t 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.

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