英文科學(xué)讀本 第三冊(cè)·Lesson 30 Gases

Lesson 30 Gases

I want you to think about the air again, Norah. said Fred.

Oh yes, she cried, "we found out that air is an actual substance, which takes up space and has weight, like all other bodies."

We have talked about solid bodies and liquids, Fred went on, "but air is a thin, light substance, which we cannot even see. It is not like either a solid or a liquid."

Teacher says there are many substances that are like air and differ from both solids and liquids. He turned on the gas-burner to show us one of them. There was nothing to be seen, but we could hear something rushing out of the pipe, and we soon smelt it, and we saw it catch fire when teacher put a match to it.

Oh, you mean the gas, said Norah.

Yes, but its proper name is coal-gas, because it is made from coal, said Willie.

All thin, light bodies like these, said Fred, "are called gases. Air is a gas, and the invisible vapor in the air is a gas—water-gas. Gases and liquids are alike in some things, but they differ in others."

What happens when we turn on the faucet over the sink?

The water runs out, said Norah, "because liquids flow."

Can you tell me then why the gas rushed out when teacher turned the burner on?

Well, I suppose it is because the gas flows along the pipes, said Norah.

Quite right, said Fred, "it does, and so gases and liquids both flow."

If you pick up the bellows and set them to work, said Will, "you will soon find that air flows, and you know, too, that the wind is only air rushing along."

Teacher made a gas, called carbonic acid gas, said Fred. "We had to take his word that the gas was in the bottle, for we could not see it. But it was very funny to see him pour it out like water from one vessel into another."

But I thought you said the gas was invisible, said Norah.

So it is, replied Fred. "But teacher held a lighted taper between the two vessels, and told us to watch while he poured out the gas. This is a gas in which no flame could live, and although we could not see it flow from one vessel to the other, we saw it put out the light as it flowed."

So then a gas is like a liquid, because they both flow.

Now think again. We catch water as it flows from the faucet. Could teacher have caught the gas in a pail as it flowed from the burner? No, it would be impossible to catch the gas, because gases always spread themselves out rapidly in all directions. Teacher says gases differ from liquids in having no surface.

Oh yes, said Norah, "and liquids always keep a level surface."

SUMMARY

Gases are fluids—they flow. Liquids always keep a level surface, but gases have no surface. They spread themselves out in all directions.