EQUILIBRIUM OF HETEROGENEOUS SUBSTANCES. 



M aero for the given fluid, and is positive for every other phase of the same 

 component*, the condition of the given fluid will be stable. 



If this condition holds for all variations of the variables the fluid will be 

 absolutely stable, but if it holds only for small variations but not for certain 

 finite variations, then the fluid will be stable when not in contact with matter 

 in any of those phases for which K is positive, but if matter in any one of 

 these phases is in contact with it, its equilibrium will be destroyed, and a 

 portion will pass into the phase of the substance with which it is in contact. 



Thus in Professor F. Guthrie's experiments, a solution of chloride of calcium 

 of 37 per cent, was cooled to a temperature somewhat below 37* C. without 

 solidification. 



In this state, however, the contact of three different solids determines three 

 different kinds of solidification. A piece of ice causes ice to separate from the 

 fluid. A piece of the cryohydrate of cMoride of calcium determines the formation 

 of cryohydrate from the fluid, and the anhydrous salt causes a precipitation of 

 anhydrous salt. 



The phase of the fluid is such that K is positive for all phases differing 

 slightly from its own phase, and its equilibrium is therefore stable, but for 

 certain widely different phases, namely, ice, cryohydrate and anhydrous salt, 

 A' is negative. 



If none of these substances are in contact with the fluid, the fluid cannot 

 alter in phase without a transport of matter through a finite distance, and is 

 therefore stable; but if any one of them is in contact with the fluid, part of 

 the fluid is enabled to pass into a phase in which K is negative. The con- 

 ditions of consistent phases are that the values of 0, p, ft,.../t n , and K are 

 equal for all phases which can coexist in equilibrium, the surface of contact 

 being plane. 



This was illustrated by Mr Main's experiments on co-existent phases of 

 mixtures of chloroform, alcohol and water. 



