When a solution of potassium iodide in liquid sulphur dioxide containing 1.49 per cent. of potassium iodide was heated, solid (potassium iodide) was deposited at a temperature of 96.4°. Solutions containing more than about 3 per cent. of the iodide separated, on being heated, into two layers, and the temperature at which the liquid became heterogeneous fell as the concentration was increased; a temperature-minimum being obtained with solutions containing 12 per cent. of potassium iodide. On the other hand, solutions containing 30.9 per cent. of the iodide, on being heated, deposited potassium iodide; while a solution containing 24.5 per cent. of the salt first separated into two layers at 89.3°, and then, on cooling, solid was deposited and one of the liquid layers disappeared. Such are, in brief, the results of experiment; their interpretation in the light of the Phase Rule is the following:-- The curve AB is the freezing-point curve of solid sulphur dioxide in contact with solutions of potassium iodide. BCD is the solubility curve of the yellow crystalline solid which is deposited from the solutions. C, the temperature-maximum, is the melting point of this _yellow_ solid, and the composition of the latter must be the same as that of the solution at this point (p. 145), which was found to be that represented by the formula KI,14SO_{2}. B is therefore the eutectic point, at which solid sulphur dioxide and the compound KI,14SO_{2} can exist together in equilibrium with solution and vapour. The curve DE is the solubility curve of the _red_ crystalline solid, and the {160} point E, at which the composition of solution and solid is the same, is the melting point of the solid. The composition of this substance was found to be KI,4SO_{2}.[238] D is, therefore, the eutectic point at which the compounds KI,14SO_{2} and KI,4SO_{2} can coexist in equilibrium with solution and vapour. The curve DE does not exhibit a retroflex portion; on the contrary, on attempting to obtain more concentrated solutions in equilibrium with the compound KI,4SO_{2}, a new solid phase (probably potassium iodide) was formed. Since at this point there are four phases in equilibrium, viz. the compound KI,4SO_{2}, potassium iodide, solution, and vapour, the system is invariant. E is, therefore, the _transition point_ for KI,4SO_{2} and KI. Passing to higher temperatures, FG is the solubility curve of potassium iodide in sulphur dioxide; at G two liquid phases are formed, and the system therefore becomes invariant (cf. p. 121). The curve GHK is the solubility curve for two partially miscible liquids; and since complete miscibility occurs on _lowering_ the temperature, the curve is similar to that obtained with triethylamine and water (p. 101). K is also an invariant point at which potassium iodide is in equilibrium with two liquid phases and vapour.