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Julius von Mayer argued for a quantitative equivalence between mechanical work and heat. His 1842 paper connects compression of a gas with its heat capacity. In a modern ideal gas example, heating at constant pressure includes expansion work that is absent at constant volume. Because an ideal gas's internal energy depends only on temperature, its molar heat capacities satisfy $C_p-C_V=R$, where $R$ is the molar gas constant. The heat capacity difference thus supplies a route to comparing thermal and mechanical units of energy.^[ref]

Physician and natural philosopher who contributed to the conservation-of-energy programme in the 1840s, including an argument for the mechanical equivalent of heat.