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:
[Illustration: 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 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?
[Illustration: 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 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.