英语听力文摘 English Digest 391、感受空气的存在（在线收听）

What is Air?
空气

D: Strictly speaking, air is mostly nitrogen–about seventy-eight percent, and about twenty-one percent oxygen, which leaves about one percent of other stuff, like water vapor and carbon dioxide.

Y: So air is mostly empty space?

D: Yep, in fact, if you ever condensed a gallon of air into a solid, you’d end up with a little over one tenth of one percent of the air’s original volume.And the reason we don’t notice all this emptiness is because air molecules are constantly moving around at the speed of sound, and bouncing against any surface that happens to fall in their path.

Y: So we’re constantly bombarded by air molecules.So how come we don't feel that?

D: Well, actually we do.The force these air molecules exert is what we measure when we measure air pressure.

D：严格地说，空气主要由大约78%的氮气和21%的氧气组成，另外还有1%的其他气体，像水蒸气和二氧化碳。

Y：所以，空气大部分都是真空吗？

D：是的。实际上，如果你将一加仑的空气凝结成固体，这个固体的体积只比空气原本体积的千分之一多一点。我们没有注意到这一点的原因是空气分子以音速在不停地移动，而当它们碰到任何表面时都会反弹回去。

Y：那么，我们一直被空气分子撞击着，可是为什么我们感觉不到呢？

D：其实我们能感觉到的。空气分子施加的压力就是我们平时测量的气压。