in the amount of the liquid phase containing excess of nitrile (_i.e._ the solution of water in fused nitrile), whereas the other liquid phase, the solution of fused nitrile in water, will gradually disappear. When it has completely disappeared, the system will be represented by the point _c'_, where the molecular concentration of nitrile is now 75 per cent., and again becomes univariant, the three phases being solid nitrile, liquid phase containing excess of nitrile, and vapour; and as the amount of the water is diminished the temperature of equilibrium rises, until at 54° the melting point of the pure nitrile is reached. Return now to the point c. At this point there exists the invariant system solid nitrile, two liquid phases, vapour. If heat be added, the solid nitrile will disappear, and there will be left the univariant system, consisting of two liquid phases and vapour.[204] Such a system will exhibit relationships similar to those already studied in the previous chapter. As the temperature rises, the mutual solubility of the two fused components becomes greater, until at _d_ (55.5°) the critical solution temperature is reached, and the fused components become miscible in all proportions. At all temperatures and concentrations lying to the right {124} of the curve _abcdc'e_ there can be only one liquid phase; in the field _cdc'_ there are two liquid phases. From the figure it will be easy to see what will be the result of bringing together succinic nitrile and water at different temperatures and in different amounts. Since _b_ is the lowest temperature at which liquid can exist in stable equilibrium with solid, ice and succinic nitrile can be mixed in any proportions at temperatures below _b_ without undergoing change. Between _b_ and _c_ succinic nitrile will be dissolved until the concentration reaches the value on the curve _bc_, corresponding to the given temperature. On adding the nitrile to water at temperatures between _c_ and _d_, it will dissolve until a concentration lying on the curve _cd_ is attained; at this point two liquid phases will be formed, and further addition of nitrile will cause the one liquid phase (that containing excess of nitrile) to increase, while the other liquid phase will decrease, until it finally disappears and there is only one liquid phase left, that containing excess of nitrile. This can dissolve further quantities of the nitrile, and the concentration will increase until the curve _c'e_ is reached, when the concentration will remain unchanged, and addition of solid will merely increase the amount of the solid phase.