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.