Atomic Theory
About 400 B.C. the Greek philosopher Democritus proposed that matter consisted of various types of tiny discrete particles and that the properties of matter were determined by the properties of these particles. This theory was later elaborated in a work by Lucretius. These philosophers did not have a method to verify the theory and it was not pursued for many centuries. The theory reappeared in the early 19th century as an explanation for several laws that had been established in the previous century when chemists had begun carefully measuring the mass of reactants and products. One of these was the law of conservation of mass, first stated by the French chemist Lavoisier, which says that there is no change in mass with a chemical reaction. Another was the observation that compounds always contain the same elements in the same proportions, the law of constant composition. Today it is known that some compounds, particularly metal oxides and sulfides, exist in ratios that vary slightly from simple whole number ratios. Some of these have the property of superconductivity. These are known as nonstoichiometric compounds.
A third law is the law of multiple proportions which states that a given mass of one element can combine with various masses of another element (or elements) but always in small whole number ratios. John Dalton revived the atomic theory in order to explain these observations. In 1808 he proposed that a chemical element (which could not be decomposed into two or more components) consisted of tiny particles (atoms), all of which had the same chemical properties. Further, the atoms of a given element have different properties than the atoms of other elements and that these atoms are not changed during ordinary chemical reactions. Compounds are formed by combination of atoms of different elements in certain simple whole number ratios.
It took many years for the concept to become widely accepted. Of course nowadays the atomic theory is fundamental to the physical sciences.