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.
[Illustration: FIG. 19. Will the water be drawn up higher in the fine
glass tube or in a tube with a larger opening?]
[Illustration: FIG. 20. The water rises through the lamp wick by
capillary attraction.]
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.