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The Wonder Book of Chemistry by  Jean Henri Fabre

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A PIECE OF CHALK

[284]

"T
O-DAY, my young friends, we shall have no thundering artillery, no furious outbursts of flame, no ear-splitting concert on pipes of glass and pasteboard, no boisterous festival to celebrate the birth of a drop of water. But the lesson will be none the less important because it is quiet. We are going to inquire what becomes of coal, or carbon, when it burns. We have seen how brilliantly it burns in oxygen, and we shall not soon forget that magnificent display. In the process of burning, an invisible gas is produced, a gas we called carbonic acid, knowing it to be an acid because, like other acids, it reddens, even though but slightly, the blue of litmus. This gas, known to us so far only by name, merits closer study. First of all I will show you how to recognize it, whatever may be its origin.

"Here is a piece of lime. I sprinkle water on it to slake it, as the masons say, and it turns hot and steaming and crumbles to powder. I add more water, much more, so as to make a thin paste. You have not forgotten that lime is slightly soluble in water, and it is a solution of that sort I desire, one that is perfectly clear and without a trace of undissolved lime. This I obtain by straining the paste through a paper filter placed in the mouth of a [285] funnel. You know that when we wish to separate fine grains from coarse in a mixture of the two, we use a sieve that allows the former to go through while it retains the latter. Well, the filter-paper is also a kind of sieve, all perforated with invisible pores that give egress to matter reduced to exceedingly fine particles by the act of dissolving, and bar the way to what is not dissolved. Druggists and grocers sell this filter-paper, which is much used in straining turbid wine and vinegar, or any other liquid that is to be freed of its dregs. It comes in rounds, larger or smaller as may be desired. By folding it over once we get a semicircle, by folding it a second time we get a quarter circle, another fold gives an eighth of a circle, and so on until it can no longer be easily folded. Then, by partly spreading out our folded disk, we obtain a sort of crinkled funnel, which we place in a chemist's glass funnel as a support, if such a funnel is at hand; otherwise a common tin funnel will do. A bottle for holding the filtered liquid receives the tube of the funnel.

"My filter is ready, and through it I strain the lime paste. Notice how thick and muddy the liquid is above the filter, and how clear and limpid below. What has gone through into the bottle looks like the very purest of water. Is not this round of paper a wonderful sieve in its perfect separation of the dissolved from the undissolved lime? The liquid that passes through it is something more than water, clear though it is to the eye; it contains a little dissolved lime? The liquid that passes through is something more than water, clear though it is to the eye; it contains a little dissolved lime, as we find by tasting it. It is called lime-water, and we shall use it as a test for carbonic acid.

[286] "We must now obtain some of this carbonic-acid gas by burning charcoal either in pure oxygen or in the mixture of oxygen and nitrogen that we call atmospheric air. We will adopt this latter method as being both simpler and quicker. Here are two bottles of the same size and both full of air. Leaving one for the present, I introduce into the other a piece of charcoal well on fire, and keep it there until it goes out, which it soon does. A little carbonic acid has formed. It is there in the bottle, invisible and mixed with what is left of the air originally in the bottle. The lime-water will betray its presence. I pour a spoonful or two into the bottle and shake it up, whereupon it loses its clearness and turns clouded and white. Is it really the carbonic acid that has caused this sudden change? In the bottle there is still some air, containing nitrogen and the little oxygen not used up by the burning of the charcoal. Could not one or the other of these gases have caused the milky appearance of the lime-water? We must make sure before asserting anything. So I pour lime-water into the bottle that has had no charcoal burned in it, and I shake it up, but with no result; it remains perfectly clear. Hence it must be the carbonic acid alone that turns the lime-water white, and neither oxygen nor nitrogen has anything to do with it. I will add that no gas except carbonic acid has this power of giving a whitish appearance to lime-water.

