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The Wonder Book of Chemistry by  Jean Henri Fabre

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ESTERDAY we found limestone to be rich in carbonic acid, and we also found that, to release this gas from the stone and obtain it by itself, all we need to do is to apply to the stone another and a stronger acid, preferably hydrochloric acid, a very cheap liquid and one that has the additional advantage of always keeping the surface of the stone clean. Our plan for to-day is to extract from limestone this gas of burnt carbon. The apparatus required will be the same as for the preparing of hydrogen—that is to say, a two-necked bottle of jar, if our laboratory can furnish this handy utensil; or else, if our resources are more modest, a bottle with a wide neck to hold a large stopper having two holes running through it from top to bottom. Into one of these holes will be fitted a straight glass tube reaching to the lower part of the bottle, and through it, by means of a small glass funnel or, lacking that, a paper cone, the hydrochloric acid will be poured little by little so that the effervescence may not be rapid enough to make the foam rise too high and run over. Through the second opening will pass a bent tube to serve as conductor of the released gas.

[301] "Here is what we are after,—a plain bottle with a large cork stopper having two holes through it. Into this bottle I put a handful of broken limestone of the hardest sort. If I had a piece of marble,—a fragment from some old bureau-top, for example,—things would go better; but, not having any, I do the best I can with common limestone, its only defect being a tendency to soil the liquid somewhat with its impurities. I add water and adjust the stopper with its two tubes. Of course the straight tube, not the bent one, is pushed down into the liquid. Now I pour in a little hydrochloric acid, and immediately we see a commotion, a kind of boiling, caused by the setting free of gas from the stone. From this point things will go of themselves, and we need only let the apparatus alone, confining our activities to the occasional addition of a little acid to keep the process from halting."

"Quick, quick, the bowl of water for collecting the gas!" cried Emile, seeing his uncle abandon the apparatus to itself in an apparently very careless fashion.

"The bowl of water is unnecessary here," his uncle assured him; "we shall get our carbonic-acid gas quite well without having to use that cumbrous thing."

"But the gas is just going to waste."

"We can afford to waste a little, so easy is it to make more and so inexpensive the process. What does it take to produce all we wish of this gas? Only a pennyworth of acid and a stone picked up by the roadside. Besides, there is a reason for [302] my inattention to waste: there is air in the bottle and I am allowing the carbonic acid to drive it out.

"Now it is done, or very nearly, and there is no more air left in the apparatus—except, perhaps, a mere trace. Accordingly, I introduce into a wide-mouthed bottle the tube conducting the gas, and make sure it reaches the bottom. In a few moments the bottle will be full of carbonic acid."


"But it will get out of the bottle, for there's no cork," objected Jules; "or at least it will mix with the air in the bottle."

"You need fear nothing of the sort," replied his uncle. "Carbonic acid is heavier than air. As fast as it arrives, conducted by the tube that reaches almost to the bottom of the receiving bottle, it accumulates and forms an increasingly thick layer, which expels the air, a lighter gas. This air comes out in an invisible stream through the mouth of the bottle. If we had a vessel full of oil and slowly poured in a stream of water at the bottom, what would happen? The water, being heavier than the [303] oil, would accumulate in the bottom of the vessel, rise gradually, and drive out the oil, the lighter liquid. A similar process goes on when carbonic acid gas is introduced at the bottom of an enclosed body of air."

"I understand," said Emile, "but I should like to ask one question. With oil, I should know by the color when it was all driven out and its place taken by water; but here there's nothing to be seen, neither carbonic acid gas nor air. How can we know, then, when all the air is driven out and the bottle filled with carbonic acid?"

"What our eyes cannot tell us, a flame will help us to see with our understanding. Carbonic-acid is a foe to combustion; it will not keep the smallest flame alive. I will light a piece of paper and thrust it into the mouth of the bottle. If it continues to burn, there is still a layer of air in the upper part of the bottle; but if it goes out, there is nothing in the bottle but carbonic-acid gas. Let us try it; now is the time. Hardly is the lighted paper inside the neck of the bottle when it is extinguished,—a sure proof that carbonic-acid gas reaches up to the mouth of the bottle. We have now a supply of this gas for further experiment. I put the apparatus aside, as we have no further use for it at present. When we need it again, all we shall have to do will be to pour in more hydrochloric acid and let it act on the limestone.

