John Dalton
1766-1844
English Chemist, Physicist and Meteorologist
John Dalton proposed the atomic theory of matter as a result of his investigations of the atmosphere. By viewing matter as made up of indivisible particles each with their own particular weight, Dalton offered a way to understand chemical reactions. This led to the rapid development of chemistry in the nineteenth century.
Dalton was born into a family of devout Quakers in a small village in the Lakes District of northwest England. His Quaker heritage, with its emphasis on education and interest in science, played a key role in Dalton's life. After completing all the schooling he could obtain in his village, Dalton left in 1781 to replace his elder brother as an assistant in a Quaker boarding school in Kendal, the principal town in the Lakes District. The school had a very good library as well as scientific equipment that allowed Dalton to continue his education by reading and experimentation.
While in Kendal Dalton was befriended by a fellow Quaker, John Gough. Although blind, Gough was highly educated and had a particular interest in the natural sciences. He tutored Dalton in mathematics, meteorology, botany, Latin, Greek, and French. Dalton also began taking on teaching duties in Kendal and giving public lectures on various scientific matters, a practice he would continue through out his life. In 1792 Dalton was appointed to the position of professor of mathematics and natural philosophy at New College in Manchester. He spent the next eight years teaching various subjects before resigning in 1800 to open his own school offeringprivate instruction in mathematics, experimental science, and chemistry.
John Dalton. (The Library of Congress. Reproduced by permission.)By 1805 Dalton had essentially produced an outline of his most important contributions, his two laws concerning the gaseous state and the atomic theory of matter. What led to these discoveries was Dalton's attempt to try to understand certain aspects of the atmosphere. In particular, why was the atmosphere a homogeneous mixture of gases instead of layers of gases arranged according to their weight, the heaviest at the bottom and the lightest on the top? Another question was how water vapor could be absorbed in the atmosphere. The key to these problems was Dalton's belief in the particulate nature of matter, a concept that had been accepted by many of his contemporaries.
Dalton's insight was that the mixing of gases need not be a chemical reaction and that gases existed independently of each other in the atmosphere. This mechanical explanation for the mixing of gases led Dalton to propose in 1801 his law of partial pressures. Dalton generalized that in any mixture of gases, each component acted independently. A further principle stated by Dalton was that at constant pressure all gases will expand equally given the same quantity of heat. A similar principle was discovered by the French physicist Jacques Charles (1746-1823), who is generally given credit for this instead of Dalton.
Dalton's unorthodox view that chemical attraction was not a force in the atmosphere was met with much criticism and disbelief. In attempting to find experimental proof for his mechanical concept of the mixing of gases, Dalton relied on the work of his friend William Henry (1774-1836), who studied the effect of pressure on the amount of gas that could be dissolved in water at constant temperature. Henry's experiments showed that the amount was related to the pressure, thus showing that mixing of gases in the atmosphere had to be a mechanical phenomenon. Dalton found in his own experiments that the nature of the substance played a role in terms of how much could be dissolved at a constant pressure. What distinguished one substance from another was its mass; thus, Dalton believed that it was the size of the particles (atoms) that was the crucial determinant of an element's chemical properties. In 1803 Dalton published these results along with his measurement of the relative weights of different gases, in the process producing the first atomic weight table.
Dalton quickly realized that the concept of elements as being ultimately made up of atoms, with each atom having a unique atomic weight, was a way to explain why compounds such as water have a constant composition. Dalton also explained how atoms could react with other atoms in more than one ratio by weight and produce a series of compounds, an observation that fueled acceptance of Dalton's atomic theory.
Dalton's theory appeared in print in A New System of Chemical Philosophy, published in two parts in 1808 and 1810. Dalton was to contribute little new after 1810 and spent the rest of his life developing the theory by making measurements of atomic weights and public lectures. Dalton lived the balance of his life in Manchester and received many honors, including election to the Royal Society (1822), the Royal Society medal (1826), and honorary degrees from Oxford (1832) and Edinburgh (1834). On his death in 1844 more than 40,000 people filed past his coffin and a public funeral was held.
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