"So, then, we have in this liquid a useful means for distinguishing carbonic acid from all other gases. Suppose, for example, a certain bottle contains a gas [287] that we know nothing about, and we wonder whether it may possibly be carbonic acid. Lime-water settles the question at once by either turning white when shaken up in the bottle or by remaining unaffected. Often when there has been some combustion of carbon without our knowledge, lime-water will decide the matter, its testimony being unimpeachable even where the presence of carbonic acid is held to be impossible. We shall have occasion to invoke this testimony in cases of the utmost importance. Let us therefore bear this truth well in mind: carbonic acid turns lime-water white and conversely, whenever lime-water is turned white by a gas, this gas is carbonic acid.

"I pour into a glass the liquid whitened by the carbonic acid in the bottle. Hold it up to the light and look through it. What do you see? Whirling white flakes, fine particles like those in curdled milk. We let the liquid stand undisturbed a while, and the white particles slowly settle at the bottom, leaving the liquid clear once more. I throw away the liquid and keep the sediment. There is not much of it, hardly a pinch. What can it be? From its appearance one might say flour, starch, or powdered chalk. And chalk it is, surely enough, chalk like that used for writing on a blackboard.

"But don't for a moment imagine that the sticks of chalk used for writing are made in the same way as this pinch of white powder. They would cost an enormous sum if, in order to make them, charcoal had to be burned and then lime-water shaken up with the carbonic acid thus obtained. Common [288] chalk occurs ready-made in nature; it only has to be separated from its impurities, pressed into a solid mass, and cut into sticks. Nevertheless we have just obtained real chalk by artificial means. And how has this come about? It is this way: the water contained lime in solution; carbonic acid came and took possession of this lime, combining with it and forming a salt known as carbonate of lime; and thus the white powder you see there is a salt, a compound of lime and carbonic acid,—in short, carbonate of lime.

"In its natural state this substance, though composed always of carbonic acid and lime, may be of very different degrees of fineness, consistency, and hardness. If it is rather soft and tends to crumble, it is chalk; if hard and coarse, limestone, building-stone, ashlar; if still harder and of fine, compact structure, it is marble. These various kinds of stony substance, so different in name, appearance, and use, are at bottom the same thing,—a union of lime and burnt carbon. Chemistry, which pays no heed to outer appearance, but considers only inner structure, gives them all alike the name of carbonate of lime. Hence we could if necessary get from chalk or marble or any piece of limestone a supply of carbonic acid that would not differ in the least from that obtained by burning charcoal.

"Now I beg you to note the curious lesson taught us by this lime-water. Suppose we wish to study the gas produced by burning charcoal. Do we heap up fuel in the fireplace and make a roaring fire? By now means. We need no mass of glowing charcoal, [289] no fireplace, no furnace. A few bits of stone will furnish us the same gas, absolutely the same. Chemistry abounds in similar examples that upset our little stock of conventional notions. To the ignorant all this seems like magic. Are you seeking the best possible fuel? Chemistry tells you to look for it in water. Do you wish for some gas such as rises from burning coals? Chemistry informs you it is to be found in stone.

"There is carbon in chalk. The blackest of black substances enters into the composition of the very whitest. To doubt this would be impossible even for Emile, who is always demanding proof after proof, whenever any statement surprises him by its novelty. It was really carbon, in the form of charcoal, that I burned in the bottle; a gas called carbonic acid was thus produced; and then, but not before, the lime-water brought into contact with this carbonic acid showed tiny chalk-flakes whirling about in it. There is carbon, I say, in chalk; but it is in the burnt state, and to burn it over again is impossible unless its present partnership with oxygen is first dissolved. Consequently, chalk is incombustible. But carbon abounds in numerous other substances, and in an unburnt condition; therefore it is combustible. It cannot be seen with the eye in these substances, nor is there anything to make one suspect its presence. Take, for example, the material of which so-called wax candles are made: this is of a beautiful white appearance, as is proved by the black smoke rising from the burning wick. But, disre- [290] garding this smoke, let us examine the substance itself and find out whether it does really contain carbon. Our course is marked out for us in advance: we have only to light the candle and see whether there is any carbonic acid formed. If so, it is an incontestable proof that there is carbon in the white substance of the candle. Let us try it.

