[Ill.u.s.tration: 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:
[Ill.u.s.tration: FIG. 22. El Capitan, Yosemite Valley, California.
If the force of cohesion were suspended, a mountain like this would immediately become the finest dust.]
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 b.u.mp on the cement bas.e.m.e.nt 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, skysc.r.a.pers, 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 s.p.a.ce, 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 s.p.a.ces, 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?
[Ill.u.s.tration: FIG. 23. The mercury does not wet the finger, and as the finger is lifted the mercury does not follow it.]
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 ap.r.o.n 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.