英文科學(xué)讀本 第三冊(cè)·Lesson 13 Vapor—What Becomes of It

Lesson 13 Vapor—What Becomes of It

Our last chat about vapor, said Fred, "showed us that evaporation is constantly going on everywhere. The air always contains vapor, and yet we cannot always see it. Teacher proved this in a very simple way today. How do you think he did it? He stood a tumbler of cold water on the table."

Please tell me all about it, Fred, said Norah, "for I can't see how that would prove it."

Well, said Fred, "after he had let the tumbler of water stand some time on the table, he showed us that the outside of the glass was covered with tiny drops of moisture."

This moisture did not come from the water in the glass. It came from the vapor in the air. That vapor was at first invisible, but when it touched the cold sides of the glass, it became visible as little round drops of water, which settled on the glass itself.

We could not at first make it out, said Willie, "for teacher had a tumbler of warm water on the table as well as the other one, and there were no drops of moisture on this."

Let me see, said Fred, "whether I can explain it. Take this slate, Norah, and breathe on it. I cannot see your breath, but look at the slate, and you will see some drops of moisture on it."

Now hold the slate over the spout of the kettle, while the steam is pouring out. What do you see? The slate is covered with little drops of moisture.

The vapor in the air leaves little drops of moisture on the sides of the tumbler; the vapor from your breath and the steam from the kettle leave drops of moisture on the slate. The tumbler and the slate were cold.

It is cold that changes the vapor into drops of moisture. Teacher says the vapor in the air is invisible only because it is split up into such extremely small particles that they cannot be seen.

Cold causes these tiny particles to rush together, so that they are pressed into a smaller space, and then we can see them. We say they are condensed.

The steam as it first comes from the spout of the kettle is invisible. But when it meets with the colder air of the room it condenses. The tiny particles of the vapor are made to rush together, and we see it as it floats away.

We do not usually see our breath as it leaves the mouth and nostrils. But when the weather is very cold, the vapor of the breath condenses and the breath at once becomes visible.

SUMMARY

Cold condenses the vapor in the air into drops of moisture. Steam is at first invisible. It becomes visible when the cold air begins to condense it. All vapor is at first invisible.