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.

The space between the threads of the wick, and especially the still finer spaces 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 glass tube. Wherever there are fine spaces 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 spaces 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 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.

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:

Things happen too swiftly for you to know much about them. The house you are in falls to dust instantly. You fall through the place where the floor has been; but you do not bump on the cement basement floor below, partly because there is no such thing as a hard floor or even hard ground anywhere, and partly because you disintegrate—fall to pieces—so completely that there is nothing left of you but a grayish film of fine dust and a haze of warm water.

With a deafening roar, rocks, skyscrapers, and even mountains tumble down, fall to pieces, and sink into an inconceivably fine dust. Nothing stands up in the world—not a tree, not an animal, not an island. With a wild rush the oceans flood in over the dust that has been nations and continents, and then this dust turns to a fine muddy ooze in the bottom of a worldwide sea.

But it is an ocean utterly different from what we have in the real world. There are no waves. Neither are there any reflections of clouds in its surface,—first because the clouds would fly to pieces and turn to invisible vapor, and second, because the ocean has no surface—it simply melts away into the air and no one can tell where the water stops and where the air begins.

Then the earth grows larger and larger. The ocean turns to a heavy, dense, transparent steam. The fine mud that used to be rocks and mountains and living things turns to a heavy, dense gas.

Our once beautiful, solid, warm, living earth now whirls on through space, a swollen, gaseous globe, utterly dead.

And the only thing that prevents all this from actually happening right now is that there is a force called cohesion that holds things together. It is the pull which one particle of anything has on another particle of the same material. The paper in this book, the chair on which you are sitting, and you yourself are all made of a vast number of unthinkably small particles called molecules, each of which is pulling on its neighbor with such force that all stay in their places. Substances in which they pull the hardest, like steel, are very hard to break in two; that is, it is difficult to pull the molecules of these substances apart. In liquids, such as water, the molecules do not pull nearly so hard on each other. In a gas, such as air, they are so far apart that they have practically no pull on each other at all. That is why everything would turn to a gas if the force of cohesion stopped. Why things would turn cold will be explained in Chapter 4.

Cohesion, adhesion, and capillary attraction, all are the result of the pull of molecules on each other. The difference is that capillary attraction is the pulling of particles of liquids up into fine spaces, as when a lamp wick draws up oil; adhesion is the pull of the particles of one substance or thing on the particles of another when they are very close together, as when water clings to your hand or when dust sticks to the ceiling; while cohesion is the clinging together of the particles of the same substance, like the holding together of the particles of your chair or of this paper.

When you put your hand into water it gets wet because the adhesion of the water to your hand is stronger than the cohesion of the water itself. The particles of the water are drawn to your hand more powerfully than they are drawn to each other. But in the following experiment, you have an example of cases where cohesion is stronger than adhesion:

Experiment 16. Pour some mercury (quicksilver) into a small dish and dip your finger into it. As you raise your finger, see if the mercury follows it up as the water did in Experiment 14. When you pull your finger all the way out, has the mercury wet it at all? Put a lamp wick or a part of your handkerchief into the mercury. Does it draw the mercury up as it would draw up water?

The reason for this peculiarity of mercury is that the pull between the particles of mercury themselves is stronger than the pull between them and your finger or handkerchief. In scientific language, the cohesion of the mercury is stronger than its adhesion to your finger or handkerchief. Although this seems unusual for a liquid, it is what we naturally expect of solid things; you would be amazed if part of the wood of your school seat stuck to you when you got up, for you expect the particles in solid things to cohere—to have cohesion—much more strongly than they adhere to something else. It is because solids have such strong cohesion that they are solids.

Application 13. Explain why mercury cannot wet your fingers; why rain falls in drops; why it is harder to drive a nail into wood than into soap; why steel is hard.

Inference Exercise

Explain the following:

51. Ink spilled on a plain board soaks in, but on a varnished desk it can be easily wiped off.

52. When a window is soiled you can write on it with your finger; then your finger becomes soiled.

53. A starched apron or shirt stays clean longer than an unstarched one.

54. When you hold a lump of sugar with one edge just touching the surface of a cup of coffee, the coffee runs up the lump.

55. A drop of water on a dry plate is not flat but rounded.

56. It is hard to write on cloth because the ink spreads out and blurs.

57. If you roughen your finger nails by cleaning them with a knife, they will get soiled much more quickly than if you keep them smooth by using an orange stick.

58. When you dip your pen in the ink and then move it across the paper, it makes ink marks on the paper.

59. If you suck the air out of a bottle, the bottle will stick to your tongue.

60. You cannot break a thick piece of iron with your hands.

Section 9. Friction.

What makes ice slippery?

How does a brake stop a car?

Why do things wear out?

It would not be such a calamity if we were to turn off friction from the world. Still, I doubt whether we should want to leave it off much longer than was necessary for us to see what would happen. Suppose we imagine the world with all friction removed:

A man on a bicycle can coast forever along level ground. Ships at sea can shut off steam and coast clear across the ocean. No machinery needs oiling. The clothes on your body feel smoother and softer than the finest silk. Perpetual motion is an established fact instead of an absolute impossibility; everything that is not going against gravity will keep right on moving forever or until it bumps into something else.

But, if there is no friction and you want to stop, you cannot. Suppose you are in an automobile when all friction stops. You speed along helplessly in the direction you are going. You cannot steer the machine—your hands would slip right around on the steering wheel, and even if you turn it by grasping the spoke, your machine still skids straight forward. If you start to go up a hill, you slow down, stop, and then before you can get out of the machine you start backward down the hill again and keep on going backward until you smash into something.

