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Dalton, John

theory atomic mass law

(1766–1844) British meteorologist and chemical theorist: proposed an atomic theory linked to quantitative chemistry.

Dalton was the son of a weaver and a Quaker and grew up in an isolated village in Cumbria. He left his village when he was 15 for Kendal in central Cumbria and thereafter made his living as a teacher. In 1793 he moved to Manchester and taught science, and from 1799 he worked as a private tutor, giving short courses to groups of students for a modest fee. Throughout his life, from 1781, he kept daily meteorological records. This interest in weather and the atmosphere led to his work on gaseous mixtures generally.

In 1794 he wrote an excellent paper on colour vision (he was colour blind); in 1799, returning to his interest in the weather, he showed that springs arise from stored rainfall. This concern with rain and the water content of the atmosphere, which appears as the origin of all his work on gases and on atomic theory, arose through his life in the wet Lake District and the influence of a childhood teacher. He made his reputation in science in 1801 with his Law of Partial Pressures. This states that the pressure of a gas mixture is the sum of the pressures that each gas would exert if it were present alone and occupied the same volume as the whole mixture. He also found the law of thermal expansion of gases, now known as Law although Dalton published it first. In 1803, at the end of a paper on gas solubility, he noted rather casually his first table of relative atomic masses. The interest this aroused led him to develop his theory further, in lectures and in his book A New System of Chemical Philosophy (1808). Briefly, his atomic theory proposed that every element consists of very small particles called atoms, which are indivisible and indestructible spheres. The atoms of one element were presumed to be identical in all respects, including mass, but to differ from atoms of other elements in their mass. Chemical compounds are formed by the union of atoms of different elements, in simple ratios (ie elements A and B would form a compound AB; and possibly A2B, AB2 , A2 B3 ). This is known as the Law of Simple Multiple Proportions.

The theory was able to interpret the laws of chemical combination and the conservation of mass; it gave a new basis for all quantitative chemistry. Each aspect of Dalton’s theory has since been amended or refined, but its overall picture remains as the central basis of modern chemistry and physics.

Dalton assumed that when only one compound of two elements exists (for example water was the only compound of hydrogen and oxygen then known) it had the simplest formula; ie HO for water. On this basis, relative atomic masses (‘atomic weights’) could easily be found; the early lists were on the scale H = 1, but now a scale on which the common isotope of carbon = 12 is used. After work, corrections were needed (eg water is H2O, not HO) and discussion by chemists on these changes, and improved analyses, greatly occupied 19th-c chemists. The unit of relative atomic mass is named the dalton for him. The dalton is equal to one-twelfth of the mass of a neutral carbon-12 atom.

Dalton himself remains a strangely dull personality. His main work came after he was 30. He was a gruff lecturer, a poor experimenter and his writing seems old-fashioned. Apart from the brilliant insight of his atomic theory, his other work seems pedestrian. He was independent, modest and attributed his success to ‘perseverance’.

Manchester was strongly aware of Dalton’s fame. His lying in state in their Town Hall was attended by 40 000 people, his funeral was a major public event and memorials (sculptural and financial) were made, marking the surprising regard in an industrial city at that time for a scientific theorist.

He had instructed that his eyes be studied to find the cause of his colour blindness. This was done 150 years after his death, and DNA from them was found to lack the genes giving the green-sensitive pigment present in the normal human eye.

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