英文科學(xué)讀本 第一冊(cè)·Lesson 19 Gutta-Percha

Lesson 19 Gutta-Percha

What a silly little girl you are, Norah, said Fred one evening. "You should not put your feet on the fender, so close to the fire." Her father had put gutta-percha soles on her boots a few days before, to keep her feet dry in the winter.

Now the boys had just had a lesson on gutta-percha at school. Fred thought this would be a good time to let Norah hear something about it.

Wait a minute, he said. "I know where I can find some gutta-percha. Father left it when he mended your boots."

Fred was soon back with the piece of gutta-percha. He brought with him, too, a basin of hot water.

He put the piece of gutta-percha into the boiling water, and after a few minutes took it out again, just as he had seen teacher do in school.

Now, said he, "take hold of the gutta-percha. Squeeze it in your hand. What have you found out?"

Norah was full of delight. "Why," she cried out, "the gutta-percha is soft and pliable now."

Yes, it is, said Fred, "I will mold it into a little cup, just as teacher did this morning."

What was it that made the clay pliable?

Water, said Norah.

What makes putty pliable?

The linseed oil.

Quite right, said Fred. "Now we have got a new substance—gutta-percha, that is made pliable by heat."

And now, added Willie, "Norah knows why she must not put her boots, with gutta-percha soles, near the fire. The soles would get soft with the heat of the fire, and break away."

Let us look, said Fred, "at our little cup. See, it is cold, and quite hard once more. It is not pliable now."

Look again, while I fill the cup with water. None of the water runs through. Gutta-percha is water-proof.

Yes, said Will, "that is why it is so useful for boots. It keeps our feet dry."

SUMMARY

Gutta-percha is a pliable substance. It becomes soft and pliable with heat. When it gets cold it becomes hard again. It makes good soles for our boots, because it is waterproof.