英文科學(xué)讀本 第四冊(cè)·Lesson 34 The Atmosphere—What It Is

Lesson 34 The Atmosphere—What It Is

We have learned a great many facts about the atmosphere as a material substance—about its weight, and its pressure in all directions," said Mr. Wilson. "We have now to study it from another point of view.

I have here a large glass jar. The jar of course contains air. I will place a small bit of phosphorus on this tin plate, and float the plate in this bowl of water. I will light the phosphorus using a taper and cover it quickly with the bell-jar, resting just below the surface of the water. The phosphorus burns with an intensely bright flame in the jar, and gives off dense white fumes. We must wait for these fumes to clear off, and then you shall tell us what you observe."

Some of the water has gone up the bell-jar, sir, said Fred.

So it has, Fred. If you come here you will see that the bell-jar is marked by lines into five equal parts, and the water now fills one of those parts. Some of the air has made room for this water, for the jar contains now only about four-fifths of the original quantity. Our next business will be to find out what we have in the jar now. Is it the same as it was before the experiment? Remove the stopper of the bell-jar, and plunge this burning taper into it. You see it is immediately extinguished.

Suppose I light a piece of phosphorus and lower that into it. The result is the same; it goes out immediately. The gas, therefore, which is now in the jar is not air as it was at first. One-fifth of its bulk has disappeared in the burning, and the remaining four-fifths is a gas which puts out a flame. It will not allow anything to burn in it. We call this gas nitrogen; it forms four-fifths of all the air around us. I have something else now to show you," he continued. "Here are some gas jars. If I hold them up they appear empty—that is to say, you would expect to find nothing in them but ordinary air. I shall soon, however, be able to prove to you first that they are not empty, and also that what they do contain is not air.

Let us begin with this one. I will light a splinter of wood, and as soon as it is well lighted, I will blow out the flame so as to leave only a red spark. Now notice what happens when I uncover the jar and plunge the splinter into it. The spark immediately bursts into flame again and burns with a very brilliant glow. It burns much more fiercely and brightly than it did in the air. Let us take another one of the bottles. Here is a piece of charcoal. We will first make it red-hot, and then plunge it as before into the jar. The result is the same: the charcoal burns with greater fierceness and brilliance immediately as it is lowered into the jar.

Lastly, I will lower a piece of phosphorus into the jar—this time without even lighting it. The phosphorus takes fire instantly, and burns with such a dazzling brightness that we cannot bear the sight of it, and we turn our eyes away. I think we have now done enough to prove that these jars are not empty, and also to prove that what they do contain is not air. I filled the jars myself with a gas which we call oxygen. One of these days you shall see me make some oxygen. At present you must be content to learn what it is, and how it acts.

You saw that the substances we put into the jars burn much more fiercely than in the air. Oxygen is a powerful supporter of burning. It forms one-fifth of the bulk of all the air all over the world. It formed one-fifth of the bulk of the air in the bell-jar, and it was this gas, oxygen, which disappeared. The burning phosphorus went out immediately all the oxygen was used up, and nothing was left but the other gas, nitrogen, which formed the remaining four-fifths. In the atmosphere the one part of oxygen is mixed with four parts of nitrogen merely to dilute or weaken it. Burning would go on too rapidly in an atmosphere of pure oxygen.

We shall have to learn presently that we and all animals live by burning. Our bodies are daily and hourly burning away. If we lived in an atmosphere of oxygen, we should burn away too fast, and live too rapidly, like the flames we saw just now. But an atmosphere of nitrogen, on the other hand, would not do, for in it there could be no burning, and without burning there could be no life. Oxygen diluted with four times its bulk of nitrogen makes just the atmosphere required.