------------------------------------+------------- | Substance. | Transition | temperature. ------------------------------------+------------- | Ammonium nitrate-- | [beta]-rhombic --> [alpha]-rhombic | 35° [alpha]-rhombic --> rhombohedral | 83° Rhombohedral --> regular | 125° Mercuric iodide | 126° Potassium nitrate | 129° Silver iodide | 145° Silver nitrate | 160° Sulphur | 95.5° Tetrabrommethane | 46.8° Thallium nitrate-- | Rhombic --> rhombohedral | 80° Rhombohedral --> regular | 142.5° Thallium picrate | 46° Tin | 20° ------------------------------------+------------- Sublimation and Vaporization Curves.--We have already seen, in the case of ice and liquid water, that the vapour pressure increases as the temperature rises, the increase of pressure per degree being greater the higher the temperature. The sublimation and vaporization curves, therefore, are not straight lines, but are bent, the convex side of the curve being towards the temperature axis in the ordinary _pt_-diagram. In the case of sulphur and of tin, we assumed vapour to be given off by the solid substance, although the pressure of the vapour has not hitherto been measured. The assumption, however, is entirely justified, not only on theoretical grounds, but also because the existence of a vapour pressure has been observed in the case of many solid substances at temperatures much below the melting point,[110] and in some cases, _e.g._ camphor,[111] the vapour pressure is considerable. {64} As the result of a large number of determinations, it has been found that all vapour pressure curves have the same general form alluded to above. Attempts have also been made to obtain a general expression for the quantitative changes in the vapour pressure with change of temperature, but without success. Nevertheless, the _qualitative_ changes, or the general direction of the curves, can be predicted by means of the theorem of Le Chatelier.