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The Wonder Book of Chemistry by  Jean Henri Fabre

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BURNING METALS

[139]

A
LL the blue flowers in the garden had been tested with phosphoric acid. After the violet had come the iris, after the iris the periwinkle, then the veronica, and others besides; and all had lost their natural color and turned red. Yellow flowers, on the other hand, such as the marigold, and white ones like the Easter daisy, and red ones such as the poppy, did not change color at all when dipped into the acid liquid. For some time, now, the contents of the glass had been used in these experiments when Uncle Paul invited his pupils to take part in some new investigations; and the summons was responded to with an outburst of joy. This time the equipment consisted of a small portable stove full of charcoal and set to burn in the fireplace, while on the table were some bits of zinc, doubtless the remnants of a worn-out watering-can. There was also an old iron spoon, in so sorry a condition that no beggar would have stopped to pick it up in the street. A bottle hardly longer than your finger contained something grayish, of metallic appearance, shaped like a narrow ribbon and wound about in the form of a skein. The boys puzzled in vain to guess what this could be. Their uncle told [140] them in due time, but before doing so he resumed his talk as follows:

"How to obtain pure oxygen freed from the nitrogen with which it is mixed in the atmosphere was the difficult problem engaging our attention at our last lesson, and it will continue to do so to-day. We know that the sour-tasting compounds, the acids, made by burning various metalloids, notably phosphorus, contain an abundance of oxygen taken from the air and stored up. That is the first stage of the journey toward our destination. Another lies ahead, and when we have accomplished it we can, with better understanding, avail ourselves of the means offered us by chemical science. Perhaps you will say I am straying from the road that leads straight to the element, oxygen, which you are so eager to learn about, and you will reproach me for dividing the journey into more stages than necessary."

"Oh, never fear," Jules hastened to reply; "you may make the journey as many stages long as you like. If they are all as interesting as the last one, when you showed us snowflakes made by fire, we shan't be the ones to complain. Oxygen will have its turn all in good time."

"To-day's stage in our journey, my young friends, will be no less interesting than the one that preceded it. I think it will give you an even greater surprise; and in the end it will show us how to obtain pure oxygen, which is what we started out for. Let us talk a little more about the burning of various substances.

[141] "A piece of phosphorus on fire is certainly a fine sight. The vigor with which it burns, the dazzling brightness of its flame, the snowy flakes of phosphoric acid resulting from the combustion—all these cannot fail to arouse interest. But, accustomed as you already were by the use of matches to seeing phosphorus burn, this spectacle had in your eyes none of the fascination of the new and unexpected. To witness the burning of a substance well known to be highly inflammable is nothing very thrilling. But to-day you will be rather surprised to see things burn that you have always thought to be fireproof. We are going to set metals on fire."

"Metals!" cried Emile with a start of astonishment.

"I said that would rather surprise you. Yes, my boy, metals, real metals."

"But metals don't burn."

"Who told you so?"

"Nobody told me, but I know what I see every day. Tongs, fire-shovel, andirons are made of iron, which is a metal; and even in the hottest kind of a fire, I have never seen any of these things burn. The stove is metal, and in winter when it gets red-hot with a roaring fire inside I have never seen a particle of it burn. Why, the whole stove would have burnt up long ago if metals could take fire as you say they do!"

"Then Emile does not believe me when I say that metals can burn?"

"What can I say, Uncle Paul? You put my faith [142] in you to a hard test. You might as well tell me that water can burn, too."

"And why not, as I am going to show you some day that it can?"

"Water burn?"

"Yes, my child; I propose to show you some day that water contains the very best kind of material for burning."

Astounded at this promise, made with undoubted sincerity, Emile said no more, but waited to see metals burn, before believing what seemed to him incredible. His uncle continued:

"If the tongs, shovel, andirons, stove, and other things made of iron do not burn in our fireplaces and kitchens, it is because the heat is not intense enough. Make more heat, and metal will not fail to take fire. You have often seen this burning of iron, but without suspecting what it really was. Let us recall what we see from time to time at the blacksmith's shop when we pass his door. The smith has just taken from the forge a bar of iron, glowing hot. As soon as it comes to the air this bar of dazzling brightness throws out showers of brilliant sparks in every direction, so that one might mistake it for a piece of fireworks. The dark shop is all ablaze with blinding flashes. What are these sparks that fly off in showers? Little scales of iron that become detached from the bar and burn as they shoot through the air. Does Emile begin to believe me now?"

"Yes, I am beginning to. I see more and more every day that anything may be expected in chemistry."

