<P> The triple point of water is used to define the kelvin, the base unit of thermodynamic temperature in the International System of Units (SI). The value of the triple point of water is fixed by definition, rather than measured . The triple points of several substances are used to define points in the ITS - 90 international temperature scale, ranging from the triple point of hydrogen (13.8033 K) to the triple point of water (273.16 K, 0.01 ° C, or 32.018 ° F). </P> <P> The term "triple point" was coined in 1873 by James Thomson, brother of Lord Kelvin . </P> <P> The single combination of pressure and temperature at which liquid water, solid ice, and water vapor can coexist in a stable equilibrium occurs at exactly 273.16 K (0.01 ° C; 32.02 ° F) and a partial vapor pressure of 611.657 pascals (6.11657 mbar; 0.00603659 atm). At that point, it is possible to change all of the substance to ice, water, or vapor by making arbitrarily small changes in pressure and temperature . Even if the total pressure of a system is well above the triple point of water, provided that the partial pressure of the water vapor is 611.657 pascals, then the system can still be brought to the triple point of water . Strictly speaking, the surfaces separating the different phases should also be perfectly flat, to negate the effects of surface tension . </P> <P> The gas--liquid--solid triple point of water corresponds to the minimum pressure at which liquid water can exist . At pressures below the triple point (as in outer space), solid ice when heated at constant pressure is converted directly into water vapor in a process known as sublimation . Above the triple point, solid ice when heated at constant pressure first melts to form liquid water, and then evaporates or boils to form vapor at a higher temperature . </P>

What three forms of ice are present at the triple point marked o