Ferguson and Merwin — The Ternary System. 1S5 



of formulae derived by thermodynamical reasoning with 

 the aid of the laws of dilute liquid solutions. 19 



The complete liquidus and solidus relations along the 

 calcium metasilicate-akermanite line, which forms a true 

 binary system, are given in fig. 4. The calcium metasili- 

 cate-diopside line does not form a binary system and 

 therefore only that part dealing with the solid solutions 

 of the diopside series is given in fig. 5. The combined 

 liquidus and solidus relations over the part of the ternary 

 system which we have been discussing in this paper are 

 depicted upon the model shown in figs. 6 and 7. 



The compositions are indicated upon the base and the 

 temperatures by the vertical distances above this base. 

 The quintuple and quadruple points are represented by 

 the black vertical wires and the boundaries of the fields 

 by the black wires connecting these uprights. The solid 

 solutions of 5Ca0.2Mg0.6Si0 2 and of wollastonite are 

 represented by the lighter-colored and lower portions of 

 the solid part of the model, whereas the upper and darker 

 portions represent the pseudowollastonite solid solu- 

 tions. Fig. 6 is given to show the relation of the fields 

 to the solid solutions and to the compounds calcium meta- 

 silicate and diopside. One can see at a glance that the 

 latter compounds do not form a binary system. Fig. 7 

 is intended to give the reader an idea of the changes in 

 the inversion and decomposition temperatures caused by 

 solid solution. The inversion interval here shown 

 between wollastonite and pseudowollastonite solutions 

 was inserted upon theoretical grounds, no direct obser- 

 vation of it being possible with the methods available. 

 The accuracy of our results is somewhat less than that 

 suggested by these models. 



Unstable Phases. — If we consider the value of the 

 thermo-dynamical potential of a phase as a measure of 

 its stability, then of all the possible phases, that phase 

 which has the minimum potential will be the stable one. 

 A system in passing from an unstable to a stable condi- 

 tion may change its potential in one step, or if phases 

 of intermediate potential are possible, in several steps, 

 and it is but natural to suppose that if several steps are 

 possible the change will probably go by means of these 

 steps. This simple conception in regard to the forma- 

 tion of unstable phases has been found of rather wide 



19 Ostwald, Lehrbueh der allgemeinen Chemic, Bd. 2, Teil 3, p. 70. 



