英文科學(xué)讀本 第二冊(cè)·Lesson 15 Adhesive

Lesson 15 Adhesive

I wish you would mend our scrap book, mother, please, said Norah. "Some of the leaves are very loose."

So after tea mother got the gum-bottle and stuck the leaves in again, and made the book quite neat.

Why did mother use gum, Norah? asked Fred. "Wouldn't water do?"

Water is not sticky, said Norah. "It would not make the leaves stick."

Try and remember a hard word that means the same as sticky, Norah. It is adhesive. Gum is adhesive or sticky. What else could we use besides gum to stick the leaves?"

Some paste would do as well.

Have you ever seen any one use paste to stick paper? Fred asked again.

Oh, yes, said Norah. "The man used paste when he put the paper on the wall of the parlor the other day."

And the men who post up the bills in the streets, said Willie, "always do it with paste."

But, said Norah, "you can stick paper with sealing wax. I have seen father melt the wax in the flame of the candle, and drop it on the paper. When it gets hard it sticks fast to the paper."

Now look at this postage stamp, Norah, said Fred. "How do we stick it on the letter?"

We wet the back of the stamp, said Norah, "and press it on the paper."

Quite right, said Fred, "and you know we stick our envelopes in the same way. The part that we wet is covered with an adhesive substance.

Teacher says, he added, "that this adhesive substance is known as British gum. Let us see what it really is.

You know what happens to starch if we put it into cold water?

Starch is insoluble in cold water, said Norah.

Yes, said Fred, "but if dry starch is baked in an oven it becomes changed. It turns brown, and will easily dissolve in cold water. In this form it is called British gum, the very substance on the back of the postage stamp and the edge of the envelope.

It is pasted on the stamps and other things with a brush, and dries very quickly. When we want to use it, we wet it, and it will stick fast.

SUMMARY

Adhesive substances make other things stick together. We might call them sticky substances. Gum, paste, and sealing wax are adhesive substances for sticking paper. The adhesive substance on an envelope and on the back of a postage stamp is British gum. It is made of starch.