"I renew the air in the bottle by filling it with pure water and then emptying it, as I have already shown you. Then I lower into the bottle a lighted candle fastened to a wire, and I keep it there until it goes out. Has any carbonic acid formed? Our lime-water will tell us. I pour a little into the bottle and shake it up. Ha! Look there: the lime-water has taken on a cloudy whiteness, and hence we know the burning of the candle has produced carbonic acid, proving that the white substance of which the candle is made contains carbon. Nothing difficult about that.

"Let us take another example. Paper contains carbon, and we have but to bur a piece of it and examine the charred fragments, to see, from the black color, that it is there. But we will take no account of this indication, which, after all, might be deceptive, as all is not carbon that is black. Refilling our bottle with pure air, I twist up a good-sized piece of paper and burn it inside the bottle, taking care not to let the ashes fall, as they would interfere with the rest of the operation. That done, I call on the lime-water to give its testimony, and again the lime-water turns white. So there must be carbonic acid [291] there, and hence carbon. It speaks for itself, you see.

"Once more: The black smoke of the burning paper and the black color of its charred fragments make us pretty sure, without the help of lime-water, that paper contains carbon, just as the black smoke rising from a smoldering candle-wick makes us think the candle has carbon in it; and this we infer despite the whiteness of both paper and candle. But now we come to a third substance that gives no similar indication of the presence of carbon. It is alcohol, or spirit of wine. Its strong winy odor proves at once that it is not water, though it is quite as clear. It takes fire readily and burns with a smokeless flame. Is there carbon in this inflammable and perfectly colorless liquid? Not a sign of anything black do we detect either in the flame or anywhere near it. There is no foul smoke, no dark-colored residue. Only lime-water can settle the question here. Into a small cup supported by a wire bent into a ring at one end I pour a little alcohol I light it and lower it into the bottle, the air in which I have first renewed. As soon as the alcohol stops burning I apply the lime-water test. The water is whitened. That settles it. I can now affirm with absolute certainty that alcohol, a liquid as colorless and transparent as water, contains that compact black, opaque substance called carbon.

"In this way we might test a great variety of substances, and all in which the gaseous products of combustion produced a clouding of lime-water would [292] be shown to have carbon in their composition. If I have laid some stress on this point, it is to show you by experiment the deceitfulness of appearances in determining the real nature of a compound substance. I have just proved to you, by facts that speak for themselves, the presence of carbon in substances that, from appearances only, would not be suspected of containing it; and thus you are prepared for the even more surprising demonstration that a piece of stone can be made to yield the gas we call carbonic acid.

"Chalk, marble, and all limestones contain carbonic acid, an acid of little strength and always ready to give up its place to any other acid that is more powerful. In chemistry the harsh law of the strongest prevails: get out of that and make room for me. If, then, we pour some strong acid on carbonate of lime, carbonic acid is set free, being ousted by the new-comer, which takes its places and forms with the lime a new salt. Sulphuric acid, for example, turns a carbonate into a sulphate, and phosphoric acid turns it into a phosphate. In both cases the carbonic acid is set free, and its release is accompanied by a foaming on the surface of the stone.

"The thing is worth seeing. Let us experiment with the pinch of white powder obtained when we shook a little lime-water in the bottle in which we had burned a piece of charcoal. Here is the powder in the bottom of this glass and not yet dry, but that will not interfere in the least with the success of our experiment. I let a drop of sulphuric acid fall on to the white paste. Immediately [293] the mixture has the appearance of boiling and is covered with froth; it foams, as we say. This foam is made of the bubbles of carbonic acid driven out by the other acid. Now let us try some real chalk, such as is used for writing on blackboards. I take a stick of it and with the help of a slender glass rod I place a drop of sulphuric acid on the chalk. Where the acid touches the chalk there is a foaming appearance, a sure sign that carbonic acid is being driven out.

