HETEROGENEOUS SUBSTANCES. 149 



manner, the other may approach it and ultimately coincide with it. 

 The phase in which this coincidence takes place is called the Critical 

 Phase. 



The region of absolutely unstable phases is in contact with that of 

 absolutely stable phases at the critical point. Hence, though it may 

 be possible by preventing the body from coming in contact with certain 

 substances to bring it into a phase far beyond the limits of absolute 

 stability, this process cannot be indefinitely continued, for before the 

 substance can enter a new region of stability it must pass out of the 

 region of relative stability into one of absolute instability, when it 

 will at once break up into a system of stable phases. 



Thus in water for any given pressure there is a corresponding tem- 

 perature at which it is in equilibrium with its vapour, and beyond 

 which it cannot be raised when in contact with any gas. But if, as 

 in the experiment of Dufour, a drop of water is carefully freed from air 

 and entirely surrounded by liquid which has a high boiling point, it 

 may remain in the liquid state at a temperature far above the boiling 

 point corresponding to the pressure, though if it comes in contact with 

 the smallest portion of any gas it instantly explodes. 



But it is certain that if the temperature were raised high enough the 

 water would enter a phase of absolutely unstable equilibrium, and that 

 it would then explode without requiring the contact of any other sub- 

 stance. 



Water may also be cooled below the temperature at which it 

 generally freezes, and if the water is surrounded by another liquid of 

 the same density the pressure may also be reduced below that of the 

 vapour of water at that temperature. If the water when in this phase 

 is brought in contact with ice it will freeze, but if brought in contact 

 with a gas it will evaporate. 



Professor Guthrie has recently discovered a very remarkable case of 

 equilibrium of a liquid which may be solidified in three different ways 

 by contact with three different substances. This is a solution of 

 chloride of calcium in water containing 37 per cent, of the salt. This 

 solution is capable of solidification at 37 C., when it forms the solid 

 cryohydrate having the same composition as itself. But it may be cooled 

 somewhat below this temperature, and then if it is touched with a bit 

 of ice it throws up ice, if it is touched with the anhydrous salt it throws 



