Читать книгу Common Science - Carleton Washburne - Страница 27

Table of Contents

SECTION 6. How liquids are absorbed: Capillary attraction.

Why do blotters pull water into themselves when a flat piece of glass will not?

How does a towel dry your face?

Suppose you could turn off nature's laws in the way that you can turn off electric lights. And suppose you stood in front of a switchboard with each switch labeled with the name of the law it would shut off. Of course, there is no such switchboard, but we know pretty well what would happen if we could shut off various laws. One of the least dangerous-looking switches would be one labeled Capillary Attraction. And now, just for fun, suppose that you have turned that switch off in order to see the effect.

At first you do not notice any change; but after a while you begin to feel perspiration collecting all over your body as if your clothes were made of rubber sheeting. Soon this becomes so uncomfortable that you decide to take a bath. But when you put your wash cloth into the water you find that it will not absorb any water at all; it gets a little wet on the outside, but remains stiff and is not easy or pleasant to use. You reach for a sponge or a bath brush, but you are no better off. Only the outside of the sponge and brush becomes wet, and they remain for the most part harsh and dry.

Then perhaps you try to dry yourself with a towel. But that does not work; not a drop of water will the towel absorb. You might as well try to dry yourself on the glossy side of a piece of oilcloth.

By this time you are shivering; so you probably decide to light the oil stove and get warm and dry over that. But the oil will not come up the wick! As a last resort you throw a dressing gown around you (it does not get wet) and start a fire in the fireplace. This at last warms and dries you; but as soon as you are dressed the clammy feeling comes again—your clothes will not absorb any perspiration. While the capillary attraction switch is turned off you will simply have to get used to this.

Then suppose you start to write your experience. Your fountain pen will not work. Even an ordinary pen does not work as well as it ought to. It makes a blot on your paper. If you use the blotter you are dismayed to find that the blot spreads out as flat as if you were pressing a piece of glass against it. You take your eraser and try to remove the blot. To your delight you find that it rubs out as easily as a pencil mark. The ink has not soaked into the paper at all. You begin to see some of the advantages in shutting off capillary attraction.

Perhaps you are writing at the dining-room table, and you overturn the inkwell on the tablecloth. Never mind, it is no trouble to brush the ink off. Not a sign of stain is left behind.

By and by you look outdoors at the garden. Everything is withering. The moisture does not move through the earth to where the roots of the plants can reach it. Before everything withers completely, you rush to the switchboard and turn on the capillary attraction again.

You can understand this force of capillary attraction better if you perform the following experiments:

Experiment 13. Fill a glass with water and color it with a little blueing or red ink. Into the glass put two or three glass tubes, open at both ends, and with bores of different sizes. (One of these tubes should be so-called thermometer tubing, with about 1 mm. bore.) Watch the colored water and see in which of the tubes it is pulled highest.

Experiment 14. Put a clean washed lamp wick into the glass of colored water and watch to see if the water is pulled up the wick. Now let the upper end of the wick hang over the side of the glass all night. Put an empty glass under the end that is hanging out. The next morning see what has happened.