The quantitative trade-off between mechanical energy and heat was even more firmly established by the mid-nineteenth century and enshrined as the first law of thermodynamics: energy is conserved when mechanical energy is converted to heat. Unlike mechanical energy, however, energy in the form of heat seemed to possess the mysterious property called entropy, which prevented some of the heat from being transformed into useful work. Like energy, entropy could be quantified experimentally: Whenever mechanical energy was turned into heat, an amount of entropy equal to the energy divided by the
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