change of a solid into a liquid when heat is applied. In a pure crystalline solid, this process occurs at a fixed temperature called the melting point (q.v.); an impure solid generally melts over a range of temperatures below the melting point of the principal component. Amorphous (non-crystalline) substances such as glass or pitch melt by gradually decreasing in viscosity as temperature is raised, with no sharp transition from solid to liquid
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