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.

[Illustration: 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 dus