John Dalton

John Dalton (1766–1844) was an English chemist, physicist, and meteorologist whose work helped establish the atomic basis of modern chemistry. He proposed that chemical elements consist of characteristic atoms and that compounds form through their combination in simple numerical ratios. He also formulated the law of partial pressures and conducted an early systematic investigation of color blindness, a condition he experienced himself. (royalsociety.org)

Life and scientific career

Dalton was born in Eaglesfield, Cumberland, England, into a Quaker family. His birthday is commonly given as September 6, 1766, but no contemporary birth registration survives, and the exact date remains uncertain. He began teaching at about twelve years of age and moved to Kendal in 1781 to work at a school associated with his brother. Elihu Robinson and John Gough encouraged his interests in mathematics and natural philosophy. (rsc.org)

In 1793 Dalton published Meteorological Observations and Essays and took a teaching position at New College in Manchester. Manchester became the principal setting of his subsequent research. The Manchester Literary and Philosophical Society provided an audience for his papers and an institutional base for his investigations. (rsc.org)

Teaching and public lecturing remained important sources of income. Among his later pupils was James Prescott Joule, whom he taught during the 1830s. Dalton served as president of the Manchester Literary and Philosophical Society from 1817 until his death. He was elected a Fellow of the Royal Society on March 7, 1822, and received its Royal Medal in 1826. He died in Manchester on July 27, 1844. (collection.sciencemuseumgroup.org.uk)

Meteorology and the behavior of gases

Dalton’s chemical investigations grew partly out of his sustained interest in meteorology. Beginning in 1787, he maintained a daily record of weather observations for approximately fifty-seven years. His studies of evaporation, atmospheric composition, and the expansion of gases helped direct his attention toward the particles composing matter. (catalogues.royalsociety.org)

In work presented around 1801, he argued that the gases in air form a physical mixture rather than a single chemical combination. Each component contributes independently to the total pressure. This relationship became known as Dalton’s law of partial pressures. (sciencehistory.org)

For a mixture of ideal gases, the law is expressed as

where $p_i$ is the pressure that component $i$ would exert if it alone occupied the mixture’s volume at the same temperature. The relationship is used, for example, to account for water vapor when gases are collected over water. Its modern formulation rests on ideal-gas behavior, rather than on all the details of Dalton’s original explanation of forces between particles. (openstax.org)

Atomic theory

Development and principal claims

Dalton began presenting his atomic ideas in 1803 and gave them a fuller exposition in the first volume of A New System of Chemical Philosophy, published in 1808. Further portions appeared through 1827. His contribution was not the first suggestion that matter consists of atoms; its significance lay in connecting atomic hypotheses with measurable chemical composition and relative particle masses. (rsc.org)

The central claims of Dalton’s atomic theory can be expressed as follows:

• Matter consists of extremely small atoms.
• Each chemical element has its own kind of atom, with characteristic properties and mass; Dalton regarded atoms of the same element as identical.
• Compounds contain atoms of different elements combined in small whole-number ratios.
• A chemical reaction rearranges atoms rather than creating or destroying them. (openstax.org)

This framework supplied a microscopic explanation of conservation of mass in chemical reactions and of the law of definite proportions: a particular compound has a fixed elemental composition because it contains its constituent atoms in a fixed ratio. (openstax.org)

Multiple proportions

Dalton also formulated the law of multiple proportions. When two elements form more than one compound, the masses of one element combining with a fixed mass of the other stand in small whole-number ratios. (openstax.org)

For example, carbon monoxide and carbon dioxide contain approximately 1.33 and 2.67 units of oxygen mass, respectively, for each unit of carbon mass. These oxygen amounts have a ratio of approximately 1:2, consistent with the formulas CO and CO₂. Such relationships connected bulk measurements to discrete atomic combinations and provided an important foundation for quantitative stoichiometry. (openstax.org)

Atomic weights and limitations

Dalton sought to calculate what are now called relative atomic masses from the measured composition of compounds. The major difficulty was that composition alone could not establish how many atoms a compound contained. A measured mass ratio therefore had to be combined with an assumption about atomic numbers. (sciencehistory.org)

His rules favored the simplest possible combination when little evidence was available. He consequently represented water as containing one hydrogen atom and one oxygen atom—HO in modern notation—rather than H₂O. Incorrect formulas produced incorrect relative atomic weights. His “compound atoms” also did not embody the fully developed modern distinction between atoms and molecules. (sciencehistory.org)

Later discoveries revised other assumptions. Atoms contain subatomic particles rather than being indivisible, and isotopes of the same element can have different masses. Nevertheless, the rearrangement of atoms remains a useful description of ordinary chemical reactions. (openstax.org)

Dalton’s theory did not immediately settle the question of whether atoms physically existed. Its chemical usefulness preceded broad acceptance of atomic reality. In the early twentieth century, quantitative studies of Brownian motion, associated with Albert Einstein and Jean Perrin, supplied additional evidence for the particulate nature of matter. (sciencehistory.org)

Investigation of color blindness

In 1794 Dalton described his own unusual color perception, including confusions between colors that other observers distinguished. His brother experienced similar difficulties. Dalton’s investigation helped establish color blindness as a subject of scientific inquiry; the term Daltonism subsequently became associated with the condition. (pubmed.ncbi.nlm.nih.gov)

Dalton hypothesized that a blue tint in the vitreous humor of his eyes selectively absorbed longer-wavelength light. He requested examination of his eyes after death, but the examination found the ocular humors to be clear, contradicting that explanation. (pubmed.ncbi.nlm.nih.gov)

A study published on February 17, 1995, analyzed DNA extracted from his preserved eye tissue. The researchers identified deuteranopia, a form of color-vision deficiency involving the absence of the normal medium-wavelength-sensitive retinal photopigment. This finding was consistent with his recorded color confusions. (pubmed.ncbi.nlm.nih.gov)

Scientific commemoration

The unit of mass called the dalton, symbol Da, bears his name. It is a non-SI unit equal to the unified atomic mass unit and is used to express atomic and molecular masses. (goldbook.iupac.org)

References

1. John Dalton & The Theory of Atomism | Science History Institutesciencehistory.org
2. Dalton; John (1766–1844) | Royal Society Cataloguecatalogues.royalsociety.org
3. John Dalton FRS - Scientists with disabilities | Royal Societyroyalsociety.org
4. John Dalton | Science Museum Group Collectioncollection.sciencemuseumgroup.org.uk
5. John Dalton and the Scientific Method | Science History Institutesciencehistory.org
6. 1 Early Ideas in Atomic Theory - Chemistry 2e | OpenStaxopenstax.org
7. 3 Stoichiometry of Gaseous Substances, Mixtures, and Reactions - Chemistry 2e | OpenStaxopenstax.org
8. 3 Atomic Structure and Symbolism - Chemistry: Atoms First 2e | OpenStaxopenstax.org
9. The chemistry of John Dalton's color blindness | PubMedpubmed.ncbi.nlm.nih.gov