“That would be, indeed, burning ‘like a house on fire,’” observed Mr. Bagges.
“‘Think,’” said Harry, continuing his quotation, “‘of the Houses of Parliament, or a steam-engine manufactory. Think of an iron-proof chest—no proof against oxygen. Think of a locomotive and its train—every engine, every carriage, and even every rail would be set on fire and burnt up.’ So now, uncle, I think you see what the use of nitrogen is, and especially how it prevents a candle from burning out too fast.”
“Eh?” said Mr. Bagges. “Well, I will say I do think we are under considerable obligations to nitrogen.”
“I have explained to you, uncle,” pursued Harry, “how a candle, in burning, turns into water. But it turns into something else besides that; there is a stream of hot air going up from it that won’t condense into dew; some of that is the nitrogen of the air which the candle has taken all the oxygen from. But there is more in it than nitrogen. Hold a long glass tube over a candle, so that the stream of hot air from it may go up through the tube. Hold a jar over the end of the tube to collect some of the stream of hot air. Put some lime-water, which looks quite clear, into the jar; stop the jar, and shake it up. The lime-water, which was quite clear before, turns milky. Then there is something made by the burning of the candle that changes the color of the lime-water. That is a gas, too, and you can collect it, and examine it. It is to be got from several things, and is a part of all chalk, marble, and the shells of eggs or of shell-fish. The easiest way to make it is by pouring muriatic or sulphuric acid on chalk or marble. The marble or chalk begins to hiss or bubble, and you can collect the bubbles in the same way that you can oxygen. The gas made by the candle in burning, and which also is got out of the chalk and marble, is called carbonic acid. It puts out a light in a moment; it kills any animal that breathes it, and it is really poisonous to breathe, because it destroys life even when mixed with a pretty large quantity of common air. The bubbles made by beer when it ferments, are carbonic acid, so is the air that fizzes out of soda-water—and it is good to swallow though it is deadly to breathe. It is got from chalk by burning the chalk as well as by putting acid to it, and burning the carbonic acid out of chalk makes the chalk lime. This is why people are killed sometimes by getting in the way of the wind that blows from lime-kilns.”
“Of which it is advisable carefully to keep to the windward,” Mr. Wilkinson observed.
“The most curious thing about carbonic acid gas,” proceeded Harry, “is its weight. Although it is only a sort of air, it is so heavy that you can pour it from one vessel into another. You may dip a cup of it and pour it down upon a candle, and it will put the candle out, which would astonish an ignorant person; because carbonic acid gas is as invisible as the air, and the candle seems to be put out by nothing. A soap-bubble of common air floats on it like wood on water. Its weight is what makes it collect in brewers’ vats; and also in wells, where it is produced naturally; and owing to its collecting in such places it causes the deaths we so often hear about of those who go down into them without proper care. It is found in many springs of water, more or less; and a great deal of it comes out of the earth in some places. Carbonic acid gas is what stupefies the dogs in the Grotto del Cane. Well, but how is carbonic acid gas made by the candle?”
“I hope with your candle you’ll throw some light upon the subject,” said Uncle Bagges.
“I hope so,” answered Harry. “Recollect it is the burning of the smoke, or soot, or carbon of the candle that makes the candle-flame bright. Also that the candle won’t burn without air. Likewise that it will not burn in nitrogen, or air that has been deprived of oxygen. So the carbon of the candle mingles with oxygen, in burning, to make carbonic acid gas, just as the hydrogen does to form water. Carbonic acid gas, then, is carbon or charcoal dissolved in oxygen. Here is black soot getting invisible and changing into air; and this seems strange, uncle, doesn’t it?”
“Ahem! Strange, if true,” answered Mr. Bagges. “Eh? well! I suppose it’s all right.”
“Quite so, uncle. Burn carbon or charcoal either in the air or in oxygen, and it is sure always to make carbonic acid, and nothing else, if it is dry. No dew or mist gathers in a cold glass jar if you burn dry charcoal in it. The charcoal goes entirely into carbonic acid gas, and leaves nothing behind but ashes, which are only earthy stuff that was in the charcoal, but not part of the charcoal itself. And now, shall I tell you something about carbon?”