Chemistry is not, as popularly supposed, a science far removed from everyday life. Everyone has some knowledge of chemistry, but the chemist has observed things more minutely and therefore more accurately understands the composition of substances and the changes that are everywhere taking place. For illustration: A cook starts a fire in a stove. She knows that the fire must have "air" or it will not burn; that when the fire is first lighted, it "smokes" heavily, but as it burns more, it smokes less; further, that if the damper in the pipe is closed the "gas" will escape in to the room. [Sidenote: Fire, gas, and smoke the result of chemical changes] The chemist also knows this, but because he has compared his observations with similar events elsewhere, he is enabled to express his knowledge in the language of science. To the chemist, fire is the process of combustion--the union of the oxygen of the air with the carbon and hydrogen compounds of the wood or of the coal. The heat of the fire is generated by this chemical union. To the chemist, the smoke is a natural phenomenon occasioned by particles of carbon which fail to unite with the oxygen gas. The gas, which to the woman suggests suffocation if enough of it escapes into the room, to the chemist suggests a compound resulting from combination of the oxygen with the carbon. CHEMICAL ELEMENTS To the chemist, all forms of matter are mere combinations of elements. Chemical analysis is a process of separating, dividing, and subdividing matter. When the chemist separates or analyzes compounds, until he can no longer simplify or subdivide them, he calls these simple products "chemical elements." [Sidenote: Common elements] Many of the chemical elements are well known, such as copper, iron, and gold. Other elements that are still more common are unknown in their elementary form, because they combine with other elements so readily that they exist in nature only as compounds. For example: Hydrogen, united with oxygen, forms water; the elements chlorin and sodium, combined or united, form common salt. [Sidenote: Number of elements] Altogether chemists have discovered about eighty-four elements, many of which are rare, and do not occur in common substances. All substances of the earth, whether dead or living, are formed of chemical elements. These elements may be found in the pure or elementary state, or they may be mixed with other substances, or they may be combined chemically. Copper, iron, and gold are elements in the pure state. If we should take iron and copper filings and mix them together, we would still have copper and iron. Were we to take copper and gold and melt them together, we would have a metal that would be neither copper nor gold. It would be harder than one and softer than the other. But this substance would still be a mixture, and its properties half way between copper and gold. [Sidenote: Examples of chemical changes]