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 scrape 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 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.
Fig. 21. As the finger is raised the water is drawn up after it.
Which do you think is the stronger, the pull of gravity which makes some of the water drip off, or the pull of adhesion which makes some of the water cling to your finger?
If the pull of gravity is stronger, would not all the water drop off, leaving your finger dry? If the pull of adhesion is the stronger, would not all the water stay on your finger, none dropping off?
The truth of the matter is that gravity is stronger than adhesion unless things are very close together; then adhesion is stronger. The part of the water that is very close to your finger clings to it in spite of gravity; the part that is farther away forms drops and falls down because of the pull of gravity.
Adhesion, then, is the force that makes things cling to each other when they are very close together.
Why it is easier to turn a page if you wet your finger. Water spreads out on things so that it gets very close to them. The thin film of water on your finger is close enough to your finger and to the page which you are turning to cling to both; so when you move your finger, the page moves along with it.
Why dust clings to the ceiling and walls. The fine particles of dust are wafted up against the ceiling and walls by the moving air in the room. They are so small that they can fit into the small dents that are in plaster and paper and can get very close to the wall. Once they get close enough, the force of adhesion holds them with a pull stronger than that of gravity.
Oily and wet surfaces catch dust much more readily than clean, dry ones, simply because the dust can get so much closer to the oil or water film and because this film flows partly around each dust particle and holds it by the force of adhesion. This is why your face gets much dirtier when it is perspiring than when it is dry.
Application 12. Explain why cobwebs do not fall from the ceiling; why dust clings to a wet broom; why a postage stamp does not fall off an envelope.
Inference Exercise
Explain the following:
41. There are no springs on the tops of high mountains.
42. People used to shake sand over their letters after writing them in ink.
43. People used to make night lights for bedrooms by pouring some oil into a cup of water and floating a piece of wick on the oil. The oil always stayed on top of the water, and went up through the wick fast enough to keep the light burning.
44. Your face becomes much dirtier when you are perspiring.
45. Ink bottles are usually made with wide bases.
46. When you spill water on the floor, you cannot wipe it up with wrapping paper, but you can dry it easily with a cloth.
47. Oiled mops are used in taking up dust.
48. Cake will stick to a pan unless the pan is greased.
49. Although the earth turns completely over every day, we never fall off it.
50. Signs are fastened sometimes to windows or to the wind shields of automobiles by little rubber "suction caps."
Section 8. The force that makes a thing hold together: Cohesion.
What makes rain fall in drops?
Why are diamonds hard?
You have not yet touched any of the most dangerous switches on the imaginary switchboard of universal laws. But if your experience in turning off the capillary attraction and adhesion switches did not discourage you, you might try turning off the one beside them labeled Cohesion:
Fig. 22. El Capitan, Yosemite Valley, California. If the force of cohesion were suspended, a mountain