英文科学读本 第四册·Lesson 44 Water

Lesson 44 Water

“Our lessons have taught us many things about water and its properties as a liquid,” said Mr. Wilson. “Today we are going to learn what water really is.

“I have here some bottles filled with a gas, which at present is quite new to you. It is called hydrogen. I cannot really show you the gas, because (like oxygen and nitrogen) it is colorless and transparent. You cannot see it in the bottles; you must, for the present, take my word that it is there. I will show it to you by its action before the lesson is over.

“First, however, I want you to think about the other gas, oxygen. What are the two chief properties of oxygen?”

“It makes other things burn, sir; but it does not take fire itself,” said Fred.

“You saw me burn carbon, i.e. charcoal, in oxygen. What did we get from the burning?”

“Carbonic acid gas was formed, sir. Carbon unites with oxygen and forms this substance.”

“Now let us return to our new gas, hydrogen. You may have noticed, perhaps, that the jars are all standing mouth downwards. This is because hydrogen is a very light gas—it is, indeed, the lightest substance in nature. If I had placed the jars with their mouths upwards, the gas would have escaped. Now watch what happens. I take the jar in my hand, still keeping it mouth downwards, and plunge this lighted taper into it. The hydrogen catches fire and burns with a blue flame in the mouth of the jar, but the flame of the taper itself is no longer burning. It has been extinguished by the gas.

“Can you now tell me any difference between hydrogen and oxygen?”

“Hydrogen is very inflammable; it burns with a blue flame. Oxygen will not take fire.”

“Hydrogen puts out the taper; it will not allow other things to burn in it; but oxygen helps all other things to burn. It was the oxygen in the air all round that made the hydrogen burn in the mouth of the jar. If there had been no oxygen present, the hydrogen would not have taken fire, for oxygen makes hydrogen burn as well as other things.

“I am now going to make some more hydrogen in this flask, and as the gas forms and passes up through this little tube, we will burn it. It takes fire instantly, and burns with a pale blue flame. I will next cover the flame with a dry cold glass, and one of you shall come and tell what you see.”

“The inside of the glass is covered with little drops of moisture, like dew-drops, sir.”

“So it is; but how did they get there? I must tell you. Hydrogen burns and forms a new substance—water, just as carbon burns and forms a new substance—carbonic acid gas.

“Carbonic acid was formed by the union or combining of carbon and oxygen when in a heated state; the new substance—water—has been formed by the union or combining of hydrogen and oxygen when in a heated state.

“You must remember that the water formed by this burning hydrogen is not liquid as we usually see it. It is in the form of steam or water gas. We could not see it until we placed the glass over the flame. It condensed into little drops of actual water when it touched the cold glass.

“So, then, we have seen that out of fire comes water. The fuel we burn in our fires, the candles, oil, and gas we use for lighting purposes, all contain hydrogen. When such things burn, therefore, water must be formed.”