All other animals, and all plants as well, are surrounded by and use practically the same things from their environment as we do. The potted plant in the window, the goldfish in the aquarium, your pet dog at home, all use, as we will later prove, the factors of their environment in the same manner. Air, water, light, a certain amount of heat, soil to live in or on, and food form parts of the surroundings of _every_ living thing. [Illustration: An experiment that shows the air contains about four fifths nitrogen.] [Illustration: Apparatus for separating water by means of an electric current into the two elements, hydrogen and oxygen.] The Same Elements found in Plants and Animals as in their Environment.--It has been found by chemists that the plants and animals as well as their environment may be reduced to about eighty very simple substances known as _chemical elements_. For example, the air is made up largely of two elements, _oxygen_ and _nitrogen_. Water, by means of an electric current, may be broken up into two elements, _oxygen_ and _hydrogen_. The elements in water are combined to make a _chemical compound_. The oxygen and nitrogen of the air are not so united, but exist as separate gases. If we were to study the chemistry of the bodies of plants and animals and of their foods, we would find them to be made up of certain chemical elements combined in various complex compounds. These elements are principally _carbon_, _hydrogen_, _oxygen_, _nitrogen_, and perhaps a dozen others in very minute proportions. But the same elements present in the living things might also be found in the environment, for example, water, food, the air, and the soil. It is logical to believe that living things use the chemical elements in their surroundings and in some wonderful manner build up their own bodies from the materials found in their environment. How this is done we will learn in later chapters. [Illustration: Chart to show the percentage of chemical elements in the human body.] What Plants and Animals take from their Environment. Air.--It is a self-evident fact that animals need air. Even those living in the water use the air dissolved in the water. A fish placed in an air-tight jar will soon die. It will be proven later that plants also need air in order to live. [Illustration: The effect of water upon the growth of trees. These trees were all planted at the same time in soil that is sandy and uniform. They are watered by a small stream which runs from left to right in the picture. Most of the water soaks into the ground before reaching the last trees.]