Diminution of pressure favours the formation of the system with the greater volume.
At 0°, and under a pressure of 1 atmo., water and ice are in equilibrium. If the pressure is increased, the ice melts, because the specific volume of the melted water is smaller than that of the ice. Usually, however, a substance when fused has a greater volume than the same substance at the same temperature in the solid state. Consequently in most cases the melting point rises with increasing pressure, as Bunsen {19) proved for parafl&n and spermaceti. As the pressure is increased this difference in volume seems to disappear, because in the liquid state the substance is more compressible than when solid. It is therefore not improbable that at very high pressures most substances would behave like water ; as Tammann (30) has pointed out, at a certain pressure the fusion is not accompanied by a change of volume.
The influence of pressure on the solubility of salts has been studied by F. Braun {21), and has been found to be in agreement with the theory. Generally the influence of pressure on the equilibrium is only very small, as we are not in a position to apply excessively high pressures. It is otherwise, of course, when we consider the relationships in the interior of the earth or of the sun, where the pressure, on account of its enormously high value, plays an extremely important part.
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