For centuries chemists and physicists believed that it was possible to transmute one element, such as lead, into another, such as gold. When the corpuscular theory of matter was developed and accepted (which could explain but not predict chemical changes in terms of transmutations), this belief was strengthened. By the middle of the eighteenth century, however, virtually all chemists and physicists believed that transmutations of matter into other kinds of matter were not possible. But lack of knowledge about the elements, the basic building blocks of all matter, hindered any real understanding of the nature of matter and the formation of new substances.
During the period between 1789 and 1803, Antoine-Laurent Lavoisier defined the elements as substances that could not be separated by fire or some other chemical means and John Dalton defined atoms as small, indestructible and invisible particles. These ideas cleared the way for understanding the makeup of all the substances in the universe. Dalton assumed that each kind of element had its own kind of atom, which was different from the atoms of all other elements. He also assumed that chemical elements kept their identity during all chemical reactions.
During the early nineteenth century, chemical experiments centered mainly around taking measurements of substances involved in chemical reactions both before and after the reaction. It was found that elements always reacted to form a new substance in the same ratio. If different ratios of the reacting substances were used, different substances were produced. Dalton's atomic theory went on to say that chemical compounds are the new substances that form when atoms combine with each other; that a specific compound always has the same kinds of atoms in the same ratio; and that chemical reactions do not involve a change in the atoms themselves but in the way they are arranged.
In 1809, French chemist Joseph-Louis Gay-Lussac and others began doing numerous experiments with gases by measuring the amounts of the gases that actually reacted. They found that two volumes of hydrogen reacted with one volume of oxygen to form two volumes of water, and that one volume of hydrogen gas reacted with one volume of chlorine gas to form two volumes of hydrogen chloride gas. In 1811, Amedeo Avogadro hypothesized that equal volumes of different gases, when at the same temperature and pressure, contained the same number of particles. These experimental results and theories eventually led to the determination of the number of atoms in the substances. The name molecule was later assigned to particles made up of more than one atom, which may be of the same or different atoms.