CHAPTER XIII.
WHY LEAD IS HEAVIER THAN CORK
Monsieur Roger continued in these words:
"We say that a body has density when it is thick and packed close. We give the name of density to the quantity of matter contained in a body of a certain size.
"Let us suppose that this bit of lead has the same bulk—that is to say, that it is exactly as big—as the cork. Suppose, also, that we have a piece of gold and a piece of stone, also of the same bulk as the cork, and that we weigh each different piece in a pair of scales. We would find that cork weighs less than stone, that stone weighs less than lead, and that lead weighs less than gold. But, in order to compare these differences with each other, it has been necessary to adopt a standard of weight.
"I now return to Miss Miette's question,—'Why is lead heavier than cork?'—a question to which I had solemnly answered, 'Because its density is greater.' Miss Miette must now understand that cork, weighing four times less than water, cannot sink in water, although that process is very easy to lead, which weighs eleven times more than water. And yet," said Monsieur Roger, "the problem is not perfectly solved, and I am quite sure that Miss Miette is not entirely satisfied."
Miss Miette remained silent.
"I was not mistaken. Miss Miette is not satisfied," said Monsieur Roger; "and she is right,—for I have not really explained to her why lead is heavier than cork."
Miss Miette made a gesture, which seemed to say, "That is what I was expecting."
"I said just now," continued Monsieur Roger, "that the density of a body was the quantity of matter contained in this body in a certain bulk. Now does Miss Miette know what matter is?"
"No."
"No! Now, there is the important thing: because, in explaining to her what matter is, I will make her understand why lead is heavier than cork."
"Well, I am listening," said Miette.
And Master Paul respectfully added, in an undertone, "We are listening."
Monsieur Roger continued:
"The name of 'bodies' has been given to all objects which, in infinite variety, surround us and reveal themselves to us by the touch, taste, sight, and smell. All these bodies present distinct properties; but there are certain numbers of properties which are common to all. Those all occupy a certain space; all are expanded by heat, are contracted by cold, and can even pass from the solid to the liquid state, and from the liquid to the gaseous state. They all possess a certain amount of elasticity, a certain amount of compressibility,—in a word, there exist in all bodies common characteristics: so they have given a common name to those possessing these common properties, and called that which constitutes bodies 'matter.' Bodies are not compact, as you may imagine. They are, on the contrary, formed by the union of infinitely small particles, all equal to each other and maintained at distances that are relatively considerable by the force of attraction.
"These infinitely small particles have received the names of atoms or molecules. Imagine a pile of bullets, and remark the empty spaces left between them, and you will have a picture of the formation of bodies. I must acknowledge to you that no one has yet seen the molecules of a body. Their size is so small that no microscope can ever be made keen enough to see them. A wise man has reached this conclusion: That if you were to look at a drop of water through a magnifying instrument which made it appear as large as the whole earth, the molecules which compose this drop of water would seem hardly bigger than bits of bird-shot. Still, this conception of the formation of bodies is proved by certain properties which matter enjoys. Among these properties I must especially single out divisibility. Matter can be divided into parts so small that it is difficult to conceive of them. Gold-beaters, for instance, succeed in making gold-leaf so thin that it is necessary to place sixty thousand one on top of the other to arrive at the thickness of an inch. I will give you two other examples of 'divisibility' that are still more striking. For years, hardly losing any of its weight, a grain of musk spreads a strong odor. In a tubful of water one single drop of indigo communicates its color. The smallness of these particles of musk which strike the sense of smell and of these particles of indigo which color several quarts of water is beyond our imagination to conceive of. And these examples prove that bodies are nothing but a conglomeration of molecules. Now, if lead is heavier than cork, it is because in an equal volume it contains a far more considerable quantity of molecules, and because these molecules are themselves heavier than the molecules of cork. And now I shall stop," said Monsieur Roger, "after this long but necessary explanation. I will continue on the day when Miss Miette will present to me the famous air-pump."
