英文科學(xué)讀本 第四冊(cè)·Lesson 09 Properties of Bodies

Lesson 09 Properties of Bodies

Are you coming with me, boys? asked Mr. Wilson, as he passed out of the playground gate, and in a moment Fred and Willie were trudging along by his side.

Well, said he, "what do you think of the force of cohesion now?"

I can see, sir, said Fred, "that it has more to do with solid bodies and their properties than I thought it had. In our early lessons we learned something about the properties of bodies. We are now able to say not only that a body has certain properties, but to tell the reason why it has these properties. If I were to scratch a piece of lead, for instance, with an iron nail, a boy in one of the lower classes might say that I was able to do it because the iron is harder than the lead. So it is, but he would not be able to say why the iron is harder than the lead.

We can now tell this from the force of cohesion. When the force of cohesion in a solid is great, the molecules are held very closely together, and the body is hard. When there is little cohesion, the molecules are more loosely held together, and the body is soft. It is because the particles of the soft body are held loosely, that a hard substance is able to force them aside, as it does when it scratches them. We learned, too, that some bodies are brittle and break with a blow; some are tough and will not break; some are flexible and bend easily without breaking; and some are elastic and will not only bend, but will even spring back to their original shape when let go.

Our old lessons taught us all this, but we can now go back to the force of cohesion to find the reason why certain bodies have these properties. When the cork, whalebone, lead, cane, and wire are bent and twisted, their molecules are forced out of their proper position. The whalebone, cork, and cane spring back to their former shape again, because each molecule takes up its old position.

I have been thinking about the metals, sir, said Willie. "Most of the metals are very tenacious, and it is this property which renders them so useful. Their tenacity, I suppose, is entirely due to the force of cohesion between their molecules."

It is, Willie, said Mr. Wilson. "A body is brittle when it possesses very little cohesion; it is tenacious when cohesion in it is strong. It is only the tenacious metals that are malleable and ductile. When a piece of metal is beaten out thin, or drawn out into wire, it really means that its molecules have been forced into different positions, and because they do this without losing their cohesion, the metal holds together. No metal can be malleable or ductile unless there is a very strong force of cohesion between its molecules."

I remember you told us too, sir, said Fred, "that cohesion will account for the different weight of different bodies. In a heavy body the force of cohesion is stronger than in a light one; the molecules are held more closely together."

Quite true, said Mr. Wilson, "and as a result of that, more matter is massed together into a given bulk, than if there were less cohesion."