英文科學(xué)讀本 第二冊(cè)·Lesson 20 About Metals

Lesson 20 About Metals

Look at this knife, Norah, said Fred. "Do you know what it is made of?"

It is made of steel, said Norah.

Quite right, said Fred. "See what a bright polished surface it has. When it gets dirty what must we do?"

We must rub it, said Norah. "That will polish it again. The more we rub the brighter it will shine."

Right, said Fred. "Now, look at the copper kettle on the hob. That, too, is very bright. How is it kept bright?"

It must be rubbed like the knife, and so must the tins that hang in the kitchen. They always look bright.

Teacher gave us a new name for this brightness of the metals, said Fred. "He calls it lustre. You must learn to say that these bright-shining metals are lustrous—full of lustre or brightness.

I found my pocket knife just now. Here it is. I lost it a few days ago in the garden; but look at it. It is no longer bright and shining. It is covered with red rust. Iron and steel soon rust if they are left out in the weather.

Do all the metals rust like this? asked Norah.

No, said her father, who had been listening to the children. "Look at these gold and silver coins. They never rust. They would not rust if I left them out in the garden for a week.

We call gold and silver the precious metals, father added, "because they never rust. All the others are called common metals."

Teacher showed us a sheet of gold leaf, thinner than the thinnest paper I ever saw, said Fred. "It was beaten out thin with hammers.

I have here a piece of lead. Watch while I beat it flat with this hammer. You know, he added, "if I had beaten a piece of coal it would have broken in pieces, for it is brittle. The metals then cannot be brittle. Teacher gave us a new word for all the metals that can be hammered out thin. We say that they are malleable."

SUMMARY

Metals have a bright, shining surface, which we call their lustre. We say they are lustrous—that is, full of lustre. Metals sometimes rust and lose their lustre. Gold and silver do not rust. We call them precious metals. Malleable means "able to be hammered out."