英文科学读本 第三册·Lesson 10 Vapor in the Air

Lesson 10 Vapor in the Air

Come to the window, Norah, said Will, "and you can see the very thing we were talking about the other day. The rain is over now, and there is so much vapor rising that the ground seems to be steaming. The air must be very dry and thirsty, for the sun has been hot, and a great deal of the rain which has fallen will evaporate. But will it all evaporate, Norah?"

No, said Norah. "Some of it will sink into the ground, and after a time bubble up again in the form of a spring. We can see too some of it is running away in streams along the gutters to the drains."

Yes, said Fred, joining in, "and the drains will carry it away to the river, and so to the sea. The whole of the water which drains off the land flows away and forms rivers, and the rivers pour their water into the sea. So that in the end it all comes to the sea."

Teacher made us think about those great bodies of water—rivers, lakes, oceans—all over the world, with the thirsty air all round. You have seen evaporation going on from the wet ground after this shower of rain.

What an immense amount of vapor must be always rising from the great surface of the sea. The air gets most of its vapor from this one source.

Don't forget that evaporation goes on over every part of its surface. Of course it is more rapid in the hot parts of the world than in the cold, but even in the cold regions there is evaporation.

Teacher gave me a surprise, Fred, about the plants, said Will.

Yes, Will, I think most of the boys were surprised to see that bottle, replied Fred.

What was that? asked Norah.

Teacher showed us a bottle with some fresh-cut leaves in it. He had put the leaves into the bottle and corked it up tight before he left home for school. The bottle was perfectly dry when he put the leaves in, but when he showed it to us the whole of the inside was covered with little drops of moisture.

Then, of course, said Norah, "the moisture must have come from the leaves."

Yes, replied Fred. "Teacher says all plants give out moisture, and this moisture is changed into vapor and absorbed by the air."

Think what a quantity of vapor must be formed and sent into the air in this way by all the trees and plants that grow on the earth.

But animals as well as plants help to fill the air with vapor, continued Fred. "Teacher made us think of a horse drawing a heavy load. You must often have seen a horse at work covered with drops of moisture or sweat. You know that we ourselves cannot work hard or run about without perspiring. We find the sweat drops on our own bodies."

Now what do you think becomes of all this moisture? I will tell you. It evaporates, and the air absorbs the vapor as it forms.

Oh yes, said Norah, "I have often seen a horse's body steaming with vapor when he has been working hard."

And not only his body, said Will, "his breath too gives off clouds of vapor. We can see it every time he breathes, and we can see the same thing if we watch our own breath. It is always loaded with vapor."

SUMMARY

Most of the vapor in the air is evaporated from the great bodies of water in the hot regions of the world. But some vapor is always rising into the air everywhere. Plants and animals send out vapor into the air.