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The Story Book of Science by  Jean Henri Fabre


 

 

CHAPTER XLVIII

THE BOILING POT

[216]

A
S their uncle finished speaking, the postman came with a letter. A friend advised Uncle Paul to go to town on pressing business, and he wished to take advantage of the occasion to give his nephews the diversion of a little journey. He had Jules and Emile dressed in their Sunday clothes, and they set out to wait for the train at the neighboring station. At the station Uncle Paul went up to a grating behind which was a very busy man, and through a wicket he handed him some money. In exchange the busy man gave him three pieces of cardboard. Uncle Paul presented these pieces of cardboard to a man who guarded the entrance to a room. The man looked and let them enter.

Here they are in what is called the waiting-room. Emile and Jules open their eyes wide and say nothing. Soon they hear steam hissing. The train arrives. At its head is the locomotive, which slackens its speed so as to stop a moment. Through the window of the waiting-room Jules sees the people passing. Something preoccupies him: he is trying to understand how the heavy machine moves, what turns its wheels, which seem to be pushed by an iron bar.

They enter the railway car, the steam hisses, the [217] train starts, and they are off. After a moment, when full speed had been gained: "Uncle Paul," said Emile, "see how the trees run, dance, and whirl around!" His uncle made him a sign to be silent. He had two reasons for this: first, Emile had just made a foolish remark, and, secondly, his uncle did not choose to notice the giddy-pate's self-betrayal in public.

Besides, Uncle Paul is not very communicative when traveling; he prefers to maintain a discreet reserve and keep silence. There are people whom you have never seen before, and perhaps will never see again, who immediately become very familiar with their traveling companions. Rather than hold their tongues they would talk to themselves. Uncle Paul does not like such people; he considers them weak-minded.

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 [218] 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 [219] 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."

Uncle Paul took a glass vial, put a finger's breadth of water into it, corked it tightly with a cork stopper, and then tied the cork with a piece of wire. The vial thus prepared was put on the ashes before the fire. Then he took Emile, Jules, and Claire, and drew them quickly into the garden, to see from a distance what would happen, without fear of being injured by the explosion. They waited a few minutes, then boom!  They ran up and found the vial broken into a thousand pieces scattered here and there with extreme violence.

"The cause of the explosion and the bursting of the bottle was the steam, which, having no way of escape, accumulated and exerted against its prison walls a stronger and stronger pressure as the temperature rose. A time then came when the vial could no longer resist the pressure of the steam, and it burst to pieces. They call elastic force the pressure exerted by steam on the inside of pots that hold it prisoner. The greater the heat, the stronger the pressure. With heat enough it may acquire an irresistible power, capable of bursting, not only a glass [220] bottle, but also the thickest, most solid pots of iron, bronze, or any other very resistant material. Is it necessary to say that under those conditions the explosion is terrific? The fragments of the pot are thrown with a violence comparable to that of a cannon-ball or a bursting bomb. Everything standing in the way is broken or knocked down. Powder does not produce more terrific results. What I have just shown you with the glass vial is also not without some danger. You can be blinded with this dangerous experiment, which it is well to see once under proper precautions, but which it would be imprudent for you to repeat. I forbid you all, understand, to heat water in a closed vial; it is a game that might cost you your eyesight. If you should disobey me on this point, good-by to stories; I would not keep you with me any longer."

"Don't be afraid, Uncle," Jules hastened to interpose; "we will be careful not to repeat the experiment; it is too dangerous."

"Now you know what makes the locomotive and a great many other machines move. In a strong boiler, tightly closed, steam is formed by the action of a hot furnace. This steam, of an enormous power, makes every effort to escape. It presses particularly on a piece placed for that purpose, which it chases before it. From that a movement results that sets everything going, as you will see in the case of the locomotive. To conclude, let us remember that in every steam engine the essential thing, the generator of the force, is a boiler, a closed pot that boils."


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