英文科學(xué)讀本 第四冊(cè)·Lesson 58 Ice

Lesson 58 Ice

We are going to have one or two further experiments with our phial of colored water today, said Mr. Wilson. "You remember I called it our water-thermometer.

I will begin by heating some water over the spirit-lamp to about 60° or 70°, and we will test it with an actual thermometer. When the thermometer tells us that this temperature is reached, we will stand the phial in the water, and in a short time the colored water inside the phial will show the same temperature. Now we have been taught that bodies expand with heat, and contract when the heat is taken away. We should expect that, in all bodies, there would be a regular step-by-step expansion, with every degree of heat added, and a corresponding, regular, step-by-step contraction, with every degree of heat taken away. If we raised the present temperature of the water in our phial by degrees we should find this to be actually the case. But we are not going to raise it this time. We will lower it.

How shall I lower it?

By adding some cold water, sir.

Very well; I will add cold water, and at the same time stand the thermometer in it. You in the meanwhile shall watch the result. As the water in the phial feels the diminution of heat, it gradually contracts, and the column in the tube falls lower and lower. This will go on till the thermometer shows us that 39° or 40° is reached. From this point, as the cooling proceeds, the colored water will be seen to rise in the tube again. What does this show?"

It shows that the water is actually expanding again, sir. It rises in the tube because it requires more room.

Quite right, said Mr. Wilson. "Now I will put some pieces of ice in the water to cool it still more. As it cools, the expansion in the tube will go on.

We will put in some more ice, and wait till the thermometer stands at 32° (the freezing-point). The expansion of the colored water inside the phial and tube still goes on, for it continues to rise, and at last, when it reaches the same temperature as the surrounding water, it assumes the solid form and becomes ice. But what has happened now?"

The phial has burst, sir.

Yes, the expansion of the water at the moment when it changed into ice was so sudden, and so violent, that the glass was not able to yield equally, and it burst. I think you will now readily see why water would not be a suitable liquid for filling thermometers. Water is unlike most other bodies in this respect, but its peculiar action, as it changes into the solid state, is of immense importance in nature.

You know that as bodies contract, their molecules are drawn more closely together; they become denser, and of course heavier. When they expand the opposite happens: their molecules are driven apart; they are less dense, and consequently lighter than they were. Water, we have seen, contracts step by step, as it cools, till it reaches about 40°, but lower than that it expands. Water, then, is heaviest when it stands at a temperature of 40°.

Imagine a pond of water in winter. The surface water cools first, and when it has cooled to about 40° it has so far contracted, that its particles are densely packed, and it is heavier, bulk for bulk, than the other water in the pond. What must happen then?"

This heavy surface water must sink to the bottom, sir.

Quite right, said Mr. Wilson, "and it does sink, and drives up to the surface that which is not so dense and heavy. This, however, soon cools and sinks in its turn, and so it goes on till the whole body of the water is cooled to that temperature. But suppose this went on till the freezing-point was reached. The coldest and heaviest water would be at the bottom, and the ice would be formed from below upwards, till the whole pond became one solid mass, killing all plant and animal life.

Instead of this, the water in cooling from 40° to 32° gradually expands, so that when the actual freezing takes place, the coldest water—that which forms the sheet of ice—is floating on the surface, because it is so much lighter than the rest. The ice which thus forms on the surface of the pond becomes a protecting coat for the still unfrozen water beneath it.