"Why secondly, wood cannot do the work."
"Why not?"
"Because it hasn't enough sunshine in it."
"How do you mean?"
"Why you know, don't you, that all the heat we get out of burning fuel of any kind, is simply so much sunshine stored up for us and released by burning?"
"I confess I didn't know that," said Tom. "Or at any rate I never thought of it. Now that I do think of it, I see how it is with wood. But what has sunshine to do with coal, buried as it is deep under rocks and earth?"
"Then you don't know what coal is, and where it comes from?" asked the Doctor. "Let me explain. There was a period in the world's remote history when the earth was much warmer than it is now—almost hot in fact. The atmosphere was filled with the gases of carbon, and the rains were an almost continuous cataclysm. Human life was impossible in these conditions. No man could have breathed such an atmosphere and lived. But the conditions were peculiarly favorable to abundant vegetable life. There were forests such as we do not dream of now even in tropical swamps. Ferns grew to the height of great trees, vines and cane and grass and air plants filled up every available inch of space, and they all grew in that carbonized atmosphere with a rapidity and luxuriance quite impossible now. All this vegetation died of course and fell to the ground as all vegetation does and has done from the beginning of time. Wherever it fell into water and was thus shielded from the air, and wherever it managed to get itself covered with earth or rock, as in that highly disturbed volcanic age often happened, it was converted into coal by pressure and by losing certain of its volatile elements, just as charcoal is made by expelling the volatile parts from wood. So, without going any further into details, you see that the coal is preserved vegetation which grew many thousands of years ago, and that the heat we get from it is simply the sunshine it stored up at a period before ever human life existed. What a pity it is that we have to waste so much of it!"
"How do you mean, Doctor?" asked Jack.
"Why you see we waste almost all the heat that coal gives us. If we could make effective use of it all, the burning of a single pound of coal would give us force enough to lift more than eleven and a half millions of pounds a foot from the earth; but the most that we actually get out of it is force enough to lift one and a half million pounds."
"What? All that from one pound of coal?" asked Jim.