"That's so," said Johnny; "but I was thinking of the coal mines which are old, buried forests packed hard under ground, where the stored heat has been preserved so long, and will be preserved for no one knows how many hundreds and thousands of years to come."
"But it will get used up sometime, and all the forests get cut off: I've heard my father say so," said Felix.
"I suppose so; but by that time we shall have found out how to burn water."
"To burn water!" exclaimed all but Pierre.
"Yes. They can burn water now, only the process is too expensive; but by that time they will have found some very cheap way. That is what the scientific men are pretty certain of. Water is composed of hydrogen and oxygen; and, you see, hydrogen is very inflammable, being what we burn for gas, and that which burns in a fire to make the flame, while oxygen is the very gas it must have in order to burn."
"Then, I don't see why water isn't very inflammable indeed," said Felix wonderingly. "I wonder the oceans and rivers and lakes haven't burned up and set all the rest of the world on fire."
"Why, you see, the hydrogen in water won't unite with the oxygen in the air, because it has all the oxygen it wants already. It has got to be separated from the oxygen it has, before it will be ready to take in oxygen, and so cause fire."
"Oh!" said Felix. "But I shouldn't think it would be so hard to get it away from its oxygen."
"It is, though," said Johnny; "for hydrogen and oxygen have a very great liking, or affinity, for each other."
"I should think, then," said Felix musingly, "if water is composed of hydrogen and oxygen, that, when hydrogen combines with oxygen in the burning of gas or a common fire, they would form water."