If a piece of iron be exposed to dampness it will soon become covered with a reddish powder called "rust." The rusting of iron is a process of chemical changes in which the original substance was wholly changed by chemically uniting with the oxygen and the moisture of the atmosphere, which is really a process of combustion. The burning of wood, the rusting of iron, the souring of milk, and the digestion of food are, in a way, all mere examples of chemical changes. [Sidenote: Difference between chemical compounds and simple mixtures] Care should be exercised to distinguish chemical compounds from simple mixtures. Air is not a compound, but a mixture of oxygen, hydrogen and nitrogen gases. Water, however, is a compound of oxygen and hydrogen. Both salt and sugar are compounds, but if we grind them together, we do not have a new compound, but a mixture of two compounds. Most of the common things around us are mixtures of different compounds or substances. Rocks are mixtures of many different compounds. Wood is, likewise, formed of many different substances. Wheat contains water, starch, cellulose, and many other compounds. Grinding the wheat into flour does not change it chemically, but if we heat the flour in an oven, some of the starch is changed into dextrin. The starch has disappeared, and dextrin, a new substance, appears in its place. Whenever elements are combined into compounds, or compounds broken up into elements, or changed into other compounds, we have true chemical action. [Sidenote: Names of elements--how derived] The names of the elements are formed in many different ways. The name chlorin is derived from a Greek word meaning _greenish-yellow_, as this is the color of chlorin. Bromin comes from a Greek word meaning _a stench_, a prominent characteristic of bromin being its bad odor. Hydrogen is formed from two Greek words, one of which means _water_ and the other to _produce_, signifying that it enters into the composition of water. Potassium is an element found in potash, and sodium in soda, etc. [Sidenote: Symbols of elements--how derived] For convenience, abbreviations are used for the names of elements and compounds. Thus, instead of oxygen, we may write simply "O"; for hydrogen, "H"; for nitrogen, "N," etc. Very frequently the first letter of the name of the element is used as the symbol. If the names of two or more elements begin with the same letter, some other letter of the name is added. In some cases the symbols are derived from the Latin names of the elements. Thus, the symbol of iron is Fe, from _ferrum_; of copper, Cu, from _cuprum_. The following table gives the names of the elements which it will be necessary to understand in pursuing this work.