"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.