英文科學(xué)讀本 第二冊(cè)·Lesson 33 Lead

Lesson 33 Lead

I had never thought till today, said Will, "what a useful metal lead is."

Father has lent me these pieces of Lead, said Fred. "Let us have a talk. Suppose we begin with this piece of sheet-lead.

What does this sheet-lead tell you, Norah?

It tells me that the lead must be malleable, because it has been hammered out into these thin sheets, said Norah.

Well, if it has not been hammered out, it has been rolled out, said Fred. "The truth is, malleable metals are more often rolled out than beaten flat with hammers. This sheet-lead was rolled flat between heavy steel rollers.

Sheet-lead is sometimes made in another way. The lead is melted and poured out on a flat table covered with fine sand. While it cools and hardens, it is rolled with wooden rollers. Take the piece of sheet-lead, Will, and see how easily you can bend it.

Lead is very flexible. See, too, how easily I can cut it with the knife. The workman can bend it, cut it, and hammer it into any shape he pleases. Sheet-lead is used for the roofs and gutters of houses, and for lining water cisterns, and you can see now why it is useful for these purposes.

Here, look at this piece of lead pipe, he added. "This is used for gas and water. Lead is more useful for this purpose than any other metal, because it can be easily fused, bent, cut, and hammered, just as the workman pleases.

Teacher showed us that the very reasons, that make lead useful for these purposes, make it useless in another way. Why don't we use lead wire?

Lead is a soft metal and breaks easily, said Willie. "It is not ductile, so that it cannot be drawn out. It is not tenacious, so that it would not bear to be pulled."

Do you see this spoon? said Fred. "It looks like lead, but it is not lead. It is made of pewter. Pewter is made by melting lead and tin together. Pewter is used for making drinking vessels, and many useful things for the house.

You must remember that lead, like most metals, is found as an ore. It must be smelted, to remove the earthy matters before it can be of use.

SUMMARY

Lead is a soft metal, and may be fused, bent, cut, or hammered with ease, but it is brittle, not ductile, and will not make useful wire. Sheet-lead is used for many purposes. Pewter is made from lead and tin.