英文科學(xué)讀本 第六冊(cè)·Lesson 24 Air, a Mixture of Gases

Lesson 24 Air, a Mixture of Gases

I want you to think for a few minutes about our experiment with the soda-water bottle, and what it taught us, said Mr. Wilson. "You remember, of course, that I filled the bottle with a mixture of the two gases, oxygen and hydrogen. I might have corked up the bottle and left it for any length of time, and in the end it would have contained, not water, but merely a mixture of the two gases, each one possessing its individual and distinctive properties. The flame, however, changed them entirely. With the explosion there was a chemical union of the two gases into a totally new compound—water—possessing the characteristics of neither.

After this little recapitulation, we may now turn to the subject of today's lesson—the air. You remember, I daresay, the experiment which helped us to find out the composition of air. I have everything ready to repeat the experiment now. You shall tell me, as before, how I am to proceed."

You have only to float the saucer in the water, sir, said Will, "with the little piece of lighted phosphorus on it, and then cover it with the bell-jar."

Mr. Wilson followed the directions, with the usual results. The water rose in the jar to take the place of the oxygen, which the phosphorus had removed by its combustion. That which was left above the water, in the upper part of the jar, was the gas—nitrogen. The jar originally contained four times as much nitrogen as oxygen. The oxygen was now all gone, and four-fifths of the jar was filled with nitrogen.

Mr. Wilson proved it to be nitrogen by plunging a lighted taper into it. The taper was immediately extinguished. Nitrogen is characterized by its negative properties. It does not support combustion, and is itself uninflammable.

But, said he, "we actually saw the phosphorus burning in the jar of air. Why did it burn?"

Because, replied Fred, "although four-fifths of the air in the jar consisted of nitrogen, a non-supporter of combustion, the remaining one-fifth was oxygen, and it was the oxygen that supported the burning."

But, said Mr. Wilson again, "water contains more oxygen in proportion than air, for one-third of its bulk is oxygen, and yet we know that the oxygen in the water cannot support combustion. Water at once extinguishes all fire. Why should the oxygen in the air be able to do what the oxygen in the water cannot do?

Let me make this clear to you. In the first place, although water contains a large amount of oxygen, that oxygen is in chemical union with the other element, hydrogen. It is not free; it forms, for the time, an inseparable constituent of the new substance—water; it has lost all its own distinctive properties. The properties of water are totally different from those of either of its constituent gases. On the other hand, the oxygen and nitrogen, as constituents of the air, have entered into no chemical union. Each is free; each retains its own individual properties. The oxygen of the air is still the active agent and promoter of burning or combustion, although by being diluted with four times its bulk of nitrogen, it has lost much of its power.

I am now going to prepare some oxygen in the usual way and, as the gas comes off, I will let it pass, by means of the delivery-tube, into the bowl of water in which our jar of nitrogen stands. You will see it rise in bubbles, and gradually displace the water in the jar.

When all the water had left the jar Mr. Wilson called attention to the fact that there was now a mixture of nitrogen and oxygen in it, the quantity of nitrogen being four times that of the oxygen. He reminded the class that there was no flash, no heat, no explosion, when these gases mingled. They were not chemically united—they were merely mixed, as we might mix salt or sugar with sand; water with wine or vinegar.

Now, said he, "we will plunge the lighted taper into the mouth of the jar." He did so, but the flame itself did not cause any explosion. The taper burned as quietly inside as it did outside the jar, but no brighter.

Remember, he added in conclusion, "the taper went out immediately when we plunged it into the pure nitrogen just now. But here we have a mixture of nitrogen and oxygen. It is the diluted oxygen which maintains the flame of the taper. We have actually made a jar of ordinary atmospheric air."