In something the same way, all the matter in the world is composed of about eighty individual substances called elements. These we might think of as the letters in a chemical alphabet which spell all the substances both organic and inorganic that are in existence. When elements unite, they form all the innumerable things that compose the world around us. -These substances, formed by the union of two or more elements, are called compounds. For example, iron is an element. Oxygen in the air is also an element. When these two unite, they form a compound which we call iron rust.
Organic substances utilize only about ten elements, but when we stop to think of the thousands of kinds of plants and of animals, and of all the different substances of which they are made, we see that ten elements are enough to make a wide variety of compounds.
The complete study of these elements and their compounds is called chemistry, but for the present we need to learn only four things about the elements which compose organic substances: (1) their names, (2) where they are found, (3) enough of their characteristics or properties so that we can recognize them, and (4) their use to living things.
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