Why does fog deaden a tennis racket?
How does cold cream keep your face from becoming chapped?
Let us now imagine that animal and plant substances have suddenly lost
their ability to be softened by oil or water.
All living things soon feel very uncomfortable. Your face and hands
sting and crack; the skin all over your body becomes harsh and dry;
your mouth feels parched. The shoes you are wearing feel as if they
had been dried over a radiator after being very wet, only they are
still harder and more uncomfortable.
A man driving a horse feels the lines stiffening in his hands; and
the harness soon becomes so dry and brittle that it cracks and perhaps
breaks if the horse stops suddenly.
The leaves on the trees begin to rattle and break into pieces as the
wind blows against them. Although they keep their greenness, they act
like the driest leaves of autumn.
I doubt whether you or any one can stay alive long enough to notice
such effects. For the muscles of your body, including those that make
you breathe and make your heart beat, probably become so harsh
and stiff that they entirely fail to work, and you drop dead among
thousands of other stiff, harsh-skinned animals and people.
So it is well that in the real world oil and water soften practically
all plant and animal tissues. Of course, in living plants and animals
the oil and water come largely from within themselves. Your skin is
kept moist and slightly oily all the time by little glands within it,
some of which, called _sweat glands_, secrete perspiration and others
of which secrete oil. But sometimes the oil is washed off the surface
of your hands, as when you wash an article in gasoline or strong soap.
Then you feel that your skin is dry and harsh.
And when you want to soften it again you rub into it oily substances,
like cold cream or vaseline.
In the same way if harness or shoes get wet and then are dried out,
they can be made properly flexible by oiling. You could wet them, of
course, and this would soften them as long as they stayed wet. But
water evaporates rather quickly; so when you want a thing to _stay_
soft, you usually apply some kind of oil or grease.
Just as diffusion and the forming of solutions are increased by heat,
this softening by oil and water works better if the oil or water
is warm. That is why you soak your hands in _warm_ water before
manicuring your nails.
_APPLICATION 67._ Explain why women dampen clothes before
ironing them; why crackers are put up in waterproof cartons;
why an oil shoe polish is better than one containing water.
INFERENCE EXERCISE
Explain the following:
421. You can shorten your finger nails by filing them.
422. You can do it more quickly after washing them than
before.
423. After a flashlight picture is taken, the smoke soon
reaches all parts of the room.
424. A jeweler wears a convex lens on his eye when he works
with small objects.
425. Shoemakers soak the leather before half-soling shoes.
426. Lightning often sets fire to houses or trees that it
strikes.
427. The directions on many bottles of medicine and of
preparations for household use say, "Shake well before using."
428. If you set a cold tumbler inside of one that has just
been washed in hot water, the outer one will crack in a few
minutes.
429. A dry cloth hung out at night becomes wet, while a wet
cloth hung out on a clear day dries.
430. Putting cold cream or tallow around the roots of your
finger nails will help to prevent hangnails.
CHAPTER TEN
CHEMICAL CHANGE AND ENERGY