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Freezing means the solidification of a liquid. It is generally applied to water only, but is applicable to all other such cases. Freezing of water occurs at a temperature so nearly constant that it is regarded as one of the fixed points required for the graduation of thermometers. On the Centigrade scale it is marked 0°, and on the Fahrenheit scale 32°. It is found to vary slightly with changes of pressure, an increase in pressure lowering the freezing-point. One atmosphere increase lowers it by about -0075° C. This variation is far slighter than that of the boiling-point per atmosphere increase or decrease. Liquids, as a rule, contract on freezing; water is a rare exception, expanding as it does to the extent of one-eighth its liquid volume. It is on this account that increase of pressure lowers the freezing-point of water; such increase usually raises the freezing-point of other liquids, and renders the process of solidification possible at higher temperatures than before pressure is applied. Thus to effect solidification the temperature may be lowered, or pressure may be applied. Both means are adopted when the liquid is very difficult to freeze. Another resource is available when the liquid has to be kept under pressure to prevent vaporisation. The pressure is suddenly removed, vaporisation immediately begins, and, requiring heat for this change of state, it takes it from the bulk of liquid. Enough may be thus abstracted to effect the freezing of the remaining liquid. Salt water freezes at a lower temperature than pure water, the exact point depending on the quantity of salt present. At this point the water solidifies as almost pure ice, leaving the salt behind. Saturated brine behaves differently, freezing of part causing the remaining liquid to lose some of its salt. This fact of the lowering the freezing-point of water by addition of salt is utilised in the formation of freezing-mixtures (q.v.). [Cold.]