of since prehistoric times, such as gold, silver, lead, copper, and sulfur (Oxtoby, Nachtrieb and Freeman, 1990, p. 7).
Chemistry is defined by a series of laws that govern all actions and reactions. The law of conservation of mass, discovered by the 18th-century French chemist Antoine Lavoisier, states that in every chemical operation, an equal quantity of matter exists before and after the operation (Oxtoby, Nachtrieb and Freeman, 1990, p. 9). Burning off a chemical in an enclosed container, therefore, does not actually destroy it; the matter is still present in the same amount as before, albeit in a different form or with different properties.
Another French chemist, Joseph Proust, discovered the law of definite proportions, that is, that in a given chemical compound, the proportions by mass of the elements that compose it are fixed, independent of the origin of the compound or its mode of preparation (Oxtoby, Nachtrieb and Freeman, 1990, p. 9). For example, sodium chloride will contain the same amount of chlorine by mass (60.66 percent) whether it is obtained from a salt mine, from seawater, or from the synthesis of sodium and chloride. Scientists have since discovered, however, that this law holds true mainly for gaseous compounds, with a range of compositions existing for some solids.
In 1808, the Englishman John Dalton published a treatise on chemistry that included the atomic theory of matter, which states that matter consists of indivisible atoms; that all the atoms of a given chemical element are identical in mass and in all other proper ties; that different chemical elements have different kinds of atoms, and in particular, that such atoms have different masses; that atoms are indestructible and retain their identity in chemical reactions; and that the formation of a compound from its elements occurs through the combination of atoms of unlike elements in small whole-number ratios (Oxtoby, Nachtrieb and Freema...