How can a camera take a picture?
Why does cloth fade in the sun?
What makes freckles?
If light could not help chemical change, nothing would ever fade when
hung in the sun; wall paper and curtains would be as bright colored
after 20 years as on the day they were put up, if they were kept
clean; you would never become freckled, tanned, or sunburned; all
photographers and moving-picture operators would have to go out of
business; but worst of all, every green plant would immediately stop
growing and would soon die. Therefore, all cows and horses and other
plant-eating animals would die; and then the flesh-eating animals
would have nothing to eat and they would die; and then all people
would die.
You will be able better to understand why all this would happen after
you do the following experiments, the first of which will show that
light helps the chemical change called bleaching or fading.
EXPERIMENT 99. Rinse two small pieces of light-colored cloth.
(Lavender is a good color for this experiment.) Lay one piece
in the bright sun to dry; dry the other in a dark cabinet or
closet. The next day compare the two cloths. Which has kept
its color the better? If the difference is not marked, repeat
the experiment for 2 or 3 days in succession, putting the same
cloth, wet, in the sun each time.
Bleaching is usually a very slow kind of burning. It is the dye that
is oxidized (burned), not the cloth. Most dyes will combine with the
oxygen in the air _if they are exposed to the sunlight_. The dampness
quickens the action.
WHY SOME PEOPLE FRECKLE IN THE SUN. When the sunlight falls for a long
time on the skin, it often causes the cells in the under part of
the skin to produce some dark coloring matter, or pigment. This dark
pigment shows through the outer layer of skin, and we call the little
spots of it _freckles_. Some people are born with these pigment spots;
but when the freckles come out from long exposure to the sunlight,
they are an example right in our own skins of chemical change caused
by the action of light. Tan also is due to pigment in the skin and is
caused by light.
The next experiments with their explanations will show you how
cameras can take pictures. If you are not interested in knowing how
photographs are made, do the experiments and skip the explanations
down to the middle of page 332.
EXPERIMENT 100. Dissolve a small crystal of silver nitrate
(AgNO_3) in about half an inch of pure water in the bottom
of a test tube. Distilled water is best for this purpose.
Now add one drop of hydrochloric acid (HCl). The white powder
formed is a silver salt, called _silver chlorid_ (AgCl); the
rest of the liquid is now a diluted nitric acid (HNO_3).
Pour the suspension of silver chlorid (AgCl) on a piece of
blotting paper or on a paper towel, so that the water will be
absorbed. Spread the remaining white paste of silver chlorid
(AgCl) out over the blotter as well as you can. Cover part of
it with a key (or anything that will shut off the light), and
leave the other part exposed. If the sun is shining, put the
blotter in the sunlight for 5 minutes. Otherwise, let as much
daylight fall on it as possible for about 10 minutes. Now take
the key off the part of the silver chlorid (AgCl) that it was
covering and compare this with the part that was exposed
to the light. What has the light done to the silver chlorid
(AgCl) that it shone on?
What has happened is that the light has made the silver (Ag)
_separate_ from the chlorine (Cl) of the silver chlorid (AgCl).
Chemists would write this:
AgCl -> Ag + Cl.
That is, silver chlorid (AgCl) has changed into silver (Ag) and
chlorine (Cl). Chlorine, as you know, is a poisonous gas, and it
floats off in the air, leaving the fine particles of silver behind.
When silver is divided into very tiny particles, it absorbs light
instead of reflecting it; so it looks dark gray or black.
HOW PHOTOGRAPHS ARE MADE. All photography depends on this action of
light. The plates or films are coated with a silver salt,--usually a
more sensitive salt than silver chlorid. This is exposed to the light
that shines through the lens of the camera. As you have learned, the
lens brings the light from the object to a focus and makes an image
on the film or plate. The light parts of this image will change the
silver salt to silver; the dark parts will not change it. So wherever
there is a white place on the object you are photographing, there will
be a dark patch of silver on the film or plate, and wherever there
is a dark spot on the object, there will be no change on the film or
plate.
[Illustration: FIG. 173. The silver salt on the paper remains white
where it was shaded by the key.]
