason, my lad. Let us make no assertions without proof to support them."

"The reason is plain enough," said Jules. "You told us and proved to us that when anything is burned it combines with something in the air called oxygen. Although an invisible gas, this oxygen is matter, and consequently has some weight, if only a very little. So that burnt phosphorous, having had oxygen added to it, ought to weigh more than phosphorous alone."

"Golden Mouth could not have said it better," applauded Uncle Paul. "Yes, my young friend, the [134] burnt phosphorous must weigh as much as it did before combustion, plus the weight of the gas combining with it in burning. A delicate pair of scales would testify to this in the most convincing fashion: it would show us that this heap of what looks like snow-flakes weighs more than the phosphorous that went to the making of it. How account for the added weight except by ascribing it to the air that played its part in the combustion? So, then, in the substance on this plate, in the burnt phosphorous, there is a small amount of oxygen taken from the atmosphere and securely retained. This oxygen has ceased to be an invisible gas occupying a great space, and has become part of a solid substance that can be seen and handled, and that occupies a comparatively small space. It is stored up as in a reservoir, where chemical combination has collected and compressed it into the least possible bulk.

"A similar chemical action attends the burning of any substance whatever. By being consumed it becomes a storehouse of oxygen. Taken in its sum total, and with no omission of any part, the resulting matter after combustion is heavier than the substance before combustion; and this excess of weight is due to the gas that took part in the combustion. Most of these burnt substances, veritable storehouses of oxygen, hold on to the latter with a tenacity that can be overcome if necessary, but that nevertheless offers very great resistance, while the lesser number surrender this oxygen easily. After a brief survey of this latter class we will select the substance best suited to our purpose of obtaining [135] oxygen in its pure form. But we will first finish our examination of burnt phosphorous while we have a specimen before our eyes.

"Although derived chiefly from phosphorous, which is highly inflammable, the snow-like powder in the plate is absolutely incombustible, the hottest fire having no effect on it, for the reason that what is once burned cannot be burned again. This phosphorous, being already combined with as much oxygen as its nature permits, cannot take on any more; that is its combustibility is at an end. Experiment will prove this better than mere words."

On a pan of glowing coals from the kitchen fire was sprinkled a little of the white powder, and the coals were then blown to a more intense heat; but the powder showed no signs of taking fire, its inflammability having been quite exhausted.

"If you had not already," Uncle Paul resumed, "some knowledge of the difference between compound substances and the simple substances composing them, this experiment would open your eyes, for the substance that at first burned so freely now refuses to burn at all. Let us proceed. You can convince yourselves that the white powder in the plate has no smell whatever, whereas the phosphorous had a very strong smell of garlic. But I would not have you touch this powder, its properties being such as to render handling harmful; still less do I wish you to taste it, as it would make you cry out with pain."

"Is it so terrible as all that?" asked Emile.

"So terrible that a drop of molten lead on your tongue would be less painful."

[136] "But that snowy stuff looks harmless enough."

"Don't trust to looks, my little friend. Innocent looks may disguise a very dangerous substance. Forewarned is forearmed. From the kitchen of the chemist there very rarely comes anything pleasant to the taste. However, it is well for you to have some idea of how phosphorous tastes, and to make it less disagreeable to the tongue I will dissolve it in water."

So saying, Uncle Paul took up the feather again and swept the contents of the plate into the glass of water. As each particle fell into the liquid it gave a hissing sound, like that produced when a black-smith plunges red-hot iron into water.

"It must be awfully hot, musn't it," asked Emile, "to hiss like that in the water?"

"Heat is not the cause of the hissing. The powder is no hotter than anything else here, —no hotter than the plate it lies on. I have already told you that burnt phosphorous has an extraordinary liking for water; and you know what extreme precautions I had to take, with the aid of quicklime, to keep it from the moisture in the atmosphere. Now I let this powder drink as much water as it pleases, and it dissolves immediately and even with violence, that sharp noise testifying to a satisfied thirst.

"Behold, now, the snowy powder all dissolved in the water. The liquid has not changed its appearance; it is still water as far as looks are concerned; but dip the tip of your finger in and taste it. You can do so without the least fear."

As the children hesitated, remembering the allu- [137] sion to the drop of molten lead, their uncle dipped the tip of his little finger into the liquid and touched it to his tongue; whereupon, emboldened by his example, Emile and Jules did likewise.

"Oh, how sour it is!" they cried, surprised at the disagreeable taste and making a wry face. "It is sourer than any vinegar they use for making salad. What would it have been like if Uncle hadn't weakened it with a lot of water?"

"Your tongues would have suffered terrible tortures, my little friends. The part touched would have been eaten away at once by this violent chemical, and you would have heard a hissing as of red-hot iron in contact with your saliva."

"Then this strong vinegar isn't real vinegar?"

"It is not vinegar at all, though it tastes much like it. Now let us go on. Phosphorous has still another property that we must test. Here are some violets just gathered from the garden. I dip one into the sour-tasting liquid, and it immediately loses its blue color and turns red. All flowers of the same color as the violet—the iris and the bluebell, for example—would likewise turn red in the sour liquid. You shall at your leisure repeat this fine experiment with all the flowers you can find in the garden, and you will always see the blue flowers turn red. Burnt phosphorous, then, always has this sour taste and this quality of turning blue flowers red.

"I will add that most of the other metalloids, such as sulphur, carbon, nitrogen, and many more, when they combine with oxygen,—or when they burn, as we say,—produce compounds of like sourness and [138] of like ability to change the color of blue flowers to red. All these compounds are called acids, from their sour or acid taste, and they are distinguished one from another by the addition of a second term indicating their origin. Thus the snowy powder resulting from the burning of phosphorous bears the name of phosphoric acid, and in future that is what we will call burnt phosphoric."


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