CHAPTER XIV.
THE AIR-PUMP.
Monsieur Roger had deferred his explanations for three days. He wasawaiting the air-pump which Monsieur Dalize, at Miette's desire, haddecided to purchase in Paris. Monsieur Roger judged that thisinterruption and this rest were necessary. In this way his hearers wouldnot be tired too soon, and their curiosity, remaining unsatisfied forthe moment, would become more eager. He was not mistaken; and when alarge box containing the air-pump and other objects ordered by MonsieurRoger arrived, a series of cries of astonishment came from the prettymouth of Miss Miette. Paul Solange, however, remained calm; butMonsieur Roger knew that his interest had been really awakened. Theyspent the afternoon in unpacking the air-pump, and Monsieur Roger wascalled upon at once to explain the instrument.
"The machine," he said, "is called an air-pump because it is intended toexhaust air contained in a vase or other receptacle. To exhaust the airin a vase is to make a vacuum in that vase. You will see that thismachine is composed of two cylinders, or pump-barrels, out of whichthere comes a tube, which opens in the centre of this disk of glass.Upon this disk we carefully place this globe of glass; and now we aregoing to exhaust the air contained in the globe."
"We are going to make a vacuum," said Miette.
"Exactly." And Monsieur Roger commenced to work the lever. "You willtake notice," he said, "that when the lever is lowered at the left theround piece of leather placed in the cylinder on the left side islowered, and that the bit of leather in the right-hand cylinder israised. In the same way, when the lever is lowered at the right, it isthe right-hand piece of leather which is lowered, while the piece ofleather at the left is raised in its turn. These round bits of leather,whose importance is considerable, are called pistons. Each piston ishollow and opens into the air on top, while at the bottom, whichcommunicates with that portion of the cylinder situated below thepiston, there is a little hole, which is stopped by a valve. This valveis composed of a little round bit of metal, bearing on top a verticalstem, around which is rolled a spring somewhat in the shape of a coil orringlet. The ends of this spring rest on one side on a little bit ofmetal, on the other on a fixed rest, pierced by a hole in which the stemof the valve can freely go up and down. When I work the lever, as I amdoing now, you see that on the left side the piston lowers itself in thecylinder, and that the piston on the right is raised. Now, what is goingon in the interior of each cylinder? The piston of the left, inlowering, disturbs the air contained in the cylinder,--it forces itdown, it compresses it. Under this compression the coiled spring givesway, the round bit of metal is raised, and opens the little hole whichputs the under part of the piston in communication with the atmosphere.The air contained in the cylinder passes in this way across the pistonand disperses itself in the air which surrounds us. But the spring makesthe bit of metal fall back again and closes the communication in theright-hand cylinder as soon as the piston commences to rise and thepressure of the air in the cylinder is not greater than the pressure ofthe atmosphere outside. Lastly, the tube which unites the cylinders tothe glass globe opens at the end of each cylinder, but a little on theside. It is closed by a little cork, carried by a metal stem whichtraverses the whole piston. When I cause one of the pistons to lower,the piston brings the stem down with it. The cork at once comes incontact with the hole, which closes; the stem is then stopped, but thepiston continues to descend by sliding over it. In the othercylinder, in which the piston is raised, it commences by raising thestem, which re-establishes communication with the glass globe; but assoon as the top of the stem comes in contact with the upper part of thecylinder, it stops and the piston glides over it and continues to rise."
"In this fashion the movements of the cork are very small, and it opensand shuts the orifice as soon as one of the pistons begins to descendand the other begins to ascend. Consequently, by working the lever for acertain space of time, I will finish by exhausting the globe of all theair which it contains."
"May I try to exhaust it?" asked Miette, timidly.
"Try your hand, Miss Miette," answered Monsieur Roger.
Miette began to work the lever of the air-pump, which she did at firstvery easily, but soon she stopped.
"I cannot do it any more," said she.
"Why?"
"Because it is too heavy."
"In fact, it is too heavy," said Monsieur Roger; "but tell me, what isit that is too heavy?"
Miette sought an answer.
"Oh, I do not know. It is the lever or the pistons which have become allof a sudden too heavy."
"Not at all; that is not it. Neither the lever nor the pistons canchange their weight."
"Then, what is it that is so heavy?"
