"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?" "With all my heart," assented Mr. Bagges. "I said that there was carbon or charcoal in all common lights--so there is in every common kind of fuel. If you heat coal or wood away from the air, some gas comes away, and leaves behind coke from coal, and charcoal from wood; both carbon, though not pure. Heat carbon as much as you will in a close vessel, and it does not change in the least; but let the air get to it, and then it burns and flies off in carbonic acid gas. This makes carbon so convenient for fuel. But it is ornamental as well as useful, uncle The diamond is nothing else than carbon." "The diamond, eh? You mean the black diamond." "No; the diamond, really and truly. The diamond is only carbon in the shape of a crystal."