"It would take a long time to explain."

"Tell me all the same."

Monsieur Roger saw at this moment that Paul was beckoning to Miette to insist.

"Goodness!" said he, smiling at Paul; "Miss Miette was right just now.

It is you that wish me to continue the questions!"

[Ill.u.s.tration]

[Ill.u.s.tration]

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 quant.i.ty 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 quant.i.ty 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 s.p.a.ce; all are expanded by heat, are contracted by cold, and can even pa.s.s 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 const.i.tutes 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 s.p.a.ces 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 quant.i.ty 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.

[Ill.u.s.tration]

[Ill.u.s.tration]

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 gla.s.s.

Upon this disk we carefully place this globe of gla.s.s; 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 pa.s.ses 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 gla.s.s 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 gla.s.s 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."

[Ill.u.s.tration]

"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 s.p.a.ce 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."