"You were already inclined to consider this white powder of the same nature as chalk, taking my word for it and also judging from the likeness of appearance between the two; and now a very decisive test proves it beyond a doubt. Both substances foam when touched with an acid, and both yield the same gas, as could easily be ascertained by operating on a larger scale and by collecting the bubbles of gas which cause the foam. The resemblance is not confined to mere appearances, but goes deeper and is fundamental; that is to say, the two substances tested are the same thing.

"Limestone, again, is the same thing. But how are we to know limestone when we see it, how distinguish it from other kinds of stone? These are questions to be answered without delay, as we are about to use this stone in obtaining a sufficient supply of carbonic acid for further experiments. We must not proceed in any haphazard fashion, trying the first stone we come to, and then another if that does not serve our purpose. We must be able at the very start to give our apparatus the right [294] material for producing carbonic acid. Where is this material, this storehouse of gas, to be found, and how can we pick out our particular kind of stone from among so many different kinds? A drop of strong acid will give the answer. Here is a hard stone picked up in the bed of a brook. I touch it with a drop of sulphuric acid: no result, no foam; hence the stone contains no carbonic acid, is not a carbonate, and will be of no use to us in producing the gas we desire. We throw it away. Here is a second stone, as hard as the first. I test it in the same manner, and the acid makes it foam at the merest touch. So there is carbonic acid here. The stone is carbonate of lime, limestone. For one who is not well acquainted with the rocks of his region and cannot distinguish one from another by outward appearances, there is—as you now see, I hope—a very convenient way of settling all doubts as to what is limestone and what is not."

"That is plain enough," Emile agreed; "if it foams with a strong acid, it is limestone; if not, it isn't limestone. The one that foams will give carbonic acid, and the other won't for the very good reason that it has none."

"In our definition," Uncle Paul continued, "I will change only one word, and I do this in order that our terms may be those used in chemistry. When I spoke of 'foaming' to indicate the bubbling or boiling produced when a gas is set free, I should more properly have said 'effervescing.' In chemical language, then, we say that limestone effervesces under the action of a strong acid, which is really the [295] same as saying that it bubbles, foams, or froths; and a stone that does not effervesce is not limestone.

"The word 'limestone' means carbonate of lime; but there are many other carbonates, each metal giving one of its own, or sometimes more than one, as occasionally there are several carbonates for the same metal. Iron, copper, lead, zinc, to name no others, have each its own carbonate, just as calcium has, this last being, of course, carbonate of lime, or limestone. This carbonate is much more plentiful than any of the others, and plays a more important part in this world of ours; therefore I particularly call your attention to it. A good half of the soil is made of it. Great mountain-chains are blocks of this salt. Whether rare or abundant, all carbonates without exception have the peculiarity of effervescing when touched with an acid. Since they all contain carbonic acid, as otherwise they would not be carbonates, they all release this acid when a stronger acid comes to take its place. Effervescence or foaming is the invariable accompaniment of this release of gas. You will see before long what lessons are to be learned from this peculiarity.

"I put into this glass a pinch or two of ashes from the fireplace. If I were to ask you what these ashes contain, what would your answer be? You could not tell me, for neither sight, taste, nor smell gives us any information. But by a skilful roundabout method we can arrive at the answer, with evidence to back it up. I pour a little sulphuric acid on the ashes, and very decided effervescence follows, the mixture of acid and ashes foaming rather violently. [296] Consequently we infer— But who will tell me?"

"I know," Emile hastened to reply; "there is carbonate of lime in the ashes."

"I think," said Jules, "that Emile has jumped to a rather hasty conclusion. All carbonates effervesce with acid, and so the foaming shows only that there is a carbonate in the ashes, but doesn't tell us which carbonate."

