The most familiar examples of equilibria in a one-component system are those furnished by the three phases of water, viz. ice, water, water vapour. The system consists of one component, because all three phases have the same chemical composition, represented by the formula H_{2}O. As the criterion of equilibrium we shall choose a definite pressure, and shall study the variation of the pressure with the temperature; and for the purpose of representing the relationships which we obtain we shall employ a temperature-pressure diagram, in which the temperatures are measured as abscissæ and the pressures as ordinates. In such a diagram invariant systems will be represented by points; univariant systems by lines, and bivariant systems by areas. Equilibrium between Liquid and Vapour. Vaporization Curve.--Consider in the first place the conditions for the coexistence of liquid and vapour. According to the Phase Rule (p. 16), a system consisting of one component in two phases has one degree of freedom, or is univariant. We should therefore expect that it will be possible for liquid water to coexist with water vapour at different values of temperature and {22} pressure, but that if we arbitrarily fix one of the variable factors, pressure, temperature, or volume (in the case of a given mass of substance), the state of the system will then be defined. If we fix, say, the temperature, then the pressure will have a definite value; or if we adopt a certain pressure, the liquid and vapour can coexist only at a certain definite temperature. Each temperature, therefore, will correspond to a definite pressure; and if in our diagram we join by a continuous line all the points indicating the values of the pressure corresponding to the different temperatures, we shall obtain a curve (Fig. 1) representing the variation of the pressure with the temperature. This is the curve of vapour pressure, or the _vaporization curve_ of water. [Illustration: FIG. 1.] Now, the results of experiment are quite in agreement with the requirements of the Phase Rule, and at any given temperature the system water--vapour can exist in equilibrium only under a definite pressure. The vapour pressure of water at different temperatures has been subjected to careful measurement by Magnus,[22] Regnault,[23] Ramsay and Young,[24] Juhlin,[25] Thiesen and Scheel,[26] and others. In the following table the values of the vapour pressure from -10° to +100° are those calculated from the measurements of Regnault, corrected by the measurements of Wiebe and Thiesen and Scheel;[27] those from 120° to 270° were determined {23} by Ramsay and Young, while the values of the critical pressure and temperature are those determined by Battelli.[28] VAPOUR PRESSURE OF WATER.