As to the upper limit of the fusion curve, the view has been expressed[121] that just as in the case of liquid and vapour, so also in the case of solid and liquid, there exists a critical point at which the solid and the liquid phase become identical. Experimental evidence, however, does not appear to favour this view.[122] The _transition point_, like the melting point, is also influenced by the pressure, and in this case also it is found that pressure may either raise or lower the transition point, so that the transition curve may be inclined either away from or towards the pressure axis. The direction of the transition curve can also be predicted if the change of volume accompanying the passage of one form into the other is known. In the case of sulphur, we saw that the transition point is raised by increase of pressure; in the case of the transition of rhombohedral into [alpha]-rhombic form of ammonium nitrate, however, the transition point is lowered by pressure, as shown by the following table.[123] -------------+---------- | Temperature. | Pressure. -------------+---------- | 85.85° | 1 atm. 84.38° | 100 " 83.03° | 200 " 82.29° | 250 " -------------+---------- So far as investigations have been carried out, it appears that in most cases the transition curve is practically a straight line. It has, however, been found in the case of Glauber's salt, that with increase of pressure the transition curve passes through a point of maximum temperature, and exhibits, therefore, a form similar to that assumed by Tammann for the fusion curve.[124] {69} Suspended Transformation. Metastable Equilibria.--Hitherto we have considered only systems in stable equilibrium. We have, however, already seen, in the case of water, that on cooling the liquid down to the triple point, solidification did not necessarily take place, although the conditions were such as to allow of its formation. Similarly, we saw that rhombic sulphur can be heated above the transition point, and monoclinic sulphur can be obtained at temperatures below the transition point, although in both cases transformation into a more stable form is possible; the system becomes metastable.