英文科學(xué)讀本 第四冊(cè)·Lesson 01 Cohesion

Lesson 01 Cohesion

Well, how did you like your science lesson today, boys? asked Mr. Wilson, as he overtook our two young friends, Fred and Willie, on their way home. The boys had been promoted to a higher class, and this was the first lesson of the new course. Mr. Wilson, their teacher, was a rare man for his boys, especially those boys who showed that they took an interest in their work. He had long been struck with the earnest attention these two boys paid, and the trouble they took to follow him intelligently, and he made up his mind to help them.

I think, sir, said Fred, "we shall soon begin to feel at home, for I am sure, from what I saw today, our lessons in the lower classes will help us very much. We are going to try hard, for father has promised to send us to the Institute by and by, if we learn all we can now."

Suppose you tell me something about today's lesson, as we walk along, said Mr. Wilson. "Well," said Willie, "the first thing we learned was that new word matter. We know now that the name matter means every substance that exists."

I think I understand, sir, said Fred, "what you mean by molecules of matter, although it seems difficult to imagine particles so small that they cannot be seen even with the help of a powerful microscope. We learned from our lesson that matter of every kind—solid, liquid, and gas—is made up of extremely small particles, and these particles are called molecules. A molecule is the name for the smallest particle of matter that can possibly exist."

Quite right, Fred, said Mr. Wilson. "If you will keep two things in your mind, you will be pretty clear about these molecules of matter.

First think of the dissolved particles of a soluble substance. They are all in the liquid, but they have been divided up into such minute particles that they are invisible. Then think of our little experiment with mercury. We boiled the mercury in the tube, and as it boiled it passed away in vapor. But we could not see the vapor, because the particles had been divided up too small to be seen. These were molecules of mercury.

We knew they were there, and we found them when we held the cold slate over the tube. The tiny drops, as they 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."