As a matter of fact, the film or plate is exposed such a short
time that there is not time for the change to be completed. So the
photographer develops the negative; he washes it in some chemicals
that finish the process which the light started.
If he exposed the whole plate to the light now, however, all the
_unchanged_ parts of the silver salt would also be changed by the
light, and there would be no picture left. So before he lets any light
shine on it, except red light which has no effect on the silver salt,
he dissolves off all the white unchanged part of the silver salt,
in another kind of chemical called the _fixing bath_. This is called
"fixing" the negative.
The only trouble with the picture now is that wherever there should
be a patch of white, there is a patch of dark silver particles; and
wherever there should be a dark place, there is just the clear glass
or celluloid, with all the silver salt dissolved off. This kind of
picture is called a _negative_; everything is just the opposite shade
from what it should be. A white man dressed in a black suit looks like
a negro dressed in a white suit.
HOW A PHOTOGRAPHIC PRINT IS MADE. The negative not only has the lights
and shadows reversed, but it is on celluloid or glass, and except
for moving pictures and stereopticons, we usually want the picture on
paper. So a print is made of the negative. The next experiment will
show you how this is done.
EXPERIMENT 101. In a dark room or closet, take a sheet of
blueprint paper from the package, afterwards closing the
package carefully so that no light can get to the papers
inside. Hold the piece of blueprint paper under your waist or
coat, to keep it dark when you go into the light. Now lay it,
greenish side downward, on a negative. Hold the two together,
or place them in a printing frame, and turn them over so that
the light will shine through the negative upon the greenish
side of the blueprint paper. Be sure that the paper is held
firmly against the negative and not moved around. Let the sun
shine through the negative upon the paper for 1 or 2 minutes
according to the brightness of the sun, or let the gray light
of the sky, if it is cloudy, shine on it for 5 or 10 minutes.
Now quickly put the blueprint paper (not the negative) into a
basin of water, face down. Wash for a couple of minutes. Turn
it over and examine it. If it has been exposed to the light
too long, it will be dark; if it has been exposed too short
a time, it will be too light; in either case, if the print
is not clear, repeat with a fresh piece of blueprint paper,
altering the time of exposure to the sunlight to improve the
print.
You can make pretty outline pictures of leaves and pressed
flowers, or of lace, by laying these on the blueprint paper in
place of the negative and in other respects doing as directed
above.
[Illustration: FIGS. 174 and 175. Where the negative is dark, the print
is light.]
In making blueprints you are changing an iron salt instead of a silver
salt, by the action of light. Regular photographic prints are usually
made on paper treated with a silver salt rather than with iron salt,
and sometimes a gold or platinum salt is used. But these other salts
have to be washed off with chemicals since they do not come off in
water, as the unchanged part of the iron salt comes off when you fix
the blueprint paper in the water bath.
Since the light cannot get through the black part of a negative, the
coating on the paper behind that part is not affected and it stays
light colored; and since the light can get through the clear parts
of the negative, the coating on the paper back of those parts _is_
affected and becomes dark. Therefore, the print is "right side
out,"--there is a light place on the print for every white place on
the object photographed, and there is a dark place on the print for
every black place on the object.
Moving-picture films are printed from one film to another, just as you
printed from a negative to a piece of paper. The negative is taken
on one film, then this is printed on another film. The second film is
"right side out."
LIGHT AND THE MANUFACTURE OF FOOD IN PLANTS. Much the most important
chemical effect of light, however, is not in making photographs, in
bleaching things, or in "burning" your skin. It is in the putting
together of carbon and water to make sugar in plants. Plants get water
(H_2O) from the earth and carbon dioxid (CO_2) from the air. When
the sun shines on chlorophyll, the green substance in plants, the
chlorophyll puts them together and makes sugar. The plant changes this
sugar into starch and other foods, and into the tissues of the plant
itself. Nothing in the world can put carbon dioxid and water together
and make food out of them except certain bacteria and the chlorophyll
of plants. And light is absolutely necessary for this chemical action.
