John Dalton (1766-1844), British chemist, studying a liquid in a glass. Dalton is most famous for his theory that matter is made of small indivisible units called atoms. This idea had also been proposed over 2000 years earlier by the Ancient Greek philosopher Democritus. However, it was not until chemical changes were scientifically studied in the 18th and 19th century that atoms were shown to be different for different elements and also to explain the simple ratios of chemical equations. Dalton published his atomic theory in A New System of Chemical Philosophy (1808). Other work included the Law of Partial Pressures for gases (1801) and an important paper on colour blindness (1794).