"That will not be very long from now," said Miss Miette to herself.
CHAPTER XIV.
THE AIR-PUMP
Monsieur Roger had deferred his explanations for three days. He was awaiting the air-pump which Monsieur Dalize, at Miette's desire, had decided to purchase in Paris. Monsieur Roger judged that this interruption and this rest were necessary. In this way his hearers would not be tired too soon, and their curiosity, remaining unsatisfied for the moment, would become more eager. He was not mistaken; and when a large box containing the air-pump and other objects ordered by Monsieur Roger arrived, a series of cries of astonishment came from the pretty mouth of Miss Miette. Paul Solange, however, remained calm; but Monsieur Roger knew that his interest had been really awakened. They spent the afternoon in unpacking the air-pump, and Monsieur Roger was called upon at once to explain the instrument.
"The machine," he said, "is called an air-pump because it is intended to exhaust air contained in a vase or other receptacle. To exhaust the air in a vase is to make a vacuum in that vase. You will see that this machine is composed of two cylinders, or pump-barrels, out of which there 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 are going 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 will take notice," he said, "that when the lever is lowered at the left the round piece of leather placed in the cylinder on the left side is lowered, and that the bit of leather in the right-hand cylinder is raised. In the same way, when the lever is lowered at the right, it is the right-hand piece of leather which is lowered, while the piece of leather at the left is raised in its turn. These round bits of leather, whose importance is considerable, are called pistons. Each piston is hollow and opens into the air on top, while at the bottom, which communicates with that portion of the cylinder situated below the piston, there is a little hole, which is stopped by a valve. This valve is composed of a little round bit of metal, bearing on top a vertical stem, around which is rolled a spring somewhat in the shape of a coil or ringlet. The ends of this spring rest on one side on a little bit of metal, on the other on a fixed rest, pierced by a hole in which the stem of the valve can freely go up and down. When I work the lever, as I am doing now, you see that on the left side the piston lowers itself in the cylinder, and that the piston on the right is raised. Now, what is going on in the interior of each cylinder? The piston of the left, in lowering, disturbs the air contained in the cylinder,—it forces it down, it compresses it. Under this compression the coiled spring gives way, the round bit of metal is raised, and opens the little hole which puts the under part of the piston in communication with the atmosphere. The air contained in the cylinder passes in this way across the piston and disperses itself in the air which surrounds us. But the spring makes the bit of metal fall back again and closes the communication in the right-hand cylinder as soon as the piston commences to rise and the pressure of the air in the cylinder is not greater than the pressure of the atmosphere outside. Lastly, the tube which unites the cylinders to the glass globe opens at the end of each cylinder, but a little on the side. It is closed by a little cork, carried by a metal stem which traverses 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 in contact with the hole, which closes; the stem is then stopped, but the piston continues to descend by sliding over it. In the other cylinder, in which the piston is raised, it commences by raising the stem, which re-establishes communication with the glass globe; but as soon as the top of the stem comes in contact with the upper part of the cylinder, 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 opens and shuts the orifice as soon as one of the pistons begins to descend and the other begins to ascend. Consequently, by working the lever for a certain space of time, I will finish by exhausting the globe of all the air 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 first very 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 is it that is too heavy?"
Miette sought an answer.
"Oh, I do not know. It is the lever or the pistons which have become all of a sudden too heavy."
"Not at all; that is not it. Neither the lever nor the pistons can change 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 could not succeed.
"Why," said she, "it is, of course, the piston on the left which has become too heavy, as I cannot make it rise again."
"You are right, Miss Miette. It is the piston in the left cylinder which cannot rise; but it has not changed its weight, as I said,—only it has now to support a very considerable weight; and it is that weight which you 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 down this 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 of air weighs about seventy-two grains; and as in the atmosphere—that is to say, in the mass of air which surrounds us—there is a very great number of gallons, you can imagine that it must represent a respectable number of pounds. It has been calculated, in fact, that each square inch of the surface of the soil supports a weight of air of a little more than sixteen pounds."
