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.