英文科學(xué)讀本 第四冊(cè)·Lesson 41 How Heat Affects Bodies

Lesson 41 How Heat Affects Bodies

I am going to amuse you with some new experiments this morning, said Mr. Wilson. "Fred may come to the front to assist me. I have here a short round bar of iron, which fits exactly into this metal groove, and will just pass through the round hole in it. Try it, Fred, and see for yourself. It fits, you see, both ways.

Now put the iron bar into the fire, and as soon as it is red-hot, try and fit it into the groove again. What, can't you get it in? Then try and push the end through the round hole. It won't go into the hole, you see, with all your trying. Now why is this? The reason is that the bar is longer and thicker than it was. The heat has made the iron expand in every direction. Put the bar into this bowl of cold water, and try again, Fred."

It fits exactly now, sir, said Fred. "I suppose the iron has got smaller again because it is cold."

Yes, Fred, you are quite right, said Mr. Wilson. "We will try another experiment. I have here a brass ball hanging from a chain. It will just pass through this metal ring. Hold it by the chain and drop it through the ring. That's right. You see it just goes through.

Now we will lower the ball into this kettle of boiling water for a minute or two. That will do; you may take it out and drop it through the ring as before.

It will not go through the ring now, sir. The heat of the water must have made it expand. It is too big to pass through the ring now.

Put it into cold water, Fred, as you did the iron bar, and then, if you try again, you will find that it will pass through the ring easily.

Yes, sir, it passes through easily enough now, said Fred.

Remember, then, that almost all solid bodies expand with heat, and contract when the heat is taken away; for what is true of the iron bar and the brass ball is equally true of nearly all solids. Now let us look at liquids. I will fill this test-tube with water, and you shall hold it over the flame of the spirit-lamp. The water, as it gets hot, swells up and overflows, Suppose we now fill another tube with mercury, and heat that. The result is the same. The mercury swells up, and, like the water, would quickly overflow. Both liquids expand, you see, with heat, and require greater space.

Here is another test-tube, which I filled with boiling water an hour ago. It is not full now. What does this mean? The water has cooled, and as it cooled it contracted into smaller bulk. Liquids, like solids, expand when heated, and contract when the heat is taken away. I have now a very interesting experiment to show you," continued Mr. Wilson. "I will place this empty glass retort over the spirit-lamp in such a position as to allow its neck to dip down into this bowl of water. When I said the retort was 'empty,' what did I really mean?"

The retort is full of air, sir, said Fred.

Just so, said Mr. Wilson. "Now our retort is getting hot. Watch what happens."

Bubbles of air are rising up and bursting on the surface of the water, sir, said Fred.

Right; but whence did these bubbles come? The flame heated not the retort only, but the air inside it, and heated air expands or swells, so as to occupy more space. There was not room enough for all the expanded air in the retort, and so some of it passed out into the water, and rose up through it in bubbles. Now let us remove the spirit-lamp, and leave the retort to cool. Tell me what you see as the cooling goes on."

The water is rising in the neck of the retort, sir. There was no water in it before.

So it is, said Mr. Wilson, "and I think you can easily tell the reason why. Some of the air was driven out in bubbles, when it expanded by the heat. Now that the air is cooling it contracts or shrinks up again; but there is not enough cool air to fill the retort. Hence the water rises in the neck. What is true of air is equally true of all gases. Gases, like liquids and solids, expand with heat, and contract when the heat is taken away."