Sulphur exists in two well-known crystalline forms--rhombic, or octahedral, and monoclinic, or prismatic sulphur. Of these, the former melts at 114.5°; the latter at 120°.[49] Further, at the ordinary temperature, rhombic sulphur can exist unchanged, whereas, on being heated to temperatures somewhat below the melting point, it passes into the prismatic variety. On the other hand, at temperatures above 96°, prismatic sulphur can remain unchanged, whereas at the ordinary temperature it passes slowly into the rhombic form. If, now, we examine the case of sulphur with the help of the Phase Rule, we see that the following systems are theoretically possible:-- I. _Bivariant Systems: One component in one phase._ (_a_) Rhombic sulphur. (_b_) Monoclinic sulphur. (_c_) Sulphur vapour. (_d_) Liquid sulphur. II. _Univariant Systems: One component in two phases._ (_a_) Rhombic sulphur and vapour. (_b_) Monoclinic sulphur and vapour. (_c_) Rhombic sulphur and liquid. (_d_) Monoclinic sulphur and liquid. (_e_) Rhombic and monoclinic sulphur. (_f_) Liquid and vapour. III. _Invariant Systems: One component in three phases._ (_a_) Rhombic and monoclinic sulphur and vapour. (_b_) Rhombic sulphur, liquid and vapour. (_c_) Monoclinic sulphur, liquid and vapour. (_d_) Rhombic and monoclinic sulphur and liquid. [Illustration: FIG. 5.] Triple Point--Rhombic and Monoclinic Sulphur and Vapour. Transition Point.--In the case of ice, water and vapour, we saw that at the triple point the vapour pressures of ice and water are equal; below this point, ice is stable; above this point, water is stable. We saw, further, that below 0° the vapour pressure of the stable system is lower than that of the metastable, and therefore that at the triple point there is a break in the vapour pressure curve of such a kind that above {35} the triple point the vapour-pressure curve ascends more slowly than below it. Now, although the vapour pressure of solid sulphur has not been determined, we can nevertheless consider that it does possess a certain, even if very small, vapour pressure,[50] and that at the temperature at which the vapour pressures of rhombic and monoclinic sulphur become equal, we can have these two solid forms existing in equilibrium with the vapour. Below that point only one form, that with the lower vapour pressure, will be stable; above that point only the other form will be stable. On passing through the triple point, therefore, there will be a change of the one form into the other. This point is represented in our diagram (Fig. 5) by the point O, the two curves AO and OB representing diagrammatically the vapour pressures of rhombic and monoclinic sulphur respectively. If the vapour phase is absent and the system maintained under a constant pressure, _e.g._ {36} atmospheric pressure, there will also be a definite temperature at which the two solid forms are in equilibrium, and on passing through which