A person on foot does not fare much better. If he is walking at the time friction ceases, the ground is suddenly so slippery that he falls down and slides along on his back or stomach in the same direction he was walking, until he bumps into something big or starts to slip up a slope. If he reaches a slope, he, like the automobile, stops an instant a little way up, then starts sliding helplessly backward.

Another man is standing still when the friction is turned off. He cannot get anywhere. As soon as he starts to walk forward, his feet slip out from under him and he falls on his face. He lies in the same spot no matter how he wriggles and squirms. If he tries to push with his hands, they slip over the rough ground more easily than they now slip through air. He cannot push sideways enough even to turn over. If there happens to be a rope within reach and one end is tied to a tree, he might try to take hold of the rope to pull himself along. But no matter how tightly he squeezes, the rope slips right through his hands when he starts to pull. If, however, there is a loop in the rope, he can slip his hand through the loop and try to pull. But the knots with which the rope is tied immediately come untied and he is as helpless as ever.

Even if he takes hold of a board fence he is no more successful. The nails in the board slip out of their holes and he is left with a perfectly slippery and useless board on the ground beside him for a companion. As it grows cold toward evening he may take some matches out of his pocket and try to start a fire. Aside from the difficulty of his being unable to hold them except by the most careful balancing or by shutting them up within his slippery hands, he is entirely incapable of lighting them; they slip over the cement beneath him or over the sole of his shoe without the least rubbing.

In the real world, however, it is fortunately as impossible to get away from friction as it is to get away from the other laws we have tried to imagine as being turned off. There is always some friction, or rubbing, whenever anything moves. A bird rubs against the air, the point of a spinning top rubs against the sidewalk on which it is spinning. Your shoes rub against the ground as you walk and so make it possible for you to push yourself forward. The drive wheels of machinery rub against the belts and pull them along. There is friction between the wheels of a car and the track they are pushing against, or the wheels would whirl around and around uselessly.

But we can increase or decrease friction a great deal. If we make things rough, there is more friction between them than if they are smooth. If we press things tightly together, there is more friction than if they touch lightly. A nail in a loose hole comes out easily, but in a tight hole it sticks; the pressure has increased the friction. A motorman in starting a trolley car sometimes finds the track so smooth that the wheels whirl around without pushing the car forward; he pours some sand on the track to make it rougher, and the car starts. When you put on new shoes, they are so smooth on the bottom that they slip over the ground because of the lack of friction. If you scratch the soles, they are rougher and you no longer slip. If you try to pull a stake out of the ground, you have to squeeze it harder than the ground does or it will slip out of your hands instead of slipping out of the ground. When you apply a brake to an automobile, the brake must press tightly against the axle or wheel to cause enough friction to stop the automobile.

There are always two results of friction: heat and wear. Sometimes these effects of friction are helpful to us, and sometimes they are quite the opposite. The heat from friction is helpful when it makes it possible for us to light a fire, but it is far from helpful when it causes a hot box because of an ungreased wheel on a train or wagon, or burns your hands when you slide down a rope. The wear from friction is helpful when it makes it possible to sandpaper a table, scour a pan, scrub a floor, or erase a pencil mark; but we don't like it when it wears out automobile tires, all the parts of machinery, and our clothes.

Experiment 17. Hold a nail against a grindstone while you turn the stone. Notice both the wear and heat. Let the nail rest lightly on the stone part of the time and press hard part of the time. Which way does the nail get hotter? Which way does it wear off more quickly? Run it over a pane of glass and see if it gets as hot as it does on the grindstone; if it wears down as quickly.

Why we oil machinery. We can decrease friction by keeping objects from pressing tightly against each other, and by making their surfaces smooth. The most common way of making surfaces smooth is by oiling or greasing them. A film of oil or grease makes things so smooth and slippery that there is very little friction. That is why all kinds of machinery will run so smoothly if they are kept oiled. And since the oil decreases friction, it decreases the wear caused by friction. So well-oiled machines last much longer than machines that are not sufficiently oiled.

Why ball bearings are used. There is much less friction when a round object rolls over a surface than when two surfaces slide over one another, unless the sliding surfaces are very smooth; think how much easier it is to pull a wagon forward than it would be to take hold of the wheels and pull the wagon sidewise. So when you want the least possible friction in a machine you use ball bearings. The bearings are located in the hub of a wheel. Then, instead of the axle rubbing against the hub, the bearings roll inside of the hub. This causes very little friction; and the friction is made still less by keeping the bearings oiled.

Application 14. Suppose you were making a bicycle,—in which of the following places would you want to increase the friction, and in which would you want to decrease it? Handle grips, axles, pedals, tires, pedal cranks, the sockets in which the handle bar turns, the nuts that hold the parts together.

Application 15. A small boy decided to surprise his mother by oiling her sewing-machine. He put oil in the following places:

On the treadle, on the large wheel over which the belt runs, on the axle of the same wheel, on the groove in the little wheel up above where the belt runs, on the joint where the needle runs up and down, on the little rough place under the needle that pushes the cloth forward. Which of these did he do well to oil and which should he have let alone?

Inference Exercise

Explain the following:

61. Rivers flow north as well as south, although we usually speak of north as "up north."

62. Tartar and bits of food stick to your teeth.

63. Brushing your teeth with tooth powder cleans them.

64. When a chair has gliders (smooth metal caps) on its feet, it slides easily across the floor.

65. When you wet your finger, you can turn a page more easily.

66. A lamp wick draws oil up from the lower part of a lamp to the burner.

67. The sidewalks on steep hills are made of rough cement.

68. Certain fish can rise in the water by expanding their air bladders, although this does not make them weigh any less.

69. When your hands are cold, you rub them together to warm them.

70. It is dangerous to stand up in a rowboat or canoe.