A detailed discussion of the effect of a continued increase of pressure will not be necessary. From what has already been said and with the help of Fig. 44, it will readily be understood that this will lead successively to the univariant system (_c_), iodine monochloride--solution--vapour; the bivariant system solution--vapour (field II.); the univariant system (_d_), iodine trichloride--solution--vapour; and the bivariant system _x'_, iodine trichloride--vapour. If the temperature of the experiment is above the melting point of the monochloride, then the systems in which this compound occurs will not be formed. Sulphur Dioxide and Water.--In the case just studied we have seen that the components can combine to form definite compounds possessing stable melting points. The curves of equilibrium, therefore, resemble in their general aspect those of calcium chloride and water, or of ferric chloride and water. In the case of sulphur dioxide and water, however, the melting point of the compound formed cannot be realized, because transition to another system occurs; retroflex concentration-temperature curves are therefore not found here, but the curves exhibit breaks or sudden changes in direction at the transition points, as in the case of the systems formed by sodium sulphate and water. The case of sulphur dioxide and water is also of interest from the fact that two liquid phases can be formed. The phases which occur are--Solid: ice, sulphur dioxide hydrate, SO_{2},7H_{2}O. Liquid: two solutions, the one containing excess of sulphur dioxide, the other excess of water, and represented by the symbols SO_{2} [wavy] _x_H_{2}O (solution I.), and H_{2}O [wavy] _y_SO_{2} (solution II.). Vapour: a mixture of sulphur dioxide and water vapour in varying proportions. Since there are two components, sulphur dioxide and water, the number of {170} possible systems is considerable. Only the following, however, have been studied:-- I. _Invariant Systems: Four co-existing phases._ (_a_) Ice, hydrate, solution, vapour. (_b_) Hydrate, solution I., solution II., vapour. II. _Univariant Systems: Three co-existing phases._ (_a_) Hydrate, solution I., vapour. (_b_) Hydrate, solution II., vapour. (_c_) Solution I., solution II., vapour. (_d_) Hydrate, solution I., solution II. (_e_) Hydrate, ice, vapour. (_f_) Ice, solution II., vapour. (_g_) Ice, hydrate, solution II. III. _Bivariant Systems: Two co-existing phases._ (_a_) Hydrate, solution I. (_b_) Hydrate, solution II. (_c_) Hydrate, vapour. (_d_) Hydrate, ice. (_e_) Solution I., solution II. (_f_) Solution I., vapour. (_g_) Solution I., ice. (_h_) Solution II., vapour. (_i_) Solution II., ice. (_j_) Ice, vapour. [Illustration: FIG. 45.]