Dalton, John published works

The existence of fundamental units of matter called atoms of different types called elements was proposed by ancient philosophers without any evidence to support the belief. Modern atomic theory is credited to the work of John Dalton published in 1803-1807. 

In England, around the same time, John Dalton studied the masses of compounds as they reacted to produce products. After Dalton read about the similar work of other scientists, such as Lavoisier and the British scientist Joseph Priestley, he contacted Gay-Lussac. He described his results and hypotheses to Gay-Lussac. In 1808, both men published their theories. After examining the theories of Dalton and Gay-Lussac, an Italian scientist named Amedeo Avogadro formulated a hypothesis that combined their theories. 

The works performed by Antoine Francois de Fourcroy (1755-1809), Louis Nicolas Vauquelin (1763-1829), Joseph Louis Proust (1754-1826) and Jons Jakob Berzelius (1779-1848) introduced new concepts in chemistry. Gay-Lussac published his Law of Combining Volumes in 1809, the year after John Dalton (1766-1844) had proposed his Atomic Theory of Matter around 1803. It was left to Amedeo Avogadro (1776-1856) to take the first major step in rationalizing Gay-Lussac s results two years later. 

For example, a clearly defined working concept of an element was essential for the elaboration of the atomic theory by John Dalton (1766-1844). In 1808, this English schoolmaster published A New System of Chemical Philosophy in which he put forth his postulates about the structure of matter. Each element, Dalton argued, is constituted of identical, immutable, and uniquely characteristic atoms. When elements combine to form compounds, then-atoms unite in a fixed ratio that is characteristic of the compound. Because these numbers are fixed, the elementary composition of the compound, by mass, is also constant. This latter property could be quite accurately determined, even in Dalton s time. However, Dalton had no direct way of ascertaining the correct atomic ratio of the elements of any compound. As a consequence, he could not calculate, with confidence, the relative masses of the atoms of the various elements—their atomic masses or atomic weights. The atomic masses of the elements and the atomic ratios characteristic of specific compounds are manifested in the elementary mass composition of the compounds. Once the correct atomic ratio is known, atomic masses can be readily calculated from mass composition. Alternatively, knowledge of atomic masses permits conversion of composition by mass to composition by atomic ratio. However, Dalton faced the dilemma of an equation with two unknowns. 

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