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John Dalton wrote about a subject of tremendous importance for the global environment: the chemical composition of the atmosphere. He began by stating some familiar laws, and then described a laboratory experiment in which two gases were mixed. Dalton recognized an important limitation of his laboratory experiment. His laboratory was small in size, but the atmosphere is much larger; thus, air pressure would vary between that at the Earth's surface and that further up in the atmosphere. Dalton then drew on another important ‘science tool’, the thought experiment, and thus arrived at his conclusions. Science is an ongoing process: a chain of experiments and reasoning. So, just as the laboratory experiment was limited, and required a new way of thinking, Dalton immediately recognized the limitations of that too: ‘All that we have said hitherto has been relating to “quiescent” atmospheres ... How the case would be with regard to the earth’s atmosphere, such as it actually is, ... is not very easy to ascertain …’ And, indeed, there was still much to learn; we return to this important topic over a century later in the 1946 timeline entry on the Brewer–Dobson circulation.

Bjoern Hassler, University of Cambridge.

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