Try this experiment:
EXPERIMENT 102. Pin together two pieces of cork on opposite
sides of a leaf that is exposed to the sun. The next day take
this leaf from the plant and heat it in a beaker of alcohol
until the green coloring matter is removed from the leaf. Then
place the leaf in a glass of water that contains iodine.
The iodine will color the leaf dark where the cells contain
starch. (See Experiment 115, page 373.) Is starch formed where
the light does not reach the leaf?
No plant can make food except with the help of light. The part of the
plant that can put carbon dioxid and water together is the green stuff
or chlorophyll, and this can work only when light is shining on it. So
all plants would die without light.
But if all plants should die, all animals would die also, for animals
cannot make food out of carbon dioxid and water, as they do not have
the chlorophyll that puts these things together. A lion does not live
on leaves, it is true, but he lives on deer and other animals that
do live on leaves and plants. If the plants died, all plant-eating
animals would die. Then there would be nothing for the flesh-eating
animals to eat except each other, and in time no animals would be left
in the world. The same thing would happen to the fish. And man, of
course, could no longer exist. The food supply of the world depends on
the fact that light can start chemical change.
OXYGEN RELEASED IN THE MANUFACTURE OF PLANT FOOD. Besides in one way
or another giving us all of our food, plants, helped by light, also
give us most of the free oxygen that we breathe. We and all animals
get the energy by which we live by _combining_ oxygen with the
hydrogen of our food (forming water) and by combining oxygen with the
carbon in our food (forming carbon dioxid). This combining (burning
or oxidizing) gives us our body heat and the energy to move. The free
oxygen is carried to the different parts of our bodies by the red
blood corpuscles that float in the liquid part of the blood. The
liquid part of the blood also carries the food to the different parts
of the body, and the food contains the carbon and hydrogen that is to
be burned. Then in a muscle, for instance, the oxygen that has been
carried by the corpuscles combines with the carbon to form carbon
dioxid, and with the hydrogen to form water. The corpuscles carry part
of the carbon dioxid back to the lungs, and the water is carried with
other wastes and the rest of the carbon dioxid in the liquid part of
the blood. In the lungs the carbon dioxid is exchanged for the free
oxygen we have just inhaled, and we exhale the carbon dioxid. A good
deal of water is also breathed out, as you can tell from the way the
mist gathers on a window pane when you blow on it.
If there were only animals (including people) in the world, all the
free oxygen in the air would in time be combined by the animals with
hydrogen to make water and with carbon to make carbon dioxid (CO_2).
As animals cannot breathe water and cannot get any good from carbon
dioxid, they would all smother.
But the plants, as we have already said, use carbon dioxid (CO_2)
and water (H_2O) to make food. They do not need so much oxygen, and
so they set some of it free. The countless plants in the world set
the oxygen free as rapidly as the countless animals combine it with
hydrogen to make water and with carbon to make carbon dioxid. Since
the water and carbon dioxid are the main things a plant needs to
make its food, the animals really are as helpful to the plants as the
plants are to the animals. For the animals furnish the materials to
the plants for making their food in exchange for the ready-made food
furnished by the plant. And both plants and animals would die if light
stopped helping to bring about chemical change.
_APPLICATION 74._ Explain why the heart of a cabbage is white
instead of green like the outside leaves; why a photographer
works in a dark room with only a ruby light; why you get
freckled in the sun.
INFERENCE EXERCISE
Explain the following:
461. If a pin is put through a lamp cord, a fuse is likely to
blow out.
462. The wall paper back of a picture is often darker than
that on the rest of the wall.
463. If you wet an eraser, it rubs through the paper.
464. Clothes are hot after being ironed.
465. If you drop candle grease on your clothes, you can remove
the grease by placing a blotter over it and pressing the
blotter with a warm iron.
466. Milliners cover hats that are on display in windows where
the sun shines in on the hats.
467. You pull down on a rope when you try to climb it.
468. In taking a picture, you expose the sensitive film or
plate to the light for a short time.
469. Good cameras have an adjustable front part so that the
lens may be moved nearer to the plate or film, or farther
from it, according to the distance of the object to be
photographed.
470. A pencil has to be resharpened frequently when it is much
used.