"But how is that?" cried Miette. "A while ago there was also a considerable quantity of air above the piston, and yet I could make it go 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 elastic force,—that is to say, that it constantly endeavors to distend its molecules, and presses without ceasing upon the sides of the vase which contained it, or upon the surrounding air. Now, when you began to work the lever there was still enough air in the globe to balance, through its elastic force, the air outside; and, as the piston receives an almost equal pressure of air from the atmosphere above and from the globe below, it is easily raised and lowered. But while you were working the lever you took air out of the globe, so that at last there arrived a time when so little air remained in this globe that its elastic force acted with little power upon the piston. So the piston was submitted to only one pressure,—that of the atmosphere; and, as I have just told you, the atmosphere weighs heavy enough to withstand your little strength. Still, all the air in the globe is not yet exhausted, and a stronger person, like Master Paul, for example, could still be able to conquer the resistance of the atmosphere and raise the piston."
Paul Solange could not refuse this direct invitation, and he approached the air-pump and succeeded in working the lever, though with a certain difficulty.
Meanwhile, Monsieur Roger was seeking among the physical instruments which had just arrived. He soon found a glass cylinder, whose upper opening was closed by a bit of bladder stretched taut and carefully tied upon the edges.
"Stop, Master Paul," said he: "we are going to exchange the globe for this cylinder, and you will see very readily that the air is heavy. Now take away the globe."
But, though Paul tried his best, he could not succeed in obeying this order. The globe remained firm in its place.
"That is still another proof of the weight of the air," said Monsieur Roger. "The globe is empty of air; and as there is no longer any pressure upon it except from outside,—the pressure of the atmosphere,—Master Paul is unable to raise it."
"He would be able to raise the glass," said Miss Miette, in a questioning tone, "but he cannot lift the air above it?"
"You are exactly right. But you are going to see an experiment which will prove it. First, however, it will be necessary to take away the globe. I am going to ask Miss Miette to turn this button, which is called 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. "Now Master Paul can take the globe away."
That was true. When Paul took away the globe, Monsieur Roger put in its place the cylinder closed by the bit of bladder. Then he worked the handle of the machine again. As the air was withdrawn from the interior of the cylinder, the membrane was heard to crackle. Suddenly it burst, with a sort of explosion, to the great surprise of Miette and the amusement of everybody.
"What is the matter?" said Miette, eagerly.
"The matter is," answered Monsieur Roger, "that the exterior air weighed so heavily upon the membrane that it split it; and that is what I want to show you. The moment arrived when the pressure of the atmosphere was no longer counterbalanced by the elastic force of the air contained in the cylinder. Then that exhausted all the air, and the atmosphere came down with all its weight upon the membrane, which, after resisting for a little while, was torn."
"Is it true, Monsieur Roger," said Miette, "that it is with this machine that you can make smoke fall?"
"Certainly."
"Well, then, won't you show that to us?"
CHAPTER XV.
DROPS OF RAIN AND HAMMER OF WATER
"I am very willing to show you that," answered Monsieur Roger; "but I must have a candle."
Miette ran to the kitchen and succeeded in obtaining that article which was once so common, and which is now so rare, known as a candle. Monsieur Roger lit the candle and placed it under the glass globe of the air-pump. Then he asked Paul to make a vacuum. At the end of a few minutes the candle went out. Monsieur Roger then told Paul to stop.
"Why has the candle gone out?" asked Miette.
"Because it needs air. Master Paul has just exhausted the air necessary to the combustion of the candle; but the wick still smokes, and we are going to see if the smoke which it produces will rise or fall."
Everybody approached the globe, full of curiosity.
"It falls," cried Miette, "the smoke falls."
And in fact, instead of rising in the globe, the smoke lowered slowly and heavily, and fell upon the glass disk of the air-pump.
