Sublimation Curve of Ice.--Vapour is given off not only by liquid water, but also by solid water, or ice. That this is so is familiar to every one through the fact that ice or snow, even at temperatures below the melting point, gradually disappears in the form of vapour. Even at temperatures considerably lower than 0°, the vapour pressure of ice, although small, is quite appreciable; and it is possible, therefore, to have ice and vapour coexisting in equilibrium. When we inquire into the conditions under which such a system can exist, we see again that we are dealing with a univariant system--one component existing in two phases--and that, therefore, just as in the case of the system water and vapour, there will be for each temperature a certain definite pressure of the vapour, and this pressure will be independent of the relative or absolute amounts of the solid or vapour present, and will depend solely on the temperature. Further, just as in the case of the vapour pressure of water, the condition of equilibrium between ice and water vapour will be represented by a line or curve showing the change of pressure with the temperature. Such a curve, representing the conditions of equilibrium between a solid and its vapour, is called a _sublimation curve_. At temperatures represented by any point on this curve, the solid (ice) will sublime or pass into vapour without previously fusing. Since ice melts at 0° (_vide infra_), the sublimation curve must end at that temperature. The following are the values of the vapour pressure of ice between 0° and -50°.[30] {25} VAPOUR PRESSURE OF ICE. --------------------------------------------------------------- Temperature. | Pressure in mm. | Temperature. | Pressure in mm. | mercury. | | mercury. -------------+-----------------+--------------+---------------- -50° | 0.050 | -8° | 2.379 -40° | 0.121 | -6° | 2.821 -30° | 0.312 | -4° | 3.334 -20° | 0.806 | -2° | 3.925 -15° | 1.279 | 0° | 4.602 -10° | 1.999 | | ----------------------------------------------------------------