By addition of heat and increase in the amount of chlorine, the iodine monochloride disappears, and the system passes along the curve DE, which represents the composition of the solutions in equilibrium with solid iodine trichloride. The concentration of chlorine in the solution increases as the temperature is raised, until at the point E, where the solution has the same composition as the solid, the maximum temperature is reached; the iodine trichloride melts. On increasing still further the concentration of chlorine in the solution, the temperature of equilibrium falls, and a continuous curve, similar to that for the monochloride, is obtained. The upper branch of this curve has been followed down to a temperature of 30°, the solution at this point containing 99.6 per cent. of chlorine.[244] The very rounded form of the curve is due to the trichloride being largely dissociated in the liquid state. One curve still remains to be considered. As has already been mentioned, iodine monochloride can exist in two crystalline forms, only one of which, however, is stable at temperatures below the melting point; the two forms are _monotropic_ (p. 44). The stable form which melts at 27.2°, is called the [alpha]-form, while the less stable variety, melting at 13.9°, is known as the [beta]-form. If, now, the presence of [alpha]-ICl is excluded, it is possible to obtain the [beta]-form, and to study the conditions of equilibrium between it and solutions of iodine and chlorine, from the eutectic point F to the melting point G. As the [beta]-ICl becomes less stable in presence of excess of chlorine, it has not been possible to study the retroflex portion of the curve represented by the dotted continuation of FG. The following table gives some of the numerical data from which Fig. 42 was constructed.[245] {165} IODINE AND CHLORINE. I. _Invariant systems._ ------------------------------------------------------------------------- | | Phases present. Temper-| Pressure.+--------------------+-----------------+-------------- ature. | | Solid. | Liquid. | Vapour. --------+----------+--------------------+-----------------+-------------- 7.9° | 11 mm. | I_{2},[alpha]-ICl | I[wavy]Cl_{0.66}| I + Cl_{0.92} 0.9° | -- | I_{2},[beta]-ICl | I[wavy]Cl_{0.72}| -- 22.7° | 42 mm. | [alpha]-ICl,ICl_{3}| I[wavy]Cl_{1.19}| I + Cl_{1.75} [-102° | <1 atm. | ICl_{3},Cl_{2} | I[wavy]Cl_{m} | I + Cl_{n}] --------+----------+--------------------+-----------------+-------------- II. _Melting points._ A. Iodine,[246] 114.15° (pressure 89.8 mm.). C. [alpha]-Iodine monochloride, 27.2° (pressure 37 mm.). E. Iodine trichloride, 101° (pressure 16 atm.). G. [beta]-Iodine monochloride, 13.9°.