"Well," said Monsieur Roger, "you see that I was right. In a vacuum smoke falls: it falls because it no longer finds itself in the midst of air which is heavier than it and forms an obstacle to its fall. In the same way the cloud in the sky above the château would fall if we could exhaust the air which is between it and us."
"I am very glad that we cannot," cried Miette.
"And why are you very glad?" asked Madame Dalize.
"Because, mamma, I don't wish any rain to fall."
"Does Miss Miette think, then," said Monsieur Roger, "that if the cloud fell rain would fall?"
"Certainly," answered Miss Miette, with a certain amount of logic. "When the clouds fall they fall in the form of rain."
"Yes; but supposing that I should exhaust the air which is between the cloud and us, the cloud would not fall in a rain, but in a single and large mass of water."
"Why?"
"Clouds, you doubtless know, are masses of vapor from water. Now, when these vapors are sufficiently condensed to acquire a certain weight, they can no longer float in the atmosphere, and they fall in the form of rain. But they fall in rain because they have to traverse the air in order to fall to the ground. Now, the air offers such a resistance to this water that it is obliged to separate, to divide itself into small drops. If there were no air between the water and the ground, the water would not fall in drops of rain, but in a mass, like a solid body; and I am going to prove that to you, so as to convince Miss Miette."
Among the various instruments unpacked from the box, Monsieur Roger chose a round tube of glass, closed at one end, tapering, and open at the other end. He introduced into this tube a certain quantity of water so as to half fill it. Then he placed the tube above a little alcohol lamp, and made the water boil.
"Remark," said he, "how fully and completely the vapors from the water, which are formed by the influence of heat, force out the air which this tube encloses in escaping by the open end of the tube."
When Monsieur Roger judged that there no longer remained any air in the tube, he begged Monsieur Dalize to hand him the blowpipe. Monsieur Dalize then handed to his friend a little instrument of brass, which was composed of three parts,—a conical tube, furnished with a mouth, a hollow cylinder succeeding to the first tube, and a second tube, equally conical, but narrower, and placed at right angles with the hollow cylinder. This second tube ended in a very little opening.
Monsieur Roger placed his lips to the opening of the first tube, and blew, placing the little opening of the second tube in front of the flame of a candle, which Monsieur Dalize had just lit. A long and pointed tongue of fire extended itself from the flame of the candle. Monsieur Roger placed close to this tongue of fire the tapering and open end of the tube in which the water had finished boiling. The air, forced out of the blowpipe and thrust upon the flame of the candle, bore to this flame a considerable quantity of oxygen, which increased the combustion and produced a temperature high enough to soften and melt the open extremity of the tube, and so seal it hermetically.
"I have," said Monsieur Roger, "by the means which you have seen, expelled the air which was contained in this tube, and there remains in it only water. In a few moments we will make use of it. But it is good to have a comparison under your eyes. I therefore ask Miss Miette to take another tube similar to that which I hold."
"Here it is," cried Miette.
"Now I ask her to put water into it."
"I have done so."
"Lastly, I ask her to turn it over quickly, with her little hand placed against its lower side in order to prevent the water from falling upon the floor."
Miss Miette did as she was commanded. The water fell in the tube, dividing itself into drops of more or less size. It was like rain in miniature.
"The water, as you have just seen," said Monsieur Roger, "has fallen in Miss Miette's tube, dividing itself against the resistance of the air. In the tube which I hold, and in which there is no longer any air, you will see how water falls."
Monsieur Roger turned the tube over, but the water this time encountered no resistance from the air. It fell in one mass, and struck the bottom of the tube with a dry and metallic sound.
"It made a noise almost like the noise of a hammer," said Paul Solange.
"Exactly," answered Monsieur Roger. "Scientists have given this apparatus the name of the water-hammer." And looking at Miette, who in her astonishment was examining the tube without saying anything, Monsieur Roger added, smiling, "And this hammer has struck Miss Miette with surprise."