[143] "I will tell you further that when the makers of fireworks wish their pinwheels, Roman candles, rockets, and squibs to throw out splendid sparks like sprays of water from a fountain, they mix with gunpowder a quantity of filings of various metals according to the colors they wish to obtain. Copper gives green sparks, iron white. Each particle of these metal filings turns into a spark as soon as it touches fire. I am planning to show you, one of these days, a conflagration of iron that will make the blacksmith's shop look dull and tame. So I will say but little more at present about this metal, merely adding a third example to the two I have just cited.

"You both know how to get bright sparks by striking flint and steel together, or flint and the back of a knife-blade. Those sparks are particles of metal, of steel, which is a variety of iron, that are struck off, becoming heated by the shock and burning as they fly through the air. In exactly the same way sparks fly off from the stone of the scissors-grinder when he sharpens a piece of cutlery on it, and from an iron horseshoe when it strikes a pebble. The violent friction of the grindstone and the shock of the horseshoe's hitting the hard pebble break off fine scales of iron, and these, raised to intense heat by the friction, catch fire as they dart off through the air. You see, you don't have to go very far to find plain proofs that iron really will burn, which at first seemed so impossible to you. The scissors-grinder sharpening a knife and the mule stumbling on a stone teach us what Emile found so hard to believe; [144] they perform for the wide-awake observer one of chemistry's fine experiments.

"I pass now to another metal, zinc. Here are some pieces, the remains of an old watering-can that has been thrown away. This metal is grayish on the surface, but if I scratch it a little with a file of the point of a knife, we get a glimpse of metallic luster like that of tin or silver. Now what we wish to do is to make this zinc burn,—quite an easy operation and readily performed with the help of a few live coals. It is the same with metals as with the things we commonly think of as inflammable, such as sulphur and phosphorus and charcoal, for example: some catch fire easily, others with difficulty. At the mere touch of a lighted match phosphorus is instantly ablaze, while sulphur is much slower and charcoal more obstinate still. In the same way, while iron requires the heat of a forge to make it burn, zinc needs only a few live coals. There are some metals that burn still more easily, as we shall soon see.

"Let us proceed, now, to make zinc burn. I place a few pieces of it in this old and discarded iron spoon, and thrust the spoon thus filled into the midst of the glowing coals in this little stove. If you have any doubts, our experiment will settle them for you."

Everything being arranged as Uncle Paul had indicated, they waited a little while. The zinc melted almost as readily as lead, and when the spoon was red-hot the coals were pushed aside a little to show the melted zinc without cooling it off. Then with a stout piece of iron wire Uncle Paul began to stir the [145] molten metal so as to bring it into better contact with the air. A flame burst forth, of a superb bluish white, dazzling to the eyes, though of no great volume. It flickered on the surface of the liquid metal, springing up brighter or dying down, according to the briskness with which the matter was stirred. The boys marveled at the brilliance of the burning zinc, and their wonder was heightened as they saw a kind of snow rise from the flame, float lightly in the air, and spread all about the room. One might have taken it for the finest of incomparably white down, or those delicate shreds of cobweb that one sees fluttering over the fields on a fine autumn morning. At the same time, on the surface of the metal in the spoon, there collected a kind of cotton-wool of unparalleled fineness. The current of hot air from the stove moved it lightly, detaching the flakes they saw rising and spreading through the room.

"This white fluff," resumed Uncle Paul, "this down or cotton, is burnt zinc, zinc combined with oxygen from the atmosphere, and it bears the same relation to this metal that the snowy flakes bear to phosphorus. We will ascertain its chief properties as soon as there is enough in the spoon."

Jules took his uncle's place in stirring the molten metal, while Emile began to blow the flakes before him, taking care not to blow hard enough to drive them too fast, even the largest of them floating so lightly in the air, despite their size, that it seemed as if they would never settle down. Soon there was no more of the shiny liquid left in the spoon, all the [146] zinc having turned into this white material. When the residue of this in the spoon had cooled off and been emptied out, Uncle Paul continued:

"Burnt zinc is a white substance such as you now see before you. It is also quite tasteless. Put some on your tongue and it will convey no sense of taste."

"That's so," affirmed Emile, after trying it cautiously, warned by his previous experience with burnt phosphorus. "It hasn't any more taste than a pinch of sand or sawdust."

"I don't taste anything, either," Jules chimed in; "and yet the burnt phosphorus, the phosphoric acid, was so sour you couldn't bear it. Now, here's something else that we've burnt, and it hasn't any taste at all."