"You are right, my boy. The ashes contain carbonate, but not that of lime; it is of another metal, potassium, a name you have already heard. If what I have just done to the ashes does not tell us the nature of the metal they contain, it at least tells us that in these ashes from our fireplace there is carbonic acid. In this connection it is well for you to learn that chemists determine the nature of a substance by just such tests as this. You hand to the chemist a rock, a mineral, an earthy substance, or anything else you deem worthy of serious examination. He tests the thing with one chemical and informs you that it contains iron; he tests it with another and says it contains copper; a third test proves the presence of sulphur; and so on. Yet neither iron, copper, nor sulphur was visible to the eye, nor did they become so in the course of the various tests. Nevertheless, that they are there is proved conclusively by the action of the various chemicals. When a piece of white marble effervesces on being touched with sulphuric acid, I conclude that the marble contains carbonic acid and, consequently, carbon. It is in a similar manner that the chemist reasons from the tests he applies, [297] ascertaining by these tests that one substance contains this and another that, without needing to see with his eyes either this or that.

"Now, then, let us get to work with our preparation of carbonic acid. For this purpose we have here a quantity of broken limestone. I put a good handful into a glass, and add water to moderate the action of the acid so as to give us a steady and not too rapid release of the gas instead of a violent effervescence, which would be difficult to manage. The acid I am going to use will not be sulphuric acid, which I used in the experiments you have just witnessed; and the reason for not using it is this: with sulphuric acid, carbonate of lime would change to sulphate of lime, or plaster of Paris, an insoluble substance that would encrust the fragments of stone and thus obstruct further action on the part of the acid, so that the release of gas would come to a standstill. The operation, though successful enough at first, would come to an untimely end. For an uninterrupted release of gas, the pieces of stone must keep their surfaces clean, not coated over with a protecting layer. In other words, the new compound formed with the acid must get out of the way as fast as it forms. This condition is fulfilled if the new compound dissolves in the surrounding water, a result obtained by the use of hydrochloric acid."

"What acid did you say?" asked Emile.

"I said hydrochloric acid."

"What a funny name! I don't believe I can remember it."

[298] "Don't blame me; I didn't invent it. We will call it, if you like, spirit of salt, the name it bears in workshops and factories where it is used. My friend the blacksmith, who is also a locksmith and a coppersmith, uses it to clean his old copper utensils, and he gave me this bottleful. Talk to him about spirit of salt, and he will know what you mean; call it hydrochloric acid, and he will give you a blank look."

"But why is it called spirit of salt? That's a rather queer name too."

"It is so called because it is made from salt, common kitchen salt. As for the word 'spirit,' which seems to puzzle you in this connection, it is a remnant of the old-time language of early chemistry, which gave this name to all invisible substances, substances that we now call gases, in fact. To those chemists of long ago the invisible and inflammable vapor that rises from hot wine was the spirit of the wine, and the pungent acid vapor derived from common salt treated in a certain way was spirit of salt. 'Spirit of wine' is a term still in common use, whereas 'spirit of salt' is heard only among artisans having occasion to use it.

"This acid has another noteworthy peculiarity: it contains no oxygen, is not a burnt metalloid as are sulphuric acid, carbonic acid, phosphoric acid, and the other acids we have had more or less occasion to speak of. It is made of chlorin and hydrogen, and this composition gives it the name you find so hard to remember. Chlorin, as I hope you have not forgotten, is the metalloid found in salt and in chloric [299] acid. As to hydrogen, it is not necessary to remind you what that is.

"Briefly, hydrochloric acid or spirit of salt, whichever you please, is a yellowish liquid of a very sour taste, evaporating in the air in white fumes of an exceedingly pungent odor. I pour some into the glass containing water and pieces of limestone, whereupon there follows a lively boiling and bubbling caused by the release of carbonic acid from the stone and its replacement by the acid of salt; and this chemical action we shall turn to our account in the next lesson."


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