Pressure-Temperature Diagram.[251]--If sulphur dioxide is passed into water at 0°, a solution will be formed and the temperature at which ice can exist in equilibrium with this solution will fall more and more as the concentration of the sulphur dioxide increases. At -2.6°, however, a cryohydric point is reached at which solid hydrate separates out, and the system becomes invariant. The curve AB (Fig. 45) therefore represents the pressure of the system ice--solution II.--vapour, and B represents the temperature and pressure at which the invariant system ice--hydrate--solution II.--vapour can exist. At this point the temperature is -2.6°, and the pressure 21.1 cm. If heat is withdrawn from this system, the solution will ultimately {171} solidify to a mixture of ice and hydrate, and there will be obtained the univariant system ice--hydrate--vapour. The vapour pressure of this system has been determined down to a temperature of -9.5°, at which temperature the pressure amounts to 15 cm. The pressures for this system are represented by the curve BC. If at the point B the volume is diminished, the pressure must remain constant, but the relative amounts of the different phases will undergo change. If suitable quantities of these are present, diminution of volume will ultimately lead to the total condensation of the vapour phase, and there will remain the univariant system ice--hydrate--solution. The temperature of equilibrium of this system will alter with the pressure, but, as in the case of the melting point of a simple substance, great differences of pressure will cause only comparatively small changes in the temperature of equilibrium. The change of the cryohydric point with the pressure is represented by the line BE; the actual values have not been determined, but the curve must slope towards the pressure axis because fusion is accompanied by diminution of volume, as in the case of pure ice. {172} A fourth univariant system can be formed at B. This is the system hydrate--solution II.--vapour. The conditions for the existence of this system are represented by the curve BF, which may therefore be regarded as the vapour-pressure curve of the saturated solution of sulphur dioxide heptahydrate in water. Unlike the curve for iodine trichloride--solution--vapour, this curve cannot be followed to the melting point of the hydrate. Before this point is reached, a second liquid phase appears, and an invariant system consisting of hydrate--solution I.--solution II.--vapour is formed. We have here, therefore, the phenomenon of melting under the solution as in the case of succinic nitrile and water (p. 122). This point is represented in the diagram by F; the temperature at this point is 12.1°, and the pressure 177.3 cm. The range of stable existence of the hydrate is therefore from -2.6° to 12.1°; nevertheless, the curve FB has been followed down to a temperature of -6°, at which point ice formed spontaneously.