"Then, the coal from which the gas came must be coke," said Felix, "something like that folks buy at the gas-works, after they have got the gas out?"
"Yes: it is the carbon of coal without the hydrogen, and so it does not burn with a flame; the flame of a fire is the burning of the hydrogen which escapes from the coal or wood when it is heated, and its burning, as it rises, makes the flames that leap up and curve about so in a fire. The anthracite coal is coal which has had most of the hydrogen driven out by heat."
"I wish you would tell us now," said Ruth, "about the stored-up heat in a cool form. I've been wondering about that most of all."
"You know plants can't grow without sunlight, Ruth?"
"Yes; that is, I know that if they don't have sunlight, they will grow very white, as they do when they are in a cellar."
"If the cellar were perfectly cold and dark, they would not grow at all; but they grow, as you have seen them, because there is some sunlight and sunheat, even in a cellar. The heat of the sun causes the plants to grow by taking in carbon from the air, and hydrogen from the water in the earth, and in rain and dew, and so the carbon and hydrogen are gradually stored up in the plants and trees; of course, there is the largest quantity in trees, because they grow large and solid. And it has been proved that you can get just as much heat by burning these things afterwards, that is, making the carbon and hydrogen unite with the oxygen from which they had been taken away, as there was heat of the sun expended in the process of their growing, or gathering up carbon and hydrogen."
"I remember about your telling that before, at the doctor's when I hurt my head; but I don't see how they ever proved it," said Felix.
"You could understand by studying it up. I have books that tell how it was proved. So, you see, it amounts to this,—that the heat from the sun is stored up some way, in the things that have grown out of the earth; and the very amount of heat stored, without a particle of waste, can be got out of the combustible a thousand or more years after it was bottled up in that way."
"Not a thousand years!" exclaimed Julia. "Trees don't live to be a thousand years old."
"Yes,—the big trees of California," said Ruth.