Death of John Dalton (1844) who investigated the physical and chemical properties of matter and deduced an Atomic Theory (1803) whereby atoms of the same element are the same, but different from the atoms of any other element.
John Dalton arrived at his view of atomism by way of meteorology, in which he was seriously interested for a long period: he kept daily weather records from 1787 until his death, his first book was Meteorological Observations (1793), and he read a series of papers on meteorological topics before the Literary and Philosophical Society between 1799 and 1801. The papers contained Dalton’s independent statement of Charles’s law (see Joseph Louis Gay-Lussac): “All elastic fluids expand the same quantity by heat.”
He proceeded to calculate atomic weights from percentage compositions of compounds, using an arbitrary system to determine the likely atomic structure of each compound. If there are two elements that can combine, their combinations will occur in a set sequence. The first compound will have one atom of A and one of B; the next, one atom of A and two atoms of B; the next, two atoms of A and one of B; and so on. Hence, water is HO. Dalton also came to believe that the particles in different gases had different volumes and surrounds of caloric, thus explaining why a mixture of gases—as in the atmosphere—would not simply layer out but was kept in constant motion. Dalton consolidated his theories in his New System of Chemical Philosophy (1808–1827).
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Excerpted with permission, Chemical Heritage Foundation