英文科學(xué)讀本 第四冊(cè)·Lesson 22 Further Properties of Liquids

Lesson 22 Further Properties of Liquids

One of the first things we learn about water is that it flows, said Mr. Wilson. "We rarely see water still. The slight cohesive force between its molecules enables it to move easily, and as a consequence wherever we meet with water, it is almost always on the move. It moves, or flows along, in the streams and rivers just as we see it flow along the gutters in the road. Let us see if we can discover the reason for this constant flowing. I have here a glass tube bent round in the form of the letter U. I will fill one arm of the tube with water. Notice what happens. The water, you see, passes at once into the other arm. It flows till it reaches exactly the same height in both arms of the tube, and there remains level. Here is a tumbler of water standing on the table. What do you notice about the surface of the water? The surface of the water remains level while the tumbler stands on the level surface. Now watch what takes place when I tilt the tumbler on one side. The water at once runs towards that side, and then remains level. You will perhaps see this better with the garden watering-pot. Here is the pot. We will fill it to the brim with water.

Now, Fred, come to the front, and tell us where the water stands in the spout.

It stands at the top of the spout, sir, just on a level with the water in the pot itself.

Is the water flowing (moving) now?

No, sir; it is quite still.

Pour some of it away, and then tell us how high the water stands in the spout.

It is not so high as it was, sir, but it is exactly on a level with the water in the pot.

Quite right, said Mr. Wilson, "and at that level it remains perfectly still. Now tilt the watering pot, as we tilted the tumbler, and let us see what will happen. The water in both spout and pot now stands level and still again, even though the pot has been tilted; but if we tilt it a little more, the water will run out of the spout, in trying to get on the same level as that in the pot.

Water flows to find its own level. It will flow till that level is found. When water is at rest its surface is always level. Fountains are constructed on this principle. All that is required is a cistern of water in some elevated position, with a pipe leading downwards from it. The water will flow down the pipe, and at the lower end rush upwards in a jet, trying to rise to the same level as the cistern from which it flowed.

It is on the very same principle that towns are supplied with water from the water-works. The reservoirs are placed on elevated ground, and the water which leaves them rushes oil through the pipes, trying to find the level from which it started. This property of liquids," continued Mr. Wilson, "is turned to account in the spirit-level. If you examine the instrument you will see there is a small glass tube in it filled with liquid. On a horizontal surface the liquid in the tube rests level. When it is tilted ever so little, the liquid flows towards that side. Workmen use this instrument to test the level of their work.

The liquid in it is not water but alcohol. It is called a spirit-level. Can you tell me why alcohol is used in preference to water?

Alcohol is a very mobile liquid, said Will. "It flows much more freely than water."

Quite true, my lad, said Mr. Wilson. "A dense liquid would flow too slowly to be of use in this instrument. There is one other property of water which I want to illustrate to you now. I have here a bottle with a cork that fits it exactly. I will remove the cork, and fill the bottle with water to the top of the neck. Now, Fred, take the bottle and push the cork in again. You can only force the cork in by forcing some of the water out, to make room for it. The water itself refuses to be squeezed into a smaller space. This is one of the most important and useful properties of water. For the present it will be sufficient for you to remember that water and all liquids are incompressible."