"Here, then, is our carbonic-acid gas. It is as colorless, as transparent, as invisible as air. We have just extracted it from limestone, where chemi- [304] cal combination held great quantities of it captive with a very narrow compass. A piece of stone hardly bigger than a walnut will yield several liters of it. We have just driven some of it our of the rock, and are now going to drive it back and make it reenter the composition of rock,—that is to say limestone, powdered chalk. I pour some lime-water into the bottle that is filled with carbonic-acid gas, close it tightly with the palm of my hand, and shake it thoroughly. The liquid turns white and thick like sour milk. We let it stand a while, and flakes settle at the bottom in a considerable layer. You know these white flakes as carbonate of lime, chalk, the compound we obtained when we shook up lime-water and carbonic acid obtained by burning charcoal. So here we have fresh proof, to add to the others, that limestone really contains the gas produced by burning charcoal.

"The gas has disappeared, being shut up once more in the stone,—or, rather, in a sort of mud that would become stone if it were dried and pressed. Again I bring out our apparatus and renew the supply of carbonic-acid gas in the bottle. How do you suppose a lighted candle would behave in such an atmosphere?"

"It would go out," was Emile's reply, "just as the lighted paper did."

"Besides," added Jules, "nothing can burn except in oxygen or in the air."

The candle did indeed go out, the very instant the flame got as far as the neck of the bottle. The results attained with nitrogen had not been quicker [305] or more complete. Not only was the candle immediately extinguished, but there was not the slightest glow left for an instant in the wick.

"Without making any cruel experiments," Uncle Paul went on, "we can feel sure that this gas, so manifestly unfit for combustion, is equally unfit for maintaining life. An animal would die in it, and very quickly, just as you saw the sparrow die in an atmosphere of nitrogen. Let us now proceed to prove that carbonic-acid gas is heavier than air. I have turned this comparative heaviness to account in collecting the gas without the help of our bowl of water, so that proof is in reality already before us. Nevertheless I will give you a still more striking demonstration.

"We will take two bottles holding equal amounts and having mouths of equal size. This one, at my right, is full of carbonic-acid gas. I lower a lighted candle into it, and the candle is immediately extinguished. This other bottle, at my left, is full of atmospheric air. I lower a lighted candle into it, and the flame continues to burn. Now, removing the candle, I take the right-hand bottle and gradually invert it, at the same time fitting its mouth to that of the other bottle. In fact, I do exactly as I should do if I were pouring water from the one into the other. I decant the carbonic-acid gas as if it were a liquid. Nothing is seen to pass from the upper to the lower bottle, nor is anything seen to rise from the lower to the upper; and yet the exchange is made, as we shall soon prove. The carbonic acid, the heavier gas, descends and fills the [306] lower bottle, while the atmospheric air the lighter gas, rises and fills the upper bottle. After waiting a few minutes for the exchange to become complete, I put the two bottles back in their places and again try the lighted candle. It burns in the right-hand bottle; hence this one no longer holds carbonic acid, but air. It goes out in the left-hand bottle, proof that the latter has exchanged its original contents of atmospheric air for the gas that will not maintain combustion. Thus it is made clear that the bottled carbonic acid has gone down and the bottled air has gone up, the two changing places without getting mixed in the process.

"Now listen to this. In various places there is a constant escape of carbonic-acid gas from the ground, especially in the neighborhood of volcanoes. There are springs of carbonic-acid gas as well as springs of water. The most celebrated spring of this sort is that at Pozzuoli, near Naples. It is known as the Dog's Grotto, from the distressing part a dog is made to play for the amusement of the curious. The grotto is hollowed out of the solid rock, and the air in this cavity is earthy, damp, and warm. Bubbles of gas rise here and there in the mud.

"The keeper of this grotto—for there is a keeper who, for money, shows people the repulsive spectacle I am going to tell you about—takes his dog, ties the animal's legs together to prevent its running away, and lays it on the ground in the middle of the grotto, where he himself remains. There is nothing to arouse the slightest suspicion of danger, [307] no foul odor, no lack of clearness in the atmosphere. Besides, is not the dog's master there, showing no sign of fear, standing in the very middle of the cave? Nevertheless there is the dog giving tokens of distress by its groans; it writhes in frightful convulsions, its eyes become dim, its head falls heavily, and it appears to be at death's door. But at this point its master carries it out of the grotto, unties its legs, and lets it breathe the pure air. Little by little the animal revives; it struggles to its feet; still giddy, it looks about in a dull, stupid manner, and then runs off as fast as its legs will carry it, evidently fearing a second ordeal.

