THE BOILING POT
AS 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 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 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.”
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
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.”
------------------------------------------------------------------------