54 Rankin and Wright — Ternary System OaO-Alfi \-SiO 



with the original 20 kilograms of OS. The percentage of S 

 and OS in e may be checked from the figures given. Solution 

 e contains 4=&6 kg. S + 20 kg. CS= 6&6 kg : 



X 100 = 70 per cent S. 



66-6 

 20- 



X 100 = 30 per cent CS. 



If the solution e is cooled further, S continues to separate 

 until point A is reached. Since this is the eutectic temperature, 

 pure S and pure OS separate together with no further change 

 in the concentration of the solution. 



Within the field A.-2-L-S, solutions containing the three 

 components will be in equilibrium with S. If a solution 

 represented, for instance, by a point b is cooled, S will crystal- 

 lize, while the composition of the residual solution will be 

 indicated by a point on a straight line passing through b and 

 the point giving the composition of the phase separating, viz., 

 S, at the apex of the triangle. If the solution is cooled further, 

 each successive, residual solution will have its composition given 

 by a point on the straight line through b and S, until finally a 

 solution b x will appear whose composition is found on the 

 boundary A-2. The line bb 1 is, therefore, the locus of the 

 points which give the compositions of the successive solutions 

 which are formed from the original solution b, on cooling, 

 with the separation of *S r as solid phase. 



The amount of S which has crystallized from solution b (a 

 ternary solution) is computed in the manner previously indicated 



for a binarv solution. In cooling from b to b x , -77- 1 X 100 per cent 



of the solution having the composition b has separated as S. 



h ^ 



So, also, -rr- X 100 per cent of the original solution b remains 



as solution having the composition b r From the phase rule 

 definition of univariant systems the solution b 1 is in equilibrium 

 with #and CS', therefore if the temperature be again lowered 

 a mixture of S and CS must crystallize. 



It has been stated that a straight line joining the points 

 representing the composition of a solid phase and that of a 

 solution from which it has separated, will pass through the 

 compositions of the residual solutions. The converse, which is 

 also true, is the form in which the principle was applied by 

 Bancroft* in his method of analysis of solid phases. If a 

 given solution is cooled, a straight line joining the successive 



*J. Phys. Chem., vi, 178, 1902. 



