1766–1844

FOR much of his life, the primary interest of John Dalton, an English Quaker, was the weather. Living in the notoriously wet country of Cumbria, he maintained a daily diary of meteorological occurrences from 1787 until his death, recording in total some 200,000 entries. It was, however, his development of atomic theory for which he is most remembered.

Different atoms – It was around the turn of the nineteenth century that Dalton started to formulate his theory. He had been undertaking experiments with gases, in particular on how soluble they were in water. A teacher by profession, who only practised science in his spare time, he had expected different gases would dissolve in water in the same way, but this was not the case. In trying to explain why, he speculated that perhaps the gases were composed of distinctly different “atoms”, or individual particles, which each had different masses. Of course, the idea of an atomic explanation of matter was not new, going way back to Democritus of Abdera (c. 460–370 BCE) in ancient Greece, but now Dalton had the discoveries of recent science to reinforce his theory. On further examination of his thesis, he realised that not only would it explain the different solubility of gases in water, but would also account for the “conservation of mass” observed during chemical reactions as well as the combinations into which elements apparently entered when forming compounds (because the atoms were simply “rearranging” themselves and not being created or destroyed).

Atomic theory – Dalton publicly outlined his support for this atomic theory in a lecture in 1803, although its complete explanation had to wait until his book of 1808 entitled A New System of Chemical Philosophy. Here, he summarised his beliefs based on key principles, including: atoms of the same element are identical; distinct elements have distinct atoms; atoms are neither created nor destroyed; everything is made up of atoms; a chemical change is simply the reshuffling of atoms; and compounds are made up of atoms from the relevant elements. In the same book he published a table of known atoms and their weights, although some of these were slightly wrong due to the crudeness of Dalton’s equipment, based on hydrogen having a mass of one. It was a basic framework for subsequent atomic tables, which are today based on carbon (having a mass of 12), rather than hydrogen. Dalton also erroneously assumed elements would combine in one-to-one ratios (for example, water being HO not H2O) as a base principle, only converting into “multiple proportions” (for example, from carbon monoxide, CO, to carbon dioxide, CO2) under certain conditions. Although scientific arguments over the validity of Dalton’s thesis would continue for decades, the foundations for the study of modern atomic theory had been laid and with ongoing refinement were gradually accepted.

Prior to atomic theory, Dalton had also made a number of other important discoveries and observations in the course of his work. These included his “law of partial pressures” of 1801, which stated that a blend of gases exerts pressure which is equivalent to the total of all the pressures each gas would wield if they were alone in the same volume as the entire mixture.

Dalton also explained that air was a blend of independent gases, not a compound. He was the first to publish the law later credited to and named after Jacques-Alexandre-César Charles (1746–1823). Although the Frenchman had been the first to articulate the law concerning the equal expansion of all gases when raised in equal increments of temperature, Dalton had discovered it independently and had been the first to publish.

Dalton also discovered the “dew point” and that the behaviour of water vapour is consistent with that of other gases, and hypothesised on the causes of the aurora borealis, the mysterious Northern Lights. His further meteorological observations included confirmation of the cause of rain being due to a fall in temperature not pressure.

Further achievements – John Dalton began teaching at his local school at the age of 12. Two years later, he and his elder brother purchased a school where they taught some 60 children.

His paper on colour blindness, which both he and his brother suffered from, and which was known as daltonism for a long while, was the first to be published on the condition. Dalton is also largely responsible for transferring meteorology from being an imprecise art on folklore to a real science.

Chronology  

. 1793 Meteorological Observations and Essays published

. 1801 Dalton states his Law of Partial Pressure

. 1803 Outlines his atomic theory in a lecture. This transformed the basics of chemistry and physics

. 1808 A New System of Chemical Philosophy published.