chlorine being -102°. Although complete measurements of the vapour pressure of the different systems of pure iodine to pure chlorine have not been made, the experimental data are nevertheless sufficient to allow of the general form of the curves being indicated with certainty. Bivariant Systems.--To these, only a brief reference need be made. Since there are two components, two phases will form a bivariant system. The fields in which these systems can exist are shown in Fig. 43 and Fig. 44, which is a more diagrammatic representation of a portion of Fig. 43. I. Iodine--vapour. II. Solution--vapour. III. Iodine trichloride--vapour. IV. Iodine monochloride--vapour. [Illustration: FIG. 44.] The conditions for the existence of these systems will probably be best understood from Fig. 44. Since the curve B'A' {168} represents the pressures under which the system iodine--solution--vapour can exist, increase of volume (diminution of pressure) will cause the volatilization of the solution, and the system iodine--vapour will remain. If, therefore, we start with a system represented by _a_, diminution of pressure at constant temperature will lead to the condition represented by _x_. On the other hand, increase of pressure at _a_ will lead to the condensation of a portion of the vapour phase. Since, now, the concentration of chlorine in the vapour is greater than in the solution, condensation of vapour would increase the concentration of chlorine in the solution; a certain amount of iodine must therefore pass into solution in order that the composition of the latter shall remain unchanged.[249] If, therefore, the volume of vapour be sufficiently great, continued diminution of volume will ultimately lead to the disappearance of all the iodine, and there will remain only solution and vapour (field II.). As the diminution of volume is continued, the vapour pressure and the concentration of the chlorine in the solution will increase, until when the pressure has reached the value _b_, iodine monochloride can separate out. The system, therefore, again becomes univariant, and at constant temperature the pressure and composition of the phases must remain unchanged. Diminution of volume will therefore not effect an increase of pressure, but a condensation of the vapour; and since this is richer in chlorine than the {169} solution, solid iodine monochloride must separate out in order that the concentration of the solution remain unchanged.[250] As the result, therefore, we obtain the bivariant system iodine monochloride--vapour.