英文科學(xué)讀本 第三冊(cè)·Lesson 26 Air

Lesson 26 Air

Norah, hand me the water bottle, please, said Fred. "Be careful; it is full of water. Thank you. Now, if I poured the water out of the bottle, what would you say about the bottle then?"

I should say the bottle would be empty, said Norah.

Ah, that's just what I should have said till today, said Fred. "But it is not empty. Is it, Will?"

Teacher proved to us that the bottle is not empty, he added. "He had a bottle which looked to be empty, and he pressed it, mouth downwards, as far as he could, into a large bowl of water."

While he held it there, he made us notice that the water would not enter very far into the mouth of the bottle. The water in the neck of the bottle was not so high as the water in the basin outside.

He told us that if the basin were deep enough, we might press the bottle down ever so far, but it would not be possible to fill it with water.

Yes, but he did fill it though at last, said Will. "He slanted the bottle on one side, and then there was a gurgling sound, and the water rushed in."

What do you think made the gurgling sound, Norah? asked Fred. "Watch, while I do it now with this bottle. As I slant the bottle in the basin of water, something seems to come bubbling up out of it through the water, and now we can see that the bottle is full of water."

That something that bubbled up was air. It was the air in the empty bottle that prevented the water from rising to fill it. The water could not enter the bottle, because it was already full of something else, and that something else was air.

This proves that air is an actual substance, although we cannot see it. It occupies all the space of the bottle, and so long as it is in the bottle, the other substance, water, cannot enter.

But, said Willie, "there is just one other thing to remember about the bottle. The water did enter a little way into the neck of it when you pressed it down. That proves that you were able to press the air into a smaller space. Teacher says air is compressible."

Then, too, when you left off pressing the bottle down, the air in it sprang back to its former bulk, and the water that had entered the neck was forced out again. This proves that air is elastic.

SUMMARY

Air is an actual substance, although we cannot see it. There is air everywhere. Air occupies space, and is compressible.