第348章 第四册(4)

"We knew they were there, and we found them when we held the cold slate over the tube. The tiny drops, asthey condensed, ran together again and again, till at last they were large enough for us to see.""We learned, too," said Fred, "that, as all matter is composed of molecules, there must be a force of some kind, which holds them together, or else everything in the world would at once fall away to the finest dust or powder. This force which holds the molecules of matter together is called cohesion. It is so named because the word cohesion means holding together.""That"s very good," said Mr. Wilson. "Try and think of our experiments with the poker, and the pieces of lead, wood, glass, and chalk, and tell me what they teach us.""Oh yes, I remember, sir," said Willie. "We can"t break or twist the poker with all our trying. This means that the force of cohesion is so strong that we cannot separate the molecules from one another. We can bend the piece of cane and the lead, but they do not easily break. We say they are tough. The glass and the chalk snap quickly. There is less cohesion between their molecules than there is between the molecules of either lead, wood, or iron. We say the glass and the chalk are brittle."Lesson 02