By evening the three travelers had returned, all much pleased with their trip. Uncle Paul had brought to a favorable conclusion his business in town. Emile and Jules each came back with an idea. When they had done honor to the excellent supper Mother Ambroisine had prepared on purpose to wind up the holiday with a little treat, Jules was the first to impart his idea to his uncle. “Of all that I saw to-day,” he began, “what struck me most was the engine at the head of the train, the locomotive that draws the long string of cars. How do they make it move? I looked well, but could not find out. It looks as if it went by itself, like a great beast on the gallop.” “It does not go by itself,” replied his uncle; “it is steam that puts it in motion. Let us, then, first learn what steam is and what its power. “When water is put on the fire, it first gets hot, then begins to boil, sending off vapor, which is dissipated in the air. If the boiling continues some time, it ends with there being nothing in the pot; all the water has disappeared.” “That is what happened to Mother Ambroisine day before yesterday,” put in Emile. “She was boiling some potatoes, and having neglected to look into the pot for some time, she found them without a drop of water, half burnt. She had to begin all over again. Mother Ambroisine was not pleased.” “By heat,” continued Uncle Paul, “water becomes invisible, intangible, as subtle as air. That is what is called vapor.” “You told us that the moisture in the air, the cause of fogs and clouds, is also vapor.” This from Claire. “Yes, that is vapor, but vapor formed only by the heat of the sun. Now, you must know that the stronger the heat, the more abundant is the vapor. If you put a pot full of water on the fire, the burning heat of the grate sets free incomparably more vapor than the temperature of a hot summer sun could. As long as it escapes freely from the pot, the vapor thus formed has nothing remarkable about it; so your attention has never been arrested by the fumes of a boiling pot. But if the pot is covered, covered tight, so as not to leave the slightest opening, then the steam, which tends to expand to an enormous volume, is furious to get out of its prison; it pushes and thrusts in all directions to remove the obstacles that oppose its expansion. However solid it may be, the pot ends by bursting under the indomitable pushing of the imprisoned steam. That is what I am going to show you with a little bottle, and not with a pot, which would not shut tight enough and the cover of which could be easily pushed off by the steam. And besides, even if I had a suitable pot, I should take care not to use it, for it might blow the house up and kill us all.”