"Come, now! Try once more, with all your strength."
Miette endeavored to lower the right-hand side of the lever: she couldnot succeed.
"Why," said she, "it is, of course, the piston on the left which hasbecome too heavy, as I cannot make it rise again."
"You are right, Miss Miette. It is the piston in the left cylinder whichcannot rise; but it has not changed its weight, as I said,--only it hasnow to support a very considerable weight; and it is that weight whichyou cannot combat."
"What weight is it?" said Miette, who did not understand.
"The weight of the air."
"The weight of the air? But what air?"
"The air which is above it,--the exterior air; the air which weighs downthis piston, as it weighs us down."
"Does air weigh much?"
"If you are very anxious to know, I will tell you that a wine gallon ofair weighs about seventy-two grains; and as in the atmosphere--that isto say, in the mass of air which surrounds us--there is a very greatnumber of gallons, you can imagine that it must represent a respectablenumber of pounds. It has been calculated, in fact, that each square inchof the surface of the soil supports a weight of air of a little morethan sixteen pounds."
"But how is that?" cried Miette. "A while ago there was also aconsiderable quantity of air above the piston, and yet I could make itgo up very easily."
"Certainly, there was above the piston the same quantity of air as now,but there was air also in the globe. Air, like gas, possesses an elasticforce,--that is to say, that it constantly endeavors to distend itsmolecules, and presses without ceasing upon the sides of the vase whichcontained it, or upon the surrounding air. Now, when you began to workthe lever there was still enough air in the globe to balance, throughits elastic force, the air outside; and, as the piston receives analmost equal pressure of air from the atmosphere above and from theglobe below, it is easily raised and lowered. But while you were workingthe lever you took air out of the globe, so that at last there arrived atime when so little air remained in this globe that its elastic forceacted with little power upon the piston. So the piston was submitted toonly one pressure,--that of the atmosphere; and, as I have just toldyou, the atmosphere weighs heavy enough to withstand your littlestrength. Still, all the air in the globe is not yet exhausted, and astronger person, like Master Paul, for example, could still be able toconquer the resistance of the atmosphere and raise the piston."
Paul Solange could not refuse this direct invitation, and he approachedthe air-pump and succeeded in working the lever, though with a certaindifficulty.
Meanwhile, Monsieur Roger was seeking among the physical instrumentswhich had just arrived. He soon found a glass cylinder, whose upperopening was closed by a bit of bladder stretched taut and carefully tiedupon the edges.
"Stop, Master Paul," said he: "we are going to exchange the globe forthis cylinder, and you will see very readily that the air is heavy. Nowtake away the globe."
But, though Paul tried his best, he could not succeed in obeying thisorder. The globe remained firm in its place.
"That is still another proof of the weight of the air," said MonsieurRoger. "The globe is empty of air; and as there is no longer anypressure upon it except from outside,--the pressure of theatmosphere,--Master Paul is unable to raise it."
"He would be able to raise the glass," said Miss Miette, in aquestioning tone, "but he cannot lift the air above it?"
"You are exactly right. But you are going to see an experiment whichwill prove it. First, however, it will be necessary to take away theglobe. I am going to ask Miss Miette to turn this button, which iscalled the key of the air-pump."
Miette turned the key, and then they heard a whistling sound.
"It is the air which is entering the globe," said Monsieur Roger. "NowMaster Paul can take the globe away."
That was true. When Paul took away the globe, Monsieur Roger put in itsplace the cylinder closed by the bit of bladder. Then he worked thehandle of the machine again. As the air was withdrawn from the interiorof the cylinder, the membrane was heard to crackle. Suddenly it burst,with a sort of explosion, to the great surprise of Miette and theamusement of everybody.
"What is the matter?" said Miette, eagerly.
"The matter is," answered Monsieur Roger, "that the exterior air weighedso heavily upon the membrane that it split it; and that is what I wantto show you. The moment arrived when the pressure of the atmosphere wasno longer counterbalanced by the elastic force of the air contained inthe cylinder. Then that exhausted all the air, and the atmosphere camedown with all its weight upon the membrane, which, after resisting for alittle while, was torn."
"Is it true, Monsieur Roger," said Miette, "that it is with this machinethat you can make smoke fall?"
"Certainly."
"Well, then, won't you show that to us?"
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