The quantitative study of heat became feasible in the early 18th century when Daniel Fahrenheit (1686-1736) invented a mercury thermometer capable of reproducible measurements. Even earlier, however, at the turn of the century, Guillaume Amontons (1663-1705) had intuitively surmised that heat flowed in solids in the direction of decreasing temperature and that temperature varied in some predictable manner with distance—Amontons himself thought the variation was linear. In 1761 Joseph Black (1728-99) introduced the concepts of latent heat and specific heat, both of which pertain to heat storage rather than to heat movement. With the invention of the calorimeter in the 1780s by Antoine Lavoisier (1743-94) and Pierre Simon Laplace (1749-1827), the latent heat of melting ice proved to be a standard for quantifying heat. 1 The nature of what had been quantified, though, would elude comprehension throughout the 19th century.

In 1776 Johann Heinrich Lambert (1728-77) took a major step...

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