Dalton, John (1766–1844)

English chemist, one of the founders of atomic theory. Some of his proposals have since proved to be incorrect, but his chief contribution was that he channeled the thinking of contemporary scientists along the correct lines, particularly in his method of using established facts to explain a new phenomenon.

Dalton was born in the village of Eaglesfield near Cockermouth, Cumbria, on or about September 6, 1766. He was the third of six children of a weaver, who was a devout Quaker and did not register the date of his son's birth. Dalton attended the village Quaker school and by the age of 12 was running it. He later became headmaster of a school in Kendal, before taking up a post in 1793 to teach mathematics and natural philosophy in Manchester. Dalton was largely self-taught, his Quaker beliefs excluding him from attending Oxford or Cambridge universities (at that time open only to members of the Church of England).

Even before he moved to Manchester, a wealthy Quaker friend, the blind philosopher John Gough, had stimulated in Dalton an interest in meteorology and for 57 years (beginning in 1787) he kept a diary of observations about the weather. He gave lectures on this subject to the Manchester Literary and Philosophical Society, of which he became honorary secretary and later president. He determined that the density of water varies with temperature, reaching a maximum at 6.1°C/42.5°F; the modern value of this temperature is 4°C). He also lectured about color blindness, a condition he shared with his brother and which for a time was known as Daltonism. He resigned his lectureship in Manchester in 1799 in order to pursue his own researches, working as a private tutor to make a living. He did, however, remain as the society's secretary and was given accommodation in a house they bought for him. This house, still containing many of Dalton's records, was destroyed in a bombing raid in 1940 during World War II. He was awarded a government pension of £150 in 1833, which was doubled three years later. He died in Manchester on July 27, 1844.

From his interest in the weather, atmosphere, and gases in general, Dalton in 1803 proposed his law of partial pressures (which states that, in a mixture of gases, the total pressure is the sum of the pressures that each component would exert if it alone occupied the same volume). He also studied the variation of a gas's volume with temperature, concluding (independently of Joseph Gay-Lussac) that all gases have the same coefficient of thermal expansion. Gaseous diffusion and the solubility of gases in water were also the subjects of his experiments.

The work on the absorption of gases led Dalton to formulate his atomic theory—he considered that gases must be made up of particles that can somehow occupy spaces between the particles that make up water, and that in a mixture of gases the different particles must intermingle rather than separate into layers depending on their density. When presented in his book New System of Chemical Philosophy (1808), the idea that atoms of different elements have different weights was supported by a list of atomic weights (atomic masses) and his newly devised system of chemical symbols. Combinations of element symbols could be made to represent compounds.

Many of the atomic weights (confused with equivalent weights) were incorrect, for example oxygen's was 8 and carbon's 6, but a pattern had been established, introducing order to a science that was hitherto little more than a collection of facts.

Taking into account later work, Dalton's atomic theory may be summarized as follows:

(a) Matter cannot be subdivided indefinitely, because each element consists of indivisible particles called atoms.

(b) The atoms of the same element are alike in every respect, having the same weight (mass), volume, and chemical properties; atoms of different elements have different properties.

(c) In chemical combinations of different elements, atoms join together in simple definite numbers to form compound atoms (now called molecules).

His formula for water—one hydrogen atom combined with one oxygen atom—was wrong, although he was more fortunate with carbon monoxide (“carbonic oxide”) and carbon dioxide (“carbonic acid”). But nevertheless he did bring a sort of order to the existing chaos, and provided a foundation for several generations of scientists. Several years later Jöns Berzelius was to supersede Dalton's system with the chemical symbols and formulas still used today.

Throughout his life Dalton retained his Quaker habits and dress, and new acquaintances were often taken aback by his appearance. He continued to keep his diary, which eventually ran to 200,000 entries. He distrusted the results of other workers, preferring to rely on his own experiences. As he grew older he became almost a recluse, with few friends and deeply involved in his pursuit of knowledge. And although he shunned fame and glory, he became famous even outside the realms of science. When his coffin stood on public display in Manchester Town Hall, more than 40,000 people filed past to pay their respects.