Rankin and Wright — Ternary System CaO-Alfi % -8iO r 15 



" The adoption of the phase rule for the purposes of classi- 

 fication has been of great importance in studying changes in 

 the equilibrium between different substances : for not only 

 does it render possible the grouping together of a large num- 

 ber of isolated phenomena but the guidance it affords has led 

 to the discovery of new substances, has given the clue to the 

 conditions under which these substances can exist and has led 

 to recognition of otherwise unobserved resemblance between 

 different systems." * 



" The phase rule informs us only as to the general conditions 

 of equilibrium and leaves the determination of the definite 

 experimental data to experiment."*) - 



Before taking up, then, the experimental study of any sys- 

 tem, it is important to consider the general conditions of 

 equilibrium for that system, as deduced from the phase rule. 



The Application of the Phase Rule to the Study 

 of Equilibria in the System CaO-Al 2 3 -Si() 2 . 



For systems, such as those involving CaO, A1 2 3 and Si0 2 , 

 certain restrictions can be imposed, which will not appreciably 

 affect the definitions of conditions of equilibrium and which 

 will reduce to a considerable extent the number of possibilities 

 which we have to consider. 



Since the change in vapor pressure of these three oxides is 

 unimportant over the range of temperatures at which we wish 

 to study them, we may assume that under constant pressure 

 (atmospheric) any system may be defined by the degrees of free- 

 dom, temperature and concentration. The vapor phase must, 

 however, always be considered to be present. As liquids these 

 three oxides are miscible in all proportions, so that for any 

 system there can be but one liquid phase. From the experi- 

 mental study we know that as solids the various oxides and 

 their compounds are practically immiscible, so that in discuss- 

 ing theoretical possibilities we will not take up solid solution. 



The systems, then, which we will have to consider are those 

 in which (1) the vapor pressure of the solid and liquid is sub- 

 stantially constant during temperature change, (2) there is but 

 one liquid phase, and (3) there are no solid solutions formed. 



According to the Phase Rule, P + F= C + 2 ; hence since 

 in this case (7=3, the number of the phases and degrees of 

 freedom (P+FJ — 5, so that i^may be 0, 1 or 2. 



In order that a three component system be nonvariant five 

 phases are necessary, of which there are two possible such sys- 

 tems, S-8-S-S- Fand S-S-S-L- V. (S= Solid ; Z= Liquid ; 

 "F-^Yapor.) In order that the system be univariant four 



*Findlay, " Phase Eule,'' pp. 17-18. 

 f Findlay, ibid., p. 53. 



