-2 



CHANGE OF STATE SOLID LIQUID. 207 



of a liquid or of a solid is slightly greater at greater pressures (see 

 chap, xix), which again implies that pressure increases the mobility. It 

 can be shown that the effect on the vapour-pressure of a given increase 

 of pressure is greater for ice than for water. Hence, we may suppose 

 the mobility of ice increased more than that of water by a given pressure, 

 and by putting a sufficient pressure on to a mixture of ice arid water 

 below 0, the mobilities can again be made equal. 



If we seek to represent the behaviour of ice and water on an indicator 

 diagram, we get curves whose general course is shown in Fig. 121. 

 There is no attempt here to draw to scale, the diagram merely represent- 

 ing the characteristic features. It is probable that the horizontal 

 portions would diminish gradually after a time as the expansion of water 

 for fall of temperature increases, and ultimately there may be a critical 

 point at which the volumes of ice and water become the same. Below 

 this temperature there would, 

 perhaps, be a gradual passage 

 from ice to water.* 



In mixtures, softening 

 may take place gradually 

 with rise of temperature 

 through the melting of one Waler 



constituent, which then, per- 260- 

 haps, dissolves the others. 

 If this occurs on reversing '30 



othe process, supersaturation \\\ . 



i r e ,*, - \ Mixture 



and superrusion may come in 



to lower the solidifying tem- 

 perature. This is observed 

 with fats. If the melting is 

 a true continuous change 

 from solid to liquid beyond Fl(J ' 121.-Ice and Water laothermals. 



the critical point, then, on 



reversing the order of change of temperature, the change of state should 

 be exactly reversed. Perhaps this kind of melting may be found to 

 occur in metals which soften as the temperature rises. 



Resemblance of Solution to Fusion. In several respects solution 

 and precipitation from solution resemble fusion and solidification. If a 

 salt is placed in excess in a solvent, a state of equilibrium is reached, 

 when the liquid is said to be saturated. If the mixture of salt and 

 solution is subjected to pressure, more salt is dissolved if the solution 

 occupies a volume less than that previously occupied by the constituents. 

 If the volume of the solution is greater than that of the constituents, the 

 pressure is accompanied by deposition from solution. 



Again, in most cases, rise of temperature increases the amount of 

 salt taken into solution. If the warm saturated solution is pcured off 

 the salt into a clean vessel, it is easy to cool the liquid without de- 

 position of the salt, though the amount dissolved is now greater than 

 that corresponding to the saturated condition. The liquid is then said to 



* Tammann (Ann. der Physik, 1900, II. 1, 424) finds that at -22 and under 2130 

 atmos. ice tends to pass into two other crystalline forms. Possibly ordinary ice 

 could not be subjected to much greater pressure than 2130 atmos. 



