John Dalton .

John Dalton ( 1766 - 1844 ) , english meteorologist who switched to chemistry when he saw the applications for chemistry of his ideas about the atmosphere. He proposed the Atomic Theoryin 1803 which stated that (1) all matter was composed of small indivisible particles termed atoms, (2) atoms of a given element possess unique characteristics and weight, and (3) three types of atoms exist: simple (elements), compound (simple molecules), and complex (complex molecules). Dalton's theory was presented in New System of Chemical Philosophy (1808-1827). This work identified chemical elements as a specific type of atom, therefore rejecting Newton's theory of chemical affinities.

Instead, Dalton inferred proportions of elements in compounds by taking ratios of the weights of reactants, setting the atomic weight  of hydrogen to be identically one. Following richter, he proposed that chemical elements combine in integral ratios. Despite the importance of the work as the first view of atoms as physically real entities and introduction of a system of chemical symbols, New System of Chemical Philosophy devoted almost as much space to the caloric theory   as to atomism .

The English teacher, chemist, and physicist John Dalton was best known for developing the ancient concept of atoms into a scientific theory that has become a foundation of modern chemistry. He considered himself primarily a teacher and earned his living by teaching and lecturing until 1833, when he was awarded an annual civil pension. A self-taught experimenter, he devised simple but effective apparatuses for his well-planned tests. Although authors have emphasized the crudeness of his results, much of his data is remarkably accurate.

 Throughout his life Dalton was interested in the Earth's atmosphere, and he recorded more than 200,000 atmospheric observations in his notebooks. These observations led Dalton to study gases, and from the results of his experiments he was able to formulate his atomic theory. In a book on meteorology, he concluded that the aurora borealis is a magnetic phenomenon. He also explained the condensation of dew and gave a table of vapor pressures of water at various temperatures. Dalton was the first to publish the generalization that all gases initially at the same temperature expand equally on going to the same higher temperature. His law of partial pressures was included in a paper (1803) on gas solubilitis.Dalton's atomic theory was expressed in public lectures in 1803 and later in his New System of Chemical Philosophy (1808). Many scientists, including William Higgins, had considered matter to be made of atoms, but Dalton provided a model from which definite predictions could be made. 

This theory incorporated additional features that have since been discarded, but the realization that each atom has a characteristic mass and that atoms of elements are unchanged in chemical processes has served chemists to the present day.

                                                                                                

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