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Little productive work on the measurement of heat changes was accomplished prior to the mid-nineteenth century for two reasons. First, the exact nature of heat itself was not well understood. Until the work of the Scottish chemist Joseph Black in the late eighteenth century, the distinction between temperature and heat was not at all clear. It then took until about 1845 before the nature of heat as a form of energy and not of matter was made clear in the experiments of James Joule and others.

Secondly, given such uncertainties, it is hardly surprising that appropriate equipment for the measurement of heat changes was not available until after the 1850s. Lavoisier and Laplace had made use of a primitive ice calorimeter to measure the heats of formations of compounds in 1780, but their work was largely ignored by their colleagues in chemistry.

In fact, credit for the development of modern techniques of calorimetry should probably be given to the French chemist Pierre Eugène Berthelot (1827-1907). In the 1860s, Berthelot became interested in the problems of heat measurement. He constructed what was probably the first modern calorimeter and invented the terms endothermic and exothermic to describe reactions in which heat is taken up or given off, respectively.

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Science EncyclopediaScience & Philosophy: Calcium Sulfate to Categorical imperativeCalorimetry - History, The Calorimeter, Calorimetry Theory