Abstract:

Accounts of how the concept of temperature has evolved typically cast the story as ancillary to the history of the thermometer or the history of the concept of heat. But then, because the history of temperature is not treated as a subject in its own right, modern associations inadvertently get read back into the historical record. This essay attempts to lay down an authoritative record not of what people in the past thought about what we call ‘temperature’ but of what they thought about what they called ‘temperature’ (or one of its cognates), from medieval times to today. I find that invention of the thermometer had little impact on the concept of temperature. Much more significant were Fahrenheit’s invention of a reliable instrument and William Thomson’s effort to make a degree of temperature a unit of measure. Overlapping definitions of temperature then emerged in the late nineteenth century, and twentieth-century scientific developments forced physicists to reconsider temperature’s conceptual boundaries. It turns out that the concept of temperature has evolved through stages that correspond to four increasingly sophisticated types of measurement. Its maturation and its maturity shed light on the philosophy of conceptual change.

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As another example, in Inventing Temperature, Chang says that something happened ‘before people could say with any confidence what it was that thermometers measured’. But in fact there was never such a time. People back then had no hesitation saying that thermometers measured what they called temperature, even if their understanding of temperature would not meet our standard for what temperature ‘really is’.

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That hot and cold are primary and independent qualities, even independent forces, was conventional thinking into the Renaissance and beyond.

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Finally, we should not presume that it took any scientific revolution or the invention of thermometers for people to distinguish how hot something really is from how hot it feels. The difference had long been understood. Many Greek philosophers engaged with the question of perception’s reliability, but it did not take a specialist to appreciate the issue. Kiln operators, of even prehistoric antiquity, used test-pieces and the colours of heated clay to objectively check how hot their kilns were. Blacksmiths and metallurgists used the melting points of known materials and the colours of heated ingots. Alchemists knew that melting and boiling points were more reliable than unaided sense experience. Even everyday cooks knew to watch for water to boil, butter to melt, or oil to sizzle.

It was not a need for precision or objectivity that led to the concept of temperature, nor was it the belief that cold is the absence of heat or heat the absence of cold. It was also not the invention of the thermometer. The concept ‘temperature’ arose because people wanted to think about and refer to combinations of qualities, especially combinations of contrary qualities.