a milestone in the development of chemistry devised by the English chemist and physicist John 
Dalton (1766-1844). He put forward the theory that all matter is made up of particles (atoms), which are identical throughout one element or substance, and that chemical reaction occurs through the attraction between atoms.
Dalton (1766-1844). He put forward the theory that all matter is made up of particles (atoms), which are identical throughout one element or substance, and that chemical reaction occurs through the attraction between atoms.Taken from Dictionary of Science
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It was in the early 1800s that John Dalton, an observer of weather and discoverer of color blindness among other things, came up with his atomic theory. Let's set the stage for Dalton's work. Less than twenty years earlier, in the 1780's, Lavoisier ushered in a new chemical era by making careful quantitative measurements which allowed the compositions of compounds to be determined with accuracy. By 1799 enough data had been accumulated for Proust to establish the Law of Constant Composition ( also called the Law of Definite Proportions). In 1803 Dalton noted that oxygen and carbon combined to make two compounds. Of course, each had its own particular weight ratio of oxygen to carbon (1.33:1 and 2.66:1), but also, for the same amount of carbon, one had exactly twice as much oxygen as the other. This led him to propose the Law of Simple Multiple Proportions, which was later verified by the Swedish chemist Berzelius. In an attempt to explain how and why elements would combine with one another in fixed ratios and sometimes also in multiples of those ratios, Dalton formulated his atomic theory.
Taken from Clackamas Community College Website
