英文科學(xué)讀本 第四冊(cè)·Lesson 02 Solids, Liquids, Gases

Lesson 02 Solids, Liquids, Gases

May we walk home with you this afternoon, sir? asked Fred.

Certainly, my boys, replied Mr. Wilson. "I was thinking of putting my museum cupboard into your charge," he continued. "You could keep the things in order, dust them regularly, and help me with the experiments during the lessons. Would you like that?"

Oh, thank you very, very much, sir, said both the boys at once. "We'll be very careful with everything."

Now, what have you got to chat about as we walk along? asked Mr. Wilson.

Our lesson on the force of cohesion, sir, said Fred, "helps us to understand, better than we have ever done, why there should be three distinct states of matter—the solid, the liquid, and the gas.

A solid is a body whose molecules are held strongly together. It is a solid simply because the force of cohesion is very strong in it. Even when we break a soft solid, like a piece of chalk, it does not fall to powder. It breaks into pieces, and these pieces still hold together.

It was so easy, sir, said Will, "to compare the cohesive force in a liquid and a solid, when you set me to take the water out of the basin, a spoonful at a time. It was no trouble to separate the molecules of water from one another with the spoon, because they are not held together firmly, as the molecules of a solid are."

Quite right, boys, said Mr. Wilson. "Now what have you to say about the gases?"

The molecules of a gas soon spread themselves out till they seem to fill the room, said Fred. "This proves that gases are quite different from either solids or liquids. Their molecules have no cohesion at all; they actually repel each other. They are always trying to get as far away from each other as possible."

Now I want you to try and tell me how the force of cohesion acts, said Mr. Wilson.

The force of cohesion can act only when the particles are in close contact, said Fred.

Do you remember, asked Mr. Wilson, "how I proved that?"

Oh, I remember, said Fred. "It is useless to try and join the two edges of a broken plate or saucer, or any other solid body, by pressing them together, because we cannot bring all the particles into actual contact, and without actual contact there can be no cohesion. But it is possible to join two perfectly smooth and level sheets of glass by pressing them together. All their particles are in actual contact, and cohesion acts and joins them."

Are we to understand, sir, asked Willie, "that cohesion acts only between the molecules of the same kind of substance?"

Yes, said Mr. Wilson, "the molecules of solid bodies are held together by cohesion, and the molecules of liquids are also held together by cohesion. When we mix two glasses of water, they immediately mingle and form a compact whole, because the molecules are brought into actual contact with each other on all sides."