Dalton, John (1766-1844), British chemist and physicist, who developed the atomic theory upon which modern physical science is founded. Dalton was born on September 6, 1766, in Eagles field, Cumberland County, England. He was the son of a weaver and received his early education from his father and at a Quaker school in his native town, where he began teaching at the age of 12. In 1781 he moved to Kendal, where he conducted a school with his cousin and his elder brother. He went to Manchester in 1793 and spent the rest of his life there as a teacher, first at New College and later as a private tutor.
He died in Manchester on July 27, 1844. Dalton began a series of meteorological observations in 1787 that he continued for 57 years, accumulating some 200, 000 observations and measurements on the weather in the Manchester area. Dalton's interest in meteorology led him to study a variety of phenomena as well as the instruments used to measure them. He was the first to prove the validity of the concept that rain is precipitated by a decrease in temperature, not by a change in atmospheric pressure. Dalton's first work, Meteorological Observations and Essays (1793), attracted little attention, however. In the following year he presented a paper on color blindness, a condition from which Dalton himself suffered, before the Manchester Literary and Philosophical Society.
This paper was the earliest description of this phenomenon of vision, which became known as Daltonism. Dalton's most important contribution to science was his theory that matter is composed of atoms of differing weights and combine in simple ratios by weight. This theory, which Dalton first advanced in 1803, is the cornerstone of modern physical science (see Atom and Atomic Theory; Chemistry; Physics). In 1808 Dalton's A New System of Chemical Philosophy was published. In this book he listed the atomic weights of a number of known elements relative to the weight of hydrogen. His weights were not entirely accurate but they form the basis for the modern periodic table of the elements (see Elements, Chemical; Periodic Law).
Dalton arrived at his atomic theory through a study of the physical properties of atmospheric air and other gases. In the course of this investigation he discovered the law of partial pressures of mixed gases, often known as Dalton's law, that is, the total pressure exerted by a mixture of gases is equal to the sum of the separate pressures that each of the gases would exert if it alone occupied the whole volume. In 1804 and 1809 Dalton was invited to deliver courses at the Royal Institution in London. He was made a fellow of the Royal Society in 1822 and was awarded the society's gold medal in 1826. In 1830 Dalton became one of the eight foreign associates of the French Academy of Sciences.