"Let us seek the reason for this lack of taste," suggested his uncle. "I drop a good pinch of the white substance into this glass of water and stir it well with a stick. It does not dissolve, does not melt in the water; it is, as we say, insoluble. You remember how readily, on the other hand, the burnt phosphorus or phosphoric acid dissolved in water."

"We are not likely," replied Emile, "to forget very soon that terrible stuff that melts in water with a hissing sound like red-hot iron, and that has to have dried air if you want to keep it."

"Let us put together these various facts: burnt phosphorus dissolves in water and has a taste; burnt zinc does not dissolve in water and has no taste. In the same way, salt and sugar readily dissolve in water, and both have a taste, the first a salt taste, the second a sweet taste. Marble and brick do [147] not dissolve in water, nor has either of them the slightest taste. Do you begin to see what these facts point to?"

"It seems to me from all that," replied Julies, "that to have any taste a thing must be able to melt in water."

"I have no fault to find with that answer. Yes, my boy, what a substance must have in order to possess any taste at all, whether strong or weak, sweet or sour, salt or bitter, or whatever flavor, is solubility in water. Anything that cannot be dissolved in water is by that very fact tasteless, and for this reason: to act on the sense of taste, to make an impression on tongue or palate, a substance must necessarily, unless it be a liquid, dissolve in the saliva, being thus divided into extremely small particles and brought into contact with the organs designed to perceive it. Now, saliva is almost entirely composed of water. If, then, a substance is insoluble in water, it is also insoluble in saliva, and therefore has no taste. In future, when you see a substance that will not dissolve in water, don't try to see how it tastes, for it has no taste and cannot have any. But if it yields to the action of water, it has a taste,—sometimes very insipid, it is true, and hardly perceptible, as gum Arabic, for example.

"To return to our two burnt substances: the white material left by the burning zinc has no taste, because it is not soluble in water, whereas the white material left by burning phosphorus is easily soluble and consequently has a very decided taste."

"Yes, very decided," assented Emile, "for it eats [148] away the part of the tongue it touches. But tell me, Uncle, if burnt zinc could melt in water, so as to have a taste, what sort of taste would it be? Would it be as strong as the taste of burnt phosphorus?"

"As to that, my little friend, neither I nor any one else can give you a sure answer, no one having experimented with such an impossibility. All we can say is that probably the taste would be detestable, as is the taste of ninety-nine chemicals out of a hundred.

"When we exhibit fireworks, we keep for the last the most beautiful piece of all, the star number on our program. This I am doing to-day, reserving to the end the prize piece in our collection, the most splendid example of metal burning. The material for this display is there in that little bottle."

"That thing that looks like a skein of narrow gray ribbon?" asked Emile.

"Yes, that's it."

"It doesn't look as if it could do anything."

"It can do much more than its appearance promises. Let us examine it closely."

So saying, Uncle Paul took the skein out of the bottle. It was a dull gray ribon[should be ribbon], narrow, very thin, and as flexible as tinfoil. When scratched with a knife it showed the brightness peculiar to metals. By this brightness and the whitish color the children though they recognized the metal.

"It is either lead or tin," declared Emile.

"I should rather say zinc or iron," said Jules.

"It is not any of those metals," their uncle told [149] them. "This is a metal you have never seen before, nor have you even heard of it."

"And what is it called, please?" was Emile's eager inquiry.

"It is called magnesium."

"Ma-ma—" stammered the boy. "Please say it again."

"Magnesium."

"Oh, what a funny name!"

"No funnier than bismuth, barium, or titanium."

"Are those the names of metals?"

"Yes, my child, they are the names of metals. If they sound strange to you, it is because you now hear them for the first time. One gets used to bismuth and titanium just as much as to copper and lead. As I have already told you, there are about fifty metals. Most of them remain unknown to us, not being in common use; their names, being seldom heard in ordinary conversation, strike the ear rather strangely at first. After you have become acquainted with magnesium, you will find its name easy to remember and it will cease to have the unwonted sound it has at present.

"A few live coals are enough to make zinc burn; a candle-flame will set fire to magnesium, and the metal will go on burning of its own accord when once it is started. It lights almost as easily as a paper spill."

"And where do they find this queer metal?" Emile inquired. "I should be willing to spend some of the pennies I have saved up if I could buy a piece."

"Magnesium is not a metal in common use. It is [150] unknown to the blacksmith, the tinsmith, and the coppersmith. It is a substance used chiefly in scientific research and in chemical experiments of an entertaining sort. It can be procured at city drug shops and toy-shops, where it is sold as one of those curiosities that instruct and amuse at the same time; and it is from the city that I brought it home for your benefit."