"Does the dog act a part taught it by its master? Has it been trained to play dead in the grotto, where its master stands upright beside it without suffering the least inconvenience? No, the dog really comes near dying, as it knows well enough from passing its wretched if in repeating the act several times a day. It knows it so well that it submits to the experiment with a very bad grace. The minute it sees a stranger approaching in the distance it turns morose and surly, growls and threatens to bite. Its master has to hold it in leash when taking it to the grotto, dragging the animal along while the poor beast shows its reluctance by the drooping of its ears and tail. But when the ordeal is over and the stranger gone, it is full of a foolish joy that shows itself in unmistakable fashion. The wretched animal is completely happy at being allowed to come to life again.

"There is nothing about the famous grotto that [308] cannot be easily explained. Carbonic-acid gas, as I said, comes up out of the ground. This gas is not fit for breathing; after a few breaths an animal dies. Furthermore, it is heavier than air; and so, instead of dispersing evenly throughout the grotto, it remains close to the ground, where it forms a layer about half a meter thick. It reaches only to the knees of the man as he stands in the middle of the grotto, whereas the dog, stretched on the ground, is entirely immersed in it. The master is not compelled to breathe the harmful gas, and so feels no inconvenience from it; the dog breathes nothing else, and in consequence almost dies. But were the dog's master to lie on the ground as does the dog itself, he would share the poor animal's fate.

"The heavier gas is constantly added to and as constantly escapes through the mouth of the grotto, running out in a sort of stream when the air is calm. No one sees this stream, and one passes through it without suspecting it to be there. It makes no mumurous sound and flows over no bed of pebbles, but runs gently and quietly over the grass. Its presence may, however, be detected with the help of a lighted candle: beyond the limits of this invisible stream of gas the candle burns, but as soon as its flame is immersed in the current it goes out as if it were plunged into water. In this way the stream of gas can be traced for some distance from the grotto, while beyond that it is dissipated by air-currents."

"If it weren't so far away," said Jules, when his uncle had finished his story, "I should like to [309] go and see that wonderful grotto; but I shouldn't ask to have the dog suffer that terrible torture. I should be satisfied with a test made with a lighted candle, first high up in the grotto and then down near the ground, to see whether it would go out when it was lowered."

"If such a test is all you wish for," replied his uncle, "a trip to Pozzuoli is quite unnecessary, for we can reproduce right here the essential conditions found in the Dog's Grotto. A glass jar shall be our grotto, and we will substitute for the carbonic acid rising from the ground the gas our apparatus yields when supplied with limestone and hydrochloric acid. Here is just the jar we need, large enough and with a wide mouth. I insert the long arm of our bent tube and let it reach to the bottom of the jar. Carbonic-acid gas flows through the tube, accumulates at the bottom of the jar, and is retained there by its weight, forming a layer of increasing depth and displacing an equal volume of air. Nothing tells us how deep this layer is at any moment, for the two gases, carbonic acid and air, are both invisible. Nevertheless, from the activity of the effervescence in our apparatus we may give a shrewd guess as to when the jar is about half full of carbonic-acid gas. Then I stop the flow of gas by disconnecting the jar from the apparatus.

"If I am not much mistaken, now is the time to make this disconnection. Having done this, we now see before us an artificial Dog's Grotto,—that is to say, a glass jar filled with carbonic-acid gas below and atmospheric air above. Look through it. It [310] appears to be all alike, the lower layer of carbonic-acid gas being as colorless, as invisible, as the upper layer of air. Our eyes cannot tell us just where the fatal layer ends and the breathable one begins. Though there is a sharp division between the two, no indication of it is to be detected by the eye.

"Slowly I lower a lighted candle into the jar to test its contents. At first it burns very well, and on being lowered farther and farther it still burns; but at last I reach a point where it begins to turn dim. This is the dividing line between the two layers, and if I lower the candle only a little more it immediately goes out, being fully immersed in the carbonic-acid gas. Here, then, we have what Jules was wishing to see,—a reproduction of the Dog's Grotto and the behavior of a lighted candle there. According to the candle's position, high or low, it burns or goes out.

"Now imagine we have in the jar two very different-sized animals, the smaller one completely immersed in the bottom layer. The former will perish in a short time, as it breathes a gas incapable of maintaining life, whereas the latter will suffer no discomfort whatever, having a plenty of pure air to breathe. That is the relative position of the dog and the man in the grotto with its lower layer of carbonic acid gas and its upper one of pure air."

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