--------------------------------------------------------------------- | Pressure in kilogms. per | Change of melting point for Temperature. | sq. cm.[34] | an increase of pressure of | | 1 kilogm. per sq. cm. --------------------------------------------------------------------- -0° | 1 | -2.5° | 336 | 0.0074° -5° | 615 | 0.0090° -7.5° | 890 | 0.0091° -10.0° | 1155 | 0.0094° -12.5° | 1410 | 0.0100° -15.0° | 1625 | 0.0116° -17.5° | 1835 | 0.0119° -20.0° | 2042 | 0.0121° -22.1° | 2200 | 0.0133° --------------------------------------------------------------------- From the numbers in the table and from the figure we see that as the pressure is increased the melting point of ice is lowered; but we also observe that a very large change of pressure is required in order to produce a very small change in the melting point. The curve, therefore, is very steep. Increase of pressure by one atmosphere lowers the melting point by only 0.0076°,[35] or an increase of pressure of 135 atm. is required to produce a lowering of the melting point of 1°. We see further that the fusion curve bends slightly as the pressure is increased, which signifies that the variation of {27} the melting point with the pressure changes; at -15°, when the pressure is 1625 kilogm. per sq. cm., increase of pressure by 1 kilogm. per sq. cm. lowers the melting point by 0.012°. This curvature of the fusion curve we shall later (Chap. IV.) see to be an almost universal phenomenon. [Illustration: FIG. 2.] [Illustration: FIG. 3.] Equilibrium between Ice, Water, and Vapour. The Triple Point.--On examining the vapour-pressure curves of ice and water (Fig. 3), we see that at a temperature of about 0° and under a pressure of about 4.6 mm. mercury, the two curves cut. At this point liquid water and solid ice are each in equilibrium with vapour at the same pressure. Since this is so, they must, of course, be in equilibrium {28} with one another, as experiment also shows. At this point, therefore, ice, water, and vapour can be in equilibrium, and as there are three phases present, the point is called a _triple point_.[36]