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 (CO2). 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.