Читать книгу Common Science - Carleton Washburne - Страница 31

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 washing, 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 eyeballs 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 glass 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.