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    "source_title": "Encyclopaedia Britannica (1911)",
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    "chunk_id": "1911:fusion:73d773fab332",
    "title": "FUSION",
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    "verified_text": "fusion, the term generally applied to the melting of a solid substance, or the change of state of aggregation from the solid to the liquid. the term \"liquefaction\" is frequently employed in the same sense, but is often restricted to the condensation of a gas or vapour. the converse process of freezing or solidification, the change from the liquid to the solid state, is subject to the same laws, and must be considered together with fusion. the solution of a solid in a foreign liquid, and the deposition or crystallization of a solid from a solution, are so closely related to the fusion of a pure substance, that it will also be necessary to consider some of the analogies which they present. 1. _general phenomena._--there are two chief varieties of the process of fusion, namely, crystalline and amorphous, which are in many ways distinct, although it is possible to find intermediate cases which partake of the characteristics of both. the melting of ice may be taken as a typical case of crystalline fusion. the passage from rigid solid to mobile liquid occurs at a definite surface without any intermediate stage or plastic condition. the change takes place at a definite temperature, the fusing or freezing point (abbreviated f.p.), and requires the addition of a definite quantity of heat to the solid, which is called the latent heat of fusion. there is also in general a considerable change of volume during fusion, which amounts in the case of ice to a contraction of 9%. typical cases of amorphous solidification are those of silica, glass, plastic sulphur, pitch, alcohol and many organic liquids. in this type the liquid gradually becomes more and more viscous as the temperature falls, and ultimately attains the rigidity characteristic of a solid, without any definite freezing point or latent heat. the condition of the substance remains uniform throughout, if its temperature is uniform; there is no separation into the two distinct phases of solid and liquid, and there is no sudden change of volume at any temperature. a change or transition from one crystalline form to another may occur in the solid state with evolution or absorption of heat at a definite temperature, and is analogous to the change from solid to liquid, but usually takes place more slowly owing to the small molecular mobility of the solid state. thus rhombic sulphur when heated passes slowly at 95.6 deg. c. into the monosymmetric form which melts at 120 deg., but if heated rapidly the rhombic form melts at 114.5. the two forms, rhombic and monosymmetric, can exist in equilibrium at 95.6 deg., the transition point at which they have the same vapour pressure. similarly a solid solution of carbon in iron, when cooled slowly, passes at about 700 deg.",
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