Common Science Part 6

30. If the s.h.i.+p fills with water faster than the engines can pump it out, the s.h.i.+p sinks.

CHAPTER TWO

MOLECULAR ATTRACTION

SECTION 6. _How liquids are absorbed: Capillary attraction._

Why do blotters pull water into themselves when a flat piece of gla.s.s 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 s.h.i.+vering; 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 gla.s.s 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 gla.s.s with water and color it with a little blueing or red ink. Into the gla.s.s put two or three gla.s.s 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 gla.s.s 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 gla.s.s all night. Put an empty gla.s.s under the end that is hanging out. The next morning see what has happened.

[Ill.u.s.tration: FIG. 19. Will the water be drawn up higher in the fine gla.s.s tube or in a tube with a larger opening?]

[Ill.u.s.tration: FIG. 20. The water rises through the lamp wick by capillary attraction.]

The s.p.a.ce between the threads of the wick, and especially the still finer s.p.a.ces between the fibers that make up the threads, act like fine tubes and the liquid rises in them just as it did in the fine gla.s.s tube. Wherever there are fine s.p.a.ces between the particles of anything, as there are in a lump of sugar, a towel, a blotter, a wick, and hundreds of other things, these s.p.a.ces act like fine tubes and the liquid goes into them. The force that causes the liquid to move along fine tubes or openings is called _capillary attraction_.

Capillary attraction--this tendency of liquids to go into fine tubes--is caused by the same force that makes things cling to each other (adhesion), and that makes things hold together (cohesion). The next two sections tell about these two forces; so you will understand the cause of capillary attraction more thoroughly after reading them.

But you should know capillary attraction when you see it now, and know how to use it. The following questions will show whether or not you do:

_APPLICATION 10._ Suppose you have spilled some milk on a carpet, and that you have at hand wet tea leaves, dry corn meal, some torn bits of a glossy magazine cover, and a piece of new cloth the pores of which are stopped up with starch.

Which would be the best to use in taking up the milk?

_APPLICATION 11._ A boy spattered some candle grease on his coat. His aunt told him to lay a blotter on the candle grease and to press a hot iron on the blotter, or to put the blotter under his coat and the iron on top of the candle grease,--he was not quite sure which. While he was trying to recall his aunt's directions, his sister said that he could use soap and water to take the grease out; then his brother told him to sc.r.a.pe the spot with a knife. Which would have been the right thing for him to do?

INFERENCE EXERCISE

Explain the following:

31. A pen has a slit running down to the point.

32. When a man smokes, the smoke goes from the cigar into his mouth.

33. A blotter which has one end in water soon becomes wet all over.

34. Cream comes to the top of milk.

35. It is much harder to stand on stilts than on your feet.

36. Oiled shoes are almost waterproof.

37. City water reservoirs are located on the highest possible places in or near cities.

38. You can fill a self-filling fountain pen by squeezing the bulb, then letting go.

39. The oceans do not flow off the world.

40. When you turn a bottle of water upside down the water gurgles out instead of coming out in a smooth, steady stream.

SECTION 7. _How things stick to one another: Adhesion._

Why is it that when a thing is broken it will not stay together without glue?

Why does chalk stay on the blackboard?

Now that you have found out something about capillary attraction, suppose that you should go to the imaginary switchboard again and tamper with some other law of nature. An innocent-looking switch, right above the capillary attraction switch, would be labeled ADHESION. Suppose you have turned it off:

In an instant the wall paper slips down from the walls and crumples to a heap on the floor. The paint and varnish drop from the woodwork like so much sand. Every cobweb and speck of dust rolls off and falls in a little black heap below.

When you try to wash, you cannot wet your hands. But they do not need was.h.i.+ng, as the dirt tumbles off, leaving them cleaner than they ever were before. You can jump into a tank of water with all your clothes on and come out as dry as you went in. You discover by the dryness of your clothes that capillary attraction stopped when the adhesion was turned off, for capillary attraction is just a part of adhesion.

But you are not troubled now with the clamminess of unabsorbed perspiration. The perspiration rolls off in little drops, not wetting anything but running to the ground like so much quicksilver.

Your hair is fluffier than after the most vigorous shampoo. Your skin smarts with dryness. Your eyes are almost blinded by their lack of tears. Even when you cry, the tears roll from your eyeb.a.l.l.s and eyelids like water from a duck's back. Your mouth is too dry to talk; all the saliva rolls down your throat, leaving your tongue and cheeks as dry as cornstarch.

I think you would soon turn on the adhesion switch again.

EXPERIMENT 15. Touch the surface of a gla.s.s of water, and then raise your finger slightly. Notice whether the water tends to follow or to keep away from your finger as you raise it. Now dip your whole finger into the water and draw it out. Notice how the water clings, and watch the drops form and fall off.

Notice the film of water that stays on, wetting your finger, after all dropping stops.