A candle having been lighted, the shutters were closed so as not to let the daylight lessen the brilliant effect of the burning metal. Then Uncle Paul cut off a short strip of the magnesium ribbon, and, taking hold of one end with a pair of pincers, touched the other to the candle-flame. A sheet of paper was spread on the table to receive what might fall from the burning metal. Catching fire very quickly, the ribbon was withdrawn from the candle and held in an upright position over the sheet of paper, after which it needed no further assistance; the magnesium burned alone. It was as if a sudden burst of dazzling sunlight had illuminated the darkened room. A superb white light, intense enough to penetrate every corner and render all objects clearly visible, came in waves from the splendid torch. No sputtering, no noise whatever, no flying sparks. It was the calm, sustained illumination of daylight. Dumb with astonishment at this brilliant display, the boys gazed in fascination. The burning continued, the flame approaching ever nearer to the pincers, while the burnt part fell off in pieces that looked like chalk. In a few seconds it was finished; the radiant flame died down for want of fuel.

[151] "Oh, how beautiful, how splendid that was!" cried the children, rubbing their eyes, dazzled by the glare.

Their uncle opened the shutters to let in the day-light.

"Why, I can't see;" said Emile, still rubbing his eyes. "I'm nearly blind from looking at the magnesium flame.

"And, I," added Jules, "am almost as dazzled as if I had been staring at the sun."

"That will pass off in a few minutes," their uncle assured them. "Wait until your eyes recover from the fatigue caused by the too-bright light of the magnesium flame."

The dazzling effect having worn off, as it did in a short time, Emile spoke of something that had struck him while the magnesium was burning.

"I was looking," said he, "at the flame of the candle, which you had left burning after setting fire to the magnesium, and all I saw was something of a reddish color, smoky and dull. How pale the candle looked, and it had been bright enough before! I could hardly made out any flame at all. Is it possible, I asked myself, that that can give light?"

"If you set a lighted candle in the full glare of the sunshine, can you see the flame?" asked Uncle Paul.

"No; it looks as pale and dim as it did in the magnesium light."

"That, my little friend, comes from the inability of the eye, when exposed to a bright light, to see, except imperfectly, a dim one. In full sunlight one cannot tell whether live coals are really alive or not. The flame that gave light in the dark ceases to do so [152] when overpowered by a greater brilliance. Our dazzled eyes and the apparently dulled flame of the candle prove to us that the magnesium light is one of the brightest possible; it bears comparison only with the sun.

"I hope I have now convinced you, including even the skeptical Emile, that metals are not hard to burn. The flying sparks of burning iron in the blacksmith's shop, the firing of zinc in our old spoon, and finally the dazzling magnesium flame, have furnished proof upon proof. Furthermore, the last experiment shows us that among the metals there are some that would give us splendid light if it were not for their price and scarcity. Instead of lamp oil or candle grease we could use a magnesium ribbon for lighting. Who knows what the future may have in store for us in this field? The history of chemistry is full of wonderful discoveries, and we already owe so many and important improvements to this science that we may well look to it for still greater things.

"But without dwelling longer on the brilliant splendors of magnesium let us see what has become of the metal after burning. The substance that has fallen on to the paper over which the ribbon burned is a white material which, when touched, crumbles into a soft powder like flour, or, better, like ground chalk of a very fine quality. It does not dissolve in water, and consequently has no taste. In addition to the metal itself, it contains, as does everything else after burning, oxygen taken from the air in the process of burning. So here is another storehouse [153] of oxygen from which this gas could be obtained by using the proper means, but not without some difficulty.

"Iron burns. Hammered on the anvil when red-hot, it gives out sparks which are tiny scales of this metal on fire. Let us get some of these particles of burnt iron from the smith, and we shall find them to be of a black material, rather hard, but still yielding to the pressure of our fingers. This black material, this burnt iron, is called oxid or iron.

"Zinc burns, being changed by the process into a white substance of which part is wafted upward from the flame and floats in the air like shreds of cotton or down. This white substance, this burnt zinc, is called oxid of zinc.

"Magnesium burns, and becomes thereby a white substance looking much like very fine chalk and extremely soft to the touch. This chalk-like substance, this burnt magnesium, is called oxid of magnesium.

"As a rule, metals are combustible, though there are exceptions; and in burning they combine with oxygen from the air or elsewhere, being thus changed into compounds which have non of the luster of metals and which take the name of oxid. An oxid is a burnt metal, just as an acid is a burnt metalloid, and both contain oxygen."


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