Section 49. _chemical Change Caused by Light._

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 (AgNO3) 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 (HNO3).

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.

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.

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 (H2O) from the earth and carbon dioxid (CO2) 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 (CO2) and water (H2O) 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.