APPLICATION OF EQUILIBRIUM LAW TO SEPARATION OF CRYSTALS. 263 



volatile solvents, equilibrium— i.e. saturation— is attained at a particular 

 pressure at a particular temperature, and vice versd, when n substances 

 are present in n + \ states or phases separable as such, each such state 

 being termed technically a phase.^ It is necessary to make this distinc- 

 tion °in order to guard against the application of the term ' phase ' to the 

 radicles of salts. The whole investigation may therefore be considered 

 independently of the modern hypothesis of solution, solely on the basis of 



I8.cts 



The real difficulty that occurs in practice is to know what are the 

 possible phases— in other words, to determine the nature of the double 

 salts or distinct hydrates that may be formed. In the case of saturated 

 solutions of non-volatile solids in a volatile solvent, as vapour of the 

 solvent IS always present, the solvent occurs in two phases, and therefore 

 the condition under which equilibrium— i.e. saturation— is determined is 

 that «— 1 solids exist in contact with the liquid. As the presence of 

 these solids determines the equilibrium, they may very properly be spoken 

 of as equilibrators, and this term may be used as the equivalent of the 

 somewhat inexpressive German phrase ' Bodenkorper.' 



The cases to be considered are the following : — 



Case I. — Solutions saturated with a single salt. 



In these two constituents (salt and water) are present in three phases— 

 the gaseous phase, one liquid phase, and one solid phase— and as a rule 

 only one solid equilibrator can act at a time ; but as not only the anhydrous 

 substance, but also its various hydrates, may equally serve as equilibrators 

 when hydrates are formed, two equilibrators— either the anhydrous sub- 

 stance and its hydrate, or two of its hydrates if there be more than one 

 possible— may act simultaneously at some particular pressure and tem- 

 perature, usually called the transition jmnt. Obviously this complication 

 arises from a variation in the behaviour of the substance relatively to the 

 solvent as the external conditions are modified. As hydrates only diifer 

 in the number of solvent molecules they contain, they are to be regarded 

 as but one substance, the molecules of the solvent attached to them being 

 left out of account. In any case, the presence in the solid state as equi- 

 librator of the particular compound or compounds with which the solution 

 is to be saturated is always the essential factor. 



To give an example : in the case of Sodium sulphate, the monohydrate 

 and decahydrate coexist in equilibrium with the solution at 32°-65 under 

 the corresponding vapour pressure ; but it follows from the above that at 

 any other temperature only one at a time of the hydrates can be in 

 equilibrium with the solution, inasmuch as a single substance cannot, a-s 

 a rule, give rise to a solution saturated with reference to two such equili- 

 brators, the existence of two such compounds, except at the transition 

 point, being only possible in presence of a second salt : this serving, in fact, 

 to condition the change in hydration. 



Case II. — Solutions saturated with tivo salts which iMSSess similar basic 

 or acid radicles, e.g., NaCl, KCl. 



' It must, however, be noted that if there be either n or fewer pliases present, 

 equilibrium is possible under every set of conditions compatible with the existence 

 of the phases considered. For example, in the case of an unsaturated solution of 

 Sodium chloride in presence of its vapour, no solid phase being present, the vapour 

 pressure of the solution at each temperature is different at different concentrations : 

 and therefore a solution and its vapour may be in equilibrium at any pressure withio 

 the possible limits at each particular temperature, 



