Another class of dimorphous substances is, however, met with as, for instance, in the case of the well-known compounds iodine monochloride and benzophenone. Each crystalline form has its own melting point, the dimorphous forms of iodine monochloride melting at 13.9° and 27.2°,[63] and those of benzophenone at 26° and 48°.[64] This class of substance differs from that which we have already studied (_e.g._ sulphur and tin), in that at all temperatures up to the melting point, only one of the forms is stable, the other being metastable. There is, therefore, no transition point, and transformation of the crystalline forms can be observed _only in one direction_. These two classes of phenomena are distinguished by the names _enantiotropy_ and _monotropy_; enantiotropic substances being such that the change of one form into the other is a reversible process (_e.g._ rhombic sulphur into monoclinic, and monoclinic sulphur into rhombic), and monotropic substances, those in which the transformation of the crystalline forms is irreversible. [Illustration: FIG. 8.] [Illustration: FIG. 9.] These differences in the behaviour can be explained very well in many cases by supposing that in the case of enantiotropic substances the transition point lies below the melting point, while in the case of monotropic substances, it lies above the melting point.[65] These conditions would be represented by the Figs. 8 and 9. In these two figures, O_{3} is the transition point, O_{1} and O_{2} the melting points of the metastable and stable forms {45} respectively. From Fig. 9 we see that the crystalline form I. at all temperatures up to its melting point is metastable with respect to the form II. In such cases the transition point could be reached only at higher pressures. Although, as already stated, this explanation suffices for many cases, it does not prove that in all cases of monotropy the transition point is above the melting point of the two forms. It is also quite possible that the transition point may lie below the melting points;[66] in this case we have what is known as _pseudomonotropy_. It is possible that graphite and diamond,[67] perhaps also the two forms of phosphorus, stand in the relation of pseudomonotropy (_v._ p. 49). The disposition of the curves in Figs. 8 and 9 also explains the phenomenon sometimes met with, especially in organic chemistry, that the substance first melts, then solidifies, and remelts at a higher temperature. On again determining the melting point after re-solidification, only the higher melting point is obtained.