"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 or 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?"